5.1 photosynthesis

Question 1

give the energy transfer process of photosynthesis.

Answer 1

light energy to chemical energy.

Question 2

state and explain three ways in which leaves are adapted to bring together the three raw materials of photosynthesis (water, carbon dioxide, and light), and remove its products (oxygen and glucose)

Answer 2

• a large surface area that adsorbs as much sunlight as possible.
• long, narrow upper mesophyll cells packed with chloroplasts.
• stomata that open and close in response to changes in light intensity.

Question 3

explain the advantage of having many air spaces in the lower mesophyll layer.

Answer 3

to allow rapid diffusion in the gas phase of carbon dioxide and oxygen.

Question 4

give the three main stages of photosynthesis.

Answer 4

• capturing of light energy.
• the light-dependent reaction.
• the light-independent reaction.

Question 5

where in the chloroplast does the light-dependent reaction take place? where are these structures found?

Answer 5

• the light-dependent stage of photosynthesis takes place in the thylakoid membrane.
• thylakoids are disc-like structures that are stacked together in groups called grana.

Question 6

give two ways in which chloroplasts are adapted to their function of capturing sunlight and carrying out the light-dependent reaction.

Answer 6

• the thylakoid membranes provide a large surface area for the attachment of chlorophyll, electron carriers, and enzymes that carry out the light-dependent reaction.
• chloroplasts contain both DNA and ribosomes, so can quickly and easily manufacture some of the proteins involved in the light-dependent reaction.

Question 7

what is a photosystem?

Answer 7

clusters of light-absorbing pigments located in the thylakoid membrane of a chloroplast.

Question 8

in the reaction centre of a photosystem, light energy is converted into chemical energy. which process does the energy captured in these reactions drive?

Answer 8

chemiosmosis, which stimulates ATP production in the chloroplasts.

Question 9

give the name of the primary photosynthetic pigment.

Answer 9

chlorophyll A.

Question 10

give the names and function of the accessory photosynthetic pigments.

Answer 10

• chlorophyll B
• carotene
• phaespytin
• xanthophlly

accessory pigments pass energy down to the primary pigment.

Question 11

give the two photosystems found within the thylakoid membrane, and their peak absorption rates.

Answer 11

photosystem II - chlorophyll A has a peak absorption rate of 680 nm.

photosysytem I - chlorophyll A has a peak absorption rate of 700 nm.

Question 12

light absorbed by the photosystems is used to do what?

Answer 12

• make ATP from ADP.
• make NADPH from NADP.
• split water into H⁺ ions.

Question 13

give the name of, and describe the first stage of the light-dependent reaction.

Answer 13

photoionisation - photons of light hit the chlorophyll A molecules in photosystem II, which causes electron excitation within the thylakoid membrane.

Question 14

describe what happens following electron excitation.

Answer 14

the excited electrons have been raised to a higher energy level, they leave photosystem II and are taken up by electron acceptors (proteins containing metals) and passed along a series of electron carriers in the thylakoid membrane, eventually ending up in photosystem I.

Question 15

what does a proton motive force cause?

Answer 15

a proton motive cause causes the protons to flow back down the gradient into the stroma through an ATP synthase molecule - this energy is used to combine ADP and inorganic phosphate to form ATP.

Question 16

describe happens during photolysis.

Answer 16

• the electrons that were lost from chlorophyll A must be replaced.
• this happens by photolysis - the splitting of water using light into oxygen gas, hydrogen ions and electrons.

Question 17

explain how NADPH is formed.

Answer 17

• light energy is absorbed by photosystem I, which excites the electrons to an even higher energy level.
• the electrons are transferred to NADP, along with a proton from the stroma, to form reduced NADP.

Question 18

what is photophosphorylation?

Answer 18

the phosphorylation of ADP to form ATP using energy from sunlight.

Question 19

which photosystem does cyclic photophosphorylation use?

Answer 19

photosystem I.

Question 20

explain what happens to the electrons during cyclic photophosphorylation.

Answer 20

during cyclic photophosphorylation the electrons from the chlorophyll molecule are not passed into NADP, but are passed back to photosystem I via electron carriers.

Question 21

give the products of cyclic photophosphorylation.

Answer 21

cyclic photophosphorylation does not produce any NADPH or oxygen, only small amounts of ATP.

Question 22

which photosystem does non-cyclic photophosphorylation use?

Answer 22

non-cyclic photophosphorylation uses both photosystem I and photosystem II, following the process of the light-dependent reaction.

Question 23

give the products of the light-dependent reaction.

Answer 23

• ATP.
• NADPH.
• oxygen.

Question 24

what are the products of the light-dependent reaction used for in the light-independent stage?

Answer 24

to generate glycerate-3-phosphate (GP)

Question 25

where in the chloroplasts does the light-independent stage take place?

Answer 25

the stroma.

Question 26

give the name of, and describe the first stage of the light-independent reaction.

Answer 26

carbon fixation - carbon dioxide from the atmosphere diffuses into the leaf through the stomata, and dissolves in water around the walls of the mesophyll cells.

Question 27

explain what happens when carbon dioxide diffuses in the stroma, and the catalyst involved.

Answer 27

• carbon dioxide reacts with the 5-carbon compound RuBP, producing two molecules of the 3-carbon glycerate-3-phosphate (GP)
• this reaction is catalysed by the enzyme rubisco.

Question 28

explain how triose phosphate (TP) is formed.

Answer 28

NADPH from the light-dependent reaction is used to reduce glycerate-3-phosphate to triose phosphate (TP), using energy supplied by ATP.

Question 29

give three of the organic substances that some of the triose-phosphate molecules are converted into.

Answer 29

• glucose.
• starch.
• cellulose.

Question 30

describe the regeneration of RuBP.

Answer 30

most TP molecules are used to regenerate RuBP using ATP from the light-dependent reaction.

Question 31

give two ways in which chloroplasts are adapted to their function of carrying out the light-independent reaction.

Answer 31

• the fluid of the stroma contains all the enzymes needed to carry out the light-independent reaction.
• the chloroplasts contains both DNA and ribosomes, so can quickly and easily manufacture some of the proteins involved in the light-independent reaction.

Question 32

explain why the calvin cycle needs to turn a total of six times in order to produce only one hexose sugar.

Answer 32

• three turns of the cycle produces six molecules of the 3-carbon sugar TP.
• 5 out 6 of these molecules are used to regenerate RuBP.
• a hexose sugar is a six-carbon sugar, so can only be formed from two molecules of TP.
• therefore, the cycle must turn a total of six times to produce these two molecules of TP.

Question 33

give the limiting factors of photosynthesis.

Answer 33

• light intensity.
• carbon dioxide concentration.
• temperature.

Question 34

explain why plants need high light intensity of a certain wavelength for photosynthesis to occur.

Answer 34

• light is needed to provide the energy for the light-dependent reaction - the higher the light intensity, the more energy it provides.
• only certain wavelengths of light can be absorbed by the photosynthetic pigments; chlorophyll A, chlorophyll B, and carotene can only absorb red and blue wavelengths of light.

Question 35

explain why the optimum temperature has to be around 25°C in order for photosynthesis to occur.

Answer 35

• photosynthesis involves the use of enzymes (including ATP synthase and rubisco)
• if the temperature falls below 10°C the enzymes become inactive, but if the temperature rises above 45°C they may start to denature, hence why around 25°C is the optimum temperature.

Question 36

explain how having too much water can affect a plant’s ability to photosynthesise.

Answer 36

too much water will cause the soil to become waterlogged, reducing the uptake of minerals such as magnesium, which is needed to manufacture chlorophyll.