2eNutrition

What photosynthesis actually does

Photosynthesis is the process in which plants convert light energy into chemical energy. The light energy absorbed by chlorophyll in the chloroplasts is transferred to the chemical bonds of glucose.

That glucose is then used to:

• be respired to release energy
• be converted to starch for storage — starch is insoluble, so it does not change the concentration of the cell contents and does not make water move into the cell by osmosis
• be converted to cellulose for cell walls
• be combined with nitrate ions to make amino acids, and hence proteins

Photosynthesis matters because it is the entry point of energy into almost every food chain, and it removes carbon dioxide from the air and releases oxygen.

The two equations you must be able to write

Word equation

carbon dioxide + water → glucose + oxygen
(with light and chlorophyll written above/below the arrow — they are not reactants)

Balanced symbol equation

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Learn C₆H₁₂O₆ by heart. Then the balancing follows: 6 carbons on the left means 6 CO₂; 12 hydrogens means 6 H₂O; that gives 18 oxygens on the left, 6 are locked into glucose, so 12 are left = 6O₂.

Common mistake: examiners repeatedly report candidates who cannot balance the equation, or who do not know the formula for glucose. Write the numbers as subscripts and count the atoms on both sides before you move on.

Photosynthesis is not respiration in reverse — don't muddle them

Common mistake: candidates have written that respiration requires the absorption of light. Respiration never involves light. A plant respires 24 hours a day; it only photosynthesises when illuminated.

Limiting factors (2.20)

The rate of photosynthesis is controlled by whichever factor is in shortest supply — the limiting factor. Increasing any other factor then has no effect.

Light intensity
As light intensity increases, rate increases steadily — the first part of the graph is close to a straight line — then levels off (plateaus) when something else — usually carbon dioxide concentration or temperature — becomes limiting. In the dark the rate is zero.

Carbon dioxide concentration
Same shape of graph. CO₂ is only about 0.04% of air, so it is very often the limiting factor in a field or greenhouse on a bright day. Growers burn fuel or add CO₂ to glasshouses for this reason.

Temperature
Different shape. Rate increases as temperature rises because the enzymes controlling photosynthesis and the substrate molecules have more kinetic energy, so there are more successful collisions. Above an optimum (around 25–35 °C in many plants) the rate falls, and above about 40 °C it drops away sharply because the enzymes are denatured — the shape of the active site changes so the substrate no longer fits.

Answering limiting-factor questions well

Always name the factor that has become limiting: "beyond point X, increasing light intensity has no further effect because carbon dioxide concentration is now the limiting factor."

For temperature above the optimum, do not stop at "the enzymes are denatured". Examiners note that candidates lose the final mark by not continuing to explain the effect on enzyme structure and function — the active site changes shape, the substrate no longer fits, so fewer enzyme–substrate complexes form and the rate falls.

Never write that heat "kills" an enzyme. Enzymes are not alive. Use denatured.

Examiner tip. Two marks are thrown away here every year: not knowing C₆H₁₂O₆ / not balancing 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, and saying high temperature 'kills the enzymes'. Say denatured, then always add what that does — active site changes shape, substrate no longer fits, rate falls.

The leaf, from top to bottom

The leaf's adaptations — in the language examiners want

• Broad and flat — large surface area to absorb light
• Thin — short diffusion distance for gases to reach the mesophyll cells
• Palisade cells at the top and packed with chloroplasts — maximum light absorption where light is strongest
• Air spaces in the spongy mesophyll — large internal surface area and a fast diffusion pathway for CO₂
• Stomata — allow gas exchange; guard cells open and close them, closing at night or when water is short
• Network of veins — supplies water for the reaction and removes the products; also supports the leaf so it stays flat

If a question says explain, you must give the adaptation and the reason ("thin, so that carbon dioxide has only a short distance to diffuse"). If it says describe, the structure alone is enough.

Mineral ions (2.22)

Glucose contains only carbon, hydrogen and oxygen. To make everything else it needs, a plant absorbs mineral ions from the soil through its root hair cells, by active transport.

You need exactly two:

That is the whole list at IGCSE. Both are absorbed as ions dissolved in soil water — say "magnesium ions", not "magnesium".

Common mistake: examiners report candidates writing that humans need nitrates to make proteins. They do not — humans get amino acids by digesting protein in the diet. Nitrate ions are a plant requirement. Keep the two lists separate in your head:

• Plant mineral ions: magnesium (chlorophyll), nitrate (amino acids)
• Human mineral ions: calcium (bones and teeth), iron (haemoglobin)

Examiner tip. Do not transfer plant mineral requirements to humans. A very common error is saying humans need nitrate ions to make protein — humans digest dietary protein into amino acids. Nitrate ions and magnesium ions belong only to plants.

Objective 2.23 names three investigations. Learn the apparatus, the variable measured and the controls for each.

1. Oxygen evolution from a water plant

Apparatus: a piece of pondweed (Elodea or Cabomba) cut at the stem, inverted in a beaker of water containing sodium hydrogencarbonate solution (supplies carbon dioxide), a lamp, a ruler.

Measure: count the number of bubbles of oxygen per minute, or collect the gas in an inverted syringe/capillary tube and measure the volume of gas per minute (more reliable — bubbles vary in size).

Vary: distance of the lamp from the plant, to change light intensity.

Control:

• temperature — put a beaker/tank of water between lamp and plant as a heat shield, or use a water bath, because a filament lamp warms the water
• carbon dioxide concentration — same volume and concentration of sodium hydrogencarbonate
• the same piece of pondweed, and leave 5 minutes to equilibrate after each move

Repeat and take a mean.

2. Testing a leaf for starch (iodine test)

Starch is the store made from glucose, so its presence shows photosynthesis has happened.

1. Boil the leaf in water for ~1 minute — kills the tissue and breaks down cell membranes so iodine can enter.
2. Boil in ethanol in a water bath — removes the chlorophyll so the colour change is visible. Ethanol is flammable: turn off the Bunsen and heat with hot water from a kettle/water bath, never a naked flame.
3. Dip in hot water — softens the brittle leaf.
4. Spread on a white tile and add iodine solution.

Blue-black = starch present. Orange-brown = no starch.

3. Showing the three requirements

First destarch the plant by leaving it in the dark for 24–48 hours, then test one leaf to prove no starch is present. Then set up the test, leave in bright light for several hours, and do the starch test.

Each one needs a control that is identical except for the single variable — otherwise you cannot say the variable caused the result.

Examiner tip. Two marks go missing here every year. First, say that the plant was destarched in the dark first — without that you cannot claim the starch was made during the experiment. Second, never write that the leaf was boiled in ethanol over a Bunsen: the ethanol is heated in a water bath with the flame turned off, because ethanol is flammable. And in every set-up, name the control and say it was identical except for the one variable being tested.

A balanced diet

A balanced diet contains appropriate proportions of all seven components: carbohydrate, protein, lipid, vitamins, minerals, water and dietary fibre. "Balanced" means the right amounts for that person — too much energy-rich food leads to obesity, too little to weight loss and, in children, poor growth.

Sources and functions — learn this table

Common mistake: examiners report candidates writing that protein is used for "insulation" or as an energy store. Insulation is lipid. Protein is for growth and repair.

Water and fibre are the two most often forgotten. Fibre is not digested and is not absorbed — do not say it gives you energy.

How energy requirements vary (2.26)

Practical (separate award): energy content of a food sample

Method

1. Measure the mass of the food sample with a balance.
2. Measure a known volume of water (e.g. 20 cm³) into a boiling tube clamped above the food.
3. Record the starting temperature of the water.
4. Hold the food on a mounted needle, set it alight in a Bunsen flame and immediately hold it under the boiling tube until it will not relight.
5. Stir, then record the highest temperature reached.

Calculation

Energy transferred (J) = mass of water (g) × 4.2 × temperature rise (°C)

Energy per gram of food (J/g) = energy transferred ÷ mass of food

(1 cm³ of water has a mass of 1 g.)

Controls: same volume of water, same distance from tube to food, same starting temperature, and repeat for each food.

Why the value is always too low: heat is lost to the surroundings and to the apparatus, and the food may not burn completely.

Improvements — name the feature:

• a draught shield / screen around the apparatus to reduce heat loss to the air
• insulate the boiling tube (e.g. with a lagged copper calorimeter)
• hold the food closer to the tube
• relight the food until it will no longer burn

Examiner tip. When asked to improve the energy-content experiment, examiners say weaker answers 'referred vaguely to reducing energy loss without identifying the features'. Name the actual change — a draught shield, insulation round the calorimeter, holding the burning food closer — and say what heat loss it prevents.

The alimentary canal, part by part

Peristalsis (2.28)

The gut wall has two layers of muscle: circular and longitudinal.

• Behind the bolus, the circular muscles contract and the longitudinal muscles relax — the gut becomes narrower, squeezing the food forwards.
• In front of the bolus, the circular muscles relax and longitudinal muscles contract — the gut becomes wider.
• These waves of contraction travel along the whole canal. Mucus lubricates the food.

Peristalsis is not gravity — you could swallow upside down. Fibre provides the bulk the muscles push against.

Digestive enzymes (2.29)

Digestion breaks large, insoluble molecules into small, soluble ones that can be absorbed through the gut wall into the blood.

Common mistake — three of them, all reported by examiners:

1. Writing that amylase breaks starch down to glucose. It does not. Amylase → maltose; then maltase → glucose. Two enzymes, two steps.
2. Saying starch digestion happens in the stomach. It happens in the mouth and the small intestine (duodenum). Stomach acid stops salivary amylase working.
3. Spelling. Maltose is the sugar (-ose); maltase is the enzyme (-ase). If your handwriting is unclear you will not be credited.

Also: never answer an enzyme question with "the enzymes" and no name, or with a vague function. Name the enzyme, name the substrate, name the products.

Bile (2.30, 2.31)

Bile is produced by the liver and stored in the gall bladder, then released into the duodenum. It contains no enzymes.

Two functions:

1. Neutralising stomach acid. Bile is alkaline, so it raises the pH of the acidic food leaving the stomach. This gives the optimum pH (slightly alkaline) for the enzymes of the small intestine, so they work at their fastest rate.
2. Emulsifying lipids. Bile breaks large lipid droplets into many small droplets. This increases the surface area for lipase to act on, so lipid digestion is faster. Emulsification is a physical change, not digestion.

Adaptations of the small intestine for absorption (2.32)

• It is very long — a large surface area.
• The lining is folded and covered in millions of villi (singular: villus), and the villus cells have microvilli — all greatly increasing the surface area for absorption.

The villus itself:

Unabsorbed material passes into the colon, where water is absorbed.

Examiner tip. The single most-penalised error in this section is 'amylase digests starch to glucose in the stomach'. It is amylase → maltose (mouth and duodenum), then maltase → glucose in the small intestine. Also watch the command word: 'describe the villus' wants the features; 'explain how the villus is adapted' wants feature plus the reason it speeds absorption.