Cells & Organisation

IGCSE Edexcel Biology
2.1–2.16 Cell structure, organisation, diffusion, osmosis and active transport
Key Concepts: Cells are the basic units of life. Organelles carry out specific functions. Cells are organised into tissues, organs, and organ systems. Substances move by diffusion, osmosis, and active transport.

Section A — Levels of Organisation

1. Place the following terms in order from simplest to most complex: organ, organ system, cell, tissue, organism. [5]
2. Give one example of each level of organisation in a mammal. [4]

a) Cell

b) Tissue

c) Organ

d) Organ system

Section B — Cell Structure

3. Complete the table comparing animal and plant cells. [6]
Structure Present in animal cells? Present in plant cells?
Cell membrane
Nucleus
Cytoplasm
Mitochondria
Cell wall
Chloroplasts
Permanent vacuole
4. State the function of each organelle. [5]

a) Nucleus

b) Mitochondria

c) Ribosomes

d) Cell membrane

e) Cell wall (plant)

5. State two features that distinguish a bacterial cell from a plant cell. [2]

Section C — Specialised Cells

6. Explain how each cell is adapted for its function. [6]

a) Red blood cell

b) Root hair cell

c) Ciliated epithelial cell

Section D — Diffusion, Osmosis and Active Transport

7. Define the following terms. [6]

a) Diffusion

b) Osmosis

c) Active transport

8. Complete the table comparing the three transport processes. [6]
Feature Diffusion Osmosis Active Transport
Requires energy (ATP)?
With or against gradient?
Substance(s) moved
9. State three factors that affect the rate of diffusion. [3]
10. A student places a piece of potato in a concentrated salt solution. After 30 minutes the potato is limp and lighter. Explain these observations. [3]
11. Explain why active transport is essential for the uptake of mineral ions by root hair cells. [2]

Total marks: 53

Mark Scheme

1. Cell → tissue → organ → organ system → organism [5]
2. a) Any named cell, e.g. muscle cell / nerve cell / red blood cell; b) Muscle tissue / nervous tissue / epithelial tissue; c) Heart / lung / kidney / stomach; d) Circulatory / nervous / digestive / respiratory system [4]
3. Cell membrane: both ✓; Nucleus: both ✓; Cytoplasm: both ✓; Mitochondria: both ✓; Cell wall: plant only; Chloroplasts: plant only; Permanent vacuole: plant only [6 — deduct 1 per error, min 0]
4. a) Controls cell activities / contains DNA; b) Site of aerobic respiration / produces ATP; c) Site of protein synthesis; d) Controls movement of substances in and out of cell; e) Provides structural support / made of cellulose [5]
5. Any two: bacterial cell has no membrane-bound nucleus; bacterial cell has circular DNA and plasmids; bacterial cell has no chloroplasts/mitochondria; bacterial cell wall is not made of cellulose; bacteria are smaller [2]
6. a) Red blood cell: biconcave disc increases surface area for O₂ absorption; no nucleus = more room for haemoglobin; haemoglobin binds oxygen [2]; b) Root hair cell: long projection increases surface area; thin wall for fast absorption; large vacuole maintains turgor; many mitochondria for active transport [2]; c) Ciliated cell: cilia beat to move mucus/particles; many mitochondria for energy [2] [6]
7. a) Net movement of particles from a region of higher concentration to lower concentration, down a concentration gradient [2]; b) Diffusion of water molecules through a partially permeable membrane from a region of high water potential (dilute solution) to low water potential (concentrated solution) [2]; c) Movement of substances against a concentration gradient using energy (ATP) and carrier proteins [2] [6]
8. Requires energy: No / No / Yes; With or against gradient: with / with (water) / against; Substance(s): any dissolved particles/gases / water only / ions/glucose etc. [6]
9. Any three: concentration gradient (steeper = faster); temperature (higher = faster); surface area (larger = faster); distance / thickness of membrane [3]
10. The salt solution has a lower water potential than the potato cells; water moves out of the cells by osmosis; cells lose water, become flaccid / the potato loses mass [3]
11. The concentration of mineral ions in the soil is lower than inside the root hair cell; so ions cannot enter by diffusion; active transport uses energy to move ions against the gradient into the cell [2]