AP Biology

AP BiologyJohn D. O’Bryant School of Mathematics and Science

November 8, 2012

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Agenda Do Now (Table of Contents) HW discussion Photosynthesis: Modeling Quiz

Table of Contents (Notes/Classwork)

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HW

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Modeling PhotosynthesisTask:

In pairs, build a dynamic model of C3 photosynthesis using cutout pieces

of paper to represent the molecules, ions, and membrane transporters or

pumps. You should be able to manipulate or move carbon dioxide and

water and its breakdown products through the various steps of the

process.

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Quiz 1. The end products of the light-dependent reactions of photosynthesis

are

A)ADP, H2O, NADPH2 B)ADP, PGAL, RuBP (RuDP) C)ATP, CO2, H2O D)ATP, NADPH2, O2

E)CO2, H+, PGAL

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Quiz 2. Which of the following enzymes is responsible for CO2

fixation in C3 plants?

A) Succinate dehydrogenase B) RuBP (RuDP) carboxylase C) Hexokinase D) Amylase E) DNA polymerase

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Quiz 3. All of the following statements about a chloroplast

and a mitochondrion are true EXCEPT:

A) Both use proton gradients for energy production. B) Both capture light energy. C) Both contain DNA. D) Both are bound by two unit membranes. E) Both synthesize ATP.

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Quiz 4. If plants are grown for several days in an atmosphere

containing 14CO2 in place of 12CO2, one would expect to find

A)very little radioactivity in the growing leaves B)large amounts of radioactive water releasedfrom the stomates C)a large increase in 14C in the starch stored in theroots D)a large decrease in the rate of carbon fixation inthe guard

cells E)an increase in the activity of RuBP carboxylasein the

photosynthetic cells

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Quiz 5. The O2 released during photosynthesis comes

from

A) CO2 B) H2O C) NADPH D) RuBP (RuDP) E) C6H12O6

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Quiz 6. Which of the following is an important difference between light-

dependent and light-independent reactions of photosynthesis?

A) The light-dependent reactions occur only during the day; the light-independent reactions occur only during the night.

B) The light-dependent reactions occur in the cytoplasm; the light-independent reactions occur in chloroplasts.

C) The light-dependent reactions utilize CO2 andH2O; the light-independent reactions produceCO2 and H2O.

D) The light-dependent reactions depend on the presence of both photosystems I and II; the light-independent reactions require only photosystem I.

E) The light-dependent reactions produce ATP and NADPH; the light-independent reactions use stored energy in ATP and NADPH.

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Quiz 7. Carbohydrate-synthesizing reactions of photosynthesis

directly require

A) light B) products of the light reactions C) darkness D) O2 and H2O E) chlorophyll and CO2

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Quiz 8. All of the following could reduce the yield of photosynthetic

products EXCEPT

A) lower concentrations of carbon dioxide in the atmosphere B) increased photorespiration C) reduced carbon dioxide concentrations inthe air spaces of

the leaf D) increased frequency of stomata openings E) fewer Calvin cycle enzymes

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Quiz 9. All of the following are common to C3 and C4 photosynthesis

EXCEPT

A) Photolysis B) Initial step of CO2 fixation C) Cyclic photophosphorylation D) Noncyclic photophosphorylation E) Chemiosmotic phosphorylation

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Photosynthesis:Variations on the Theme

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Remember what plants need… Photosynthesis

light reactions light H2O

Calvin cycle CO2

What structures have plants evolved to

supply these needs?

sun

ground

air

O

OC

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Leaf Structure

H2O

CO2

O2 H2O

phloem (sugar)xylem (water)

stomate guardcell

palisadeslayer

spongylayer

cuticleepidermis

O2 CO2

Transpiration

vascular bundle

Gas exchange

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Controlling water loss from leaves Hot or dry days

stomates close to conserve water guard cells

gain H2O = stomates open

lose H2O = stomates close

adaptation to living on land, but…

creates PROBLEMS!

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When stomates close…

xylem (water)

phloem (sugars)

H2OO2 CO2

CO2

O2

Closed stomates lead to… O2 build up from light reactions

CO2 is depleted in Calvin cycle causes problems in Calvin Cycle

The best laidschemes of

mice and men…and plants!

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Inefficiency of RuBisCo: CO2 vs O2

RuBisCo in Calvin cycle carbon fixation enzyme

normally bonds C to RuBP CO2 is the optimal substrate

reduction of RuBP building sugars

when O2 concentration is high RuBisCo bonds O to RuBP O2 is a competitive substrate

oxidation of RuBP breakdown sugars

photosynthesis

photorespiration

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6Cunstable

intermediate

1C CO2

Calvin cycle when CO2 is abundant

5CRuBP

3CPGA

ADP

ATP

3CNADP

NADPH

ADP

ATP

G3Pto make glucose

3CG3P

5C

RuBisCo

C3 plants

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Calvin cycle when O2 is high

5CRuBP

3C2C

to mitochondria

–––––––lost as CO2

without making ATP

photorespiration

O2

Hey Dude,are you highon oxygen!

RuBisCo

It’s so sad to see a

good enzyme,go BAD!

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Impact of Photorespiration Oxidation of RuBP

short circuit of Calvin cycle loss of carbons to CO2

can lose 50% of carbons fixed by Calvin cycle reduces production of photosynthesis

no ATP (energy) produced no C6H12O6 (food) produced

if photorespiration could be reduced, plant would become 50% more efficient strong selection pressure to evolve

alternative carbon fixation systems

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Reducing photorespiration Separate carbon fixation from Calvin cycle

C4 plants PHYSICALLY separate carbon fixation from Calvin cycle

different cells to fix carbon vs. where Calvin cycle occurs store carbon in 4C compounds

different enzyme to capture CO2 (fix carbon) PEP carboxylase

different leaf structure CAM plants

separate carbon fixation from Calvin cycle by TIME OF DAY fix carbon during night

store carbon in 4C compounds

perform Calvin cycle during day

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C4 plants A better way to capture CO2

1st step before Calvin cycle, fix carbon with enzymePEP carboxylase store as 4C compound

adaptation to hot, dry climates have to close stomates a lot different leaf anatomy

sugar cane, corn, other grasses…

sugar cane

corn

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C4 leaf anatomy

PEP (3C) + CO2 oxaloacetate (4C)

CO2

CO2

O2

light reactions

C4 anatomy

C3 anatomy

PEP carboxylase enzyme higher attraction for CO2 than O2

better than RuBisCo

fixes CO2 in 4C compounds

regenerates CO2 in inner cells for RuBisCo keeping O2 away from RuBisCo

bundlesheath

cell RuBisCo

PEPcarboxylase

stomate

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Comparative anatomy

C3 C4

Location,location,location!

PHYSICALLY separate C fixation from Calvin cycle

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CAM (Crassulacean Acid Metabolism) plants

Adaptation to hot, dry climates separate carbon fixation from Calvin cycle by TIME

close stomates during day

open stomates during night

at night: open stomates & fix carbonin 4C “storage” compounds

in day: release CO2 from 4C acids to Calvin cycle increases concentration of CO2 in cells

succulents, some cacti, pineapple

It’s all inthe timing!

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CAM plants

succulents

cacti

pineapple

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C4 vs CAM Summary

C4 plants separate 2 steps of C fixation anatomically in 2 different cells

CAM plants separate 2 steps of C fixation temporally =2 different times

night vs. day

solves CO2 / O2 gas exchange vs. H2O loss challenge

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Why the C3 problem? Possibly evolutionary baggage

Rubisco evolved in high CO2 atmosphere there wasn’t strong selection against active site of

Rubisco accepting both CO2 & O2

Today it makes a difference 21% O2 vs. 0.03% CO2

photorespiration can drain away 50% of carbon fixed by Calvin cycle on a hot, dry day

strong selection pressure to evolve better way to fix carbon & minimize photorespiration

We’ve all gotbaggage!

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It’s not so easy as it looks…

Any Questions??

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Ghosts of Lectures Past(storage)

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A second look inside a leaf… Gas exchange & water flow

CO2 in O2 out H2O out

photosynthesis

gas exchangewater loss

xylem (water)

O2 CO2

for Calvin cyclewaste from light reactionsfor light reactions

phloem (sugars)

H2O O2 CO2

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C4 photosynthesis

CO2O2

CO2

O2

Outer cells light reaction &

carbon fixation pumps CO2 to inner

cells keeps O2 away from

inner cells away from RuBisCo

Inner cells Calvin cycle glucose to veins

PHYSICALLY separated C fixation from Calvin cycle

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Supporting a biosphere

On global scale, photosynthesis is the most important process for the continuation of life on Earth each year photosynthesis…

captures 121 billion tons of CO2

synthesizes 160 billion tons of carbohydrate heterotrophs are dependent on plants as

food source for fuel & raw materials

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The poetic perspective… All the solid material of every plant

was built by sunlight out of thin air All the solid material of every animal

was built from plant material

Then all the plants, cats, dogs, elephants & people …

are really particles of air woven together by strands of sunlight!

sunair

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If plants can do it…You can learn it!

Ask Questions!!

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Plant pigment lab