Cell membranes and transport

• Heads out, tails in, sugar outside only. Two questions sort every transport process: which way is the gradient, and is ATP used? Water always moves from higher to lower water potential.
• Heads out, tails in, sugar outside only. Down or against, ATP or not. Higher to lower water potential; the wall makes the difference.

• 4.1.1 I can describe — describe the fluid mosaic model of membrane structure with reference to the hydrophobic and hydrophilic interactions that account for the formation of the phospholipid bilayer and the arrangement of proteins
• 4.1.2 I can describe — describe the arrangement of cholesterol, glycolipids and glycoproteins in cell surface membranes
• 4.1.3 I can describe — describe the roles of phospholipids, cholesterol, glycolipids, proteins and glycoproteins in cell surface membranes, with reference to stability, fluidity, permeability, transport (carrier proteins and channel proteins), cell signalling (cell surface receptors) and cell recognition (cell surface antigens – see 11.1.2)
• 4.1.4 I can outline — outline the main stages in the process of cell signalling leading to specific responses: • secretion of specific chemicals (ligands) from cells • transport of ligands to target cells • binding of ligands to cell surface receptors on target cells
• 4.2.1 I can describe — describe and explain the processes of simple diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis
• 4.2.2 I can investigate — investigate simple diffusion and osmosis using plant tissue and non-living materials, including dialysis (Visking) tubing and agar
• 4.2.3 I can illustrate — illustrate the principle that surface area to volume ratios decrease with increasing size by calculating surface areas and volumes of simple 3-D shapes (as shown in the Mathematical requirements)
• 4.2.4 I can investigate — investigate the effect of changing surface area to volume ratio on diffusion using agar blocks of different sizes
• 4.2.5 I can investigate — investigate the effects of immersing plant tissues in solutions of different water potentials, using the results to estimate the water potential of the tissues
• 4.2.6 I can explain — explain the movement of water between cells and solutions in terms of water potential and explain the different effects of the movement of water on plant cells and animal cells (knowledge of solute potential and pressure potential is not expected)

Precise vocabulary is part of the biology. Water moves down a water potential gradient; an active site is complementary to its substrate; enzymes are denatured, not killed; ATP releases energy when it is hydrolysed, and respiration never produces energy. Give a calculated answer to the same number of significant figures as the least precise data, or one more, with its unit. A fifth of the qualification is experimental: Papers 3 and 5 test AO3 only, and their questions may be set in contexts outside the syllabus content, so the practical work in this chapter is set out as variables, method, recording, graphs and evaluation rather than as theory.

• “Water moves from a high water concentration to a low water concentration.” Correct Use water potential, never “water concentration”. Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.
• “−800 kPa is a higher water potential than −300 kPa, because 800 is bigger.” Correct Water potentials are negative. −300 kPa is higher (less negative). Pure water, 0 kPa, is the highest of all.
• “Facilitated diffusion uses energy because it uses proteins.” Correct It is passive: down the concentration gradient, no ATP. The protein only provides a route through the hydrophobic core.
• “Glycoproteins and glycolipids have carbohydrate on both sides of the membrane.” Correct The carbohydrate chains are on the outer (extracellular) face only.
• “The ligand fits the receptor because it is the same shape.” Correct The receptor’s binding site is complementary in shape to the ligand, so the receptor is specific.
• “A plant cell bursts in pure water, like a red blood cell.” Correct The cell wall resists expansion, so the plant cell becomes turgid. Only a cell with no wall bursts.
• “The acid diffused more slowly into the bigger agar block.” Correct It reached the same depth in every block. What differs is the percentage of the volume reached, which is smaller in a block with a smaller SA:V.
• “The phospholipid tails face the water.” Repair The hydrophilic heads face the water on both sides; the hydrophobic tails point inwards, towards each other, held by hydrophobic interactions.
• “The membrane is fluid because it is made of liquid.” Repair Fluid means the phospholipids and many proteins move sideways within their layer.
• “Glycoproteins are found on both sides of the membrane.” Repair Their carbohydrate chains are on the outer face only, like those of glycolipids.
• “Cholesterol makes the membrane more fluid.” Repair It regulates fluidity — less fluid when warm, still fluid when cold — and reduces permeability to ions and polar molecules.
• “The ligand enters the target cell and causes the response.” Repair It binds a complementary cell surface receptor; binding triggers events inside the cell that give the response.
• “Facilitated diffusion uses energy because it uses proteins.” Repair It is passive — down the gradient, no ATP. Only active transport, endocytosis and exocytosis use ATP.
• “Active transport speeds up diffusion.” Repair It moves particles against their concentration gradient, using ATP and carrier proteins. It is not a faster form of diffusion.
• “Diffusion stops when the concentrations are equal.” Repair The particles keep moving randomly in both directions; the net movement stops.
• “Water moves from a high concentration of water to a low concentration of water.” Repair From a higher water potential to a lower water potential, through a partially permeable membrane.
• “−800 kPa is a higher water potential than −300 kPa.” Repair −300 kPa is higher (less negative); pure water, 0 kPa, is the highest.
• “Plant cells burst in pure water.” Repair The wall resists expansion, so the cell becomes turgid. Animal cells, with no wall, burst.
• “In plasmolysis the cell wall shrinks.” Repair The protoplast shrinks and the membrane pulls away from the wall, which keeps its shape; outside solution fills the gap.
• “Larger cubes have a larger SA:V because they have more surface.” Repair SA grows with l2 but V with l3, so SA:V falls as size rises.
• “The acid diffused more slowly into the big agar block.” Repair It reached the same depth; a smaller proportion of the big block’s volume was reached.

• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. State, give and identify want a fact and nothing more. Define wants a precise meaning. Outline wants the main points only; describe wants the points or the features in full — and when you describe a graph, the trend with figures quoted from it. Explain wants the reasons and the mechanism — a describe-level answer to an explain question is incomplete however well written it is. Compare wants similarities and differences, each stated for both things side by side; contrast wants differences only. Discuss wants the issue written about in depth, in a structured way; assess wants an informed judgement. Suggest asks you to apply what you know to a situation where there is a range of valid responses, making proposals or putting forward considerations, so any sound biological reasoning is creditable.
• Cholesterol regulates fluidity. “Cholesterol makes the membrane more fluid” and “cholesterol makes the membrane rigid” are each half the truth. Write that it regulates fluidity, and say how at high and at low temperature.
• Interleave with the chapters that use this one. Chapter 7 (transport in plants) uses water potential and active loading: re-answer “which way does water move between a root hair cell and soil water?”. Chapter 9 (gas exchange) uses diffusion and SA:V: re-answer “why does a large organism need an exchange surface?”. Chapter 11 (immunity) uses cell surface antigens. At A Level, chapter 14 continues cell signalling inside the target cell, and chapter 15 uses channel and carrier proteins in neurones. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Biology 9700 has five components. Topic 4 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E. There is no data booklet in Biology. At A Level, the statistical formulae (Hardy–Weinberg, the Lincoln index, Simpson’s index, standard deviation, standard error, 95% confidence intervals, the χ² test, the t-test, and Pearson’s and Spearman’s correlation) are printed in a question when it needs them, and so are the tables of critical values; degrees of freedom you must work out yourself. Everything else — magnification, surface area to volume, RQ, Rf, rates — you must recall, and this chapter says which is which.
• A Paper 1 item on this topic can turn on a single word: facilitated diffusion that “uses ATP”, water moving to the higher water potential, carbohydrate chains on both faces, SA:V that “rises” with size. Paper 2 asks you to describe the fluid mosaic model, explain how the bilayer forms, outline the three stages of cell signalling, define diffusion, osmosis and active transport, and explain what happens to plant and animal cells in a solution.
• Expect a membrane diagram to label; a graph of rate of uptake against concentration to sort into simple diffusion, facilitated diffusion or active transport; a table of water potentials to turn into arrows; drawings or photomicrographs of turgid and plasmolysed cells; and surface areas, volumes and SA:V of cubes, cuboids and cylinders, calculated from formulae you must recall.
• The topic supplies Paper 3 investigations directly. Independent variable: concentration of the outside solution (at least five values, by proportional dilution) or block size. Dependent variable: percentage change in mass, or depth or time of the colour change. Standardise the tissue source, size, volume, time and temperature, and the blotting. Main random error: blotting; main systematic error: evaporation. Read the tissue’s water potential where the mass change is zero.
• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. State, give and identify want a fact and nothing more. Define wants a precise meaning. Outline wants the main points only; describe wants the points or the features in full — and when you describe a graph, the trend with figures quoted from it. Explain wants the reasons and the mechanism — a describe-level answer to an explain question is incomplete however well written it is. Compare wants similarities and differences, each stated for both things side by side; contrast wants differences only. Discuss wants the issue written about in depth, in a structured way; assess wants an informed judgement. Suggest asks you to apply what you know to a situation where there is a range of valid responses, making proposals or putting forward considerations, so any sound biological reasoning is creditable.