Green plants don't absorb food from the soil - make their own food using sunlight - process called photosynthesis

Photosynthesis occurs in the cells of green plants & algae that are exposed to light

During photosynthesis, light energy is absorbed by green chlorophyll (found in chloroplasts in some plant cells & algae)

Four things are needed for photosynthesis:

  • light from the sun 
  • carbon dioxide from the air 
  • water from the soil 
  • chlorophyll in the leaves 

The light energy is used to convert carbon dioxide from the air & water from the soil into sugar (glucose). Oxygen released as a by-product

Word equation: carbon dioxide + water > glucose + oxygen

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How Plants Use Glucose

Glucose produced in photosyntheses may be:

  • changed into insoluble starch & stored in the stem, leaves or roots
  • used by the plant during respiration to provide energy

Some gluocse in platnts and alagae is:

  • used to produce fat or oil & stored
  • used to produce cellulose to strengthen cell walls 
  • used to produce proteins

To produce proteins, plants also use nitate ions, which are absorbed from the soil

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Factors Affecting Photosynthesis


As temp. increases, so does rate of photosynthesis. Temp. limits rate of photosynthesis.  As temp. approaches 45degrees celcius, rate of photosynthesis drops to 0 bc enzymes controlling photosynthesis have been destroyed

CO2 Concentration

As rate of CO2 concentration increases, so does rate of photosynthesis. CO2 limits rate of photosynthesis. After reaching a certain point, an increase in CO2 has no further effect. CO2 isn't the limiting factor - must be either lighting of temperature 

Light Intensity

As light intensity increases, so does rate of photosynthesis. Light intensity limits rate of photosynthesis. After reaching a certain point, an increase in light intensity has no further effect. 

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Artificial Controls

Greenhouses can be used to control the rate of photosynthesis. By controlling lighting, temperature & carbon dioxide, gardeners can increase the rate of photosynthesis. 

This can result in plants:

  • growing more quickly
  • becoming bigger & stronger 
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