Topic 5 Energy transfers between organisms

Question 1

How is the structure of the leaf adapted for photosynthesis?

Answer 1

• Large surface area → to maximise light absorption.
• Thin → short diffusion distance for gases.
• Transparent cuticle & epidermis → allows light through to mesophyll cells.
• Palisade mesophyll cells → packed with chloroplasts to maximise light absorption.
• Many stomata → open/close in response to changes in light intensity.
• Interconnected air spaces within spongy mesophyll → allow rapid diffusion of CO2 and O2 through leaf.
• Xylem → transports water and mineral ions up the plant.
• Phloem → transports solutes from one part of the plant to another.

Question 2

How is the structure of a chloroplast adapted for its function in photosynthesis?

Answer 2

• Disc shape → provides large surface area for light absorption.
• Contain circular DNA and ribosomes → enables rapid synthesis of specific proteins involved in photosynthesis.
• Two distinct regions:
• GRANA: stacks of thylakoids
• Thylakoid membranes = site of LIGHT-DEPENDENT REACTION.
• Thylakoid membranes provide large surface area for photosynthetic pigments, electron carriers and ATP
  synthase channels to be embedded → required for light-dependent reaction.
• Thylakoid membranes are selectively permeable → allows a concentration gradient to exist.
• STROMA: fluid-filled matrix
• Stroma = site of LIGHT-INDEPENDENT REACTION.
• Contains all the enzymes needed for light-independent reaction.
• Also contains enzymes required for starch synthesis/hydrolysis (carbohydrates produced by photosynthesis
  and not used straight away are stored as starch grains).
• Stroma fluid surrounds grana → allows products of light-dependent reaction to easily diffuse into the stroma.

Question 3

Why is all light not used in photosynthesis?

Answer 3

1. Not all wavelengths of light can be absorbed and used for photosynthesis.
2. > 90% Sun’s energy is reflected back into space or absorbed by atmosphere.
3. Light may not fall on a chloroplast.
4. Limiting factors may limit rate of photosynthesis e.g. low CO2.

Question 4

Light dependent reaction

Answer 4

1. Photosynthetic pigments (in PSII), e.g. chlorophyll a, absorb light energy.
2. This excites electrons, which leave the chlorophyll [photoionisation].
3. Electrons move down the electron transport chain (to PSI), losing energy at each stage.
4. This energy is used to pump protons into the thylakoid space, creating a proton gradient across the
   membrane.
5. Protons diffuse down their concentration gradient through an ATP synthase channel, which drives the
   synthesis of ATP from ADP + Pi.
6. (Light energy is absorbed by PSI, exciting electrons to an even higher energy level.)
7. NADP is the final electron acceptor and accepts an electron and a proton to form reduced NADP.
8. Photolysis of water produces protons, electrons and oxygen. These electrons replace the excited electrons in
   chlorophyll.
9. ATP and reduced NADP enter the light independent reaction.

Question 5

Chemiosmosis

Answer 5

Energy lost by electrons as they pass down an electron transport chain is used to pump protons across a membrane, creating a concentration gradient of protons. Protons diffuse through ATP synthase channels down
their concentration gradient, driving the synthesis of ATP from ADP + Pi.

Question 6

Light Independent Reaction

Answer 6

1. CO2 fixation: CO2 (1C) diffuses into leaf through stomata and combines with 5C ribulose bisphosphate
   (RuBP) → catalysed by rubisco (an enzyme). This produces an unstable 6C compound → breaks down into 2 x
   3C molecules of glycerate-3-phosphate (GP).
2. Reduction of GP to TRIOSE PHOSPHATE: reduced NADP from light-dependent reaction is used to reduce GP to TP
   (triose phosphate) using energy supplied by ATP (the resulting NADP is recycled back to light-dependent
   reaction).
3. RuBP regeneration: 5 out of 6 TP used to regenerate RuBP, 1 out of 6 TP is converted to useful
   organic substances e.g. starch, cellulose, glucose, amino acids etc.

Question 7

Limiting factors/optimum conditions

Answer 7

High light intensity
0.4% CO2
25 degrees temperature
High water concentration

Question 8

Where do the 4 stages of respiration take place?

Answer 8

1. Glycolysis (cytoplasm)
2. Link reaction (matrix of mitochondria)
3. Krebs cycle (matrix of mitochondria)
4. Oxidative phosphorylation (cristae of mitochondria)

Question 9

Glycolysis

Answer 9

PHOSPHORYLATION:
* Glucose (6C) is phosphorylated to glucose phosphate and then hexose bisphosphate by the addition
of two phosphate molecules from 2 molecules of ATP.
* Hexose bisphosphate is unstable and splits into 2 molecules of triose phosphate (3C).
* The phosphorylation step of glycolysis USES 2 ATP.
* OXIDATION:
* Triose phosphate is oxidised (loses hydrogen) forming 2 molecules of pyruvate (3-carbon).
* The hydrogens are transferred to 2 NAD forming 2 reduced NAD.
* 4 ATP are produced from this redox reaction (oxidation of triose phosphate and reduction of NAD).

Question 10

The link reaction

Answer 10

Pyruvate is actively transported into the
mitochondrial matrix.
* Happens once for each pyruvate molecule (therefore
twice for each glucose molecule).
* Pyruvate is OXIDISED (loses electrons, which are
collected by NAD) and DECARBOXYLATED (loses
CO2) to form acetate (2C) → CO2 and reduced NAD are
produced.
* Acetate combines with coenzyme A (CoA) → Acetyl
coenzyme A.
* NO ATP is produced.

Question 11

The Krebs cycle

Answer 11

• Happens once for each pyruvate molecule (therefore twice for each glucose molecule).
• 2C acetyl CoA from link reaction combines with a 4C molecule to form a 6C molecule (coenzyme A is recycled
  back to link reaction).
• 6C molecule loses CO2 (decarboxylation) and hydrogen (dehydrogenation) to form a 5C molecule, then a 4C
  molecule.
• For each glucose molecule: produces 2 ATP, 4 reduced NAD and 2 reduced FAD.
• The ATP was produced as result of substrate-level phosphorylation (a phosphate group is transferred directly
  from one molecule to another).
• Regeneration of the 4C molecule, allows it to combine with a new acetyl CoA → cycle begins again.

Question 12

What other substances can be used in respiration?

Answer 12

• FATTY ACIDS (from lipids) and AMINO ACIDS (from proteins) can be converted into molecules that can
  enter Krebs cycle.

Question 13

Oxidative phosphorylation

Answer 13

1. Reduced NAD and reduced FAD (produced from earlier stages in respiration) are oxidised to NAD and FAD →
   releasing hydrogen atoms which split into protons (H+) and electrons (e-
   ).
2. The electrons pass along chain of electron transfer carriers in series of redox reactions → losing energy at
   each carrier.
3. The energy they release is to pump protons across the inner mitochondrial membrane and into the intermembrane space → creating an electrochemical gradient across the membrane.
4. Protons diffuse back into the mitochondrial matrix down their concentration gradient, through ATP synthase
   channels (which are embedded in the inner mitochondrial membrane). This drives the synthesis of ATP from
   ADP+Pi.
   * CHEMIOSMOSIS = the process of ATP production driven by the movement of hydrogen ions across
   a membrane, as a result of electrons moving down an electron transport chain.
5. At the end of the chain, electrons combine with protons and oxygen to form H2O → OXYGEN is the FINAL
   ACCEPTOR in the election transport chain.

Question 14

Anaerobic respiration

Answer 14

• Pyruvate formed in glycolysis is
  converted into CO2 and
  ethanol (in plants / yeast) or
  lactate (in animal cells).
• Production of ethanol/lactate
  regenerates oxidised NAD, so
  glycolysis can continue even if
  not much oxygen is present,
  allowing small amounts of ATP
  to still be produced.

Question 15

How many ATP in total from respiration?

Answer 15

32

Question 16

What is biomass?

Answer 16

BIOMASS = chemical energy stored in the plant

Question 17

BIOMASS CAN BE MEASURED IN TERMS OF DRY MASS OF TISSUE PER GIVEN AREA

Answer 17

• Dry a sample of the biomass at a low temperature (low temperature avoids combustion which would
  cause biomass to be lost in form of CO2).
• Weigh the sample at regular intervals until mass becomes constant (i.e. all water has been removed)
• Typical unit of dry mass → kg m-2.
• As biomass often changes over time (e.g. tree lose their leaves in the winter), it is often more appropriate
  to use units of kg m-2 yr-1.

Question 18

Explain how the chemical energy store in dry biomass can be estimated using calorimetry

Answer 18

• Burn sample of biomass completely.
• Heat a known volume of water using the heat from combustion.
• Measure the temperature change of water and calculate the energy released (measured in J or kJ).

Question 19

Explain how mass of carbon can be used to measure biomass

Answer 19

• Organisms contain carbon because they are made from organic compounds.
• Mass of carbon is approx 50% of the dry biomass

Question 20

Give reasons why most of the Sun’s energy is NOT converted to biomass by photosynthesis

Answer 20

1. Not all wavelengths of light can be absorbed and used for photosynthesis.
2. > 90% Sun’s energy is reflected back into space or absorbed by atmosphere.
3. Light may not fall on a chloroplast.
4. Limiting factors may limit rate of photosynthesis e.g. low CO2.

Question 21

What is Gross Primary Production?

Answer 21

The total amount of chemical energy store in plant biomass, in a given area
or volume.

Question 22

What is Net Primary Production?

Answer 22

The chemical energy store in plant biomass after respiratory losses (R) back to
the environment have been taken into account.
* i.e. NPP is the energy available to the plant for growth/reproduction and so the amount of energy available to
organisms at other tropic levels e.g. herbivores.

Question 23

How do you calculate net productivity of consumers?

Answer 23

N = I - (F + R)
net productivity = chemical energy in ingested food - (chemical energy lost in faeces/urine + energy lost through respiration)

Question 24

Reasons for low percentage energy transfer at each stage of food chain

Answer 24

1. Not all of the food is eaten e.g. roots, bones → energy it contains is not ingested.
2. Some parts are eaten but cannot be digested and are lost as faeces → energy in these parts lost to
   environment.
3. Some energy is lost to the environment in excretory materials e.g. urine.
4. Some energy is lost as heat from respiration.

Question 25

How do farming practices increase the productivity of energy transfer? 1. The energy lost to other organisms can be reduced

Answer 25

• Farmers reduce pest numbers using
• Chemical pesticides:
• Insecticides → kill insect pests that damage crops → less biomass lost from crops → crops grow
  larger and NPP is greater.
• Herbicides → killing weeds eliminates direct competition with the crop for energy from the sun and
  removes food source and/or habitat of pests → further reduces pest numbers.
• Biological agents
• Pathogens used to kill pests.
• Parasites allowed to live on a pest insect → reduces the pest’s ability to function.
• Combination of both chemical and biological methods → reduces pest numbers more than either
  method alone.

Question 26

How do farming practices increase the productivity of energy transfer? 2. The energy lost through respiration can be reduced

Answer 26

• To reduce respiratory losses:
• Keep animals in pens so movement is restricted.
• Keep animals indoors where it is warmer → less energy is wasted generating body heat.
• This = more biomass = more chemical energy stored → increases net productivity and increases efficiency of
  energy transfer to humans.
• Advantage = more food produced in shorter time at lower cost.
• Disadvantage = unethical to keep animals in pens → distress / limits natural behaviours.

Question 27

Ammonification

Answer 27

• Saprobionts (decomposers) hydrolyse nitrogen-containing compounds (e.g. urea, proteins) in dead organic
  material → producing ammonia.
• Ammonia forms ammonium ions in the soil.

Question 28

Nitrification

Answer 28

• Nitrifying bacteria convert ammonium ions in the soil into nitrate ions.
• Two step process: Ammonium ions → nitrite ions (NO2-
  ) → nitrate ions (NO3-
  ).
• Nitrates can then be absorbed by the plant via active transport.

Question 29

Nitrogen fixation

Answer 29

• Nitrogen-fixing bacteria convert nitrogen gas into nitrogen-containing compounds.
• Some nitrogen-fixing bacteria are found in root nodules of leguminous plants e.g. peas, beans and clover.
• These nitrogen-fixing bacteria have a mutualistic relationship with the plants → they provide the plant with
  nitrogen compounds and the plant provides them with carbohydrates.

Question 30

Denitrification

Answer 30

Denitrifying bacteria convert nitrates in the soil into nitrogen gas (released back into the atmosphere).
* Reduces the availability of nitrogen-containing compounds for plants.
* Happens under anaerobic conditions (no oxygen) → i.e. in waterlogged soil.
* Soil must be kept well-aerated to prevent build up of denitrifying bacteria e.g. ploughing fields.

Question 31

Phosphorous cycle

Answer 31

1. Phosphate ions in rocks are released by weathering.
2. Phosphate ions are taken into plants through roots (mycorrhizae dramatically increase the rate at which
   phosphorus ions can be absorbed).
3. Phosphate ions are transferred through food chain as one organism eats another.
4. Phosphate ions are lost from animals via waste products.
5. Saprobionts break down dead organic material and waste products, releasing phosphate ions into the soil for
   assimilation by plants.
6. Weathering of rocks also releases phosphate ions into aquatic environments, where the phosphate ions are taken
   up by aquatic producers e.g. algae and passed along food chains to birds (guano = excretory product of seabirds
   that is very high phosphate ions → used as a fertiliser).
7. Phosphate ions in waste products are transported by rivers into lakes/oceans where they form sedimentary rocks
   → completes the phosphorus cycle

Question 32

How do mycorrhizae benefit plants?

Answer 32

Mycorrhizae = the mutualistic association between certain types of fungi and the vast majority of plants.
* Hyphae of fungi act like extensions of the plant’s root system and increase the surface area for absorption
of water and minerals.
* Fungi receives organic compounds such as sugars and amino acids from the plant.
* Fungi hold water and minerals near the roots → makes plant more drought resistant and allows plants to take
up inorganic ions more readily.
* Mycorrhizae improve the uptake of relatively scarce ions e.g. phosphate ions.

Question 33

Natural fertiliser

Answer 33

Natural fertilisers = organic and come from living organisms e.g. manure, compost.
* Nutrients released over a long period of time.
* Large amounts needed as nutrients are not very concentrated.

Question 34

Artificial fertiliser

Answer 34

Artificial fertilisers = inorganic, contain pure chemicals, mined from rocks/deposits.
* Contain known quantities of minerals → easier to predict how much is needed and the effects they will have on
crop growth.
* Concentrated → smaller quantities needed → cheaper to transport than natural fertilisers.
* BUT inorganic ions in artificial fertilisers are very soluble → rain can wash artificial fertilisers into waterways
(leaching).

Question 35

Environmental issues with the use of nitrogen-containing fertilisers:

Answer 35

1. Reduced species diversity
   * Nitrogen-rich soils favour the growth of certain rapidly growing species which out compete other species.
2. Leaching
   * The process by which water-soluble nutrients are washed away from the soil into ponds and lakes etc.
   * Leached nitrate ions cause eutrophication.
3. Eutrophication = the process by which nutrient concentrations increase in bodies of water.
4. Changes balance of nutrients in soil → too much of a particular nutrient can cause plants to die.

Question 36

Describe the process of eutrophication

Answer 36

1. Leaching causes nitrate ion concentration to increase in ponds/rivers → growth of plants and algal increases.
2. Leads to algal bloom near surface of water → blocks light to plants and algae at lower depths.
3. Plants at lower depths die because they can’t photosynthesise.
4. Saprobiontic bacteria feed on the dead plant matter.
5. Saprobiontic bacteria respire aerobically → reduce the oxygen concentration in the water.
6. Aquatic organisms die due to lack of dissolved oxygen.