Photosynthesis

Photosynthesis is the production of organic compounds from inorganic molecules using light energy trapped by chlorophyll.

CHLOROPLAST

Carbon Dioxide + Water + Light >> Glucose + Oxygen

chlorophyll:

6CO2 + 6H2O + Light >> C6H12O6 + 6O2

Location of Chlorophyll in Photosynthetic Plant Cells

Chlorophyll is present in the internal green membranes of the chloroplast.

The material within the chloroplast is called the stroma. Within the stroma are stacks of thylakoids, the sub-organelles which are the site of photosynthesis. The thylakoids are arranged in stacks called grana (singular: granum). A thylakoid has a flattened disk shape. Inside it is an empty area called the thylakoid space or lumen. The light phase of photosynthesis takes place on the thylakoid membrane. The dark phase of photosynthesis takes place in the stroma. Note that the chloroplast is surrounded by a double membrane.

ENERGY FOR PHOTOSYNTHESIS

The energy needed for photosynthesis is obtained from sunlight (or artificial light). The green plant stores this energy in the form of ATP (Adenosine TriPhophate) and then uses the energy to carry out photosynthesis.

 

MAIN EVENTS OF PHOTOSYNTHESIS (ordinary level)

1. Light is absorbed by the leaf s chloroplasts.

2. The light energy is used to split water molecules.

3. The break up of water releases oxygen, electrons and protons (H+, H ions)
a. The electrons go to the chlorophyll.
b. The protons are stored in the chloroplast.
c. The oxygen passes from the chloroplast. Some will be used by the mitochondria for aerobic respiration and the remainder will be passed into the atmosphere.

4. Some of the light energy is carried to the chlorophyll to form high energy electrons.

5. The high energy electrons and the stored protons combine with carbon dioxide to form glucose. (C6H12O6)

This diagram represents the 5 events listed above:

Source of Carbon Dioxide in Leaf Cells

Carbon dioxide in air is the major source for the leaves of plants. Also carbon dioxide produced by the mitochondria of the plant during aerobic respiration is a minor source. Carbon dioxide dissolved in water is the major source for aquatic plants.

Source of Water in Leaf Cells

Water from the soil is the major source. It is carried to the leaves in xylem tissue. Some water is also produced by the mitochondria of leaf cells during aerobic respiration.

THE MAIN EVENTS OF PHOTOSYNTHESIS (higher level)

Photosynthesis takes place in two stages; the light stage and the dark stage. The light stage needs light so it is called the light-dependent stage. The dark stage is called the light-independent stage.

THE THE LIGHT STAGE

The light stage takes place in the chloroplast s thylakoid membranes. The events of the light stage are as follows:

1. Light Absorption: All the colours of white light (except green light) are absorbed by pigments in the chloroplast.

2. Energy Transfer: The energy is transferred to electrons. These electrons become high energy electrons. Within the chloroplasts are electron acceptors.

After the electron acceptor receives the high energy electrons the electrons will flow to one of two paths:

Pathway 1 (Cyclic Electron Transport): The high energy electrons travel through a series of electron acceptors (electron carrier system) and then back to the chlorophyll molecule. As they travel through the acceptors they lose energy. This energy is in ADP molecules. This is used to combine with 1 more phosphate molecule to form ATP and water. This is a high energy molecule.

The addition of a phosphate molecule to ADP is called phosphorylation. Since light is needed this addition of phosphate is called photophophoylation.

THIS DIAGRAM DEPICTS PATHWAY 1:

Pathway 2 (Non-cyclic Electron Transport): As in Stage 1 the electron has gained energy from sunlight. The electron moves from acceptor to acceptor but does not return to the chlorophyll. As an electron moves from acceptor to acceptor it is passed to NADP+. This causes the NADP+ to become neutral. This NADP molecule receives another electron and becomes NADP- (negative). The negative NADP- attracts the proton + which was previously released when water was broken up. The molecule now becomes NADPH.

THIS DIAGRAM DEPICTS PATHWAY 2:

SUMMARY OF THE LIGHT STAGE

Light energy is used to make ATP.
Light energy is used to produce NADPH from NADP+.
Oxygen gas is produced as a by-product. This will leave the plant through the leaves or be used by the plant for respiration.

THE DARK STAGE (THE CALVIN CYCLE)

The dark stage takes place in the stroma of the chloroplast. Unlike the light stage, the dark stage is controlled by enzymes and therefore affected by temperature. The enzyme is ribulose bisphosphate carboxylase oxygenase
(RUBISCO). In this cycle ATP and NADPH, produced in the light stage, provide the energy and electrons to change carbon dioxide (CO2) to carbohydrates (C H O) molecules. Although this stage is called the dark stage it occurs in light and also in darkness as long as the products of the light phase are still available.

The dark stage functions if ATP, NADPH and carbon dioxide are present. At the completion of this stage ADP, Phosphorous, and NADP+ are recycled for use in the light stage.

IN SUMMARY:

This diagram summarises the Light and Dark Stages’ dependence on each other.

The light stage produces NADPH and ATP which is used in the dark stage. The dark stage sends ADP, P, and NADP+ back to the thylakoids for use in the light stage.