C1.3 Photosynthesis

Almost all the energy that flows through living things on Earth was captured first by a leaf. Photosynthesis is the process that converts light energy into the chemical energy of carbohydrates, and in doing so it builds the base of nearly every food chain and releases the oxygen we breathe. For C1.3 the examiner wants you to see photosynthesis as an energy conversion — light energy in, chemical energy out — and to be able to explain how pigments absorb that light, how the overall reaction is summarised, and how environmental factors limit the rate. Keep returning to that single idea and the topic holds together.

Light energy to chemical energy: the overall reaction

Photosynthesis transforms light energy into chemical energy stored in organic compounds. The familiar word equation summarises it: carbon dioxide and water are combined, using light, to make glucose and oxygen. As a balanced symbol equation:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Two points matter for the exam. First, this is an endothermic, energy-requiring process: the products contain more chemical energy than the reactants, and that extra energy came from light. Second, the oxygen released comes from the splitting of water (photolysis), not from carbon dioxide — a classic point that examiners test. Photosynthesis is therefore essentially the reverse, in energy terms, of cell respiration, which releases the chemical energy locked away here.

Pigments and the absorption of light

Light is only useful once it is absorbed, and that job falls to photosynthetic pigments in the chloroplasts. Chlorophyll is the main pigment; accessory pigments such as carotenoids absorb additional wavelengths and pass the energy on. Each pigment absorbs some colours of light and reflects others — chlorophyll absorbs strongly in the red and blue regions of the spectrum but poorly in the green, which is why it reflects green light and leaves look green.

The syllabus expects you to distinguish two graphs that look similar but mean different things:

• An absorption spectrum shows how much light of each wavelength a pigment absorbs.
• An action spectrum shows the rate of photosynthesis achieved at each wavelength.

The two curves match closely: photosynthesis is fastest at the red and blue wavelengths that chlorophyll absorbs best, and slowest in the green. This close correspondence is the evidence that the pigments doing the absorbing are the pigments driving photosynthesis.

Limiting factors and the rate of photosynthesis

The rate of photosynthesis depends on the environment, and at any moment one factor usually holds it back — the limiting factor, defined as the factor in shortest supply that controls the rate. The three you must know are light intensity, carbon dioxide concentration and temperature.

• Light intensity: as it increases the rate rises, then levels off (plateaus) when another factor becomes limiting.
• Carbon dioxide concentration: the same pattern — raising CO₂ raises the rate until something else limits it.
• Temperature: the rate rises with temperature up to an optimum, then falls steeply as the enzymes involved denature.

To interpret a graph, look at the plateau: the factor that, when increased, lifts the plateau higher is the one that was limiting. Greenhouse growers exploit this by adding artificial light, extra carbon dioxide and warmth to keep all three factors high at once.

Measuring photosynthesis

Because photosynthesis consumes carbon dioxide and produces oxygen, its rate can be estimated by measuring any of three changes over time:

• Oxygen production — for example, counting or collecting the bubbles of gas released by an aquatic plant such as Elodea or Cabomba.
• Carbon dioxide uptake — often followed using a pH or carbon dioxide indicator, since removing CO₂ changes the pH of the surrounding solution.
• Biomass increase — the dry mass gained over a longer period reflects the carbohydrate produced.

When designing such an experiment, identify the independent variable (the factor you change, such as light intensity altered by moving a lamp), the dependent variable (what you measure, such as bubbles per minute) and the variables you must keep constant (for example temperature, often controlled with a water bath that also acts as a heat shield). A sodium hydrogen carbonate solution is commonly supplied so that carbon dioxide does not become the limiting factor when you are investigating light.