Chapter 6
WHERE IT STARTS
– PHOTOSYNTHESIS
Introduction
Before photosynthesis evolved, Earth’s
atmosphere had little free oxygen
Oxygen released during photosynthesis
changed the atmosphere
Favored
evolution of new metabolic
pathways, including aerobic respiration
Sunlight as an Energy Source
Visible light
A
small part of a spectrum of electromagnetic
energy radiating from the sun
Travels in waves
Wavelength – The distance between the crest
of two successive waves of the light is called
wavelength
Measured in nanometer (nm)
It is organized as photons or packets of
electromagnetic energy
Electromagnetic Spectrum
Photosynthetic Pigments
Photosynthesis begins when photons are
absorbed by photosynthetic pigment
molecules
Pigments absorb nearly all wavelengths of
visible light
The wavelength of light that are not
absorbed are reflected as its characteristic
color
Major Photosynthetic Pigments
Chlorophyll a
Main photosynthetic pigment
Absorbs violet and red light
(appears green)
Chlorophyll b, carotenoids, phycobilins
Absorb additional wavelengths
Overview of Photosynthesis
Chloroplasts are organelles of
photosynthesis in plants
They have three membranes
Two enclose a semifluid matrix called
stroma
Folded thylakoid membrane
Chloroplasts contain two kinds of
photosystems, type I & type II
Sites of Photosynthesis
6.3 Overview of Photosynthesis
Photosynthesis proceeds in two stages
Light-dependent
reactions
Light-independent reactions
Summary equation:
energy
6H2O + 6CO2 Light
6O2 + C6H12O6
enzymes
Sites of Photosynthesis:
Chloroplasts
Light-dependent reactions occur at a
much-folded thylakoid membrane
Forms
a single, continuous compartment
inside the stroma (chloroplast’s semifluid
interior)
Light-independent reactions occur in the
stroma
Products of Light-Dependent and Light
–Independent Reactions
Light - Dependent
Typically, sunlight energy drives the
formation of ATP and NADPH
Oxygen is released from the chloroplast
(and the cell)
Light – Independent
Synthesis of sugar and other
carbohydrates
sunlight
O2
CO2
H2O
CHLOROPLAST
lightdependent
reactions
NADPH, ATP
NADP+, ADP
lightindependent
reactions
sugars
CYTOPLASM
In chloroplasts, ATP and NADPH form in the light-dependent stage of
photosynthesis, which occurs at the thylakoid membrane. The second stage,
which produces sugars and other carbohydrates, proceeds in the stroma.
Light-Dependent Reactions
Two types of photosystems
In thylakoid membrane
Light-harvesting complexes
Absorb light energy and pass it to photosystems which
then release electrons
Photosynthesis begins when photon energy is
captured by light harvesting complexes and
transferred to photosystem II
P700 – Photosystem I, absorb energy of 700nm
P680– Photosystem II, absorb energy of 680nm
Noncyclic Photophosphorylation
Electrons released from photosystem II
flow through an electron transfer chain
At
end of chain, they enter photosystem I
Photon energy causes photosystem I to
release electrons, which end up in NADPH
Photosystem II replaces lost electrons by
pulling them from water
Photolysis – The process by which the
energy of light breaks down a molecule
ATP Formation
In both pathways, electron flow through electron
transfer chains causes H+ to accumulate in the
thylakoid compartment
A hydrogen ion gradient builds up across the
thylakoid membrane
H+ flows back across the membrane through
ATP synthases into stroma
The ATP and NADPH formed are used in
sugar-synthesizing, light-independent
reactions in the stroma
Noncyclic Photophosphorylation
Cyclic Photophosphorylation
Electrons released from photosystem I enter an
electron transfer chain, then cycle back to
photosystem I
It yields only ATP
NADPH does not form, oxygen is not released
Energy Flow in
Light-Dependent Reactions
Light Independent Reactions:
The Sugar Factory
Calvin – Benson cycle build sugars in the
stroma of chloroplast
Light-independent reactions proceed in the
stroma
Carbon fixation: Enzyme rubisco attaches
carbon from CO2 to RuBP (a 5 carbon
molecule) to start the Calvin–Benson cycle
Calvin–Benson Cycle
Cyclic pathway makes phosphorylated glucose
Uses energy from ATP, carbon and oxygen
from CO2, and hydrogen and electrons from
NADPH
Reactions use glucose to form photosynthetic
products (sucrose, starch, cellulose)
Six turns of Calvin–Benson cycle fix six carbons
required to build a glucose molecule from CO2
Light-Independent Reactions
Adaptations:
Different Carbon-Fixing Pathways
Environments differ
Plants
have different details of sugar
production in light-independent reactions
On dry days, plants conserve water by closing
their stomata
Stomata is a small opening across the surface
of leaves
O2 from photosynthesis cannot escape
Plant Adaptations to Environment
C3 Plant
At high O2,rubisco attaches oxygen (not
carbon) to RuBP in a pathway called
photorespiration
This reduces the efficiency of sugar
production
CO2
O2
glycolate
RuBP
CalvinBenson
cycle
PGA
sugar
ATP
NADPH
C3 plants. On dry days, stomata close
and oxygen accumulates in air spaces
inside leaves. The high concentration of
oxygen makes rubisco attach oxygen
instead of carbon to RuBP. Cells lose
carbon and energy as they make sugars.
Plant Adaptations to Environment
C4 plants
Carbon
fixation occurs twice
First reactions release CO2 near rubisco, limit
photorespiration when stomata are closed
CO2
from
inside
plant
C4 oxaloacetate
cycle
CO2
RuBP
CalvinBenson
PGA
cycle
sugar
C4 plants. Oxygen also builds
up in the air spaces inside the leaves
when stomata close. An additional
pathway in these plants keeps the
CO2 concentration high enough to
prevent rubisco from using oxygen.
Plant Adaptations to
Environment
CAM plants (Crassulacean Acid Metabolism)
Open
stomata and fix carbon at night