How do organisms obtain and process their food?
Modes of nutrition, the human digestive system and digestion of food, absorption, and adaptations for different diets.
A focused answer to WJEC A-Level Biology Unit 2, covering autotrophic and heterotrophic nutrition, the human gut and digestive enzymes, absorption at the ileum, and the dental and gut adaptations of herbivores and carnivores.
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What this dot point is asking
WJEC wants you to distinguish modes of nutrition, describe the human digestive system and the enzymes that digest carbohydrates, proteins and lipids, explain absorption at the ileum, and describe how guts and teeth adapt to different diets.
Modes of nutrition
Digestion in humans
Food is broken down both physically (chewing, churning, emulsification) and chemically (enzymes) as it passes along the gut. Carbohydrases start with salivary and pancreatic amylase, which hydrolyse starch to maltose; membrane-bound maltase then completes the job to glucose. Proteases include pepsin in the acidic stomach and trypsin from the pancreas, with peptidases finishing the breakdown to amino acids. Lipase from the pancreas digests lipids to fatty acids and glycerol, greatly helped by bile salts that emulsify large fat globules into many tiny droplets, increasing the surface area for lipase to act on.
Absorption at the ileum
Co-transport works because the sodium-potassium pump keeps sodium low inside the cell; sodium then diffuses back in through a co-transporter, dragging glucose with it against its own gradient. This indirectly uses ATP, linking absorption to active transport.
Adaptations for different diets
Carnivores have sharp canines and shearing carnassial teeth for catching prey and cutting meat, plus a relatively short gut because meat is protein-rich and quick to digest. Herbivores have ridged grinding molars, often a diastema, a sideways-grinding jaw, a long gut and an enlarged caecum or rumen housing microbes that secrete cellulase to digest cellulose, which mammals cannot break down themselves.
Examples in context
Example 1. Ruminant digestion in cattle. A cow's four-chambered stomach holds billions of microbes in the rumen that ferment cellulose into fatty acids the cow can absorb, and produce methane as a by-product. The cow regurgitates and re-chews the cud to expose more surface area. This shows how a herbivore outsources cellulose digestion to microbes, a classic WJEC nutrition case.
Example 2. Oral rehydration therapy. Sachets given for severe diarrhoea contain glucose and salt together precisely because glucose absorption is co-transported with sodium. Adding glucose drives sodium (and therefore water) uptake across the gut wall, rehydrating the patient. This direct application of the co-transport mechanism has saved millions of lives.
Try this
Q1. State the products of lipid digestion. [1 mark]
- Cue. Fatty acids and glycerol.
Q2. Explain the role of bile salts in the digestion of lipids. [2 marks]
- Cue. They emulsify fats into small droplets, increasing the surface area so lipase digests them faster.
Q3. Explain why glucose absorption in the ileum stops if the sodium-potassium pump is inhibited. [3 marks]
- Cue. The pump keeps sodium low in the cell; this gradient drives co-transport of glucose; without the pump the gradient collapses, so glucose can no longer be dragged in against its gradient.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC 20184 marksExplain how the structure of the ileum is adapted for the efficient absorption of digested food.Show worked answer →
The ileum wall is folded into villi and each epithelial cell has microvilli, giving a very large surface area for absorption.
The epithelium is one cell thick, giving a short diffusion distance into the blood.
Each villus has a dense capillary network (and a lacteal) that carries absorbed products away, maintaining a concentration gradient, while co-transport of glucose and amino acids with sodium ions actively takes them up.
Markers reward villi and microvilli for surface area, thin epithelium, good blood supply, and co-transport.
WJEC 20215 marksCompare the teeth and gut of a herbivore and a carnivore, and explain how each is adapted to its diet.Show worked answer →
A carnivore has sharp pointed canines and scissor-like carnassial teeth for gripping prey and shearing meat, with a relatively short gut because protein-rich meat is quickly and easily digested.
A herbivore has ridged grinding molars and premolars and often a diastema (gap) that lets the tongue manipulate grass, with the lower jaw moving sideways to grind tough plant material.
A herbivore has a much longer gut and an enlarged caecum or rumen housing symbiotic microbes that secrete cellulase to digest cellulose, which mammals cannot break down themselves; the longer gut gives more time for this slow digestion and absorption.
Markers reward correct teeth differences, the longer herbivore gut, and microbial cellulose digestion.
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Sources & how we know this
- WJEC A-level Biology specification — WJEC (2015)