Introduction to Photosynthesis
Chapter 10p. 181-188
Autotrophs: Producers of the Biosphere
Autotroph: “self-feeding”; produce own organic molecules from CO2 & inorg. molec. in environment i.e.: plants, algae, some
bacteria Heterotroph: “feeds on
others”; must consume other orgs to obtain nutrients, O2, & energy i.e.: animals, fungi, most
bacteria
Chloroplasts: Sites of Photosynthesis
Plants appear green due to pigment chlorophyll inside thylakoid space of chloroplasts Found in mesophyll tissue
of leaves 30-40 per cell Stomata: pores through
which CO2 enters & O2
leaves
Stomata
Chloroplast Structure
Tracking Atoms through Photosynthesis
O2 given off by plants comes from H2O; NOT CO2 Chloroplast splits H2O → 2H + O
C.B. van Niel: proved 2H from split H2O goes to glucose; O released as atmospheric O2
Photosynthesis is a Redox Reaction
Unlike cell respir., photosynthesis is endergonic Energy comes from sun Reverses direction of e- flow from H2O →
CO2 (oxidized)
CO2 is reduced to glucose
CO2 + H2O + energy → C6H12O6 + O2
The 2 Stages of Photosynthesis
1) Light Reactions (“photo”) e- + H+ transferred to NADP+ (cousin of
NAD+) O2 given off as byproduct Produces 1 ATP
(“photophosphorylation”) Occurs in the thylakoid
The 2 Stages of Photosynthesis
2) Calvin Cycle (“synthesis”) CO2 incorporated into organic molecules
already present (“carbon fixation”) “Fixed” C is reduced to glucose (add e-)
Powered by NADPH & ATP from light rxns Occurs during day in most plants; relies
on light rxns Occurs in stroma
Photosynthesis Overview
Nature of Sunlight
Light: electromagnetic energy (“radiation”) Travels in waves; distance
between called wavelength Also acts as photons:
particles of light energy Electromagnetic
Spectrum: entire radiation spectrum
Visible light = 380-750nm Amt energy inversely
proportional to wavelength Purple photon > red photon
Photosynthetic Pigments
Pigment: substance that absorbs visible light at different wavelengths Reflected/transmitted
wavelength is color we see
Spectrophotometer: measures absorbed wavelengths
Photosynthetic Pigments
Chlorophyll a: main photosynthetic pigment Absorbs red & blue
photons; reflects green Only pigment directly
involved in light rxns Other pigments
(Chlorophyll B & Carotenoids) transfer photons to chlorophyll a & provide photoprotection
Excitation of Chlorophyll by Light
When molecules absorb light energy (photons), e- “jump” to next orbital Ground state → excited
state Specific to wavelength Unstable e- will “fall” back
quickly, releasing energy (heat)
Fluorescence: energy released as light
Reactions of Photosynthesis
Chapter 10p. 189-198
Photosystems Consists of 3 sections: 1) Light-Harvesting Complex:
contain all 3 types pigments; ↑ surface area to absorb more light
2) Reaction-Center: at center; receives energy from light-harvesting complex & becomes excited Contains special chlorophyll a molecules whose e-’s
move to higher energy level 3) Primary Electron Acceptor: receives e-s
from excited chlorophyll a molecules & “catches” them e-’s then enter into Noncyclic Electron Flow
Types of Photosystems
Photosystem II: absorbs 680nm best (“P680”) P700 & P680 identical, but surrounded
by diff. proteins Work together to make ATP & NADPH
for Calvin Cycle Photosystem I: Reaction center
chlorophyll a absorbs 700nm best (“P700”)
Noncyclic Electron Flow Predominant route for e-s Steps:
1) Photots. II absorbs light, P680 excited 2) e-s captured by Primary e- Acceptor
P680 now has e- “hole” 3) e-s replaced in P680 by split H2O
molecule; O2 released inside thylakoid 4) ETC takes e-s from Primary e- Acceptor to
Photosystem I Composed of plastoquinone (Pq), 2
cytochromes, & plastocyanin (Pc)
Noncyclic Electron Flow
5) As e-s “fall” down chain, energy is harvested to make ATP by chemiosmosis outside thylakoid “Noncyclic Photophosphorylation”
6) Final e- acceptor is P700 P700 e-’s excited by light energy are captured by
Primary e- Acceptor Fills “hole” created by Primary e- Acceptor of Photo II
7) Primary e- Acceptor passes e-s to 2nd ETC → ferredoxin (Fd)
8) NADP+ reductase transfers e-’s from Fd to NADP+→ makes NADPH in stroma
Summary of Noncyclic Electron Flow
P680 → Primary e- Acceptor → 1st ETC → P700 → Primary e-
Acceptor → Fd → NADP+ reductase → NADPH
Cyclic Electron Flow
Calvin cycle uses more ATP than NADPH If ATP runs low,
chloroplast switches to cyclic
Involves Photosystem I (P700) only Fd takes e-s to
cytochrome complex of 1st ETC & returns them to P700
No NADPH produced; no O2 released
Calvin Cycle
C enters as CO2, leaves as sugar (G3P) Cycle must turn 3 x’s to make glucose
Must “fix” 3 C’s into org. molecules Occurs in 3 phases:
1) C Fixation: 3C’s from 3CO2 are incorporated into RuBP, catalyzed by rubisco
2) Reduction: e-’s from NADPH reduce 6 1,3 biphosphate → 6 G3P (↑ energy)
Spends 6 ATP 3) Regeneration of RuBP: G3P rearranged → RuBP
(can pick up CO2 again) Spends 3 ATP
Calvin Cycle - Summarized
For each turn of Calvin: In: Out:
9 ATP 9 ADP 6 NADPH 6 NADP+
3 CO2 1 G3P (will become
glucose)
Alternate Methods of C Fixation
In hot, dry climates, stomata remain closed to prevent H2O loss Also prevents CO2
in & O2 out Result is
Photorespiration
Photorespiration Most plants are called C3 because C
fixation creates a 3-C compound Closed stomata ↓ [CO2] inside leaf, and ↑
[O2] O2 will be picked up by rubisco (instead of
CO2) Photorespiration: uses light (“photo”)
to consume O2 (“respiration”) No ATP produced; no sugar made May be ancient evolutionary adaptation
C4 Plants
C fixed by PEP carboxylase to form 4-C compound (oxaloacetate → malate) PEP carbox. has ↑↑ affinity for CO2; can
“fix” CO2 when rubisco can’t 4-C cmpnd (malate) enters Bundle Sheath
cells where CO2 breaks off & enters Calvin Keeps CO2 levels ↑ for rubisco Minimizes photorespiration & ↑ sugar
production
CAM Plants
Water-storing plants (cacti, pineapple, etc.) close stomata during day, open at night Store org. molec.
until day when light rxns can provide ATP & NADPH