Chapter 17: Energy for Biological Processes

Question 1

Explain the importance of the Carbon-Hydrogen bond in small inorganic molecules (e.g. H2O, CO2).

Answer 1

• Joined by strong bonds.
• Release lots of energy when formed.
• Require lots of energy to break.

Question 2

Explain the importance of the Carbon-Hydrogen bond in large organic molecules (e.g. glucose, amino acids).

Answer 2

• Joined by more + weaker bonds than inorganic.
• Release less energy when formed.
• Require little energy to break.

Question 3

Define respiration.

Answer 3

• The process by which large organic molecules are broken down into small inorganic molecules.
• Process by which organisms break down biomass necessary to provide ATP necessary for metabolic reactions that take place in cells.
• Forms ATP –> used to supply energy needed to break bonds in metabolic reactions of the cells.

Glucose + Oxygen —–> CO2 + H2O

Question 4

Define photosynthesis.

Answer 4

• Process by which energy in the form of light from the sun is used to build complex organic molecules such as glucose.

CO2 + H2O —–> Glucose + Oxygen

• Energy used to form chemical bonds in ATP –> broken down to release energy needed to make bonds as glucose is formed.

Question 5

Define exothermic.

Answer 5

• Heat transferred from system to surroundings.

- Energy released when bonds formed.

Question 6

Define endothermic.

Answer 6

• Heat transferred from surroundings to system.

- Energy taken in when bonds broken.

Question 7

Outline chemiosmosis.

Answer 7

• Process by which ATP synthesised for photosynthesis + respiration.
• Involves diffusion of protons from region of high conc. to low conc. through partially permeable membrane down proton conc. gradient.
• Movement of protons down conc. gradient releases energy used in attachment of inorganic phosphate to ADP forming ATP.

Question 8

How are electrons excited to higher energy levels?

Answer 8

• Electrons present in pigment molecules (e.g. chlorophyll) –> excited by absorbing light from sun.
• High energy electrons released when chemical bonds broken in respiratory substrate molecules (e.g. glucose).

Question 9

How is the proton conc. gradient created in the electron transport chain?

Answer 9

• Electron transport chain = made up of series of electron carriers each with progressively lower level.
  1. High energy electron move from one carrier chain to another energy is released.
  2. Energy released –> used to pump protons across membrane.
  3. Create conc. difference across membrane + therefore proton gradient.
  4. Proton gradient maintained as a result of the impermeability of the membrane to H+ ions
  5. Only way protons move back through membrane down conc. gradient is through hydrophilic membrane channels linked to ATP synthase enzymes

Question 10

What is the difference between autotrophic and heterotrophic organisms?

Answer 10

• Autotrophic = capable of photosynthesis –> e.g. plant, algae.
• Heterotrophic = obtain complex organic molecules by eating/consuming other organisms.

Question 11

Outline the structure of chloroplasts.

Answer 11

• Pigment –> chlorophyll –> absorb light –> embedded within thylakoid membranes.
• Network of membranes –> large s.a. to vol for increased absorption of sunlight.
• Thylakoids –> flattened sacs –> stack together to form grana.
• Grana joined by lamellae.
• Stroma –> fluid within chloroplasts where most chemical reactions happen –> form complex organic molecules.

Question 12

Function of chlorophyll?

Answer 12

• Absorb specific wavelengths of light and reflect others.
• Absorb red + blue light but reflect green hence the green plant colour.
• Primary pigment is chlorophyll a.

Question 13

Outline the light harvesting system that chlorophyll b, xanthophyll and carotenoids are a part of.

Answer 13

1. Chlorophyll b pigments are in photosystem/antenna complex.
2. Light energy absorbed by chlorophyll b pigment.
3. Electron excited and moves to a higher energy level then returns to pigment.
4. Energy passed from one pigment to another.
5. Energy passed to reaction centre/chlorophyll a/PSI/PSII.
6. Range of pigments allow range of wavelengths to be absorbed.

Question 14

Why do plants turn yellow/orange when there is intense sunlight or shorter days + cooler nights?

Answer 14

• Intense sunlight –> destroys chlorophyll
• Shorter days + cooler nights –> chlorophyll no longer produced
• Carotenoids responsible for yellow/orange colour (normally masked by green colour) –> still produced in these conditions + not broken down by intense sunlight.

Question 15

When is anthocyanin produced and what does it do?

Answer 15

• Produced when sugar conc. is high.
• pH dependent.
• Responsible for red/purple pigment formed from reactions between sugars and proteins present in cell sap.
• Production promoted by strong light intensity.
• Act as sunscreen –> absorb UV light + blue-green light –> prevent chlorophyll destruction.
• Camouflage leaves from herbivores blind to red wavelength.

Question 16

Define the light-dependent stage of photosynthesis.

Answer 16

• Energy from sunlight absorbed + used to form ATP.

- Hydrogen from water reduces coenzyme NADP to form reduced NADP

Question 17

Define the light-independent stage of photosynthesis.

Answer 17

• Hydrogen from reduced NADP used to build complex organic molecules.
• ATP supplies the energy.

Question 18

Outline the process of non-cyclic phosphorylation (part of light-dependent stage).

Answer 18

1. Absorbed light –> excites electrons at reaction centres of the photosystem.
2. Excited electrons –> released from PSII reaction centre and transferred to electron transport chain (ETC) –> produce ATP by chemiosmosis.
3. Released electrons replaced at PSII reaction centre by water molecules that have been broken down by sun energy.
4. Excited electrons –> released from PSI reaction centre and transferred to another electron transport chain –> produce ATP by chemiosmosis.
5. Released electrons replaced at PSI by electrons that have just been released from PSII into and have travelled along the first ETC.
6. Electrons leaving ETC following PSI –> accepted by coenzyme NADP forming reduced NADP.
7. Reduced NADP –> provides reducing power or hydrogen necessary to build complex organic molecules such as glucose.

Question 19

Outline the process of photolysis.

Produces O2

Answer 19

• Oxygen evolving complex that forms a part of PSII reaction centre is an enzyme that catalyses break down of water:
• Here water is broken down into H+ ions, electrons and oxygen molecules using sun energy.
  1. Protons released into lumen of thylakoids –> increasing proton conc. across membrane.
  2. As protons move across membranes down proton conc. + electrochemical gradient they release energy used to make ATP.
  3. Once H+ ions are returned to stroma they combine with NADP and a single electron to form reduced NADP.
  4. Reduced NADP –> involved in light-independent stage –> removes H+ ions from stroma in order to maintain proton conc. gradient across thylakoid membrane.

Question 20

Outline cyclic phosphorylation.

Answer 20

• When electrons leaving ETC after PSI can be returned to PSI instead of being used to form reduced NADP.
• ATP still produced but without any electrons being supplied from PSII.

Question 21

Outline the steps of the Calvin cycle –> a part of the light-independent stage of photosynthesis. (9 marks)

Calvin cycle produces carbohydrates/protein/lipid.

Answer 21

1. CO2 enters intercellular spaces between spongy mesophyll by diffusion from atmosphere through stomata.
2. CO2 diffuses into cells + stroma of chloroplasts where its combined with ribulose biphosphate (RuBP) –> 5 carbon molecule.
3. Carbon in CO2 is fixed –> it is incorporated into an organic molecule.
4. Enzyme ribulose biphosphate carboxylase (RuBisCO) —> inefficient enzyme (competitively inhibited by O2) –> catalyses reaction –> unstable 6 carbon intermediate formed.
5. Unstable six carbon compound formed breaks down immediately –> forms two 3 carbon glycerate 3-phosphate (GP) molecules.
6. Each GP converted to another 3 carbon molecule –> triose phosphate (TP) –> using H atom from reduced NADP + energy supplied by ATP.
7. TP –> carbohydrate, 3 carbon sugar –> used to regenerate RuBP.

Question 22

Summarise Calvin cycle very concisely. (3 marks)

Answer 22

1. Fixation –> CO2 is fixed (incorporated into an organic molecule) in the first step.
2. Reduction –> GP reduced to TP by addition of Hydrogen from reduced NADP using energy supplied from ATP.
3. Regeneration –> RuBP regenerated from recycled TP.

Question 23

Outline photorespiration.

Answer 23

• RuBisCO is competitively inhibited by O2 –> leads to phosphoglycolate + reduced GP production –> only happens when CO2 conc. low:
• e.g. high temp + low humidity –> high water loss so stomata close –> prevent CO2 entry but photosynthesis continues –> CO2 levels fall + O2 levels rise.
• Phosphoglycolate –> toxic 2 carbon compound –> converted to other organic molecules using energy from ATP.
• RuBisCO –> higher CO2 affinity than O2.

Question 24

Define limiting factor with regards to photosynthesis.

Answer 24

• When factor needed for photosynthesis is in short supply it reduces ROR of photosynthesis and becomes a limiting factor.

Question 25

How can light intensity be a limiting factor?

Answer 25

• Light needed as energy source.

- As light intensity increases ATP + reduced NADP produced at a higher rate + vice versa.

Question 26

How can CO2 conc. be a limiting factor?

Answer 26

• CO2 –> carbon source.

- Increasing CO2 conc. –> increases rate of carbon fixation in Calvin cycle + increases triose phosphate production.

Question 27

How can temperature be a limiting factor?

Answer 27

• Affects rate of enzyme controlled reactions.
• As temp increases –> enzyme activity increases up until temp at which proteins denature.
• Increase in temp –> increases rate of carbon fixation
• Photorespiration rate also increases above 25 degrees celsius but higher rates not seen at higher temps even if enzymes are not denatured.

Question 28

State the law of limiting factors

Answer 28

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

Question 29

What is the effect of reducing light intensity on the Calvin cycle?

Answer 29

• Reduce rate of light dependent stage –> less ATP + reduced NADP produced.
• GP conc. increase so TP conc. decrease
• Less RuBP regenerated due to there being less TP so lower RuBP conc.

Question 30

What is the effect of low CO2 conc. on the Calvin cycle?

Answer 30

• Low GP conc. + TP conc. –> less CO2 to be fixed

- RuBP conc. increases as it is still being formed from TP but not being used to fix CO2

Question 31

What is the effect of temp on the Calvin cycle?

Answer 31

• Low temp –> enzyme + substrate have low KE –> low frequency of successful collisions –> few ESCs formed + reduced ROR.
• Decreasing temp –> decreases GP, TP, RuBP conc.
• Same effect seen at high temps where enzymes denatured except at high temp this is irreversible.

Question 32

How are the grana adapted for photosynthesis?

Answer 32

• Contain chlorophyll pigments/photosystems.
• Contain ETC/electron carriers/ATP synthase.
• Large s.a. to vol ratio for light absorption/light dependent stage/electron transport.

Question 33

Why is the theoretical rate of photosynthesis not achieved at higher light intensities?

Answer 33

• At high light intensity –> another factor becomes a limiting factor.
• Temp becomes limiting as Calvin cycle requires enzymes/relies on KE of molecules.
• CO2 becomes limiting as it is required for Calvin cycle/reaction with RuBP/fixation by RuBisCO.

Question 34

Outline the ways in which heterotrophic organisms are dependent on plants.

Answer 34

• Heterotrophs obtain organic material by eating plants.
• Autotrophs produce more organic molecules (glucose) during photosynthesis/Calvin cycle/ light-independent stage.
• Autotrophs/plants produce O2 during photosynthesis/photolysis/light dependent stage.
• Glucose/carbohydrate/O2 produced in photosynthesis are used in respiration by heterotrophs.

Question 35

Suggest the role of DNA and ribosomes in chloroplasts.

Answer 35

• DNA coding for genes/proteins/enzymes.
• Protein synthesis.
• Enzymes/proteins for production of chlorophyll.
• Protein for electron carriers.
• ATP synthase.
• Enzyme for photolysis.
• Enzyme for Calvin cycle –> e.g. RuBisCO.

Question 36

Discuss the fate of triose phosphate in the Calvin cycle.

Answer 36

• Regenerates RuBP –> so cycle continues for further CO2 fixation.
• Formation of glucose/starch/cellulose/sucrose.
• Formation of triglyceride/lipid/fatty acid/glycerol/amino acid/protein/nucleic acids/nucleotides.
• 10x TP for RuBP formation.
• 2x TP for production.
• More TP used to produce RuBP and rest for production.