Photosynthesis Part 1 - (Week 3)

Question 1

Give 3 reasons why photosynthesis is a significant process

Answer 1

• Atmospheric O2 (the ‘ingredient’ to multicellular life)
• Global carbon cycle
• Food, fuel & medicine

Question 2

What are autotrophs?

Answer 2

• ‘Self feeders’
• They are living organisms that sustain themselves without feeding/eating anything derived from other organisms
• Only need water, CO2 & minerals from soils
• Synthesise organic products/sugars

Question 3

What are photoautotrophs?

Answer 3

They are organisms that get energy from the sun to produce organic compounds to feed on

Question 4

What are heterotrophs?

Answer 4

They are organisms that rely either on:

• Compounds produced by other organisms (autotrophs are the ultimate source organic compounds for non-autotrophs)
• Other autotrophs or heterotrophs for energy

Question 5

What is photosynthesis?

Answer 5

• The process of converting light energy to chemical energy
• It can be carried out by both uni/multicellular organisms
  (Life on earth ultimately depends on the sun’s energy and this is the only process that can harvest this energy)

Question 6

How are cells in a leaf arranged to maximise the products of photosynthesis?

Answer 6

Cells are packed closely together to increase CO2 absorption (apart from spongy layer - to allow CO2 & O2 to go in/out of leaf in respiration/photosynthesis)

Question 7

What is the balanced equation for photosynthesis?

Answer 7

6 CO2 + 6 H20 -> 6 C6H12O6 + 6 O2

above arrow should be light energy

Question 8

What is found in the stroma of chloroplasts?

Answer 8

Most enzymes for the kelvin cycle

Question 9

What parts of photosynthesis take place in the thylakoid membrane?

Answer 9

• Electron Transport Chain (ETC)

- All of the ‘light reaction’

Question 10

What happens in the thylakoid space that is important for photosynthesis?

Answer 10

Where electrochemical gradient is generated, required for the production of ATP

Question 11

Why are there 2 parts to the process photosynthesis?

Answer 11

ATP & NADPH (energy carrier molecules) cannot be simply transported out of the chloroplasts so are channeled into the dark (carbon) reaction

Question 12

Why is photosynthesis an endergonic process?

Answer 12

• Water is split & electrons are transferred
• H+ ions from water is transferred to CO2, reducing it to sugar
• Electrons increase in energy as they move from water to sugar
• This process requires energy from sunlight which is endergonic as energy is absorbed

Question 13

Name the fundamental processes in photosynthesis

Answer 13

• Light harvesting
• Excitation of electrons
• Carbon fixation
• Kelvin cycle

Question 14

Name the 3 key components of the light reactions

Answer 14

• Pigments
• Photosystems
• Electron Transport Chain (ETC)

Question 15

What are photosynthetic pigments?

Answer 15

They are pigments that absorb the light that powers photosynthesis

Question 16

Name the 5 types of pigment that can be found in photosynthetic organisms

Answer 16

1. Bacteriochlorophyll a (usually in bacteria)
2. Chlorophyll a
3. Chlorophyll b
4. Phytoerythrobilin (usually in cyanobacteria & algae)
5. β-carotene

Question 17

Which pigment causes the appearance of yellow leaves? Why? What is the reason???

Answer 17

β-carotene
- Lower temp. destroys chlorophyll so green fades & yellow of this pigment remains
(In plants usually when there is less sunlight & less chlorophyll produced)

Question 18

Describe the role of chlorophyll in light reactions.

Answer 18

• Green pigment
• Absorbs light that powers photosynthesis through the excitation of electrons (in its porphyrin-like ring - electrons around ring will be excited by light)
• Makes up an antenna complex that is associated to a photochemical reaction centre, forming a photosystem

Question 19

Describe the role of chlorophyll b in the light reactions

Answer 19

• It has the same structure & functions as chlorophyll a but the -CH3 is replaced by aldehyde group (-CHO)
• absorbs at 500-640nm (appears olive green)

Question 20

Describe the role of carotenoids (β-carotene) in the light reactions

Answer 20

• Are yellow, orange, red or brown pigments
• Absorb strongly in the blue-violet range
• Pass absorbed light energy to chlorophyll A
• Provide photoprotection

Question 21

What does photo protection do in carotenoids?

Answer 21

Absorbs & dissipate excessive light energy that would destroy chlorophyll a or react with O2 to form other O2 species (oxidative damage)

Question 22

What type of pigment is chlorophyll a?

Answer 22

Primary photosynthetic pigment (& the most abundant pigment)

Question 23

Define the term ‘accessory pigment’ and the two that are found in photosynthetic organisms

Answer 23

They are pigments that broaden the range of light absorbed

• Chlorophyll b
• Carotenoids

Question 24

What is the antenna complex?

Answer 24

It is a part of the photosystem that is formed from the 3 pigments; carotenoids, chlorophyll a & b

Question 25

In what order is light absorbed by the 3 pigments?

Answer 25

Carotenoids -> Chlorophyll b -> Chlorophyll a

Question 26

How does the antenna complex act as a ‘funnel’?

Answer 26

• It funnels energy from sunlight to the reaction centre via excitation of electrons (through ‘resonance energy transfer’)
• Some energy is lost as heat/light (but is very quick to reduce/avoid energy lost)

Question 27

What is resonance energy transfer?

Answer 27

When the sun hits the electrons & energy is transferred, electrons don’t move - the energy is transferred from electron to electron

Question 28

Give the 3 possible fates of the energy absorbed by the pigments

Answer 28

• Converting extra energy to heat/combo. of heat & light
• Transferring energy from neighbouring chlorophyll molecules in antenna complex to reaction centre (via resonance energy transfer)
• When energy arrives in reaction centre, energy can power the movement of an electron (negatively charged high-energy electron) to another nearby molecule (electron acceptor)