Ch. 8.2
• Section Objectives
• Relate the structure of chloroplasts to the events in
photosynthesis
• Describe light-dependent reactions.
• Explain the reactions and products of the lightindependent Calvin cycle.
Photosynthesis:
Life from Light and Air
2006-2007
Trapping Energy from Sunlight
• The process that uses the sun’s energy to make
simple sugars is called photosynthesis.
AN OVERVIEW OF
PHOTOSYNTHESIS
• Photosynthesis is the process by which
autotrophic organisms use light energy to make
sugar and oxygen gas from carbon dioxide and
water
PHOTOSYNTHESIS
Plants are energy producers
• Like animals, plants need energy to live
– unlike animals, plants don’t need to eat food to make
that energy
• Plants make both FOOD & ENERGY
– animals are heterotrophs(consumers)
– plants are autotrophs (producers)
How do plants make energy & food?
• Plants use the energy from the sun
– to make ATP energy
– to make sugars
• glucose, sucrose, cellulose, starch, & more
sun
ATP
sugars
Building plants from sunlight & air
• Photosynthesis
– 2 separate processes
– ENERGY building reactions
sun
• collect sun energy
• use it to make ATP
ATP
– SUGAR building reactions
• take the ATP energy
• collect CO2 from air &
H2O from ground
• use all to build sugars
carbon dioxide
CO2
water
+ HO
2
H2O
+
CO2
sugars
C6H12O6
sugars
What do plants need to grow?
• The “factory” for trapping
energy & making sugars
– chloroplast
• Fuels
– sunlight
– carbon dioxide
– water
• The Products
- Oxygen & Glucose
The Helpers
– Enzymes
sun
O2
CO2
enzymes
H2 O
sugars
•
So what does a plant
Bring In need?
– light
– CO2
– H2O
leaves
• Let Out
shoot
– O2
• Move Around
– sugars
roots
C6H12O6 + 6O2
6CO2 + 6H2O + light

energy
Photosynthesis
sun
ENERGY
building
reactions
ATP
ADP
SUGAR
building
reactions
H 2O
used immediately
to synthesize sugars
sugar
CO2
Chloroplasts absorb
Leaf
Leaves
sunlight & CO2
sun
CO2
Chloroplasts
in cell
Chloroplast
Chloroplasts
contain
Chlorophyll
Chloroplast
make
ENERGY & SUGAR
Stomates & Guard Cells
in the leaf
• Function of stomates
– CO2 in
– O2 out
– H2O out
• gets to leaves for photosynthesis
• Function of guard cells
– open & close stomates
guard cell
stomate
An overview of photosynthesis
Chloroplast
CO2
H2O
NADP+
ADP
+P
LIGHT
REACTIONS
(in grana
ATP
CALVIN
CYCLE
(in stroma
NADPH
O2
O2
Light
Sugar
Trapping Energy from Sunlight
•
Photosynthesis happens in two phases.
1. The light-dependent reactions convert light
energy into chemical energy. (ATP)
2. The molecules of ATP produced in the light-dependent
reactions are then used to fuel the Calvin Cycle or
light-independent reactions that produce simple
sugars.
•
The general equation for photosynthesis is written as
6CO2 + 6H2O→C6H12O6 + 6O2
THE LIGHT REACTIONS: CONVERTING SOLAR
ENERGY TO CHEMICAL ENERGY
Visible radiation drives the light reactions
• Certain wavelengths of visible light drive the light
reactions of photosynthesis
Gamma
rays
X-rays
UV
Infrared
Visible
light
Wavelength (nm)
Microwaves
Radio
waves
pigments in the chloroplast
• To trap the energy in the sun’s light, the thylakoid membranes
contain pigments, molecules that absorb specific wavelengths of
sunlight.
• Wavelengths that are NOT absorbed are reflected (bounce off) or
transmitted (pass through)
• So the material in which the pigment is found appears to be the
color of the wavelengths that are NOT absorbed
Photosynthetic pigments: can absorb light energy & make it available
for conversion to chemical energy
Light
Reflected
light
Chloroplast
•
• .
Absorbed
light
pigments in the chloroplast
• Chlorophyll a: most common pigment in chloroplast
(absorbs blue &red light and reflects green light thus
giving the chloroplast a green color)(chloro = green
phylla + leaf)
• Accessory pigments: additional pigments that absorb
different wavelengths (carotene, chlorophyll b, &
xanthophyll)
Light-Dependent Reactions
absorption of light energy by chlorophyll
• As sunlight strikes the chlorophyll molecules in a
photosystem of the thylakoid membrane, the energy
in the light is transferred to electrons.
• These highly energized, or excited, electrons are
passed from chlorophyll to an electron transport
chain, a series of proteins embedded in the thylakoid
membrane.
• At each step along the transport chain, the electrons
lose energy.
Energy from the energized electrons pump
H+ ions and change NAPD+ to NADPH.
The H+ ions move from high to low and turn
the ATP synthase and change ADP + P to ATP
Energized electrons
Oxygen
by-product
Photolysis
Light-Dependent Reactions
• Chemiosmosis: This “lost” energy can be used to
form ATP from ADP, or to pump hydrogen ions into
the center of the thylakoid disc.
• .
Light-Dependent Reactions
• The electrons are transferred to the stroma of the
chloroplast. To do this, an electron carrier molecule
called NADP is used.
• NADP can combine with two excited electrons and a
hydrogen ion (H+) to become NADPH.
• NADPH will play an important role in the lightindependent reactions.
Restoring electrons
• To replace the lost electrons, molecules of water are
split in the first photosystem. This reaction is called
photolysis.
• The O2 liberated by photosynthesis is made from the
oxygen in water
H2O
+ +
O2 +
2e-
2e-
2
Chlorophyll
O2 + 2H+
H2O
Restoring electrons
• The oxygen produced by photolysis is released into the air and
supplies the oxygen we breathe.
• The electrons are returned to chlorophyll.
• The hydrogen ions are pumped into the thylakoid, where they
accumulate in high concentration.
Light Dependent Reactions: Summary
• Absorptions of light energy by chlorophyll-takes
place in thylakoid
1. Split water molecule (PHOTOLYSIS) Oxygen
combines with other oxygen to produce O2 which is
given off as a by-product
2. Hydrogen produced by splitting of water is
attached to hydrogen carrier NADP -> NADPH
(energy in this molecule)
3. Then energy from “excited electrons”is used to
take ADP + P ->ATP
Light Independent Reactions or The Calvin Cycle
• CO2 put into organic molecules called Carbon
fixation
• Does not require light but must have ATP and
NADPH which are produced by light
dependent reactions
• Called Calvin Cycle (Melvin Calvin won Nobel
prize in chemistry)
• Takes place in stroma of chloroplast
Light Independent Reactions or The Calvin
Cycle
• The Calvin cycle
constructs G3P (a
sugar) using
– carbon from
atmospheric CO2
– electrons and H+ from
NADPH
– energy from ATP
• Energy-rich sugar
(G3P) is then
converted into
glucose
INPUT
CALVIN
CYCLE
OUTPUT:
•
So what does a plant
Bring In need?
– light
– CO2
– H2O
leaves
• Let Out
shoot
– O2
• Move Around
– sugars
roots
C6H12O6 + 6O2
6CO2 + 6H2O + light

energy
An overview of photosynthesis
Chloroplast
Light
CO2
H2O
NADP+
ADP
+P
LIGHT
REACTIONS
(in grana
ATP
CALVIN
CYCLE
(in stroma
NADPH
O2
Sugar
Factors Affecting Rate of Photosynthesis
1.
2.
3.
4.
5.
Temperature: increases rate up to a certain point
Light Intensity: increases rate up to a certain point
CO2 level: Increases rate up to a certain point
Water: decrease water, decrease photosynthesis
Minerals; Ex. Magnesium, Nitrogen