Ch. 20 Photosynthesis

Question 1

What is the basic idea of photosynthesis?

Answer 1

Start with water and work backwards to make ATP.

Question 2

What do photosynthesis reactions need to be driven forward? (2)

Answer 2

Reactions need LOTS of energy from the sun because they are the opposite of decarboxylations. It also uses NADP⁺ (bcs. it is anabolic).

Question 3

What are chloroplasts?

Answer 3

Green structures made of two membranes; OM is leaky and IM is a barrier.

Question 4

What is the stroma?

Answer 4

The inner compartment of chloroplasts. It is similar to the matrix of mitochondria.

Question 5

What are thylakoids?

Answer 5

Structures inside the stroma.

Question 6

Where does photosynthesis occur in the cell?

Answer 6

In the thylakoid membranes of chloroplasts.

Question 7

Where doe the enzymes requiring light exist? What about the enzymes that don’t require light?

Answer 7

Enzymes needing light exist in the thylakoid membrane. Enzymes that DON’T need light exist in the stroma (the dark reactions).

Question 8

How do protons play a role in photosynthesis? (2)

Answer 8

Enzymes that harvest energy from light pump protons INTO the thylakoid. Protons will then flow out INTO the stroma where ATP is made.

Question 9

Why do we care that chloroplasts are easy to isolate? (2)

Answer 9

We can study photosynthesis with isolated chloroplasts. We can also look at the medicinal use of metabolic intermediates.

Question 10

How can chloroplasts be isolated?

Answer 10

Easily. Total homogenate→differential centrifugation→save pellet.

Question 11

What is chlorophyll?

Answer 11

A hydrophobic compound found in chloroplasts. It is similar to heme in hemoglobin.

Question 12

What does chlorophyll do?

Answer 12

They can absorb light due to their conjugated double bonds.

Question 13

Are all chlorophyll the same?

Answer 13

No. Light wavelengths absorbed depends on the type of chlorophyll. (e.g. When chlorophyll b is gone, green is no longer the main color and tree leaves change color)

Question 14

What are photosystems?

Answer 14

Big complexes that contain multiple different chlorophylls and harvest light in thylakoid membranes.

Question 15

How does energy from a photon make it to a photosystem? (4)

Answer 15

1. light strikes a chlorophyll exciting an electron (not stable)
2. energy is transferred to a neighboring chlorophyll…. and again…. and again….
3. chlorophyll next to a reaction venter transfers the electron (energy) to the reaction center
4. reaction center now has an excited electron and a hole (battery with energy that can be used in redox reactions)

Question 16

What are antenna chlorophylls?

Answer 16

They are chlorophylls at the edge of the photosystem; kind of like a satellite dish.

Question 17

What are reaction centers?

Answer 17

Integral membrane proteins that are a part of a photosystem.

Question 18

What is the net reaction of photosynthesis?

Answer 18

2 H₂O + 2 NADP⁺ (light)→ 2 NADPH + 2 H⁺ + O₂

(O₂ kills plants; animals detox atmosphere by consuming O₂ and giving off CO₂)

Question 19

What are the two photosystems?

Answer 19

PSII and PSI

Question 20

What is the reaction/process of PSII? (5)

Answer 20

1. take an electron from water
2. move energy from light into the reaction center
3. hands the excited electron down through electron carriers (e- going downhill energetically)
4. cytochrome b₆f transfers excited electron to plastocyanin
5. plastocyanin passes excited electron to PSI

Question 21

What is the reaction/process of PSI? (3)

Answer 21

1. light “bumps up” the electron from PSII for a second time
2. electron is handed off through electron carriers/intermediates (energetically downhill)
3. energy is used to reduce NADP⁺ to NADPH

Question 22

Where does the energy from PSII and PSI go to?

Answer 22

Energy released during the processes of PSII and PSI is used to pump protons into the thylakoid

Question 23

Why aren’t the photosystems excited at the same time?

Answer 23

Different photosystems absorb different wavelengths of light.

Question 24

What happens to water in photosystems and why is it needed?

Answer 24

Water is oxidized. Water provides the electrons that are excited by photosystems.

Question 25

What does the chemiosmotic gradient produced by the photosystems do?

Answer 25

It drives ATP production, very similar to oxidative phosphorylation.

Question 26

What is the oxygen evolving complex? | What is the net reaction?

Answer 26

A complex found inside the thylakoid lumen. It splits water to electrons, protons and O₂. | 2 H₂O → 4 e- + 4 H⁺ + O₂

Question 27

How many electrons can PSII accept and why does this tie into the oxygen evolving complex?

Answer 27

PSII can only accept ONE electron at a time. The oxygen evolving complex will only pass one electron to PSII at a time and hold onto all of the intermediates (including oxygen free radicals).

Question 28

How many protons are produced by the oxygen evolving complex?

Answer 28

4 total protons are produced in the lumen of thylakoids. There are also protons being pumped by cytochrome b₆f in the photosystems, so there is a LOT of energy here!

Question 29

Why do protons stay in the thylakoid? How does this effect the energy of photosynthesis?

Answer 29

The thylakoid membrane is impermeable to protons. The difference in pH between the inside and outside of the thylakoid is 3 pH units (1000-fold difference). This equates to ~200 kJ of energy released from the gradient.

Question 30

What is the ATP/H⁺ conversion for photosynthesis?

Answer 30

~3 ATP / 12 H⁺

Question 31

What is CF₀CF₁ ATPase?

Answer 31

It is basically the same as the mitochondrial ATPase; the energy from the protons is needed to release ATP, not make it.

Question 32

What do the light reactions produce? (3)

Answer 32

1. O₂: can leave without a problem because it is permeable and soluble (toxic to plants) 2. ATP: can drive reactions forward 3. NADPH: strong reductant (provides reducing power)

Question 33

What is ribulose-1,5-bisphosphate?

Answer 33

A five carbon doubly phosphorylated molecule where the Calvin cycle begins.

Question 34

What is the first step of the Calvin Cycle?

Answer 34

ribulose-1,5-bisphosphate (5C) is carboxylated to produce two 3-phosphoglycerates (6C) by rubisco

Question 35

What is rubisco?

Answer 35

An enzyme with 8 large subunits and 8 small subunits that is responsible for carbon assimilation. Most abundant enzyme on Earth!

Question 36

Where is rubisco found?

Answer 36

It is found in the stroma in a VERY high concentration. (50% of the protein in chloroplasts is rubisco)

Question 37

Why is the lysine in rubisco important?

Answer 37

The lysine can be carbamolated: can have a CO₂ group added to a nitrogen of lysine. (CO₂ group in the active site is perfectly oriented with ribulose-1,5-bisphosphate)

Question 38

What is rubisco inhibited by and why is that?

Answer 38

Rubisco is inhibited by ribulose-1,5-bisphosphate (its substrate) because CO₂ MUST be covalently attatched to rubisco before ribuloose-1,5-bisphosphate.

Question 39

What is rubisco activase?

Answer 39

An enzyme that activates rubisco by removing ribulose-1,5-bisphosphate from the active site if the lysine is bare (no CO₂).

Question 40

What is the second step of the Calvin Cycle? (2 processes here)

Answer 40

Two 3-phosphoglycerate is phosphorylated to two 1,3-bisphosphoglycerate by 3-phosphoglycerate kinase. The two 1,3-bisphosphoglycerate can be reduced and dephosphorylated to two glycaraldehyde-3-phosphate by G3P dehydrogenase.

Question 41

What does rubisco activase require? What does it cause?

Answer 41

It burns ATP because the reaction is unfavorable. The active site of rubisco is now empty and CO₂ and Mg²⁺ can come in.

Question 42

What does step two of the Calvin Cycle require?

Answer 42

Step two burns ATP and NADPH.

Question 43

Where does step two of the Calvin Cycle occur?

Answer 43

It happens in the stroma.

Question 44

What is the ΔG of step two of the Calvin Cycle? Why does it occur?

Answer 44

Slightly positive ΔG. Still occurs because the concentration of ATP and NADPH is VERY high.

Question 45

What is step three of the Calvin Cycle?

Answer 45

Two glyceraldehyde-3-phsophate are converted to two dihydroxyacetone phosphate (DHAP) (3C) by triose phosphate isomerase.

Question 46

Where can triose phosphates (like DHAP) go once produced? (5)

Answer 46

1. triose phosphates can stay in the stroma and condense to make F1,6BP 2. carbons can make starch 3. DHAP can make sucrose 4. three carbons from DHAP can go to glycolysis 5. MOST carbons go back to regenerating ribulose-1,5-bisphosphate

Question 47

What is starch?

Answer 47

A midterm energy storage molecule produced in plant seeds. Stores glucose.

Question 48

What is sucrose?

Answer 48

A sugar made of glucose and fructose. "Table sugar"

Question 49

What does one revolution of the Calvin Cycle consume (2) and why is that okay (3)?

Answer 49

One cycle consumes 6 NADPH and 9 ATP. This is okay because sunlight provides enough energy, rubisco is abundant and cranking out molecules, and the concentration of ATP and NADHP is very high because PS's are good at making gradients.

Question 50

What is the net reaction of the Calvin Cycle (dark reactions)?

Answer 50

3 CO₂ + H₂O → glyceraldehyde-3-phosphate

Question 51

How does DHAP get out of the stroma and why does it need to?

Answer 51

Triose phosphate antiporter transports DHAP out of the stroma and Pᵢ into the stroma. ATP is transported out of the stroma in equivalents of DHAP.

Question 52

What is the citric acid cycle in plants (cat. or ana.)?

Answer 52

Think of the citric acid cycle in plants as anabolism (get intermediates out of the cycle!)

Question 53

What happens to excess triose phosphates in plants?

Answer 53

Plants make more triose phosphates than they need when in the presence of sunlight, so the excess is converted into starch or glucose.

Question 54

What are the two forms of starch? Define each.

Answer 54

Amylose: all linear ɑ1→4 glucose linkages Amylopectin: has branched ɑ1→6 glucose linkages

Question 55

Where is starch synthesized?

Answer 55

Starch is synthesized in chloroplast stroma (interior).

Question 56

How is starch synthesized? (3)

Answer 56

Almost identical to glycogen synthesis, except it used ADP-glucose not UDP-glucose. glucose + ATP → ADP-glucose ADP-glucose + amyloseₙ → amyloseₙ₊₁

Question 57

How are ɑ1→6 linkages made in starch?

Answer 57

Made by branching enzyme like in glycogen.

Question 58

Where do most of the triose phosphates NOT replenishing ribulose-1,5-bisphosphate go?

Answer 58

They go to producing sucrose.

Question 59

How is sucrose synthesized? (4)

Answer 59

1. aldolase condenses DHAP and G3P to F1,6BP (gluconeogenesis) 2. F1,6BP is converted to F6P by F1,6BPase (gluconeogenesis) 3. F6P and UDP-glucose are converted to sucrose-6-phosphate by S6P Synthase 4. S6P is dephosphorylated to sucrose by S6P Phosphotase

Question 60

What is the ΔG of the sucrose-6-phosphate phosphotase step of sucrose synthesis?

Answer 60

It is negative and pulls the other three reactions forward.

Question 61

Is sucrose reducing or non-reducing?

Answer 61

Sucrose is a non-reducing sugar, unlike glucose.

Question 62

What is a reducing sugar?

Answer 62

A sugar that can form a free aldehyde; it can linearize. It is chemically active.

Question 63

Why do plants make sucrose? (2)

Answer 63

1. It is NOT reactive so it is safe and water soluble (easy to transport) 2. It is an energy source, carbon source, glucose source, and a sucrose source in the plant vascular system

Question 64

Where will excess triose phosphates go? (2)

Answer 64

Either stored in the cell as starch or converted to sucrose that can be transported.

Question 65

What do plants do if triose phosphates are in excess?

Answer 65

When triose phosphates are in excess, plants do gluconeogenesis to get to F1,6BP.

Question 66

What does fructose-2,6-bisphosphate inhibit in photosynthesis? Stimulate?

Answer 66

It inhibits F1,6BPase. (Don't make F6P, keep it at F16BP) It stimulates PFK. (Make F16BP)

Question 67

What is phosphofructokinase 2?

Answer 67

An enzyme that converts F6P to F2,6BP.

Question 68

What is PFK2 inhibited by?

Answer 68

Inhibited by triose phosphates (DHAP and G3P). When triose phosphates are high, plants make sucrose. When triose phosphates are low, plants make F26BP and inhibit S6P synthase.

Question 69

What is sucrose-6-phosphate synthase stimulated by?

Answer 69

Allosterically stimulated by glucose-6-phosphate.

Question 70

What is sucrose-6-phosphate synthase inhibited by?

Answer 70

Inhibited by Pᵢ because that means triose phosphate concentration is low.

Question 71

How does S6P Synthase Kinase impact S6P Synthase?

Answer 71

It inactivates it by phosphorylating it.

Question 72

What inhibits S6P Synthase Kinase?

Answer 72

G6P inhibits the kinase.

Question 73

What does S6P Synthase Phosphatase do?

Answer 73

It activates S6P Synthase by dephosphorylating it.

Question 74

What inhibits S6P Synthase Phosphatase?

Answer 74

Phosphate inhibits the phosphotase.

Question 75

What is cellulose?

Answer 75

A glucose polymer of β1→4 linkages that humans can't break.

Question 76

What is the structure of cellulose? (2)

Answer 76

Individual filaments are held together by 1000s of H-bonds. Monomers (strands) are stacked and they H-bond with themselves and neighboring strands. This is strong. (almost all donors and acceptors are involved in H-bonds)

Question 77

Where is cellulose synthesized?

Answer 77

Cellulose is synthesized on the cytosolic face of the plasma membrane. One strand is secreted OUT of the p.m.

Question 78

What is cellulose synthase?

Answer 78

An integral membrane protein in the plasma membrane that synthesizes cellulose.

Question 79

How many strands of cellulose can be synthesized by cellulose synthase?

Answer 79

18 strands. There are 6 trimers that can each synthesize a strand.

Question 80

Where does the glucose to make cellulose come from and how (2)?

Answer 80

Glucose comes from sucrose. Sucrose can go backwards to UDP-glucose in plants. UDP-glucose feedstocks cellulose synthase.

Question 81

What do the cellulose synthase "rosettes" do? (3)

Answer 81

They move together along the cytoskeletal proteins while synthesizing the 18 strands of cellulose that are coming together (linear microfibrils). This makes a strong extracellular matrix (cell wall) in plants.