Singapore-Cambridge N(A)-Level Combined Science, Biology: Cells and Human Physiology, from cell structure and the movement of substances to digestion and transport in humans

What this module is about

The Cells and Human Physiology module is the foundation of N(A)-Level Combined Science Biology (SEAB 5106 and 5107). It builds from the smallest unit of life, the cell, up to the organ systems that keep a whole human supplied with food and energy. The idea that ties it together is that structure is matched to function: a thin capillary wall speeds exchange, a long folded intestine speeds absorption, and a biconcave red blood cell holds plenty of haemoglobin. Learn that idea once and it explains most of the marks in this module.

This overview pulls the threads together and links to every dot point page in the module, each with its own worked answers and practice questions.

The cell and how it is organised

Everything starts with cell structure and organisation. You need to identify the parts of animal and plant cells and state what each one does: the nucleus controlling the cell and holding the genetic material, the cytoplasm where reactions happen, the cell membrane controlling what enters and leaves, and the mitochondria where respiration releases energy. Plant cells add a cellulose cell wall for support, a large permanent vacuole, and chloroplasts for photosynthesis. From the single cell, life is organised into a hierarchy: cells form tissues, tissues form organs, and organs form organ systems.

How substances enter and leave cells

A cell is only useful if it can take in what it needs and remove waste, which is covered in movement of substances. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a gradient, and needs no energy. Osmosis is the same idea applied to water: the net movement of water molecules across a partially permeable membrane from a dilute solution to a concentrated one. Knowing the effect of osmosis on cells matters: a plant cell in pure water gains water and becomes turgid, while a cell in a concentrated solution loses water and becomes flaccid.

Digestion: breaking food into absorbable molecules

Large food molecules cannot cross cell membranes, so they must be broken down, which is the job of the human digestive system. Digestion uses enzymes to break large molecules into small, soluble ones: starch is broken into glucose, proteins into amino acids, and fats into fatty acids and glycerol. The small soluble products are then absorbed across the wall of the small intestine, whose villi give the large surface area, thin wall and rich blood supply needed for fast absorption.

Transport: carrying food and oxygen

Once absorbed, materials must reach every cell, which is the role of the system described in transport and the circulatory system. The heart pumps blood through three types of vessel: arteries carry blood away from the heart at high pressure and have thick muscular walls; veins return blood at low pressure and have thinner walls, wider lumens and valves; capillaries are one cell thick to allow exchange. The blood itself carries red cells (oxygen, via haemoglobin), white cells (defence), platelets (clotting) and plasma (the liquid that carries dissolved food and waste).

How this module is examined

• Link structure to function. Almost every "explain how X is adapted for Y" question wants surface area, a thin wall, a short diffusion distance, or a maintained concentration gradient. Use the same checklist for villi, capillaries and red blood cells.
• Be precise with diffusion and osmosis. Both are passive and need no energy from respiration; osmosis is the special case for water crossing a partially permeable membrane. Stating which way the particles move is the key mark.
• Know your enzymes and products. Carbohydrase gives simple sugars, protease gives amino acids, and lipase gives fatty acids and glycerol.

Check your knowledge

A mix of recall and application questions covering the module. Attempt them under timed conditions, then check against the solutions, and use the dot point pages for fuller practice.

1. State two structures found in a plant cell but not an animal cell, and give the function of each. (2 marks)
2. Define diffusion. (1 mark)
3. Name the three classes of digestive enzyme and the small molecule each one produces. (3 marks)
4. Give two structural differences between an artery and a vein, and explain each in terms of its function. (2 marks)
5. A potato cylinder loses mass in a concentrated salt solution. Explain why, using osmosis. (2 marks)
6. Explain why a capillary is well adapted for the exchange of substances. (2 marks)