Topic 5 - Energy Transfers

Question 1

How is the structure of a leaf adapted for efficient photosynthesis?

Answer 1

• A large surface area that absorbs as much sunlight as possible
• Thin, as most light is absorbed in the first few micrometres of the leaf and diffusion distance for gases is kept short.
• Numerous stomata for gaseous exchange so that all mesophyll cells are only a short distance from one.

Question 2

Give an outline of what happens during photosynthesis:

Answer 2

Overall equation is 6CO2 + 6H2O —> C6H12O6 + 6O2
Stage 1 = Capturing of light energy by chlorophyll
Stage 2 = Light dependent reaction reducing NADP, ATP and oxygen
Stage 3 = Light independent reaction - H+ ions used to produce sugars

Question 3

Can you explain how you would use chromatography to investigate pigments separated from leaves of different plants?

Answer 3

• Cut a section of a leaf and place in a mortar. Add 20 drops of acetone and use the pestle to grind up the sample and release the pigments.
• Use a capillary tube to extract some of the pigment and blot it onto the centre of the pencil line approx. 1cm above the bottom of your filter paper.
• Suspend the paper in chromatography solvent until it has run up the paper to near the top.
• Remove the paper from the solvent and draw a pencil line marking where the solvent moved up to. The pigment should have separated out and there should be different spots on the paper at different heights above the pencil line.
• Calculate the Rf value for each spot.

Question 4

Describe oxidation and reduction.

Answer 4

Oxidation = When a substance gains oxygen or loses hydrogen (loses electrons)
Reduction = When a substance loses oxygen or gains hydrogen. (gains electrons)

Question 5

How is ATP produced?

Answer 5

• Light energy excites electrons and they are removed from chlorophyll
• They then move along the electron transfer chain and undergo a series of redox reactions where energy is released at each stage.
• Energy released is used to join ADP and Pi to form ATP at an ATP synthase channel within the membrane.

Question 6

What happens during photolysis?

Answer 6

• The process of splitting water within the thylakoid
• H2O —> H2 + O2 + e-
• E- produced replaces electron in PSII
• Reduced H+ helps with the chemiosmotic gradient

Question 7

Describe the chemiosmotic theory

Answer 7

Difference in H+ concentration across a membrane results in a chemical gradient.
ATP synthase enzymes embedded in the thylakoid membrane allows for the formation of ATP from ADP and Pi as long as the gradient is there.

Question 8

How are the chloroplasts adapted to carry out the LDR?

Answer 8

• Thylakoids provide large SA for chlorophyll, photosystems, enzymes and electron carriers

Question 9

Explain the steps of the Calvin cycle:

Answer 9

Stage 1 - Carbon Fixation. RuBP combined with Co2 catalysed by RUBISCO. This forms 2xGP
Stage 2 - Reduction - GP is reduced to triose phosphate. ATP provides energy and NADPH provides 2 electrons and hydrogen ions
Stage 3 - Regeneration - 5/6 triose phosphate used to regenerate RuBP (uses ATP) and 1/6 triose phosphate used in biosynthesis to produce sugars, lipids or amino acids etc…

Question 10

How is the chloroplast adapted to carry out the light independent reaction?

Answer 10

The fluid of the stroma contains all the enzymes necessary for the light independent reaction to occur.

Question 11

Explain the law of limiting factors?

Answer 11

The rate of a physiological process will be limited by the factor which is in shortest supply.

Question 12

How does light intensity affect the rate of photosynthesis?

Answer 12

Increasing light intensity will increase rate of photosynthesis until the graph plateaus showing increasing light intensity has no effect on the rate of photosynthesis as rate is limited by lack of chlorophyll, water or carbon dioxide.

Question 13

How to investigate the effect of a factor on the rate of photosynthesis:

Answer 13

• Use aquatic plant to measure volume of oxygen as rate of respiration = rate of photosynthesis..
• Place plant in a test tube with potassium hydrocarbonate solution to provide plant with CO2
• Put test tube in water bath maintaining constant temp
• A source of light whose intensity can be adjusted.
• Apparatus left in dark for 2 hours before
• Light source for 30 mins to allow air spaces in the leaves to fill with oxygen.
• Oxygen released collects in capillary tube
• Draw up gas into the syringe and measure the volume in mm3
• Repeat 4/5 times and calculate mean. Then repeat steps with different intensities to calculate means then plot your graph.

Question 14

What are the main stages of glycolysis?

Answer 14

Takes place in the cytoplasm of the cell and involves trapping glucose in the cell by phosphorylating the molecule and then splitting the glucose molecule in 2.
Stage 1 - Glucose is activated by phosphorylation
Stage 2 - Phosphorylated glucose splits into 2x triose phosphate molecules.
Stage 3 - Oxidation and dephosphorylation of triose phosphates produces 2 x pyruvates.
Stage 4 - Production of ATP as the dephosphorylation of triose phosphates produces 2 x ATP each but because of the energy investment in the phosphorylation the net production is 2 x ATP

Question 15

What are the stages of respiration?

Answer 15

Glycolysis
Link Reaction
Krebs cycle

Question 16

What are the products of glycolysis?

Answer 16

2 Pyruvate (3C) molecules
Net gain 2 ATP
2 Reduced NAD

Question 17

What happens during the link reaction?

Answer 17

• Takes place in the matrix of the mitochondria
• Pyruvate is oxidised by enzymes to produce acetate and carbon dioxide, requiring the reduction of NAD to NADH
• Combination with coenzyme A to form acetyl coenzyme A
  Produces : Acetyl CoA, CO2, Reduced NAD (NADH)

Question 18

What happens during the Krebs cycle?

Answer 18

• 4C molecule regenerated in every cycle.
• Acetyl CoA + 4C = 6C molecule citrate.
• Reduced NAD and CO2 and ATP products

Question 19

Why is the Kreb’s cycle so significant?

Answer 19

To produce energy, stored and transported as ATP or GTP

Question 20

What is the role of the mitochondrion in respiration?

Answer 20

The electron transport chain is a series of four protein complexes that couple redox reactions, creating an electrochemical gradient that leads to the creation of ATP in a complete system named oxidative phosphorylation. It occurs in mitochondria in both cellular respiration and photosynthesis.

Question 21

How is ATP synthesised in the electron transfer chain (in respiration)

Answer 21

NAD+ and FAD+ are reduced which produces electrons and they are passed from one molecule to another in the ETC, and energy released in these electron transfers in the form of H+ ions which form an electrochemical gradient. In chemiosmosis, the energy stored in the gradient is used to produce ATP via ATP synthase.

Question 22

What is the role of oxygen in aerobic respiration?

Answer 22

Allows for oxidative phosphorylation. In the presence of oxygen, pyruvate can be reduced to acetate in the link reaction and then used in the krebs cycle and then ETC, producing ATP. In the absence of oxygen, the pyruvate is broken down into lactate.

Question 23

How is lactate produced in animals?

Answer 23

Anaerobic respiration : In the absence of oxygen, the pyruvate is reduced into lactate.

Question 24

How is ethanol and carbon dioxide produced in plants and some micro-organisms?

Answer 24

Ethanol and carbon dioxide are produced in plants and some microorganisms through a process called fermentation, which is a type of anaerobic respiration:

In fermentation, microorganisms like yeast, bacteria, and fungi break down sugars like glucose, fructose, and sucrose to produce ethanol and carbon dioxide.

Question 25

Can you explain how energy is released by anaerobic respiration?

Answer 25

Each molecule of glucose ultimately yields 2 lactic acid molecules and 2 molecules of ATP which can then be used for muscle contraction.