5. Energy Transfer In & Between Organisms

Question 1

General adaptations of the leaf for photosynthesis

Answer 1

• Large SA to absorb as much light as possible
• Arrangement of leaves to minimise overlapping (avoids shading)
• Transparent cuticle & epidermis to let light through to the mesophyll cells below
• Long narrow upper mesophyll cells packed with chloroplasts to collect sunlight
• Many air spaces in lower mesophyll layer for rapid diffusion in the (g) phase of co2 and h2O
• Network of xylem to carry h2O to leaf cells and phloem to carry sugars produced away

Question 2

What is the overall equation for photosynthesis

Answer 2

6CO2 + 6H2O ———> C6H12O6 + 6O2
. light

Question 3

What are the 3 main stages to photosynthesis

Answer 3

1. Capturing of light energy (chlorophyll)
2. Light-dependant reaction (splitting of water)
3. Light independent reaction (reduction of co2)

Question 4

What are the 2 purposes for the energy from the capture of light in the LD reaction of photosynthesis

Answer 4

• to add an inorganic phosphate (Pi) molecule to ADP making ATP. Process known as photophosphorylation
• to split water into H+ ions (protons) and OH- ions. Process known as photolysis

Question 5

Explain briefly how ATP is made (photosynthesis: LD stage)

Answer 5

Chlorophyll molecule absorbs light = energy of pair of electrons is boosted= they move to a higher energy level
Results in chlorophyll molecule becoming ionised (photoionisation)
Electrons are taken up and passed along a number of electron carriers (forms a transfer chain) in a series of oxidation&reduction reactions
Each new carrier is at a slightly lower energy level so the electrons lose energy
Some of this energy is used to combine Pi with ADP to make ATP

Question 6

Which theory describes and explains the precise mechanism by which ATP is produced in the LD reaction?

Answer 6

Chemiosmotic theory

Question 7

Step by step process of the mechanism for the production of ATP in the LD stage (chemiosmotic theory)

Answer 7

• Protons are pumped from the stroma into each thylakoid using proton pumps in the thylakoid membrane
• Photolysis of water also produces protons which increases their conc inside the thylakoid space
• A conc gradient is created & maintained across the thylakoid membrane (high conc of protons inside thylakoid space, low conc in stroma)
• Protons pass through ATP synthase protein channels and as they do cause changes to the structure of the enzyme which then catalyses the combination of ADP with Pi to form ATP

Question 8

Where does the energy used to drive the process of pumping protons into the thylakoids come from

Answer 8

Energy from electrons released when water molecules are split by light (photolysis)

Question 9

Why is the photolysis of water necessary

Answer 9

Loss of electrons when light strikes chlorophyll molecule leaves it short of electrons. If the chlorophyll molecule is to continue absorbing light energy, these electrons must be replaced.
Replacement electrons are provided from photolysis
Protons are also produced

Question 10

What is the equation for photolysis of water

Answer 10

2H2O ———> 4H+ + 4e- + O2

Question 11

Describe and explain the role of protons produced from the photolysis of water (LD stage)

Answer 11

• Protons are passed out of the thylakoid space through ATP synthase channels and are taken up by an electron carrier called NADP
• By taking up the protons NADP becomes reduced. (Allows transport of hydrogen into light independant reaction)
• The reduced NADP is important because it is a further potential source of chemical energy to the plant.
• The oxygen by-product is either used for respiration or diffuses out of the leaf as a waste product of photosynthesis

Question 12

How are chloroplasts structurally adapted for capturing sunlight and carrying out the LD reaction of photosynthesis

Answer 12

• Thylakoid membranes provide a large SA for attachment of chrlophyll, electron carriers and enzymes used in the LD reaction
• Network of proteins in grana hold chlorophyll in a precise manner for max absorption of light
• Granal membranes have ATP synthase channels within them and are selectively permeable which allows proton gradient to be established
• Chloroplasts contain both DNA & ribosomes so they can quickly & easily manufacture some of the proteins involved in the LD reaction

Question 13

What are the products of the LD reaction (ATP and reduced NADP) used for in the second stage of photosynthesis?

Answer 13

To reduce glycerate 3-phosphate

Question 14

What does the Calvin cycle refer to

Answer 14

The second stage of photosynthesis : Light independent reaction that takes place within the stroma of the chloroplasts

Question 15

Give the step by step process of the Calvin cycle

Answer 15

• CO2 reacts with the 5 carbon compound ribulose biphosphate (RuBP), a reaction catalysed by an enzyme known as rubisco
• This reaction produces two molecules of the 3-carbon, glycerate 3-phosphate (GP)
• Reduced NADP, produced in the LD reaction, is used to reduce GP to triose phosphate (TP) using energy supplied by ATP
  - The NADP is re-formed and returned to the LD reaction to be reduced again by accepting more protons
• Some TP molecules are converted to organic substances that the plant needs (eg. starch cellulose lipids glucose amino acids & nucleotides)
• Most TP molecules are used to regenerate ribulose bisphosphate using ATP from the LD reaction

Question 16

How is the chloroplast adapted to carry out the LID reaction of photosynthesis

Answer 16

• fluid of stroma contains all enzymes needed to carry out the LID reaction.
• The stroma fluid surrounds the grans so the products of the LD reaction in the grana can readily diffuse into the stroma
• Contains both DNA & ribosomes so it can quickly & easily manufacture some of the proteins involved in the LID reaction

Question 17

Define the law of limiting factors

Answer 17

At any given moment the rate of physiological process is limited by the factor that is at its least favourable value

Question 18

What does compensation point mean

Answer 18

The point at which the volume of o2 produced & co2 absorbed in photosynthesis is exactly balanced by the o2 absorbed & co2 produced by cellular respiration

Question 19

What is the effect of temperature on the rate of photosynthesis

Answer 19

Provided that other factors are not limiting, the rate of photosynthesis increases in direct proportion to the temperature
Above optimum temperature, the rate falls steeply

Question 20

How does CO2 concentration affect the rate of photosynthesis

Answer 20

No photosynthesis at very low levels
At low - fairly low concentrations, the rate is positively correlated with CO2
At very high concentrations the rate reaches a plateau
CO2 affects enzyme activity, in particular the enzyme that catalyses the combination of ribulose bisphosphate with co2 in the LID reaction

Question 21

How does light intensity affect the rate of photosynthesis 

Answer 21

At low – medium light intensities, the rate is directly proportional to light intensity
At high light intensity the rate reaches a plateau

Question 22

What are the 2 different forms of cellular respiration ?

Answer 22

Aerobic respiration
Anaerobic respiration

Question 23

What 4 stages make up aerobic respiration ?
Where do they take place ?

Answer 23

Glycolysis - cytoplasm
Link reaction - mitochondrial matrix
Kerbs cycle - mitochondrial matrix
Oxidative phosphorylation - inner mitochondrial membrane

Question 24

Describe in simple terms what happens in glycolysis

Answer 24

The splitting of the 6-carbon glucose molecules into x2 3-carbon pyruvate molecules

Question 25

Describe in simple terms what happens in the link reaction

Answer 25

The 3-carbon pyruvate molecules enter into a series of reactions which lead to the formation of acetylcoenzyme A, a 2-carbon molecule

Question 26

Describe in simple terms what happens in the Krebs cycle

Answer 26

The introduction of acetylcoenzyme A into a cycle of oxidation-reduction reactions that yield some ATP and a large quantity of NADH & FADH

Question 27

Describe in simple terms what happens in oxidative phosphorylation

Answer 27

The use of the electrons, associated with NADH & FADH, released from the Krebs cycle, to synthesise ATP. Water is produced as a by-product

Question 28

Give the step by step process of glycolysis

Answer 28

1. Glucose is phosphorylated to glucose phosphate. Phosphate molecules come from the hydrolysis of x2 ATP molecules to ADP
2. Each glucose phosphate is split into x2 triose phosphate molecules
3. Oxidation of triose phosphate. Hydrogen is removed and transferred to NAD to form NADH
4. Production of ATP. Enzyme controlled reactions convert each triose phosphate into a pyruvate molecule. Each time x2 ATP is regenerated from ADP

Question 29

What is the overall yield from one molecule of glucose undergoing glycolysis ?

Answer 29

• x2 ATP
• x2 NADH
• x2 molecules of pyruvate

Question 30

Give the step by step process of the link reaction

Answer 30

1. Pyruvate molecules produced in the cytoplasm during glycolysis are actively transported into the mitochondrial matrix
2. Pyruvate is oxidised to acetate. Loses x1 co2 and x2 hydrogens. Hydrogens are accepted by NAD to form NADH
3. The acetate combines with coenzyme A (CoA) to produce acetylcoenzymeA

Question 31

Give the step by step process of the Krebs Cycle

Answer 31

1. The 2-carbon acetylcoenzymeA from the link reaction combines with a 4-carbon molecule to form a 6-carbon molecule
2. In a series of reactions this molecule loses co2 and hydrogen to give a 4-carbon molecule, NADH, FADH and x1 ATP produced as a result of substrate level phosphorylation
3. The 4-carbon molecule can now combine with a new molecule of acetylcoenzymeA and the cycle can begin again

Question 32

What is the yield of both the link reaction and Krebs cycle ?

Answer 32

• reduced coenzymes such as NAD and FAD
• one molecule of ATP
• x3 molecules of co2

As there are x2 pyruvate molecules produced for each single glucose molecule the yield is double the quantities above

Question 33

What are coenzymes

Answer 33

Molecules that some enzymes require in order to function

Question 34

Give the step by step process of oxidative phosphorylation.

Answer 34

1. The hydrogen atoms produced during glycolysis & the Krebs cycle combine with the coenzymes NAD & FAD
2. NADH & FADH donate the electrons of the hydrogen atoms they’re carrying to the first molecule in the electron transfer chain
3. The electrons pass down the electron transfer chain in a series of oxidation-reduction reactions.
4. The energy they release allows for the active transport of protons across the inner mitochondrial membrane and into the inter-membrane space
5. Protons accumulate in the inter-membrane space before they diffuse back into the mitochondrial matrix through ATP synthase channels
6. At the end of the chain the electrons combine with these protons & oxygen to form water

Question 35

What is the importance of oxygen in respiration ?

Answer 35

Act as the final acceptor of the hydrogen atoms produced in glycolysis and the Krebs cycle.
Without it the hydrogen ions (protons) and electrons would back up along the chain and the process of respiration would halt

Question 36

What are the 2 alternative respiratory substrates

Answer 36

Lipids and proteins

Question 37

How does respiration using lipids as a substrate work?

Answer 37

Lipids are first hydrolysed to glycerol and fatty acids.
Glycerol is then phosphorylated and converted to triose phosphate which enters through the glycolysis pathway
The fatty acid component is broken down into 2-carbon fragments which are converted to acetylcoenzyme A which enters the Krebs cycle
Many hydrogen atoms are also produced and used to produce ATP during oxidative phosphorylation

Question 38

How does respiration using proteins as a substrate work ?

Answer 38

First hydrolysed to its constituent amino acids.
They have their amino group removed before entering the respiratory pathway at different points depending on the amount of carbon atoms they contain

Question 39

In eukaryotic cells, what are the two types of anaerobic respiration that occur with any regularity ?

Answer 39

• In plants and in microorganisms such as yeast, pyruvate is converted to ethanol and to co2
• In animals, the pyruvate is converted to lactate

Question 40

Give the summary equation for anaerobic respiration in plants and some microorganisms

Answer 40

Pyruvate + NADH —> ethanol + co2 + oxidised NAD

Question 41

Why must anaerobic respiration be used during periods of strenuous activity

Answer 41

Energy demand is high
Oxygen is being used up quicker than it can be supplied (oxygen debt occurs)

Question 42

What is the role of the pyruvate molecules in anaerobic respiration

Answer 42

When oxygen is in short supply, NADH from glycolysis can accumulate and must be removed.
**The pyruvate molecule takes up the two hydrogen atoms from the reduced NAD to form lactate **
NAD is recycled

Question 43

Give the summary equation for anaerobic respiration in animals

Answer 43

Pyruvate + NADH —-> lactate + oxidised NAD

Question 44

Why does a person often breathe deeply for a few mins after strenuous exercise

Answer 44

Anaerobic respiration produces lactate which must be broken down by oxygen in aerobic respiration

Question 45

When oxygen is available again what happens to the lactate

Answer 45

Oxidised back to pyruvate, further oxidised to release energy
If o2 isn’t available it’s removed by the blood and taken to the liver where it’s converted to glycogen

Question 46

What does lactate cause if it accumulates in the muscle tissue

Answer 46

Cramp and muscle fatigue
As it is acidic it also causes pH changes which affects enzymes

Question 47

Define producers

Answer 47

Photosynthetic organisms that manufacture organic substances using light energy, water co2 and mineral ions

Question 48

Define consumers

Answer 48

Organisms that obtain their energy by feeding on (consuming) other organisms rather than using energy from the sunlight directly.
Primary, secondary or tertiary

Question 49

Define saprobionts

Answer 49

Group of organisms that break down the complex minerals in dead organisms into simple ones.
In doing so they release valuable minerals & elements in a form that can be absorbed by plants and so contribute to recycling

Question 50

Define food chain

Answer 50

Describes the feeding relationship in which producers are eaten by primary consumers, which are then eaten by secondary consumers etc.

Question 51

What is each stage in the food chain referred to as

Answer 51

Trophic level

Question 52

What do they arrows in a food chain represent

Answer 52

The direction of energy flow

Question 53

Define biomass

Answer 53

The total mass of living material in a specific area at a given time

Question 54

Why does the volume of water in living organisms cause issues when calculating biomass?
What do we use instead as a result ?

Answer 54

Volume of water varies so makes measuring biomass unreliable
Instead we measure the mass of carbon or dry mass

Question 55

What are the disadvantages of using dry mass or mass of carbon to calculate biomass?

Answer 55

Involves killing an organism which means only small samples can be measured which are not representative

Question 56

How can bomb calorimetry be used to estimate the chemical energy store in biomass

Answer 56

Sample of biomass is weighed and burnt in pure oxygen within a sealed chamber (bomb)
Bomb is surrounded by a known vol of water and the heat of combustion causes temp to rise.
Temp rise is measured and used to calc energy released from mass of burnt biomass

Question 57

Why can plants only capture 1-3% of the light energy from the sun ?

Answer 57

• over 90% of the sun’s energy is reflected back into space by clouds/dust or is absorbed by the atmosphere
• not all wavelengths of light can be absorbed and used for photosynthesis
• light may not fall on a chlorophyll molecule
• a factor, such as low co2 levels, may limit rate of photosynthesis

Question 58

How do you calculate net primary production

Answer 58

net primary production = gross primary production - respiratory losses

Question 59

Why is so little energy transferred between each trophic level ?

Answer 59

• some of the organism isn’t consumed
• some parts consumed can’t be digested and are therefore lost in faeces
• some is lost in excretory materials eg. Urine
• some us lost as heat from respiration

Question 60

How do we calculate the net production of consumers

Answer 60

N = I - (F + R)

N : net production
I : chemical energy store of injected food
F : energy lost in faeces and urine
R : energy lost in respiration

Question 61

How do we calculate the efficiency of energy transfers

Answer 61

energy available after the transfer
————————————————- x100
Energy available before the transfer

Question 62

How does intensive farming increase the energy conversion rate ?

Answer 62

• movement is restricted and so less energy is used in muscle contraction
• environment can be kept warm to reduce heat loss from body
• feeding can be controlled so animals receive optimum amount and type of food for maximum growth (no wastage)
• predators are excluded so there is no loss to other organisms in the food web