Photosynthesis

The leaf is the main photosynthetic structure. The chloroplasts are the cellular organelles within the leaf where photosynthesis takes place. Leaves are adapted for this:

• large SA that absorbs as much sunlight as possible
• arrangement of leaves that minimises overlapping and shadowing
• thin: short diffusion distance
• transparent cuticle and epidermis that lets light through to the mesophyll cells beneath
• long, narrow upper mesophyll cells packed with chloroplasts
• numerous stomata for gaseous exchange that open and close with light intensity changes
• many air spaces in lower mesophyll layer to allow diffusion of O2 and CO2
• network of xylem and phloem to carry water and sugars

Oxidation: when a substance combines with oxygen.

Reduction: when a substance loses oxygen

When a substance is oxidised it loses electrons and when it is reduced it gains elections.

Oxidation results in energy being given out, reduction results in energy being taken.

1. Add a Pi molecule to ADP, making ATP

• When a chlorophyll molecule absorbs light energy, it boosts the energy of a pair of electrons within this chlorophyll molecule, raising them to a higher energy level, 'excited'. This makes them leave the chlorophyll and are taken up my an electron carrier molecule. The chlorophyll molecule has been oxidised. The electron carrier has been reduced.
• The electrons are not passed along a number of electron carriers in a series of oxidation-reduction reactions. The carriers form a transfer chain in the thylakoid membranes. Each new carrier is at a lower level of energy at each stage. This energy is used to combine a Pi molecule with an ADP molecule to make ATP.

2. Split water into H+ ions and OH- ions. This is photolysis as it is caused by light

• The loss of electrons in the chlorophyll are replaced by electrons from water molecules that are split using light energy. This photolysis also yields hydrogen ions.
• These hydrogen ions are taken up by NADP, which becomes reduced. Reduced NADP enters the light-independent reaction with the electrons. The O2 by-product from photolysis is used in respiration or diffuses out the leafe as a waste product.

Chloroplasts are adapted to capture sunlight and carrying out the light dependent reaction:

• thylakoid membranes provide a large surface area for the attatchment of chlorophyll, electron carriers and enzymes
• network of proteins in the grana hold the chlorophyll in a precise manner allowing maximum absorption of light
• granal membranes have enzymes attatched to them, that help manufacture ATP
• contain DNA and ribosomes so they can quickly manufacture proteins

Chloroplast are adapted to carrying out the light independent reaction:

• fluid in the stroma contains all the enzymes needed
• the stroma fluid surrounds the grana and so the products of the light dependent reaction in the grana can readily diffuse into the stroma
• it contains both DNA and ribosomes so can quickly manufacture proteins

• Carbon dioxide from the atmosphere diffuses into the leaf through stroma and dissolves in water around the mesophyll. It diffuses through the plasma membrane, cytoplasm and chloroplast membranes into the stroma
• In the stroma, the CO2 combines with the 5-carbon ribulose biphosphate (RuBP) 
• The combination of CO2 and RuPB produces two molecules of 3-carbon glycerate 3-phosphate (GP)
• ATP and reduced NADP from the light dependent reaction are used to reduce the activated glycerate 3-phosphate to triose phosphate (TP)
• The NADP is re-formed and goes back to the light-dependent reaction to be reduced again by accepting hydrogen
• some TP molecules are converted to useful organic substances, e.g glucose
• most of the TP molecules are used to regenerate RuBP using ATP from the light dependent reaction