Photosynthesis

Photosynthesis

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

chlorophyll:

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

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.

Photosynthesis

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 Light Stage

• Light Absorption: All the colours of white light (except green light) are absorbed by pigments in the chloroplast.
• Energy Transfer: The energy is transferred to electrons. These electrons become high energy electrons. Within the chloroplasts are electron acceptors. 

Pathway 1 (Cyclic Electron Transport): 

• The high energy electrons travel through a series of electron acceptors and then back to the chlorophyll molecule(reaction centre). 
• 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. .

The light stage

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). 
• Water splits into electrons, protons and oxygen
• The negative NADP- attracts the proton + from the water splitting. 
• The molecule now becomes NADPH.

The Dark Stage

• 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 (Glucose). 
• 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.