Cell: Structure and Functions

All living organisms are made of cells, the basic structural and functional units of life. In 1665, Robert Hooke observed thin slices of cork under a simple microscope and saw tiny box-like compartments, which he called cells. In this chapter you will learn that some organisms, such as Amoeba, Paramecium and bacteria, consist of a single cell, while others, like humans and mango trees, have millions of cells organised into tissues and organs. Cells vary greatly in number, shape and size: an Amoeba changes shape using pseudopodia, nerve cells are long and branched, and the egg of an ostrich is the largest single cell. You will study the parts of a cell: the cell membrane, which separates the cell from its surroundings; the cytoplasm; and the nucleus, which contains chromosomes carrying genes. You will compare plant cells, which have a cell wall, plastids such as chloroplasts and a large vacuole, with animal cells, and distinguish prokaryotic cells from eukaryotic cells.

Detailed Worksheet: Cell: Structure and Functions Section A - Definitions (10 marks) 1. What is a cell? Who first observed cells, in which year, and in what material? (2 marks) 2. Distinguish between unicellular and multicellular organisms, giving two examples of each. (2 marks) 3. State one function each of the cell membrane and the cell wall. In which type of cells is the cell wall found? (2 marks) 4. What are chromosomes and genes? Where are they found in the cell? (2 marks) 5. Define prokaryotic and eukaryotic cells, giving one example of each. (2 marks) Section B - Calculations and Applications (15 marks) 6. A student views onion peel cells through a microscope with a 10x eyepiece and a 40x objective lens. (i) What is the total magnification? (ii) If one cell appears 40 mm long in the image, what is its actual length in mm and in micrometres? (1 mm = 1000 micrometres) (3 marks) 7. A human red blood cell has a diameter of about 7 micrometres. Approximately how many red blood cells, placed side by side, would cover a length of 1 mm? Why can we not see a single red blood cell with the naked eye? (3 marks) 8. The size of a bacterium is about 0.5 micrometre, a human red blood cell is about 7 micrometres and an ostrich egg is about 170 mm long. Arrange them in increasing order of size, and calculate how many times longer the ostrich egg is than the bacterium. (3 marks) 9. A thin peel of a plant leaf is kept in strong salt solution for some time and then observed under a microscope. Explain what happens to the cytoplasm and why. Which part of the cell prevents it from bursting when it is placed in plenty of water? (3 marks) 10. In a laboratory, onion peel is stained with safranin and mounted in glycerine, and cheek cells are stained with methylene blue. Explain why stains are used, why glycerine is added, and why a cover slip is placed carefully without trapping air bubbles. (3 marks) Section C - Diagrams (10 marks) 11. Draw a neat labelled diagram of a plant cell showing the cell wall, cell membrane, cytoplasm, nucleus, plastids and vacuole. (4 marks) 12. Draw a neat labelled diagram of an animal cell, such as a human cheek cell, showing the cell membrane, cytoplasm and nucleus. (3 marks) 13. Draw the shapes of a red blood cell, a muscle cell and a nerve cell, and write one feature of each shape that suits its function. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Write three differences between a plant cell and an animal cell. Explain why plant cells need a cell wall in addition to a cell membrane, and why vacuoles are larger in plant cells. (5 marks) 15. A student claims that the cells of an elephant must be much larger than the cells of a rat because the elephant is much larger. Evaluate this claim. Using the nerve cell, red blood cell and white blood cell as examples, explain how the shape of a cell is related to its function. (5 marks) 16. A hen's egg is said to be a single cell. Explain this statement, naming the parts of the egg that you can see when it is boiled and peeled. Compare the size of the hen's egg with that of most other cells, and explain how a single-celled organism like Amoeba carries out all its life processes. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw all diagrams neatly in pencil and label every part clearly.