(e) Nutrition
- Syllabus
- 2024
- Topic
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- Level
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Learning objectives
2.18Photosynthesis and energy conversionUnderstand the process of photosynthesis and its importance in the conversion of light energy to chemical energy.2.19Photosynthesis equationsKnow the word equation and the balanced chemical symbol equation for photosynthesis.2.20Factors affecting photosynthesisUnderstand how varying carbon dioxide concentration, light intensity and temperature affect the rate of photosynthesis.2.21Leaf adaptations for photosynthesisDescribe the structure of the leaf and explain how it is adapted for photosynthesis.2.22Mineral ions for plant growthUnderstand that plants require mineral ions for growth, and that magnesium ions are needed for chlorophyll and nitrate ions are needed for amino acids.2.23Photosynthesis practicalPractical: investigate photosynthesis, showing the evolution of oxygen from a water plant, the production of starch and the requirements of light, carbon dioxide and chlorophyll.2.24Balanced dietUnderstand that a balanced diet should include appropriate proportions of carbohydrate, protein, lipid, vitamins, minerals, water and dietary fibre.2.25Dietary sources and functionsIdentify the sources and describe the functions of carbohydrate, protein, lipid (fats and oils), vitamins A, C and D, the mineral ions calcium and iron, water and dietary fibre as components of the diet.2.26Energy requirementsUnderstand how energy requirements vary with activity levels, age and pregnancy.2.27Alimentary canal structure and functionDescribe the structure and function of the human alimentary canal, including the mouth, oesophagus, stomach, small intestine (duodenum and ileum), large intestine (colon and rectum) and pancreas.2.28PeristalsisUnderstand how food is moved through the gut by peristalsis.2.29Digestive enzymesUnderstand the role of digestive enzymes, including the digestion of starch to glucose by amylase and maltase, the digestion of proteins to amino acids by proteases and the digestion of lipids to fatty acids and glycerol by lipases.2.30Bile production and storageUnderstand that bile is produced by the liver and stored in the gall bladder.2.31Roles of bileUnderstand the role of bile in neutralising stomach acid and emulsifying lipids.2.32Small intestine adaptations for absorptionUnderstand how the small intestine is adapted for absorption, including the structure of a villus.233B Food energy practicalPractical: investigate the energy content in a food sample.
Convert light energy into chemical energy
Photosynthesis transfers light energy into chemical energy stored in glucose. It occurs mainly in chloroplasts, where chlorophyll absorbs light.
The absorbed energy drives the conversion of carbon dioxide and water into glucose; oxygen is released. The glucose can be respired, stored as starch, transported as sucrose, or used to make substances such as cellulose that support growth.
This conversion supplies plant biomass and stores energy in chemical bonds. That chemical energy can pass to consumers through food chains and can be released during respiration.
Photosynthesis does not create energy: it converts and stores it. Chlorophyll absorbs light energy; glucose is the energy-containing chemical product.
Write both equations for photosynthesis
Photosynthesis uses carbon dioxide and water to make glucose and oxygen. Light supplies energy and chlorophyll absorbs it; neither is a reactant consumed in the equation.
ext{carbon dioxide}+ ext{water}\longrightarrow ext{glucose}+ ext{oxygen}
6\mathrm{CO}{2}+6\mathrm{H}{2}\mathrm{O}\longrightarrow\mathrm{C}{6}\mathrm{H}{12}\mathrm{O}{6}+6\mathrm{O}{2}
The coefficients balance six carbon atoms, twelve hydrogen atoms and eighteen oxygen atoms on each side. Changing a subscript would change the substance, so balance an equation only by changing coefficients.
Explain limiting factors of photosynthesis
The rate of photosynthesis is controlled by the factor in shortest effective supply: the limiting factor. Increasing that factor raises the rate until a different factor becomes limiting.
| Factor increased | Why rate can rise | Why the rise stops or reverses |
|---|---|---|
| light intensity | more light energy is absorbed by chlorophyll | another factor, such as carbon dioxide or temperature, becomes limiting |
| carbon dioxide concentration | more reactant is available for carbon fixation | light or temperature becomes limiting |
| temperature | enzyme-controlled reactions proceed faster as particles have more kinetic energy and successful collisions increase | above the optimum, enzymes denature and the rate falls |
A plateau does not mean photosynthesis has stopped; it means changing the plotted factor no longer changes the rate under those conditions. Two curves can plateau at different rates because their other conditions differ.
State which factor is limiting from the evidence. More light or carbon dioxide cannot keep increasing the rate indefinitely, and high temperature can reduce rather than increase the rate.
Link leaf structures to photosynthesis
A leaf is broad and thin, giving a large surface for light absorption and a short diffusion distance for gases.
| Structure | Adaptation for photosynthesis |
|---|---|
| transparent cuticle and upper epidermis | allow light to reach mesophyll cells while the waxy cuticle limits water loss |
| palisade mesophyll near the upper surface | tightly packed cells contain many chloroplasts to absorb light |
| spongy mesophyll | large air spaces create internal surfaces and allow carbon dioxide and oxygen to diffuse |
| stomata and guard cells | pores allow gas exchange; guard cells control opening |
| xylem in veins | supplies water and mineral ions |
| phloem in veins | carries products such as sucrose away from the leaf |
Palisade cells contain more chloroplasts than spongy cells, not all the leaf's chloroplasts. Air spaces aid gas movement; they are not empty because the leaf is poorly packed.
Use nitrate and magnesium ions for growth
Plants need mineral ions because carbohydrates made by photosynthesis do not supply every element required to build new cells.
| Mineral ion | Required to make | Likely effect of deficiency |
|---|---|---|
| nitrate | amino acids, then proteins needed for growth and enzymes | stunted growth |
| magnesium | chlorophyll needed to absorb light for photosynthesis | yellow leaves (chlorosis) and reduced photosynthesis |
A valid mineral-deficiency comparison uses a complete mineral solution as the control and a solution lacking only one ion as the treatment. Any growth difference can then be linked to that missing ion if other conditions are controlled.
Mineral ions do not provide energy. Nitrate supplies nitrogen for amino acids, while magnesium is a component needed for chlorophyll; their functions are not interchangeable.
Investigate the evidence for photosynthesis
Photosynthesis can be demonstrated by measuring oxygen production and by showing that starch forms only where light, carbon dioxide and chlorophyll are available.
| Claim tested | Method and comparison | Positive evidence |
|---|---|---|
| oxygen evolves from a water plant | illuminate equal lengths of pondweed; collect gas by displacement or measure its volume over time | gas volume rises; collected oxygen relights a glowing splint |
| light is required | destarch a plant, cover part of a leaf, then expose it to light | only the uncovered region tests blue-black for starch |
| carbon dioxide is required | enclose a destarched leaf with potassium hydroxide or soda lime; compare with a control containing water | control leaf forms starch; carbon-dioxide-free leaf does not |
| chlorophyll is required | expose a destarched variegated leaf to light | green regions form starch; non-green regions do not |
For a starch test, boil the leaf in water, heat it in ethanol using a water bath to remove chlorophyll, rinse it, then add iodine solution. Blue-black shows starch; orange-brown shows no starch.
Destarching in darkness makes any later starch attributable to the experiment. Keep temperature, exposure time and plant material comparable, use repeats, and never heat flammable ethanol directly over a flame.
Build a balanced diet
A balanced diet supplies carbohydrate, protein, lipid, vitamins, minerals, water and dietary fibre in proportions that meet the body's needs without persistent excess or deficiency.
| Component group | Main contribution to balance |
|---|---|
| carbohydrate and lipid | energy supply; lipid also provides longer-term energy storage |
| protein | material for growth and repair |
| vitamins and mineral ions | required in small amounts for specific body functions |
| water | medium for transport and chemical reactions |
| dietary fibre | supports movement of material through the gut |
Appropriate proportions are not identical for everyone. Energy and nutrient needs vary with factors such as activity, age and pregnancy, while the overall diet must still include all required component groups.
Balanced does not mean equal amounts of every component or a single 'healthy' food. A food rich in one useful nutrient cannot by itself supply the full diet.
Match dietary components to sources and functions
Each dietary component has characteristic sources and functions, so a varied diet reduces the risk that one essential function is left unsupported.
| Component | Example sources | Main functions |
|---|---|---|
| carbohydrate | bread, rice, pasta, potatoes | energy released in respiration |
| protein | meat, fish, eggs, beans, pulses | growth and tissue repair; making enzymes |
| lipid | oils, butter, nuts, seeds | energy store, insulation and organ protection |
| vitamin A | liver, eggs, dairy, orange or dark-green vegetables | normal vision |
| vitamin C | citrus fruits and vegetables | healthy connective tissue; prevents scurvy |
| vitamin D | oily fish, eggs, fortified foods; made in skin in sunlight | helps calcium absorption and healthy bones |
| calcium ions | milk and dairy foods, leafy vegetables | bones and teeth |
| iron ions | red meat, liver, beans, leafy vegetables | haemoglobin in red blood cells |
| water | drinks and water-rich foods | solvent, transport medium and reaction medium |
| dietary fibre | wholegrains, fruit and vegetables | adds bulk and supports peristalsis, reducing constipation |
Fibre is not digested for energy, but it still has an important gut function. Vitamins and mineral ions have specific roles and cannot replace energy-providing carbohydrate or lipid.
Explain changing energy requirements
Energy requirements change when the amount of energy used for movement, growth or reproduction changes.
| Factor | Effect on requirement | Why |
|---|---|---|
| greater activity level | increases | contracting muscles need more ATP, so respiration releases more energy |
| age | varies | children and adolescents require energy for growth; requirements often fall when growth ends or activity decreases |
| pregnancy | increases | energy supports growth of the fetus and changes in maternal tissues |
If energy intake persistently exceeds energy use, surplus energy is stored, mainly as fat, and body mass can increase. If intake is lower than use, stored fat and glycogen are respired and body mass can decrease.
These factors change requirements, not a fixed amount for every person. Age alone does not determine need: body size, activity and physiological state also matter.
Trace food through the alimentary canal
The alimentary canal is a continuous tube that ingests food, digests large molecules, absorbs useful products and egests undigested material.
| Part | Main function |
|---|---|
| mouth | ingestion, chewing, mixing with saliva and starting starch digestion |
| oesophagus | carries the bolus to the stomach by peristalsis |
| stomach | churns food; acid kills many pathogens and provides conditions for protease |
| duodenum | receives pancreatic enzymes and bile; much chemical digestion occurs |
| ileum | completes digestion and absorbs soluble products through villi |
| colon | absorbs water and compacts undigested material |
| rectum | stores faeces before egestion |
| pancreas | produces digestive enzymes released into the duodenum |
The pancreas is an associated digestive organ, not part of the tube food passes through. Digestion breaks molecules down; absorption moves soluble products into blood or lymph.
Move food by peristalsis
Peristalsis is a coordinated wave of muscle contraction that pushes food along the gut.
Circular muscle contracts behind a bolus and relaxes ahead of it, narrowing the gut behind the food. Coordinated contractions of the gut wall then move the bolus forward through the oesophagus and intestines.
Because the movement is produced by muscle, food can travel through the digestive system even when gravity is not acting in the direction of movement. Dietary fibre adds bulk that helps the gut contents stimulate this process.
Peristalsis moves food; it does not chemically digest it. The wave is coordinated contraction and relaxation, not the food sliding down only because of gravity.
Match digestive enzymes to their products
Digestive enzymes hydrolyse large, insoluble food molecules into smaller soluble molecules that can be absorbed.
| Substrate | Enzyme | Product |
|---|---|---|
| starch | amylase | maltose |
| maltose | maltase | glucose |
| protein | proteases | amino acids |
| lipid | lipases | fatty acids and glycerol |
Carbohydrate digestion therefore uses two named steps: amylase changes starch to maltose, then maltase changes maltose to glucose. Each enzyme is specific to its substrate because its active site has a complementary shape.
Amylase does not convert starch directly to glucose in the named syllabus pathway, and bile is not an enzyme. Mechanical breakdown increases surface area but does not replace chemical digestion.
Locate bile production and storage
Bile is produced continuously by the liver and stored in the gall bladder until it is needed.
After food enters the duodenum, bile is released from the gall bladder through the bile duct into the small intestine. Storage allows a supply to be released when lipid-containing food arrives.
The gall bladder stores and releases bile; it does not make it. The pancreas produces digestive enzymes, not bile.
Explain the two roles of bile
Bile helps digestion in the duodenum by neutralising acid and emulsifying lipids.
| Role | Mechanism and benefit |
|---|---|
| neutralisation | bile neutralises acidic chyme from the stomach, providing a more suitable pH for enzymes in the small intestine |
| emulsification | bile separates large lipid globules into many small droplets, increasing surface area for lipase action |
Emulsification is physical subdivision, not chemical digestion. Bile contains no digestive enzyme and does not convert lipid into fatty acids and glycerol; lipase does that.
Adapt the small intestine for absorption
The ileum absorbs digested nutrients rapidly because its inner surface is folded into villi, and each villus has structures that shorten transport paths and maintain gradients.
| Feature | How it improves absorption |
|---|---|
| many villi and microvilli | provide a very large surface area |
| epithelium one cell thick | gives a short diffusion distance |
| dense capillary network | quickly carries away glucose and amino acids, maintaining concentration gradients |
| lacteal | absorbs and transports products of lipid digestion |
| many mitochondria in epithelial cells | supply ATP for active transport when needed |
Small soluble products cross the epithelium by diffusion or active transport. Blood capillaries carry water-soluble nutrients away, while lipid products enter lacteals and are transported in lymph.
Villi absorb products after digestion; they do not produce the digestive enzymes listed in the previous step. A large surface area is effective only because it is paired with a thin barrier and rapid transport away.
Measure energy content in food
Food energy can be estimated by burning a measured mass of food and measuring the temperature rise of a known mass of water.
| Step | Action |
|---|---|
| 1 | record the mass of food and the mass or volume of water |
| 2 | record the initial water temperature, ignite the food and heat the water |
| 3 | stir the water and record its highest temperature |
| 4 | reweigh the remaining food to find the mass actually burned |
| 5 | repeat, calculate a mean and compare energy transferred per gram |
E=mc\Delta T
For water, use c=4.2Jg−1∘C−1. Divide E by the mass of food burned to obtain joules per gram. Keep water mass, flame-to-container distance and apparatus the same when comparing foods.
The estimate is usually lower than the food's true energy value because combustion may be incomplete and heat is lost to the air and apparatus. Insulation, a lid, stirring and an oxygen supply improve accuracy; wear eye protection and secure hair around flames.