11 Photosynthesis and 12 Respiration Flashcards
Covers topics of photosynthesis and respiration
Describe the steps of the light-dependent reaction
- Light energy excites electrons in a chlorophyll molecule and the chlorophyll molecule becomes photoionised.
- The electrons move along the electron carriers in the electron transport chain (in the thylakoid membrane) releasing some of their energy to these carriers.
- This energy is used to pump protons (H+ ions) against their concentration gradient in the thylakoid disc.
- The hydrogen ions then diffuse out of the disc (chemiosmosis) through a molecule of ATP synthase which causes ADP and Pi to join and form ATP.
- NADP is then reduced by the lower energy electrons from the electron transport chain and hydrogen ions to form NADPH.
- Photolysis of water produces protons, electrons and oxygen, these electrons then replace those lost by the chlorophyll. ATP and NADPH then go on to the Calvin Cycle (light independent reactions).
Describe the light independent reaction of photosynthesis
- In the Calvin Cycle, carbon dioxide combines with a 5 carbon RuBP to form an unstable 6 carbon molecule. This immediately splits into two 3 carbon Glycerate Phopshate molecules (GP).
- Then the GP molecules are reduced to triose phosphate using NADPH and energy from ATP.
- Triose Phosphate molecules can then be converted into two useful organic substances like glucose.
- Two triose phosphate molecules are needed to make glucose. The RuBP must be regenerated otherwise photosynthesis would stop.
- If the Calvin Cycle turns 6 times, 12 molecules of TP are made, 2 of which are useful and 10 of which would be needed to regenerate the 6 molecules of 5 carbon RuBP.
What are the main limiting factor of photosynthesis
light intensity, enzymes, CO2, Water, minerals, magnesium ions, temperature, chlorophyll
Outline what the four stages of aerobic respiration are.
- Glycolysis- the splitting of the 6 carbon glucose molecule into 2x 3 carbon pyruvate molecules.
- Link reaction- The 3-Carbon pyruvate molecules enter into a series of reactions which lead to the formation of acetylcoenzyme A, a 2 carbon molecule.
- Krebs cycle- the introduction of acetylcoenzyme A into a cycle of oxidation-reduction reactions that yield some ATP and a large quantity of reduced NAD and FAD.
- Oxidative phosphorylation- the use of the electrons, associated with reduced NAD and FAD, released from the Krebs cycle to synthesise ATP with water as a produced by-product.
Describe the main stages of glycolysis and its products. Where does it take place?
- Phosphorylation of glucose to glucose phosphate- Glucose must be made reactive before it can split into two, so two phosphate molecules are added on. These come from the hydrolysis of 2ATP. This provides energy to activate glucose and lowers the activation energy for the enzyme-controlled reactions that follow.
- Splitting of phosphorylated glucose- Each glucose molecule is split into 2 three carbon molecules known as triose phosphate.
- Oxidation of triose phosphate- Hydrogen is removed from each of the two triose phosphate molecules and is transferred to a hydrogen-carrier molecule known as NAD to for, reduced NAD.
- Production of ATP- Enzyme-controlled reactions convert each triose phosphate into another 3 carbon molecule called pyruvate. In this process two molecules of ATP are regenerated from ADP.
NET PRODUCTION OF ATP= 2
GROSS PRODUCTION OF ATP= 4 (2 are used in phosphorylation of glucose)
Other products: 2x reduced NAD and 2x pyruvate
Takes place in the cytoplasm.
Outline the events of the link reaction
The pyruvate molecules are first actively transported into the mitochondrial matrix.
1. Pyruvate is oxidised to acetate, by losing a carbon dioxide molecule and 2H. These hydrogens are accepted by NAD to form reduced NAD (NADH), which is late used to produce ATP.
2. The 2-carbon acetate combines with a molecule called coenzyme A (CoA) to produce a compound called acetylcoenzyme A.
Overall equation: pyruvate + NAD + CoA = acetylcoenzyme A + reduced NAD + Carbon dioxide.
Describe the events of the Kreb’s Cycle
- A 2-Carbon acetate group from the acetylcoenzyme A combines with a 4-Carbon molecule to form a 6-Carbon molecule.
- This molecule is then decarboxylated and dehydrogenated back to a 4-Carbon molecule.
- This yield 3 molecules of reduced NAD, one molecule of reduced FAD and two carbon dioxide molecules per turn of the cycle.
- One molecule of ATP is formed via a substrate level phosphorylation.
How many ATPs in theory should be created from the whole of aerobic respiration? And why is this not always the case?
38
Some energy is lost as heat whilst transporting ATP.
Leakage of H+ through phospholipid belayer into matrix, meaning that less energy is generated as they are not leaving through the ATP synthase molecule.
ATP is used to actively transport pyruvate into the mitochondrial matrix.
Define respiratory substrate
An organic substance that can be used for respiration.
Explain why lipids are so energy dense.
They have a high proportion of hydrogens to carbons, meaning that more of reduced NAD (coenzyme) are produced and therefore ATP is generated.
What happens to excess amino acids that are not needed?
They are deaminated in the liver, this involves the removal of the amine group and it’s conversion to urea, which is then excreted by the kidneys. The rest of the molecule is converted to glycogen or fat. However, in periods of starvation or prolonged exercise, proteins from muscle can be converted to pyruvate which can then be respired.
Explain the effect of a lack of oxygen on the process of oxidative phosphorylation
- Oxygen acts as the terminal electron receptor in the ETC, meaning that it combines with protons and electrons to form water.
- Therefore electrons would not be able to be transported down the ETC.
- Reduced NAD and FAD would not be oxidised.
- Protons cannot be moved into the inter membranal space to establish the electrochemical gradient.
- Flow of protons through the ATP synthase would slow/stop.
ATP synthesis would therefore slow/stop.
Explain the effect of the lack of oxygen in the mitochondria during the Link reaction and the Kreb’s Cycle
Reduced NAD and FAD are not reoxidised back to NAD and FAD, the pool of oxidised coenzymes in the cell would be reduced.
Steps involving dehydrogenation reactions in the Link reaction and Krebs cycle are unable to occur.
Describe the stages of mammal anaerobic respiration.
- Glycolysis- glucose is phosphorylated, splits into two triose phosphates, removal of 2H to form reduced NAD, the oxidation of triose phosphate to pyruvate.
- The enzyme lactate dehydrogenase converts pyruvate to lactic acid (hydrogens are removed from reduced NAD to form oxidised NAD.
Describe the stages of fermentation that occurs in plants and yeast.
- Glycolysis- glucose is phosphorylated, then splits into two triose phosphate molecules, 2H leave and form reduced NAD. Triose phosphate is then oxidised to become pyruvate.
- The enzyme pyruvate decarboxylase converts pyruvate to ethanal, then ethanol dehydrogenase converts this to ethanol.
