Introduction to Photosynthesis
Chapter 10
p. 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 10
p. 189-198
Photosystems


Consists of 3 sections:
1) Light-Harvesting Complex:



2) Reaction-Center: at center; receives
energy from light-harvesting complex &
becomes excited


contain all 3 types pigments;
↑ surface area to absorb more light
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


6) Final e- acceptor is P700




“Noncyclic Photophosphorylation”
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 eAcceptor → 1st
ETC → P700 →
Primary eAcceptor → 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:




9 ATP
6 NADPH
3 CO2
Out:
9 ADP
6 NADP+
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