The Cell – SS1 Biology Lesson Note (First Term)

The cell is the simplest, smallest, basic structural and functional unit of life. It can carry out all life activities such as growth and respiration. Cells may exist alone (unicellular), in colonies, in filaments, or as part of multicellular organisms, where cells form tissues, organs and organ systems.

Behavioural objectives

By the end of the lesson, students should be able to:

• define the cell and state the forms in which cells exist;
• describe the features of Chlamydomonas, Amoeba, Paramecium and Euglena;
• state the functions of the nucleus and of the main cell organelles;
• list the similarities and differences between plant and animal cells;
• state the cell theory.

In what forms do cells exist?

Cells can exist as a single cell, as a colony of cells, as a filament of cells, or as part of a multicellular organism.

Unicellular forms

A unicellular organism is a single cell. Examples are Amoeba, Chlamydomonas, Chlorella and Euglena.

Chlamydomonas (Chlamydomonas reinhardtii)

Chlamydomonas is a unicellular plant-like protist. It moves using its flagellum and has a cup-shaped chloroplast. It is sensitive to light through its eye spot. In the presence of light, starch grains appear in the cytoplasm. These disappear in a dark environment.

Chlamydomonas cells respond to changes in their surroundings through their flagella (whip-like), for example by moving from a dark region to a region of light. The organism disposes of its excess water using its contractile vacuoles. It reproduces asexually and sexually when it reaches its maximum size.

Amoeba (Amoeba proteus)

Amoeba is a unicellular, animal-like protist and one of the simplest organisms. It feeds heterotrophically and has no fixed shape. It digests its complex food in a food vacuole. Undigested food is egested through the cell surface, and excess water is removed through its contractile vacuole.

Amoeba moves about and captures its food with the aid of its pseudopodia (“false feet”). At maximum growth, Amoeba reproduces asexually by dividing into two daughter cells (binary fission).

Diagram of an amoeba

Paramecium

Paramecium is a more complex unicellular organism than Amoeba. It is slipper-shaped and moves with the aid of its cilia, which also direct food into its gullet. Undigested food is removed through the anal pore. Excess water is removed through the anterior and posterior contractile vacuoles. Two types of nuclei are visible in this organism: the micronucleus and the macronucleus.

The structure of Paramecium caudatum

Euglena (Euglena viridis)

Euglena viridis is a protist that has both plant and animal features. As a plant, it has chloroplasts (star-shaped) for photosynthesis, pyrenoids for starch storage and paramylon granules (the form in which food is stored).

As an animal, it has no cellulose cell wall. It has a plasma membrane called a pellicle, a gullet, an eye spot, a contractile vacuole and a flagellum for movement.

Colonial forms

Examples of colonial forms are Volvox, Sponges and Pandorina.

Volvox: Volvox is made up of several Chlamydomonas-like cells arranged in an angular layer to form a hollow ball-like structure. The cells are connected by cytoplasmic strands.

The cells of Volvox move about through the beating of their flagella. Most cells of Volvox lose their ability to reproduce. Only a few of them can reproduce to form two daughter cells.

Filamentous forms

Examples of filamentous forms are Spirogyra, Oscillatoria and Oedogonium.

Spirogyra: Spirogyra is a filament of identical cells joined end to end without branching. Each cell carries out its function independently of the other cells. Spirogyra has a spirally arranged chloroplast for photosynthesis.

The filament grows indefinitely. Cells that break off from the filament can reproduce asexually to form a new filament. Spirogyra can also reproduce sexually.

Part of a multicellular organism

A multicellular organism is made up of interdependent cells carrying out several special functions. The cells are arranged to perform similar functions and form tissues. A group of specialised tissues performing similar functions forms an organ. Organs are further organised to carry out special functions as a system, or organ system. Ultimately, the interdependent biological systems result in complex multicellular organisms.

What are the cell structures and their functions?

The study of the cell is called cytology. A cell consists of a living material called protoplasm, which is surrounded by a membrane called the plasma membrane. In plants, a cell wall (a non-living part) also surrounds the plasma membrane.

The protoplasm is made up of two major parts:

1. Cytoplasm
2. Nucleus

Cytoplasm

The cytoplasm contains cell organelles or inclusions, which are cellular structures with specific functions. Cells that contain a nucleus and organelles are called eukaryotic cells. These show a high level of cellular organisation.

Some cells lack definite nuclei and organelles. In such cells (prokaryotic cells, e.g. bacteria), the materials containing instructions for growth and development are found in a particular part of the cell and are not enclosed in any membrane.

Nucleus

The nucleus is the largest cellular organelle. It is enclosed by a double-layered nuclear membrane with many pores. It contains chromosomes and the nucleolus, which are rich in proteins and nucleic acids.

The nucleus performs the following functions:

1. Exchange of materials between the nucleus and the cytoplasm through the nuclear pores.
2. Storage of hereditary information by the chromosomes.
3. Control centre of cellular activities.

Structure of a typical animal cell

What are the cell components and their functions?

Structure of a typical plant cell

What are the similarities between plant and animal cells?

Both plant and animal cells have the following in common:

• Nucleus
• Golgi bodies
• Mitochondria
• Cytoplasm
• Chromosomes
• Endoplasmic reticulum
• Nucleolus
• Ribosomes
• Lysosomes
• Cell membrane

What are the differences between plant and animal cells?

What is the cell theory?

The cell theory is the summary of the research efforts of many scientists, such as Robert Hooke, Matthias Schleiden and Theodor Schwann (1838 and 1839), and Rudolf Virchow (1855).

The cell theory states that:

1. All living organisms are composed of one or more cells.
2. The cell is the structural and functional unit of all living organisms, that is, the basic organisational unit of life.
3. All existing cells come from pre-existing ones.
4. A cell contains information for its structural and functional development in its nucleic acids. This information is passed down from the parents to the offspring.

Evaluation

1. State the four points of the cell theory.
2. Name two unicellular organisms. Describe how Amoeba moves and captures its food.
3. State three functions of the nucleus.
4. State three differences between a plant cell and an animal cell.
5. State the function of the mitochondria.

Assignment

1. A cell that is capable of self-existence is a/an
   A. amoeba
   B. ovum
   C. amoeba
   D. onion epidermal cell
2. Which of these organelles is common to both plant and animal cells?
   A. Chloroplast
   B. Centriole
   C. Mitochondrion
   D. Pyrenoid
3. Which scientist did not contribute to the formation of the cell theory?
   A. Isaac Newton
   B. Robert Hooke
   C. Matthias Schleiden
   D. Theodor Schwann
4. The organelle which is contractile in function in Euglena is the
   A. eye spot
   B. gullet
   C. myoneme
   D. paramylon
5. Which of the following cells are not regarded as specialised?
   A. Muscle cells
   B. Root tip cells
   C. Somatic cells
   D. Sperm cells
6. Differentiate between prokaryotic and eukaryotic cells.
7. Describe and state three functions each of:
   • rough endoplasmic reticulum
   • vacuoles