What is being taught lesson by lesson:

Key Terms for this topic (Tier 3 vocabulary)

Photosynthesis – chloroplast – respiration – cellulose – starch – respiration – limiting factor – hydroponics – inverse square law – aerobic – anaerobic – lactic acid – muscle fatigue – metabolism.


Are you ready for your assessment in this topic? Try out this simple quiz.

What everyone needs to know:

Biologists favourite process, photosynthesis, is represented by the equation:

Carbon dioxide + Water → Glucose + Oxygen . Or

6CO2 + 6H2O → C6H12O6 + 6H2O

This is an endothermic reaction and needs uv light for it to happen. The reaction takes place in the chloroplasts of cells. The rate of photosynthesis is affected by:

  • Light intensity,
  • Temperature,
  • Availability of CO2.

You need to be able to both calculate and actually measure the rate of photosynthesis, interpret graphs and explain trends/results using given data.

Required practical 5 – investigate the effect of light intensity on the rate of photosynthesis. Using pondweed underwater and collecting oxygen through water.

Glucose, which is produced during photosynthesis is used for the following:

  • Respiration,
  • Converted then stored as insoluble starch,
  • Manufacture of amino acids and proteins (need nitrogen from the soil too),
  • Production of cellulose (plant cell walls),
  • Making plant oils (olive oil, sunflower oil etc).

There are two types of respiration:

  • Aerobic happens in the presence of oxygen and gives out more energy. This process is occuring in cells continuously and the energy is used for living organisms.
  • Glucose + Oxygen → Carbon dioxide + Water (reverse of photosynthesis so must be exothermic),
  • Used for movement, keeping warm and making larger molecules (storage and growth).
  • Anaerobic respiration occurs when there is not enough oxygen produces less energy.
  • Glucose → lactic acid
  • This lactic acid needs to be later broken down, we call this the oxygen debt. It occurs in muscles after anaerobic exercise.
  • In plant and animals cells, another product is formed
  • Glucose → Ethanol + Carbon dioxide
  • If completed by yeast cells, this process is called fermentation.

When exercising, the heart and breathing rates increase so more oxygen gets to the muscles for respiration, if there is not enough oxygen arriving , the mitochondria in cells are forced to undergo anaerobic respiration.

Metabolism is the sum of all of the reactions happening in your body, you need to know the roles of sugars, amino acids, fatty acids and glycerol in making and breaking carbohydrates, proteins and lipids. Ensure that you know the metabolic process of:

  • Glucose → Starch → Glycogen → Cellulose
  • Glycerol → Glycerol + 3 Fatty acids
  • Nitrate ions + Glucose → Amino acids
  • Amino acids → Proteins
  • Respiration
  • Excess proteins → urea (excreted)

Extra topics needed for the Higher papers:

Photosynthesis has limiting factors, given variables of the factors that affect the rate, you need to work out which is the limiting factor. You may be given graphs showing the affect of several variables and you need to pick out which is the limiting factor.

In relation to limiting factors, ensure that you can explain the inverse square law in relation to light intensity. These factors are crucial as they affect the production rates of commercial greenhouses and it will have more and more importance with our population growing so that we can maximise food production to feed everyone.

After anaerobic respiration, the lactic acid is transported to the liver where it is converted back to glucose using more oxygen. It is this extra oxygen that is the oxygen debt.