Singapore-Cambridge GCE O-Level Combined Science, Biology: Cells and Human Physiology, from cell structure and the movement of substances to digestion, transport and respiration in humans

What this module is about

The Cells and Human Physiology module is the foundation of O-Level Combined Science Biology (SEAB 5077 and 5078). It builds from the smallest unit of life, the cell, up to the organ systems that keep a whole human supplied with food, oxygen and energy. The thread that ties it together is the idea that structure is matched to function: a thin alveolus wall speeds gas exchange, a long folded intestine speeds absorption, and a muscular ventricle wall pumps blood hard. Master 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 what each one does: the nucleus controlling activities, the cytoplasm where reactions happen, the cell membrane controlling what enters and leaves, and the mitochondria where aerobic respiration releases energy. Plant cells add a cellulose cell wall for support, a large vacuole, and chloroplasts for photosynthesis. From the single cell, life is organised into a hierarchy: cells form tissues, tissues form organs, organs form systems, and systems form the organism.

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. There are three processes to compare. Diffusion is the net movement of particles from high to low concentration, down a gradient, and needs no energy. Osmosis is the same idea applied to water moving across a partially permeable membrane from a dilute to a concentrated solution. Active transport moves particles against their concentration gradient and so requires energy from respiration, for example when root cells take up mineral ions or when the gut absorbs glucose that is already at a low concentration in the blood.

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. Physical digestion (teeth and churning) increases surface area, and chemical digestion uses enzymes to break the bonds. The three classes to know are carbohydrases (such as amylase) that break carbohydrates to simple sugars, proteases that break proteins to amino acids, and lipases that break fats to 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, oxygen and waste

Once absorbed, materials must reach every cell, which is the role of the system described in transport in humans. The heart is a double pump driving a double circulation: the right side sends deoxygenated blood to the lungs, the left side sends oxygenated blood to the body. Arteries carry blood away from the heart at high pressure and have thick muscular elastic 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 transport fluid for dissolved food, carbon dioxide and waste).

Respiration: releasing the energy

The whole point of supplying glucose and oxygen is to release energy, the subject of respiration in humans. Aerobic respiration breaks glucose down fully using oxygen: glucose + oxygen gives carbon dioxide + water, releasing a large amount of energy. When oxygen is short, as in muscle during hard exercise, anaerobic respiration takes over, partly breaking glucose to lactic acid and releasing far less energy, which builds an oxygen debt. Breathing and gas exchange supply the oxygen and remove the carbon dioxide, and the alveoli are adapted for it with a large surface area, a moist, thin wall one cell thick, and a dense capillary network that keeps the gradient steep.

How this module is examined

• Link structure to function. Almost every "explain how X is adapted for Y" question wants surface area, thin wall, short diffusion distance, or a maintained concentration gradient. Use the same checklist for alveoli, villi and capillaries.
• Be precise with the three transport processes. Diffusion and osmosis are passive and need no energy; active transport goes against the gradient and needs energy from respiration. Stating which way the particles move is the key mark.
• Get the equations and comparisons right. Know the word equation for aerobic respiration, and be able to contrast aerobic and anaerobic respiration on energy released, products and the need for oxygen.

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 active transport and state one example in the human body. (2 marks)
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. Write the word equation for aerobic respiration. (1 mark)
6. Explain why an alveolus is well adapted for gas exchange. (3 marks)