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The Site of Photosynthesis

The leafe is the main photosynthetic structure; the chloroplasts are the cellular organisms within the lead where photosynthesis takes place.

Structure of the Leaf:

Leaves are adapted to bring together the three materials of photosynthesis; water, carbon dioxide and light, and its products; oxygen and glucose. These adaptations include:

  • A large surface area that collects as much sunlight as possible.
  • An arrangement of leaves on the plant that minimises overlapping, and so avoids the shadowing of one leaf by another.
  • Thin, as most light is absorbed in the first few millimetres of the leaf and the diffusion distance is kept short.
  • A transparent cuticle and epidermis that let light through to photosynthetic mesophyll cell beneath.
  • Long, narrow upper mesophyll cells packed with chloroplasts that collect sunlight.
  • Numerous stomata for gaseous exchange.
  • Stomata that open and close in response to changes in light intensity.
  • Many air spaces in the lower mesophyll layer to allow diffusion of carbon dioxide and oxygen.
  • A network of xylem that brings water to the leaf cells and phloem that carries away the sugars produced in photosynthesis.

Outline of Photosynthesis:

6CO2 + 6 H2O ----- light -----> glcose + 6H2O

  • Photosynthesis is a complex metabolic pathway involving many intermediate reactions.
  • There are three main stages of photosynthesis:
  • Capturing of light energy by the chlorophyll.
  • The light dependent reaction - this is when light energy is converted into chemical energy. An electron flow is created by the effect of light on the chlorophyll and this causes photolysis - the splitting of water molecules, into protons, electrons and oxygen. Producing the products reduced NADP, ATP and oxygen.
  • The light independent reaction - this is when hydrogen ions are used to reduce carbon dioxide into sugars and other useful organic molecules.

Structure and Role of Chloroplasts in Photosynthesis:

  • Photosynthesis occurs in the chloroplasts of the leaf, they vary in shape and size but at typically disc-shaped and are also surrounded by a double membrane.
  • There are two distinct regions of the chloroplasts:
  • The grana are stacks of up to 100 disc-like structures called thylakoids where the light-dependent reaction occurs. Within the thylakoids is the photosynthetic pigments called chlorophyll. Some thylakoids have tubular extentions that join with thylakoids in adjacent grana, called inter-granual llamellae.
  • The stroma is a fluid-like matrix where the light independet reaction occurs. There are a number of structures such as starch grains, within the stroma.

The Light Dependent Reaction:

This involves the capture of light whose energy is used for two purposes:

  • To add inorganic phosphate molecule to ADP to make ATP.
  • To split water into hydrogen ions and electrons. As the splitting is caused by light, it is known as photolysis.

Oxidation = when a substance combines with oxygen and loses electrons, resulting in energy being given out and so energy is lost.

Reduction = when a substance loses oxygen and gains electrons, resulting in energy being taking in and so energy is gained.

The light dependent reaction occurs as the



Amazing notes- really helpful! Thanks alot...

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