The Cell Structure and Functions
Cells vary a great deal in size and shape depending on their function. Nevertheless, it is possible to make a drawing to show the generalized features of a cell, though there is no such thing as a typical plant cell or animal cell.
Generally, all cells have a cell membrane which is a thin layer of the cytoplasm. Most cells have a nucleus. The main cell organelles and their function are shown below.
Most cells contain one nucleus seen as a rounded structure enclosed in a membrane (nuclear membrane) and embedded in the cytoplasm. The nucleus contains chromosomes (made of DNA).
The nucleus is surrounded by a selectively/semi-permeable membrane that contains pores and allows for the transport of large molecules such as RNA.
i. It controls the type and quantity of enzymes produced by the cytoplasm.
ii. The nucleus controls cell division.
iii. It controls and coordinates many activities of the cell.
Contained within the nucleus is a dense, membrane-less structure composed of RNA and proteins called the nucleolus.
There are tangles of chromatin and unfinished bits of ribosomes. They are prominent inside the nucleus of non-dividing cells.
Function: They are concerned with RNA transcription (copying of gene sequence to make an RNA molecule).
It is a jelly-like material made up of cytoplasmic organelles such as Golgi bodies, endoplasmic reticulum, etc. Organelles are suspended in the cytoplasm.
Function: Many chemical reactions takes place here which keep the cell alive by providing energy and making substances that the cell needs.
This Is the living material inside the cell membrane. It is made up of the cytoplasm and the nucleus.
i. Transforms food into living matter
ii. Removing waste products
iii. Provides nutrients and oxygen and renews worn-out parts.
iv. Helps in producing new cells.
Cytoplasm Vs. Protoplasm:
|It is located between the |
cell membrane and the nucleus.
|It surrounds the cell membrane, |
cytoplasm and nucleus.
|Does not contain a nucleus.||Contains a nucleus.|
|It is a type of jelly-like fluid |
which contains water and proteins.
|It is a thick fluid which contains |
lipids, organic salts and carbohydrates.
5. Cell Membrane:
It consists of two layers of lipids molecules plus protein. It is a partially permeable layer that forms a boundary around the cytoplasm.
i. Selectively controls the movements of substances in and out of the cell.
ii. It prevents cell contents from escaping.
6. Endoplasmic Reticulum:
This is a system of membrane channels that transverse the cytoplasm.
There are two varieties of the endoplasmic reticulum; rough and smooth endoplasmic reticulum.
a. Rough Endoplasmic Reticulum is studded with ribosomes.
Function: It is the site for protein synthesis and transport of materials throughout the cytoplasm.
b. Smooth Endoplasmic Reticulum (endoplasmic reticulum without ribosomes.)
i. Synthesizes steroid hormones and other lipids.
ii. Detoxifies the cell.
iii. Carbohydrate metabolism.
iv. Connects the rough endoplasmic reticulum with the Golgi apparatus.
7. Mitochondrion: (plural; Mitochondria)
Mitochondria contain Mitochondrial deoxyribonucleic acid (Mitochondrial DNA) and ribosomes, protein-producing organelles in the cytoplasm.
It consists of an outer double membrane and an inner folded membrane called cristae. Some enzymes important in cellular respiration are embedded in the cristae membrane. They usually appear slipper-shaped, circular or oval. They are most numerous in regions of rapid chemical activity.
Functions: It is often labelled the powerhouse of the cell. It is the site for aerobic respiration; responsible for producing energy (ATP – the cell’s energy currency) from food substances during aerobic respiration.
Within the mitochondria, the DNA directs the ribosomes to produce proteins as enzymes, or biological catalysts, in ATP production.
NB: Prokaryotes do not possess mitochondria or rough endoplasmic reticulum in their cytoplasm
They are suspended freely in the cytoplasm to bind to the endoplasmic reticulum. They are tiny and spherical.
Function: Site for protein synthesis.
9. Golgi Apparatus:
Golgi Apparatus consists of flattened sacs of membranes stacked together and surrounded by vesicles.
Function: They modify, store and package substances that are produced in the rough endoplasmic reticulum. In a sense, they act as a factory in which proteins received from the endoplasmic reticulum are further processed and stored before they are transported to their eventual destinations i.e the plasma membrane, lysosomes, or secretion.
A lysosome is a vesicular sac containing digestive or hydrolytic enzymes. It is enclosed by a single membrane. Plant cells do not usually have lysosomes.
i. It is the site for intracellular digestion and respiratory enzymes.
ii. Storage compartment for powerful digestive enzymes known as lysozymes.
iii. Help remove unwanted organelles and entire cells from the body.
iv. Ingest and destroy foreign substances.
11. Plastids in Plant Cell:
Plastids are a class of small organelles that contain pigment or food and are found in the cytoplasm of cells.
- Chloroplasts are plastids that contain the green pigment called chlorophyll. Plants and other photosynthetic organisms use chlorophyll to absorb light (usually solar energy) and convert it into chemical energy.
- Chloroplasts contain different colours e.g. colour of a flower’s petals.
- Colourless plastids are called leucoplasts. An amyloplast is a leucoplast. It doesn’t have pigment or colour. Leucostictes are colourless plastids for storage of starch and lipids.
12. Cell Wall:
It is a tough, non-living layer made up of cellulose, surrounding the cell membrane. It is present only in plant cells. It is made of cellulose in plants and chitin in fungi.
i. It gives shape and mechanical support to the cell.
ii. It prevents plant cells from bursting.
iii. It is freely permeable to salts and water.
They are two small, dark cylindrical bodies found near the nucleus. They are present mainly in animals.
i. They are important in cell division and they help in the formation cilia and flagella.
ii. They help determine the locations of the nucleus and other organelles within the cell.
A fluid-filled space surrounded by a membrane. Vacuoles may contain water and food substances. Animal cells may have many small vacuoles which contain water and food substances, while plant cells usually have large central vacuoles containing cell sap. The large vacuole is enclosed by a membrane called a tonoplast.
i. It contains salts and sugars and helps to keep the plant cells firm.
ii. It acts as storehouse for many substances including excretory products.