Photosynthesis – Revision Pack (C4)

The Chemistry behind Photosynthesis:

Simple sugars like glucose can be used in a number of ways; for example:

  • In respiration, releasing energy
  • Can be converted into cellulose to make cell walls
  • Can be used to make proteins for growth and repair
  • Can be converted into starch, fats and oils for storage

Starch is insoluble so it is used for storage. Glucose can affect the water concentration of cells and cause osmosis; starch does NOT do this and doesn’t move from where it is being stored.

Photosynthesis happens in a few simple steps:

STEP 1 – Water (H20) is split up

STEP 2 – This releases oxygen gas and hydrogen ions

STEP 3 – Carbon dioxide (CO2) combines with the hydrogen ions forming glucose (and water)

The History of Photosynthesis:

Priestley was another scientist who conributed to the understanding of photosynthesis. His experiment showed that plants must produce oxygen.

A more modern experiment was conducted using a green alga (plant) and an isotope of oxygen (O18). This formed part of a water molecule. The experiment showed that the light energy is used to split up the water, rather than the carbon dioxide. The oxygen gas made was O18 while the oxygen present in glucose was normal oxygen (O16). An isotope is a different form of a certain element.

The Rate of Photosynthesis and Limiting Factors:

Generally, three things can increase the rate of photosynthesis; these are: more carbon dioxide, more light and a higher temperature (increases enzyme action).

Photosynthesis only happens in day time because it needs light. Respiration however continues to happen at all times – this is because plants are living organisms; this means they are releasing energy at all times. REMEMBER – During respiration takes in oxygen and releases carbon dioxide.

During the day (in light), photosynthesis takes place – this is basically the same gas exchange as respiration but in reverse. Photosynthesis takes in carbon dioxide and releases oxygen. By comparison, the rate of gas exchange is a lot higher for photosynthesis than it is for respiration. Respiration is only really noticed at night.

Past Papers:

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