UNIT 5: Energy transfers in and between organisms

Give the equation for photosynthesis.

The equation for photosynthesis is:
6CO₂ + 6H₂O —> C₆H₁₂O₆ + 6O₂

Where does the light-dependent reaction take place?

The light-dependent reaction takes place in the thylakoid membranes.

Describe the first step of the light-dependent reaction.

In the first step of the light-dependent reaction, chlorophyll absorbs light energy, leading to the photoionisation of chlorophyll.
The light energy excites electrons in chlorophyll.
The high-energy electrons are released from the chlorophyll and move down the electron transfer chain.

Describe the second step of the light-dependent reaction.

In the second step of the light-dependent reaction, the excited electrons that have been released are replaced.
Photolysis of water occurs, which produces protons, electrons, and oxygen.

Give the equation for the photolysis of water.

The equation for the photolysis of water is:
H₂O —> 2H⁺ + ½O₂ + 2e^-

Describe the third step of the light-dependent reaction.

In the third step of the light-dependent reaction, the electrons released lose energy.
This energy is conserved in the production of ATP, where it is used to actively transport protons into the thylakoid.
This forms a proton gradient across the thylakoid membrane, where there is a greater concentration of protons in the thylakoid compared to the stroma.

Describe the fourth step of the light-dependent reaction.

In the fourth step of the light-dependent reaction, protons diffuse down their concentration gradient, and into the stroma via ATP synthase.
The energy from this movement combines ADP and an inorganic phosphate to form ATP.

Describe the fifth step of the light-dependent reaction.

In the fifth step of the light-dependent reaction, the electrons are transferred to NADP along with a proton, from the stroma, to form reduced NADP.

What does the chemiosmotic theory describe?

The chemiosmotic theory describes the transfer of electrons down the electron transfer chain and the movement of protons across chloroplast membranes as being necessary for the synthesis of ATP.

Where are the ATP synthase enzymes located?

The ATP synthase enzymes are embedded in the thylakoid membranes.

Where does the light-independent reaction take place?

The light-independent reaction takes place in the stroma.

In the light-independent reaction, how is reduced NADP from the light-dependent reaction used?

In the light-independent reaction, reduced NADP from the light-dependent reaction is used to form a simple sugar.

In the light-independent reaction, how is ATP from the light-dependent reaction used?

In the light-independent reaction, ATP from the light-dependent reaction is hydrolysed to provide additional energy for the reaction that forms a simple sugar.

Describe the first step of the light-independent reaction.

In the first step of the light-independent reaction, carbon dioxide reacts with ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP).
This reaction is catalysed by rubisco.

Describe the second step of the light-independent reaction.

In the second step of the light-independent reaction, ATP and reduced NADP from the light-dependent reaction are used to reduce GP to triose phosphate.

Describe the third step of the light-independent reaction.

In the third step of the light-independent reaction, some of the triose phosphate is used to regenerate RuBP in the Calvin cycle.
Some of the triose phosphate is converted to useful organic substances.

Identify the environmental factors that limit the rate of photosynthesis.

The environmental factors that limit the rate of photosynthesis are light intensity, temperature, and carbon dioxide concentration.

Explain how light intensity affects the rate of photosynthesis.

The higher the light intensity the faster the rate of photosynthesis, as more energy is provided for the light-dependent reaction.

Explain why only light of a certain wavelength is used for photosynthesis.

Only light of a certain wavelength is used for photosynthesis because the photosynthetic pigments chlorophyll a, chlorophyll b, and carotene only absorb red and blue light.

Explain how and why temperature affects the rate of photosynthesis.

Temperature affects the rate of photosynthesis because enzymes are involved.
If the temperature falls below 10°C the enzymes have too little kinetic energy, so few enzyme-substrate complexes form.
If the temperature is greater than 45°C the enzymes may denature. At high temperatures stomata also close, so less CO₂ enters the leaf.

Explain how carbon dioxide concentration affects the rate of photosynthesis.

The higher the carbon dioxide concentration the faster the rate of photosynthesis.
However if the concentration is too high, the stomata close.

What does respiration produce?

Respiration produces ATP.

What is the first stage of anaerobic and aerobic respiration?

Glycolysis is the first stage of anaerobic and aerobic respiration.

Where does glycolysis occur?

Glycolysis occurs in the cytoplasm.

True or false:
Glycolysis is an anaerobic process.

True.
Glycolysis is an anaerobic process.

Describe the first step of glycolysis.

In the first step of glycolysis, glucose is phosphorylated to glucose phosphate using ATP.

Describe the second step of glycolysis. glycolysis.

In the second step of glycolysis, glucose phosphate is phosphorylated to hexose biphosphate using ATP.
The hexose biphosphate is then split into two molecules of triose phosphate.

Describe the third step of glycolysis.

In the third step of glycolysis, triose phosphate is oxidised to 2 molecules of pyruvate.
NAD collects the hydrogen ions from this oxidation, forming 2 reduced NAD.
There is a net gain of ATP and reduced NAD.

Explain why there is a net gain of ATP in glycolysis.

There is a net gain of ATP in glycolysis because 4 ATP molecules are produced, and 2 are used up. Hence there is a net gain of 2 ATP molecules.

Fill in the blanks.
If respiration is only anaerobic, pyruvate can be converted to ________ or ________ using ________ ___.

If respiration is only anaerobic, pyruvate can be converted to ethanol or lactate using reduced NAD.

Describe which organisms produce ethanol during anaerobic respiration and which produce lactate.

During anaerobic respiration, plants and yeast produce ethanol whereas animals and some bacteria produce lactate.

When pyruvate is converted to ethanol or lactate, what is the oxidised NAD produced used for?

When pyruvate is converted to ethanol or lactate, the oxidised NAD produced is used in further glycolysis.

Fill in the blanks.
If respiration is aerobic, ________ from glycolysis enters the __________ ______ by _____ ________.

If respiration is aerobic, pyruvate from glycolysis enters the mitochondrial matrix by active transport.

Describe the first step of the link reaction.

In the first step of the link reaction, pyruvate is oxidised to acetate, which produces reduced NAD.
One carbon atom is removed from pyruvate in the form of CO₂).

Describe the second step of the link reaction.

In the second step of the link reaction, acetate combines with coenzyme A to produce acetyl coenzyme A.

What must occur before the Krebs cycle?

Before the Krebs cycle, acetyl coenzyme A must react with a four-carbon molecule.
This releases coenzyme A and produces a six-carbon molecule that enters the Krebs cycle.

What does the Krebs cycle involve?

The Krebs cycle involves a series of oxidation-reduction reactions.

Describe the first step of the Krebs cycle.

In the first step of the Krebs cycle, the six-carbon molecule is converted to a five-carbon molecule.
One molecule of CO₂ and one hydrogen is removed.
This hydrogen is used to produce reduced NAD from NAD.

Describe the second step of the Krebs cycle.

In the second step of the Krebs cycle, the five-carbon molecule is converted to a four-carbon molecule.
One molecule of reduced FAD and two molecules of reduced NAD are produced.
ATP is produced by substrate-level phosphorylation.

Describe the first step of oxidative phosphorylation.

In the first step of oxidative phosphorylation, reduced NAD and reduced FAD are oxidised to NAD and FAD.
This releases H atoms, which split into protons and electrons.

Describe the second step of oxidative phosphorylation.

In the second step of oxidative phosphorylation, electrons are transferred down the electron transfer chain and lose energy.

Describe the third step of oxidative phosphorylation.

In the third step of oxidative phosphorylation, the energy from the electrons is used by the electron carriers to actively transport protons from the mitochondrial matrix into the intermembrane space.
The concentration of protons is now higher in the intermembrane space than in the mitochondrial matrix.

Describe the fourth step of oxidative phosphorylation.

In the fourth step of oxidative phosphorylation, protons move down the concentration gradient, across the inner mitochondrial membranes and into the mitochondrial matrix, via ATP synthase.
This movement causes the synthesis of ATP from ADP and inorganic phosphate, known as the chemiosmotic theory.

Describe the fifth step of oxidative phosphorylation.

In the fifth step of oxidative phosphorylation, the protons, electrons, and oxygen (from the blood) combine to form water.
This occurs in the mitochondrial matrix, at the end of the transport chain.

Fill in the blanks.
Other respiratory substrates include the breakdown products of ______ and _____ _____, which enter the Krebs cycle.

Other respiratory substrates include the breakdown products of lipids and amino acids, which enter the Krebs cycle.

Fill in the blanks:
In any ecosystem, plants synthesise organic compounds from ______ or ______ carbon dioxide.

In any ecosystem, plants synthesise organic compounds from atmospheric or aquatic carbon dioxide.

Fill in the blanks:
Most of the sugars synthesised by plants are used by the plant as _______ _______.

The rest are used to make other groups of ______ _______.
These biological molecules form the _____ of the plants.

Most of the sugars synthesised by plants are used by the plant as respiratory substrates.
The rest are used to make other groups of biological molecules.
These biological molecules form the biomass of the plants.

Give two ways biomass can be measured.

Biomass can be measured in terms of mass of carbon or dry mass of tissue per given area.

What can calorimetry be used to estimate?

Calorimetry can be used to estimate the chemical energy store in dry biomass.

What is gross primary production (GPP)?

Gross primary production (GPP) is the chemical energy store in plant biomass, in a given area or volume.

What is net primary production (NPP)?

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

Give the equation for net primary production.

The equation for net primary production is
NPP = GPP – R
where GPP represents gross production and R represents respiratory losses to the environment.

Give three things the net primary production is available for.

The net primary production is available for plant growth, plant reproduction, and other trophic levels in the ecosystem, such as herbivores and decomposers.

Give the equation for the net production of consumers.

The equation for the net production of consumers is N = I – (F+R)
where I represents the chemical energy store in ingested food
F represents the chemical energy lost to the environment in faeces and urine
R represents the respiratory losses to the environment.

What is primary productivity?

Primary productivity is the rate of primary production.

What is secondary productivity?

Secondary productivity is the rate of secondary production.

What are primary and secondary productivity measured as?

Primary and secondary productivity is measured as biomass in a given area in a given time
eg kJ ha^-1 year^-1.

Give three reasons for the low efficiency of energy transfer in an ecosystem.

One reason for the low efficiency of energy transfer in an ecosystem is due to the heat loss from respiration.
Another reason is that not all of the organism is eaten.
Another reason is that energy is lost via excretion.

Give two ways the efficiency of energy transfer is increased.

One way the efficiency of energy transfer is increased by simplifying food webs to reduce energy losses to non-human food chains.
Another way is by reducing respiratory losses within a human food chain.

What is recycled within natural ecosystems?

Nutrients are recycled within natural ecosystems.

Give two examples of nutrient cycles.

Two examples of nutrient cycles are the nitrogen cycle and the phosphorus cycle.

Fill in the blank:
____________ play a vital role in recycling chemical elements such as phosphorus and nitrogen.

Microorganisms play a vital role in recycling chemical elements such as phosphorus and nitrogen.

Describe the role of saprobionts in decomposition.

In decomposition, saprobionts carry out saprobiotic nutrition.
They secrete enzymes to digest food externally.
Here, organic molecules are broken down into inorganic ions by extracellular digestion.
These nutrients are then absorbed.

Define the term mycorrhizae.

Mycorrhizae are symbiotic relationships between fungi and the roots of plants.

Describe the structure of the fungi involved in the mycorrhizae.

The fungi involved in the mycorrhizae have long, thin strands called hyphae which connect to the roots of the plant.

Explain why mycorrhizae facilitate the uptake of water and inorganic ions by plants.

Mycorrhizae facilitate the uptake of water and inorganic ions by plants because the hyphae on the fungi increase the surface area of the plant’s root system.
In turn, the fungi obtain organic compounds such as glucose from the plant.

Name the five processes involved in the nitrogen cycle.

The five processes involved in the nitrogen cycle are nitrogen fixation, saprobiotic nutrition, ammonification, nitrification, and denitrification.

Describe the process of nitrogen fixation.

In the process of nitrogen fixation, nitrogen gas in the atmosphere is converted to ammonia by nitrogen-fixing bacteria.
The ammonia forms ammonium ions in solution.

Where are nitrogen-fixing bacteria found?

Nitrogen-fixing bacteria are found inside the root nodules of leguminous plants.

Describe how nitrogen-fixing bacteria form a mutualistic relationship with plants.

Nitrogen-fixing bacteria form a mutualistic relationship with plants since the bacteria provide the plant with nitrogen-containing compounds and the plant provides carbohydrates.

Describe the process of ammonification.

In the process of ammonification, DNA, RNA, and proteins from dead organisms or urea are converted to ammonia by saprobtioic nutrition.
The ammonia then forms ammonium ions.

Describe the process of nitrification.

In the process of nitrification, ammonium ions in the soil are converted to nitrites, which are then converted to nitrates.
This is done by nitrifying bacteria.

Describe the process of denitrification.

In the process of denitrification, nitrates in the soil are converted to nitrogen gas by denitrifying bacteria, during respiration.
This occurs under anaerobic conditions.

Describe two other ways nitrogen is cycled in an ecosystem.

One other way nitrogen is cycled in an ecosystem is by lightning, which fixes nitrogen gas.
Another way is by artificial fertilisers.

Describe how phosphate ions in rocks are released into the soil or bodies of water.
What happens to these ions?

Phosphate ions in rocks are released into the soil or bodies of water by weathering.
These phosphate ions are assimilated by plants or aquatic producers.

Describe how phosphate ions are transferred through the food chain.

Phosphate ions are transferred through the food chain by feeding.

Describe how phosphate ions are released BACK into the soil.

Phosphate ions are released back into the soil by saprobionts, which digest organic compounds in animal waste and dead plants and animals.

Give two ways nitrates and phosphates are lost.

Nitrates and phosphates are lost by harvesting plants and removing livestock.

What is used to replace nitrates and phosphates in the soil?

Natural and artificial fertilisers are used to replace nitrates and phosphates in the soil.

Describe the difference between natural and artificial fertilisers.

Natural fertilisers are organic matter whereas artificial fertilisers are inorganic.

Give two environmental issues that arise from the use of fertilisers.

Two environmental issues that arise from the use of fertilisers are leaching and eutrophication.

What is leaching?

Leaching is when water-soluble compounds in the soil are washed away into nearby ponds or rivers by the rain or irrigation systems.

A fertiliser is applied just before heavy rainfall.
Suggest how this can affect the likelihood of leaching.

Applying a fertiliser just before heavy rainfall can make leaching more likely to occur.

Explain why leaching is less likely to occur with natural fertilisers, compared to artificial fertilisers.

Leaching is less likely to occur with natural fertilisers compared to artificial fertilisers because the nitrogen and phosphorus are still contained in organic molecules.
They must be decomposed by saprobionts before they can be absorbed by plants, controlling the rate of uptake and preventing them from being washed away.

Phosphate ions are less soluble than nitrate ions.
Suggest how this can affect the likelihood of the leaching of phosphates.

As phosphate ions are less soluble than nitrate ions, the leaching of phosphates is less likely.

Explain what causes eutrophication.

Mineral ions leached from fertilised fields cause eutrophication.
They stimulate the rapid growth of algae in ponds and rivers, known as algal blooms.

Describe the effects of algal blooms.

Algal blooms result in large amounts of algae, which block light from reaching the plants below.
The plants are unable to photosynthesise enough and eventually die.

Describe the effects of dead plants in ponds and lakes.

Dead plants in ponds and lakes are decomposed by saprobionts.
The saprobionts carry out lots of aerobic respiration, lowering the oxygen concentration in the water. This lack of oxygen causes fish and other aquatic animals to die.