SCIENCE

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SCIENCE

• PHOTOSYNTHESIS
• CHELSTON HIGH SCHOOL

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PHOTOSYNTHESIS

• What is Photosynthesis ?It is a process
  where green plants make their own food by
  using inorganic substances to build up
  organic substances. During this process,
  carbon dioxide and water react together to
  produce glucose and oxygen. This reaction is
  called photosynthesis (photo means light and
  synthesis means manufacture)
• Photosynthesis is the process by which
  green plants make glucose from carbon
  dioxide and water in the presence of light
  energy.
• The light energy is trapped by a green
  pigment called chlorophyll. Chlorophyll is
  found in small sacs called chloroplasts.

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Sunlight

• Carbon dioxide Water
  glucose oxygen
• The balanced chemical equation is written
  as
• 6Co2 6H2o C6H12O6 6O2

Absorbed by chlorophyll
sunlight
Absorbed by chlorophyll
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Light and dark reaction

• The equation above summarises a series of
  chemical reactions which take place in
  chloroplasts. In fact, carbon dioxide and water
  do not combine directly .The reaction occurs
  in two stages called light reaction and
  the dark reaction.

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• The energy from light is used to split
  water molecules into hydrogen and oxygen.
  This is called photolysis .Since this stage
  requires light, it is called the light
  reaction. Oxygen is given off while hydrogen
  combines with carbon dioxide to produce
  glucose in the second stage. This stage
  does not require light and it is the dark
  reaction , although it also takes place
  during day time .

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External structure of a leaf .

• The main organ of photosynthesis in green
  plants is the leaf. It is for this reason
  that chloroplasts are present in abundance
  in leaves. Water and carbon dioxide combine
  in the chloroplasts . The structure of the
  leaf enables light , carbon dioxide and
  water to reach the chloroplasts efficiently.

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External structure of a leaf
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• A leaf usually has a leaf stalk (petiole)
  and a leaf blade or lamina with the
  midrib from which veins branch .The leaf
  stalk exposes the leaf to light as much
  as possible and acts as a channel for
  water and dissolved mineral salts from the
  stem .The lamina provides a large surface
  area through light and carbon dioxide can
  be absorbed .The midrib provides support for
  the leaf . The leaf is also quite thin .

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Internal structure of a leaf

• The midrib contains vascular bundles .
  Vascular bundles are made up of xylem
  vessels and phloem tubes. Xylem vessels
  provides support and bring water to the
  leaf while phloem tubes take manufactured
  food away from the leaf to other parts of
  the plant .