Human Nutrition

The One Thing

Human nutrition depends on obtaining a balanced diet, digesting food into small soluble molecules, absorbing these molecules mainly through the small intestine, and assimilating them for energy, growth, repair and storage.

Reactions, Processes and Experiments

Key Terms

• Balanced diet: A diet containing the correct amounts and proportions of carbohydrates, proteins, fats, vitamins, minerals, fibre and water for a person's needs.
• Carbohydrate: A nutrient used mainly as an energy source; it is digested into simple sugars such as glucose.
• Protein: A nutrient needed for growth and repair; it is digested into amino acids.
• Lipid: A nutrient used for energy storage, insulation and forming cell membranes; it is digested into fatty acids and glycerol.
• Vitamin: An organic substance needed in small amounts to maintain health; vitamins do not usually provide energy.
• Mineral ion: An inorganic nutrient needed in small amounts for specific body functions, such as calcium for bones and iron for haemoglobin.
• Fibre: Indigestible plant material that adds bulk to food and helps move it through the alimentary canal.
• Digestion: The breakdown of large, insoluble food molecules into small, soluble molecules that can be absorbed.
• Mechanical digestion: The physical breakdown of food into smaller pieces, such as chewing and stomach churning, without changing the molecules chemically.
• Chemical digestion: The breakdown of food molecules by enzymes through chemical reactions.
• Enzyme: A protein that acts as a biological catalyst, increasing the rate of a reaction without being used up.
• Amylase: An enzyme that digests starch into maltose.
• Protease: An enzyme that digests proteins into amino acids.
• Lipase: An enzyme that digests lipids into fatty acids and glycerol.
• Bile: An alkaline fluid made by the liver and stored in the gall bladder; it neutralises stomach acid and emulsifies fats.
• Emulsification: The division of large fat droplets into smaller droplets, increasing the surface area for lipase action.
• Peristalsis: Wave-like muscular contractions that push food along the alimentary canal.
• Absorption: The movement of digested, soluble nutrients through the wall of the small intestine into the blood or lymph.
• Villus: A finger-like projection in the small intestine that increases surface area for absorption.
• Assimilation: The uptake and use of absorbed nutrients by cells, for example using glucose in respiration or amino acids to make proteins.
• Deamination: The removal of the nitrogen-containing part of excess amino acids in the liver, producing urea and a carbohydrate part that may be respired.
• Deficiency disease: An illness caused by not obtaining enough of a particular nutrient over time.

Easily Confused

• Digestion vs absorption: Digestion breaks large, insoluble molecules into small, soluble molecules; absorption transfers these digested molecules through the small-intestine wall into the blood or lymph.
• Absorption vs assimilation: Absorption moves nutrients into body fluids; assimilation is their uptake and use by cells.
• Mechanical vs chemical digestion: Mechanical digestion changes the size of food without changing its molecules; chemical digestion uses enzymes to change food molecules.
• Bile vs lipase: Bile emulsifies fats and is not an enzyme; lipase chemically digests lipids into fatty acids and glycerol.
• Amylase vs protease: Amylase digests starch into maltose; protease digests proteins into amino acids.
• Protease vs lipase: Protease acts on proteins; lipase acts on lipids.
• Blood capillaries vs lacteals: Glucose and amino acids enter blood capillaries; fatty acids and glycerol are transported mainly through lacteals in the lymphatic system.
• Diffusion and facilitated diffusion vs active transport: Diffusion and facilitated diffusion move substances down a concentration gradient; active transport uses energy to move substances against a concentration gradient.
• Villi vs microvilli: Both increase absorptive surface area, but villi are finger-like projections of the small intestine and microvilli provide additional surface area at the epithelial-cell surface.
• Deficiency vs excess: Deficiency results from insufficient nutrient intake, whereas excess nutrients can also cause harm, such as coronary heart disease, tooth decay or obesity.

What Gets Asked

• Define or explain the sequence of nutrition: questions may require balanced diet, digestion, absorption and assimilation to be distinguished; marks are lost by treating absorption as the breakdown of food.
• Complete digestion equations: starch must be linked to maltose, maltose to glucose, protein to amino acids, and lipid to fatty acids and glycerol; do not state that bile digests lipids.
• Explain small-intestine adaptations: include villi and microvilli for large surface area, villus walls one cell thick for a short diffusion distance, and a good blood supply for maintaining concentration gradients.
• Compare nutrient transport routes: glucose and amino acids enter blood capillaries, whereas fatty acids and glycerol enter epithelial cells and are transported mainly through lacteals; assigning all nutrients directly to blood loses the distinction.
• Explain active transport: it requires energy from respiration and can move substances against a concentration gradient; describing it as ordinary diffusion is incorrect.
• Identify deficiency diseases and their effects: vitamin C deficiency causes scurvy, vitamin D deficiency causes rickets in children, and iron deficiency causes anaemia; the specific nutrient and consequence must be matched correctly.