Energy Transfers

Question 1

What are the stages of photosynthesis and where does each occur?

Answer 1

1. Light dependent reaction - Occurs in the thylakoid membrane of chloroplasts
2. Light independent reaction - Occurs in the stroma of chloroplasts

Question 2

Describe what happens during the LDR.

Answer 2

1. Chlorophyll absorbs light energy which excites its electrons to a higher energy level, and therefore the electrons leave the chlorophyll, meaning chlorophyll is oxidised, becomes positively charged.
2. Electrons move along electron transfer chain, releasing energy
3. This energy is used to actively pump protons (H+ ions) from across the thylakoid membrane into the thylakoid space, creating a proton gradient across the thylakoid membrane
4. Protons move by facilitated diffusion down electrochemical gradient into stroma via ATP synthase (Chemiosmosis). This phosphorylates ADP to form ATP (Photophosphorylation)
5. NADP takes up a proton and an electron to become reduced NADP, which moves to the stroma for the light independent reaction
6. Light causes water to split inti protons, electrons and oxygen (Photolysis). The electrons are used to replace those lost during photoionisation, the protons form part of the proton gradient that is used to form ATP and reduce NADP, and O2 is released as a by-product

Question 3

Describe the light-independent reaction of photosynthesis (Calvin cycle)

Answer 3

1. CO2 reacts with ribulose bisphosphate (RuBP), catalysed by the enzyme rubisco. This forms an unstable 6 Carbon molecule.
2. This dissociates into 2 Glycerate 3-Phosphate (GP) molecules
3. GP is reduced to triose phosphate (TP) using reduced NADP and energy from ATP (from the LDR)
4. Some TP is converted to useful organic substances such as glucose, but most of the TP is used to regenerate RuBP (using energy from ATP)

Question 4

Describe and explain how temperature affects rate of photosynthesis

Answer 4

As temperature increases, rate of photosynthesis increases. As temperature increases, enzymes such as Rubisco gain kinetic energy, so there are more frequent successful collisions and more enzyme-substrate complexes form. However, if the temperature rises above the optimum temperature, rate decreases. At temperatures that are high above the optimum, enzymes denature as H bonds in tertiary structure break, so fewer enzyme-substrate complexes form

Question 5

Describe and explain how light intensity affects rate of photosynthesis

Answer 5

As light intensity increases, rate of photosynthesis increases. As light intensity increases, the rate of light-dependent reaction increases, so more ATP and reduced NADP is produced. Therefore, the rate of the light-independent reaction increases as more GP reduced to
TP and more TP regenerates RuBP. When the light intensity reaches a certain light intensity, rate stops increasing because another factor is limiting eg. temperature / CO2 concentration

Question 6

Describe and explain how CO2 concentration affects rate of photosynthesis

Answer 6

As CO2 concentration increases, rate of photosynthesis increases. As CO2 concentration increases, the light-independent reaction increases because more CO2 combines with RuBP to form GP, so there is more GP reduced to TP, and more TP is converted to organic substances and more RuBP is regenerated. When the CO2 concentration reaches a certain level, the rate stops increasing because another factor is limiting eg. temperature / light intensity

Question 7

What factors affect rate of photosynthesis?

Answer 7

Temperature, Light Intensity and Carbon Dioxide concentration.

Question 8

Why is respiration important?

Answer 8

Respiration produces ATP which is hydrolysed to release energy. This energy is used for important processes like protein synthesis and active transport

Question 9

What are the stages of aerobic and anaerobic respiration and where do they occur?

Answer 9

• Aerobic Respiration
  1. Glycolysis - cytoplasm (anaerobic)
  2. Link reaction - mitochondrial matrix
  3. Krebs cycle - mitochondrial matrix
  4. Oxidative phosphorylation - inner
  mitochondrial membrane
• Anaerobic Respiration
  1. Glycolysis - cytoplasm
  2. NAD regeneration - cytoplasm

Question 10

Give 2 similarities and 2 differences between Chloroplasts and Mitochondria.

Answer 10

Similarities
* Both organelles are double membrane bound
* Both organelles contains their own DNA and ribosomes
Differences
* The fluid inside the organelles is called the matrix in mitochondria and is called the stroma in chloroplasts
* Chloroplasts contain thylakoid membrane and grana, whereas mitochondria contain cristae

Question 11

Describe the process of glycolysis

Answer 11

Glucose is phosphorylated to glucose phosphate using inorganic phosphates from 2 ATP molecules. This glucose phosphate is hydrolysed to 2 x 3C triose phosphate molecules, which are then oxidised to 2 x 3C pyruvate, and the hydrogen removed is transferred to the co-enzyme NAD to form 2 x reduced NAD. 4 molecules of ADP are phosphorylated, forming ATP. Therefore, overall net gain is 2 x ATP, 2 x reduced NAD and 2 x 3C Pyruvate