Unit 5- Photosynthesis

Question 1

photosynthesis and respiration involve enzymes

Answer 1

a coenzyme used in photosynthesis is NADP. NADP transfers hydrogen from one molecule to another- this means it can reduce or oxidise a molecule.
- examples of coenzymes used in respiration are: NAD, FAD and Coenzyme A.
- NAD and FAD transfer hydrogen from one molecule to another, this means they can reduce or oxidise a molecule.
-Coenzyme A transfers acetate between molecules

Question 2

chloroplasts 1

Answer 2

-small flattened organelles found in plant cells
-they have a double membrane called the chloroplast envelope.
-thylakoidds are stacked up in the chloroplast into structures called grana. the grana are linked together by bits of thylakoid membrane called lamellae.
-contains photosynthetic pigments (chlorophyll A,B, carotene). coloured substances that absorb the light energy needed for photosynthesis. the pigments are found in the thylakoid membranes- theyre attached to protein.
the protein and pigment= photosystems.

Question 3

chloroplasts 2

Answer 3

-photosystems contain 2 types of photosynthetic pigments: primary pigments and accessory pigments. primary pigments are reaction centres, where electrons are excited during the light dependent reaction. Accessory pigments make up light harvesting systems. Surrounds reaction centres and transfer light energy to them to boost the energy available for electron excitement to take place.
- there are two photosystems used by plants. photosystem I absorbs light best at a wavelength of 700nm and photosystem II - 680nm.
- contained within inner membrane of chloroplast is stroma which contains enzymes, sugars, organic acids etc.
-chloroplasts have their own DNA which is located in the stroma and is circular.
-carbohydrates can be stored as starch grains in stroma.

Question 4

the light-dependent reaction

Answer 4

this reaction needs energy. takes place in the thylakoid membranes of the chloroplasts. light energy is absorbed by the photosynthestic pigments in the phtotosystems and converted to chemical energy. the light nergy is used to add a phosphate group to ADP to form ATP and to reduce NADP to form reduced NADP. ATP transfers energy and reduced NADP transfers hydrogen to the light independent reaction. during this process H2O is oxidised to O2.

Question 5

the light independent reaction

Answer 5

also called the calvin cycle. doesnt directly used light energy. but does rely on the products of the light dependent reaction. takes place in the stoma of the chloroplasts. here the ATP and reduced NADP from the light dependent reaction supply the enrgy and the hydrogen to make glucose from CO2.

Question 6

Thin layer chromatography (PRACTICAL)

Answer 6

1. grind up several leaves with some anhydrous sodium sulfate and some propanone.
2. transfer the liquid to a test tube, add some petroleum ether and gently shake the tube. two distinct layers will form in the liquid- the top layer is the pigments mixed in with the petroleum ether.
3. transfer some of the liquid from the top layer into a second test tube wtih some anhydrous sodium sulfate
4. draw a horizontal pencil line near the bottom of a chromatography plate. build up a single concentrated spot of the liquid on the line by aplying several drops and ensuring each one is dry before the next is added. this is the point of origin
5. then put the plate into a glass beaker with some prepared solvent- just enough so that the point of origin is a liitle bit above the solvent. ut a lid on the beaker and leave the plate to develop. as the solvent spreads up the plate, the different pigments move with it but at different rates so they separate.
6. when the solvent has nearly reached the top, take the plate out and mark the solvent front with a pencil and leave the plate to dry in a well ventilated place.
7. you can calculate the Rf value of the pigments and look them up in a database to identify what the pigments are

Question 7

what is ATP made by in the light dependent reaction?

Answer 7

Phosphorylation. there are two types: cyclic and noncyclic.
light energy is absorbed by the photosystems for three things:
1. making ATP from ADP and inorganic phosphate. this reaction is called photophosphorylation.
2. making reduced NADP from NADP.
3. splitting water into protons and, electons and oxygen. this is called photolysis.

Question 8

non-cyclic photophosphorylation 1

Answer 8

light energy excites electrons in chlorophyll.
light energy is absorbed by PSII. the light energy excites electrons in the chlorophyll. the electrons move to a higher energy level. these high energy electrons move along the electron transport chain to PSI

Question 8

non-cyclic photophosphorylation 2

Answer 8

photolysis of water produces electrons, protons and oxyfen.
as the excited electrons from chlorophyll leave PSII to move along the electron transport chain, the must be replaced. light energy splits water into protons, electrons and oxygen. the reaction is:
H2O—-> 2H+ + 1/2 O2

Question 9

non-cyclic photophosphorylation 3

Answer 9

energy from the excited electroms makes ATP.
the excited electrons lose energy as they move the electron transport chain. this energy is used to transport protons into the thylakoid, via membrane protons called proton pumps, so that the thylakoid has a higher concentration of protons than the stroma. this forms a proton gradient across the membrane. protons move down their concentration gradient, into the stroma via an enzyme called ATP synthase. the energy from this movement combines ADP and inorganic phosphate to form ATP.

Question 10

non-cyclic photophosphorylation 4

Answer 10

.. and generates reduced NADP.
light energy is absorbed by PSI which excites the electrons again to an even higher energy level. finally the electrons are transferred to NADP, along with a proton from the stroma to form reduced NADP.

Question 11

cyclic phosphorylation

Answer 11

only produces ATP. only uses PSI. its called cyclic because the electrons from the chlorophyll molecule arent passed onto NADP, but are passed back to PSI via electron carriers. this means the electrons are recycled and can repeatedly flow through PSI. this process doesnt produce any reduced NADP or O2- it only produces small amounts of ATP.

Question 12

the calvin cycle- co2 is combined with RuBP to form two molecules of GP

Answer 12

co2 enters the leaf through the stomata and diffuses into the stroma of the chloroplast. here, its combined with ribulose biphosphate, a 5 carbon compound. this gives an unstable 6- carbon compound, which quickly breaks down into 2 molecules of a 3 carbon compound called glycerate 3-phosphate (GP).ribulose biphosphate carboxylase (RuBisCO) catalyses the reaction between CO2 and ribulose bisphosphate.

Question 13

the calvin cycle- ATP and NADP are required for the production of GP to TP

Answer 13

now ATP from the light dependent reaction provides energy to turn the 3 carbon compound, GP into a different 3 carbon compound called triose phosphate (TP). this reaction also requires H+ ions whcih comes from reduced NADP. reduced NADP is recycled to NADP (for use in the light dependent reaction again). TP is then converted into many useful compounds

Question 14

the calvin cycle- RuBP is regenerated

Answer 14

5 out of 6 molecules of TP produced in the cycle arent used to make hexose sugars, but to regenerate RuBP. regenerating RuBP uses the rest of the ATP porduced by the light independent reaction.

Question 15

what useful organic substances like TP and GP converted to?

Answer 15

carbohydrates- hexose sugars are made by joining two TP molecules together and larger cabohydrates are made by joining hexose sugars together in different ways
lipids- these are made using glycerol which is synthesised from TP and fatty acids which are syntheised from GP
amino acids- some amino acids are made from GP

Question 16

how many turns does the calvin cycle have to make in ouder to form one hexose sugar

Answer 16

three turns of the cycle produces six molecules of TP because two molecules of TP are made for every one co2 molecule used. five out of six of these TP molecules are used to regenerate RuBP. this means that for three turns of the cycle only one TP is produced thats used to make a hexose sugar. this means that the cycle must turn 6 times to produce 2 molecules of TP that can be used to make one hexose sugar. six turns of the cycle needs 18 ATP and 12 reduced NADP from the light dependent reaction

Question 17

what are the optimum conditions for photosynthesis?

Answer 17

1. high light intensity of a certain wavelength
2. temperature around 25’C
3. carbon dioxide at 0.4%

Question 18

high light intensity of a certain wavelength

Answer 18

light is needed to provide the energy for the light dependent reaction - the higher the intensity of the light, the more energy it provides. only certain wavelengths of light are used for photosynthesis. the photosynthetic pigments chlorophyll a, b and carotene only absorb the red and blue light in sunlight (green light is reflected which is why plants look green)

Question 19

temperature around 25’C

Answer 19

photosynthesis involvesenzymes. if the temperature fakks below 10’C the enzymes become inactive, but if the temperature is more than 45’C they might start to denature. also at high temperatures:
1. stomata close to avoid losing too much water. this causes photosynthesis to slow down because less co2 enters the leaf when the stomata are closed
2. the thylakoid membranes may be damaged. this could reduce the rate of the light dependent stage reactions by reducing the number of sites available for electron transfer.
3. the membranes around the chloroplasts could be damaged, which could cause enzymes important in the calvin cycle to be released into the cell. this would reduce the rate of the light independent stage reactions
4. chorophyll could be damaged. this would reduce the amount of pigment that can absorb light energy, which would reduce the rate of the light dependent stage reactions

Question 20

carbon dioxide at 0.4%

Answer 20

carbon dioxide makes up 0.04% of the gases in the atmosphere. increasing this to 0.4% gives a higher rate of photosynthesis but any higher and the stomata start to close.