The Concept of Cell

Cells are the basic units of life. Based on the organization of their nucleus, cells are classified into two main types:

Prokaryotic cells

A prokaryotic cell is a type of cell in which the genetic material (DNA) is not enclosed within a membrane. These cells are found in organisms known as prokaryotes.

Examples of prokaryotes

1. Bacteria
2. Blue-green algae (also known as cyanobacteria)

Generalised structure of a bacterium

Characteristics of prokaryotic cells

1. Lack a true nucleus; they have no membrane-bound nucleus.
2. Do not contain membrane-bound organelles, such as mitochondria, chloroplasts, or the endoplasmic reticulum.
3. Reproduce mainly by binary fission and sometimes by conjugation.
4. Are usually smaller in size compared to eukaryotic cells.
5. Some prokaryotes have flagella (whip-like structures) for movement or pili (hair-like structures) for attachment.
6. Have a variety of shapes including: Rod-shaped (bacilli), Spherical (cocci), Spiral (spirilla), Comma-shaped (vibrios), Corkscrew-shaped (spirochetes).
7. Contain circular DNA (not linear, and not enclosed in a nucleus).

Eukaryotic cells

Eukaryotic cells are cells that have their internal structures enclosed within membranes. These structures include a nucleus and other membrane-bound organelles.

These cells are found in organisms known as eukaryotes.

Examples of eukaryotes

1. Unicellular organisms such as Amoeba and Euglena
2. Multicellular organisms such as animals and plants

Structure of Euglena sp.

Characteristics of eukaryotic cells

1. They have a membrane-bound nucleus that contains the cell's genetic material.
2. They are usually larger and more complex than prokaryotic cells.
3. They contain specialized organelles enclosed by membranes, such as:
   • Mitochondria – for energy production
   • Chloroplasts (in plant cells) – for photosynthesis
   • Endoplasmic reticulum – for protein and lipid processing
   • Golgi apparatus – for packaging and transporting substances
4. They have linear DNA arranged in chromosomes inside the nucleus.
5. They are capable of both asexual and sexual reproduction, depending on the organism.

Both animals and plants differ in structure and function in order to be able to meet the requirements of each organism.

Plant cell

Animal cell

Cell membrane

1. Is also known as plasma membrane or plasma lemma.
2. Is a thin layer that encloses the whole cell.

Function of cell membrane

1. Communicate with other cells.
2. It allows selective movement of substances into and out of the cell. It is said to be a selectively permeable membrane or differential membrane.
3. It encloses the contents of the cell.

Cytoplasm

This is a jelly-like substance made up of water and dissolved chemical substances. The cytoplasm is the site for many chemical reactions in the cell. Cell organelles such as the vacuoles, nucleus and mitochondria are suspended in the cytoplasm. The movement of cytoplasm is known as cytoplasmic streaming.

Nucleus

This is a round or oval organelle suspended in the cytoplasm. The nucleus is made up of nucleolus and fluid called nucleoplasm. It is surrounded by a membrane called the nuclear membrane.

Functions of nucleus

1. To determine the chemical processes that take place in the cell.
2. To control the functions of all parts of the cell.
3. To determine the cell size, shape and function.
4. To determine the hereditary characteristics of a cell.

Mitochondrion

The function of mitochondrion is the site for respiration, reactions which yield energy for the cell.

Cell wall

1. This is a strong covering made of cellulose.
2. The cell wall is fully permeable. It allows the passage of water and minerals, gases (freely permeable).
3. Protects and gives the cell a definite shape.

Chloroplast

1. They are sites of photosynthesis.
2. They contain green pigment called chlorophyll.
3. Chlorophyll absorbs the light energy needed for photosynthesis.

Vacuole

Vacuoles maintain a balance between water molecules and solute molecules in the cell; contain color pigments, which give color to flower. The fluid inside vacuole is known as cell sap.

Mitochondria

The function of mitochondria is the site for respiration, reactions which yield energy for the cell.

Nucleus

This is a round or oval organelle suspended in the cytoplasm. The nucleus is made up of nucleolus and fluid called nucleoplasm. It is surrounded by a membrane called the nuclear membrane.

Cytoplasm

This is jelly-like substance made up of water and dissolved chemical substances. The cytoplasm is the site for many chemical reactions in the cell. Cell organelles such as the vacuoles, nucleus and mitochondria are suspended in the cytoplasm. The movement of cytoplasm is known as cytoplasmic streaming.

Ribosomes

This is the site of protein synthesis.

1. Both have cell membrane.
2. Both have cytoplasm.
3. Both have nucleus.
4. Both have vacuoles, ribosomes and mitochondria.