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Structure of a cell

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Structure Of A Cell

The Structure Of A Plant Cell

Some plant cells differ in structure from one type to another.
When a plant cell is seen through a compound light microscope, its cell consists of the following major parts which are, The cell membrane, the cell wall, the nucleus and the cytoplasm.

The Cell Wall

The outermost part of the plant cell is the cell wall which is made up of cellulose and very thin especially in a young cell but as the cell grows older, the cell was is thickened with cellulose.
Note in some tissues, other chemicals may be deposited on the cellulose wall.top↑
E.g
a) suberin is deposited on cellulose cell walls in cork tissue.

b) lignin is deposited on cellulose cell wall in xylem and also in pericycle tissues.

Deposition of lignin and suberin on the cell walls causes the protoplasm in these cells to die. Cellulose cell walls are permeable to water and oxygen.
Suberin is not permeable, hence in cork tissue, water and air diffuse through the lenticles.

The Cell Membrane

Singer and Nicholson's model of the cell membrane

The cell membrane is located below the cell wall.
Cell membrane is seen as a line when view through a light microscope, it is semipermeable, that is, it is selective in allowing small molecules, but do not allow large molecules to pass through.top↑
The cell membrane encloses the cytoplasm, colloidal liquid which contains water, mineral salts, protein, enzymes and other organic molecules, sometimes the cell membrane reserves food particles.

A young plant cell is filled with cytoplasm, as the cell grows, the growth of the cytoplasm becomes lower than the growth of the cell due to the presence of spaces in the cytoplasm called the cell vacuoles, which are filled with cell sap. The membrane that separates the cell vacuoles from the cytoplasm is called tonoplast, the cytoplasm also consist of Chloroplasts.top↑

The Structure Of A Chloroplast

Structure of a chloroplast

In higher plants, each cell in the green parts of the plants contains many small and spherical chloroplasts. The chloroplasts consists of chlorophyll and the centres for photosynthesis.
The nucleus is roughly spherical in shape and is denser than the cytoplasm in which it is embedded, the nucleus is surrounded by a double membrane, which together make up the nuclear envelope.top↑

At certain points, the two membranes come together, creating spaces which are called nuclear pores. The nucleus communicate with the cytoplasm through the nuclear pores.
In the nucleus there are numerous fine threads which form a network of material called chromatin, which are invisible to naked eye and only visible through microscope in dividing cells.

The Structure Of A Generalised Plant Cell

structure of a generalised plant cell

In a dividing cell, the chromatin network condenses into chromosomes, whose number is constant in all species. Chromosomes are visible through the compound light microscope but only in dividing cells.
It is the chromosomes that carries hereditary factors called genes.top↑

Note: it is the nucleus of a cell that controls all the functional activities of a cell, and it is also the nucleus that contains the chromosomes that transmit heritable characters.

The Structure Of An Animal Cell As Seen Through A Light Microscope

The animal cell is similar in some aspects to a plant cell except that, the animal cell lacks cell vacuoles, chloroplasts and cell wall. For example a cheek cell;

The Structure Of A Generalised Animal Cell

 cheek cell

The cheek cell is roughly irregular in shape, where the outermost layer as the cell membrane which encloses the granular cytoplasm

Note : the nucleus is embedded in the cytoplasm.top↑

Compound Microscope

A compound light microscope is used observe cell because light rays passes through the thin specimen and then through the the lenses that produces a magnified image. A compound light microscope gives a maximum magnification of about 2000 times and it uses combination of lenses.
Another microscope that can also be used to observe cell is the electron microscope.
The electron microscope was invented with a beam of electrons passing through the specimen instead of light. The image produced by the electron microscope is not seen with the naked eye, but it is produced on a sensitive screen.top↑

The electron microscope can magnify up to 500,000 times of the cell structure which make it easier for the cell organelles which had been seen through the light microscope to be better understood, with this instrument, cell organelles that are too small to be seen with the compound light microscope have well be seen on the sensitive screen.
Cell organelles which are seen through electron microscope but not through compound light microscope are Golgi body, ribosomes, endoplasmic reticulum and mitochondria.top↑

The Structure Of A Cell As Seen Through The Electron Microscope

the structure of a cell as seen through the light microscope

Endoplasmic Reticulum, Golgi Body Mitochondrion And Ribosomes

a] Endoplasmic Reticulum : it is a system of membrane bound spaces which form an extensive network in the cytoplasm. The endoplasmic reticulum extends from the nuclear envelope to the cell membrane and provides channels of communication within the cell.
The endoplasmic reticulum is also considered that some materials may pass through these channels from the nucleus to the cell membrane.

b] Golgi body : is a system of membrane bound sacs, packed together. Several golgi bodies occur in the cytoplasm of a cell. It is the golgi body that causes removal of cells.
Below is the structure of a golgi bodytop↑

structure of a golgi body

c] Mitochondrion : is a spherical shape system. Several mitochondria are present in each cell, they are the centres for respiration because they secrete the enzymes that bring about respiration. They are numerous in metabolic active cells than in other cells.
Below is the structure of Mitochondrion

structure of a mitochondrion

d] Ribosomes : are very tiny particles which may occur in free cell or may be attached to the endoplasmic reticulum. Ribosomes synthesise proteins.top↑

The Structure Of Rough And Smooth Endoplasmic Reticulum

 smooth and round endoplasmic reticulum

Functions Of Cell Components

1] Cell Membrane : it forms the outer boundary of the protoplasm, being semipeameable, it allows some substances to pass in and out of the cell and prevents other components from doing so.

2] Cell wall : it gives shape and firmness to the cell, it allows gases and water to pass through the cell except where substances such as lignin and suberin are deposited on the cell wall.

3] Cytoplasm : it contains cell organelles such as chloroplasts, mitochondria, golgi bodies and endoplasmic reticulum. It also contains organic molecules such as enzymes, food substances which are used by the cell to carry out its activities.

Note : Many metabolic activities are carried out by the cytoplasmtop↑

4] Chloroplast. (plant cell) : it is the site for the synthesis of sugar.

5] Cell vacuole (plant cell) it contains cell sap which helps to make the cell turgid. Excretory waste are also stored in the cell sap.

6] Mitochondrion : it is the organelle in which reactions in respiration take place.

7] Food vacoule (animal cell) : it contains and stores food during digestion.

8] Nucleus : it controls the shape, size and functions of the cell, that is, it contains chromosomes by which hereditary characters are transmitted from the parent to the offspring.top↑

Differences Between Plant And Animal Cells

1] Chloroplasts are present in green plant cells while chloroplasts are absent in animal cells.

2] Mature plant cells are not filled with cytoplasm while mature animal cells contain plentiful cytoplasm and are denser (heavier) than in plant cells.

3] Cell wall of cellulose are present in plant cell while cell wall of cellulose are absent in animal cell.

4] In plant cell, few large cell vacuoles are present in mature cells while in animal cell, cell vacuoles are usually absent but food vacuole or contractile vacuole may be present.

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