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
-lamellae= thylakoid membrane channels that link the grana
-stroma= fluid-like region, site of many chemical reactions, contain enzymes, starch granules(storage molecules for carbs), lipid droplets, sugars + circular DNA

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, carotene
-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 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

Question 12

Redox reactions

Answer 12

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

Light-dependent stage

Answer 14

-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 15

Photolysis

Answer 15

-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 16

Non cyclic phosphorylation: electrons

Answer 16

-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
-electrons produced from photolysis replace the electrons lost in PSII

Question 17

Non cyclic phosphorylation: protons

Answer 17

-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 18

Non cyclic phosphorylation: Chemiosmosis

Answer 18

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

Question 19

Non cyclic phosphorylation: electrons part 2

Answer 19

-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
-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 20

Cyclic phosphorylation/photophosphorylation

Answer 20

-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 21

Light independent stage

Answer 21

• 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 22

Formation of glycerate 3-phosophate/ Carbon fixation

Answer 22

-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 23

more on RuBisCO

Answer 23

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

Question 24

Formation of triose phosphate/ Reduction

Answer 24

-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 come from reduced NADP from the light dependent reaction(reduced NADP is recycled to NADP for the light dependent reaction)
-TP can be converted to many useful organic compounds i.e glucose

Question 25

Regeneration (of RuBP)

Answer 25

-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