Cells and Transport.
Notes
Cell types
| Feature | Eukaryotic (plant, animal) | Prokaryotic (bacteria) |
|---|---|---|
| Nucleus | yes, enclosed in a membrane | no, DNA is a loop in the cytoplasm |
| Plasmids | rare | often present |
| Size | larger | much smaller |
| Cell wall | plants and fungi only | yes (not cellulose) |
Plant cells also have chloroplasts, a permanent vacuole and a cellulose cell wall.
Organelle functions
Nucleus (controls the cell, holds DNA), mitochondria (aerobic respiration), ribosomes (protein synthesis), cell membrane (controls what enters and leaves), chloroplasts (photosynthesis).
Microscopy and magnification
Magnification = image size ÷ actual size. Convert to the same units first: 1 mm = 1000 µm.
Electron microscopes give higher magnification and resolution than light microscopes, so we can see sub-cellular structures such as ribosomes.
Transport
| Process | Direction | Energy needed? |
|---|---|---|
| Diffusion | high to low concentration | no |
| Osmosis (water only, through a partially permeable membrane) | high to low water concentration | no |
| Active transport | low to high concentration (against the gradient) | yes, from respiration |
Rate of diffusion increases with: a steeper concentration gradient, higher temperature, and larger surface area.
Surface area to volume ratio
As an organism gets bigger, its SA:V ratio falls. Large organisms need specialised exchange surfaces and transport systems (for example alveoli, villi, circulatory system) because diffusion alone is too slow.
Cube of side 2 cm: SA = 24 cm², V = 8 cm³, SA:V = 3:1. Cube of side 4 cm: SA = 96, V = 64, SA:V = 1.5:1.
Required practical: osmosis in potato
Potato cylinders in different sugar solutions. Calculate percentage change in mass = (final − initial) ÷ initial × 100. The concentration where the percentage change is zero is the point where the solution matches the potato cell sap.
Common mistakes
- Saying water moves "to where there is more sugar" without explaining it in terms of water concentration.
- Forgetting that active transport needs energy and carrier proteins.
- Mixing units when calculating magnification.
Exam tips
- "Explain" questions: link a feature to a function (for example thin walls give a short diffusion distance).
- Give a direction for every transport question.
Worksheet
Total: 30 marks.
- A drawing of a cell is 45 mm long. The magnification is ×300. Calculate the real length of the cell in mm and in µm. (3)
- A cube has side 2 cm and another has side 4 cm. Calculate the surface area to volume ratio of each cube and explain what this shows about large organisms. (5)
- A potato cylinder has an initial mass of 2.50 g. After 24 hours in a sugar solution its mass is 2.15 g.
(a) Calculate the percentage change in mass. (2)
(b) Explain this result using the term osmosis. (3) - Explain why a single-celled organism does not need a transport system but a large multicellular organism does. (3)
- Root hair cells take up mineral ions from soil that has a lower concentration of those ions than the cell. Name this process and explain why it needs energy. (3)
- Compare an animal cell and a bacterial cell. Give three differences. (3)
- Alveoli and villi are both adapted for efficient exchange. Describe three features they share and explain how each increases the rate of diffusion. (6)
- An electron microscope can show ribosomes, but a light microscope cannot. Explain why. (2)
Show mark scheme and answers (tutor copy)
Try the worksheet first, then check your work here.
- Real size = image ÷ magnification = 45 ÷ 300 (M1) = 0.15 mm (A1) = 150 µm. (A1)
- 2 cm cube: SA = 6 × 4 = 24 cm², V = 8 cm³, ratio 3:1 (B1, B1). 4 cm cube: SA = 6 × 16 = 96 cm², V = 64 cm³, ratio 1.5:1 (B1, B1). As size increases, SA:V decreases, so large organisms cannot rely on diffusion across the surface and need specialised exchange surfaces and transport systems. (B1)
- (a) (2.15 − 2.50) ÷ 2.50 × 100 (M1) = −14%. (A1)
(b) The solution had a lower water concentration than the potato cells (B1). Water moved out of the cells by osmosis (B1) through the partially permeable membrane, down the water concentration gradient (B1), so the potato lost mass. - A single-celled organism has a large SA:V ratio, so substances diffuse in and out fast enough (B1). A multicellular organism has a small SA:V ratio and a long diffusion distance to inner cells (B1), so it needs a transport system to deliver materials and remove waste (B1).
- Active transport (B1). Ions move against their concentration gradient, from low to high concentration (B1). This needs energy from respiration, and carrier proteins in the membrane (B1).
- Any three: animal cell has a nucleus, bacterium has a DNA loop with no nucleus; bacteria are much smaller; bacteria have a cell wall (not cellulose), animal cells do not; bacteria may have plasmids; animal cells have mitochondria, bacteria do not. (B1 each)
- Any three features with explanations (B1 feature + B1 explanation each): large surface area (more area for diffusion); thin walls, one cell thick (short diffusion distance); rich blood supply (maintains a steep concentration gradient); ventilation or good blood flow (maintains the gradient).
- Electron microscopes have a much higher resolution (B1) and a higher magnification than light microscopes (B1).