A2 Biology - Unit 4: On The Wild Side

Photosynthesis stores energy as glucose, via a series of reactions which use light to break apart strong bonds in water molecules.

6(CO2) + 6(H2O) + Energy = C6H12O6 (glucose) + 6(O2)

Cells release energy from glucose via respiration (aerobic/anaerobic)

C6H12O6 (glucose) + 6(O2) = 6(CO2) + 6(H2O) + Energy

ATP IS IMMEDIATE SOURCE OF ENERGY IN A CELL

ATP is broken down into ADP and an inorganic P molecule using ATPase, the reaction is exothermic (give off energy). ATP needs to be resynthesised, but the reaction is endothermic (requires energy).

Photosynthesis occurs in the chloroplasts (small, flattened organelles). Chloroplasts have a double membrane and contain thylakoid stacks called grana, The grana are linked together with lamellae.

Chloroplasts contain photosynthetic pigements called chlorophyll. These are found in the thylakoid membranes and are attached to proteins. The protein and the pigment together are called photosystems. Photosystem 1 absorbs 700nm light best and photosystem 2 absorbs 680nm light best.

Phosphorylation - Adding phosphate to a molecule (ADP is phosphorylated into ATP)

Photophosphorylation - Adding of a phosphate molecule using light

Photolysis - Splitting (lysis) of a molecule using light

Hydrolysis - Splitting of a molecule using water (ATP hydrolysed to ADP)

Redox Reactions - Reactions that involve oxidation (losing of electrons) and reduction (gaining of electrons)

Chloroplast is adapted for photosynthesis because; thylakoids have large surface area = more light energy to be absorbed, lots of ATPsynthase present in thylakoids, stroma contains all enzymes needed for light independent reaction and chloroplast envelope keeps reactants fot photosynthesis close to reaction sites.

ONLY PRODUCES ATP + ONLY USES PHOTOSYSTEM 1 (PS1)

Electrons from chlorophyll molecules aren't passed onto NADP, but are passed back to PS1 via electron carriers. So electrons are recycled and repeatedly flow through PS1.

Electrons are excited by 700nm light energy absorbed by PS1, the excited electrons are sent to the electron transport chain, as they travel down the ETC the electrons lose energy. This lost energy is used to transport protons into the thylakoid, forming a proton gradient across the membrane. The energy from this movement of phosphates causes ATP synthase to resynthesis ADP back to ATP.

However PS1 only produces a small amount of ATP.

Occurs in Photosystem 2 in 4 stages:

1) Light energy excites electrons in chlorophyll. Absorbed light energy excites electrons, causing them to have a higher energy level, which then move down the ETC.

2) Photolysis of water, produces H ions and Oxygen. Used ot replace lost electrons, light energy spilts water into protons (H ions), electrons and Oxygen. H20 = (2H+)+(1/2O2)

3) Energy from excited electrons make ATP. Excited electrons travel down the ETC and the electrons lose energy. This lost energy is used to transport protons into the thylakoid, forming a proton gradient across the membrane. The energy from this movement of phosphates causes ATP synthase to resynthesis ADP back to ATP.

4) Generates reduced NADP. Light energy from PS1 excites electrons to an even higher energy level. The electrons are then tranferred to NADP along with a proton (H ion) to form reduced NADP.

Stage 1: Carbon Fixation

Co2 combines with RuBP to form an unstable 6Carbon molecule,which immediately breaks down into 2 molecules of glycerate 3-phosphate (GP).Combination of Co2 and RuBP is catalysed by RUBISCO (ribulose biphosphate carboxylase).

Stage 2: ATP and NADP reduce GP to a triose phosphate.

ATP uses energy to reduce GP to triose phosphate (TP). The reaction also requires reduced NADP, which adds a H ion to the TP. The NADP is then recycled. The TP is converted into glucose and other useful organic compounds.

Stage 3: Regeneration

Regenerating RuBP requires more energy via ATP. 5 out of every 6 molecules of TP arent used to make hexose sugars but are used to regenerate RuBP. 

Needs to turn 6 times in order to make one glucose molecule. 3 turns of the calvin cylce produces 6 TP molecules (2 TP formed for every 1 Co2). However 5/6 of the TP molecules are used in regeneration of RuBP.

So for every 3 turns of the calvin cycle, only one molecule of TP is produced to make glucose with. Glucose however needs 6C, so 2 TP molecules are needed, meaning the calvin cycle has to turn 6 times to produce one glucose molecule,

6 turns of the calvin cycle requires; 18 ATP and 12 reduced NADP molecules.