Energy transfers

What are the 2 stages of photosynthesis?

• Light dependent reaction (LDR)

• Light independent reaction (LIR)

Where do the 2 photosynthesis stages occur in plants?

Light dependent reaction: thylakoids of chloroplasts

Light independent reaction: stroma of chloroplasts

What is the purpose of the LDR?

To convert light energy into chemical energy (ATP and NADPH)

What are the 4 stages of the LDR?

1. Photoionisation of chlorophyll

2. Photolysis

3. Chemiosmosis

4. Production of ATP and reduced NADP

What happens during photoionisation?

• Light energy is absorbed by chlorophyll

• This energy causes 2 electrons to become excited and raise up to a higher energy level by an electron carrier to leave chlorophyll

• Therefore the chlorophyll has been oxidised by light and the electron carrier has been reduced

• Some of the energy from the released electrons is used to make ATP and NADPH in chemiosmosis

What happens during photolysis? Give the equation to show this

• Light energy is absorbed by chlorophyll and splits water into oxygen, H+ and e-

• 2H2O → 4H+ + 4e- + O2

• The H+ is picked up by NADP to form NADPH

• The e- are produced to replace the ones lost during photoionisation and passed along a chain of electron carrier proteins

• The oxygen is either used for respiration or diffuses out of the leaf through the stomata

What happens during chemiosmosis and how does this lead to the production of ATP and reduction of NADP?

• The electrons that gained energy and left the chlorophyll move along a series of proteins embedded within the thylakoid membrane (ETC)

• As they move along, they release energy and some is used to pump protons from stroma into thylakoid - creating a proton gradient

• Photolysis also produces protons which further increases their concentration inside the thylakoid space

• Protons then diffuse back into stroma via the channel proteins ATP synthase, also known as stalked granules - causes changes to the structure of the enzyme which catalyses the combination of ADP with inorganic phosphate which results in production of ATP

• In the stroma, the protons, as well as electrons, combine with the co-enzyme NADP to form NADPH

Where is reduced NADP produced in the LDR?

Stroma of chloroplasts

Whats another name for the LIR?

The Calvin Cycle

Where does the Calvin Cycle occur?

In the stroma

What enzyme is found in the fluid of the stroma, which catalyses the Calvin Cycle?

RuBisCo

Name the 3 stages in the Calvin Cycle

1. Carbon fixation

2. Reduction

3. Regeneration

What happens during carbon fixation in the Carbon Cycle?

CO2 reacts with a 5C molecule ribulose biphosphate (RuBP), catalysed by the enzyme rubisco to form 2 molecules of glycerate 3-phosphate (GP)

What happen during reduction in the Calvin Cycle?

• Reduced NADP from the LDR reduces 2x GP to 2x triose phosphate (TP)

• This uses ATP from the LDR

• Forms 2x NADP and 2x ADP

• Some TP leaves the cycle and is involved in the production of organic substances that the plant requires

What happens during regeneration in the Calvin Cycle?

• Most of the TP is used to regenerate RuBP using ATP from LDR

• NADP is re-formed and goes back to LDR

State the roles of ATP and NADPH in the LDR

• ATP: reduction of GP to TP and regeneration of RuBP from TP

• NADPH: reduction of GP to TP

State the number of carbon atoms in RuBP, GP and TP

• RuBP: 5

• GP: 3

• TP: 3

How does the structure of the chloroplast maximise the rate of the light-dependent reaction?

• Granal membrane contains ATP synthase channels

• Large surface area of thylakoid membrane for ETC

• Photosystems position chlorophyll to enable maximum absorption of light

How does the structure of the chloroplast maximise the rate of the light-independent reaction?

• Contains own DNA and ribosomes for synthesis of enzymes eg rubisco

• Concentration of enzymes and substrates in stroma is high

Define limiting factor

A factor that determines the maximum rate of a reaction

Name 3 environmental factors that can limit the rate of photosynthesis

• Light intensity

• CO2 concentration

• Temperature

Give some common agricultural practices used to overcome the effect of limiting factors in photosynthesis

• Artificial light

• Artificial heating

• Burning fuel -releases CO2 to greenhouse atmosphere

Why do farmers try to overcome the effect of limiting factors?

• To increases yield

• Additional cost must be balanced with yield to ensure maximum profit

What are the 4 stages of aerobic respiration and where do they occur?

1. Glycolysis - cytoplasm

2. Link reaction - mitochondrial matrix

3. Krebs cycle - mitochondrial matrix

4. Oxidative phosphorylation - membrane of cristae

What are the 3 steps of glycolysis?

What’s the net gain?

1. Phosphorylation of glucose to glucose phosphate, using ATP to make it more reactive

2. Glucose phosphate splits into two 3C molecules called triose phosphate

3. Hydrogen is removed from each triose phosphate, oxidising them to produce pyruvate. ATP is regenerated for this reaction. The hydrogen is transferred to a hydrogen-carrier molecule called NAD to produce NADH

   • Net gain (per glucose):

   • 2x reduced NAD

   • 2x ATP

   • 2x pyruvate

How does pyruvate from glycolysis enter the mitochondria?

Via active transport

Where does glycolysis take place?

In the cytoplasm

What are the 3 steps of the link reaction?

What’s the net gain?

1. Pyruvate is actively transported into matrix and oxidised to acetate, which produces CO2 (decarboxylation) and 2 hydrogens

2. NAD picks up hydrogen and becomes reduced NAD

3. Acetate combines with coenzyme A to produce acetylcoenzyme A

   Net gain (per pyruvate):

   1x CO2

   1x reduced NAD

   1x acetyl coA

Where does the link reaction take place?

In the mitochondrial matrix

Give a summary equation for the link reaction

pyruvate + NAD + CoA → acetyl CoA + NADH + CO2

What are the 2 steps of the Krebs Cycle?

What’s the net gain?

1. Acetyl Co-A reacts with oxaloacetate (4 carbon molecule)

   This releases coenzyme A and produces citrate (6 carbon molecule) that enters the Krebs cycle

2. A series of redox reactions occur

   Net gain (per acetyl Co-A):

   2x CO2

   1x ATP

   1x FADH

   3X NADH

Explain the chemiosmotic theory of oxidative phosphorylation in aerobic respiration

• Hydrogen atoms produced during glycolysis and the Krebs cycle combine with the coenzymes NAD and FAD

• NADH and FADH donate electrons which are passed along the ETC through successive redox reactions

• Some of the energy released is used for the active transport of H+ ions from the matrix to the inter-membrane space, creating an electrochemical gradient

• These H+ ions diffuse back into the matrix via the channel protein ATP synthase, which catalyses the production of ATP

• At the end of the chain, the electrons combine with the protons and oxygen to form water (a by-product)

• Therefore, oxygen is the final electron acceptor in the ETC

What is the benefit of an electron transfer chain rather than a single reaction?

• Energy is released gradually

• Less energy is released as heat

Name 2 types of molecules that can be used as alternative respiratory substrates

• Proteins

• Lipids

Explain how lipids can act as an alternative respiratory substrate

• Lipids are first hydrolysed to glycerol and fatty acids

• The glycerol is then phosphorylated and converted to triose phosphate which enters glycolysis

• The fatty acid is broken down into 2C fragments which are converted to acetyl coenzyme A which enters the Krebs cycle

• The breakdown of fatty acids also produces hydrogen atoms which are used to produce ATP during oxidative phosphorylation

Explain how proteins can act as an alternative respiratory substrate

• Proteins are hydrolysed into amino acids

• These have their amino group removed (deamination) before entering respiration at different points

• 3 carbon compounds are converted into pyruvate for the link reaction

• 4/4 carbon compounds are converted to intermediates in the Krebs cycle

Why is it a problem if no oxygen is available for respiration? How does anaerobic respiration overcome this problem?

Because in aerobic respiration, oxygen is the final electron acceptor in oxidative phosphorylation

Without oxygen, the ETC stops and NADH/FADH cant be oxidised

NAD is needed for glycolysis

Anaerobic respiration overcomes this by finding a way to regenerate NAD

Where does anaerobic respiration occur?

In the cytoplasm only

What happens during anaerobic respiration in animals?

• Pyruvate produced in glycolysis is reduced by lactate dehydrogenase to form lactate by gaining the hydrogen from reduced NAD

• This oxidises NAD so that it can be reused in glycolysis

Give a word equation to how what happens during anaerobic respiration in animals

Pyruvate + reduced NAD → lactate + oxidised NAD

What happens to the lactate produced in anaerobic respiration in animals?

• Transported to liver via bloodstream where it is oxidised back into pyruvate

• Can enter link reaction in liver cells or be converted to glycogen

Why is lactate a problem?

• If it accumulates in muscle tissue, it will cause cramp and muscle fatigue

• As it is an acid, it also causes pH changes which affects enzymes

What happens in anaerobic respiration in plants and other microorganisms such as yeast?

• Pyruvate produced in glycolysis is decarboxylated (loses a CO2) to form ethanal

• Ethanal is reduced to ethanol by accepting a hydrogen from NADH - happens via ethanol dehydrogenase

• Oxidised NAD is used for further glycolysis

Give a word equation to show what happens during anaerobic respiration in plants cells and yeast

Pyruvate + NADH → ethanol + CO2 + NAD

What is the advantage of producing ethanol/lactate during anaerobic respiration?

Converts NADH back into NAD so glycolysis can continue

What is the disadvantage of producing ethanol during anaerobic respiration?

• Cells die when ethanol concentration is above 12%

• Ethanol dissolves cell membranes

What are 3 similarities between aerobic and anaerobic respiration?

• Both involve glycolysis

• Both require NAD

• Both produce ATP

Compare how much ATP is produced in aerobic and anaerobic respiration

• Aerobic: approx 30-32 ATP

• Anaerobic: 2ATP

Compare how energy is yielded from aerobic and anaerobic respiration

• Aerobic: ATP is produced by substrate-level phosphorylation in glycolysis and the Krebs cycle as well as by oxidative phosphorylation in the ETC

• Anaerobic: substrate-level phosphorylation only

How do plants use the sugar that they synthesise?

Most of the sugars are used as respiratory substances

The rest are used to make other groups of biological molecules - these form the biomass of plants

What is biomass?

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

What are the units for biomass?

When an area is being sampled: gm-2

When a volume is being sampled: gm-3

Biomass can be measured in terms of what?

Mass of carbon or dry mass of tissue per given area

What method can be used to measure the chemical energy store in dry biomass?

Calorimetry

Explain how bomb calorimetry can be used to estimate the chemical energy store in dry mass

• A sample of dry material is weighed and then burnt in pure oxygen within a sealed chamber called a bomb

• The bomb is surrounded by a water bath and the heat of combustion causes a temperature rise in this water

• As we know how much heat (energy) is required to raise the temperature of 1g of water by 1C, if we know the volume of water and temperature rise, we can calculate the energy released from the mass of burnt biomass

Why is bomb calorimetry preferable to simple calorimetry?

Reduces heat loss to surroundings

How could you ensure that all water had been removed from a sample before weighing?

Heat the sample and reweigh it until the mass reading is constant

How much (%) of the suns light energy us converted into organic matter?

1-3%

Give 4 reasons why most of the suns energy is not converted to organic matter

• Most solar energy is absorbed by atmosphere or reflected by clouds

• Photosynthetic pigments cannot absorb some wavelengths of light

• Not all light falls directly on a chlorophyll molecule

• Energy lost as heat during respiration/photosynthesis

What is GPP?

Gross primary production is the chemical energy store in plant biomass, in a given area or volume. It is the total energy resulting from photosynthesis.

What is NPP?

Net primary production is the chemical energy store in plant biomass after respiratory losses to the environment have been taken into account

Give the equation relating GPP and NPP

NPP = GPP - R

Where R represents respiratory losses

What is NPP available for?

Plant growth, reproduction and for other trophic levels in the ecosystem

How can the net production of consumers be calculated?

N = I - (F+R)

• I: chemical energy stored in ingested food

• F: energy lost as faeces and urine

• R: respiratory losses

Why does biomass decrease along a food chain?

• Energy lost in urine and faeces

• Some of the organism is not consumed

• Energy lost to surroundings as heat

Give an equation that can be used to calculate the percentage efficiency of the energy transfer between a trophic level of a food chain

Percentage efficiency = energy available after the transfer/energy available before the transfer x100

Name a method used by farmers to increase the efficiency of energy transfer along the food chains which produce our food and give 4 reasons why it works

Factory farming

• Restricts movement so less energy used in muscle contraction

• Environment kept warm in order to reduce heat loss

• Controlled feeding so animals receive the optimum amount and type of food for maximum growth

• No predators so no loss to other organisms

What are primary and secondary productivity?

Primary productivity: the rate at which energy is converted by producers into biomass

Secondary productivity: the rate at which consumers convert the chemical energy in their food into biomass

What are the units for rates of productivity?

kJha-1year-1

• kJ is the unit for energy

• ha-1 is per unit area to standardize the results to enable environments to be compared

• year-1 is per year to take into account the impact seasons will have on rain, light and heat. Provides an annual average to allow fair comparisons

Outline 4 farming practices used to increase the efficiency of energy transfer

• Exclusion of predators

• Artificial heating

• Restriction of movement

• Feeding is controlled

Explain why the length of food chains is limited

• Energy is lost at each trophic level

• So there is insufficient energy to support a higher trophic level

What is a pyramid of biomass?

Diagram that shows the biomass at each trophic level

Why is a pyramid of biomass preferable to a pyramid of numbers?

Shape of pyramid of numbers may be skewed since a small number of producers can support many consumers

Why can’t organisms use nitrogen directly from the atmosphere?

N2 is very stable due to strong covalent triple bond

Which biological molecules contain nitrogen?

Proteins, ATP and nucleic acids

What are the 4 key processes in the nitrogen cycle?

1. Nitrogen fixation

2. Ammonification

3. Nitrification

4. Denitrification

How do plants take up the nitrogen they require?

In the form of NO3- ions which they absorb from the soil using active transport

How do animals obtain a source of nitrogen?

By eating and digesting plants

What happens during nitrogen fixation in the nitrogen cycle?

• Nitrogen gas is converted into nitrogen-containing compounds

• Atmospheric fixation of nitrogen: when lightning passes through the atmosphere, high energy breaks N2 into N. N reacts with oxygen to form NO2-. NO2- dissolves in water to form NO3-

• Nitrogen fixation is mainly carried out by 2 types of bacteria, which use nitrogenase to reduce nitrogen into ammonia:

• Free-living nitrogen-fixing bacteria - reduce gaseous nitrogen to ammonia, which they use to manufacture amino acids

• Mutualistic nitrogen-fixing bacteria - live in nodules on the roots of plants and provide usable nitrogen to the plant, as ammonium, while gaining carbohydrates from the plant

What happens during Ammonification in the nitrogen cycle?

• Saprobiontic microorganisms (eg fungi and bacteria) feed on faeces and dead organisms materials containing nitrogen (eg urea, proteins, nucleic acids)

• This releases ammonia which dissolves in water in soil to form ammonium ions

What happens during Nitrification in the nitrogen cycle?

• Occurs in aerobic soil conditions

• Nitrifying bacteria cause the oxidation of ammonium ions to nitrite ions (NO2-), and then the oxidation of nitrite ions to nitrate ions (NO3-)

What happens during denitrification in the nitrogen cycle?

• There is an increase in anaerobic denitrifying bacteria

• These convert soil nitrates back into gaseous nitrogen

• This reduces soil fertility and the availability of nitrogen-containing compounds for plants

What is phosphorus used for in organisms?

• DNA/RNA

• ATP

• Phospholipid bilayer

What form is phosphorus mainly found in?

• As a phosphate ion, in mineral form in sedimentary rocks

• Not as a gas in the atmosphere

What does the phosphorus cycle show?

How phosphorus is passed through an ecosystem