Module 8: Photosynthesis

Question 1

What is photosynthesis?

Answer 1

the conversion of light energy to chemical energy
- photosynthetic organisms build carbohydrates using sunlight and CO2 from the air
- it is a major entry point of energy into biological systems

Question 2

Supporting Photosynthesis

Answer 2

• photosynthetic organisms in a wide range of environments can carry out photosynthesis, if sunlight is available
• ex: in the ocean photosynthesis occurs in the surface layer about 100m deep, the photic zone
• food webs are supported by the biomass of photosynthetic organisms

Question 3

What type of reaction is photosynthesis, what does it do and require?

Answer 3

its a redox reaction
- CO2 is reduced to form high energy carbohydrate molecules
- requires energy in the form of sunlight

Question 4

What are the two stages of photosynthesis?

Answer 4

1. Light capture
2. Carbon fixation

Question 5

What is the electron donor in the reaction in photosynthesis?

Answer 5

H2O

Question 6

The oxidation of water results in the production of what?

Answer 6

Electrons
Protons
O2

Question 7

Oxidation of water and reduction of CO2 are linked through…

Answer 7

the photosynthetic electron transport train

Question 8

In photosynthetic eukaryotes both stages take place where?

Answer 8

in the chloroplast

Question 9

Chloroplasts are _____ & ______

Answer 9

semi-autonomous
self replicating

Question 10

Makeup of the chloroplast

Answer 10

1. bound by 2 membranes separated by a narrow space (outer and inner membrane)
2. center of chloroplast highly folded flattened membrane sacs, thylakoid
3. space inside thylakoid membrane is a fluid filled space, lumen
4. Orderly stacks of thylakoids, grana
5. Space surrounding thylakoids, stroma
   - carbon fixation occurs here

Question 11

Photosynthesis balanced chemical reaction

Answer 11

Energy + 6 CO2 + 12 H2O –(light)–> C6H12O6(glucose) + 6 O2 + 6 H2O

Oxidation: 12 H2O to 6 O2
Reduction: 6 CO2 to C6H12O6(glucose)

Question 12

What are the 2 series of reactions?

Answer 12

1. Light-dependent reaction:
   - sunlight energy converted into chemical energy, occurs in the thylakoid membranes
   - products are ATP and NADPH
2. Light-independent reaction:
   - ATP & NADPH used to synthesize carbohydrates
   - occurs in the stroma

Question 13

How is light absorbed?

Answer 13

energy from the sun is a form of electromagnetic radiation, travels in photons
- when a photon becomes absorbed, the compound is converted to a higher energy state, an excited state

Question 14

What are the three ways to re-establish ground state?

Answer 14

1. Dissipate energy as heat
2. Re-emit energy in a longer wavelength, fluorescence
3. Transfer energy to another molecules, this is what happens with photosynthetic pigments

Question 15

What are pigments?

Answer 15

pigments are molecules that contain a chromophore, a chemical group capable of absorbing light of wavelengths

Question 16

Leaves efficiently absorb what?

Answer 16

the leaf of a green plant efficiently absorbs light energy over most of the spectrum

Question 17

Why green in plants?

Answer 17

because the pigment chlorophyll is poor at absorbing green wavelengths

Question 18

Chlorophyll

Answer 18

-major light capturing molecules
- absorb light of blue and red wavelengths
- reflects green wavelengths

Question 19

What are the two parts of chlorophyll?

Answer 19

Porphyrin ring
- light absorption
- specific atom in center of the ring is Mg
- different side groups on the ring, give different types of chlorophyll

Phytol side chain
- insertion of chlorophyll in lipid bilayer, thylakoid membrane
- hydrocarbon side chain

Question 20

Accessory Pigments

Answer 20

found in thylakoid membrane
- can absorb light from regions of the visible spectrum that are poorly absorbed by chlorophyll
– increases efficiency of light absorption
– protects photosynthetic components from damage

Question 21

What happens when chlorophyl absorbs light?

Answer 21

one of its electrons is elevated to a higher energy state

Question 22

What happens if a chlorophyl is beside another chlorophyll?

Answer 22

energy can be transferred
- the energy is passed from chlorophyl to chlorophyll until it reaches a pair of chlorophyll molecules, the reaction center

Question 23

the Antenna chlorophylls do what?

Answer 23

transfer energy, not electrons

Question 24

Reaction-center chlorophyll

Answer 24

a specific chlorophyll capable of transferring electrons to an electron donor

Question 25

Before the reaction center can take in more light energy it has do to what?

Answer 25

it must first acquire an electron from an electron donor since it lost one electron - electron comes from H2O

Question 26

Role of Photosystems

Answer 26

- photosynthesis begins with the absorption of light by protein-pigment complexes, i.e. photosystems

Question 27

Absorbed light drives what reactions?

Answer 27

redox reactions

Question 28

What are the 2 photosystems?

Answer 28

photosystem ll or PSll photosystem l or PSl

Question 29

How are the two photosystems connected? What do they do?

Answer 29

connected by the photosynthetic electron transport chain - drives the formation of ATP and NADPH - these are energy sources needed to synthesize carbohydrates from CO2 in the Calvin cycle

Question 30

What does each photosystem have?

Answer 30

a reaction center chlorophyll

Question 31

How do the electrons and energy move?

Answer 31

the electrons move from water to PSll - initial high energy level, but this drops as it goes through the ETC - more light energy coming in PSl raises the electron energy high enough to be used to reduce NADP+ - the energy trajectory resembles a "z", called the Z scheme

Question 32

Flow of electrons

Answer 32

a. between H2O and PSll b. between PSll and PSl (ETC) c. between PSl and NADP+

Question 33

As electrons flow along the Z pathway...

Answer 33

H+ ions are moved from stroma to inner compartment of thylakoids, sets up a proton gradient - proton concentration **increases** in *lumen* of *thylakoid* and **decreases** in *stroma*

Question 34

ATP Production in the Chloroplast

Answer 34

- the thylakoid membrane has an ATP synthase - **the ATP production in the chloroplast remains in the chloroplast** -- it is used in carbon fixation reactions - plants also have a mitochondria to produce ATP needed elsewhere in the cell -- because ATP produced in chloroplast stays in chloroplast

Question 35

ETC in Plants

Answer 35

electrons pass between PSll and PSl through the cytochrome b6f complex

Question 36

Plastiquinnone

Answer 36

carries electrons from PSll to the cytochrome b6f complex

Question 37

Plastocyanin

Answer 37

carries electrons from the cytochrome b6f complex to the PSl by diffusing through the thylakoid lumen

Question 38

Formation of ATP in plant cell

Answer 38

results from electrons moving through PSll and PSl - proton gradient drives ATP formation, enzyme is ATP synthase - protons pass through the enzyme ATP synthase and ATP is generated

Question 39

The Calvin cycle take a ___ energy carbon compound (_) and converts it into a _____ carbon compound

Answer 39

low potential CO2 high energy

Question 40

What does the Calvin Cycle require?

Answer 40

NADP and ATP 15 chemicsl reactions that synthesize carbohydrates from CO2

Question 41

What are the three stages of the Calvin cycle?

Answer 41

1. Carboxylation 2. Reduction 3. Regeneration

Question 42

Step 1 Calvin cycle

Answer 42

**Carboxylation** - CO2 from the air is combined with **RuBP** to form a *6-carbon molecule* that is catalyzed by *rubisco* - the 6-carbon molecule is then broken down into two-3 carbon molecules of PGA

Question 43

Step 2 Calvin cycle

Answer 43

**Reduction** - the potential energy of PGA needs to be increased, it must be reduced - the reduction occurs in two steps 1. the PGA is phosphorylated by ATP, produce triose phosphates 2. the triose phosphate molecules are then reduced by NADPH and exported out of the chloroplast

Question 44

Step 3 Calvin cycle

Answer 44

**Regeneration** - for every six triose phosphate molecules that are produced, only one can be withdrawn from the Calvin cycle - this is because RuBP (a 5-carbon molecule) needs to be regenerated using the other triose phosphates - that regeneration requires energy from ATP - regeneration produces three 5-C RuBP molecules to be used in the carboxylation portion of the Calvin cycle

Question 45

Excess carbohydrates in the Calvin cycle

Answer 45

excess carbohydrates produced in the Calvin cycle are converted into storage, STARCH - the formation of starch granules during the day is a source of carbohydrates that can be used during the night when there is no sunlight

Question 46

Evolution of Photosynthesis

Answer 46

- early photosynthetic bacteria had only a single photosystem, cannot capture enough energy with one photosystem

Question 47

Evolution of two photosystems

Answer 47

two possibilities 1. Horizontal gene transfer 2. gene duplication and divergence of one of the genes

Question 48

Photosynthetic Eukaryotes

Answer 48

- the first organisms to use water as the electron donor were the cyanobacteria - the endosymbiotic theory proposes that a cyanobacterium was engulfed by a eukaryotic cell - over time the cyanobacterium lost the ability to love outside the host, thus it became the organelle we know as the chloroplast