5.1.5 Photosynthesis

Question 1

Why is energy important?

Answer 1

-living organisms need energy for biological/metablolic processes i.e plants require energy for active transport, photosynthesis, DNA replication + cell division

Question 2

Respiration

Answer 2

-energy from releasing glucose is used to power all the biological processes in a cell
-aerobic= using oxygen
-anaerobic= without oxygen

Question 3

Word equation for respiration

Answer 3

C6H12O6 + 6O2 –} 6CO2 + 6H2O

Question 4

What is photosynthesis?

Answer 4

-process by which energy from light is used to make glucose from water and CO2
-this energy is stored in glucose* until plants release it for respiration
*light energy is transferred into chemical energy trapped in the bonds of glucose produced

Question 5

Equation for photosynthesis

Answer 5

6CO2 + 6H2O –} C6H12O6 + 6O2

Question 6

Autotrophs vs heterotrophs

Answer 6

-autotrophs can produce their own food in the form of complex organic molecules i.e glucose, using light/chemical energy
-heterotrophs obtain complex organic molecules by eating other organisms

Question 7

Structure of chloroplasts

Answer 7

-small flattened organelles in plant cells
-double membrane(chloroplast envelope)
-thylakoids(fluid-filled sacs forming a network of membranes)–} stacked to form the grana, site of light dependent stage
-lamellae= thylakoid membrane channels that link the grana
-stroma= fluid-like region, site of many chemical reactions i.e light independent stage: contain enzymes, starch(energy storage of glucose), lipid droplets, ribosomes(protein synthesis), sugars + circular DNA(codes for enzymes needed for photosynthesis)

Question 8

How do thylakoids being a network of membranes help the chloroplast?

Answer 8

-provide a large SA to maximise light absorption for light-dependent photosynthesis

Question 9

Chlorophyll

Answer 9

-photosynthetic pigments that absorb the light energy needed for photosynthesis–} mainly absorbs red + blue light and reflects green light
-found in the thylakoid membranes, attached to proteins–} form a photosystem
-photosystems can either be primary pigments or accessory pigments
-i.e chlorophyll a, chlorophyll b, carotenoids , xanthophylls
-form light harvesting systems that absorb light energy of different wavelengths and transfer this efficiently to the reaction centre for photosynthesis

Question 10

Primary pigments

Answer 10

-reaction centres where electrons are excited after sunlight is absorbed during the light-dependant reaction
-mostly chlorophyll a

Question 11

Accessory pigments

Answer 11

-make up light-harvesting systems
-surround reaction centres and transfer light energy to them to boost the energy available for electron excitement to take place
-i.e chlorophyll B, xanthophylls and carotenoids

Question 12

Redox reactions

Answer 12

-oxidation= lost electrons, and may have gained hydrogen/lost oxygen
-reduction= has gained electrons, and may have lost hydrogens/gained oxygen

Question 13

Coenzyme

Answer 13

-molecule that aids the function of an enzyme
-usually work by transferring a chemical group from one molecule to another
-NADP transfers hydrogen from one molecule to another

Question 14

What are photo systems?

Answer 14

-structures that absorb light energy
-in the thylakoid membrane
-made up of photosynthetic pigments(primary and accessory)

Question 15

Light-dependent stage

Answer 15

-energy from sunlight is absorbed and used to form ATP from ADP + an inorganic phosphate
-hydrogen from water is used to reduce NADP to reduced NADP

Question 16

Photolysis

Answer 16

-photons of light are used to split water molecules into oxygen, electrons and protons(H+ ions) in the thylakoid lumen
-oxygen is released as a by-product and diffused out of the chloroplast to the atmosphere via the stomata

Question 17

Equation for photolysis

Answer 17

2H2O —} 4e- + 4H+ +O2

Question 18

‘Excited’ electrons

Answer 18

-light energy/ photons of light is absorbed by PSII(P680), which excites electrons in the thylakoid membrane
-the electrons move to a higher energy level so they move along the electron transport chain to PSI via electron carriers where a series of redox reactions take place
-electrons produced from photolysis replace the electrons lost in PSII

Question 19

Proton pump

Answer 19

-electrons lose energy as they move down the chain as its used to transport protons across the membrane via proton pumps to form a proton conc gradient
-in the thylakoid lumen, the protons released from photolysis increase the conc gradient higher than the stroma–} protons move into the stroma via the enzyme ATP synthase

Question 20

Chemiosmosis

Answer 20

-ATP synthase combines an inorganic phosphate with ADP to form ATP–} used in the Calvin cycle

Question 21

electrons part 2

Answer 21

-electrons have lost lots of energy when they reach PSI
-photons of light/light energy is absorbed which excites the electrons again to an even higher energy level
-are transferred with a proton in the stroma to NADP(a coenzyme) forming reduced NADP—} NADP + H+ +2e- = NADPH
-transfers protons and electrons to the Calvin cycle–} can be recycled itself to be used again in the light-dependent reaction(maintains proton conc gradient in thylakoid membranes)

Question 22

Cyclic phosphorylation/photophosphorylation

Answer 22

-the electrons leaving the electron transport chain after PSI can be returned to PSI via electron carriers instead of being used to reduce NADP
-leads to a small amount of ATP being produced without electrons being supplied by PSII

Question 23

Light independent stage

Answer 23

• H+ ions from reduced NADP and CO2 is used to make organic molecules i.e glucose
  -ATP supplies the required energy for this
  -takes place in the chloroplast stroma

Question 24

Formation of glycerate 3-phosophate/ Carbon fixation

Answer 24

-Carbon dioxide enters the spaces within the spongy mesophyll of leaves by diffusion from the atmosphere through stomata
-diffuses into the stroma of the chloroplast
-CO2 combines with a 5 carbon molecule= ribulose bisphosphate(RuBP)–} carbon becomes fixed(is incorporated into an organic molecule)
-the 6 carbon compound formed is very instable and breaks down into 2 molecules of glycerate 3-phosphate(GP)
-Ribulose bisphosphate carboxylase(RuBisCO) catalyses reaction between CO2 and RuBP

Question 25

more on RuBisCO

Answer 25

-inefficient enzyme as it is competitively inhibited by oxygen–} lots of it is needed for photosynthesis

Question 26

Formation of triose phosphate/ Reduction

Answer 26

-each GP molecule is reduced to another 3-carbon molecule, TP
For this reaction:
-2 ATP molecules from the light dependent reaction provide the energy(produce 2 ADP + inorganic phosphates)
-requires H+ ions, which is donated by reduced NADP from the light dependent reaction(reduced NADP is recycled to NADP for the light dependent reaction)
-TP is the starting point for synthesis of many useful biological molecules

Question 27

Biological molecules

Answer 27

-carbohydrates/hexose sugars are made from 2 TP molecules and larger ones i.e cellulose, starch, sucrose are made from condensation reactions
-lipids are made from glycerol(synthesised from TP) and fatty acids(synthesised from GP)
-some amino acids are made from GP combined with nitrogen

Question 28

Regeneration (of RuBP)

Answer 28

-5 out of 6 TP molecules produced are used to regenerate RuBP, to continue the Calvin cycle
-requires ATP
-starting point for the synthesis of many biological molecules i.e carbs, lipids, proteins and nucleic acids

Question 29

How is a hexose sugar produced?

Answer 29

-has 6 carbon atoms–} 6 turns of the Calvin cycle
-produced by joining 2 TP molecules
-every 3 turns, 6 TP molecules are produced, 5 of which are used to regenerate RuBP
and 1 for glucose

Question 30

Calculating carbons produced

Answer 30

10 x 3-carbon TP= 30 carbons—} 6 x 5= 30 carbons in total
(10 from the 5 carbons every 3 turns to regenerate, 6 is the amount of turns, 5 is the number of carbons RuBP has before cycle restarts)

Question 31

Why is light intensity needed for photosynthesis?

Answer 31

-needed as energy source for the light-dependent stage
-higher light intensity= ROR for light dependent increases, faster rate of ATP and NADPH production, ROR light independent increases

Question 32

The effect of reducing light intensity on the Calvin cycle

Answer 32

-will reduce the rate of the light-dependent stage
-reduce the quantity of ATP and reduced NADP
produced(products of light dependent)–} needed to convert GP to TP
-conc of GP will increase and conc of TP will decrease
-conc of RuBP will decrease due to less TP

Question 33

Why is carbon dioxide conc needed for photosynthesis?

Answer 33

-needed as a source of carbon
-increases rate of carbon fixation in the Calvin cycle and conc of GP if all other conditions are met
-has a conc of around 0.04% in the atmosphere but having a conc of 0.4% increases the rate of photosyntheis(concentration gradient)
-any higher and the stomata starts to close

Question 34

The effect of CO2 concentration on the Calvin cycle

Answer 34

-at low concs, conversion of RuBP to GP is slow as there is less CO2 to be ‘fixed’
-less concs of GP and TP as they are being used to make RuBP but are not made back
-increasing conc of RuBP as it is not being used up

Question 35

Why is optimum temp needed for photosynthesis?

Answer 35

-affects the rate of enzyme controlled reactions i.e ATP synthase, RuBisCO
-optimum(25-30) increases rate of reactions i.e carbon fixation

Question 36

Temperature above 45

Answer 36

-rate increases until enzymes denature
-stomata close to avoid losing too much water via transpiration–} less CO2 can diffuse in
-damage to thylakoid membranes–} reduce no. of sites available for electron transfer
-membrane around chloroplast could break down–} enzymes for the Calvin cycle release into cell
-damaged chlorophyll–} reduce pigment that can absorb light energy

Question 37

The effect of temp on the Calvin cycle

Answer 37

-at low temps, enzyme and substrate molecules have less kinetic energy resulting in less successful collisions–} lower concs of GP and TP
-at very high temps, enzymes will denature

Question 38

Water stress

Answer 38

-NOT A LIMITING FACTOR because if water levels were very low, the plant closes its stomata and ceases photosynthesis
-leads to less CO2 entering the leaf as the stomata is closed

Question 39

Increasing plant growth commercially

Answer 39

-CO2 is added to the air i.e burning small amount of propane in a CO2 generator
-sunlight + artificial lamps at night time
-glasshouses trap heat energy from sun which warms the air
-heaters + cooling systems

Question 40

How do we investigate the pigment in leaves?

Answer 40

-using thin layer chromatography

Question 41

Investigating the rate of photosynthesis

Answer 41

-A source of white light is placed at a specific distance from the pondweed
-the pondweed is left to photosynthesise for a set amount of time–} oxygen released will collect in the capillary tube
-at the end, the syringe is used to draw the gas bubble in the tube up alongside a ruler so the length of the bubble is measured–} proportional to vol of O2 produced
-repeated and average length of gas bubble is calculated(precise)
-repeated at light sources placed at different distances