Cell: The Unit of Life

The cell is the fundamental structural and functional unit of all living organisms. The cell theory developed as Matthias Schleiden (1838) and Theodor Schwann (1839) proposed that plants and animals are made of cells, and Rudolf Virchow (1855) explained that new cells arise from pre-existing cells (omnis cellula-e cellula). You will compare prokaryotic cells, such as bacteria with their cell envelope, mesosomes, plasmids, 70S ribosomes, flagella, pili and fimbriae, with eukaryotic cells. Cells vary greatly in size, from Mycoplasma (about 0.3 micrometres) to the ostrich egg, the largest isolated single cell. The chapter then describes each part of a eukaryotic cell: the cell membrane and its fluid mosaic model (Singer and Nicolson, 1972), the plant cell wall and middle lamella, and the endomembrane system of endoplasmic reticulum, Golgi apparatus, lysosomes and vacuoles. You will also study mitochondria and plastids with their own DNA, 80S cytoplasmic ribosomes, the cytoskeleton, the 9+2 arrangement of cilia and flagella, centrioles, and the nucleus with nuclear pores, nucleolus, chromatin, and chromosome types based on centromere position.

Detailed Worksheet: Cell: The Unit of Life Section A - Definitions (10 marks) 1. State the cell theory as modified by Rudolf Virchow. (2 marks) 2. What are mesosomes? State two functions. (2 marks) 3. What is the fluid mosaic model of the cell membrane? Who proposed it? (2 marks) 4. What are plasmodesmata? What is their function? (2 marks) 5. Distinguish between the cis face and the trans face of the Golgi apparatus. (2 marks) Section B - Calculations and Applications (15 marks) 6. A bacterial cell is 3 micrometres long. Under a microscope it appears 1.2 mm long. Calculate the magnification. A human RBC is 7 micrometres in diameter. How large would it appear at the same magnification? (3 marks) 7. Consider two cube-shaped cells, one of side 1 micrometre and the other of side 10 micrometres. Calculate the surface area, volume and surface area to volume ratio for each. Explain why cells remain small. (3 marks) 8. A mitochondrion is about 0.5 micrometres in diameter and 3 micrometres long. Express these dimensions in nm. If a cell has 1,500 mitochondria arranged end to end, what total length would they cover in mm? (3 marks) 9. The 80S ribosome consists of 60S and 40S subunits, and the 70S ribosome of 50S and 30S subunits. Explain why 60 + 40 does not equal 80. Where are 70S ribosomes found in eukaryotic cells? (3 marks) 10. The membrane of human RBCs has about 52% protein and 40% lipids. If a membrane sample has a mass of 50 micrograms, calculate the mass of protein and lipids and the mass of the remaining component. Name the remaining component. (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of a typical prokaryotic (bacterial) cell. (4 marks) 12. Draw a labelled diagram of the fluid mosaic model of the plasma membrane. (3 marks) 13. Draw labelled diagrams of the ultrastructure of a mitochondrion and a chloroplast. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Compare prokaryotic and eukaryotic cells. Why are mitochondria and chloroplasts called semi-autonomous organelles? (5 marks) 15. Explain how the endomembrane system works as a coordinated unit, from protein synthesis at the RER to packaging and secretion by the Golgi apparatus and digestion by lysosomes. (5 marks) 16. Describe the structure of the nucleus. Classify chromosomes on the basis of the position of the centromere, with labelled diagrams. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw diagrams neatly with labels. Show the steps of calculations with units.