Chapter 11 Photosynthesis

Adaptations of the leaf structure

• large surface area - absorbs as much sunlight possible
• specific arrangement of leaves - minimises overlapping so avoids shadowing
• Thin - diffiusion distance for gases is keot short so rate of photsynthesis increases
• transparent cuticle and epidermis  - lets light through the mesophyll cells.
• long narrow upper mesophyll cells - packed with chloroplasts 
• numerous stomata - for gaseous exhange
• Air spaces - located in the lower mesophyll layer to allow rapid diffusion 
• network of phloem

Structure and role of chloroplasts in photosynthesis

• Grana - stacks of thylakoids where the light dependent stage of photosynthesis occurs

Thylakoids contain chlorophyll which is a photosynthetic pigment that absorbs sunlight. Some thylakoids contain intergranal lamellae which  acts as tubular extensions to join up with other thylakoids in the adjacent grana. 

Main purpose: synthesise reduced NADP and ATP to supply fot the light independent reaction. 

In this process, light energy is used to add an inorganic phosphate to ADP and split water into H+ ions and OH- ions

Process:

• In the non-cyclic phosphorylation, photosystem II absorbs photons from light. This causes the electrons to get excited since they absorb more energy
• The elctron leaves PS II (oxidation) and gets taken by the electron carrier(reduction) 
• The electron passes through the electron transport chain in a series of redox reactions until it reaches the ATP synthase to be used in the process ADP + Pi = ATP
• At the same time, the photolysis of water occurs producing H+ ions, oxygen, and electrons. 
• Some of these electrons are taken by PS II to compensate for the lost electrons and the rest are taken by NADP to produce reduced NADP. 
• The H+ ions passes through the proton pumps to get pumped intro the stroma for chemiosmosis
• In the cyclic phosphorylation. PSI absorbs light energy. Electrons get excited causing photoionisation. 
• The electron is taken up by NADP to produce reduced NADP  and the rest are taken by the ATP synthase to produce ATP. 

Main purpose: Pump H+ ions to the stroma

• Proton pumps actively transport protons from the stroma to the thylakoid space. 
• Due to photolysis of water, the concentration of H+ ions increases in the thylakoid space. 
• this creates a concentration gradient of H+ ions from the thylakoid space to the stroma. 
• The thylakoid space has a high concentration while the stroma has a low concentration
• the H+ ions then needs to cross the thylakoid membrane to cross to the stroma through ATP synthase channels
• As protons pass through ATP synthase channels. a change in the enzyme structure is caused which catalyses the combination of ADP and Pi.

Main purpose: to use reduced NADP and ATP to make glucose. This takes place in the stroma of chloroplasts

Process:

• Carbon dioxide enters the leaf and diffuses into the stroma. 
• CO2 combines with ribulose biphosphate which is catalyse by RuBisCo
• This produces a 6 carbon compound which breaks down into 2 molecules of glycerate 3 - phosphate
• Reduced NADP is used to reduce glycerate 3 -phosphate into triose phosphate
• one molecule of triose phosphate is converted to glucose using ATP
• 5 out of every 6 molecules of triosphosphate is used to regenerate RUBP which uses the rest of ATP

NOTE: The cycle must turn 6 times in order to make one hexose sugar because for every 3 turns, 6 TPs are produced but only one is used to make glucose.