Exam 2

Question 1

What is photorespiration?

Answer 1

when Rubisco fixes O2 (oxygenase activity) instead of CO2. This lowers the carbon count and “undoes” photosynthesis. This makes the plant less efficient

Question 2

What are the effects in plants due to photorespiration?

Answer 2

One of the molecules produced from the addition of O2 to RuBP is processed in sections that consume ATP and release CO2 in order to regenerate 3PGA. Part of the pathway occurs in the chloroplasts and part of it occurs in peroxisome/mitochondria. Reaction sequence resembles respiration b/c it consumes O2 and produces CO2. It undoes photosynthesis b/c it consumes energy and releases fixed CO2—>decreases rate of photosynthesis
It can be protective for when plants are under high light and low CO2 conditions

Question 3

Why might RUBISCO have both carboxylase and oxygenase activity?

Answer 3

O2 was the result of the creation of Rubisco
Carboxylase activity fixes CO2 (CO2+RuBP a 2PGA (3 carbons each)) —leads to sugar/starch synthesis w/new carbon
Oxygenase activity binds with oxygen (photorespiration)
O2 + RuBP a PGA (3 carbons) + PG (2 carbons)
—no new carbon gained, energy is consumed and CO2 released

Question 4

Why might RUBISCO have both carboxylase and oxygenase activity?

Answer 4

Carboxylase activity is efficient in the current environment b/c there is a high concentration of CO2 and plants need CO2 in order to fix carbon to create energy under photosynthesis. Oxygenase activity is included b/c RUBISCO evolved under conditions in which the environment didn’t have oxygen or very little of it. Rubisco’s active sites are limited, the active site for O2 and the active site for CO2 seem to be connected, if you change one, you change the other

Question 5

What is a C3 plant?

Answer 5

C3 plants fix CO2 in Rubisco to make a 3 carbon PGA in the bundle sheath fibers
C3 photosynthesis is the normal photosynthesis
C3 photosynthesis occurs in mesophyll cells on surface of leaves

Question 6

What is a C4 plant?

Answer 6

C4 plants use PEP carboxylase to fix CO2 to make a 4 carbon compound in the mesophyll
C4 photosynthesis occurs in a different space than C3
C4 photosynthesis also occurs in the mesophyll cells but the 4-carbon compound that is produced is then transferred to the bundle sheath cells; consumes energy
C4 plants have Kranz anatomy—thicker bundle sheath cells due to increase in concentration of chloroplast in that organelle
enzyme that fixes CO2 into its 4-carbon compound is PEP carboxylase (PEPC)

Question 7

What is the function of Kranz anatomy is C4 leaves?

Answer 7

has thicker bundle sheath cells in C4 leaves due to increased concentration of chloroplast.
4-carbon compound is transferred from the mesophyll leaf cells to the bundle sheath cells to concentrate the amount of CO2. In this way, there is no loss of CO2 due to photorespiration and plants would need less Rubisco for photosynthesis

Question 8

What are the benefits of C4 photosynthesis?

Answer 8

It acts as a CO2 concentrator, it allows for the stomata to be open less which results in less water loss
There is a higher concentration of CO2 in the bundle sheath cells which means that there will be a decreased change in O2 binding to the active site and setting off photorespiration.
B/c there is a high concentration of CO2, plants need less amounts of Rubsico for photosynthesis
In C4 photosynthesis, CO2 is fixed more efficiently by PEPC which allows for stomata to be open less and this conserves water

Question 9

What are the costs of C4 photosynthesis?

Answer 9

It is more energy costly
It is also only more advantageous in environments that are hot and dry
Uses more ATP

Question 10

Where would you expect to find C4 plants?

Answer 10

In hot and dry environments

Found in conditions of drought, high temperatures, and nitrogen or CO2 limitation

Question 11

In measuring photosynthesis in crop species you find that two species respond differently to a decrease in the concentration of O2 in the gas surrounding the leaf. One species (Species A) maintains a constant rate of photosynthesis in both high and low O2 concentration while the other exhibits a significant increase in photosynthesis in the low O2 environment. Propose hypothesis to explain observation

Answer 11

Hypothesize that Species B is a C4 plant since its photosynthetic rate increases in a low O2 environment. PEPC doesn’t perform well under conditions of high concentrations of O2

Question 12

Compare and contrast C4 and CAM photosynthesis by following a molecule of CO2 from initial fixation from the atmosphere to synthesis of sugar following the Calvin-Benson cycle

Answer 12

Same in C4 and CAM: initial CO2 fixation is with PEP carboxylase
Difference in C4 and CAM: release of CO2 for fixation with Rubisco is at a different time
Rubisco and PEPC reactions are separated in time
For a C4 plant: these reactions are separated by location
In CAM they are separated by time

Question 13

CAM plants

Answer 13

Stomata is only open at night
It can fix CO2 with PEP carboxylase at night in the mesophyll and then perform the calvin cycle during the day when there is solar energy to power it

Question 14

C4 plants

Answer 14

The binding to PEP carboxylase can happen at the same time but it occurs in the bundle sheath rather than the mesophyll

Question 15

Describe the cost of mycorrhizal associations for plants.

Answer 15

They have to maintain bacteria in their cells

Costs to the plant are often a percentage of their carbon, b/c they have to supply mycorrhizae with carbon and energy

Question 16

Describe the benefits of mycorrhizal for plants

Answer 16

Increase the amount of surface area for a plant to absorb nutrients and water from the soil.
There is a net benefit when the soil is unmanaged and there is a normal amount of light available
Major benefit is that there is an increased phosphate supply.
The fungus is able to break down macromolecules that the plant is unable to break down, itself, so it is able to obtain N from being bound to the mycorrhizae

Question 17

What is endomycorrihzae?

Answer 17

When the hyphae penetrates the root cortical cells.
Endo is when the hyphae penetrate the root cortical cells and access the vesicle arbusicle for storage and exchange.
Occurs mostly in herbs, grasses, and some trees

Question 18

What is ectomycorrhizae?

Answer 18

When the hyphae doesn’t penetrate the root cortical cell.
In ecto, they grow between outer root cortical and form a Hartig net sheath
Happens primarily in trees

Question 19

Describe the interaction between legume roots and Rhizobium.

Answer 19

Rhizobium nodulates the roots and creates a pathway for nitrogen-fixing bacteria to weave itself in the root cells, but does not work well when O2 is present. Leghemoglobin regulates the concentration of O2 inside the root nodule
They work mutualistically

Question 20

What are the advantages to the legumes and and the bacteria?

Answer 20

mutualism

the nodulation creates pockets of low oxygen

Question 21

What are the disadvantages to the mutualism between Rhizobium and legumes?

Answer 21

The root cells have to expend energy to maintain the bacteria culture in the root cells.
Isn’t beneficial when the legumes are in an environment that is nitrogen rich and also have a high concentration of O2

Question 22

In what kind of habitats are most carnivorous plants found?

Answer 22

Found in environments of low pH and low nutrient environments.
They capture insects as nutrient supplements
Soils that have a low pH aren’t good at dissolving nutrients, so often the plant is unable to get the nutrients it needs
Carnivorous plants still perform photosynthesis, but supplement that with carnivory to obtain Nitrogen
When nitrogen is really low, they increase their rate of carnivroy

Question 23

When might the costs of carnivory exceed its benefits?

Answer 23

If the plant is expending more energy to consume these insects than they are making from consuming the insects
It might exceed its benefits if there is Nitrogen available in the soil, this would decrease the gains that it is getting from expending energy to eat insects

Question 24

How did epiphytes acquire minerals?

Answer 24

Epiphytes are structural parasites that attach themselves to the canopies of trees to compete for light
It has a container for water and can catch other materials and decompose those materials for their own gain, it is creating its own ecosystem
They are able to gather rainwater and dust from the air to obtain nutrients

Question 25

How damaging are parasitic plants to their hosts?

Answer 25

Extremely damaging
Compete with their hosts for sunlight—epiphytes
ex. Mistletoes tap into their host’s xylem and extract water from it.
ex. Dodders tap into their host’s phloem and extracts sugar, water and nutrients from their host