Plants

Biology B2 Unit 2

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Photosynthesis

(http://www.bbc.co.uk/schools/gcsebitesize/science/images/ocr_gate_lightenergy.gif)         (http://www.bbc.co.uk/schools/gcsebitesize/science/images/ocr_gate_lightenergy2.gif)

Cross section of a leaf:

  • Waxy cuticle - (epidermis layer) waterproof layer which stops water loss
  • Palisade cells - (palisade layer) tightly packed cells full of chloroplasts to collect as much sunlight as possible
  • Air spaces - (spongy layer) makes sure the cells in the spongy layer are not tightly packed to have a larger surface area for gas exchange. Few chloroplasts
  • Guard cells - (lower epidermis) open and close the stomata to control water loss
  • Stomata - (lower epidermis) allows gases to move in and out of cells
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Limiting Factors of Photosynthesis - light intensi

  • Light intensity:
    • the brighter it is, the faster the rate of photosynthesis
    • once it reaches a certain intensity, light is no longer a limiting factor
    • photosynthesis stops altogetherif the light intensity is too low or none at all, regardless of carbon dioxide and temperature

        (http://www.bbc.co.uk/schools/gcsebitesize/science/images/photosyn_1.gif)

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Limiting Factors of Photosynthesis - temperature

  • Temperature:
    • as it increases, the rate of photosynthesis increases
    • once it reaches about 40ºC, it is at it's optimum where it is no longer a limiting factor
    • if it goes beyond this temp, enzymes denature so the rate of photosynthesis decreases because there are no enzymes to control it
  •               (http://www.bbc.co.uk/schools/gcsebitesize/science/images/photosyn_3.gif)
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Glucose & Transport systems

Plants make glucose when they photosynthesise

Some of the glucose made is used by cells of the plant for respiration which goes on 24/7.

It provides energy for cell functions, reproduction and growth

Glucose is soluble in water so it would affect the water balance in the cells so it is converted to starch which is insoluble for storage


Phloem in the plant transports sugars made by photosynthesis to the rest of the plant. They have thin walls made of living cells

Xylem in the plant transports water and minerals from the soil around the plant. They have thick strong walls that are non-living.

In between the phloem and xylem are cambium cells which grow into new xylem and phloem.

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Limiting Factors of Photosynthesis - carbon dioxid

  • Carbon dioxide levels/concentration:
    • as it increases, the rate of photosynthesis increases
    • once it reaches a certain level, CO2 is no longer a limiting factor
    • photosynthesis stops alltogether if C02 levels are too low or none at all, regardless of light intensity and temperature

            (http://www.bbc.co.uk/schools/gcsebitesize/science/images/photosyn_2.gif)

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