Nutrition for O-Level Biology (SEAB 6093): photosynthesis and leaf structure in plants, and the human digestive system, digestion, absorption and assimilation

What this module covers

Nutrition is about how organisms obtain and use food, and in O-Level Biology (SEAB 6093) it has two halves: how green plants make food by photosynthesis, and how humans break food down and use it. You need the word equation and conditions for photosynthesis, how the leaf is built to carry it out, the path of food through the gut with the difference between mechanical and chemical digestion, and how the small intestine absorbs digested food that the body then assimilates.

This overview links every dot point in the module; work through them, then test yourself at the end. See the full set at /sg-o-level/biology/syllabus.

Photosynthesis: making food from light

Plants are producers, so start there. The page on photosynthesis in plants gives the word equation, the raw materials and conditions, the role of chlorophyll, and how light, carbon dioxide and temperature act as limiting factors. Learn the equation with its conditions and be ready to read a rate graph and identify which factor is limiting.

The leaf: built for photosynthesis

Structure follows function. The page on leaf structure and adaptations relates the tissues of the leaf, from the upper epidermis to the lower epidermis, to their roles, including the palisade layer packed with chloroplasts, the air spaces of the spongy mesophyll, and the stomata and guard cells that control gas exchange and water loss. Every feature is an adaptation you can be asked to explain.

The human digestive system

Now turn to the consumer. The page on the human digestive system describes the path of food through the gut and the difference between mechanical and chemical digestion, with the main digestive enzymes and what they break down. Learn each enzyme with its substrate and product as a set: amylase breaks starch to maltose, protease breaks protein to amino acids, and lipase breaks fats to fatty acids and glycerol.

Absorption and assimilation

Finally, the products must be taken up and used. The page on absorption and assimilation explains how the small intestine is adapted for absorption with villi, what each digested product is used for, and the difference between absorption (into the blood) and assimilation (use by cells). This links back to the surface area to volume ratio idea and forward to how the blood transports nutrients.

How this module is examined

• Equation with conditions. The photosynthesis equation only scores full marks with the conditions (light and chlorophyll) shown above the arrow.
• Adaptation with reason. Leaf and villus questions reward linking each structural feature to the function it serves.
• Enzyme, substrate, product. Digestion questions reward stating the enzyme, what it acts on, and what it produces, all three together.
• Absorption versus assimilation. Define each term separately: absorption is into the blood, assimilation is use by the cells.

Check your knowledge

A mix of recall, application and definition questions. Attempt them, then check against the solutions.

1. Write the word equation for photosynthesis, including the conditions. (3 marks)
2. State two ways the leaf is adapted for photosynthesis and explain each. (2 marks)
3. Name the enzyme that digests protein and state the product. (2 marks)
4. Explain the difference between absorption and assimilation. (2 marks)