Nutrition

Photosynthesis is the process by which plants make glucose from carbon dioxide and water, using light energy absorbed by chlorophyll. Glucose is the starting point for all the organic molecules in the plant (and ultimately for all heterotrophs that eat plants).

The equation.

Word: carbon dioxide + water $\xrightarrow{\text{light, chlorophyll}}$ glucose + oxygen

Balanced symbol: $6CO_2 + 6H_2O \xrightarrow{\text{light, chlorophyll}} C_6H_{12}O_6 + 6O_2$

Light and chlorophyll are conditions — they are NOT used up in the reaction. They sit ABOVE the arrow.

Where it happens. In chloroplasts, found mainly in the palisade mesophyll cells just below the upper surface of the leaf. Chloroplasts contain the green pigment chlorophyll which absorbs red and blue light from sunlight.

Sources of the reactants:

• CO₂ — diffuses in through stomata (mainly on the lower epidermis of the leaf).
• H₂O — absorbed from the soil by root hair cells, transported up the stem in xylem.

Fates of the products:

• Glucose — used immediately in respiration to release ATP; converted to starch for storage (insoluble; does not affect water potential); built into cellulose for cell walls; built into sucrose for transport in the phloem; combined with nitrate to make amino acids (then proteins).
• Oxygen — by-product. Used in respiration or released through stomata.

Demonstrating the requirements of photosynthesis:

• Light is needed. Destarch a plant by leaving it in the dark for 48 h. Cover part of a leaf with foil for 24 h in the light. Test the leaf for starch: only the uncovered region turns blue-black.
• Chlorophyll is needed. Use a variegated leaf (with green and white patches). After destarching and exposing to light, test for starch: only the green regions turn blue-black; the white (chlorophyll-free) regions do not.
• CO₂ is needed. Place a destarched plant in a sealed jar with soda lime (absorbs CO₂). After exposure to light, test the leaf: NO starch produced. A control with no soda lime DOES produce starch.

Required practical — rate of photosynthesis in pondweed (e.g. Cabomba or Elodea). Place pondweed in water with sodium hydrogencarbonate (NaHCO₃ — provides CO₂). Shine a lamp at known distance d. Count oxygen bubbles per minute OR collect O₂ in an inverted measuring cylinder. Vary the IV (light intensity = 1/d², distance from lamp). Rate = bubbles per minute OR cm³ O₂ per minute.

Leaves are the main photosynthetic organ. Their structure is optimised for light absorption and gas exchange.

Feature	Adaptation	Reason
Large, flat lamina	Maximises surface area	More light absorbed; more area for gas exchange
Thin	Short distance from surface to chloroplasts	Light penetrates all cells; gases diffuse quickly
Waxy cuticle	Transparent waterproof coating on upper surface	Reduces water loss without blocking light
Upper epidermis	Single layer of transparent cells	Lets light pass through to mesophyll
Palisade mesophyll	Column-shaped cells PACKED with chloroplasts, just below upper epidermis	Maximum chloroplasts in maximum light → most photosynthesis happens here
Spongy mesophyll	Loosely packed cells with large AIR SPACES	Massive internal surface area for gas exchange (CO₂ in, O₂ out)
Stomata	Pores on the LOWER epidermis, opened/closed by guard cells	Gas exchange in/out; lower position reduces water loss
Vascular bundle (vein)	Xylem + phloem	Xylem brings water from roots; phloem carries sucrose away

Standard exam answer. Pick FIVE features and for each give: (feature) + (adaptation/where) + (why = function). Examiner reports flag candidates who name a feature without saying WHY it helps photosynthesis.

The three factors that limit the rate of photosynthesis are light intensity, CO₂ concentration and temperature.

Rule: at any given moment, ONE factor is limiting the rate. Increase that factor and the rate increases. Increase the others — no effect, because they're already in excess.

Light intensity.

• At low light intensity, light is the limiting factor → rate is proportional to light intensity (linear rise).
• At high light intensity, light is no longer limiting → rate plateaus → another factor (CO₂ or temperature) now limits.
• To push the plateau higher: raise the limiting factor (more CO₂, or warm the air).

CO₂ concentration.

• Normal atmospheric CO₂ is only ~0.04 % — usually the limiting factor for plants in bright light.
• Adding CO₂ (e.g. burning paraffin heaters in a glasshouse) raises the rate substantially.

Temperature.

• Photosynthesis is enzyme-catalysed (e.g. RuBisCO in the Calvin cycle).
• Rate rises with temperature up to the optimum (~25-30 °C for most temperate plants).
• Above the optimum, enzymes denature → rate falls sharply.

Commercial glasshouses. Growers manipulate ALL THREE factors to maximise yield:

1. Light — supplementary lamps in winter.
2. CO₂ — released into the air from gas burners or canisters (typically 0.1 % vs 0.04 % in normal air).
3. Temperature — heated to ~25 °C; ventilated if it rises above optimum. The cost of providing these conditions must be balanced against the increased value of the crop (commercial decision).

Iodine cannot penetrate a fresh waxy leaf, so the leaf must first be prepared.

Procedure:

1. Boil the leaf in water for ~1 min. Kills the cells, denatures enzymes, softens, breaks the membranes — allows chemicals through.
2. Boil the leaf in ETHANOL in a boiling tube in a hot water bath (NEVER direct heat — ethanol is flammable). The chlorophyll dissolves out → the leaf is decolourised (white/pale yellow). Decolourising makes the iodine colour change easy to see.
3. Rinse the leaf in cold water. It is brittle after ethanol; cold water softens it.
4. Spread the leaf flat on a white tile and add iodine solution (orange-brown).
5. Observe the colour: blue-black = starch present (so photosynthesis has happened). Orange-brown = no starch.

Safety: ethanol is flammable. Turn off Bunsen burners while pouring ethanol; heat using a water bath.

Use in experiments: confirms whether photosynthesis took place in a particular leaf region. Used in the foil-strip experiment (light needed), variegated leaf (chlorophyll needed) and soda-lime jar (CO₂ needed).

Plants take mineral ions from the soil through their roots. Two specifically named on the Edexcel spec:

Nitrate ions ($\text{NO}_3^-$).

• Used to make amino acids → proteins (e.g. enzymes, transporters, structural proteins).
• Deficiency symptoms: stunted growth (no protein → no growth); yellowing of older leaves (chlorophyll broken down to scavenge nitrogen for the new growth).

Magnesium ions ($\text{Mg}^{2+}$).

• Used to make chlorophyll (Mg sits at the centre of the chlorophyll molecule).
• Deficiency symptoms: yellow leaves (chlorosis) — can't make chlorophyll → can't photosynthesise → poor growth.

Distinguishing the two deficiencies in exams. Both cause yellowing. Key distinguishing words:

• Nitrate → stunted growth + yellow older leaves.
• Magnesium → chlorosis + yellow leaves overall (no growth phrase).

Fertilisers. Soil minerals are depleted as plants grow; farmers add NPK fertiliser (Nitrogen + Phosphorus + Potassium) to replace them. Magnesium is sometimes added separately if soil is deficient.

A balanced diet contains seven components in the right proportions. The proportions vary with age, gender, activity level and state of health.

Component	Function	Sources
Carbohydrates	Main energy source (used in respiration)	Bread, rice, pasta, potatoes
Lipids (fats)	Concentrated energy store, insulation, cell membranes	Butter, oils, oily fish
Proteins	Growth and repair; enzymes, hormones, antibodies	Meat, fish, eggs, beans, pulses
Vitamin A	Vision in dim light; healthy skin	Carrots, liver
Vitamin C	Healthy skin and gums; collagen synthesis	Citrus fruits, tomatoes, peppers
Vitamin D	Absorption of calcium	Oily fish, eggs, sunlight on skin
Calcium	Strong bones and teeth; blood clotting	Milk, cheese, leafy greens
Iron	Haemoglobin in red blood cells	Red meat, liver, spinach
Fibre	Bulks faeces; stimulates peristalsis; prevents constipation	Wholegrains, fruit, vegetables
Water	Solvent for all reactions; transport (blood); temperature regulation	Drinks, fruit, vegetables

Deficiency diseases to know:

• Scurvy — vitamin C deficiency. Bleeding gums, slow wound healing, loose teeth.
• Rickets — vitamin D / calcium deficiency. Soft, bowed bones in children.
• Anaemia — iron deficiency. Pale, tired, breathless on exertion; not enough haemoglobin → less O₂ carried.

Energy requirements. Different people need different amounts:

• Age: children/teenagers need lots of energy AND protein for growth.
• Gender: males typically need more energy due to greater muscle mass.
• Activity: manual workers / athletes need ~3000+ kcal/day; sedentary office workers ~2000 kcal.
• Pregnancy/lactation: mothers need extra energy and protein for the baby.

Body Mass Index (BMI): $BMI = \dfrac{\text{mass (kg)}}{\text{height}^2\,(\text{m}^2)}$. <18.5 underweight; 18.5-24.9 healthy; 25-29.9 overweight; ≥30 obese.

The digestive system is a long muscular tube with associated glands (salivary glands, liver, gall bladder, pancreas).

Mouth. Teeth chew (mechanical digestion). Saliva (from salivary glands) contains salivary amylase → begins digestion of starch into maltose. Tongue rolls food into a bolus.

Oesophagus. Muscular tube from mouth to stomach. Peristalsis (waves of muscle contraction) moves the bolus down.

Stomach. Muscular bag that churns food (mechanical) and secretes gastric juice:

• Hydrochloric acid (HCl) — pH 2: kills most bacteria in food + gives optimum pH for pepsin.
• Pepsin (a protease) — begins protein digestion → polypeptides / amino acids. The thick mucus lining protects the stomach wall from the acid + the enzymes.

Small intestine — duodenum (first part). Receives two secretions:

• Bile from the liver (stored in the gall bladder, released via the bile duct).
• Pancreatic juice from the pancreas: contains pancreatic amylase, trypsin (protease) and lipase. Also alkaline (contains hydrogencarbonate).

Small intestine — ileum (the long part). Wall produces maltase which converts maltose → glucose. ABSORPTION of digested nutrients happens here through villi.

Large intestine (colon). Absorbs water from the remaining material → solid faeces.

Rectum. Stores faeces.

Anus. Egestion — faeces removed via the anus.

Digestive enzymes summary:

Enzyme	Substrate	Product	Where produced	Where acts
Salivary amylase	Starch	Maltose	Salivary glands	Mouth
Pancreatic amylase	Starch	Maltose	Pancreas	Duodenum
Maltase	Maltose	Glucose	Wall of ileum	Ileum
Pepsin (protease)	Proteins	Polypeptides / amino acids	Stomach wall (gastric glands)	Stomach
Trypsin (protease)	Proteins	Polypeptides / amino acids	Pancreas	Duodenum
Lipase	Lipids	Fatty acids + glycerol	Pancreas	Duodenum

The role of bile (spec 2.31). Bile is made by the liver, stored in the gall bladder, and released into the duodenum. It has two functions:

1. Emulsifies lipids — breaks large lipid droplets into many small droplets. This increases the surface area of lipid exposed to lipase, so lipid digestion is much faster.
2. Neutralises the acidic chyme from the stomach — bile is alkaline (contains hydrogencarbonate). This provides the optimum pH (~8) for pancreatic enzymes (amylase, trypsin, lipase) to work in the duodenum.

Bile is NOT an enzyme. It does NOT chemically digest lipids — it just breaks them physically into smaller droplets.

Villi adaptations (spec 2.30). The ileum is lined with millions of finger-like villi:

• Large surface area (villi + microvilli on each villus cell). Total intestinal SA is ~250 m².
• Thin wall — one cell thick (epithelium). Short diffusion distance.
• Rich capillary network. Carries absorbed glucose and amino acids away rapidly → maintains a steep concentration gradient → fast diffusion.
• Lacteal (lymph vessel) in the centre — absorbs fatty acids and glycerol.

Aim: to determine how much energy is stored in a sample of dry food (e.g. peanut, dry pasta, crisp).

Method:

1. Weigh the food sample → mass m_food (g).
2. Measure 25 cm³ of water into a boiling tube. (1 cm³ water ≈ 1 g, so m_water = 25 g.)
3. Clamp the boiling tube above the bench. Record the initial water temperature T₁.
4. Mount the food on a mounted needle / pin. Ignite in a Bunsen flame and IMMEDIATELY hold under the boiling tube until the food stops burning.
5. Record the final water temperature T₂.
6. Calculate temperature rise: $\Delta T = T_2 - T_1$.
7. Calculate energy released by the food: $E = m_{water} \times c \times \Delta T$, where c = 4.2 J g⁻¹ °C⁻¹.
8. Energy per gram = $E / m_{food}$ → units J g⁻¹ (or divide by 1000 for kJ g⁻¹).
9. Repeat with fresh water and a new food sample and take a mean.

Variables:

• IV — type of food (if comparing foods).
• DV — temperature rise of water.
• CVs — volume of water (25 cm³ each time), distance from flame to tube, type of tube, length of burning (until the flame goes out).

Limitations / why the method UNDER-estimates the true energy content:

• Heat is lost to the surroundings (the air, the tube wall).
• Some food may not burn completely.
• The flame from the Bunsen also adds heat to the tube.

Improvements:

• Use a calorimeter (a copper container with a built-in thermometer and stirrer; sometimes insulated). Much less heat lost.
• Shield apparatus from draughts.
• Use a bomb calorimeter (in industry — completely sealed, surrounded by water, food burnt in pure O₂ at high pressure).