PhotosynthesisHow do

photoautotrophs use light to make their own food?

Autotrophs

• Literally means “self-feeders”

• Include all plants, some bacteria, archaea, and protists (algae)

• make organic food molecules (e.g. glucose) from inorganic raw materials (e.g. water and carbon dioxide)

http://iusd.k12.ca.us/uhs/cs2/images/Leaf%20Cross-section.jpg Chloroplast

How can carbon dioxide get into the leaf?

• Through tiny holes on leaf surface called stomata

• Oxygen produced from photosynthesis can also escape out of leaves through stomata

Leaf Structure

Stomata

http://www.macleans.school.nz/students/science/F4/plants/closed_stoma.jpg

Closed Stomate

Regulation of Guard Cells

A. Open stoma B. Closed stoma

http://www.phschool.com/science/biology_place/labbench/lab9/images/stoma1.gif

How can we tell if a plant is actively photosynthesizing?

http://www.bbc.co.uk/schools/ks3bitesize/science/images/image33.gif

Photosynthesis and oxygen atoms

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

6 CO2 + 12 H2O C6H12O6 + 6H2O + 6O2Label

No label

Redox reactions…Which one is photosynthesis?

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

reduction

oxidation

C6H12O6 + 6O2 6 CO2 + 6 H2O

reduction

oxidation

Electromagnetic Spectrum

http://upload.wikimedia.org/wikipedia/en/thumb/8/8a/Electromagnetic-Spectrum.png/350px-Electromagnetic-Spectrum.png

Green plants are green because…

• Chlorophyll reflects green light, but absorbs light in the red, orange, blue, indigo and violet wavelength range

• Other pigments absorb light of different wavelengths, e.g. carotenoids are yellow-orange pigments

Chlorophyll embedded in a photosystem

Two photosystems

1. Photosystem I was discovered first and the chlorophyll a of its reaction center absorbs light best at 700 nanometers (far-red zone)

2. Photosystem II reaction center chlorophyll absorbs best at 680 nm (red zone)

Chemiosmosis

Electron flow in the light reactions

http://library.thinkquest.org/C004535/media/photosystem.gif

Two types of photosystems cooperate in the light reactions

Ph

oto

n

Ph

oto

n

Water-splittingphotosystem

NADPH-producingphotosystem

ATPmill

Two stages of Photosynthesis

• Light Reactions

• takes place in thylakoid membranes (grana) of chloroplasts

• Dark Reactions (Calvin Cycle)

• takes place in the stroma (thick fluid inside chloroplasts)

I. Light Reactions• Input:

• H2O (from roots)

• NADP+

• ADP and Phosphate

• photons (packets of light energy)

• Output:

• O2

• NADPH (carrying electrons)

• ATP

http://student.ccbcmd.edu/courses/bio141/lecguide/unit6/metabolism/photosyn/images/u4fg45.jpg

II. Calvin Cycle

• Input:• CO2

• ATP• NADPH• RuBP (ribulose

biphosphate: a 5-carbon sugar)

• Output:• G3P

(glyceraldehyde 3-phosphate) used to make glucose

• NADP+• ADP and Phosphate

http://www.daviddarling.info/images/Calvin_cycle.jpg

http://edu.tebyan.net/biology/Cells/images/calvin_cycle.gif

Calvin Cycle

• Carbon is reduced to glucose (carbon fixation)

• Electrons and protons are added to carbon

• NADP+ is regenerated so it can be reused in the Light Reactions

C3 plants like rice, wheat, and soybeans Rubisco can accept O2 instead of CO2 resulting in photorespiration

Stomates on hot dry daysdecrease carbon dioxide in leaf starves Calvin Cycle

oxygen concentrations in the leaf overtake CO2photorespiration occurs decreasing photosynthetic

output taking organic material away from Calvin Cycle

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/C4plants.html

95 percent of the plant species on earth

The equation for the Calvin Cycle:CO2 (Carbon dioxide in from stomata) + RuBP (Ribulose bisphosphate already in plant) + the enzyme RUBISCO (Ribulose bisphosphate carboxylase) “fixes” carbon from the atmosphere 2PGA (phospholygerate)  PGA enters Calvin cycle in Mesophyll cells more RuBP (to fix more CO2) + sugar (CH2O)  C3 are inefficient at CO2 fixation because RUBISCO has a greater affinity for oxygen than CO2

Mesophyll cells are packed with RUBISCO Stomata open during day (CO2, oxygen, and water can all flow out)   Photorespiration undoes CO2 assimilation

2PGA CO2 + RuBPincreases when there is lots of O2, low levels of CO2, and increased temperature

http://wc.pima.edu/~bfiero/tucsonecology/plants/plants_photosynthesis.htm

crop plants—sugar cane and corn 2nd most prevalent photosynthetic type

C4 plants—carbon fixation and photosynthesis split between the mesophyll cells and bundle sheath cells.  The equation:In mesophyll (carbon fixation):CO2 + PEP (phosphoenol pyruvate) + PEP carboxylase fixes carbon OAA (oxaloacetic acid) OAA diffuses to bundle sheath cells  In bundle sheath (Calvin Cycle):OAA malic acid and aspartic acid is decarboxylated CO2 + pyruvateThen the Calvin Cycle CO2 + RuBP + the enzyme RUBISCO 2PGA RuBP + CH2O Pyruvate with ATP is moved back to mesophyll and turned into PEP (to fix more CO2)

C4 Photosynthesis

Feature of many grasses (i.e. big blue stem back campus), corn, and many arid/semi arid shrubs

Calvin cycle in bundle sheath cells where there is no oxygen to be bound by RUBISCO

Very high concentration of CO2 in bundle sheath cells

PEP carboxylase has a high affinity for CO2 so plants must open their stomata less to get CO2 and hence lose less water (especially important in arid regions)

Low levels of photorespiration and higher net photosynthesis than C3 because of low photorespiration

Costly adaptation because it requires lots of ATP (energy)—however, benefits outweigh energy costs.

Stomata are open during the day

Fixation and the Calvin cycle are physically separate