Plants Lecture 2 - The Carbon Assimilation Reactions of Photosynthesis

Question 1

What is a photoautotroph? What is an example of it?

Answer 1

Organisms that carry out photosynthesis e.g. plants

Question 2

What is a chemoheterotroph? What is an example of it?

Answer 2

Organisms which derives its energy from chemicals, and needs to consume other organisms in order to live e.g. humans

Question 3

Plants reduce atmospheric CO2 to _____, which then acts as a precursor for synthesis of ____, ____, ____, _____ and ____.

Answer 3

trioses

cellulose, starch, lipids, proteins and other organic molecules.

Question 4

What are the 3 steps to the Calvin cycle?

Answer 4

1. Fixation of CO2 - by RuBisCO
2. Reduction of 3-phosphoglycerate to a triose - in this stage synthesis of a carbohydrate occurs
3. Regeneration of the carbon acceptor, ribulose 1,5-bisphosphate (RuBP) - involves interconversions of triose and pentose phosphates

Question 5

Of the triose molecules produced in the Calvin cycle, how many are put towards regenerating RuBP? How many are put towards actually growing the plant?

Answer 5

5/6 –> RuBP

1/6 –> growing plant e.g. make sucroses, hexoses, starch

Question 6

What is the name of the enzyme that catalyses the fixation of CO2 in the cycle (basically what enzyme lets you go from step 3 to step 1)?

Answer 6

RuBisCO

Question 7

How many forms of RuBisCO are there? Where is each form found?

Answer 7

Form I: has 2 subunits - found in vascular plants, algae, cyanobacteria
Form II: more complicated - only found in certain photosynthetic bacteria

Question 8

What are C3 plants?

Answer 8

Plants with 3- phosphosphoglycerate as the first stable product of photosynthesis

Question 9

Plant RuBisCO has ___ identical large subunits encoded by ____ genome, and ___ identical small subunits encoded in genome.

Answer 9

8 large, chloroplast DNA

8 small, nuclear DNA

Question 10

RuBisCO has an extremely low/high Kcat (turnover rate). Therefore you need a lot/not much of it.

Answer 10

Low, need lots

Question 11

For RuBisCO to work, ___ and ___ need to be _____ in its active site.

Answer 11

Lys and Mg2+ need to be carbamyolated

Question 12

NADPH and ATP are produced in the light reactions in about the SAME/DIFFERENT ratio as they are consumed in the Calvin cycle.

Answer 12

Same

Question 13

Can production of ATP and NADPH occur in the dark.

Answer 13

No

Question 14

Can the reactions that occur in the Calvin Cycle occur in animals? can they make glucose from CO2?

Answer 14

Yes these reactions happen in animals except animals are missing 3 key enzymes (which plants have) meaning they can’t do the last step which is converting CO2 to glucose.

Question 15

What is photorespiration?

Answer 15

A process in plant metabolism where the enzyme RuBisCO oxygenates RuBP (instead of assimilating the CO2 it assimilates the O2 in step 1 instead), causing some of the energy produced by photosynthesis to be wasted. Because the molecule created by adding the O2 can’t be used for anything.

Question 16

What does photorespiration consume? What does it produce? What is it dependent on? Does it use a lot of energy?

Answer 16

Consumes O2
Produces CO2
Dependent on light
Yes - substantial metabolic cost!

Question 17

RuBisCO has a Km for CO2 of ~9μM while the Km for O2 is ~350 μM – so the enzyme is quite specific for CO2. However there are 3 things that can occur to make the affinity for CO2 less allowing more O2 to bind therefore allowing photorespiration to occur. What are these 3 things?

Answer 17

1. Atmosphere has a greater concentration of O2 compared to CO2
2. At higher temps, there is more O2 and the affinity of RuBisCo for CO2 lowers
3. As CO2 is consumed in the carbon assimilation reactions the greater the proportion of O2 to CO2 becomes.

Question 18

What is the name of the cycle that photorespiration occurs through? What 3 organelles are involved?

Answer 18

Oxidative phosphosynthetic carbon cycle.

Chloroplast, peroxisome, mitochondrion.

Question 19

What is the name of the protein that makes up half the total protein in the mitochondrial matrix?

Answer 19

Glycine decarboxylase

Question 20

What is so special about C4 plants? How does this occur?

Answer 20

They concentrate CO2 around RuBisCO, thus minimizing photorespiration.
PEP carboxylase is an enzyme used in these plants and they cannot use O2 as a substrate.

Question 21

What are plasmodesmata?

Answer 21

Special plant-specific cell junctions. Protein lined channels that take specific molecules from cell to cell. Animals don’t have these.

Question 22

C4 plants require more energy than C3 plants and therefore photosynthesis has a greater metabolic cost. Is it worth it just to reduce photorespiration by a bit?

Answer 22

Yes! Easily worth it especially at temps. at or above 28 degrees.

Question 23

Noncyclic photophosphorylation (from the light reaction) can be represented by the following overall equation:

Answer 23

2H2O + 8 photons + 2NADP+ + ~3ADP+ + ~3Pi —>

O2 + ~3ATP + 2NADPH

Question 24

Bacteria have __ membrane. They pump protons into the ____.
Mitochondria have __ membrane. They pump protons into the ____.
Chloroplast have __ membrane. They pump protons into the ____.

Answer 24

1, outside the cell.
2, intermembrane space.
3, stroma.