Biology, Section 3
Notes
Date Submitted: 12/15/11
Chapter 6 Section 2 Notes
Photosynthesis: The Calvin Cycle
Carbon Fixation
A. Calvin Cycle: series of enzyme-assisted chemical reactions that produces 3-carbon sugar
a. Carbon fixation: Carbon atoms from CO2 in atmosphere are bonded, or “fixed,” into
organic compounds
b. Total of 3 CO2 must enter Calvin cycle to make each 3-carbon sugar to make organic
compounds
1) 3 CO2 molecules diffuse into STROMA from CYTOSOL
a. Enzyme combines each CO2 molecule with 5-carbon molecule called ribulose biphosphate
(RuBP)
b. Resulting 6-carbon molecule is UNSTABLE and immediately splits into TWO 3-carbon
molecules called 3-phosphoglycerate (3-PGA) = 6 3-PGA
2) Each of 6 molecules of 3-PGA is converted into another 3-carbon molecule called glyceraldehyde 3phosphate (G3P) in 2-part process = 6 G3P
a. Each 3-PGA molecule receives a phosphate group from 1 ATP
b. Resulting compound receives a proton (H+) from NADPH (NADP+ + H+ + 2 e-) and releases
phosphate group. This produces G3P.
c. 6 ADP, 6 NADP+, and 6 Phosphate that are also produced can be used again in light
reactions to make more ATP and NADPH.
3) 1 of G3P molecules leaves CC and is used to make organic compounds (carbohydrates) in which
energy is stored for later
4) Remaining G3P molecules are converted back into RuBP through addition of Phosphate groups from
ATP
a. Resulting RuBP molecules enter CC again.
B. Calvin cycle (named after Melvin Calvin) is most common pathway for carbon fixation.
C. Plant species that fix carbon exclusively through Calvin cycle are known as C3 plants b/c of 3-carbon
compound that is initially formed
Alternative Pathways
A. Many plant species that evolved in HOT, DRY climates fix carbon in other
ways
B. Plants can rapidly lose water to air through small pores called stomata
a. Usually located on undersurface of leaves
b. Plants can reduce water loss by partially closing stomata when air
is arid
C. Stomata are major passageways through which CO2 enters and O2 leaves
plant
D. When partly closed,
a. Level of CO2 in plant ↓ (since CO2 is consumed in CC)
b. Level of O2 in plant ↑ (light reactions generate O2)
c. Both ↓CO2 and ↑O2 levels INHIBIT carbon fixation
1) C4 Pathway
a. Allows plants to fix CO2 into 4-carbon compounds (known as C4 plants)
b. During hottest part of day, C4plants have stomata partially closed
c. Certain cells in plant have enzyme that can fix CO2 into 4-C compounds even when ↓CO2
and ↑O2 levels
d. Compounds are then transported to other cells where CO2 is released and enters CC
e. C4 plants: corn, sugar cane, crab grass
i. Lose only about ½ as much water as C3 plants when producing same amount of
carbohydrates
ii. Many evolved in TROPICAL climates
2) CAM Pathway
a. Cacti, pineapples, and others adapt to HOT, DRY climates
b. Fix carbon through CAM: crassulacean acid metabolism (b/c water-conserving pathway was
discovered in plants of family Crassulaceae)
c. Open stomata at night & close during day (opp. of what other plants do)
i. b/c open @ night, temp. is lower, grow slowly
d. At night, CAM plants take CO2 and fix it into a variety of organic compounds
e. During day, CO2 is released from these compounds and enters CC
f. Lose less water than C3 or C4 plants
Summary of Photosynthesis
A. Light reactions (thylakoids) – Energy is absorbed from sunlight, converted into chemical energy,
which is temp. stored in ATP and NADPH
B. Calvin cycle (stroma) – CO2 and chemical energy stored in ATP and NADPH are used to form
organic compounds
C. Ongoing cycle: Products of LR used in CC, some products of CC used in LR
D. Other products of CC used to produce variety of organic compounds (aa, lipids, carbohydrates)
E. Most plants produce more carbohydrates than they need  can be stored as starch in chloroplasts
and in structures such as roots and fruits  provide chemical energy that auto/hetero-trophs need
F. Water is split during LR, yielding electrons, protons, and oxygen as byproduct
G. Simplest equation including LR and CC:
H. CH2O represents general formula for CARBOHYDRATE (often replaced by glucose):
I. However, glucose is not actually a direct product
J. Glucose mainly used to emphasize relationship b/w photosynthesis and cellular respiration (glucose
plays key role). Photosynthesis & Cellular Respiration create ongoing cycle too.
K. LR aka light-dependent reactions b/c light energy is required
L. CC aka light-independent / dark reactions b/c doesn’t require light directly
a. But usually proceed during daytime when LR are producing materials for CC
Factors that Affect Photosynthesis
A. Light Intensity
a. Rate of photosynthesis increases as light intensity increases
b. Higher LI excites more electrons in both photosystems
c. However, at some point, all of available electrons are excited, and max. rate of
photosynthesis is reached
B. CO2 Levels
a. Increase of CO2 stimulates photosynthesis until rate levels off (similar to graph of LI)
C. Temperature
a. Increasing temperature accelerates chemical reactions involved
b. Thus, rate of photosynthesis increases BUT over a certain range
c. The rate peaks @ certain temp. @ which many enzymes that catalyze the reactions denature
d. Also, stomata begin to close, limiting water loss and CO2 entry
e.