Photosynthesis¶

Part of Bioenergetics.

Photosynthesis is how plants transfer energy from light into the chemical energy of glucose. The topic is easier to understand when you connect the equation to leaf structure and limiting factors.

What You Need to Learn¶

Photosynthesis turns light energy into a supply of organic material for the plant. Connect its reactants and products to glucose use, and explain how environmental conditions change the rate. Measure and interpret those changes; Higher Tier also includes interacting limits, distance and light intensity, and the economics of greenhouse conditions.

Photosynthetic Reaction¶

Photosynthesis is an endothermic process — the plant takes in light energy and stores it as chemical energy.

Photosynthesis takes place in chloroplasts and uses light energy absorbed by chlorophyll to convert carbon dioxide and water into glucose and oxygen.
Word equation: carbon dioxide + water → glucose + oxygen
Photosynthesis involves linked reactions: light is absorbed to supply energy, and carbon dioxide provides carbon for making glucose. Oxygen is released from water.
Leaves are adapted for photosynthesis because they are broad, thin and packed with chloroplast-containing cells near the upper surface.
Photosynthetic organisms are the primary producers of biomass on Earth. The glucose they make passes energy along food chains to all other organisms.

The chemical symbols are CO₂ for carbon dioxide, H₂O for water, C₆H₁₂O₆ for glucose and O₂ for oxygen. The balanced relationship is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with light energy supplied to the reaction.

Respiration and Photosynthesis in Plants¶

Plants respire all the time — day and night — to obtain energy for their internal processes. Photosynthesis requires sufficient light, whether natural or artificial.

In sufficiently bright light, photosynthesis can run faster than respiration, so the plant shows net carbon dioxide uptake and oxygen release. Dim light may not provide enough energy for this net gain.
During the night, with no light available, only respiration occurs. Plants consume oxygen and release carbon dioxide.

Hydrogencarbonate Indicator Investigation¶

Hydrogencarbonate indicator is used to detect changes in carbon dioxide concentration. It starts orange; it turns yellow when CO₂ increases and purple when CO₂ decreases.

A typical experiment uses four sealed tubes containing the indicator:

A control tube with no plant — remains orange.
A tube with a leaf wrapped in foil (no light) — turns yellow, as respiration releases CO₂ with no photosynthesis to remove it.
A tube with a leaf wrapped in gauze (dim light) — little colour change, as respiration and limited photosynthesis roughly balance.
An uncovered tube with a leaf (full light) — turns purple, as rapid photosynthesis removes more CO₂ than respiration produces.

Rate of Photosynthesis¶

A limiting factor is whichever condition is in shortest supply at a given moment; increasing it will raise the rate until a different factor becomes limiting.

Higher Tier: when comparing curves measured under several conditions, identify which change raises the rate at that point. If extra light no longer helps but extra carbon dioxide does, carbon dioxide is limiting. A factor can limit one part of a graph while another limits a different part.

Light Intensity¶

Greater light intensity → more light captured by chlorophyll → faster rate of photosynthesis.
If the rate levels off as light increases, another factor limits further increase; adding still more light alone will not overcome that limit.
Inverse square law: light intensity is inversely proportional to the square of the distance from the light source (intensity ∝ 1/distance²). Doubling the distance quarters the light intensity; halving the distance quadruples it.

Carbon Dioxide Concentration¶

Higher CO₂ drives the reaction forward, increasing the rate.
A plateau as carbon dioxide increases means another factor now restricts the rate; carbon dioxide is no longer limiting.

Temperature¶

Warming increases molecular movement and enzyme-substrate collisions, raising the rate up to an optimum that depends on the plant and conditions.
Excessive heat can denature enzymes, changing their active sites and reducing the rate.

Amount of Chlorophyll¶

More chlorophyll means more light can be captured, increasing the rate.
Disease and infection can reduce the amount of functional chlorophyll and thus limit photosynthesis.

Practical Investigation: Pondweed¶

The effect of limiting factors on photosynthesis can be measured using Canadian pondweed submerged in water. The rate of oxygen production (measured by counting bubbles or observing displacement of a gas bubble in a capillary tube) is proportional to the rate of photosynthesis.

Light intensity: move a lamp to different distances; apply the inverse square law to calculate intensity.
Temperature: place the boiling tube in water baths of different temperatures.
CO₂ concentration: dissolve different amounts of sodium hydrogencarbonate in the water (it releases CO₂).
Always control variables not being tested, and repeat at least three times to calculate a mean.

Rate = oxygen volume ÷ collection time. Collecting 12 cm³ in 4 min gives 3 cm³/min. Bubble counting is less precise because bubbles vary in size. Plot the changed factor on the horizontal axis and measured rate on the vertical axis, using labelled units and suitable scales; read a value from the curve to convert graphical data back into a numerical result.

Measure Photosynthesis as Light Changes

Move the lamp and measure the model pondweed’s oxygen production. Keep other conditions constant and compare rate readings, remembering that unequal bubble sizes make bubble counts less precise than collected gas volume. The model illustrates a trend; its readings are not measurements from a real plant. Open full interactive.

Apply it: The measured rate doubles when a lamp moves closer. Does that prove the plant would keep doubling its rate with every further move?

Answer

No. Light may limit the initial rate, but another factor can become limiting, so the rate can level off even while light intensity continues to increase.

Starch Test for Photosynthesis¶

Starch is a storage product of photosynthesis in leaves. Begin with a plant kept in darkness long enough to use its stored starch, then expose the test leaf to the chosen light conditions. The iodine test checks for starch formed during the investigation:

Dip the leaf in boiling water to stop reactions.
Place in ethanol heated in a water bath to remove chlorophyll (the leaf turns pale/white). Keep ethanol away from flames because it is flammable.
Rinse with cold water.
Add iodine solution — blue-black = starch present; orange-brown = no starch detected. Interpret this with the starting condition and control leaves; absence of starch alone does not prove that photosynthesis never occurred.

Higher Tier: use the inverse-square relationship above to compare relative light intensities. For the same lamp, a distance of 20 cm gives four times the intensity at 40 cm.

Greenhouses and Commercial Optimisation¶

Greenhouses allow growers to manipulate all major limiting factors to maximise yield year-round:

Temperature: glass panels trap solar heat via the greenhouse effect. Heaters maintain warmth in winter. Blinds prevent overheating and enzyme denaturation in summer.
CO₂ concentration: burning paraffin heaters raises CO₂ levels inside the greenhouse while also providing heat.
Light intensity: artificial lighting supplements natural daylight, extending the photoperiod and allowing growth during winter months.

Monitoring and adjusting these conditions involves significant cost, so growers must balance efficiency against expenditure to make production economically viable.

Higher Tier: maximum photosynthesis does not automatically mean maximum profit. Extra heating, lighting or carbon dioxide is worthwhile only if the additional crop value exceeds its cost. Improving a condition already above its limiting level wastes money.

Uses of Glucose from Photosynthesis¶

Respiration — glucose is broken down to release energy for cell processes.
Starch — insoluble storage form; built up when glucose supply exceeds demand.
Cellulose — used to make cell walls.
Lipids — formed for energy storage, especially in seeds.
Amino acids — made when glucose combines with nitrate ions absorbed from the soil; used to build proteins.

Common Confusions¶

Endothermic vs endergonic: Photosynthesis is described as endothermic because the plant takes energy in from light. Do not confuse this with a reaction that simply releases heat.
The inverse square law direction: doubling the distance reduces intensity to one quarter — not one half. A common error is to halve rather than quarter.
Plants only photosynthesise, not respire: Plants always respire. In daylight they also photosynthesise; at night only respiration occurs. The net exchange depends on which process is faster.
Limiting factor plateau: A graph line levelling off does not mean photosynthesis has stopped — it means that factor is no longer limiting. Another factor is now the bottleneck.

Key Terms¶

Photosynthesis: the process by which plants use light energy to make glucose from carbon dioxide and water.
Chlorophyll: the green pigment that absorbs light energy for photosynthesis.
Limiting factor: a factor in shortest supply that restricts the rate of a process.
Glucose: a simple sugar made during photosynthesis and used for respiration and growth.
Starch: an insoluble storage carbohydrate made from glucose in plants.
Endothermic reaction: a reaction that takes in energy from the surroundings; photosynthesis is endothermic because it absorbs light energy.
Chloroplast: the organelle in plant cells where photosynthesis takes place; contains chlorophyll.
Inverse square law: the rule that light intensity is inversely proportional to the square of the distance from the source (intensity ∝ 1/d²).
Hydrogencarbonate indicator: a pH-sensitive indicator used to detect changes in CO₂ concentration; orange at normal levels, yellow when CO₂ increases, purple when CO₂ decreases.
Optimum temperature: the temperature at which enzyme-controlled reactions, including photosynthesis, proceed at the fastest rate.