Photosynthesis (Edexcel A Level Biology (A) SNAB): Flashcards
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Define photosynthesis.
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Define photosynthesis.
Photosynthesis is a series of reactions in producers that converts light energy into chemical energy stored in biomass.
What is light energy used to split during photosynthesis?
Water molecules (H₂O).
This releases hydrogen and oxygen.
What happens to the hydrogen and oxygen produced from splitting water?
Hydrogen is combined with carbon dioxide to make glucose.
Oxygen is released into the atmosphere as a waste product.
Where is chemical energy stored in the products of photosynthesis?
In the bonds of glucose molecules.
Glucose can then act as a fuel for respiration.
Give two examples of producers that photosynthesise.
Plants and algae.
What is the word equation for photosynthesis?
Carbon dioxide + water → glucose + oxygen (using light energy).
During photosynthesis, oxygen is released into the atmosphere as a product.
During photosynthesis, oxygen is released into the atmosphere as a waste product.
True or False?
Photosynthesis converts chemical energy into light energy.
False.
It does the reverse — it converts light energy into chemical energy.
Why is glucose a useful product of photosynthesis?
It stores chemical energy in its bonds.
This energy can be released as a fuel for respiration.
What is combined with carbon dioxide to produce glucose?
Hydrogen from the splitting of water.
True or False?
The oxygen released in photosynthesis comes from carbon dioxide.
False.
The oxygen comes from the splitting of water molecules.
What form of energy does light energy get converted into and stored as?
Chemical energy.
It is stored in the biomass of producers, such as in glucose.
Define ATP.
ATP (adenosine triphosphate) is the molecule used to transfer and supply energy within cells — the universal energy currency.
Why is ATP described as the universal energy currency?
It is used to transfer energy within cells in all known forms of life.
What are the three components of an ATP molecule?
The base adenine, a ribose sugar, and three phosphate groups.
How is ATP formed from ADP?
By adding an inorganic phosphate (Pi) to ADP.
This is called phosphorylation and requires energy from respiration.
What is released when ATP is hydrolysed?
An inorganic phosphate and a small amount of energy that the cell can use.
ADP is also formed.
Which enzyme catalyses the hydrolysis of ATP?
ATPase.
Give two processes in a cell that require energy from ATP.
Any two of:
Active transport
Muscle contraction
Building molecules in anabolic reactions
ATP is formed by adding an inorganic phosphate to .
ATP is formed by adding an inorganic phosphate to ADP.
Why is ATP a good short-term energy store rather than glucose?
Its hydrolysis releases a small, manageable amount of energy in a single step.
It can also diffuse to where energy is needed in the cell.
True or False?
ADP contains three phosphate groups.
False.
ADP contains two phosphate groups; ATP contains three.
What happens to the ADP and phosphate produced when ATP is hydrolysed?
They are recycled.
They are rejoined to reform ATP using energy from respiration.
To which group of molecules does ATP belong?
It is a phosphorylated nucleotide, structurally similar to the nucleotides in DNA and RNA.
What is the function of a chloroplast?
It is the site of photosynthesis in a plant cell.
Define stroma.
The stroma is the fluid filling the chloroplast, containing enzymes, sugars, DNA and ribosomes — the site of the light-independent reactions.
Define thylakoid.
A thylakoid is a flattened fluid-filled sac inside a chloroplast; its membrane is the site of the light-dependent reactions.
Define granum.
A granum is a stack of thylakoids inside a chloroplast, connected to other grana by lamellae.
How do the grana help maximise photosynthesis?
They give a large surface area for photosystems, so more light is absorbed.
They also provide membrane area for electron carriers and ATP synthase.
What is found embedded in the thylakoid membranes?
ATP synthase enzymes.
Photosystems containing photosynthetic pigments such as chlorophyll.
Why do chloroplasts contain their own DNA and ribosomes?
The DNA codes for some photosynthesis proteins.
The ribosomes translate these proteins.
Why is the small volume of the thylakoid space useful?
A proton (H⁺) gradient can build up quickly across the thylakoid membrane.
This drives ATP production.
What is the chloroplast envelope, and what does it do?
A double membrane surrounding the chloroplast.
It encloses the components of photosynthesis and controls what enters and leaves.
Stacks of thylakoids inside a chloroplast are called .
Stacks of thylakoids inside a chloroplast are called grana.
True or False?
The light-independent reactions take place in the thylakoid membranes.
False.
The light-independent reactions take place in the stroma; the light-dependent reactions occur at the thylakoid membranes.
Why do the two photosystems absorb light at different wavelengths?
They contain different combinations of pigments.
This maximises the range of light absorbed for photosynthesis.
Where in the chloroplast do the light-dependent reactions occur?
Across the thylakoid membrane.
Define photolysis.
Photolysis is the splitting of water using light energy, producing hydrogen ions, electrons and oxygen.
What three products are formed when one water molecule undergoes photolysis?
2 hydrogen ions (2H⁺).
2 electrons (2e⁻).
One atom of oxygen.
What are the two useful products of the light-dependent reactions?
ATP and reduced NADP (NADPH).
Define photophosphorylation.
Photophosphorylation is the production of ATP using light energy during the light-dependent reactions.
What is the difference between cyclic and non-cyclic photophosphorylation?
Non-cyclic uses photosystems I and II and makes ATP and NADPH.
Cyclic uses only photosystem I and makes ATP only.
In non-cyclic photophosphorylation, what happens when light hits photosystem II?
Two electrons are excited to a higher energy level and leave the photosystem.
They are replaced by electrons from the photolysis of water.
How is ATP produced by chemiosmosis in the light-dependent reactions?
Energy from electrons pumps H⁺ ions into the thylakoid space, building a gradient.
H⁺ diffuse back through ATP synthase, which catalyses ATP production.
How is reduced NADP (NADPH) formed?
Electrons from photosystem I combine with H⁺ ions and NADP.
This reduces NADP to NADPH.
What is the role of NADP in the light-dependent reactions?
It is a coenzyme that transfers hydrogen.
When reduced to NADPH, it carries hydrogen to the light-independent reactions.
The splitting of water using light energy is called .
The splitting of water using light energy is called photolysis.
True or False?
Cyclic photophosphorylation produces both ATP and NADPH.
False.
Cyclic photophosphorylation produces ATP only — non-cyclic produces both ATP and NADPH.
By what other name are the light-independent reactions known?
The Calvin cycle.
Where in the chloroplast does the Calvin cycle take place?
In the stroma.
What two products of the light-dependent reactions does the Calvin cycle require?
ATP and reduced NADP (NADPH).
What happens in the first step of the Calvin cycle?
Carbon dioxide combines with RuBP (a 5C compound).
This forms an unstable 6C compound that splits into two molecules of GP (3C).
Which enzyme catalyses the fixation of carbon dioxide in the Calvin cycle?
Rubisco.
Define carbon fixation.
Carbon fixation is the removal of carbon dioxide from the environment and its incorporation into an organic molecule inside the plant.
How is GP reduced to GALP in the Calvin cycle?
Using energy from ATP and hydrogen from reduced NADP.
Both come from the light-dependent reactions.
What are the two fates of the GALP produced in the Calvin cycle?
Some is used to make useful organic molecules such as glucose.
Most is used to regenerate RuBP.
How many turns of the Calvin cycle are needed to make one glucose molecule?
Six turns.
Glucose has six carbons, and only one carbon is fixed per turn.
What is regenerated at the end of the Calvin cycle, and what does this require?
RuBP is regenerated from GALP.
This requires ATP.
In the Calvin cycle, carbon dioxide combines with the 5-carbon compound .
In the Calvin cycle, carbon dioxide combines with the 5-carbon compound RuBP.
True or False?
The Calvin cycle directly requires light energy.
False.
The Calvin cycle is light-independent — it needs ATP and NADPH, but not light itself.
Which two intermediate molecules of the Calvin cycle are used to make other biological molecules?
Glycerate 3-phosphate (GP) and glyceraldehyde 3-phosphate (GALP).
What biological molecules can GP be used to produce?
Amino acids for protein synthesis.
Fatty acids for lipids.
What can GALP be used to produce?
Hexose sugars such as glucose.
Glycerol for lipids.
Which two polysaccharides can be made by joining hexose sugars from photosynthesis?
Starch and cellulose.
Why is glucose converted to sucrose in a plant?
Sucrose is the form in which sugar is transported in the phloem.
What are two uses of the glucose produced in photosynthesis?
It can be used in respiration to release energy.
It can be used to build new plant biomass (e.g. as starch or cellulose).
How do the products of photosynthesis reach consumers?
They form the plant's biomass.
This biomass is passed on to consumers when the plant is eaten.
Which products of photosynthesis form the basis of DNA and RNA?
Nucleic acids.
Glucose made in photosynthesis is converted to for transport in the phloem.
Glucose made in photosynthesis is converted to sucrose for transport in the phloem.
True or False?
The only useful product of photosynthesis is glucose.
False.
Calvin cycle intermediates are also used to make amino acids, lipids, nucleic acids and polysaccharides.
What is GP used to make that is needed for protein synthesis?
Amino acids, which are joined to build polypeptides.
What is glycerol, made from GALP, used to build?
Lipid molecules such as triglycerides and phospholipids.
Define the Hill reaction.
The Hill reaction is the formation of reduced NADP during the light-dependent reactions, using electrons released from chlorophyll.
What is the natural electron acceptor in the Hill reaction?
NADP, which is reduced to NADPH.
How can DCPIP be used to study the rate of the Hill reaction?
DCPIP accepts the electrons instead of NADP.
As it is reduced it changes from blue to colourless, and the rate of this shows the reaction rate.
What colour change does DCPIP undergo as it is reduced?
From blue to colourless.
What does a faster colour change of DCPIP indicate?
A faster rate of the Hill reaction.
Why is an isolation solution used when extracting chloroplasts?
To prevent damage to the chloroplasts.
Its sucrose and buffer protect against osmosis and extreme pH.
Why is the chloroplast extract kept in an ice-cold water bath?
To slow down the activity of the chloroplasts.
This keeps them intact until the experiment begins.
Why is a beaker of water placed between the lamp and the chloroplast extract?
To absorb heat from the lamp.
This prevents a temperature increase affecting the results.
What happens to the colorimeter absorbance reading during the experiment, and why?
It decreases over time.
As DCPIP is reduced it becomes colourless, so less light is absorbed.
Name two variables you could change to study their effect on the Hill reaction rate.
Light intensity and light wavelength.
(Temperature of the extract is another.)
In the Hill reaction practical, DCPIP acts as an artificial acceptor in place of NADP.
In the Hill reaction practical, DCPIP acts as an artificial electron acceptor in place of NADP.
True or False?
The reduced solution may appear green rather than colourless.
True.
The chlorophyll in the chloroplast extract can make the reduced solution look green.
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