Chapter 10:
Photosynthesis
Essential Knowledge


2.a.1 – All living systems require constant input of
free energy (10.1-10.3).
2.a.2 – Organisms capture and store free energy for
use in biological processes (10.1-10.3).
Photosynthesis
Process
by which plants use
light energy to make food
A reduction process that makes
complex organic molecules
from simple molecules
Ps General Equation
6 CO2 + 6 H2O  C6H12O6 + 6 O2
Requires:
Chlorophyll
Light
♦Notice:
This is VERY
similar to the
cell resp eq.
Ps: a redox process
Hydrogens
are added to
Carbons-reduces CO2 to sugar
Water is split; becomes source
of hydrogens
Complex covalent bonds made
Requires energy
Ps
Has
two chemical reactions:
1. Light Reaction
2. Dark Reaction
Names are from “light” as a
requirement, not where or
when they occur
Light
A
form of electromagnetic
radiation/energy
Visible light has the right
energy for use in Ps
Travels in waves
Too Hot
Too Cold
Just Right
Detected by our eye-hence the term visible
Action Spectrum
Not
all colors are useable to
the same degree for Ps
Red and Blue light - absorbed
and used in Ps
Green light - reflected or
transmitted
Comment
In
oceans, red light is lost or
filtered out early because it
has lower energy
Only blue light which has
higher energy can reach the
lower depths
Result
Many
deep ocean
fish are bright red in
color.
Why?
They
can’t be seen
because there is no
red light to reflect
their color
Photosynthesis
Pigments
Pigment:
absorb light
Different
pigments absorb
different wavelengths
Types:
Chlorophylls
(a and b)
Accessory Pigments
(carotenoids)
Chlorophylls
Have
C,H,O,N and Mg
Two major types: a, b
Molecule has a lipophilic tail
(allows it to dissolve into
membranes)
Contains Mg in a reaction
center.
Accessory Pigments
Absorb
light energy and
transfer it to chlorophyll
Various shades of
yellow/orange
Function: photoprotection
Ex: Carotene (orange)
Xanthophyll (yellow)
Fall Leaf Colors
Chlorophyll
breaks down
N and Mg salvaged and
moved into the stem for next
year
Accessory pigments remain
behind, giving the various fall
leaf colors
Parts of Plant:
Ps function
Stomata
Roots
Mesophyll
Veins
Site of Ps
Chloroplast
Contain
chlorophyll pigment
All green parts of plants
contain chloroplasts
Most are found in leaves
Why? Larger surface area
Chloroplast Structure
Double
outer membrane
Inner membrane folded and
stacked into grana
Stroma - liquid that
surrounds the thylakoid
membranes
Photosystems
Collection
of pigments that
serve as a light trap
Made of chlorophyll and the
accessory pigments
Two types known:
PSI
& PSII
Parts of Photosystem
1)
Rxn center/e- acceptor
Chlorophyll
uses light energy to
move e- to next energy level
2)
Pigment
Light
3)
absorption, photon acceptor
Light-harvesting complex
Proteins,
contain pigment
Light Reaction
Same
thing as Noncyclic
Photophosphorylation
Location - grana of the chloroplast
Function - to split water and
produce ATP and NADPH (provide
chem energy for next cycle)
Light Reaction
Requirements:
Light
Water
ADP + Pi
NADP+
Products:
O2
ATP
NADPH
Chemiosmosis Model
The
chloroplast produces
ATP in the same manner as
the mitochondria in Rs
Light energy is used to pump
H+ across a membrane
When the H+ diffuses back,
ATP is generated
Chemiosmosis
H+ are
pumped into the
thylakoid space
ATP and NADPH are made
when the H+ diffuse into the
stroma
Both Rs and Ps use ETC to
pump H+
Dark Reactions
How
plants actually make food
(carbohydrates)
Don't require light directly to
run
Also known as the
Calvin cycle or C3 Ps
Calvin cycle/Dark reaction
– builds sugars from
smaller molecules
Function - use ATP and
NADPH to build food from CO2
Location - stroma of the Calvin
Cycle
chloroplast
movie
Anabolic
Rubisco
Ribulose
BisPhosphate
Carboxylase
Enzyme that adds CO2 to an
acceptor molecule
Most
important enzyme on earth
Catalyzes first step of dark rxn
C3 Ps
C3
plants are plants that DO use
Rubisco enzyme “normally”
Keep stomata open during day
MOST plants are C3 plants
When
you “think of Ps”, you think of a
C3 plant performing Ps
Adaptive value: Require less
Less
enzyme activity, no specialized
organs/anatomy
Photorespiration
When
Rubisco accepts O2
instead of CO2 as the substrate
Generates no ATP
Actually
uses ATP
Decreases
Ps output by as
much as 50%
Photorespiration
May
reflect a time when O2 was
less plentiful and CO2 was more
common
Important
to evolution of autotrophs
Photo=occurs
in light
Respiration=consumes oxygen
while producing CO2
Alternate Ps Methods
1. C4 Ps
2. CAM Ps
Intro to C4 and CAM
Remember:
MOST plants are
C3 plants that performs Ps
“normally”
Both are adaptations to arid
conditions
CAM
plants “idle” and save
energy while C4 plants go
through Ps faster
C4 Ps
Uses
a different enzyme to
initially capture CO2
Separates CO2 capture from
carbon fixation into sugar
Still uses C3 Ps to make
sugar, but only does so in the
bundle sheath cells
C4 Ps
Found
in 19 plant families
Characteristic of hot regions
with intense sunlight
Examples - sugarcane,
Bermuda grass, crab grass
C3 Ps
 Can
vs
use
photorespiration
 Shade to full sun
 High water use
 Cool temperatures
 Slow to moderate
growth rates
 Cool season crops
C4 Ps
 No
Photorespiration
 Full sun only
 Moderate water use
 Warm temperatures
 Very fast growth
rates
 Warm season crops
CAM plants
Found
in plants from arid
conditions (where water stress is
a problem)
Avoid H2O stress by keeping
stomates closed during the day
Generally have slow growth
Ex: pineapple, cacti, orchids
CAM Plants, cont.
Store
CO2 as C4 acid
During the day, the acid is
broken down and CO2 is
made into sugar.
Ps:Rs Ratios
Reflect
a plant’s balance in
making food and using food
1. Ps > Rs, energy available
for growth and
reproduction
2. Ps = Rs, no growth, but
don’t die either
3. Ps < Rs, death by starvation
Comments - Ps:Rs
Rs
happens 24 hours a day
Ps only in light
Plants overwinter on stored
food when Ps > Rs
If Ps < Rs, best solution is to
increase the amount of light
Factors Affecting Ps
1. Light - quantity and quality
2. Temperature - too hot or too
cold
3. CO2 - often limits C3 plants
4. Minerals - especially NPK
and Mg
Importances of Ps
1. Food - either directly or
indirectly comes from plants
2. Oxygen in the air
3. CO2 balance
4. Plant products
5. Life on Earth
Summary

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
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Identify the basic equation of photosynthesis.
Identify the main reactions of photosynthesis.
Recognize the function, location, requirements
and products of the photosynthesis reactions.
Recognize the role of light and pigments in the
photosynthesis process.
Recognize the effects and importance of
photorespiration.
Recognize various types of photosynthesis and
their importance.
Identify the biological and economic importance
of photosynthesis.
Exclusion Statements
You
do NOT need to
memorize the steps in the
Calvin Cycle, the structure of
the molecules, and the names
of the enzymes (except ATP
synthase).