THECELLThe basic unit of structure andfunction in living organisms is the cell, cells are very minute, and they canonly be seen with the aid of a microscope. Thecell is defined as the structural and functional unit of a living organism. Living organisms are classified into two(2) major groups based on the number of cells.

These groups are: -·        UnicellularOrganism.·        MulticellularOrganism.Unicellular Organisms: These are organisms which consist ofonly one cell. Examples of unicellular organisms are Amoeba, Chlamydomonas,Euglena and Paramecium. The single celled organisms can carry out all lifeactivities such as feeding, reproduction, excretion, growth, adaptation,respiration, definite life span, sensitivity and movement.Multicellular Organisms: These are organisms which consist oftwo (2) or more cells. Examples are Volvox, Spirogyra, Plants, Fish, Bird andMan. Multicellular Organisms have many cells and these cells also carry out alllife activities.

Some cells will be involved in feeding, some reproduction, andsome growth and so on.       There are 2 types of cells; theseare prokaryotic and eukaryotic cells. Prokaryoticcells are simpler and smaller than eukaryotic cells. They do not have anorganized nucleus or the organelles found in eukaryotic cells.

  Examplesof prokaryotes are bacteria. Eukaryotic cells are more complex and bigger thanprokaryotic cells, they have an organized nucleus as well as several organellesthat are not found in prokaryotic cells. Examples of eukaryotes are plants,animals, fungi and protists.The diagrambelow shows the differences between a prokaryotic and eukaryotic cell. Fig 2.1 The eukaryotic and prokaryotic cellsThe Cell TheoryRobert Hooke is the father of cell.

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Hewas the first human being to discover the honey-comb structure of the cell in1665. He explained his observations of magnified thin slice of a cork of an oaktree. He described that the cork is made up of thin component. He then namedthe components cells.  Fig 2.

2 Robert Hooke (1665). The cell theory states that:·        Thecell is the structural and functional unit of life.·        Allliving organisms are made of one (unicellular) or more cells (multicellular).·        Allcells come from cells that have previously existed.·        Thereis no life apart from the life of cells.Forms in Which Living Cells ExistThere are four (4) forms in whichliving cells exists.

These are:·        Asindependent or single free living organisms e.g. Amoeba, Paramecium etc.

·        Asa colony.·        Asa filament.·        Cellsas part of living organisms.1.

As independent or single freeliving organisms: Independent and free living organisms areorganisms that possess only one cell and are able to live freely on their own.Examples are Amoeba, Euglena, and Paramecium etc.(i) Amoeba: Amoeba is a single cell organism (i.e.unicellular). It has irregular shape and changes constantly. The protoplasm ismade of nucleus and cytoplasm. It is a cell of about 3000um across and ithouses the machinery with which it carries out all the functions of life.

It feedsitself and exchanges materials with its environment.                                                      It responds to stimuli in itsenvironment. It grows and reproduces. Amoeba moves by means of finger-likeprojections of the body wall called pseudopodia.

It also possesses foodand contractile vacuole. Fig 2.3 Amoeba(ii) Paramecium: Paramecium is a single-celled or unicellularorganism. As a unicellular organism, it has to carry out within one cell allthe functions performed by differentiated cells, tissues and organs in amulticellular organisms. The macronucleus exercises control over metabolicfunctions, including growth, while the micronucleus is necessary for sexualreproduction.

                                                    Thecilia is responsible for locomotion (i.e. movement), the contractile vacuolesfor osmoregulation. The trichocysts are used for defense. Food particles sweptinto the oral groove by ciliary action, and taken up into food vacuole formedat the base of the gullet.

(iii) Euglena Viridis: Euglena Viridisis a microscopic organism found in fresh waters. The spindle-shaped body isbroad in the middle. The presence of a gullet, flagellum, eye spot, contractilevacuole, pellicle and myonemes often qualifies Euglena as an animal.                        Equally, the presence ofstellate chloroplast, pyrenoids and paramylum granules also qualifies it as aplant. It is therefore capable of independent existence. It is an organismsharing the characteristics of both plants and animals. It moves with the aidof flagellum. Fig 2.

5 EuglenaAnimal Characteristics of    EuglenaThe characteristics of Euglena which makeit an animal include:·        Possessionof flagellum used for movement.·        Possessionof gullet for passage of food and as reservoir.·        Possessionof contractile vacuole for osmoregulation.·        Presenceof eye spot which enables it to respond to light.·        Possessionof pellicle which makes it flexible.·        Ithas the ability to carry out holozoic mode of nutrition in the absence ofsunlight.

Plant Characteristics of    EuglenaThe characteristics of Euglena which makeit a plant include:·        Possessionof chloroplast which enables it to carry out photosynthesis.·        Possessionof pyrenoids where starch is stored.·        Presenceof paramylum granules which is the form in which starch is stored.2. Asa colony:Some organisms are made up of many similar cells which are clustered together,these cells cannot be differentiated from each other. This aggregation orconstitution of independent cells is called a colony. Examples of organismswhich exist as colonies are Volvox, Pandorina and sponges.

3. As a filament: Some cells areorganized into filaments in which identical cells are joined end-to-end to forman unbranched filament. Each cell functions as an independent living cell.Filamentous organisms are Spirogyra, Zygnema, and Oscillateria etc. 4. Cellsas part of living organisms: Living things are made up ofcells. In multicellular organisms, a group of similar cells are arrangedtogether to form a tissue, a group of similar tissues are arranged together toform an organ and group of organs are aggregated together to form a system.Cells form the unit of living organisms.

CELLSTRUCTURES AND ORGANELLES            Thecell can only be seen with the aid of the microscope. The cell is made up theprotoplasm; the protoplasm is made up of two parts which are the cytoplasm(i.e. fluid in the cell where cytoplasmic organelles like the mitochondria,golgi bodies, endoplasmic reticulum etc are embedded) and the nucleus. Theanimal cell is bounded by the cell membrane while the plant cell is envelopedby the cell wall.

The organelles present in the cell include the following:1. Cell Surface Membrane (plasmamembrane)The cell membrane is the outermoststructure of the cell that seperates the cell from its external environment. Allcells possess a cell membrane (both prokaryotic and eukaryotic). It also helpsto regulate the entrance and exit of material between the cell and itsenvironment.2.

Cytoplasm: Thisis a fluid enveloped by the cell membrane. It is jelly-likeand supports the cell structures located within its semi-liquid medium. About70% of the cytoplasm is water. 3. Nucleus: Thenucleus is a very important cell organelle.

It is the largest cell organellethat is present in both the plant and animal cell.  The nucleus is boundedby an envelope called nuclear membrane.The nucleus consists of protein and deoxyribonucleic acid (DNA). DNA is thegenetic material of the cell and controls its structure and function.  DNAis organized into linear units called chromosomes. The chromosomes contain thebasic parts of inheritance called genes.

It is through these genes that geneticmaterial is passed from parent to offspring.4. Endoplasmic Reticulum: Theendoplasmic reticulum is network of membrane found in both plants and animalcells. It is responsible for protein and lipid synthesis and transportation ofmaterials around the cells. There are 2 types of endoplasmicreticulum these are rough and smooth endoplasmic reticulum.

The rough endoplasmic reticulum hasribosomes and is responsible for the synthesis of proteins. The smooth endoplasmic reticulum doesnot have ribosomes.  5. The Mitochondrion (pluralmitochondria)Mitochondria are rod shaped organelles that playsan important role in releasing energy in the cell. Itis referred to as the “power house of the cell” because it providesenergy in the form of ATP through the process of respiration. This organelle ispresent in all eukaryotic cells.6.

Chloroplast: Theseare structures found in plant cells containing chlorophyll. The chlorophyll aidthe process of photosynthesis in green plants.7. Vacuoles: Theseare small sacs bounded by the vacuolar membrane. Vacuoles contain cell sap,which is made up of variable amounts of water, mineral salts, sugars, pigmentsand enzymes depending on the cell.

It acts as an osmoregulator; it removesexcess water in the cell. Plant cells have a large vacuole. It is the presenceof a large vacuole in a plant cell that causes it to become turgid(swollen). 8. Cell wall:The cell wall is found only in plant cells. This rigid cell structure gives the cell a regular shape and prevents itfrom bursting when the plant cells become swollen. The cell wall is madeof cellulose, which is a polysaccharide.

The cell wall encloses/ covers thecell membrane and its contents (protoplasm). Due to its structural make up itsprimary function is to provide protection and support to the cell.9.Ribosomes: They are responsible for protein synthesis.

10. Golgi apparatus: Theyfunction in synthesis, packaging and distribution of materials.11. Lysosomes: Lysosomes are thin-wall bodiesthat contain enzymes that eliminate worn-out mitochondria (i.e. the power houseof the cell) and other redundant organelles. They destroy other worn out cellorganelles.

Theyplay an important role in the death of cells because enzymes in them bringabout autolysis.12. Centriole: It plays animportant role in cell division.13. Starch granules: Theystore starch for the cell.

14. Nucleolus: They produce theribosome for protein synthesis. Fig 3.6 Organelles of the cellDifferencesbetween Plant and Animal CellsPlant and animal cells are botheukaryotic cells. However, there are distinct differences between the cellsfound in plants and those found in animals. The differences between the twotypes of cells can be seen with a light microscope.  Below is a list ofthe major differences:                              PLANT CELL                               ANIMAL CELL Plant cell has chloroplast. Animal cell has no chloroplast.

Plant cell has rigid cell wall. It has no cell wall. Plant cell contains conspicuous or large vacuoles.

Animal cell has small or no vacuoles. It has no Centriole. It has Centriole. It has no flexible cell membrane. It has flexible cell membrane.  Plant cell contain plastids including chlorophyll.

Animal cell has no chlorophyll. Plant cell have carbohydrates stored as starch granules. Animal cell have carbohydrates stored as glycogen.     Fig3.7 A typical animal cell Fig3.8 A typical plant cellSimilarities between plant andanimal cells:·        Both have a cell surface membranethat surrounds the cell.·        Both contain endoplasmic reticulum·        Both have cytoplasm·        Both contain ribosomes·        Both contain a nucleus·        Both contain mitochondria·        Both contain golgi bodies·        Both contain chromosomes·        Both contain lysosomes·        Both contain nucleolus.