Photosynthesis

Photosynthesis

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…