Photosynthesis

Question 1

Structure of a chloroplast

Answer 1

• Usually disc-shaped
• Double membrane (envelope)
• Thylakoids : flattened discs stack to form grana
• Stroma : fluid-filled matrix with high enzyme and substrate concentration and own loop of DNA

Question 2

Where do the light dependant and light information reactions occur in plants

Answer 2

Light dependant : Thylakoids of chloroplasts

Light independent : Stroma of chloroplasts

Question 3

What is the role of photosynthetic pigments and name the 2 main groups

Answer 3

Embedded within thylakoid membrane, absorb different wavelengths of light to machines rate of photosynthesis.
Primary pigment - chlorophyll (made of chlorophyll a and b) found in photo systems
Accessory pigments - carotenoids (carotene and xanthophylls) which are found in light harvesting systems

Question 4

Name the processes in the light dependent reaction

Answer 4

• Photo-ionisation
• Electron transfer chain
• Chemiosmosis
  Non-cyclic
• Reduction of NADP
• Photolysis of water

Question 5

Explain the role of photoionisation

Answer 5

• Chlorophyll molecule absorbed energy from photons of light exciting 2 electrons causing them to be released from the chlorophyll

Question 6

What happens in the electron transfer chain

Answer 6

Electrons released from chlorophyll move down a series of carrier proteins embedded in the thylakoid membrane and undergo a series of redox reactions which release energy

Question 7

How does chemiosmosis produce ATP in the light-dependant?

Answer 7

Some energy released from the electron transfer chain u so coupled to active transport H+ ions into thylakoid space. H+ ions move down concentration gradient from thylakoid space into Stroma via transmembrane channel protein ATP synthase. ATP catalyses ADP and Pi to ATP

Question 8

Describe non-cyclic photo phosphorylation

Answer 8

Uses photo systems I & II. Excited electrons enter the electron transfer chain to produce ATP. NADP acts as a final electron acceptor and is reduced. Water is photolyses to release electrons and replace this lost from PSII. Purpose is to proceed ATP and reduced NADP for Calvin cycle

Question 9

Describe cyclic photo phosphorylation

Answer 9

• Uses PSI
• Excited electrons enter ETC to produce ATP then return directly to the photosystem. Purpose is to produce additional ATP to meet surplus energy demands of the cell.

Question 10

What is the equation for photolysis of water

Answer 10

Light energy splits molecule water

2H2O -> 4H+ +4e- +O2

Question 11

What happens to the products of the photolyses of water

Answer 11

H+ ions : move out the thylakoid space via ATP synthase and are used to reduce NADP (coenzyme)
e- : replace electrons lost from chlorophyll
O2 : used for respiration or diffuse out of leaf as waste gas.

Question 12

How and where is reduced NADP produced in the light independent reaction

Answer 12

• NADP + 2H+ (from photolysis) + 2e- (from acting as final electron acceptor in ETC) -> reduced NADP
• Catalysed by dehydrogenase enzymes
• Stroma of chloroplasts

Question 13

Name the 3 stages in the light independent stage

Answer 13

• Carbon fixation
• Reduction
• Regeneration

Question 14

What happens during carbon fixation

Answer 14

• Reaction between CO2 and ribulose biphosphate (RuBP) catalysed by ribulose biphosphate carboxylase (RuBisCo)
• Forms unstable 6C intermediate that breaks down into 2 * glycerate 3 phosphate (GP)

Question 15

What happens during reduction in the light independant reaction

Answer 15

2 * GP are reduced to 2 * triode phosphate (TP)

• Requires 2 * reduced NADP & 2 * ATP
• Forms 2 * NADP and 2 * ADP

Question 16

Outline the roles of TP from the light independent stage

Answer 16

Raw material : 1C leaves the cycle to procure monosaccharides, amino acids and other biological molecules
- Involved in the regeneration of RuBP : After 1C leaves the cycle, the 5C compound RuP forms. RuPis converted into RuBP using 1ATP and forms 1ADP

Question 17

State the number of carbon atoms in RuBP, GP and GALP

Answer 17

RuBP : 5
GP : 3
GALP : 3

Question 18

Define limiting factor

Answer 18

Factor that determines maximum rate of a reaction, even if other factors change or become more favourable.

Question 19

Define limiting factor

Answer 19

Factor the determines maximum rate of a reaction, even if other factors change to become more favourable

Question 20

Name 4 environmental factors the limit the rate of photosynthesis

Answer 20

• Light intensity (light independent stage
• CO2 levels (light independent stage)
• Temperature (enzyme controlled steps)
• Mineral/magnesium levels (maintain normal function of chlorophyll)

Question 21

How does light intensity affect the rate of photosynthesis

Answer 21

Low light intensity = slower light dependant reaction so less ATP and reduced NADP are provided to convert GP to TP in light independent reaction

• GP level rises
• TP level falls = RuBP levels falls

Question 22

Describe water stress

Answer 22

1) Abscisic acid binds to complementary receptors on guard cell membrane, causing Ca2+ ion channels on tonoplast to open. Ca2+ ions diffuse from vacuole into cytosol) Positive feedback triggers other ions channels to open such as K+.
3) Water potential of guard cells become more positive so water diffuses out via osmosis
4) Guard cells become flaccid so stomata close

Question 23

State the purpose of paper chromotigraphy

Answer 23

• Molecules in a mixture are separated based pj their relative attraction to the mobile phase against the stationary phase

Question 24

Outline a method for extracting photosynthetic pigments

Answer 24

Use a pestle and mortar to grind a leaf with an extraction solvent e.g. propanone

Question 25

Explain how TLC can be used to separate photosynthetic pigments

Answer 25

1) Use a capillary tube to spot pigment extract onto the pencil line 1cm above the plate
2) Place chromatography paper in the solvent
3) Allow solvents to run until it almost touches the end of the paper