Photosynthesis

Photosynthesis is the chemical reaction in green plants and algae that uses light energy to convert simple inorganic molecules (carbon dioxide and water) into a sugar (glucose) and oxygen.

Location. The reaction takes place inside chloroplasts — small green organelles found mainly in the palisade mesophyll cells of leaves (and in some other green parts of the plant). Each chloroplast contains stacks of membranes packed with the pigment chlorophyll, which gives plants their green colour.

The reactants come from outside the leaf:

• Carbon dioxide diffuses in from the air through the stomata (tiny pores on the underside of leaves).
• Water is absorbed from soil by root hair cells, then carried up the plant in xylem vessels.

The products go to opposite places:

• Glucose is used inside the plant — for respiration, to make starch (storage), cellulose (cell walls) and other compounds.
• Oxygen diffuses out of the leaf through the stomata and back into the atmosphere.

Word equation:

$\text{carbon dioxide} + \text{water} \xrightarrow[\text{chlorophyll}]{\text{light energy}} \text{glucose} + \text{oxygen}$

Symbol equation (must balance):

$6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow[\text{chlorophyll}]{\text{light energy}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Checking the balance:

Atom	Left side	Right side
C	6 × 1 = 6	6 (in glucose)
H	6 × 2 = 12	12 (in glucose)
O	6 × 2 + 6 × 1 = 18	6 (glucose) + 6 × 2 = 18

All three atom types balance — that's why both 6's are essential.

Why the 'light energy' and 'chlorophyll' labels matter.

• They are conditions, not reactants — they go ABOVE the arrow, not on the same line.
• Light energy is what makes the reaction go (without it, the reaction stops).
• Chlorophyll is the catalyst-like pigment that absorbs the light energy and passes it to the chemistry — it is NOT consumed.

AQA exam wording. When asked to 'write the symbol equation for photosynthesis', the marks usually break down as:

• Correct reactants and products (1).
• Correct balancing numbers (1, Higher tier).
• Light energy and chlorophyll labelled correctly (1, often).

Reactions either give out energy (exothermic, like respiration) or take in energy (endothermic, like photosynthesis).

Photosynthesis takes in energy. It is therefore endothermic. The energy comes from the Sun's light and is stored in the chemical bonds of the glucose molecule that's produced. That stored energy can later be released by respiration when the plant — or anything that eats the plant — breaks down the glucose.

This is why:

• Plants need light to photosynthesise. Without it, the energy supply is cut and the reaction stops.
• Photosynthesis is the start of nearly every food chain on Earth — light energy is captured here, then passed along as chemical energy.
• The Sun is the original energy source for most life. Plants are the ones that lock that energy into a form animals (including us) can use.

Cross-link to physics. This is the same idea as 'energy is transferred from a thermal store to a chemical store' from the AQA physics specification — the language of 'energy stores' is identical across the sciences.

Photosynthesis is a chemical reaction, so the same kinds of factors that affect any reaction rate apply — but with biology-specific twists.

1. Light intensity.

• Light provides the energy. More light → more energy → more reaction → higher rate.
• At low light, doubling the light roughly doubles the rate.
• Eventually the curve levels off — another factor (e.g. CO₂) has become limiting.

2. Carbon dioxide concentration.

• CO₂ is one of the reactants. More CO₂ → more raw material → higher rate.
• Same shape of curve: rise then plateau.
• Atmospheric CO₂ (~0.04%) is low enough to limit photosynthesis in most crops.

3. Temperature.

• Enzymes inside the chloroplast catalyse photosynthesis. Higher temperature → enzymes work faster → higher rate.
• BUT: above about 45 °C the enzymes start to denature (change shape and stop working). Rate then falls sharply.
• This gives an 'optimum' temperature curve, not a plateau.

4. Amount of chlorophyll.

• More chlorophyll = more light absorbed = potentially higher rate.
• Reduced by disease (e.g. tobacco mosaic virus discolours leaves) or mineral deficiency (magnesium deficiency means less chlorophyll). Both lower the rate of photosynthesis.

Limiting factor. Whichever factor is in shortest supply is the limiting factor — increasing it raises the rate; increasing the others has no effect until the limiting one is fixed.

Reading a rate graph.

• The rising portion = the named factor on the x-axis is the limiting one. Increasing it raises the rate.
• The plateau = another factor has taken over as the limiting factor.

Higher-tier interactions. At the plateau, raising CO₂ (or temperature, within enzyme limits) will push the plateau UP. Multiple lines on the same axes can show this clearly — high CO₂ gives a higher plateau than low CO₂.

The inverse-square law for light. Light from a point source (like a lamp or the sun) spreads out in all directions. Intensity (light energy per unit area) follows:

$I \propto \frac{1}{d^2}$

where $d$ is the distance from the source.

Worked numbers. If a lamp 10 cm from a pond plant produces intensity $I$:

• At 20 cm (2× distance), intensity = $I / 2^2 = I/4$.
• At 30 cm, intensity = $I / 3^2 = I/9$.
• At 5 cm (½ distance), intensity = $I / (0.5)^2 = 4I$.

This is critical when planning the pondweed practical (or the AQA Required Practical 5 in Combined Science) — distance from the lamp dominates the rate.

Quick rule to memorise. Doubling distance → quarter the intensity. Halving distance → four times the intensity.

Commercial growers (e.g. UK tomato and cucumber producers) use this knowledge to maximise crop yields:

1. Light — large glass roof + artificial LED lighting at night and on dull winter days, keeping light intensity high all year.

2. Carbon dioxide — burn paraffin heaters or pump in industrially produced CO₂ to raise the indoor concentration well above the 0.04% in normal air. Some greenhouses run at 0.1–0.2% CO₂.

3. Temperature — thermostatically controlled heaters keep the air at the enzyme optimum (~25 °C for many crops). Ventilation prevents overheating.

4. Water — automated irrigation prevents wilting (closed stomata stop CO₂ getting in).

5. Disease and mineral status — sterilising surfaces, monitoring nutrient feed (especially nitrate and magnesium, see 4.3.3.1) keeps leaves green and chlorophyll healthy.

The economic question. Greenhouse heating, lighting and CO₂ enrichment all cost money. A grower has to balance the increased yield against the cost of inputs: maximum rate is not always maximum profit. Exam questions often ask you to identify a sensible compromise (e.g. heat to 22 °C, not 28 °C, because the extra rate doesn't justify the fuel cost).

Glucose is the basic 'sugar building block' produced in photosynthesis. Plants don't just stockpile it — they immediately put it to work in several ways.

1. Respiration. Most of the glucose made each day is broken down in mitochondria by aerobic respiration to release energy for the plant's own life processes (active transport, growth, building proteins, etc.). See spec 4.4.2.1 for the full reaction.

2. Stored as starch. Excess glucose is converted to starch — a long polymer of glucose molecules. Starch is insoluble in water, so:

• It doesn't dissolve and leak out of cells.
• It doesn't affect the cell's water balance (no osmosis problems).
• It packs into compact storage granules in the cytoplasm.

Starch is stored in leaves overnight, in roots (e.g. potato tubers), in stems and in seeds (e.g. wheat grain). Animals do the same idea but use glycogen instead.

3. Cellulose for cell walls. Glucose molecules are joined together (in a different way from starch) to form cellulose — long fibre-like chains that build the plant cell wall. The cellulose wall:

• Gives the cell its rigid shape.
• Withstands the pressure when the cell is full of water (turgor).
• Allows the plant to stand upright without a skeleton.

4. Lipids (oils and fats). Glucose can be converted into lipids — fatty molecules that store about TWICE as much energy per gram as carbohydrates do. Plants store lipids especially in seeds (sunflower, oilseed rape, olive, peanut). Energy-dense seed oils provide energy for the embryo to germinate.

5. Amino acids → proteins. To build proteins, plants combine glucose (carbon, hydrogen, oxygen) with nitrate ions (NO₃⁻) absorbed from soil. The nitrate provides the NITROGEN that all amino acids contain. The resulting amino acids are joined into proteins for:

• Enzymes (every metabolic reaction).
• Structural proteins.
• Antigens and other functional molecules.

Without enough nitrate in the soil, plants cannot make proteins → stunted growth (links to 4.3.3.1 plant mineral deficiencies).

Glucose itself dissolves in water — that would cause two big problems if a plant just stored it as glucose:

Problem 1: osmosis. A high glucose concentration inside cells would make water rush IN by osmosis (water moves from low-concentration solution to high-concentration solution across a partially permeable membrane). Cells would swell and could burst (animal cells) or build excessive turgor pressure (plant cells).

Problem 2: leakage. Soluble molecules dissolve and may diffuse or be transported away. Storage should stay put.

The solution: convert to starch.

• Starch is a long-chain polymer of glucose units, joined into a coiled structure.
• It is insoluble in water — so it doesn't dissolve, doesn't affect osmosis, doesn't leak out.
• It can be packaged into dense starch granules visible inside potato or rice cells under a microscope.
• When the plant needs energy, enzymes break starch back into glucose, ready for respiration.

Animals do the same trick, but with glycogen (stored in muscle and liver) — also insoluble and broken down to glucose on demand.

Iodine test for starch. A drop of iodine solution turns from brown-orange to blue-black in the presence of starch. This is the food test from 4.2.1.5 (Food tests) — and is often used to show that a part of a leaf has been photosynthesising (de-starch the plant first, then expose part of the leaf to light, then test for starch).

Glucose is made of carbon, hydrogen and oxygen only (C₆H₁₂O₆). Amino acids — and the proteins made from them — also need nitrogen.

Plants get the nitrogen they need by absorbing nitrate ions (NO₃⁻) from the soil through their root hair cells. This is an active transport process (4.1.3.3) — nitrate is usually in lower concentration in soil than in the cell, so it has to be moved AGAINST its concentration gradient using energy from respiration.

Inside the plant, glucose + nitrate ions → amino acids → proteins.

Why this matters. Soils that are short of nitrate produce small, weak plants. Farmers use fertilisers (especially NPK fertilisers: nitrogen, phosphorus, potassium) to replace ions used up by previous crops. Crop rotation with legumes (peas, beans, clover) also helps — they host nitrogen-fixing bacteria that put nitrate-style ions back into the soil naturally.

Don't forget magnesium. A separate mineral, magnesium, is needed for chlorophyll itself (centre of every chlorophyll molecule). So magnesium deficiency → less chlorophyll → less photosynthesis → less glucose → less of everything on this page. Nitrate and magnesium together cover most of the mineral-nutrition exam questions.