Cell membranes and transport — Eduqas A-Level Biology

Cell membranes and transport is a fundamental topic in A-Level Biology, focusing on the structure and function of the plasma membrane. The fluid mosaic model describes the membrane as a dynamic bilayer of phospholipids with embedded proteins, cholesterol, and glycolipids/glycoproteins. This structure allows the membrane to be selectively permeable, controlling the movement of substances into and out of cells. Understanding this topic is crucial for grasping how cells maintain homeostasis, communicate with each other, and respond to their environment.

The topic covers various transport mechanisms: passive processes like diffusion (simple and facilitated) and osmosis, and active processes like active transport, endocytosis, and exocytosis. Each mechanism is essential for different physiological roles, such as nutrient uptake, waste removal, and signal transduction. For example, the sodium-potassium pump is a classic example of active transport that maintains electrochemical gradients in nerve cells. Mastery of these concepts is vital for understanding more complex topics like nerve impulses, muscle contraction, and immune responses.

In the WJEC A-Level specification, this topic appears in Unit 1 (Basic Biochemistry and Cell Organisation) and is assessed through multiple-choice, short-answer, and extended-response questions. Students must be able to explain how the structure of the membrane relates to its function, compare and contrast transport mechanisms, and interpret experimental data (e.g., from osmosis experiments). A strong grasp of cell membranes and transport also underpins topics like respiration, photosynthesis, and homeostasis, making it a cornerstone of the biology curriculum.

• →Fluid mosaic model: phospholipid bilayer with embedded proteins (integral and peripheral), cholesterol (for stability and fluidity), and glycoproteins/glycolipids (for cell recognition and signalling).
• →Selective permeability: the membrane allows some substances to pass through freely (e.g., small non-polar molecules like oxygen) while restricting others (e.g., ions and large polar molecules).
• →Diffusion: passive movement of molecules down a concentration gradient; simple diffusion through the bilayer, facilitated diffusion via channel or carrier proteins.
• →Osmosis: the net movement of water across a partially permeable membrane from a region of higher water potential to lower water potential.
• →Active transport: movement of substances against a concentration gradient using ATP and carrier proteins (e.g., sodium-potassium pump).

• Components of the plasma membrane and the fluid-mosaic model
• Factors affecting membrane permeability
• Mechanisms of diffusion and factors affecting its rate
• Osmosis and the concept of water potential
• Facilitated diffusion, pinocytosis, and phagocytosis
• Exocytosis (secretion)
• Active transport and the influence of cyanide

• 💡Use the term 'water potential' consistently when discussing osmosis; avoid 'concentration of water'
• 💡Be prepared to interpret data from colorimeter experiments regarding membrane permeability
• 💡Understand the relationship between surface area, volume, and transport rates
• 💡Practice calculating rates of change from graphs showing linear relationships
• 💡When describing the fluid mosaic model, always mention both 'fluid' (phospholipids move laterally) and 'mosaic' (proteins scattered throughout). Use specific examples like cholesterol in animal cells to show depth.
• 💡For osmosis questions, always define water potential and use the correct terminology: 'water moves from high water potential to low water potential'. Avoid saying 'water moves from dilute to concentrated' as this is less precise.
• 💡In experiments (e.g., potato strips in sucrose solutions), calculate percentage change in mass to account for initial differences. Explain results in terms of water potential gradients and the effect on turgor pressure.

• Confusing water potential with solute potential
• Incorrectly describing the movement of water in terms of concentration rather than water potential
• Failing to link active transport to energy requirements (ATP) and the effect of metabolic inhibitors like cyanide
• Misinterpreting the fluid-mosaic model structure
• Misconception: Osmosis is the movement of water from low to high solute concentration. Correction: Osmosis is the movement of water from a region of higher water potential (lower solute concentration) to lower water potential (higher solute concentration).
• Misconception: Facilitated diffusion requires ATP. Correction: Facilitated diffusion is a passive process; it does not require energy. The movement is still down a concentration gradient, but via protein channels or carriers.
• Misconception: All proteins in the membrane are fixed in place. Correction: Many membrane proteins are mobile and can move laterally within the bilayer, as described by the fluid mosaic model.