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
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).
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