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Best Self-Teaching Biology Guide for Dummies; Understand the Human Science the Best Way

By Anton Romanov

Copyright © 2015 by Anton Romanov

All rights reserved. No part of this publication may be reproduced, distributed, or transmitted in any form or by any means, including photocopying, recording, or other electronic or mechanical methods, without the prior written permission of the publisher, except in the case of brief quotations embodied in critical reviews and certain other noncommercial uses permitted by copyright law.

Table of Contents


Thank you for choosing to read the book, “Biology: Best Self-Teaching Biology Guide for Dummies; Understand the Human Science the Best Way.”

This book contains a comprehensive, self-teaching, easy-to-grasp biology lesson and guide.

The term “biology” was derived from the Greek words bios, meaning life, and logos, meaning study. Biology is literally the study of life. It is a branch of natural science that includes the anatomy, physiology, morphology, and origin of living organisms. Organisms encompass all living entities that either consist of one cell—like bacteria and protozoa—or multiple cells, such as plants and animals.

There are three major branches of biology, each of them based on different life forms: zoology, botany, and microbiology. Biology was established on four foundations of study: cell theory, evolution, gene theory, and homeostasis. Biology covers many more areas of study, which this book seeks to put together in one place, in a way that makes the science easy to follow and understand.

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Chapter 1: Atoms and Molecules

To understand biology, you must first consider the science at a molecular level.


Atoms are the infinitesimal building blocks of every object, both living and nonliving alike. Your own body is made up of billions upon billions of atoms. Elements, such as carbon and oxygen, consist of only one type of atom. For example, pure 24-karat gold is composed of only gold atoms and nothing else. Atoms are the smallest particle into which an element can be split.

Because an atom is thinner than a strand of hair, you would think atoms were the smallest possible objects in the world. It gets smaller, however. Atoms contain even tinier bits and pieces known as subatomic particles.

Parts of an Atom

The core of an atom is called the nucleus, which houses the protons and neutrons. The nucleus is dense, holding most of an atom’s mass. Atoms contain three very distinct subatomic particles:

  1. Protons: These are stable subatomic particles that are found in the nucleus of all atoms. Protons have a positive electric charge that is equal in scale to the charge of an electron but has the opposite sign. Atoms contain at least one proton.

  1. Electrons: These are the negatively-charged polar opposite of protons. Like protons, electrons are also stable subatomic particles, except they are found outside the nucleus of an atom. Electrons act as the primary carrier of electricity in solids.

  1. Neutrons: Neutrons are uncharged and share the nucleus with protons. They have roughly the same mass as a proton.


A molecule is what you get when you join two or more atoms together by chemical bonds. Molecules may contain a mixture of different elements. You can create anything out of atoms, from beetles to freight trains; it’s just a matter of bonding atoms in different ways.

Molecules represent the smallest particle in a chemical compound. A compound is the term for a molecule that contains at least two different elements. While all compounds are molecules, not all molecules are necessarily compounds.

Some common molecules include:

  1. Water: H₂O

  1. Ozone: O₃

  1. Nitrogen: N₂

  1. Calcium oxide: CaO

  1. Glucose: C₆H₁₂O₆

Chapter 2: Cells: The Building Blocks of Life

Cells are the basic building blocks of life. They developed on Earth some 3.5 billion years ago. Typically microscopic, cells are the smallest structural and functional components of an organism. Cells are able to replicate independently. The amount of cells composing an organism is varies from order to order. Human beings are made of roughly 100 trillion cells.

Cell Theory

Cell theory is a biological theory that defines the properties of cells. It was developed by Matthias Jacob Schleiden, a German botanist, and Theodore Schwann, a German physiologist. Cell theory consists of four major tenets:

  1. All organisms are made of at least one cell.

  1. Cells are the basic unit of function and structure in all living things.

  1. All cells spring from preexisting cells—which is a process described as replication.

  1. All cells contain the hereditary data necessary for regulating cell functions and for passing that information on to the next generation of cells.

Cell Classification

Organisms are classified as one of two cell types, based on the number of cells of which they are composed.


  1. Prokaryotes are single-celled organisms.

  1. They were the very first life forms to prosper on Planet Earth.

  1. Typical prokaryotes include archaea and bacteria—the two domains of life.

  1. Prokaryotes represent the simplest and smallest type of organisms.

  1. Prokaryotic cells possess very few structures and do not contain a nucleus.


  1. Eukaryotes are multicellular organisms.

  1. They are more complex and are roughly 15 times the size of prokaryotes.

  1. Unlike prokaryotes, eukaryotic cells contain a double-membrane nucleus.

  1. Eukaryotic cells have specialized compartments—membrane-bound organelles in which specific metabolic processes occur.

  1. A eukaryotic cell consists of multiple chromosomes, whereas a prokaryotic cell only possesses one chromosome.

Both prokaryotic and eukaryotic cells are encased in a membrane that filters material traveling in and out of the cell.

Parts of a Cell

Both prokaryotic and eukaryotic cells consist of specialized structures that all work together in harmony to maintain normal cell function. The parts of a cell are also referred to as organelles.

  1. Nucleus: The nucleus is the command center of eukaryotic cells. It directs all cell activity. The nucleus contains the majority of the cell’s DNA, which are organized into chromosomes.

  1. Nuclear Membrane: This is referred to as a thin, bilayer, semi-permeable sheath that envelops the cell’s nucleus. The nuclear membrane keeps DNA safe within the nucleus and protects it from external cell materials.

  1. Cytoplasm: The cytoplasm is a jelly-like substance that fills the inside of a cell. It is primarily composed of water and salt. Cytoplasm is present in both eukaryotic and prokaryotic cell types.

  1. Golgi Apparatus: Consider this organelle the delivery service of a cell. It sorts and packs the proteins produced by the ribosomes to be sent out of the cell.

  1. Mitochondrion: This organelle is often found in great numbers within a cell. It is frequently referred to as the powerhouse of a cell. The mitochondrion is responsible for converting glucose molecules into energy. It is also the site of the biochemical process of cellular respiration.

  1. Vacuole: Like mitochondrion, a vacuole is usually found in multiple quantities within a cell. Vacuoles act as a cell’s storage tank. They are capable of storing food, water, and cellular waste.

  1. Ribosome: This cell organelle is like a micro-machine, responsible for protein production. Ribosomes are found either floating freely in the cytoplasm of a cell or attached to the endoplasmic reticulum.

  1. Endoplasmic Reticulum: The ER serves as a transportation network that carries materials, such as synthesized protein, around the cell. There are two types:

  1. Rough ER: Endoplasmic reticulum that does not have any ribosomes attached to it.

  1. Smooth ER: Endoplasmic reticulum that has ribosomes attached to it.

  1. Lysosome: This is a cell organelle that is often described as the stomach of a cell. It facilitates cellular digestion of food, waste, and worn out cellular components.

  1. Nucleolus: Consider this organelle the brain of a nucleus. Its main function is ribosome production, as well as making rRNA—the substance of which ribosomes are composed.

  1. Cell Membrane: This bilayer membrane can be referred to as a cell’s gatekeeper. It serves as a barrier that protects the cell from its environment. The cell membrane is selectively permeable; meaning, it allows some materials to pass, but not others.

  1. Chloroplast: This is an organelle found in large numbers within eukaryotic plant and algae cells. Chloroplast is a unique chlorophyll-containing organelle that conducts photosynthesis.

  1. Cell Wall: Composed of cellulose, the cell wall is a rigid outer layer of a cell that surrounds the cell membrane. The cell wall offers a cell adequate protection and support.

  1. Vesicle: This small organelle is composed of a lipid bilayer and may be considered the messenger of a cell. Vesicles transport material within the cytoplasm of a cell.

  1. Cytoskeleton: This structure is made up of proteins and lends shape and support to a cell. It is composed of three major structural components: microfilaments, intermediate filaments, and microtubules.

  1. Centriole: The centriole is a set of microtubules, which migrate to opposite poles of the cell in preparation for cell division.

  1. Flagella: A flagellum is a whip-like structure that is present in both eukaryotic cells and prokaryotic cells. It lends locomotion to a cell.

  1. Chromosomes: These are thread-like arrangements of DNA in the nucleus of a cell. Chromosomes are made of tightly-coiled DNA.

Chapter 3: Cell Cycle and Cell Division

The Cell Cycle

Growth and the ability to reproduce are important properties of both unicellular and multicellular organisms. Cell growth is made possible through the synthesis of new proteins, carbohydrates, lipids, and nucleic acids. When these molecules pile up, the accumulation causes an increase in cell volume. The plasma membrane, in turn, expands in order to prevent the cell from erupting. However, cells cannot continue to enlarge forever, therefore, cell growth results in cell division.

When cells divide, the daughter cells are genetic duplicates of the parent cell; daughter cells contain the same DNA arrangements. All the genetic data within the parent cell must be carefully replicated and distributed among the daughter cells. This is accomplished by the cell cycle—a series of stages that a cell must cross.

The cell cycle of eukaryotic cells is divided into four distinct phases in eukaryotic cell division: G1, S phase, G2, and M phase. Collectively, the first three phases are referred to as interphase.

G1 Phase

Also called the Gap 1 phase, G1 is the first of the four cell cycle phases. During G1, the cell increases in size. The cell synthesizes mRNA and proteins. Messenger RNA (mRNA) is a family of RNA molecules that transmits genetic information from DNA to the ribosome.

S Phase

From G1, the cell transitions into S phase—also known as DNA synthesis. This transition is a major checkpoint in the cell cycle, as the cell will only enter the S phase provided it has sufficiently grown during G1 and the DNA contained within it is undamaged.

When S phase begins, DNA replication occurs. The goal of DNA replication is to create two identical chromosomes. The complete DNA information in the cell must be duplicated precisely and accurately in order to produce two identical daughter cells. Accurate replication is essential for the prevention of genetic abnormalities. The cell leaves S phase upon completion of DNA replication.

G2 Phase

From S phase, the cell enters G2, or Gap 2—the last phase of interphase. In G2, the cell prepares to enter mitosis. It is a period of rapid cell growth and protein synthesis. At the G2 checkpoint, the cell analyzes DNA for any damage that might have occurred during S phase. The cell also certifies the presence of the proteins required for cell division.


Mitosis is the process of cell division that produces two identical daughter cells. It is divided into four phases: prophase, metaphase, anaphase, and telophase.


  1. The nucleolus and nuclear membrane of the cell disappear.

  1. Chromatin condenses into tightly coiled chromosomes that are visible under a light microscope.

  1. The formation of the spindle occurs, which is an intercellular structure that is required for cell division.

  1. Centrioles divide and migrate.


  1. Chromosomes attach themselves to the cell’s spindle fibers.

  1. The spindle prods the centromeres—sites on a chromosome by which it is attached to a spindle fiber—until the chromosomes are lined up across the equator of the spindle.


  1. Spindle fibers pull apart each pair of sister chromatids. They separate and begin to migrate to opposite poles.


  1. Chromosomes arrive at the poles of their respective spindles, unwinding into chromatin form.

  1. The nuclear envelope and nucleolus reappear.

  1. The spindle disintegrates.

  1. The cell undergoes the cytokinesis process, which divides the cell into two identical daughter cells.

Chapter 4: Laws of Thermodynamics

Thermodynamics is the branch of physics that is concerned with the relation between heat and other energy forms. Simply put, thermodynamics is the study of energy. Energy is the power that is derived from the use of physical or chemical sources. Energy is a vital resource upon which the world is dependent as it provides heat, light, and electricity to power machines.

The laws of thermodynamics are central principles to biology. These laws regulate the metabolic processes in all organisms. There are two major principles of thermodynamics.

The First Law of Thermodynamics

  1. The first law is also known as the law of conservation of energy.

  2. It states that energy cannot be created, nor can it be destroyed. Energy may change forms, but the total energy of a system will always remain constant.

  3. The total amount of energy in the world remains constant.

The Second Law of Thermodynamics

  1. The second law states that when energy is transferred, the available energy at the end of the transfer will be less than the amount of energy at the beginning of the process.

  2. Because of entropy—the measure of the degree of “disorder” of a system—all of the available energy at the end of the transfer process will be unusable to the organism. When energy is transferred, there is an upsurge in entropy.

Chapter 5: Photosynthesis


Organisms—chiefly plants—use an essential process known as photosynthesis to convert light energy from the sun into food. The process is accomplished through the ability of plants to trap light energy with their leaves. Plants employ the sun’s energy in order to convert carbon dioxide and water into glucose. In turn, plants use glucose for energy, as well as in the production of other fundamental substances, such as starch and cellulose. While plants store starch in their seeds as a food source, cellulose is used in the formation of cell walls.

Conversions That Occur

When glucose is produced through photosynthesis, cellular respiration converts the sugar into ATP. ATP, or adenosine triphosphate, is an energy form that organisms produce. It is often referred as the fuel on which all living organisms run. Photosynthesis uses chlorophyll in the process as well. Chlorophyll absorbs light energy and converts it into chemical energy.

The photosynthesis process can be written as a chemical equation:

6H₂O + 6CO₂  C₆H₁₂O₆ + 6O₂

The above equation can be understood and read as: Six molecules of water plus six molecules of carbon dioxide produce one molecule of glucose sugar plus six molecules of oxygen.

The Two Stages of Photosynthesis

Photosynthesis is not a one-step process. It takes place in two successive stages: light-dependent and light-independent reactions.

The Light Dependent Process

The first process requires direct light energy to produce the energy-carrier molecules that will be used in the following process. The process involves capturing light energy and using that energy to produce ATP and NADPH—nicotinamide adenine dinucleotide phosphate—which are both stored in the plant.

The Light Independent Process

This final process is known as the Calvin Cycle. It occurs when products from the first process are utilized in the construction of sugar molecules from carbon dioxide. Although this process does not require any sunlight, it does require the products of the light dependent process.

Chapter 6: Plant Structures and Nutrition

Plants are classified as eukaryotes of the kingdom Plantae. They have two organ systems: the shoot system, which consists of the plant parts that are above ground, and the root system, which is comprised of plant structures that lie underground.

Parts of a Plant

All plants are composed of various structures, each of them contributing to the plant’s overall function. The following lists the main plant parts:

  1. Flower: Also known as blossoms, flowers offer plants a unique appeal that differs from other eukaryotes. A flower’s petals are designed to be showy and attractive in order to attract pollinators—birds, bees, and other insects. Plants depend on pollen for fertilization. In short, the flower is responsible for producing seeds.

  1. Leaf: The leaves of a plant serve as sunlight-collectors for the photosynthesis process. The delicate veins in a leaf distribute water and nutrients throughout the entire leaf.

  1. Fruit: The ovules within a plant grow into seeds when the plant has been fertilized. The ovary matures into a fleshy fruit.

  1. Stem: The primary functions of a plant’s stem are to lend support to the leaves, carry water and nutrients to the leaves, and transport the products of photosynthesis from the leaves to other plant parts.

  1. Root: The roots of a plant anchor and support the plant, absorb and manage water and minerals, and store the products of photosynthesis, such as carbohydrates and sugars.

Parts of a Flower

The flower is the site of reproduction in flowering plants. Sexual reproduction involves the production of male and female gametes and the transfer of male gametes to the female ovules through the pollination process. Fertilization occurs after pollination wherein the ovules develop into seeds within a fruit.

The anatomy of a flower is slightly more complex than that of its host plant. The key components of a flower are as follows:

  1. Petals: The petals of a flower function as both a source of protection for the flower and as a means by which to attract pollinators.

  1. Pistil: The female reproductive organs of a flower is made of three distinct components.

  1. Stigma: The stigma of a flower acts as a pollen receptor. The tip is sticky so as to enable pollen to adhere to it.

  1. Style: The style is the long, thin stalk that connects the stigma and ovary. This structure basically checks pollens’ capability of fertilizing the plant.

  1. Ovary: This structure lies at the base of the pistil. The ovary contains one or more ovules, which becomes the plant’s seeds when the ovary grows into a fruit.

  1. Stamen: The stamen is the reproductive system of a flower. It is responsible for producing the pollen grains that house male sex cells necessary for reproduction. The stamen is composed of two parts:

  1. Anther: The anther is a small, oval-shaped structure where pollen is produced.

  1. Filament: This is a thin hair-like tube that supports the anther.

Plant Hormones

A hormone is a term for a chemical that is produced in one part of the body to be directed elsewhere in the body. In terms of plants, hormones that are produced by a plant regulate plant growth. Plant hormones are also known as phytohormones. While plants only produce five hormones, animals and humans produce many more.

Abscisic Acid

Abscisic acid is the first of five plant hormones that plays a key role in plant development and plant stress. Unlike animals, plants do not have a fight-or-flight response to danger like drought and salinated soil. They are required to either adapt or die. Abscisic acid mediates a plant’s adaptation to stress.

During the winter months, abscisic acid mediates the conversion of a flower’s meristem into a dormant bud in order to protect it from the harsh cold and mechanical damage. Abscisic acid also triggers the stomata—pores on leaves—of a flower to close in the event that water from the soil is insufficient to keep up with transpiration. Abscisic acid promotes seed dormancy—a state in which seeds are prevented from germinating during unsuitable ecological conditions.


Auxins were the first class of plant growth regulators discovered. They are primarily produced in a flower’s stem and buds. Auxins influence cell growth, root initiation, and bud formation. They promote stem elongation and maintain apical dominance as well. Auxins control the growth of roots, stems, and fruits, too.


Also known as CKs, cytokinins are a family of chemicals that positively influence shoot formation and cell division. They are produced in the meristem at the tip of a shoot.


Ethylene is a gas that is mainly produced by ripened fruits. It affects cell shape and cell growth. If, for example, a growing shoot hits an obstacle, it triggers an increase in ethylene. This increase in the gas inhibits cell elongation and causes the stem to swell. Due to the resulting thickness, the stem now has the capability to exert more pressure against whatever object is impeding its path.


The main function of gibberellins is to initiate the mobilization of storage materials within seeds during germination. Gibberellins promote stem elongation, and they also stimulate filament growth.

Plant Nutrition

Various chemical elements and compounds are necessary for plant growth. Plants acquire nutrients and minerals from the atmosphere and the soil. Just by simply using sunlight and modifying the products attained through photosynthesis, plants are capable of making all the organic macronutrients they require. However, plants also depend on vital minerals from the soil, which they absorb through their root systems. Without these minerals, plants are incapable of completing a normal life cycle. A plant’s balanced diet includes the following elements:

Essential Elements

  1. Carbon: This element acts as a necessary support for a plant’s biomolecules, such as starches and cellulose. Carbon fixation occurs when carbon dioxide is converted to organic compounds through photosynthesis. Carbon is included in the carbohydrates that a plant stores in its leaves and seeds for energy.

  1. Hydrogen: Plants require hydrogen for its formation, as well as sugar production. This element is primarily obtained from water. It facilitates respiration and photosynthesis.

  1. Oxygen: This element is essential for cell respiration within a plant.

Primary Macronutrients

  1. Nitrogen: Plants absorb this element from nitrate or ammonium ions in the soil through their roots. Plants use nitrogen to make nucleic acids and for protein production in the form of enzymes. These proteins are necessary for plant nutrition.

  1. Phosphorous: Plants rely on phosphorous for growth and other vital processes. The element is important in cell division and the development of new tissue.

  1. Potassium: This element helps plants in the formation of proteins and carbohydrates and accelerates enzyme action. Potassium also contributes to photosynthesis.

Secondary Macronutrients

  1. Calcium: This element is involved in photosynthesis. Calcium regulates the transport of nutrients throughout a plant.

  1. Sulfur: Sulfur is essential in the manufacturing of chloroplasts. It is involved in the production of proteins and in the activation of enzymes.

  1. Magnesium: Magnesium is not only involved in photosynthesis, but in enzyme reactions as well. It is part of the chlorophyll contained in plants.


While micronutrients—or trace elements—are not as essential to plants as macronutrients, they exhibit properties that positively affect plant growth. Small quantities of micronutrients are all a plant needs to function; large quantities are toxic to plants.

  1. Boron: This trace mineral is involved in cell division, sugar transport, and enzyme synthesis. Small quantities of boron are needed for pollen germination, membrane functioning, and cell elongation.

  1. Chlorine: Chlorine plays a role in photosynthesis. It is necessary for plant osmosis and ionic balance.

  1. Manganese: This element is also involved in photosynthesis. Manganese helps plants build chloroplasts.

  1. Iron: Though hardly needed, iron is important for healthy plant life. Iron helps transport essential elements within a plant. Without iron, a plant would be incapable of getting oxygen and producing chlorophyll.

  1. Zinc: An essential trace element, zinc helps plants produce chlorophyll. Without zinc, the leaves of a plants would be discolored.

  1. Copper: Plant growth partially depends on the presence of copper in the soil. Copper promotes seed formation and is involved in various enzyme activities.

  1. Nickel: Nickel plays an important role in seed germination and in the activation of several enzymes.

  1. Molybdenum: Molybdenum helps a plant build amino acids.

Chapter 7: Genetics

Genetics is the study of heredity, genes, and genetic variation in organisms. It focuses on the manner and means of passing traits, like hair color and eye color, from parents to offspring. Genetics influence how inherited traits vary from person to person. Your own personal genetic information is referred to as your genome, or genetic code. DNA is the chemical that makes up your genome.

Gregor Mendel

Gregor Mendel was both a scientist and an Augustinian friar, who is known as the father of genetics. Mendel conducted a series of pea plant experiments showing that the crossbreeding of living organisms favored certain desirable traits. His experiments established the rules of heredity, which are now referred to as the laws of Mendelian inheritance.

Pea Plant Experiment

Gregor Mendel’s pea plant experiment involved seven distinct characteristics of pea plants: plant height, pod color, pod shape, seed color, seed shape, flower color, and flower position.

When experimenting with seed colors, Mendel showed that yellow peas and green peas always produced yellow plants. Mendel explained this phenomenon by creating the terms “recessive traits” and “dominant traits”—the color yellow being a dominant trait.

Mendel’s Laws of Inheritance

After careful analysis, Mendel devised three laws of inheritance:

  1. The Law of Dominance: This law states that some alleles—variant forms of a gene—are dominant while other alleles are recessive. This means that recessive alleles will always be overshadowed by dominant alleles.

  1. The Law of Segregation: This law states that when an organism produces gametes, the copies of a gene segregate during cell division so that each gamete receives just one copy.

  1. The Law of Independent Assortment: This law states that genes from different traits are inherited independent of each other.


Heredity is the genetic passing of traits—both mental and physical characteristics—from one generation to the next through sexual or asexual reproduction. This is why siblings often resemble one another. Heredity is a process that occurs in various eukaryotic organisms. Offspring cells acquire the characteristics of their parent cells through heredity.

Genes are what control inherited traits. Genotype is the term for the complete set of genes within an organism’s genome. Examples of inherited characteristics include eye color, prominent ears, dimples, curly hair, and freckles. A human may inherit any of those traits plus more from either of his parents. Heritable traits are passed from parents to offspring through a channel known as DNA.

An organism’s phenotype refers to the complete set of observable traits of his structure and behavior. Observable traits stem from the interaction between an organism’s genotype and the environment. Therefore, phenotypic traits are not inherited. For example, suntanned skin arises from the interaction between a person’s genotype and sunlight; thus, suntanned skin cannot be passed on to that person’s children.


A trait can be defined as a genetically determined characteristic, or a distinguishing quality that belongs to a person. It is sometimes called a phenotypic trait. Simply put, a phenotypic trait is the expression of genes in an observable way. Traits can be inherited, environmentally determined, or a combination of both.

Traits such as hair color are controlled by fundamental genes, which make up an individual’s genotype. The actual hair color that is observable, however, is the phenotype. Basically, the phenotype is the physical characteristics of an organism.

Traits are the final products of a succession of molecular and biochemical processes. The process begins with DNA. DNA travels to RNA and finally to protein. This current of genetic information can also be followed through the journey of a cell as it travels from the nucleus, to the cytoplasm, to the ribosomes, to the ER, and finally, to the Golgi Body. The Golgi Apparatus packages the final products in preparation for release into an organism’s tissue and organs. There, the products will affect the physiology and produce a trait.


A gene serves as the basic unit of heredity. Genes are often referred to as instruction manuals for the bodies of living organisms. They are made up of DNA; one strand of DNA contains numerous genes. A human being has about 25,000 genes. Some genes may hold instructions for producing hemoglobin proteins while others contain instructions for hair or eye color.


A chromosome is a threadlike structure that contains DNA and is found in the nucleus of living cells. It carries genetic data in the form of genes. The number of chromosomes one cell carries depends on the species. One human cell, for example, contains exactly 46 chromosomes. In humans, DNA is organized into two sets of 23 chromosomes. In the representation above, matching chromosomes are paired up—one each from the male parent and the female parent.

Chapter 8: Reproduction

Reproduction is a biological process by which organisms create offspring. All organisms exist as the products of reproduction. Reproduction can either be a sexual or asexual process.

Sexual Reproduction

In sexual reproduction, organisms create offspring by combining the genetic material of a male parent and a female parent. The biological process begins with meiosis—cell division that results in four daughter cells, each cell containing half the number of chromosomes of the parent cell. Male and female gametes must fuse in order to form a zygote, which in turn, develops into genetically distinct offspring. Most eukaryotes reproduce sexually.

The sexual reproduction process is explained using various terms:

  1. Ovum: This is the female reproductive cell of animals. Ovum is also referred to as the female gamete. After fertilization, an ovum is capable of dividing, which gives rise to an embryo.

  1. Spermatozoon: This is the male reproductive cell of animals by which the ovum is fertilized. It is composed of a compact head and one or more flagella for locomotion.

  1. Zygote: When an ovum is fertilized, it is referred to as a zygote. A zygote is a diploid cell that is the product of fertilization.

  1. Haploid: Haploid cells are those that contain only one complete set of chromosomes and are produced by meiosis. Ovum and spermatozoon are haploid cells. Human haploid cells contain 23 chromosomes.

  1. Diploid: Diploid cells result from the fusion of ovum and spermatozoon—or fertilization. Diploid cells have two sets of chromosomes—one set obtained from the male parent and the other obtained from the female parent. Human diploid cells contain a complete set of 46 chromosomes.

Asexual Reproduction

In asexual reproduction, organisms create genetically identical copies of themselves without the contribution of genetic material from another organism. Offspring arise from a single organism, inheriting only the genes of that parent. Bacteria typically reproduce asexually. Some species are capable of reproducing both sexually and asexually, such as jellyfish and yeast.

Types of Sexual Reproduction

There are four types of sexual reproduction:

  1. Autogamy: Autogamy is self-fertilization, such as the self-pollination of a flower. Organisms that possess both male and female reproductive parts that are both fully functioning are called hermaphrodites. Earthworms and most plants are capable of reproduction via autogamy.

  1. Allogamy: In plants, allogamy is the fertilization method of a flower by another flower’s pollen. In humans, however, allogamy is defined as cross-fertilization. In allogamy, an ovum is fertilized by the spermatozoa of another. Humans always reproduce via allogamy.

  1. Internal Fertilization: This is fertilization that occurs inside the female womb through sexual intercourse via copulation.

  1. External Fertilization: By contrast, external fertilization occurs outside the female body. It is a strategy of fertilization in which the male and female egg cells fuse in the open. Species, such as plants and animals that live in water, usually undergo external fertilization. A female oceanic species may lay her eggs in the water; a male of the same species will then spray his sperm over the eggs to fertilize them.

Types of Asexual Reproduction

Asexual reproduction occurs via one of five common modes:

  1. Binary Fission: This type of asexual reproduction can be described as division in half. Binary fission occurs primarily in prokaryotes. The process is fairly similar to the mitosis process that occurs in eukaryotes. In binary fission, a single cell duplicates its DNA then divides into two identical daughter cells.

  1. Budding: Budding is another form of asexual reproduction in which offspring grow on the side of the adult through an element known as a bud. A new organism remains attached to its parent until it reaches maturity wherein it breaks off and becomes a self-sufficient individual organism.

  1. Fragmentation: In fragmentation, a new organism develops from a viable fragment of its parent. Starfish undergo fragmentation when one of their five arms breaks off and regenerates into offspring.

  1. Parthenogenesis: In parthenogenesis, an ovum reproduces without the help of a male gamete and without undergoing fertilization. This form of asexual reproduction occurs in lower plants and most invertebrates.

  1. Spore Formation: Plants and fungi typically use spores as a means of asexual reproduction. During their life cycle, these organisms form spores through the process of sporogenesis. They reproduce without undergoing fertilization. Organisms that reproduce through spore formation include ferns and mushrooms.

Chapter 9: Biological Molecules

Life on Earth utilizes three distinct biological molecules. Each molecule contributes to cellular function in critical ways. Proteins, collectively, are the rock that carries out most of the cell’s processes. Nucleic acids DNA and RNA hold essential biological data, which they transfer from one generation to another.


Deoxyribonucleic acid (DNA) is a molecule that contains the biological instructions that make every organism unique. This molecule explains why dogs give birth to dogs, hippos to hippos, humans to humans, and so on and so forth. DNA is passed from an organism to its offspring during sexual reproduction. When organisms reproduce, offspring inherit exactly half of their nuclear data from the male parent, and half from the female parent. Scientists refer to DNA as nuclear DNA.


Frederich Miescher, a Swiss biochemist, was the first to observe DNA in 1869. It wasn’t until nearly a century later, however, that researchers discovered the structure of DNA, and that it retains life’s biological instructions.

In 1953, James Watson and Francis Crick discovered DNA’s double helix structure. It is this distinctive structure that enables DNA to transfer biological data from one generation to another.

Structural Shape of DNA

Double helix is a term used to describe DNA’s twisting, two-stranded chemical structure that can be likened to a twisted ladder. The double helix shape enables DNA to pass along precise biological directions. The double helix contains a sugar phosphate backbone, hydrogen bond, and base pairs.

Its unique structure also gives DNA the power to replicate itself during cell division. In preparation for cell division, the DNA double helix splits itself straight down the middle and becomes two separate strands. These strands then become templates for the construction of two new double-stranded DNA molecules, each of which are exact replicas of the original DNA. The process of chemical pairing is highly specific. Base A always pairs with base T, and base C always pairs with base G. Wherever there is a base C, a base G will be added; likewise with base C and base G. The pairing process continues until all the bases have partners again.

DNA’s Dwelling

All known living organisms contain DNA. Eukaryotes store their DNA inside cell nuclei with small amounts within the mitochondria. Prokaryotes, on the other hand, store their entire supply of DNA in the cytoplasm. Large quantities of DNA inhabit the nucleus of every cell in an organism’s body. Each DNA molecule is tightly coiled within the nucleus. This bundled form of DNA is known as a chromosome.

DNA uncoils itself during certain cellular processes. DNA unravels during replication so that it may be duplicated. The molecule also uncoils at specific cell cycle phases where the instructions contained in them are used for biological processes, such as protein production. During cell division, however, DNA remains in its tightly packaged chromosome form so that it can be transferred to new cells.

DNA Constituents

DNA is composed of chemical building blocks known as nucleotides. These nucleotides are linked into chains to form a single strand of DNA. Each nucleotide is made up of three components:

  1. Phosphate Group

  1. Sugar Group

  1. Nitrogen Bases:

  1. Adenine (A)

  2. Thymine (T)

  3. Guanine (G)

  4. Cytosine (C)

The arrangement or sequence of nitrogen bases determines the biological instructions that are contained in a DNA strand. For example, the sequence ATCGTT may command for blue eyes while the sequence ATCGCT is the command for brown eyes.

A human being’s complete set of DNA contains approximately three billion bases and 20,000 genes on 23 pairs of chromosomes.

The Role of DNA

DNA is the sole vessel that contains the essential biological instructions that an organism needs to develop, survive, and reproduce. DNA sequences are converted into messages before they can be used to produce proteins.

DNA sequences are used to make proteins through a two-step process:

  1. Enzymes scan the data contained in a DNA molecule. The enzymes then write out the information into mRNA that serves as an intermediary molecule.

  1. The mRNA molecule deciphers the information obtained from DNA and passes it on to amino acids. Various amino acids can be linked in numerous orders to form a range of proteins.


Like DNA, ribonucleic acid (RNA) is present in the cells of every known living organism. RNA is considered to be a messenger that carries instructions from DNA to be used in protein synthesis. The molecule’s job description includes decoding, transcription, regulation, and gene expression.

Difference between RNA and DNA

While the chemical structure of these two molecules share many similarities, they differ in three main ways:

  1. RNA is single-stranded, whereas DNA is double-stranded.

  1. RNA contains ribose, whereas DNA contains deoxyribose.

  1. In DNA, adenine pairs with thymine, but in RNA, adenine pairs with uracil.

RNA Structure

RNA is assembled in the form of a nucleotide chain much like DNA. RNA nucleotides contain three components:

  1. A Five-Carbon Sugar

  1. A Phosphate Group

  1. A Nitrogenous Base

  1. Adenine (A)

  2. Guanine (G)

  3. Cytosine (C)

  4. Uracil (U)

RNA nucleotides are attached to one another by covalent bonds between the phosphate of one nucleotide and the sugar of another nucleotide. RNA has the ability to fold back upon itself, forming complex three-dimensional shapes as the nitrogenous bases bind to one another. Base A always pairs with base U, and base C always pairs with base G.

Types of RNAs

Three main types of RNA exist:

  1. Messenger RNA (mRNA): mRNA serves as a messenger molecule in DNA transcription. It carries DNA instructions that are used in making proteins. At the end of transcription, mRNA travels to cytoplasm to complete protein synthesis.

  1. Transfer RNA (tRNA): This type of RNA serves as a translator. It is responsible for translating the message within the nucleotide sequences of mRNA into the “language” of amino acids. Amino acids are then chained together according to the assembly instructions to form a protein.

  1. Ribosomal RNA (rRNA): Found in the ribosomes of a cell, rRNA performs functions, which are critical to protein synthesis.


Proteins are large, complex molecules—or macromolecules—that play various key roles in organisms. They are the building blocks of organisms. Proteins are like micro-machines that enable all living organisms to function. Proteins are responsible for the majority of eukaryotic cellular processes. An organism’s tissues and organs both depend on proteins for their function, structure, and regulation.

Cells use genetic information as a blueprint for manufacturing proteins. Every single gene in DNA encodes instructions on how to make a particular protein. Specialized machinery scans the gene then uses the info to produce a sort of molecular message in the form of RNA. RNA then travels from the nucleus and into the cytoplasm. There, the cell’s ribosome reads the message and makes a protein according to the specifications outlined in the gene.

Proteins are made up of thousands of minuscule fundamental units known as amino acids. The amino acids that make up proteins are linked to one another in long chains. There exists precisely twenty types of amino acids that are capable of being combined to make a protein. Amino acid sequences are the determining factors of a protein’s unique structure and specific function.

Proteins Types and Their Function

  1. Enzymes: Enzymes are biological molecules of protein that serve as efficient catalysts. Catalysts are substances that speed up the rate of a chemical reaction without undergoing any chemical change. If enzymes did not exist, chemical reactions would occur far too slowly to sustain life.

One example of an enzyme is lactase, which many eukaryotes produce. Lactase breaks down a sugar called lactose, thus facilitating digestion of whole milk.

  1. Structural Proteins: This type of protein is a fibrous protein, the most familiar being the keratins that form protective layers such as fur, horns, and scales. Structural proteins lend support and strength to organism components like cells and tissues.

  1. Signaling Proteins: Signaling proteins allow cell communication. Signals, receptors, and relay proteins work hand in hand.

  1. Regulatory Proteins: Any protein that influences genes that are transcribed by RNA during the transcription process is a regulatory protein. For example, androgen and estrogen receptors influence the genes that trigger the onset of puberty.

  1. Transport Proteins: These are proteins that serve the function of moving materials, such as nutrients and molecules, within an organism.

  1. Sensory Proteins: Sensory proteins help an organism detect environmental factors such as light and heat. Olfactory receptors in the nose are sensory proteins.

  1. Motor Proteins: Motor proteins enable cells to move and change shape. These proteins also transport materials around within cells.

  1. Defense Proteins: These are proteins that protect an organism and help them fight infection, evade predators, and heal damaged tissue. Antibodies are one example of defense proteins. They combat the viruses that make a human ill by targeting invading microbes for destruction.

  1. Storage Proteins: These proteins serve as storage tanks, which stow nutrients and molecules. Casein and gluten are both storage proteins.

Chapter 10: Animal Tissue

Tissue is an aggregation of similar cells and intercellular matter that work together to perform specialized bodily functions. There are four types of animal tissue:

Epithelial Tissue

Epithelial tissue is found covering the surface the body as well as organs, airways, and reproductive tracts. The epithelial cells that make up epithelium can be one of three shapes: cuboidal, squamous, or columnar. If the epithelium consists of only one cell layer, it is described as “simple.” If it has more than one cell layer, however, it is described as “stratified.”

Types of Epithelial Tissue

  1. Simple Squamous Epithelium: This type of epithelium covers the inner lining of blood vessels, arteries, and veins, providing a surface that is conducive to smooth blood flow.

  1. Simple Cuboidal Epithelium: Kidney tubules are lined with a single layer of cuboidal cells, which are active in both secretion and absorption.

  1. Simple Columnar Epithelium: This epithelium is found as the lining of the small intestine. Its main function is nutrient absorption.

  1. Stratified Squamous Epithelium: This type of epithelium is comprised of multiple cell layers. Stratified squamous epithelium lends protection to the esophagus.

Connective Tissue

Connective tissue is characterized by the presence of an extracellular matrix—nonliving material composed of protein fibers and ground substance. Connective tissue gives organs shape and support.

Types of Connective Tissue

  1. Loose Connective Tissue: This type of connective tissue is primarily composed of a loosely knitted mix of cells, fibers, and ground substance.

  1. Areolar tissue

  2. Adipose tissue

  3. Reticular tissue

  1. Dense Connective Tissue: This type of connective tissue is also referred to as fibrous tissue. Fiber is its primary matrix element. Tendons and ligaments are made of dense connective tissue.

  1. Bone Tissue: This is a type of connective tissue that is primarily composed of calcium and phosphorous salts. There are two types of bone tissue, each differing in density:

  1. Compact bone tissue

  2. Spongy bone tissue

  1. Blood Tissue: Blood tissue is a specialized form of connective tissue.

  1. Lymphatic Tissue: This is another specialized form of connective tissue, which contains large quantities of lymphocytes.

Muscular Tissue

Muscular tissue is made of muscle cells, which are elongated and are the largest group of cells. It functions as a force and motion producer—both locomotion and movement of internal organs.

Types of Muscular Tissue

  1. Skeletal Muscle Tissue: This type of muscular tissue can be found throughout the body. Skeletal muscle tissue enables movement of body structures.

  1. Cardiac Muscle Tissue: This muscle tissue is only found in the heart. Cardiac muscle tissue facilitates blood flow through the heart.

  1. Smooth Muscle Tissue: This is an involuntary muscle tissue that is located around the stomach walls and intestines. Smooth muscle tissue aids fluid passage.

Nervous Tissue

Nervous tissue enables response to stimuli. It also coordinates body functions.

Types of Nervous Tissue

  1. Nerve Cells: Also known as neurons, nerve cells transmit electrical nerve impulses that carry information throughout the body.

  1. Neuroglia: The primary purpose of neuroglia is to protect and support neurons. Examples of neuroglia include astrocytes, Schwann cells, and oligodendrocytes.

Chapter 11: Homeostasis

Homeostasis is a self-regulating process by which higher animals, like humans, tend to maintain stability while they adjust to conditions that are most favorable for survival. Life continues if homeostasis is successful. If homeostasis is unsuccessful, however, it results in ether death or disaster.

In humans, for example, the regulation of body temperature can be described as homeostasis. Homeostasis is a biological process that maintains the stability of a human’s internal body temperature in response to external factors such as exposure to cold or disease.

Control Mechanisms

Homeostasis control mechanisms have three primary components:

  1. Receptor: The receptor is referred to as the sensing component. It monitors and responds to environmental changes. The receptor sends information to a control center when it senses a stimulus.

  1. Control Center: The control center determines the appropriate response to the stimulus. Once that is done, the control center sends signals to an effector.

  1. Effector: The effector may be muscles, organs, or other bodily structures. Once it receives the signal from the control center, the effector triggers a change to occur, which will rectify the problem by depressing it with negative feedback.

Negative Feedback

Negative feedback is a biological mechanism that reduces the productivity of any organ or bodily system back to its normal range of functioning.

One example of this process is blood pressure regulation. When blood pressure increases, the blood vessels sense the resistance of blood flow. Acting as receptors, the blood vessels then relay this message to the brain, which, in this case, is the control center. The brain determines the appropriate response before sending a message to the effectors—the blood vessels and the heart. The blood vessels will then undergo vasodilation, causing the heart rate to decrease. This change, assuming homeostasis succeeds, will cause the blood pressure to go back to normal.

Chapter 12: Organ Systems

Organ systems work together to keep an organism functioning. Within an organ system, a group of anatomical structures work in harmony to perform a set of specific functions.

The Circulatory System

Also known as the cardiovascular system, the circulatory system is responsible for blood circulation and the transport of nutrients, hormones, blood cells, oxygen, and carbon dioxide to and from cells. It also carries away the carbon dioxide waste produced by cells. Circulation and transportation of these vital materials provide nourishment to the body, help combat diseases, and help maintain homeostasis.

Vertebrates typically have a closed circulatory system; blood never leaves the cardiovascular network. Some invertebrates, on the other hand, have an open circulatory system. The circulatory system is composed of three essential parts:

The Heart

The heart is both a muscle and an organ that is roughly the same size as the average human’s fist. The heart pumps blood while ensuring smooth blood flow throughout the body. While the heart pumps oxygenated blood to the body, it pumps deoxygenated blood to the lungs.

Four chambers make up the heart: left atrium, left ventricle, right atrium, and right ventricle. Other parts of the heart include four valves, a superior vena cava, an inferior vena cava, left pulmonary veins and pulmonary arteries, and right pulmonary veins and pulmonary arteries.

The Blood

Blood is the substance that is constantly flowing and pumping through the human body. Blood is pumped by the heart. It travels through the blood vessels of the body, carrying nutrients, water, and oxygen from one cell to another. The average adult contains roughly five quarts of blood.

Blood is made of various substances including liquids and solids:

  1. Red Blood Cells: A single drop of blood contains approximately five million red blood cells. The primary function of red blood cells is to transport oxygen and carbon dioxide.

Red blood cells gather oxygen from the lungs, which they then carry to all the cells of the body. Once a red blood cell reaches its destination, it picks up the carbon dioxide byproducts of cells. The red blood cell carries the carbon dioxide back to the lungs where it will be removed through the act of exhalation.

  1. White Blood Cells: These blood cells are the counterparts of red blood cells. White blood cells help the body combat invading germs. When a person is sick with an infection, the quantity of white blood cells in his body increases in order to help destroy the infection. Antibiotics are prescribed to aid white blood cells in attacking a large-scale infection.

One drop of blood contains roughly 10,000 white blood cells.

  1. Platelets: Platelets are a type of blood cell that is responsible for impeding blood flow. When a person gets a cut, blood leaks out of the wound due to damaged blood vessels. In this case, the platelets’ job is to adhere to the opening of the damaged blood vessels. It forms a sort of plug that seals broken blood vessels shut, thus staunching blood flow. The result of this platelet plug is what you call a scab.

One drop of blood is estimated to contain approximately 250,000 platelets.

  1. Plasma: Plasma is the term for the liquid component of blood. It makes up roughly half of a human’s blood supply. Plasma acts as the vessel that carries blood cells and platelets throughout the body.

While plasma is produced in the liver, red and white blood cells and platelets are produced by bone marrow.

The Blood Vessels

There are three types of blood vessels in the human body:

  1. Arteries: Arteries are a type of blood vessel that carry oxygenated blood away from the heart.

  1. Veins: Unlike arteries, veins carry blood toward the heart.

  1. Capillaries: Capillaries are the smallest of the three blood vessels. They connect arteries and veins together. Capillary walls are permeable; nutrients, waste, and oxygen pass in and out of blood via these walls.

The Endocrine System

The endocrine system is an organ system in that is composed of hormone-secreting glands. These hormones are secreted directly into the circulatory system of an organism where they will be carried toward target organs. The endocrine system controls the functions of organs, tissues, and cells through chemical means.

Major Endocrine Glands

Each endocrine gland is associated with the secretion of a specific hormone.

  1. Hypothalamus: The hypothalamus is the part of the brain that serves as the control center of the autonomic nervous system. This gland helps regulate sleep cycles, body temperature, and appetite, to name a few.

The hormones secreted by the hypothalamus include:

  1. Dopamine: Dopamine is a neurotransmitter—a substance that transmits nerve impulses. It inhibits the production of the hormone prolactin so as to stop the release of breast milk.

  2. Somatostatin: This hormone regulates the endocrine system. It inhibits the secretion of growth hormone.

  3. Growth Hormone-Releasing Hormone: Also known simply as GHRH, this peptide hormone activates the release of hormones which stimulate growth.

  1. Pineal Body: This is a small endocrine gland that is situated in the brain area.

  1. Melatonin: Melatonin is a hormone that regulates sleep-wake cycles.

  1. Pituitary Gland: This gland is considered the main endocrine gland. It is responsible for regulating the growth, development, and function of its fellow endocrine glands. The pituitary gland secretes hormones that maintain homeostasis.

  1. Growth Hormone: This hormone enables children to grow to their final height. In adults, GH manages the amount of muscle and fat in the body.

  2. Thyroid Stimulating Hormone: TSH triggers the release of hormones from the thyroid gland that control metabolism and temperature.

  3. Oxytocin: During childbirth, oxytocin enables the womb to contract.

  4. Follicle Stimulating Hormone: FSH increases estrogen levels in women and urges ovum production. In terms of men, FSH triggers sperm production.

  1. Thyroid: The thyroid is a butterfly-shaped gland directly beneath the Adam’s apple. It secretes hormones that regulate growth and development through metabolism.

  1. Thyroxine: Also known as T4 hormone, this is one of the major thyroid hormones. It stimulates the metabolism of all the cells and tissues of the body.

  2. Triiodothyronine: Also known as T3, this hormone influences nearly every single physiological process in the human body including heart rate, metabolism, and body temperature.

The Excretory System

This organ system flushes excess materials from the body of an organism. This biological process of waste removal maintains homeostasis, as well as prevents internal body damage. In humans and higher animals, waste leaves the body through the urine, sweat, and sometimes, exhalation.


Kidneys are a pair of excretory organs located on each side of the abdominal cavity. These organs produce urine, thus regulating fluid balance. When the kidneys filter out waste products—urea and salts—from the blood, they leave the body in the form of urine. This process is made possible by the millions upon millions of nephrons that are present in each kidney. Excretory tubes, called the ureter, collect the urine from the kidneys and transfer it to the urinary bladder. The bladder then stores the substance until urination occurs, whereupon the urine is excreted from the body via the urethra.


The liver is an organ situated in the abdomen that is involved in excretory processes. Its primary function is to secrete bile to aid digestion, but the liver also stores sugars and fat as energy. The liver also converts harmful substances to forms that are less noxious and regulates the blood supply.


The skin is also an excretory organ. It secretes sweat via sweat glands throughout mammalian bodies. As part of the excretory system, the skin regulates body temperature. Sweating cools down body temperature, thus, helping to maintain homeostasis.


The lungs play a key role in the excretory system. Lungs use alveoli cells to remove the carbon dioxide that accumulates in blood. Each time a human breathes, the gas is expelled from the body.

The Digestive System

The digestive system is comprised of the organs and other structures, which are responsible for digestion. Digestion is a process that begins with the mouth, extending to other bodily structures before ending with the anus. Food can be followed as it journeys through the digestive system:

  1. Mouth: The mouth marks the beginning of the digestive tract. Here is where the saliva mixes with the food you eat in order to make it easier to digest.

  1. Pharynx: The pharynx and the throat are one in the same. It allows the passage of food and liquids.

  1. Esophagus: The esophagus is a hollow, muscular tube that serves one simple function in the digestive system: to carry swallowed liquids and solids from the mouth to the stomach.

  1. Stomach: The stomach acts as a sac-like container that receives food from the esophagus. The stomach secretes the necessary acids and enzymes that will digest food.

  1. Small Intestine: The small intestine is where the majority of digestion and food absorption occurs. Its primary function is to absorb the nutrients and minerals from foods.

  1. Large Intestine: The remaining food travels from the small intestine to the large intestine. The large intestine’s primary function is to absorb water from any indigestible food matter. It then transfers the useless waste from the body.

  1. Rectum and Anus: These two are the final components of the digestive system. Whatever solid waste is left over from digestion will be expelled from the body through the rectum and anus.

The Musculoskeletal System

The musculoskeletal system in humans is an organ system that is responsible for locomotion. It is made up of the muscular and skeletal systems combined, providing support, stability, and movement.


  1. Skeletal System: The skeletal system is made up of over 200 bones in human adults. This system serves as a framework for tissues and organs. It is what gives shape to the human body. Not only does it provide you with movement, the skeletal system also stores minerals and produces red and white blood cells.

The skeletal system is comprised of two types of bone tissue:

  1. Compact Bone: This dense osseous tissue makes up the outer layer of bones. It stores and releases chemical elements, protects organs, and supports the entire body. Compact bone consists of blood vessels and nerves.

  2. Spongy Bone: The spicules of this osseous tissue form a latticework. Spongy bone is weak in comparison with compact bone. It contains red bone marrow.

  1. Muscular System: The muscular system lends movement and maintains posture. It also helps circulate blood throughout the body.

The muscular system includes three types of muscles:

  1. Cardiac Muscle: This muscle type is found in the walls of the heart. Cardiac muscles expand and contract, enabling the heart to pump blood in and out as blood circulates throughout the body.

  2. Skeletal Muscle: Tendons attach skeletal muscles to bones. Skeletal muscles are under voluntary control, unlike the other two muscle types. They produce all body movement.

  3. Smooth Muscle: This involuntary muscle type is responsible for the contractility of hollow organs like the blood vessels, uterus, and bladder.

The Integumentary System

This is the organ system that gives the body protection from damage. The integumentary system consists primarily of skin, which is the largest bodily organ, but it also includes hair, fur, nails, and claws. The system serves to protect, cushion, or waterproof deeper tissues and regulates temperature. It is the attachment site for sensory receptors that detect pressure, pain, temperature, and sensation.


The epidermis is composed of the outer layers of skin cells. The thickness varies, but the epidermis is typically thinnest on the eyelids and thickest on the soles of the feet. The epidermis is comprised of five layers, stratum corneum being the outermost layer:

  1. Stratum basale

  2. Stratum spinosum

  3. Stratum granulosum

  4. Stratum licidum

  5. Stratum corneum

While the bottom layer, stratum basale, is composed of column-shaped cells, the top layer of the epidermis is made of dead skin cells.


The dermis is the second layer of skin, and it also varies in thickness. It is made up of three types of tissue:

  1. Collagen

  2. Elastic tissue

  3. Reticular fibers


The hypodermis is also called subcutaneous tissue. It is the innermost layer of the integumentary system, and it is the thickest of the three layers. It acts as an energy reserve and is primarily composed of adipocytes—fat cells.

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