Topic 5A- Photosynthesis and Respiration

What two stages make up photosynthesis?

• Light Dependent Reaction (LDR)

• Light Independent Reaction (LIR)

Where does the LDR and LIR take place?

• LDR- Thylakoid membrane

• LIR- Stroma

What is photophosphorylation?

Adding a phosphate to a molecule using light energy

What is a coenzyme?

A molecule that aids the function of an enzyme

How do coenzymes work and what is an example of one in photosynthesis?

• The transfer a chemical group from one molecule to another

• NADP is a coenzyme used in photosynthesis

  • NADP transfers hydrogen from one molecule to another

  • It can reduced (give hydrogen) or oxidised (take hydrogen from) a molecule

What happens in the first stage of the light dependant rection/non-cyclic phosphorylation (photoionisation)?

• Light energy absorbed by chlorophyll

  • This excites electrons and they move up an energy level, then leave the chlorophyll charged

  • This is called photoionisation

  • Excited electrons move down the electron transport chain to PSI

• The positively charged chlorophyll needs to be discharged

• Light energy splits water into protons, electrons and oxygen

• The electrons discharge the chlorophyll

What happens in the second step of photosynthesis (photophosphorylation)?

• Energy is used to actively transport protons from the stroma into the thylakoid

  • This energy is provided by electrons that have lost energy while moving down the electron transport chain

• This establishes a proton gradient

• Protons diffuse down their concentration gradient by facilitated diffusion through ATP synthase 

• This produces ATP from ADP and Pi (photophosphorylation)

What happens in the third stage of the light dependent reaction (reduction of NADP)?

• Light energy absorbed by PSI

• Electrons excited to even higher energy levels

• Electrons transferred to NADP along with a proton from the stroma to form reduced NADP/NADPH

  • If NADP is reduced, PSI must have been oxidised

What is chemiosmosis?

• Movement of electrons down the electron transport chain is coupled with the transfer of protons into the thylakoid to build a proton gradient

• Movement of protons across a membrane generates ATP

What happens in cyclic photophosphorylation? (might need to rewrite this one)

• PSI absorbs light energy which excites electrons

• Electrons gain energy and move to a higher energy level

• Energy lost from electrons is used to actively transport protons into the thylakoid

• Protons diffuse down their concentration gradient by facilitated diffusion through ATP synthase to produce ATP from ADP and Pi

  • This is cyclic because electrons are passed to an electron acceptor, then to the electron transport chain

  • The electrons will then cycle back to PSI

What are the products of cyclic photophosphorylation?

• Only produces ATP

• No NADPH produced

What are the products at the end of the LDR?

• Oxygen

• ATP

• NADPH

What happens in the light-independent reaction?

• Ribulose biphosphate (a 5 carbon compound) joins with CO₂ catalysed by rubisco

• This forms an unstable 6 carbon compound

• This splits into 2 molecules of glycerate 3-phosphate

• Each Glycerate 3-phosphate is then reduced by NADPH (which is oxidised to NADP) to form 2 3 carbon compounds called triose phosphate, with energy provided from the hydrolysis of ATP into ADP and Pi

• 1/6th of the time, triose phosphate is used to make glucose

• 5/6th of the time, triose phosphate is used to regenerate ribulose biphosphate, meaning that the reaction has to turn 6 times to make 1 molecule of glucose

• The NADP and ADP + Pi produced go back to the LDR

Why to plants contain a range of photosynthetic pigments?

• In order to absorb as many varieties of light as possible

• This maximises how much photosynthesis can take place

How does light intensity affect the rate of photosynthesis, assuming that it is a limiting factor?

• As light intensity increases, the rate of photosynthesis increases

• Increasing light energy increases how much light energy can be absorbed for photoionisation and photolysis

  • This increases the rate of the LDR which makes products needed for the LIR, which in turn increases the rate of the LIR

  • More ATP and NADPH produced in a certain amount of time

• At the point that increasing light intensity has no effect on the rate of photosynthesis, something else like temperature of CO₂ concentration is limiting the rate of photosynthesis

Why is water never going to be a limiting factor of photosynthesis?

Only 1% of water uptake in a plant is used for photosynthesis

How does CO₂ concentration affect the rate of photosynthesis, assuming that it is a limiting factor?

• As CO₂ increases, the rate of photosynthesis increases

  • More CO₂ can be used for the LIR, which in turn increases the rate of the LDR as more ADP, Pi and NADP is produced

• When increasing the concentration no longer has an effect, there must be another limiting factor affecting the rate of photosynthesis

How does temperature affect the rate of photosynthesis, assuming it is a limiting factor?

• As temperature increases, the rate of photosynthesis increases

• This is because as enzymes and substrates gain more kinetic energy, there are more successful collisions between substrate and enzyme active sites so more products are formed

• Beyond the optimum temperature, the rate begins to decrease as hydrogen and ionic bonds begin to break due to a lot of vibration of atoms within the tertiary structure

• This causes the shape of the tertiary structure to change which denatures enzymes, so the rate of photosynthesis falls

What is decarboxylation?

Removing CO₂ from a molecule

What are the 4 stages of aerobic respiration and where do they take place?

• Glycolysis- cytoplasm

• Link reaction- matrix

• Krebs cycle- matrix

• Electron transport chain- cristae membrane

What part of aerobic respiration doesn’t require oxygen?

Glycolysis 

What happens in glycolysis

• Glucose is phosphorylated by hydrolysing 2 molecules of ATP that 2 provides phosphate groups

• This phosphorylated glucose then splits into 2 molecules of triose phosphate

• Both TP are oxidised by 2 molecules of NAD (which is reduced to NADH) which forms 2 molecules of pyruvate

• This process also forms 4 molecules of ATP in total (2 from each pyruvate), with a net gain of 2 ATP

What happens in the link reaction?

• Pyruvate is decarboxylated (carbon dioxide is removed)

• NAD is reduced to NADH as it collects hydrogens from the pyruvate which is oxidised 

• Pyruvate converted to acetate

• The acetate combines with coenzyme A to from acetyl-coA

What happens to the products of glycolysis?

• 2 NADH goes to the electron transport chain

• 2 pyruvate goes to the matrix for the link reaction

• A net gain of 2 ATP can be used for the cell

What are the products of the link reaction and what happens to them?

• CO₂ is a waste product

• NADH enter the electron transport chain

• Acetyl-coA enters the Krebs cycle

What happens in anaerobic respiration in general?

• There is no oxygen so the process stops at glycolysis

• This produces ethanol and carbon dioxide in plants and yeast, and lactic acid in animals and bacteria

• The production of both produces ATP so respiration can continue when oxygen is in low supply- small amounts of ATP are produced keeping biological processes going

What happens in alcoholic fermentation in plants and algae?

• Pyruvate is decarboxylated, releasing CO₂

• This forms ethanal

• Ethanal is then reduced by NADH (which is oxidised to NAD) to form ethanol

  • This NAD can then return to glycolysis in order to oxidised more triose phosphate into pyruvate

• The reduction of ethanal to ethanol allows for the coenzyme NAD to be regenerated which can return to glycolysis and ensure a net 2 ATP is produced

Why can alcoholic fermentation not continue indefinitely?

Ethanol is toxic and would kill the yeast/plant cells

What happens in lactic acid fermentation?

• Pyruvate is reduced into lactate (lactic acid)

• NADH is oxidised to regenerate NAD

  • NAD goes back to glycolysis to be reduced again, producing a net 2 ATP by oxidising TP

• This reaction is reversible

What problem can lactic acid build up cause?

• Lactic acid dissociates into lactate and H+ ions

• The increase in H+ ions decreases pH

• This causes the tertiary structure of proteins in muscle tissue to change as H+ ions interact with ionic and hydrogen bonds

• So proteins lose their tertiary structure

Where do the products of the link reaction go?

• 2 acetyl-coA goes to the Krebs cycle

• 2 carbon dioxide is a waste product

• 2 reduced NAD goes to the electron transport chain

What is the purpose of the Krebs cycle?

To produce as much NADH and FADH₂ as possible for the electron transport chain/oxidative phosphorylation

What happens in the Krebs cycle?

• Acetyl coA joins with a 4 carbon compound

• This produces a 6 carbon compound and coenzyme A is released

• This compound is oxidised and decarboxylated, forming a 5 carbon compound and carbon dioxide

  • This causes NAD to become reduced

• This 5 carbon compound is oxidised and undergoes decarboxylation to form a 4 carbon compound

• More NAD is reduced to NADH

• 1 molecule of ATP is produced by substrate level phosphorylation where a phosphate group is directly transferred from one molecule to ADP to form ATP

• More NAD and FAD is reduced to form NADH and FADH₂

What are the products of the Krebs cycle and where do they go?

• 1 coenzyme A goes back to the Link reaction

• The 4 carbon compound is reused in the Krebs cycle

• 2 carbon dioxide are waste products that could be used in photosynthesis

• 1 molecule of ATP is produced which can be used by the cell

• 3 reduced NAD and 1 reduced FAD is made when can be used for oxidative phosphorylation in the electron transport chain

What happens in oxidative phosphorylation?

• Reduced NAD and reduced FAD are oxidised- NAD and FAD return to link and Krebs

  • Hydrogen released splits into H+ and e-

  • Electrons reduced the first carrier protein and move down the ETC, releasing energy

• Energy is used to actively transport the protons from the matrix into the intermembrane space

• This builds up a proton gradient across the membrane which means there is a higher concentration gradient of protons in the intermembrane space than the matrix

• Protons diffuse through ATP synthase by facilitated diffusion down their concentration gradient, producing ATP from ADP and Pi (oxidative phosphorylation)

• Oxygen joins with protons and electrons to form water

  • Oxygen is final electron acceptor in the electron transport chain

What can we say about the gradient that protons move down in oxidative phosphorylation?

They move down an electrochemical gradient

Why is oxygen needed for the ETC?

• If there is no oxygen, link, Krebs and the ETC will stop

• This is because there would be no oxygen to accept electrons- reduced NAD and FAD cannot be oxidised in the ETC so they are not recycled