THE CELL
The Cell Theory
•1. All organisms are made up of one or more cells and the products of those cells.
•2. All cells carry on life activities.
•3. New cells arise only from other living cells by the process of cell division.
Main types of cells
There are two main types of cells – Prokaryotic and Eukaryotic. Page 32 and 33 demonstrates the structure of these cells. A prokaryotic cell has no organelles, cell wall and is small (1 – 2 micrometers). A eukaryotic cell has membrane bound organelles (these are described below). Plant and animals cells are eukaryotic.
Cell Structure
•The basic material from which all cells are made is called protoplasm. There are two forms:
•1. Cytoplasm: Protoplasm found between the nucleus and the cell membrane.
•2. Nucleoplasm: Protoplasm found inside the nucleus.
Major Organelles of a Cell
A cell must carry out a large number of activities to sustain life. These activities are carried out by specialized cell parts called organelles. See page 25 for the diagram of the animal cell and page 32 for a diagram of the plant cell.
1. Cell Membrane
•Location: surrounds the cell.
•Structure: consists of two layers composed of lipids, proteins, and carbohydrates.
•1. Lipids: There is a double layer of lipid molecules called a bilipid. This is the basic unit of the membrane.
•2. Proteins: Some are on the outer surface of the membrane, some on the inner surface, and some extend through the membrane.
2. Nucleus
•Location: round membrane structure located in the cytoplasm.
•Structure: consists of a nuclear envelope (membrane), nucleolus, and chromosomes.
•Function: serves as the control center for metabolism and reproduction.
3. Nucleolus
•Location: inside the nucleus
•Structure: dense granular bodies made up of DNA, RNA, and protein
•Function: produce ribosomes
4. Nuclear Envelope (Membrane)
•Location: surrounds the nucleus
•Structure: double membrane that contains dozens of pores
•Function: controls which materials enter and exit the cell
5. Chromosomes
•Location: found in the nucleus
•Structure: fine fibers composed of DNA and protein
•Function: contain genetic information that is passed on in cell division to each new generation
6. Endoplasmic Reticulum
•Location: found in the cytoplasm
•Structure: complex network of fluid-filled channels enclosed by membranes. Two forms:
7. Ribosomes
•Location: found in the cytoplasm and on the endoplasmic reticulum.
•Structure: tiny, round organelles. Among the smallest organelles in the cell.
•Function: Protein production. The proteins are mostly enzymes that function in the cell’s cytoplasm.
8. Mitochondria
•Location: found in the cytoplasm. Numbers vary from 300 to 800 per cell depending on cellular activity.
•Structure: oval or slipper shaped with a double membrane:
9. Golgi Bodies
•Location: found in the cytoplasm. Animal cells usually have only one which is frequently located near the nucleus. Certain plant and animal cells may have several hundred.
•Structure: take the shape of stacked, flattened sacs.
•Function: serve as processing, packaging, and storage centers for the products to be released from the cell.
10. Vacuoles
•Location: found in the cytoplasm.
•Structure: fluid filled spaces surrounded by a single membrane.
•Function: used for the storage of food, water, and minerals. Certain vacuoles control water content within the cell and are referred to as contractile vacuoles.
11. Lysosomes
•Location: found in the cytoplasm.
•Structure: sac-like structures surrounded by a single membrane containing strong digestive enzymes.
•Function:
12. Microtubules
•Location: found in the cytoplasm.
•Structure: long, hollow, cylindrical structures composed of proteins.
•Function:
13. Microfilaments
•Location: found in the cytoplasm.
•Structure: long, solid, thread-like strands composed of protein.
•Function: 1. Support the cell.
•2. Aid in the movement of cytoplasm within the cell (cyclosis).
•3. Those containing the protein actin are involved in muscle contraction.
14. Cilia
•Location: extend from the cell membrane while being attached to the interior of the cell by a basal body.
•Structure: short hair-like projections usually present in large numbers. They contain 9 pairs of microtubules surrounding a central pair,
•Function:
15. Flagella
•Location: extend from the cell membrane; attached to the interior of the cell by a basal body.
•Structure: long hair-like structures usually present in small numbers. Microtubules: 9 pairs + 1 central pair.
•Function: important in movement.
16. Centrioles
•Location: found in the cytoplasm near the nucleus.
•Structure: cylindrical organelles with two bundles at right angles to each other. They are composed of microtubules.
•Function: plays an important role in cell division within animal cells.
17. Cell Wall (plant cells only)
•Location: outside the cell membrane
•Structure: rigid covering composed of cellulose. Contains many small openings which allow materials to pass to and from the cell membrane.
•Function: gives cell its shape and provides protection.
18. Plastids (plant cells only)
•Location: found in the cytoplasm
•Structure: bound by a double membrane, and has a system of membranes within the organelle
•Function: chemical factories and store-house for food and colour pigment
•Types of plastids include: