Photosynthesis: equations + limiting factors
Photosynthesis makes glucose from carbon dioxide and water using light energy absorbed by chlorophyll; oxygen is a by-product.
Word equation: carbon dioxide + water → glucose + oxygen
Symbol equation: 6CO2 + 6H2O → C6H12O6 + 6O2
Limiting factors: at any moment only ONE limits the rate — light intensity, CO2 concentration or temperature. Raising it raises the rate until another becomes limiting; above about 40 °C the enzymes denature and the rate falls.
Balanced diet, digestive enzymes and bile
A balanced diet has seven components: carbohydrate (energy), protein (growth/repair), lipid, vitamins, minerals, water and fibre.
Enzymes: amylase digests starch to maltose (maltase then gives glucose); protease digests protein to amino acids; lipase digests lipids to fatty acids and glycerol.
Bile (from the liver, stored in the gall bladder) neutralises stomach acid and emulsifies lipids into small droplets, raising the surface area for lipase; it is not an enzyme.
Leaf, mineral ions and villus adaptations
Leaf (2.21): broad and flat (large surface area); thin (short diffusion path); palisade cells packed with chloroplasts.
Mineral ions (2.22): nitrate makes amino acids for proteins; magnesium makes chlorophyll — magnesium deficiency yellows the leaves (chlorosis).
Villus (2.32): finger-like shape plus microvilli (large surface area); one cell thick (short diffusion path); dense capillaries maintain the gradient; a lacteal absorbs fatty acids and glycerol.
Drawn from real examiner reports.
Define photosynthesis; balance the equation
A definition of photosynthesis needs both the reactants and product (carbon dioxide + water → glucose, plus oxygen) AND the energy source (light absorbed by chlorophyll). "Plants make food using sunlight" gives neither and scores zero. The oxygen must balance as 6O2, not O2: 6CO2 + 6H2O → C6H12O6 + 6O2 — with the correct glucose formula.
June 2023 Paper 1BR Q6(a): an unbalanced photosynthesis equation and a wrong glucose formula were common errors.
Nitrates make protein in plants, not humans
Nitrate ions are used by PLANTS to make amino acids, which build proteins; magnesium ions are used to make chlorophyll. A common category error is to say nitrates are needed to make proteins in HUMANS — humans get their amino acids from the protein they eat and digest, not from nitrate ions. Keep plant mineral requirements separate from human dietary needs.
June 2024 Paper 2B Q4(c)(ii): candidates wrote that nitrates are needed to make proteins in humans, confusing plant mineral needs with human ones.
Vitamin and mineral deficiency mix-ups
Learn each deficiency exactly: vitamin C deficiency causes scurvy; calcium (or vitamin D) deficiency causes rickets or weak, brittle bones; vitamin A deficiency causes night blindness; iron deficiency causes anaemia. The frequent slip is to confuse vitamin C with vitamin A or vitamin D, or to blame bone problems on the wrong nutrient.
June 2023 Paper 1BR Q2(b)(ii): vitamin C deficiency was confused with vitamin A or D, and calcium-deficiency effects with vitamin D.
Peristalsis: naming it is not enough
Peristalsis moves food along the gut, but naming it does not earn the description mark. You must say it is waves of muscular contraction — the circular and longitudinal muscles in the gut wall contracting in sequence to squeeze and push food along. "Peristalsis moves the food" with no mechanism will not gain the description mark.
June 2023 Paper 1BR Q2(a): candidates named peristalsis but did not describe it as waves of muscular contraction.
Protein digestion: enzyme, site, product
Protein is digested by protease (pepsin in the stomach; trypsin from the pancreas), NOT amylase. Common errors: naming amylase; placing protein digestion in the mouth (there is no protease there); and describing absorption in the small intestine instead of digestion. State the product as amino acids — not "smaller molecules" or "smaller proteins".
June 2023 Paper 1BR Q2(b)(iv): candidates named amylase, placed protein digestion in the mouth, or described absorption rather than digestion.
Emulsion test needs ethanol AND water
The emulsion test for lipids needs ethanol AND water: add ethanol and shake to dissolve any lipid, then pour into water — a cloudy white emulsion shows lipid is present. Ethanol alone, or "it turns cloudy" with no water step, is incomplete (Sudan III giving a red layer is also accepted). Bile only emulsifies lipids; it is not an enzyme and does not digest them.
June 2024 Paper 2B Q6(a)(ii): the very common error in the lipid test was using ethanol without adding water to form the emulsion.
Protein is for growth and repair
The dietary role of protein is growth and repair of tissues. Do not write that protein is for insulation (that is lipid) or that it is a main energy source (carbohydrate is the primary fuel, and protein is used for energy only as a last resort). Examiners specifically reject "insulation" and "energy" as the function of protein.
June 2024 Paper 2B Q6(a)(iii): candidates gave insulation or energy as the role of protein instead of growth and repair.
Describe vs explain — give a direction
Match the command word: "describe" states what happens, "explain" needs a reason. Give a direction — "less light reduces photosynthesis", not "light affects it". Also comment on sample size and controls in long questions.
Explain "how adapted" as a linked chain
For "explain how [structure] is adapted", give a linked chain, not a list: name the feature, say what it does, then link it to the process (many microvilli → large surface area → more glucose absorbed). A bare list with no "so" or "therefore" scores only half.
Calorimetry: use the mass of WATER
In the food-energy (calorimetry) calculation, use energy = mass of WATER × 4.2 × temperature change — not the mass of the food — and always state the unit (J or J/g). Repeating the experiment improves reliability, not accuracy; changing the thermometer would affect accuracy.
This topic covers both plant nutrition (photosynthesis, leaf adaptations, mineral ions) and human nutrition (balanced diet, alimentary canal, digestion, absorption). Spec statements 2.18–2.33B.
Photosynthesis — the process by which plants manufacture glucose from carbon dioxide () and water () using light energy absorbed by the green pigment chlorophyll; oxygen () is released as a by-product.
Word equation:
carbon dioxide + water glucose + oxygen
Balanced symbol equation:
Limiting factors for photosynthesis (at any moment, only ONE factor is rate-limiting):
| Factor | Effect of increasing it | How it limits |
|---|---|---|
| Light intensity | Increases rate (up to a plateau) | Provides the energy to drive the reaction |
| concentration | Increases rate (up to a plateau) | is a raw material |
| Temperature | Increases rate up to ~40 °C; then enzymes denature and rate falls sharply | Affects enzyme activity |
Mineral ions for plant growth:
| Feature | Adaptation | Benefit |
|---|---|---|
| Broad, flat shape | Large surface area | More light can be absorbed |
| Thin lamina | Short diffusion path | reaches palisade cells quickly |
| Transparent waxy cuticle | Lets light through; prevents water loss | Maximises light to palisade layer |
| Palisade mesophyll (top layer) | Many chloroplasts, tightly packed | Maximum light absorption for photosynthesis |
| Spongy mesophyll with air spaces | Large internal surface area; air spaces connected to stomata | diffuses to cells; diffuses out |
| Stomata on lower epidermis | Pores for gas exchange; guard cells open/close them | Controls entry of and exit of |
| Vascular bundles (xylem + phloem) | Xylem supplies water/minerals; phloem removes sucrose | Sustains photosynthesis |
A balanced diet contains the correct proportions of seven components:
| Component | Key sources | Main function(s) | Deficiency |
|---|---|---|---|
| Carbohydrate | Rice, bread, pasta, sugar | Energy (primary fuel) | Fatigue, weight loss |
| Protein | Meat, fish, eggs, beans | Growth and repair of tissues | Stunted growth, kwashiorkor |
| Lipid (fats/oils) | Butter, oil, nuts | Energy store, insulation, cell membranes | Deficiency rare |
| Vitamin A | Liver, carrots, dairy | Night vision (makes visual pigment) | Night blindness |
| Vitamin C | Citrus fruit, peppers | needed for healthy skin and gums (prevents scurvy) | Scurvy |
| Vitamin D | Oily fish, sunlight | Calcium absorption; bone health | Rickets (weak/bowed bones) |
| Calcium | Dairy, leafy greens | Bones and teeth; blood clotting | Weak bones |
| Iron | Red meat, spinach | Making haemoglobin (carries ) | Anaemia |
| Water | Drinks, fruit | Metabolic reactions, solvent, transport | Dehydration |
| Dietary fibre | Wholegrain, vegetables | Prevents constipation; aids peristalsis | Constipation |
Energy requirements (statement 2.26) vary with: activity level (more activity = more energy), age (teenagers growing rapidly need more protein; older people generally need fewer calories), and pregnancy (increased energy and nutrient demands).
The alimentary canal (gut) is a tube from mouth to anus. Key organs and their functions:
| Organ | Role |
|---|---|
| Mouth | Mechanical digestion (teeth); chemical digestion (salivary amylase breaks starch → maltose) |
| Oesophagus | Carries food to stomach by peristalsis |
| Stomach | Mechanical churning; pepsin (protease) digests proteins → amino acids; acidic pH (~2) kills bacteria |
| Duodenum (first part of small intestine) | Bile arrives from gall bladder; pancreatic enzymes (amylase, protease, lipase) complete digestion |
| Ileum (rest of small intestine) | Absorption of soluble products (glucose, amino acids, fatty acids, glycerol) via villi |
| Large intestine (colon + rectum) | Absorbs water; compacts faeces |
| Anus | Egestion of faeces |
| Pancreas | Produces amylase, protease and lipase — all released into the duodenum |
| Liver | Produces bile (no enzymes in bile) |
| Gall bladder | Stores bile |
Peristalsis (statement 2.28) — waves of muscular contraction of the circular and longitudinal muscles in the gut wall, which squeeze and push food along the alimentary canal. Naming peristalsis alone does not score the description mark; the mechanism (waves of muscular contraction) must be stated.
| Enzyme | Substrate | Product(s) | Site of production | Site of action |
|---|---|---|---|---|
| Amylase | Starch | Maltose | Salivary glands + pancreas | Mouth + duodenum |
| Maltase | Maltose | Glucose | Small intestine wall | Ileum |
| Protease (pepsin, trypsin) | Protein | Amino acids | Stomach (pepsin); pancreas (trypsin) | Stomach + duodenum |
| Lipase | Lipids | Fatty acids + glycerol | Pancreas | Duodenum + ileum |
Bile (statements 2.30–2.31):
Villi are finger-like projections of the small intestine lining. Their adaptations for absorption:
| Feature | How it aids absorption |
|---|---|
| Finger-like shape (many villi) | Greatly increases the surface area of the small intestine wall |
| One-cell-thick epithelium | Shortens the diffusion distance for soluble molecules |
| Dense capillary network | Glucose and amino acids enter capillaries; blood carries them away, maintaining a steep concentration gradient |
| Lacteal (lymph vessel) | Fatty acids and glycerol are absorbed and re-formed into fat droplets (chylomicrons) that enter the lacteal |
| Microvilli on epithelial cells | Further increase the surface area (forms the "brush border") |
Define photosynthesis (mark-scheme form, two elements).
State the word equation for photosynthesis and name the energy source used. (2 marks)
