Photosynthesis 2 Flashcards by shweta rajam

Light Independent Reactions names

Calvin cycle
Calvin Benson cycle
Carbon fixing reactions
Carbon reducing reactions
Dark reactions

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Dark reactions

a misnomer as the reactions
do not occur in the dark. They are sometimes
called this as the reactions do not TRAP light
but they occur in the light.

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B) Light Independent Reactions what happens

-Carbon fixation:
B) Occurs in light (not in dark)
C) The energy that was trapped as ATP and NADPH in light reactions
powers the Calvin cycle and ends up in carbohydrates
D) Regeneration of substrate =
RuBP

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Carbon fixation:

Carbon dioxide that enters through stomates is trapped.
Carbon from CO2 is fixed into chemical bonds of carbohydrates e.g. glucose

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Regeneration of substrate in the light-independent reactions =

RuBP (Ribulose bisphosphate)

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rubisco

*Rubisco is the most abundant enzyme in the world!!
*Rubisco is partially coded for by chloroplast DNA and nuclear DNA.
*Rubisco is light activated (why the reactions do not occur in dark)
*All photosynthesizing plants use Rubisco in the Calvin Cycle to make carbohydrates
( but not all plants use Rubisco to fix atmospheric CO2!)

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do all plants use rubisco the fox atmospheric co2

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Calvin Cycle step 1

Six molecules of CO2 combine with six
molecules of RuBP (ribulose 1,5-bisphosphate)
in the carbon fixing step.

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calvin cycle step 2

For every 6 CO2’s fixed, one molecule of glucose
(which contains 6 carbons) is produced

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calvin cycle step 3

NADPH and ATP from the light dependent
reactions supply energy and electrons that
result in the production of glucose

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calvin cycle step 3

The cycle regenerates the substrate RuBP.

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Photorespiration

A special type of respiration that occurs in plants
in the light (in addition to aerobic cellular
respiration)

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RuBP carboxylase/oxygenase
has 3 possible substrates

RuBP (always one of the substrates)
CO2
O2

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co2 in

Calvin cycle (Rubisco acting as carboxylase)

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o2 in

Photorespiration (Rubisco acting as oxygenase)

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what 2 chemicals Compete for the Active Site

co2 and o2

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Photorespiration versus Aerobic Cellular Respiration

photoresperation

Photorespiration is a seemingly
wasteful process
1. No ATP is produced
2. Release of CO2 (loss of CO2 is of
significance to plants)
3. No net carbon fixation.
4. When rubisco is active in
photorespiration, it is not
available for the Calvin cycle.
5. However allows plants that have
it (C3 plants) to survive under
hot dry conditions and helps
prevent photooxidative damage

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Photorespiration versus Aerobic Cellular Respiration

Aerobic cellular respiration

ATP produced
Release of CO2

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Does Rubisco tend to act more as a
carboxylase or as an oxygenase?

calvin cycle

carboxylase

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Does Rubisco tend to act more as a
carboxylase or as an oxygenase?

photoresperation

oxygenase

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Atmospheric concentrations

of o2

20-21 percent o2

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Atmospheric concentrations

of co2

0.04 percent and raising

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Does Rubisco tend to act more as a
carboxylase or as an oxygenase?

The high O2 concentration in the atmosphere means
that Rubisco often acts as an oxygenase Photorespiration is promoted by the relative O2 /CO2
concentrations in atmosphere

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Different Photosynthetic Strategies

A. C3
B. C4
C. CAM

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C4 and CAM plants have

evolved strategies to deal with photorespiration

C3 Plants

-Produce 3-carbon compounds during first step of atmospheric carbon fixation (3PGA) -Use Calvin cycle to fix atmospheric CO2 * C3 plants can have much photorespiration * If one pumps CO2 into a greenhouse or a growth chamber with C3 plants, that will cut down on photorespiration.

C3 Plants examples

* Spinach * Soybeans * Oats * Wheat * Rye * Many grasses (not all) * Oaks * Maples

Some plants don’t use Rubisco to

fix atmospheric CO2 .

All photosynthesizing plants use Rubisco in

the Calvin cycle to make carbohydrates

C4 Plants

C4 plants have way to deal with photorespiration * They have additional pathway

additional pathways c4 plants

Have C3 pathway (Calvin cycle just like all photosynthesizing plants) and C4 pathway

When c4 plants fix atmospheric co2 they produce

they produce a 4 carbon compound

C4 Plants number of species and famalies how many monocots dicots

> 1000 species in 18 different plant families – 3 monocot families, 15 dicot families – All of these 18 plant families have C3 and C4 members – Suggests C4 syndrome has arisen a number of times

The 4-Carbon Pathway (C4 pathway)

Produces 4-carbon compound OAA instead of 3-carbon PGA during fixation of atmospheric CO2

c4 plants ex

C4 Plants: – Corn – Sugar cane – Cat tails – Crab grass (weedy), more of a problem in heat of summer – Pigweed (weedy) – Many tropical grasses and arid-region plants

Kranz anatomy

C4 plants have Kranz anatomy (Kranz = German for wreath). – Enlarged bundle sheath cells with large chloroplasts and numerous starch grains * © Kingsley Stern

what is combine, with aid of the enzyme PEP carboxylase in mesophyll cells

PEP (phosphoenolpyruvate) and CO2

PEP carboxylase acts as a

carboxylase only, not an oxygenase

step 1 of c4 pathway

Oxaloacetic acid (4C) is produced as first stable product of atmospheric CO2 fixation instead of PGA (3C)

step 2 of c4 pathway

CO2 is transported as malic acid to bundle sheath cells

step 3 of c4 pathway

CO2 is released and is re-fixed in Calvin cycle.

step 4 of c4 pathway

CO2 is concentrated in bundle sheath cells around Rubisco, so Rubisco acts like a carboxylase and photorespiration is minimized.

C4 plants have spatial separation between:

Calvin cycle in Bundle Sheath cells & C4 pathway in Mesophyll cells

c4 or c3 plants which is better

c4 plants perform better in hotter weathere

At low temperatures, C3 plants are more

efficient, and they outcompete C4 plants because Costs extra ATPs for C4 photosynthesis

C4 plants do best when it is

hot and dry. They can have stomates partially open to conserve water but they do well since they are so efficient at trapping CO2 * E.g. Crab grass does better in heat of summer

At lower temperatures,

C3 plants outcompete C4 plants

At higher temperatures,

C4 plants outcompete C3 plants

Research to try to Convert C3 Plants to C4 Plants

* Need to change anatomy of leaf to Kranz anatomy * Need to introduce more PEP carboxylase as well as the C4 pathway into mesophyll cells * Need to take Rubisco out of the mesophyll cells and put in bundle sheath cells

CAM Plants

Tend to grow in dry environments * In about 30 plant families

CAM Plants plant famalies

Crassulaceae Orchidaceae Cactaceae Bromeliaceae

Crassulaceae

* Jade plants * Mother of thousands

Orchidaceae

orchids

Cactaceae

cacti

Bromeliaceae

* Bromeliads * A lot of air plants = epiphytes; e.g. Spanish moss

step 1 CAM Photosynthesis stomates

are opened at night

step 2 CAM photosynthesis Atmospheric CO2 is fixed at

night by PEP carboxylase into malic acid (4C

step 3 cam photosynthesis Malic acid is stored in the

vacoule

step 4 Stomates are closed during day to (CAM photosynthesis)

conserve water

step 5 (CAM Photosynthesis ) CO2 is released from malic acid during the

day

step 6 of cam photosynthesis Now there is a high concentration of CO2 inside the plant, and

Rubisco will act like a carboxylase and re-fix the CO2 into carbohydrates Organic acids accumulate at night (stomata open).

CAM plants have time-based separation between activity of:

PEP carboxylase fixes CO2 at night and Rubisco fixes CO2 during day

CAM photosynthesis allows plants to function well under

limited water supply, as well as high light intensity.

However CAM plants have low

productivity

CAM plants Grow slowly

under natural conditions (in hot, dry environments)

Obligate CAM =

always show CAM photosynthesis

Facultative CAM plants exhibit

CAM photosynthesis under certain conditions (e.g. hot, dry) and the rest of the time they act like C3 plants – E.g. Mother of thousands