Form 1:
The Cell

Menu|Table of content

Cell Structures as seen under the Light Microscope

The structures within the cell are referred to as organelles.

Some of the cell organelles that can be observed under the light microscope include the cell wall, cell membrane, cytoplasm, nucleus, vacuole and chloroplasts.

These cell organelles perform specific functions within the cell.

Image:animal cell seen under light microscope

Image:Plant cell seen under light microscope

The cell as seen under the Electron Microscope

The electron microscope is more powerful than the light microscope.

It uses a beam of electrons to illuminate the specimen instead of light as in the case of light microscope.

Electron microscope can magnify an object up to 500, 000 times.

Image:animal cell seen under Electron microscope

Image:Plant cell seen under Electron microscope

It also has a very high resolving power. Resolving power is the ability to distinguish between separate things which are close to each other.

The high resolving power makes the electron microscope a very important research tool in microbiology.

Through the electron microscope, very fine details of the cell can be observed.

Structure and Functions of the Cell Organelles

a) Cell membrane

The cell membrane, also known as plasma membrane or plasmalemma consists of three layers when viewed under the electron microscope.

The three layers are composed of one layer of phospholipid sandwiched between two protein layers.

It is flexible and has pores. The cell membrane is important in that:

1) It encloses the cell contents.

2) It allows for selective movement of materials in and out of the cells. The pores allow materials particularly of small molecular size to move in and out of the cells.

Image:Cell Membrane

b) Cytoplasm

Cytoplasm consists of a fluid medium in which chemical reactions take place.

It contains organelles and other inclusions such as starch, glycogen, fat droplets and many other dissolved substances.

Cytoplasm is not static; it undergoes a movement known as cytoplasmic streaming.

It provides a suitable medium for cellular reactions to take place.

c) Mitochondrion

Mitochondrion is a sausage shaped organelle that provides sites for respiratory reactions that yield energy for the cell. Mitochondria is thus, referred to as the powerhouse of the cell.

It is bound by two membranes. The inner membrane is greatly folded into cristae to increase surface area for respiration.

The arrangement and number of mitochondria in a cell depends on the cell energy requirements. Cells that require large amounts of energy contain high amount of mitochondria.

Such cells include muscle cell, sperm cell, apical meristem cells, and kidney cell.

Mitochondria are self replicative that is they can divide to form new ones.

d) Endoplasmic Reticulum

Endoplasmic reticulum appears as a series of interconnected channels, running throughout the cytoplasm.

Their membranes are continuous with the outer membrane of the nuclear membrane.

Some endoplasmic reticula have granules called ribosomes on their surfaces and are referred to as rough or granular endoplasmic reticula.Endoplasmic reticula that are not associated with ribosomes are called smooth endoplasmic reticula.

The rough endoplasmic reticulum transports proteins.

Smooth endoplasmic reticulum transports lipids.

Endoplasmic reticula also act as storage areas for synthesized molecules such as enzymes.

They also contribute to mechanical support.


e) Ribosomes

These are spherical in shape. While some are bound to the endoplasmic reticula, some ribosomes are scattered within the cytoplasm (free ribosomes). Their largest dimension is 25 nanometres.

They are synthesised in the nucleolus.

They form sites for protein synthesis.


f) Lysosomes

These are spherical sac-like organelles bound by a single membrane.

They contain lytic enzymes which break down large molecules, destroy worn out organelles or even the entire cells.

Lysosomes also play crucial role in digestion in unicellular organisms.

The lysosomes are also vital in breakdown of bacteria and other harmful microbes that might have been ingested in food.

This explains their high relative abundance in injured or infected cells.

The membrane of the lysosomes are intact. This is important because if the enzymes leak out, they may destroy the whole cell.

g) Golgi bodies/Golgi apparatus

These are stacks of membrane bound tube like sacs. They are found close to the cell membrane.

Golgi bodies perform the following functions:

1) They package and transport glycoproteins.
2) They are involved in secretion of synthesized proteins and carbohydrates.
3) They manufacture lysosomes.

Note: Golgi bodies are abundant in cells that are active in secretion. For instance pancreatic cells which secrete enzymes and the nerve cells which secrete neuro-transmitter substances.

h) Centrioles.

These are rod shaped structures located just outside the nuclear membrane.

They take part in cell division and also in the formation of cilia and flagella in lower organisms.

Plant cells lack centrioles.

i) Chloroplasts

Chloroplasts are egg-shaped structures surrounded by two membranes and contain a gel like stroma through which runs a system of membranes that are stacked together to form grana.

The granum contains chlorophyll which traps light energy that is used during photosynthesis.

It is in the chloroplasts that photosynthesis takes place.


j) Vacuoles

These are sacs that are filled with fluid called cell sap.

Vacuoles vary in size.

Animal cells contain small vacuoles which may be numerous in the cells while plant cells contain one large centrally placed vacuole.

Sap vacuoles store sugars and salts thereby contributing to the osmotic properties of the cell. This influences how materials move in and out of the cell.

In some unicellular organisms, food vacuole stores and digests food substances while the contractile vacuole excretes unwanted materials from the cell.

k) Cell wall

This is the rigid outer cover of plant cells and some lower organisms.

In plants it is composed of cellulose fibres.

Cell wall is important in that:

1. It gives plant cells their definite shape
2. It provides mechanical support and protection against mechanical injury.
3. The cell wall allows gases, water and other substances to pass through it.

l) Nucleus

Nucleus is a double membrane bound structure made up of a viscous fluid known as nucleoplasm in which nucleolus and chromatin materials are suspended. The nuclear membrane has minute pores, nuclear pores which allow materials to move in and out of the nucleus.

Nucleus controls all the activities of the cell.

Nucleolus is responsible for manufacture of ribosomes while chromatin contains hereditary materials.

Nucleus generally takes a sperical or oval shape.

<< How to use a microscope|Comparison between plant cells and animal cells >>