Chapter 7

PHOTOSYNTHESIS

Life Depends on Photosynthesis What if there is a nuclear winter?

What is itWhat would cause it?What are the repercussions?

What is a photosynthate?Glucose and other

carbohydratesRuns photorespirationMakes amino acids, starch, cellulose, rubber, quinine, spices, etc

Who came first: Autotrophs or Heterotrophs?Heterotrophs CREATED a heavy

CO2 atmosphere.Almost became extinct when resources disappeared.

Environmental pressure allowed heterotrophs to begin to photosynthesize

Autotrophes kick out TONS of O2

Who came first: Autotrophs or Heterotrophs?, con’t How did the rise of autotrophes change the

earth?Decrease CO2 in atmosphere so climate

got coldCreated polar ice capsLowered ocean levelsO2 levels increased to above today’s

levels Jurassic – much higher

ARE LEVELS STABLE NOW?????

A. Light

Visible light makes up only a small portion of

the electromagnetic spectrum.

Sunlight consists of:

4% Ultraviolet (UV) radiation

44% Visible light

52% Infrared (IR) radiation

Characteristics of Visible Light:• is a spectrum of colors ranging from violet to

red (ROY G BIV)• consists of packets of energy called photons • photons travel in waves, having a measurable

wavelength (λ) λ = distance a photon travels during a complete vibration [measured in nanometers (nm)]

A photon’s energy is inversely related to its wavelength......the shorter the λ, the greater the

energy it possesses.Which of the following photons

possess the greatest amount of energy?

Green photons λ = 530nm

Red photons λ = 660nm

Blue photons λ = 450nm

What happens to light when it strikes an object?

• reflected (bounces off)

Only absorbed wavelengths of light function in photosynthesis.

Visible light provides just enough energy to “excite” or energize molecules

• transmitted (passes through)

• absorbed

B. Photosynthetic PigmentsMolecules that capture photon energy

by absorbing certain wavelengths of light.

1. Primary pigments• Bacteriochlorophyll - green pigment

found in certain bacteria.• Chlorophylls a & b - bluish green

pigments found in plants, green algae & cyanobacteria.

Chlorophyll a is the dominant pigment in plant cells.

Central Mg atom with 4 N atoms –area where energy transfer occurs

Tail anchors molecule to chloroplast

2. Accessory Pigments• Carotenoids - red, orange, yellow pigments

found in plants, algae, bacteria & archaea.• Xanthophylls – red and yellow pigments

found in plants, algae & bacteria.• Fucoxanthin –brown pigment found in brown

algae, diatoms, & dinoflagellates• Phycoerythrin - red pigment found in red

algae.• Phycocyanin - blue pigment found in red

algae & cyanobacteria.• Bacteriorhodopsin – purple pigment found in

halophilic archaeaEach pigment absorbs a particular range

of wavelengths.

C. Chloroplasts (hyperlink chloroplasts)Sites of photosynthesis in plants & algae.Concentrated in mesophyll cells of most

plants.

Chloroplast structure:

• Stroma - gelatinous matrix; contains ribosomes, DNA & various enzymes.

• Thylakoid - flattened membranous sac; embedded with photosynthetic pigments.

D. Photosynthesis

Occurs in two stages:• Light reactions (pink hyperlink)- harvest

photon energy to synthesize ATP & NADPH.

• Carbon reactions (Calvin cycle) - use energy from light reactions to reduce CO2 to carbohydrate.

6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O

Overview of Photosynthesis

1. Light Reactions• require light• occur in thylakoids of chloroplasts• involve photosystems I & II (light

harvesting systems).

Photosystems contain antenna complex that captures photon energy & passes it to a reaction center.

Light Reactions of Photosynthesis

LIGHT REACTION

•Uses light and water•Synthesizes ATP and NADPH•O2 released here (comes from water)•Begins in thylakoid membrane•4 clusters of proteins involved•2 clusters make 2 photosystems•other 2 clusters form electron transport chain (ETC)•notice different light lengths

Light Reaction, con’t

Photosystem II happens first

SUN Antenna Complex

Chlorophyll a in reaction center P680

e-

1st carrier molecule in ETC

Energy (electron) goes to Photosystem I

ATP, produced from ADP, comes out of thylakoid and into stroma for Calvin cycle

Powers Calvin

Collects photons sends electrons

Receives electrons sends electrons

Redox reactions

Oxygen is released here when H2O is split

Light reaction, con’t

Photosystem I is next

Sun

1st carrier molecule in new ETC

Antenna complex

Electrons change (reduce) NaDP+ to NaDPH

Chlorophyll a in reaction center P700

e-

Powers Calvin

ATP Production by Chemiosmotic Phosphorylation

2. Carbon Reactions (Calvin cycle; C3 cycle) • do NOT require light (occur in both

darkness & light as long as ATP & NADPH are available)

• occur in stroma of chloroplasts• require ATP & NADPH (from light

reactions), and CO2

Calvin Cycle

CALVIN CYCLE (aka carbon cycle, aka C3 cycle)

•Found in plants that only use calvin: cereals, peanuts, tobacco, spinach, most trees and lawn grasses•C3 named for 3 carbon compound: phosphoglyceric acid (PGA)- 1st stable compound in pathway•Calvin – discovered this system•Goal of calvin cycle: Fix C from CO2 into organic compounds (glucose)•Occurs in both light and dark (as long as ATP and NADPH is available•CO2 enters through stoma (plural: stomata)•Powered by ATP/NADPH from light reaction•Diagram on pg 117 is good•Rubisco – most abundant and important protein in the world•85% of plants use this system

Calvin cycle, con’tRuBp (ribulose Biphosphate

CO2 from the atmosphere

Unstable 6 Carbon molecule

PGA

3-C

PGA

3-C

Combines with

(assisted by rubisco- an enzyme)

(uses ATP and NADPH as energy from LR)

Becomes

More ATP & NADPH used here

PGAL

C3H6O3

Converts to glucose then to other carbohydrate

Can be rearranged to form RuBP (starts cycle over)

(Phosphoglyceraldehyde: 1st carbohydrate product of Calvin) (C3H6O3)

Plants that use only the Calvin cycle to fix carbon are called C3 plants.

Ex. cereals, peanuts, tobacco, spinach, sugar beets, soybeans, most trees & lawn grasses.

Photosynthetic Efficiency Only .037% of atmosphere is CO2

Yet 200 billion tons of carbon (from CO2 used to make glucose

If each photosystem gets all the CO2 possible, efficency is only 30%- reality is much lower!

Cloudy days are only .1% efficient Cultivated plants only 3% efficient Greatest natural efficiency – evening primrose

– 8% Sugarcane – 7%

E. PhotorespirationProcess that counters

photosynthesis.Occurs when stomata close under

hot, dry conditions:• O2 levels in plant increase• CO2 levels in plant decrease

Under these conditions, rubisco fixes O2 (rather than CO2).Thus, PGAL is NOT produced.

Photorespiration Occurs when conditions are low CO2 or

high O2 Rubisco uses O2 instead of CO2 as

substrate Results in loss of Carbon for calvin

(carbon) reaction High temperatures complicate things:

fixing and releasing CO2 occurs at the same rate. No net C gain, no glucose produced

Stomata too open (trying to get more CO2) means dehydration

C4 Photosynthesis

Adaptations that allow certain plants to conserve water and reduce photorespiration at higher temperatures.

Found in sugarcane, corn, millet, sorghum, all flowering plants in hot, open environments (about .4% of all plants)

Use 4-carbon compound to concentrate carbon within special cells

CO2 is fixed in mesophyll cells first This keeps CO2 concentration 20-120 times greater than in

C3 cycle Not efficient in normal climates: used 2 ATP for every

carbon that is moved from the mesophyll cells Then it is fixed as normal via Calvin cycle These plants require about half as much water due to

recycling of compounds during the pathway

C4 Respiration

C4 plants reduce photorespiration by physically separating the light reactions and Calvin cycle.

C4 plants even fix CO2 when stomata begin to close – preserves water – plants require ½ the water of C3

CO2 malic acid migrates to bundle sheath cells and enters C3 glucose (instead of PGA)

Leaf anatomy of a C4 plant

C4 Photosynthesis:

• Light reactions occur in chloroplasts of mesophyll cells.

• Calvin cycle occurs in chloroplasts of bundle sheath cells.

2. CAM PhotosynthesisCAM plants reduce photorespiration

by acquiring CO2 at night.

Night:• mesophyll cells fix

CO2 as malic acid• malic acid is stored

in vacuoles.

Day:• malic acid releases

CO2 which enters Calvin cycle.

Malic acid

CAM (crassulcean acid metabolism)Respiration

Includes cacti, pineapple, Spanish moss, orchids, some firns and wax plants (10% of all plant species)

Take in CO2 at night Fix it in calvin cycle the next day Stomata stay closed during the day to

preserve water Plants acidic at night – more alkaline in

day Malic acid formed in large vacuoles in

same cells that contain chloroplasts Malic acid enters chloroplast during the

day where C3 begins