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

Question 12

Explain what happens after glycolysis if respiration is anaerobic

Answer 12

Fermentation:
1. Pyruvate is converted to lactate (animals & some bacteria) or ethanol (plants & yeast)
2. This oxidises reduced NAD, so NAD is regenerated
3. So glycolysis can continue (which requires
NAD) allowing continued production of ATP

Question 13

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

Answer 13

Only glycolysis is involved which produces a small amount of ATP. There is no oxidative phosphorylation which forms majority of ATP.

Question 14

What happens after glycolysis if respiration is aerobic?

Answer 14

Pyruvate is actively transported into the mitochondrial matrix

Question 15

Describe the link reaction

Answer 15

Pyruvate is oxidised and decarboxylated to 2C Acetate. CO2 is produced and NAD is reduced. Acetate combines with coenzyme A, forming Acetyl Coenzyme A. This reaction happens twice per glucose molecule and the products per glucose molecule are 2 x Acetyl Coenzyme A, 2 X CO2 and 2 X reduced NAD

Question 16

Describe the Krebs cycle

Answer 16

Acetyl Coenzyme A combines with a 4C molecule, releasing Coenzyme A and producing a 6C molecule. The 6C molecule is decarboxylated and dehydrogenated to a 5C molecule, so Carbon Dioxide and reduced NAD is released. The 5C molecule is decarboxylated and dehydrogenated into the 4C molecule from the start. This process creates ATP, 3 molecules of reduced NAD, a molecule of reduced FAD and 2 molecules of carbon dioxide. This process occurs twice per glucose molecule.

Question 17

Describe the process of oxidative phosphorylation

Answer 17

• Reduced NAD/FAD is oxidised to release H atoms, which then split into protons (H+) and electrons (e-)
• Electrons are transferred down electron transfer chain by redox reactions, as they move down, energy is released.
• Energy released by electrons is used to actively pump protons from matrix to the inner membrane, so proton accumulate in the intermembrane space, creating a proton gradient.
• Protons diffuse into matrix down the electrochemical gradient, via ATP synthase
• In the matrix at the end of the ETC, oxygen is the final electron acceptor. The oxygen combines with the hydrogen ions to form water

Question 18

How do metabolic poisons affect respiration?

Answer 18

Poisons such as Cyanide disrupts respiration by binding to the electron carriers and inhibiting the movement of electrons down the ETC. This reduces chemiosmosis as the proton gradient can not be established and inhibits the Krebs Cycle as reduced FAD/NAD can not release the electrons to the ETC so they can not return to the Krebs cycle. This results in ATP production stopping.

Question 19

Describe how biomass is formed in plants

Answer 19

During photosynthesis, plants make organic (carbon) compounds from CO2. Most sugars synthesised are used by the plant as respiratory substrates, but the rest are used to make other groups of biological molecules (e.g. carbs, lipids & proteins), which form biomass

Question 20

How can biomass be measured?

Answer 20

Mass of carbon or dry mass of tissue per given area

Question 21

Describe how dry mass of tissue can be measured

Answer 21

Dry sample in an oven until mass remains constant (all water evaporated)

Question 22

Why is dry mass is more representative than fresh (wet) mass?

Answer 22

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

Question 23

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

Answer 23

Using a calorimeter:
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 the biomass

Question 24

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

Answer 24

• Stirrer → evenly distributes heat energy in the water
• Air / insulation → reduces heat loss & gain
• Water → has a high specific heat capacity

Question 25

What is gross primary production (GPP)?

Answer 25

Chemical energy store in plant biomass, in a given area or volume, in a given time

Question 26

What is net primary production (NPP)?

Answer 26

Chemical energy store in plant biomass after respiratory losses to environment taken into account

Question 27

State the formula for NPP

Answer 27

NPP = GPP – R

Question 28

Explain the importance of NPP in ecosystems

Answer 28

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

State the units used for primary or secondary productivity and explain why these units are used

Answer 29

kJ ha-1 year-1 (unit for energy, per unit area, per year).
* Per unit area → takes into account that different environments vary in size, enabling comparison between environments
* Per year → takes into account effect of seasonal variation (temperature etc.) on biomass. so its more representative and enables comparison between environments

Question 30

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

Answer 30

• Some light will be reflected or is wrong wavelength.
• Light may miss chlorophyll / chloroplasts.
• CO2 concentration or temperature is a limiting factor

Question 31

State the formula for net production of consumers (N)

Answer 31

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 32

State the formula for efficiency of energy transfer

Answer 32

Energy or biomass available after transfer / energy or biomass available before transfer x 100 if a %

Question 33

Explain why energy transfer between trophic levels is inefficient

Answer 33

• 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 34

What crop farming practices can increase efficiency of energy transfer

Answer 34

• Add fertilisers to soil
• Plough fields
• Crop rotation

Question 35

Explain how livestock farming practices increase efficiency of energy
transfer

Answer 35

• 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 36

Explain the role of saprobionts

Answer 36

They decompose organic compounds by secreting digestive enzymes for extracellular digestion. They also absorb soluble nutrients and release minerals ions.

Question 37

Explain the role of mycorrhizae

Answer 37

• 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
• Fungi receive organic compounds eg. carbohydrates

Question 38

Give examples of biological molecules that contain nitrogen

Answer 38

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

Question 39

Describe the role of bacteria in nitrogen fixation

Answer 39

Nitrogen gas (N2) is converted into ammonia (NH3), which forms ammonium ions (NH4+) in soil by nitrogen-fixing bacteria

Question 40

Describe the role of bacteria in ammonification

Answer 40

Nitrogen-containing compounds from dead
organisms and waste are decomposed by saprobionts (which secrete digestive enzymes) and converted to ammonia, which forms ammonium ions in soil.

Question 41

Describe the role of bacteria in nitrification

Answer 41

Ammonium ions in soil are 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

Question 42

Describe the role of bacteria in denitrification

Answer 42

Nitrates in soil are converted into nitrogen gas via reduction by denitrifying bacteria in anaerobic conditions

Question 43

Suggest why ploughing (aerating) soil increases its fertility

Answer 43

More ammonium is converted into nitrite and nitrate (more nitrification). Less nitrate is converted to nitrogen gas (less denitrification)

Question 44

Give examples of biological molecules that contain phosphorus

Answer 44

Phospholipids, DNA, RNA, ATP, ADP, NADP, TP, GP, RuBP

Question 45

Describe the nitrogen cycle

Answer 45

Ammonification: Saprobionts decompose nitrogen containing compounds from dead organisms and waste into ammonia. The saprobionts secrete protease enzymes for extracellular digestion so proteins are converted to ammonia which then form ammonium ions in the soil

Nitrification: Nitrifying bacteria convert the ammonium ions from ammonification into nitrite ions and then nitrate ions via a two step oxidation. The nitrate ions are absorbed by plants by active transport via the roots. Nitrifying bacteria require aerobic conditions.

Denitrification: If conditions are anaerobic, nitrate ions are converted to nitrogen gas by denitrifying bacteria. This is wasteful and can be prevented by aerating and draining soil

Nitrogen fixation: Nitrogen gas from the atmosphere can be fixed into other compounds by nitrogen fixing bacteria. These bacteria can reduce nitrogen gas into ammonia which then dissolves to ammonium ions.

Question 46

Describe the phosphorous cycle

Answer 46

Phosphate ions are found in rocks and are released into the soil/water via erosion and weathering. The phosphate ions are taken up by producers and become incorporated in their biomass. This absorption is increased by mycorrhizae. The phosphate ions are moved through the tropic levels and are excreted by animals, then saprobionts decompose the organic material and release the phosphate ions.

Question 47

What is leaching and eutrophication?

Answer 47

Use of fertilisers can lead to mineral ions e.g. phosphates and nitrates dissolving into rainwater and leaching into rivers . This can lead to eutrophication, where the excess mineral ions in the water causes a rapid growth of algae which blocks light. This means water plants die as they do not receive sufficient light for photosynthesis. When they die, saprobionts decompose the dead matter, using up oxygen for aerobic respiration. This reduces the level of oxygen in the water, so fish and other organisms die due to lack of oxygen for respiration.