CELL BIOLOGY

Objectives

The content provides the following objectives to students. Students will be able to:

o             Explain the concept of cell as the basic unit of life and all cells evolving from pre-existing cell (s)

o             Distinguish between akaryotes, and prokaryotes.

o             Describe mode of life of viruses and bacteria.

o             Describe eukaryotic cell structure and functions.

o             Identify specialized eukaryotic cells and outline their functions.

o             Explain the terms cell, tissue, organ, organ system and organism.

o             List the forms in which cells can exist.

Cell and Cell Theory

Cell 

Cell is the basic structural and functional unit of all living organisms. Cell is considered to be the smallest structure that is alive. The cell was discovered by Robert Hooke in 1665, when he observed thin sections of bottle cork under an early light microscope. Some living organisms are composed of a single cell, therefore referred to as unicellular organism e.g., Amoeba, Paramecium, Euglena. Others are made up of many cells and are multicellular e.g., man, bird.

Cell Theory

The cell theory first developed in 1839 by Schileiden and Schwann. The cell theory states that:

1. All organisms are composed of cells.
2.  A cell is the smallest unit of living matter.
3.  Cells come only from preexisting cells.
4.  Hereditary information passes from parent cell to daughter cell.

¨5. The fundamental biochemical reactions of life take place within cells.

Types of Cells

1. Akaryotic cells: Akaryotic cells do not possess nucleus and reproduce only in the cell of another organism. They are non-cellular. E.g., viruses

2. Prokaryotic Cells: Prokaryotic cells do not have a true nucleus. They have few organelles which are not membrane-bound. Prokaryotes include the bacteria and cyanobacteria.

3. Eukaryotic cells: Eukaryotic cell has nucleus and a complex internal structure with membrane-bounded organelles. These include cells of protoctists, fungi, plants, and animals.

Akaryotes (Viruses)

A virus is a small infectious agent that can replicate only inside the living cells of an organism. They have nucleic acid core surrounded by a protein coat
 

Structure of Viruses

A complete viral particle, known as a virion, consist of two or three parts:

o             genetic material (either DNA or RNA) that carry genetic information;

o             a protein coat called a capsid, that protects these genes;

o             a lipid envelope that surrounds the protein coat of some viruses

They lack ribosome, and enzymes needed for protein synthesis or metabolism. They can therefore reproduce only within a living cell by using the metabolic equipment of the host. 

They are classify based on

1. type of nucleic acids - (either DNA or RNA) and whether it is single or double stranded
2. viral size and shapes

3.  presence or absences of outer envelope

Viruses are much smaller than bacteria and can only be seen under electronic microscope. They have a variety of shapes e.g., rods, spheres, spirals and hexagons.

Parasitic Nature

Viruses are referred to as obligate intracellular parasites that live within the cells of all kinds of organisms. Viruses are very host specific. That is, they infect only certain cells or groups of organisms. E.g., bacteriophage infects only bacteria; tobacco mosaic virus infects only plants and rabies infects only mammals. All known viruses cause disease e.g., sore throat, yellow fever, poliomyelitis, measles, small pox, AIDS, influenza, rabies, foot and mouth disease, mosaic disease in plants etc. In human, vaccination is a cheap and effective way of preventing viral infections.

Types of viruses

Characteristics viruses share with non-living things

1. they are non-cellular (accellular) and have no cytoplasm, membranes nor organelles
2. they lack the enzymes necessary for protein synthesis and energy transfer
3. they do not grow, excrete or respire
4. they cannot reproduce on their own
5. they are not motile

6. they can crystallize like chemicals

Characteristics viruses share with living things

o   they contain nucleic acids

o   they reproduce

Why viruses are not considered as a cell

o   they contain either RNA or DNA never both

o   they cannot reproduce outside of a living cell

Viral Replication

The two basic replication cycles are

o  Lytic cycle: During this cycle the host cell undergoes lysis which is the breakdown of the host cell to release new viruses. It involves the following stages

□    Attachment: the phage attaches to cell membrane of host.

□   Penetration: viral enzyme digests part of host cell wall and viral nucleic acid (DNA) injects into the host cell

□   Replication: viral DNA inactivates host cell DNA and uses host raw materials and ribosomes to synthesis viral DNA, capsids, tails etc.

□   Assembly (Maturation): viral DNA and capsids assembled to produced several new viral particles

□   Lysis: viral enzymes destroy the host cell membrane releasing new viruses. The host cell dies as a result.

o  Lysogenic cycle: During this cycle, the virus stays inactive inside the host cell, replication does not take place and doesn’t immediately kill the host cell. This cycle involves: attachment, penetration, recombination, cell reproduction, activation, replication, assembly and lysis. E.g., is HIV.

Prokaryotes (Bacteria)

Prokaryotes are single-celled organisms. They are the smallest, simplest organisms. They are abundant in the air, water, soil, and on most objects. 

Generalized Structure of Bacteria

Structure of generalized bacterium

Bacteria are small and can be seen using light microscope. They have cell wall composed peptidoglycan (complex fat and protein). The cell wall is surrounded by a capsule, a short, hair-like projection called pilli. These structures enable bacteria to attach themselves to environmental surfaces. 

They have plasma membrane which regulates the movement of substance into and out of the cell.

Bacteria do not have a nuclear membrane. They also lack membrane-bound organelles.  Bacteria have a single circular chromosome. They also have accessory rings of DNA called plasmids. Some move by means of long whip-like structures called flagella.

The shape of a cell is used to classify bacteria.

1.   Round or sphere-shaped bacteria are called cocci (singular: coccus), 
2.   Rod-shaped bacteria are called bacilli (singular: bacillus), and  
3.   Rigid, spiral-shaped bacteria are called spirilla (singular: spirillum). 

4.  Bacteria shapes may appear in particular arrangement. Bacteria can occur in pairs called diplo; in chains called strepto or in cluster called staphylo (for instance cocci may form cluster called staphylococcus or in chain called streptococcus).

 

False colour Electron micrograph of a colony of Bacillus cereus

Mode of Nutrition in Bacteria

Prokaryotes are also classified as Autotrophs or Heterotrophs

1.  Autotrophs: make their own food. Mostly Chemosynthetic; they make their food using energy from inorganic chemicals

2.  Heterotrophs: consume ready-made food. Heterotrophs feed on organic matter by secreting enzymes and absorbing the digested material. The types of heterotrophs;

a. Saprotrophs are decomposers. They play a critical role in decaying dead organisms and recycling (releasing) nutrients into the soil. 

b. Parasites are organisms that live in close association with another species and one species benefits at the expense of the other. Parasitic prokaryotes cause disease in plant and animals

Reproduction

Prokaryotes reproduce by binary fission. Some bacteria form endospores when environmental conditions become unfavorable. Endospores are DNA and a portion of cytoplasm enclosed in a tough cell wall. They are resistant to extremes in temperature, drying, and harsh chemicals.

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Structure and Function of Eukaryotic Cells 

Microstructure of a typical Animal cell

Organelles are minute structures in a cytoplasm bounded by a membrane, which perform specific function. E.g., mitochondrion, endoplasmic reticulum, Golgi body, lysosome, vacuole and chloroplast. 

Cell Wall

The cell wall is thick, rigid structure which support and protect the cell. It is permeable. Plant cell walls are made up of cellulose; fungi have walls composed of chitin.

Functions

1.      Gives rigidity to cells
2.      Allows free passage of materials
3.     It forms frame work which protects, support and gives linkage to the cell

4.      Gives definite shape 

Plasma Membrane 

Plasma membrane or cell membrane is a thin flexible structure which encloses the cell contents. It is semi-permeable. The fluid-mosaic model states that, membranes are fluids (thin flexible) with proteins embedded within the membrane. Plasma membrane consists of phospholipid bilayers interspersed with globular protein molecules. 

Functions of Cell Membrane

1.  Regulates the passage of molecules into and out of the cell
2.  Protects the cytoplasm or delimits the content of the cytoplasm

3. Forms a barrier between cell and its surrounding

Cytoplasm

Cytoplasm is a transparent fluid within which various organelles and chemicals substances can be found. It refers to the material enclosed by the plasma membrane, except the nucleus.

Function of cytoplasm 

It is the site for most cellular reactions.

Structure of Cellular organelles and Their Functions 

Nucleus

This is a prominent, spherical or oval structure covered by double membrane. The nucleus is composed of the following structures:

1. Nuclear membrane: is a double-layered membrane which separates the nucleoplasm from the cytoplasm. It has minute pores which regulates materials in and out of the nucleus.

2.  Nucleoplasm: is a clear, semi-solid, granular substance or matrix inside the nucleus. The nucleolus and the chromatin are suspended in the nucleoplasm.

3.  Nucleolus: is dense, spherical granule found in the nucleus contains RNA (ribonucleic acid) which is responsible for protein synthesis in the cytoplasm.

4.  Chromatin: are fine thread-like, coiled filaments uniformly distributed in the nucleoplasm. During of cell division, the chromatin becomes thick and ribbon like and are known as chromosomes. The chromosomes contain genes, which are composed of DNA (deoxy-ribonucleic acid). Genes are responsible for transmitting hereditary characteristics from parents to offspring.

 

Functions of Nucleus

1. It controls all cellular activities

2.  It regulates all metabolic and hereditary activities of the cell.

3.  It initiates cell division

3.  It controls protein synthesis

Endoplasmic Reticulum (ER)

The endoplasmic reticulum is a complex network of tubes that extends throughout the cell. It connects the nuclear membrane with plasma membrane. There are two types of ER; Rough and Smooth. Tubes with a smooth surface are called smooth endoplasmic reticula. The rough appearance of rough endoplasmic reticula is due to the presence of ribosomes on the membrane.

Electron Micrograph of Rough Endoplasmic
 Reticulum

Electron micrograph of smooth endoplasmic
 reticulum

Structure of Endoplasmic Reticulum

Functions of Endoplasmic Reticulum

1.  It transports proteins from one part of the cell to another

2.  It functions in protein synthesis

3.  It synthesizes new membrane 

4.  It secretes lipids such as phospholipids, steroids, and fatty acids

5.  It forms the skeletal framework of the cell.

Structure and Functions of Mitochondrion

Structure of Mitochondrion

Mitochondrion is a rod-shaped or spherical structure bound by double membrane. The outer membrane is smooth whiles inner membrane is folded into ridges called cristae. Cristae increase the surface area of the membrane. The cristae project into the gel-like matrix. Enzymes involved in cellular respiration are found in the matrix.

Electron Micrograph of Mitochondrion

Function of Mitochondrion 

It is the sites for cellular respiration (energy or ATP production). Hence, it is termed as the “power house” or the “power plant” of the cell. 

Structure and Functions of Golgi Apparatus (Golgi Body)

Structure of Golgi Apparatus

Golgi apparatus also called Golgi complex or Golgi body. Golgi apparatus consists of tiny, elongated flattened sacs which are stacked parallel to one another. Each Golgi body is a stack of flattened, membrane bound sacs (cisternae, singular: cisterna) with a cluster of vesicles at the edges. They are present in the cytoplasm of all eukaryotic cells but absent in those of bacteria (prokaryotes). Golgi bodies produce cell membranes. They are also involved in the secretion and packaging of complex carbohydrates and proteins; thus they are conspicuous in electron micrographs of actively secreting cells. Complex carbohydrates are both made (synthesized) in and secreted from the Golgi bodies.

Diagram of Golgi Body or Golgi Apparatus

Functions of Golgi Apparatus

o   It involves packaging and secretion of certain proteins or carbohydrates.

o   It produces and repairs cell membrane.

o   It forms lysosomes and peroxisomes

Ribosomes

These are spherical, granular particles which occur freely in the matrix or remain attached to the rough endoplasmic reticulum. Each ribosome is composed of two subunits. One subunit is large in size and has a dome like shape. The other subunit is smaller in size and it occurs above the larger subunit forming a cap-like structure.

Ribosomes contain RNA and proteins.

Structure of Ribosome

Function of Ribosome

It  is site for protein synthesis.

Lysosomes

Lysosomes are tiny spherical sac-like membrane-bound structures scattered in the cytoplasm. Lysosome is a small vesicle surrounded by single membrane and contains hydrolytic enzymes. These enzymes are capable of digesting all organic materials hence called “digestive bags”.

Function of Lysosome 

The main function is digestion of organic material

1.  It is capable of digesting worn out organelles or even its own damaged cell, hence called “suicide bag”

2.  It fuses with vesicle and digest the content of the vesicle. E.g., bacteria or viruses are engulfed by white blood cells to form vesicles and destroyed by lysosomal enzymes

Plastids

These organelles are found only in plant cells. Plastids are of three types:

1.  Leucoplasts: are colorless and found in roots, seeds and underground stems. Leucoplasts store food in the form of carbohydrates, fats and proteins.

2.  Chromoplasts: are yellow, orange or red and found in flowers and fruits. Chromoplasts impart color to flowers to attract insects for pollination.

3.  Chloroplasts: are oval shaped or disc shaped surrounded by double membrane. They are found only in green plants and photosynthetic protoctists. Chloroplast contains membranous disk-like structures called thylakoids that are stacked together to form structures called grana (singular: granum). Molecules that absorb light energy (chlorophyll; photosynthetic pigments) are located in the thylakoid membranes. The fluid-filled space surrounding the grana is the stroma. The stroma contains nucleic acids, enzymes and starch grains. 

 

Function of Chloroplast

It is the site of photosynthesis (where organic food is synthesized)

Centrioles and Centrosome

Centrioles occur in pairs; each one oriented at a right angle to the other. Centrioles are contained within a structure called a centrosome. The centrosome and centrioles are involved in the formation of the microtubules called spindle fibres in cell division. They found in animal and fungal cells but absent in plant cells.

Structure of Centrioles

 

Vacuoles

Vacuoles are membranous sacs found in the cytoplasm. Vacuole encloses a watery fluid called cell sap, containing dissolved substances. Plant cell is occupied by a large central vacuole
surrounded by membrane called tonoplast. Vacuoles in animal cells are relatively small. Some protoctists have specialized contractile vacuoles for eliminating excess water and food vacuoles.

Functions of Vacuole

1.  It provides structural support to plant cells by making the cells turgid

2.  It serves as storage regions for reserve food or waste products

3.  It controls water content of fresh water protozoans

Structural Differences between Prokaryote and Eukaryotes

Prokaryote	Eukaryotes
Genetic material is present in cytoplasm	Genetic material is found in the nucleus
Absence of nuclear membrane	Presence of nuclear membrane
Lack of membrane bound organelles	Have membrane bound organelles
Simpler and smaller	Complex and larger
Few organelles	Many organelles
Circular DNA is present	Linear DNA is present
DNA is not associated with RNA	DNA is associated with RNA protein

Difference between Plant and Animal Cells

Plant cell	Animal cell
Presence of cellulose cell wall	Absence of cell wall
Has definite shape	Indefinite shape
No centriole	Has centriole
Protoplasm is less dense	Protoplasm is more dense, more granular and occupies most of the space in the cell
Chloroplast present	Chloroplast absent
Contain large permanent vacuoles	Contain small temporary vacuoles
Carbohydrates are stored as starch	Carbohydrates are stored as glycogen
Plastids are usually present	Plastids are absent

Similarities between Plant and Animal Cells

Both have

1. nucleus
2. vacuoles
3. mitochondrion
4. cytoplasm
5. cell membrane

6. endoplasmic reticulum 

Levels of Organization of Cells in Living Organism

o Cell: is the structural and functional unit of life. All living organisms are composed of one or more cells. E.g., Amoeba, Paramecium, sperm cell, nerve cell, Euglena, red blood cell.

o Tissue: is a group of similar cells which together carry out a specific function. E.g., epidermis, bone, blood tissue, epithelium, nerve tissue  

o Organ: is composed of different tissues which are closely related and perform the same or multiple function.  E.g., kidney, eye, leaf, lung, skin, liver, brain, heart, ear, penis

o Organ system: is a collection of organs that perform a specific function. Some organ systems are: the reproductive, nervous, skeletal, muscular, circulatory, digestive, excretory, root, shoot systems

o All organ systems combined to form a whole organism. E.g., humans

 

Specialized Cells 

These are cells that perform specific function. E.g., sperm cells, nerve cells, white blood cells, muscles cells, red blood cells, leaf epidermal cells and root tip cells.

Animal Specialized Cells

1.  Blood Cells: Blood cells are formed in the bone marrow. All blood cells arise from the same stem cells in the bone marrow. These stem cells give rise to erythrocytes, leukocytes and platelets.

a.  Erythrocytes (red blood cells): red blood cell is a tiny, disc-like cell (biconcave shape) which has no nucleus. It contains red pigment called haemoglobin which aid in transport of oxygen.

b.  Leukocytes (white blood cells):  white blood cell involves in immune function. It destroys and removes old cells as well as attack infectious agents (pathogens). Leukocytes include neutrophils, eosinophils, basophils, lymphocytes and monocytes. 

c.   Thrombocytes (platelets): are responsible for blood clotting (coagulation). They change fibrinogen into fibrin. 

2.  Ciliated Cells: are found in the trachea, uterus, bronchi, lungs and fallopian tubes. Ciliated cells have tiny hairs called cilia (singular: cilium) on their surface. The cilia sweep mucus with trapped dust and bacteria back up the throat. Ciliated cells also function in the movement of released egg in the fallopian tube into the uterus.

3.  Spermatozoa (sperm cells): sperm cell is made up of head, middle piece and a tail. The head contains an acrosome, which is a specialized lysosome that releases enzymes. The enzymes help sperm cell to penetrate the ovum coat of the egg by dissolving the egg membrane. Middle piece is rich with mitochondria which produces energy for active swimming of the sperm toward the egg to bring about fertilization. The tail propels the cell forward during swimming.  

4.  Nerve Cells: A nerve cell (or a neurone) is the basic functional unit of the nervous system. It receives and transmits impulse from one part of the body to another. A neurone consists of three main parts: The cell body which contains the cell membrane, cytoplasm and the nucleus; Axon a long extension of the cell body; and Dendron which arise from the cell body and branch to small, fine fibres called dendrites. 

5.  Muscle Cells: A muscle cell is elongated, spindle-shaped and elastic containing large number of mitochondria. The elongated and elastic feature helps muscle tissues to contract and relax. Contraction and relaxation of muscle tissues help in movement. The large number of mitochondria is very important in tissue respiration in the muscle cells.

Plant Specialized Cells

1.  Root Tip Cells: It has the ability to divide and give rise to different tissues.

2. Root Hair Cells: Root hair cell is a modified epidermal cell of the roots. It absorbs water and minerals in the soil. A root hair cell has a hair-like projection, a large vacuole and numerous mitochondria which provide energy for active transport. It has large surface area, due to the hair-like projections, which eases uptake.  

3. Epidermal Cells: are rectangular shaped cells which form transparent layer covering the upper and lower part of the leaf. They allow passage of light through them to photosynthetic cells. The cells are closely fitted without intercellular spaces. The cells secrete cuticle on the outer surface of the leaf. Epidermal cells together with cuticle protect the plant from bacterial and fungal attack.

4. Palisade Mesophyll Cells: They have long, narrow, tightly packed and cylindrical cells. The cells contain many chloroplasts with chlorophyll for absorption of sunlight for photosynthetic activities.

5. Spongy Mesophyll Cells: Spongy mesophyll cells are beneath the palisade mesophyll cells. The cells are irregular shaped, thin walled and loosely arranged with numerous intercellular spaces. The cells allow rapid diffusion of respiratory gases.

 

Plant Tissues and Animal Tissues 

Examples of Animal Tissues 

Embryonic Tissue

Ectoderm, mesoderm, and endoderm are embryonic tissues that give rise to all of the tissues, organs, and organ systems in the body.

Embryology is the study of the development of an organism from fertilization until birth. After fertilization, the fertilized egg (zygote) undergoes repeated mitotic cell divisions called cleavage. The zygote divides to form solid-ball of cells, called the morula. As division continue, the center of the ball becomes hollow in a stage called the blastula. The blastula stage leads to the formation of double layer cells called gastrula. The inner cell layer of gastrula is called the endoderm. The outer cell layer is called the ectoderm. A third cell layer, the mesoderm, forms between the endoderm and ectoderm. These three layers are called primary germ layers which give rise to all the tissues, organs, and organ systems of the animal.

1. Animals with radial symmetry produce two germ layers (ectoderm and endoderm) and referred to as diploblastic. E.g., cnidarians,

2.  Animals with bilateral symmetry produce three germ layers, hence called triploblastic.

3. An acoelomate animal does not have a body cavity e.g., flatworms

4. A pseudocoelomate animal has a body cavity (called a pseudocoelom) located between endoderm and mesoderm. E.g., roundworms

5. A coelomate animal has body cavity (called a coelom) located within the mesoderm e.g., vertebrates.

Nervous Tissue

Nervous tissue responds to stimuli and transmits impulses from one part of the body to another. Neurons are the basic unit of nervous tissue.
 

Epithelial Tissue

Epithelial tissue covers external and internal surfaces of organs. E.g., the outer layer of the skin and the inner linings of the digestive tract and blood vessels are made of epithelial tissue. The cells of the epithelial tissue are tightly packed and rest on a thin basement membrane. The free surface of the epithelium is exposed to air or fluid. No blood vessels are present.
 

Classification of Epithelial

Epithelial tissues are classified according to the shape and arrangement of the component cells.

1.  Squamous: flat, scale-like cells.

2. Cuboidal: cells appear square or cube-shaped with nucleus in a central position.

3. Columnar: cells appear rectangular or elongated with the nucleus displaced toward the base of the cell.

4. Simple epithelium: is one cell thick (single layer).

5.  Stratified epithelium: has more than one layer.

6.  Pseudostratified: epithelium appears to be layered but each cell touches the same basement membrane.

Function of Epithelial Tissue

o Ciliated epithelium lining the respiratory tract sweep impurities toward the throat

o Epithelium lining the gut absorb nutrients from food

o Epithelial tissue helps to protect organisms from microorganisms, injury, and fluid loss

Muscle Tissue

Muscle tissue or muscular tissue is made of cells that are capable of contraction. Muscle tissue contains

numerous microfilaments composed of actin and myosin, which are contractile proteins. Muscle tissue

is used for locomotion, food movement in gut, and heat production.

Types of Muscle Tissue

□ Smooth Muscle: Smooth muscle is involuntary. It surrounds the gut and moves food through the digestive tract. It also surrounds the blood vessels where it controls the distribution of blood.

□ Skeletal Muscle: Skeletal muscle is voluntary. The cells are very long, extending the length of the muscle. They are multinucleate and striated.

□ Cardiac Muscle: Cardiac muscle is found in the heart, allowing it to contract and pump blood throughout an organism. It is striated and branched.

Connective Tissue

Connective tissue binds and supports body parts, protects, stores fat (for energy), and transports materials. They are made up of different kinds of cells embedded in large amount of non-living material called matrix. Connective tissue contains three kinds of fibers. Collagen fibers provide strength and flexibility. Elastic fibers provide elasticity. Reticular fibers provide a support framework for organs such as the liver and lymph nodes.

Classification of Connective Tissue

1.  Connective Tissue Proper: This is a very large and diverse group of tissues and includes adipose tissue (fat), areolar (loose) tissue, and dense regular tissue, among others.

2. Specialized Connective Tissues: this group includes cartilage, bone, and blood.  Cartilage and bone form the skeletal framework of the body while blood is the vascular (transport) tissue of animals.

Types of Connective Tissue

1. Adipose tissue: contains cells that store fat.

2.  Cartilage: is found in the ends of bones where it prevents friction within the joints. In the nose, external ear, and the walls of the trachea it functions to support the softer tissues. Cells of cartilage are called chondroblasts (chondrocytes) embedded in matrix of chodrion.

3.  Bone: forms when calcium salts are deposited around protein fibers. The calcium salts provide rigidity while the fibers provide elasticity and strength. Bone cells called osteoblasts are embedded in calcified matrix.

4. Blood:  contains cells that are separated by a non-living material. The non-living material is called the plasma. The blood tissue transport gasses and food nutrients to all parts of the body. Blood cells are erythrocytes, leucocytes and thrombocytes.

5.  Ligament: It holds bones together firmly at joints.

6. Tendon: it connects skeletal muscles to bones.

Example of Plant Tissues 

1.  Epidermal Tissue: The entire surface of the plant consists of a single layer of cells called epidermis or surface tissue. Most of the epidermal cells are relatively flat. The cells form a continuous sheet without inter cellular spaces. It protects all parts of the plant. 

2.  Vascular Tissue (Xylem and Phloem): Xylem and Phloem are the transport tissues of plants. Xylem transports water and minerals up the plant, and Phloem carry sugars up and down the plant. Both are found in Vascular Bundles. 

3  Photosynthetic Tissue: Photosynthetic tissue manufactures nutrients by photosynthesis and stores reserve nutrients. Several Tissues and Cells are specialized to work together to maximize the rate of Photosynthesis.

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