3.5

Question 1

What are the stages
of photosynthesis?

Answer 1

1. Light dependent reaction
   ○ Thylakoid membrane of chloroplast
2. Light independent reaction
   ○ Stroma of chloroplast

Question 2

Describe photoionisation in the light-dependent reaction (LDR)

Answer 2

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

Question 3

Describe what happens after photoionisation in the LDR

Answer 3

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 into 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 photolysis of water in the LDR

Answer 4

● Water splits to produce protons, electrons and oxygen
○ 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)
   ○ Catalysed by the enzyme rubisco
2. Forming 2 glycerate 3-phosphate (GP) molecules
3. GP reduced to triose phosphate (TP)
   ○ Using products from light-dependent reaction - reduced NADP and energy from ATP
4. Some TP converted to useful organic substances (eg. 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 eg. rubisco gain kinetic energy
   ○ So more enzyme-substrate complexes form
2. Above an optimum temperature, rate decreases
   ○ Enzymes denature as H bonds in tertiary structure break
   ○ So fewer enzyme-substrate complexes form

Question 7

Describe and explain how light intensity affects rate of photosynthesis

Answer 7

1. As light intensity increases, rate increases
   ○ Light-dependent reaction increases (eg. 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 eg. 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 combines 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 eg. 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 eg. 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 & anaerobic respiration

Answer 11

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 12

Describe the process of glycolysis

Answer 12

1. Glucose phosphorylated to glucose phosphate
   ○ Using inorganic phosphates from 2 ATP
2. Hydrolysed to 2 x triose phosphate
3. Oxidised to 2 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 involved which produces little ATP (2 molecules)
● No oxidative phosphorylation 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 combines with coenzyme A, forming Acetyl
   Coenzyme A

Question 17

Describe the Krebs cycle

Answer 17

1. Acetyl coenzyme A (2C) 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:
   ○ 2 x CO2
   lost

○ Coenzymes NAD & FAD reduced
○ Substrate level phosphorylation
(direct transfer of Pi from
intermediate compound to ADP)
→ ATP produced

Question 18

Describe the process of oxidative phosphorylation

Answer 18

1. Reduced NAD/FAD oxidised to release H atoms → split into protons
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 end of ETC, oxygen is 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

● 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 (eg. 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 eg. at 100oC (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 representative 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 temperature 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 environment taken into account

Question 28

State the formula for NPP

Answer 28

NPP = GPP – R

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 30

What is primary or secondary productivity?

Answer 30

The rate of primary or secondary production, respectively

Question 31

State the units used for primary or secondary productivity

Answer 31

kJ ha-1 year-1 (unit for energy, per unit area, per year)

Question 32

Explain why these units for primary or secondary productivity are used

Answer 32

● Per unit area → takes into account that different environments 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 33

Explain why most light falling on producers is not used in photosynthesis

Answer 33

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

Question 34

State the formula for net production of consumers (N)

Answer 34

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 35

State the formula for efficiency of energy transfer

Answer 35

Energy or biomass available after transfer / energy or biomass available before transfer

Question 36

Explain why energy transfer between trophic levels is inefficient

Answer 36

● Heat energy is lost via respiration ● Energy lost via parts of organism that aren’t eaten (eg. bones) ● Energy lost via food not digested → lost as faeces ● Energy lost via excretion eg. urea in urine

Question 37

Explain how crop farming practices increase energy transfer efficiency

Answer 37

● Simplifying food webs to reduce energy / biomass losses to non-human food chains eg. ○ Herbicides kill weeds → less competition (eg. 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 38

Explain how livestock farming practices increase energy transfer efficiency

Answer 38

● 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 39

Explain the role of saprobionts in recycling chemical elements

Answer 39

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

Question 40

Explain the role of mycorrhizae

Answer 40

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 eg. carbohydrates

Question 41

Give examples of biological molecules that contain nitrogen

Answer 41

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

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 eg. 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 oxidation 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, eg. 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 denitrification / 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 erosion / 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 eg. as herbivores eat producers 4. Some phosphate ions lost from animals in waste products (excretion) 5. Saprobionts decompose organic compounds eg. DNA in dead matter / organic waste, releasing phosphate ions

Question 49

Explain why fertilisers are used

Answer 49

● To replace nitrates / phosphates 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 artificial and natural fertilisers

Answer 50

Natural ● Organic, eg. manure, compost, sewage ● Ions released during decomposition by saprobionts Artificial Contain inorganic compounds of nitrogen, phosphorus 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 nutrients 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 photosynthesise 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 advantage of using natural fertiliser over artificial fertiliser

Answer 52

● Less water soluble so less leaching → eutrophication less likely ● Organic molecules require breaking down by saprobionts → slow release of nitrate / phosphate etc.