[3.5] Energy Transfers In & Between Organisms

Question 1

What are the stages of photosynthesis and where do they happen?

Answer 1

1. Light dependant reaction.
   
   • Thylakoid membrane of chloroplast.
2. Light independent reaction.
   
   • Stroma of chloroplast.

Question 2

Describe photoionisation in the light-dependant reaction (LDR).

Answer 2

1. Chlorophyll absorbs light energy which excites its electrons (higher energy level).
2. So electrons are released from chlorophyll (chlorophyll becomes positively charged).

Question 3

Describe what happens after photoionisation in the LDR.

Answer 3

• Some energy from electrons released in photoionisation is conserved in the production of ATP / reduce NADP (chemiosmotic theory).

1. Electrons move along electron transfer chain (electron carriers), releasing energy.
2. This energy is used to actively pump protons from stroma into thylakoid.
3. Protons move by facilitated diffusion down electrochemical gradient intro stroma via ATP synthase.
4. Energy used to join ADP and Pi to form ATP (photophosphorylation).
5. NADP accepts a proton and an electron to become reduced NADP.

Question 4

Describe the photolysis of water in the LDR.

Answer 4

• Water splits to produce protons, electrons and oxygen (H₂O -> 1/2 O₂ + 2e⁻ + 2H⁺).
  
  • Electrons replace those lost from chlorophyll.

Question 5

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

Answer 5

1. CO₂ reacts with ribulose bisphosphate (RuBP) which is catalysed by the enzyme rubisco.
2. Forming 2x glycerate 3-phosphate (GP).
3. GP reduced to triose phosphate (TP) using products from light-dependant reaction (reduced NADP & energy from ATP)
4. Some TP converted to useful organic substances (e.g. glucose).
5. Some TP used to regenerate RuBP in the Calvin cycle (using energy from ATP).

Question 6

Describe and explain how temperature affects rate of photosynthesis.

Answer 6

1. As temperature increases, rate increases.
   
   • Enzymes e.g. rubisco gain kinetic energy.
   • So more enzyme-substrate (E-S) complexes form.
2. Above an optimum temperature, rate decreases.
   
   • Enzymes denature as H bonds in tertiary structure break.
   • So fewer enzyme-substrate (E-S) complexes form.

Question 7

Describe and explain how light intensity affects rate of photosynthesis.

Answer 7

1. As light intensity increases, rate increases.
   
   • Light-dependant reaction increases (e.g. more photoionisation of chlorophyll) so more ATP and reduced NADP produced.
   • So light-independent reaction increases as more GP reduced to TP and more TP regenerates RuBP.
2. Above a certain light intensity, rate stops increasing.
   
   • Another factor is limiting e.g. temperature / CO₂ concentration.

Question 8

Describe and explain how CO₂ concentration affects rate of photosynthesis.

Answer 8

1. As CO₂ concentration increases, rate increases.
   
   • Light-independent reaction increases.
   • As more CO₂ combined with RuBP to form GP.
   • So more GP reduced to TP.
   • So more TP converted to organic substances and more RuBP regenerated.
2. Above a certain CO₂ concentration, rate stops increasing.
   
   • Another factor is limiting e.g. temperature / light intensity.

Question 9

Explain the key consideration when evaluating data relating to agricultural practices used to overcome the effect of limiting factors.

Answer 9

• Agricultural practice should increase rate of photosynthesis, leading to increased yield.
  
  • As more glucose produced for faster respiration.
  • So more ATP to release energy for growth e.g. cell division, protein synthesis.
• But profit from extra yield should be greater than costs (money & environmental costs).

Question 10

Why is respiration important?

Answer 10

• Respiration produces ATP (to release energy).
• For active transport, protein synthesis etc.

Question 11

Summarise the stages of aerobic and anaerobic respiration.

Answer 11

Aerobic Respiration

1. Glycolysis - cytoplasm (anaerobic).
2. Link reaction - mitochondrial matrix.
3. Krebs cycle - mitochondrial matrix.
4. Oxidation phosphorylation - inner mitochondrial matrix.

Anaerobic Respiration

1. Glycolysis - cytoplasm.
2. NAD regeneration- cytoplasm.

Question 12

Describe the process of glycolysis.

Answer 12

1. Glucose phosphorylated to glucose phosphate.
   
   • Using inorganic phosphates from 2 ATP.
2. Hydrolysed to 2x triose phosphate.
3. Oxidised to 2x pyruvate.
   
   • 2 NAD reduced.
   • 4 ATP regenerated (net gain of 2).

Question 13

Explain what happens after glycolysis if respiration is anaerobic.

Answer 13

1. Pyruvate converted to lactate (animals & some bacteria) or ethanol (plants & yeast).
2. Oxidising reduced NAD -> NAD regenerated.
3. So glycolysis can continue (which needs NAD) allowing continued production of ATP.

Question 14

Suggest why anaerobic respiration produces less ATP per molecule of glucose than aerobic respiration.

Answer 14

• Only glycolysis invovled which produces little ATP (2 molecules).
• No oxidative phosphorlyation which forms majority of ATP (around 34 molecules).

Question 15

What happens after glycolysis if respiration is aerobic?

Answer 15

• Pyruvate is actively transported into the mitochondrial matrix.

Question 16

Describe the link reaction.

Answer 16

1. Pyruvate oxidised (and decarboxylated) to acetate.
   
   • CO₂ produced.
   • Reduced NAD produced (picks up H).
2. Acetate combined with coenzyme A, forming Acetyl Coenzyme A.

Question 17

Describe the Krebs cycle.

Answer 17

1. Acetyl coenzyme A reacts with a 4C molecule.
   
   • Releasing coenzyme A
   • Producing a 6C molecule that enters the Krebs cycle.
2. In a series of oxidation-reduction reactions, the 4C molecule is regenerated and:
   
   • 2x CO₂ lost.
   • Coenzymes NAD & FAD reduced.
   • Substrate level phosphorylation (direct transfer of Pi from intermediate compound to ADP) which produces ATP.

Question 18

Describe the process of oxidative phosphorylation.

Answer 18

1. Reduced NAD/FAD oxidised to release H atoms -> split into protons (H⁺) and electrons (e⁻).
2. Electrons transferred down electron transfer chain (chain of carriers at decreasing energy levels).
   
   • By redox reactions.
3. Energy released by electrons used in the production of ATP from ADP + Pi (chemiosmotic theory):
   
   • Energy used by electron carriers to actively pump protons from matrix -> intermembrane space.
   • Protons diffuse into matrix down an electrochemical gradient, via ATP synthase (embedded).
   • Releasing energy to synthesise ATP from ADP + Pi.
4. In matrix at the end of electron transfer chain, oxygen is the final electron acceptor (electrons can’t pass along otherwise).
   
   • So protons, electrons and oxygen combine to form water.

Question 19

Give examples of other respiratory substrates.

Answer 19

• Breakdown of products of lipids and amino acids, which enter the Krebs cycle. For example:
  
  • Fatty acids from hydrolysis of lipids -> converted to Acetyl Coenzyme A.
  • Amino acids from hydrolysis of proteins -> converted to intermediates in Krebs cycle.

Question 20

Describe how biomass is formed in plants.

Answer 20

• During photosynthesis, plants make organic (carbon) compounds from atmospheric or aquatic CO₂.
• Most sugars synthesised are used by the plant as respiratory substrates.
• Rest used to make other groups of biological molecules (e.g. carbs, lipids & proteins) -> form biomass.

Question 21

How can biomass be measured?

Answer 21

• Mass of carbon or dry mass of tissue per given area.

Question 22

Describe how dry mass of tissue can be measured.

Answer 22

1. Sample dried in an oven e.g. 100°C (avoid combustion).
2. Sample weighed and reheated at regular intervals.
3. Until mass remains constant (all water evaporated).

Question 23

Explain why dry mass is more represenantive than fresh (wet) mass.

Answer 23

• Water volume in wet samples will vary but will not affect dry mass.

Question 24

Describe how the chemical energy stored in dry biomass can be estimated.

Answer 24

Using calorimetry:

1. Known mass of dry biomass is fully combusted (burnt).
2. Heat energy released heats a known volume of water.
3. Increase in temeprature of water is used to calculate chemical energy of biomass.

Question 25

Explain how features of a calorimeter enable valid measurement of heat energy released.

Answer 25

• Stirrer -> evenly distributes heat energy (in water).
• Air/insulation -> reduces heat loss & gain to & from surroundings.
• Water -> has a high specific heat capacity.

Question 26

What is gross primary production (GPP)?

Answer 26

• Chemical energy store in plant biomass, in a given area or volume, in a given time.
  
  • Total energy transferred into chemical energy from light energy during photosynthesis.

Question 27

What is net primary production (NPP)?

Answer 27

• Chemical energy store in plant biomass after respiratory losses to enivronment taken into account.

Question 28

State the formula for NPP.

Answer 28

NPP = GPP = R
R = respiratory losses to the environment.

Question 29

Explain the importance of NPP in ecosystems.

Answer 29

• NPP is available for plant growth and reproduction.
• NPP is also available to other trophic levels in the ecosystem, such as herbivores and decomposers.

Question 29

What is primary or secondary productivity?

Answer 29

• The rate of primary or secondary production, respectively.

Question 30

State the units used for primary or secondary productivity.

Answer 30

KJ ha⁻¹ year⁻¹ (unit for energy, per unit area, per year)

Question 31

Explain why these units for primary or secondary productivity are used.

Answer 31

• Per unit area -> takes into account that different envrionments vary in size.
  
  • Standardising results to enable comparison between environments.
• Per year -> takes into account effect of seasonal variation (temperature etc.) on biomass.
  
  • More representative and enables comparison between environments.

Question 32

Explain why most light falling on prodcuers is not used in photosynthesis.

Answer 32

• Light is reflected or wrong wavelength.
• Light misses chlorophyll / chloroplasts / photosynthetic tissue.
• CO₂ concentration or temperature is a limiting factor.

Question 33

State the formula for net production of consumers (N).

Answer 33

N = I - (F + R)
I = the chemical energy store in ingested food.
F = the chemical energy lost to the environment in faeces and urine.

Question 34

State the formula for efficiency of energy transfer.

Answer 34

Energy or biomass available after transfer / energy or biomass available before transfer (x100 if %)

Question 35

Explain why energy transfer between trophic levels is inefficient.

Answer 35

• Heat energy is lost via respiration.
• Energy lost via parts of organism that aren’t eaten (e.g. bones).
• Energy lost via food not digested -> lost as faeces.
• Energy lost via excretion e.g. urea in urine.

Question 36

Explain how crop farming practices increase energy transfer efficiency.

Answer 36

• Simplifying food webs to reduce energy / biomass losses to non-human food chains. For example:
  
  • Herbicides kill weeds -> less competition (e.g. for light) so more energy to create biomass.
  • Pesticides kill insects (pests) -> reduce loss of biomass from crops.
  • Fungicides reduce fungal infections -> more energy to create biomass.
• Fertilisers e.g. nitrates to prevent poor growth due to lack of nutrients.

Question 37

Explain how livestock farming practices increase energy transfer efficiency.

Answer 37

• Reducing respiratory losses within a human food chain (so more energy to create biomass):
  
  • Restrict movement and keep warm -> less energy lost as heat from respiration.
  • Slaughter animal while still growing / young, when most of their energy is used for growth.
  • Treated with antibiotics -> prevent loss of energy due to pathogens.
  • Selective breeding to produce breeds with higher growth rates.

Question 38

Explain the role of saprobionts in recycling chemical elements.

Answer 38

• Decompose (break down) organic compounds e.g. proteins / urea / DNA in dead matter / organic waste.
• By secreting enzymes for extracellular digestion (saprobiotic nutrition).
• Absorb soluble needed nutrients and release mineral ions e.g. phosphate ions.

Question 39

Explain the role of mycorrhizae.

Answer 39

Mycorrhizae = symbiotic association between fungi and plant roots.

• Fungi (hyphae) act as an extension of plant roots to increase surface area of root system.
• To increase rate of uptake / absorption of water and inorganic ions .
• In return, fungi receive organic compounds e.g. carbohydrates.

Question 40

Give examples of biological molecules that contain nitrogen.

Answer 40

• Amino acids / proteins or enzymes / urea / DNA or RNA / chlorophyll / ATP or ADP / NAD or NADP.

Question 41

Draw a diaram to show the key stages of the nitrogen cycle.

Answer 41

Question 42

Describe the role of bacteria in nitrogen fixation.

Answer 42

• Nitrogen gas (N₂) converted into ammonia (NH₃), which forms ammonium ions (NH₄⁺) in soil.
• By nitrogen-fixing bacteria (may be found in root nodules).

Question 43

Describe the role of bacteria in ammonification.

Answer 43

• Nitrogen-containing compounds e.g. proteins / urea from dead organisms / waste are broken down / decomposed.
• Converted to ammonia, which forms ammonium ions in soil.
• By saprobionts - secrete enzymes for extracellular digestion.

Question 44

Describe the role of bacteria in nitrification.

Answer 44

• Ammonium ions in soil converted into nitrites then nitrates, via a two-step oxidiation reaction.
  
  • For uptake by plant root hair cells by active transport.
• By nitrifying bacteria in aerobic conditions (oxygen).

Question 45

Describe the role of bacteria in denitrification.

Answer 45

• Nitrates in soil converted into nitrogen gas (reduction).
• By denitrifying bacteria in anaerobic conditions (no oxygen, e.g. waterlogged soil).

Question 46

Suggest why ploughing (aerating) soil increases its fertility.

Answer 46

• More ammonium converted into nitrite and nitrate / more nitrification / more (active) nitrifying bacteria.
• Less nitrate converted to nitrogen gas / less dentrification / fewer (active) denitrifying bacteria.

Question 47

Give examples of biological molecules that contain phosphorus.

Answer 47

• Phospholipids / DNA or RNA / ATP or ADP / NADP / TP or GP / RuBP.

Question 48

Describe the phosphorus cycle.

Answer 48

1. Phosphate ions in rocks released (into soils/oceans) by erosions / weathering.
2. Phosphate ions taken up by producers / plants / algae and incorporated into their biomass.
   
   • Rate of absorption increased by mycorrhizae.
3. Phosphate ions transferred through food chain e.g. as herbivores eat producers.
4. Some phosphate ions lost from animals in waste products (excretion).
5. Saprobionts decompose organic compounds e.g. DNA in dead matter / organic waste, releasing phosphate ions.

Question 49

Explain why fertilisers are used.

Answer 49

• To replace nitrates / phosphate lost when plants are harvested and livestock are removed.
  
  • Those removed from soil and incorporated into biomass can’t be released back into the soil through decomposition by saprobionts.
• So improve efficiency of energy transfer -> increase productivity / yield.

Question 50

Describe the difference between artifical and natural fertilisers.

Answer 50

NATURAL

• Organic, e.g. manure, compost, sewage.
• Ions released during decomposition by saprobionts.

ARTIFICAL

• Contain inorganic compounds of nitrogen, phosphorous and potassium.

Question 51

Explain the key environmental issue arising from use of fertilisers.

Answer 51

• Phosphates / nitrates dissolve in water, leading to leaching of nutritents into lakes / rivers / oceans.
• This leads to eutrophication.
  
  1. Rapid growth of algae in pond / river (algal bloom) so light blocked.
  2. So submerged plants die as they cannot photosynthesis.
  3. So saprobionts decompose dead plant matter, using oxygen in aerobic respiration.
  4. So less oxygen for fish to aerobically respire, leading to their death.

Question 52

Explain the key advantages of using natural fetiliser over artifical fertiliser.

Answer 52

• Less water soluble so less leaching -> eutrophiciation less likely.
• Organic molecules require breaking down by sapriobionts -> slow release nirate / phosphate etc.