Photosynthesis (OCR A Level Biology): Flashcards

Energy is never created or destroyed; it is only converted from one form to another. In respiration, energy stored in organic molecules is released into usable forms rather than being made.

Simple inorganic compounds are converted into complex organic compounds.

False — respiration occurs in all living organisms.

True

The organelle in plant cells where photosynthesis occurs. It is surrounded by a double-membrane envelope and is roughly 2–10 μm in diameter.

• A double-membrane envelope, with each membrane being a phospholipid bilayer

• The outer membrane is permeable to a range of ions and small molecules

• The inner membrane contains transport proteins that only allow certain molecules or ions to enter or leave the chloroplast

The cytosol-like fluid that fills the chloroplast. It contains dissolved CO₂, sugars, enzymes and other molecules, and is the site of the light-independent stage of photosynthesis.

A series of flattened, fluid-filled sacs that make up the membrane system inside the stroma. Their membranes contain pigments, enzymes and electron carriers.

Stacks of thylakoids within the chloroplast. They create a large surface area for many photosystems, maximising light absorption and providing membrane space for electron carriers and ATP synthase.

Membranous channels that connect the grana.

The large surface area provides a large number of pigment molecules, ensuring that as much light as necessary is absorbed for photosynthesis.

A light-harvesting cluster of photosynthetic pigment molecules in the thylakoid membrane. The pigments are arranged in funnel-like structures, passing energy from one pigment to the next until it reaches the primary pigment reaction centre.

• Small (70S) ribosomes

• A loop of DNA

• Starch grains

The loop of chloroplast DNA codes for some of the chloroplast proteins (others are coded for by the DNA in the plant cell nucleus).

Sugars are stored as starch inside the starch grains in the stroma.

• Light-dependent stage: on the thylakoid membranes of the grana, where light is trapped within the reaction centres

• Light-independent stage: in the stroma

The selective transport proteins in the inner membrane of the chloroplast envelope control the flow of molecules between the stroma and the cytosol.

Enzymes in the stroma catalyse the reactions of the light-independent stage of photosynthesis.

They ensure the stacks of thylakoid sacs are connected but distanced from each other.

The chloroplast proteins coded for by chloroplast DNA are translated at the 70S ribosomes in the stroma.

True

False — the inner membrane contains the selective transport proteins; the outer membrane is permeable to a range of ions and small molecules.

Within the thylakoids.

• Chlorophylls

• Carotenoids

A pigment that surrounds the primary pigment and absorbs both similar and different wavelengths of light to chlorophyll, expanding the range of wavelengths that can be absorbed for use in photosynthesis.

• Chlorophyll a - blue-green

• Chlorophyll b - yellow-green

• β carotene - orange

• Xanthophyll - yellow

The blue-violet and red regions of the spectrum.

Mainly the blue-violet region of the spectrum.

Stacks of thylakoids within a chloroplast (singular: granum).

A light-harvesting cluster of photosynthetic pigment molecules held in the thylakoid membrane, arranged so that as much light as necessary is absorbed.

In funnel-like structures in the thylakoid membrane.

• Photosystem I (PSI) = P700; its chlorophyll a has maximum light absorption at 700 nm

• Photosystem II (PSII) = P680; its chlorophyll a has maximum light absorption at 680 nm

The two types of photosynthetic pigment are chlorophylls and carotenoids.

Stacks of thylakoids within a chloroplast are known as grana.

They absorb different wavelengths of light for use in photosynthesis.

Chlorophylls reflect green light, which is why plants appear green.

Each pigment molecule passes energy down to the next until it reaches the primary pigment reaction centre.

True

False — Photosystem II is P680; P700 is Photosystem I.

An experimental technique used to separate mixtures. Different components travel through the material at different speeds due to their size and charge, causing them to separate.

Within the thylakoids.

• Chlorophyll a – blue-green

• Chlorophyll b – yellow-green

• β Carotene – orange

• Xanthophyll – yellow

• Paper chromatography – the mixture of pigments is passed through paper (cellulose)

• Thin-layer chromatography (TLC) – the mixture is passed through a thin layer of adsorbent (e.g. silica gel), through which the mixture travels faster and separates more distinctly

Pen ink would separate into pigments during the experiment and obscure the results. Pencil (graphite) does not dissolve in the solvent.

Acetone is an organic solvent, so fats such as the lipid membranes dissolve in it.

Acetone and mechanical pressure break down the cell, chloroplast and thylakoid membranes to release the pigments.

• Mobile phase – the solvent, in which the mixture is dissolved and carried

• Stationary phase – the static material (e.g. the paper) through which the dissolved mixture passes

If the solvent were above the pencil line, the pigment spot would dissolve directly into the solvent rather than travelling up the paper, so the pigments would not separate.

Rf value = distance travelled by the component (pigment) ÷ distance travelled by the solvent.

Always measure to the centre of each spot.

• Molecules with a higher affinity to the stationary phase (e.g. large molecules) travel slower and have a smaller Rf value

• Molecules that are more soluble in the mobile phase travel faster and have a larger Rf value

• Carotenoids – highest Rf value (usually close to 1)

• Chlorophyll a – intermediate, between carotenoids and chlorophyll b

• Chlorophyll b – much lower Rf value

A small Rf value indicates that a pigment is less soluble and/or larger in size.

• It is not as specific as other chromatography techniques (though sufficient to separate pigments and calculate Rf values)

• It gives no data on the amount of each pigment present or the wavelengths they absorb (colorimetry can be used for these)

Each pigment absorbs different wavelengths of light.

True

False — it must be drawn in pencil, as pen ink separates into pigments and obscures the results.

In the thylakoids — both the thylakoid membranes and the thylakoid lumen.

The breakdown of water using light energy, producing hydrogen ions, electrons and oxygen in the thylakoid lumen.

The overall process of using light energy and the electron transport chain to phosphorylate ADP to ATP.

The movement of protons down their concentration gradient from the thylakoid lumen to the stroma by facilitated diffusion through ATP synthase, providing the energy to synthesise ATP.

A collection of photosynthetic pigments that absorbs light energy and transfers it onto electrons; each contains a primary pigment.

As excited electrons pass through the electron transport chain they gradually release their energy.

This energy is used by a proton pump to transport protons (H⁺) across the thylakoid membrane, from the stroma to the thylakoid lumen.

This creates a high concentration of protons in the thylakoid lumen and a low concentration in the stroma.

Protons return from the thylakoid lumen to the stroma down their concentration gradient by facilitated diffusion through ATP synthase (chemiosmosis).

This provides the energy to add an inorganic phosphate group (Pᵢ) to ADP:

ADP + Pᵢ → ATP

They are alternately:

• reduced as they gain an electron

• oxidised as they lose the electron by passing it to the next carrier

Hydrogen ions (from photolysis of water) and electrons from the electron transport chain combine with the carrier molecule NADP:

2H⁺ + 2e⁻ + NADP → reduced NADP

Reduced NADP then passes to the light-independent reactions for the synthesis of carbohydrates.

Photosystem I (PSI) only.

• Light is absorbed by photosystem II; electrons in the primary pigment (P680) are excited and emitted (photoionisation)

• Excited electrons pass down an electron transport chain to photosystem I, driving chemiosmosis and ATP synthesis

• Electrons lost from PSII are replaced by electrons from the photolysis of water

• Photosystem I also absorbs light and its electrons are excited; these combine with H⁺ and NADP to form reduced NADP

• Cyclic involves only photosystem I, whereas non-cyclic involves both photosystems I and II

• Cyclic does not produce reduced NADP, whereas non-cyclic does

It is a water-splitting enzyme that catalyses the photolysis of water:

H₂O → 2H⁺ + 2e⁻ + ½O₂

The electrons released replace those lost from the primary pigment of photosystem II.

In non-cyclic photophosphorylation, the electrons lost by photosystem II are replaced by electrons from the photolysis of water.

No — the excited electron is passed along an electron transport chain and returned to PSI (hence 'cyclic'), driving ATP synthesis via chemiosmosis but producing no reduced NADP.

False — cyclic photophosphorylation involves only photosystem I.

True

• The light-dependent stage, which takes place in the thylakoids

• The light-independent stage (Calvin cycle), which takes place in the stroma

• ATP

• Reduced NADP

Hydrogen ions combine with the carrier molecule NADP, using electrons from the photolysis of water, to form reduced NADP.

ATP is produced from ADP and P_i by the enzyme ATP synthase, in a process called photophosphorylation (ADP + P_i → ATP).

The production of ATP from ADP and P_i during the light-dependent stage of photosynthesis, using the energy from the proton gradient generated by the photolysis of water.

The proton (H⁺) gradient generated by the photolysis of water drives photophosphorylation.

The proton (H⁺) gradient used for photophosphorylation is generated by the photolysis of water.

• Energy, in the form of ATP

• Hydrogen, in the form of reduced NADP

The light-independent reactions of photosynthesis, which use the energy from ATP and the hydrogen from reduced NADP to produce complex organic molecules such as carbohydrates.

• Starch – for storage

• Sucrose – for translocation around the plant

• Cellulose – for making cell walls

To produce ATP and reduced NADP, which are then used in the light-independent stage (Calvin cycle) to complete photosynthesis by making complex organic molecules.

False — the light-dependent stage takes place in the thylakoids; the light-independent stage takes place in the stroma.

True

The light-independent stage of photosynthesis; a cycle of reactions that fixes carbon dioxide and produces complex organic molecules, using ATP and reduced NADP supplied by the light-dependent stage.

The incorporation of carbon dioxide from the external environment into an organic molecule. In the Calvin cycle, CO2 combines with ribulose bisphosphate (RuBP).

The enzyme that catalyses the fixation of carbon dioxide by combining it with ribulose bisphosphate (RuBP).

• Carbon fixation: rubisco catalyses the combination of CO2 with ribulose bisphosphate (RuBP) to yield two molecules of glycerate 3-phosphate (GP)

• Reduction: GP is reduced to triose phosphate (TP) using reduced NADP and ATP

• Regeneration: RuBP is regenerated from TP in reactions that use ATP

Carbon dioxide combines with the five-carbon (5C) sugar ribulose bisphosphate (RuBP).

This reaction is catalysed by the enzyme rubisco.

The resulting six-carbon (6C) compound is unstable and splits into two molecules of the three-carbon (3C) compound glycerate 3-phosphate (GP).

It is unstable and immediately splits into two molecules of the three-carbon (3C) compound glycerate 3-phosphate (GP).

GP is reduced to triose phosphate (TP) using:

• Energy from ATP

• Hydrogen from reduced NADP

Both of these are produced during the light-dependent stage of photosynthesis.

• One-sixth of the TP molecules are used to produce useful organic molecules needed by the plant (e.g. carbohydrates)

• Five-sixths of the TP molecules are used to regenerate ribulose bisphosphate (RuBP)

Five-sixths of the triose phosphate (TP) molecules are used to regenerate RuBP.

This process requires ATP.

It does not directly require light, so it can occur in light or darkness. However, it requires inputs of ATP and reduced NADP from the light-dependent stage. In darkness these inputs run out, so the cycle cannot continue indefinitely.

• Starch – for storage

• Sucrose – for translocation around the plant

• Cellulose – for making cell walls

Carbon dioxide is fixed by combining with the 5C compound ribulose bisphosphate to form two molecules of glycerate 3-phosphate (GP), a 3C compound.

The reduction of glycerate 3-phosphate to triose phosphate requires ATP and reduced NADP from the light-dependent stage.

False — it does not directly require light, so it can occur in light or darkness, but it stops once the ATP and reduced NADP from the light-dependent stage run out.

False — GP is not a carbohydrate, but the next step of the Calvin cycle converts it into one.

A phosphorylated three-carbon (3C) sugar produced during the light-independent stage of photosynthesis by the reduction of glycerate 3-phosphate (GP).

Glycerate 3-phosphate (GP) is reduced to triose phosphate (TP) using:

• energy from ATP

• hydrogen from reduced NADP

Both are products of the light-dependent stage.

One-sixth of the triose phosphate molecules are used to produce useful organic molecules needed by the plant.

Five-sixths of the triose phosphate molecules are used to regenerate ribulose bisphosphate (RuBP).

Five-sixths of the triose phosphate molecules are used to regenerate ribulose bisphosphate (RuBP), a process that requires ATP.

ATP provides the energy needed to convert triose phosphate back into ribulose bisphosphate (RuBP), ensuring the Calvin cycle can continue to fix carbon dioxide.

Two triose phosphates (3C) can condense to form a hexose phosphate (6C).

• Triose phosphate is converted to glycerol

• Glycerate 3-phosphate (GP) is converted to fatty acids

Glycerol and fatty acids join together to form lipids for cell membranes.

Triose phosphate can be used in the production of amino acids, which are then used for protein synthesis.

• starch

• sucrose

• cellulose

Two triose phosphate molecules can condense to form a hexose phosphate, which is a six-carbon (6C) sugar phosphate.

Triose phosphate can be used to make:

• carbohydrates (via hexose phosphates)

• lipids for cell membranes

• amino acids for protein synthesis

Hexose phosphates can be used to produce:

• starch

• sucrose

• cellulose

True

False — only one-sixth leaves to form useful organic molecules; the other five-sixths regenerates RuBP.

A factor that, when in short supply or below the optimum level, restricts the rate of photosynthesis, even if all other factors are at their optimum level.

• Light intensity

• Carbon dioxide concentration

• Temperature

Although a water shortage can reduce the rate of photosynthesis, it usually affects other processes in the plant before it affects photosynthesis, so it is not classed as a main limiting factor.

Greater light intensity supplies more energy, so the light-dependent stage occurs faster.

This produces more ATP and reduced NADP for the Calvin cycle (light-independent stage), which can then also occur at a greater rate.

Light intensity is no longer the limiting factor.

Another factor is now limiting the rate, e.g. temperature being too low or too high, or an insufficient supply of carbon dioxide.

Carbon dioxide is required in the light-independent stage, where it is combined with the five-carbon compound ribulose bisphosphate (RuBP) during carbon fixation.

More carbon dioxide allows this step of the Calvin cycle to occur faster, increasing the overall rate of photosynthesis.

At concentrations much higher than the natural atmospheric level, carbon dioxide can become toxic to the plant.

The light-independent reactions (Calvin cycle) are enzyme-controlled, e.g. rubisco catalyses the reaction between CO2 and RuBP.

Up to an optimum temperature, increasing temperature increases the rate of these reactions and so the rate of photosynthesis.

Beyond the optimum, the enzymes begin to denature and the rate decreases.

The light-dependent reactions are driven by energy from light rather than by the kinetic energy of the reacting molecules, so temperature does not significantly affect them.

• Increasing temperature can cause stomata to close to reduce water loss; this stops CO2 entering the leaf, slowing photosynthesis.

• Extreme temperatures can change thylakoid membrane permeability, dissipating the proton gradient needed for the light-dependent reaction and slowing photosynthesis.

GP increases slightly; TP and RuBP decrease.

With less light, the light-dependent stage slows, producing less ATP and reduced NADP.

GP builds up because it is not converted to TP; the lack of TP means less RuBP is regenerated.

GP and TP decrease; RuBP increases.

RuBP accepts carbon dioxide, so when there is very little CO2 it remains unfixed and builds up.

The lack of carbon fixation prevents GP and then TP from forming.

In a glasshouse, sensors can monitor light intensity, humidity and carbon dioxide concentration so that limiting factors can be adjusted to allow the crop to photosynthesise at the highest rate possible.

Where the line is rising, the limiting factor is whatever is plotted on the x-axis.

Where the line is horizontal, the limiting factor is something other than the x-axis variable (choose from temperature, light intensity or carbon dioxide concentration).

False — a water shortage usually affects other processes in the plant before photosynthesis, so it is not classed as a main limiting factor.

False — the light-dependent reactions are driven by light energy, not the kinetic energy of molecules, so temperature has little direct effect on them.

• Light intensity

• Carbon dioxide concentration

• Temperature

Aquatic plants (pondweed) such as Elodea or Cabomba.

The distance (d) of the light source from the plant is changed.

Different quantities of sodium hydrogencarbonate (NaHCO₃) are added to the water surrounding the plant.

The NaHCO₃ dissolves to produce CO₂, a reactant in photosynthesis.

The boiling tube containing the submerged plant is placed in water baths set to different temperatures.

When investigating one limiting factor, the other two factors must be kept constant to ensure a valid result.

The glass tank absorbs heat from the lamp.

This keeps the temperature of the solution constant, so it does not act as a confounding variable.

Measure the volume of oxygen gas collected in the gas syringe over a set period of time (e.g. 5 minutes).

It ensures the oxygen given off by the plant forms bubbles that can be collected, rather than dissolving in the water.

• Known surface area and volume, making it easier to standardise the quantity of algae

• Cheap and easy to grow

• Can be kept alive for several weeks and reused in different experiments

Light releases high-energy electrons from chlorophyll during the light-dependent reaction.

Instead of being taken up by NADP, these electrons are accepted by the redox indicator, which becomes reduced and changes colour.

It changes from blue (oxidised) to colourless (reduced). The solution may appear green due to the chlorophyll present.

It is kept in the dark to act as a control.

This confirms that any colour change in the other tubes is due to light driving the light-dependent reaction.

A liquid in which leaves are crushed to produce a concentrated extract containing a suspension of intact, functional chloroplasts. It has the same water potential as the leaf cells, contains a buffer to keep pH constant, and is kept ice-cold to protect the chloroplasts.

Light intensity is proportional to 1/d², so moving the lamp closer increases the light intensity.

Rate of photosynthesis = volume of oxygen produced ÷ time elapsed

The rate of dehydrogenase activity, and therefore the rate of the light-dependent stage of photosynthesis.

True

False — DCPIP changes from blue (oxidised) to colourless (reduced).