THE CELL – Cell membrane, Cell wall, Plasma Membrane, Endoplasmic Reticulum & Ribosomes, Goigi Bodies, Mitochondria

Introduction

What is a cell? The cell is the basic unit of an organism. All living organisms are made up of cells. Some organisms are made up of one cell and others are said to be multicellular. Other organisms are made of many cells and are said to be multicellular. Cells are too little to see with the naked eye. They can only be seen with the aid of a microscope

Table of Contents

The microscope

The microscope is used to magnify objects

Magnification

The magnifying power is usually inscribed on the lens
To find out how many times a specimen is magnified, the magnifying power of the objective lens is multiplied by that of the eye piece lens
If the eye piece magnification lens is x10 and the objective lens is x4, the total magnification is x40
Magnification has no units
It should always have the multiplication sign

e.g.x40

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Microscope parts and their functions

To View the Object

Turn the low power objective lens until it clicks into position
Looking through the eye piece, ensure that enough light is passing through by adjusting the mirror
This is indicated by a bright circular area known as the field of view
Place the slide containing the specimen on stage and clip it into position
Make sure that the specimen is in the centre of the field of view
Using the coarse adjustment knob, bring the low power objective lens to the lowest point
Turn the knob gently until the specimen comes into focus
If finer details are required, use the fine adjustment knob
When using high power objective always move the fine adjustment knob upwards

Care of a Microscope

Great care should be taken when handling it
Keep it away from the edge of the bench when using it
Always hold it with both hands when moving it in the laboratory
Clean the lenses with special lens cleaning paper
Make sure that the low power objective clicks in position in line with eye piece lens before and after use
Store the microscope in a dust-proof place free of moisture

Cell membrane (Plasma membrane):

This is a thin membrane enclosing cell contents
It controls the movement of substances into and out of the cell

Cytoplasm:

This is a jelly-like substance in which chemical processes are carried out
Scattered all over the cytoplasm are small structures called organelles
Like an animal cell, the plant cell has a cell membrane, cytoplasm and a nucleus

vacuole

Plant cells have permanent, central vacuole

It contains cell sap where sugars and salts are stored

Cell wall:

This is the outermost boundary of a plant cell
It is made of cellulose
Between the cells is a middle lamella made of calcium pectate

Chloroplasts;

With special staining techniques it is possible to observe chloroplasts
These are structures which contain chlorophyll, the green pigment responsible for trapping light for photosynthesis

The Electron Microscope (EM)

Capable of magnifying up to 500,000 times
The specimen is mounted in vacuum chamber through which an electron beam is directed
The image is projected on to a photographic plate
The major disadvantage of the electron microscope is that it cannot be used to observe living objects
However, it provides a higher magnification and resolution (ability to see close points as separate) than the light microscope so that specimen can be observed in more detail

The Plasma Membrane

Under the electron microscope, the plasma membrane is seen as a double layer
This consists of a lipid layer sandwiched between two protein layers
This arrangement is known as the unit membrane and the shows two lipid layers with proteins within
Substances are transported across the membrane by active transport and diffusion

The Endoplasmic Reticulum (ER)

This is a network of tubular structures extending throughout the cytoplasm of the cell
It serves as a network of pathways through which materials are transported from one part of the cell to the other
An ER encrusted with ribosomes it is referred to as rough endoplasmic reticulum
An ER that lacks ribosomes is referred to as smooth endoplasmic reticulum
The rough endoplasmic reticulum transports proteins while the smooth endoplasmic reticulum transports lipids

The Ribosomes

These are small spherical structures attached to the ER
They consist of protein and ribonucleic acid (RNA)
They act as sites for the synthesis of proteins

Goigi Bodies

Golgi bodies are thin, plate-like sacs arranged in stacks and distributed randomly in the cytoplasm
Their function is packaging and transportation of glycol-proteins
They also produce lysosomes

Mitochondria

Each mitochondrion is a rod-shaped organelle
Made up of a smooth outer membrane and a folded inner membrane
The foldings of the inner membrane are called cristae
They increase the surface area for respiration
The inner compartments called the matrix
Mitochondria are the sites of cellular respiration, where energy is produced

Lysosomes

These are vesicles containing hydrolytic enzymes
They are involved in the breakdown of micro-organisms, foreign macromolecules and damaged or worn-out cells and organelles

The Nucleus

The nucleus is surrounded by a nuclear membrane which is a unit membrane
The nuclear membrane has pores through which materials can move to the surrounding cytoplasm
The nucleus contains proteins and nucleic acid deoxyribonucleic acid (DNA) and RNA
The chromosomes are found in the nucleus
They are the carriers of the genetic information of the cell
The nucleolus is also located in the nucleus but it is only visible during the non-dividing phase of the cell

The Chloroplasts

These are found only in photosynthetic cells
Each chloroplast consists of an outer unit membrane enclosing a series of interconnected membranes called lamellae
At various points along their length the lamellae form stacks of disc like structures called grana
The lamellae are embedded in a granular material called the stromal
The chloroplasts are sites of photosynthesis
The light reaction takes place in the lamellae while the dark reactions take place in the stroma

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