CCEA A Level Biology
CCEA A Level Biology 1.5 Cell Physiology Exam Questions
Free CCEA A Level Biology revision for 1.5 Cell Physiology: what you need to know, common mistakes and exam-style questions with answers. Covers diffusion, osmosis and water potential, plasmolysis, facilitated diffusion, active transport, endocytosis and exocytosis, membrane permeability and potato and onion practicals.
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About CCEA A Level Biology 1.5 Cell Physiology
Cell physiology, section 1.5 of Unit AS 1, is about how substances cross membranes. CCEA uses water potential terminology throughout: you must use solute potential, pressure potential and the equation for water potential of a cell, and explain turgor, incipient plasmolysis, plasmolysis, lysis and crenation. You also need facilitated diffusion, active transport, endocytosis and exocytosis, and how the structure of the membrane controls permeability. Two required practicals are common: finding the water potential of potato tissue from mass changes and finding solute potential by counting plasmolysed onion epidermis cells. This content is used again in transport in plants and the kidney.
What you need to know
- Water potential Water moves by osmosis from higher (less negative) to lower (more negative) water potential across a partially permeable membrane. For a plant cell, water potential = solute potential + pressure potential. Pure water has a water potential of 0.
- Plant and animal cells in solutions Plant cells in water become turgid; in a strong solution they lose water and the protoplast pulls from the wall (plasmolysis). Incipient plasmolysis is when pressure potential has just reached zero. Animal cells burst (lysis) in water and shrink (crenation) in strong solutions.
- Passive transport Lipid-soluble substances and water cross the bilayer by diffusion. Water-soluble and charged substances use channel proteins (facilitated diffusion) or carrier proteins, down a concentration gradient with no ATP.
- Active transport Carrier proteins move substances against a concentration gradient using ATP from respiration. Rates fall if respiration is inhibited.
- Bulk transport Endocytosis takes in material in vesicles: phagocytosis for solids and pinocytosis for liquids. Exocytosis releases material, for example secretion of enzymes, when vesicles fuse with the membrane.
- Potato practical Find percentage change in mass of potato cylinders in a range of sucrose solutions, plot against concentration, and read off where the line crosses zero. That solution has the same water potential as the tissue.
- Onion practical Count plasmolysed cells in onion epidermis in a range of solutions. At 50 percent plasmolysis, pressure potential is taken as zero, so the solute potential of the cells equals the water potential of that solution.
Common mistakes in 1.5 Cell Physiology
- Saying water moves from a high to a low concentration. Use water potential: water moves from higher to lower water potential.
- Thinking more negative means higher water potential. -800 kPa is lower than -300 kPa.
- Using actual mass change instead of percentage change in the potato practical. Percentage change allows for different starting masses.
- Saying plant cells burst in pure water. The cellulose wall resists expansion, so pressure potential rises and the cell becomes turgid.
- Calling facilitated diffusion active because it uses proteins. It is passive and uses no ATP.
Try three 1.5 Cell Physiology exam-style questions
1. A plant cell has a solute potential of -1200 kPa and a pressure potential of 450 kPa. Calculate its water potential. (1 mark)
Show the mark scheme answer
-1200 + 450 = -750 kPa.
2. The cell above is placed in a solution with a water potential of -900 kPa. Explain what happens. (3 marks)
Show the mark scheme answer
Water potential of solution (-900 kPa) is lower than that of the cell (-750 kPa); water leaves the cell by osmosis; across the partially permeable membrane; pressure potential decreases and the cell may become flaccid or plasmolysed.
3. Explain why onion cells are considered to be at incipient plasmolysis when 50% of cells are plasmolysed. (2 marks)
Show the mark scheme answer
Cells vary in solute potential; at 50% plasmolysis the average cell has just lost turgor so pressure potential is zero; therefore solute potential equals the water potential of the external solution.
1.5 Cell Physiology FAQs
What does CCEA A Level Biology 1.5 Cell Physiology cover?
It covers diffusion, osmosis and water potential, plasmolysis, facilitated diffusion, active transport, endocytosis and exocytosis, plus the potato and onion practicals.
Which unit is 1.5 Cell Physiology examined in?
It is examined in Unit AS 1: Molecules and Cells, a 1 hour 30 minute written paper of structured questions and an essay. Related practical skills can also be tested in Unit AS 3.
What formula do I need for 1.5 Cell Physiology?
Water potential of a cell = solute potential + pressure potential. Remember all values are in kPa, solute potential is negative and pressure potential is usually positive.
