How do organisms obtain and digest their food, and how is the gut adapted for absorption?
Adaptations for nutrition: autotrophic and heterotrophic nutrition; the human digestive system; the digestion of carbohydrates, proteins and lipids; and adaptations of the small intestine for absorption.
A focused answer to the Eduqas Component 3 statement on nutrition. Covers autotrophic and heterotrophic nutrition, the human digestive system, the enzymes that digest carbohydrates, proteins and lipids, and the adaptations of the small intestine for absorption.
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What this dot point is asking
Eduqas wants you to distinguish autotrophic and heterotrophic nutrition, describe the human digestive system, explain the digestion of carbohydrates, proteins and lipids, and explain how the small intestine is adapted for absorption. This completes the exchange-and-transport core of Component 3.
Autotrophic and heterotrophic nutrition
The human digestive system and digestion
Food passes through the mouth, stomach, small intestine (duodenum and ileum) and large intestine, digested by enzymes secreted along the way (from the salivary glands, stomach, pancreas and the gut lining). Digestion is hydrolysis of large insoluble molecules into small soluble ones.
- Carbohydrates: amylase (saliva and pancreas) hydrolyses starch to maltose; membrane-bound disaccharidases (maltase, sucrase, lactase) on the epithelial cells finish the job to monosaccharides.
- Proteins: endopeptidases (pepsin in the acidic stomach, trypsin from the pancreas) break internal peptide bonds into shorter chains; exopeptidases remove terminal amino acids; dipeptidases on the epithelial membranes hydrolyse dipeptides to amino acids.
- Lipids: bile (from the liver) emulsifies fats into droplets (increasing surface area), then lipase (pancreas) hydrolyses them to fatty acids and glycerol.
Adaptations of the small intestine for absorption
Glucose and amino acids are absorbed by co-transport (with sodium ions) into the epithelial cells and then into the blood; fatty acids and glycerol are absorbed and reform triglycerides, entering the lacteal (lymph).
Examples in context
Example 1. Lactose intolerance. People lacking the membrane disaccharidase lactase cannot digest lactose, so it passes undigested into the large intestine where bacteria ferment it, causing symptoms, a direct application of membrane-bound carbohydrate digestion.
Example 2. Why emulsification speeds fat digestion. Bile salts break large fat globules into many small droplets, vastly increasing the surface area exposed to lipase, so digestion is faster, a classic surface-area argument.
Try this
Q1. Distinguish between autotrophic and heterotrophic nutrition. [2 marks]
- Cue. Autotrophs make organic molecules from inorganic ones using an energy source (light or chemical); heterotrophs obtain organic molecules by consuming other organisms.
Q2. Name the enzyme that emulsifies fats and state whether it is an enzyme. [1 mark]
- Cue. Trick: bile (a secretion, not an enzyme) emulsifies fats; lipase then digests them.
Q3. State two ways the small intestine is adapted to increase its surface area for absorption. [2 marks]
- Cue. Villi (folds of the lining) and microvilli (folds of the epithelial cell membrane).
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20195 marksDescribe how a protein in food is digested into amino acids in the human gut, naming the enzymes and where they act.Show worked answer →
In the stomach, endopeptidases (such as pepsin, which works in acidic conditions) hydrolyse peptide bonds within the protein chain, breaking it into shorter polypeptides.
In the small intestine, more endopeptidases (from the pancreas, such as trypsin) continue to break polypeptides into shorter chains.
Exopeptidases then remove amino acids from the ends of the chains.
Dipeptidases, located in the membranes of the epithelial cells lining the small intestine, hydrolyse dipeptides into single amino acids.
Markers reward endopeptidases breaking internal bonds, exopeptidases removing terminal amino acids, dipeptidases on the epithelial membranes finishing the job, and producing amino acids.
Eduqas 20214 marksExplain how the structure of the small intestine is adapted for the efficient absorption of digested food.Show worked answer →
The inner surface is folded and covered in villi, and the epithelial cells have microvilli, which together greatly increase the surface area for absorption.
The epithelium is only one cell thick, giving a short diffusion distance.
Each villus has a dense network of capillaries (and a lacteal) to carry absorbed products away, maintaining a concentration gradient.
The epithelial cells have many mitochondria to provide ATP for active transport (for example co-transport of glucose with sodium ions).
Markers reward villi and microvilli increasing surface area, the thin epithelium for a short diffusion distance, a good blood supply maintaining the gradient, and mitochondria for active transport.
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Sources & how we know this
- Eduqas A Level Biology Specification (A400) — Eduqas (2015)