EXPLAIN:
CONVERTING
SUNLIGHT INTO
ENERGY
PHOTOSYNTHESIS
Energy: Capacity to do work
Forms of energy:
• Chemical (Food and Gas)
• Light
• Electrical
• Mechanical
Organisms store energy in the organic molecules
from which mechanical organisms are made
(Chemical Energy)
Remember: Energy is transferred from
one form to another, never destroyed
WEEEEE!!!
WHAT PLANTS DO
Photosynthesi
s
PHOTOSYNTHESIS
• Photosynthesis: is an energy-storing process where light
energy is converted to chemical energy in the form of
sugars
• Plants use Light Energy and convert it to Chemical
Energy
• Photosynthesis uses only certain wavelengths of visible
light
• In plants most light is absorbed by a green pigment called
Chlorophyll
• Plants are green because they reflect green light
CHLOROPLASTS
• Thylakoid: are located in the chloroplasts and are highly
organized arrangement of internal membranes.
• Chlorophyll and enzymes are embedded
• Grana: thylakoids form flattened stacks of disk shaped
structures
• Stroma: Solution that surrounds the thylakoid
ADENOSINE TRIPHOSPHATE (ATP)
• Chemical form of energy used in cells
• Most important of several transfer compounds in all
organisms
• Is efficient and useful for energy transfer
• Stores energy in high energy bonds between phosphates
ATP = 3 Phosphates
Breaking down ATP Releases
Energy
ADP = 2 Phosphates
Adding a phosphate Stores
Energy
HAPPENS IN TWO PHASES

Light-Dependent Reaction


Light-Independent Reaction


Converts light energy into chemical energy
Produces simple sugars (glucose)
General Equation
6 CO2 + 6 H2O  C6H12O6 + 6 O2
FIRST PHASE

Requires Light = Light Dependent Reaction
Sun’s energy energizes an electron in chlorophyll
molecule
 Electron is passed to nearby protein molecules in the
thylakoid membrane of the chloroplast


Electron from Chlorophyll is passed from protein
to protein along an electron transport chain
Electrons lose energy (energy changes form)
 Finally bonded with electron carrier called NADP+ to
form NADPH or ATP


Energy is stored for later use
Carbon
Dioxide
H2O
CO2
Light
NADP 
ADP
+ P
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH
Chloroplast
Figure 10.5
O2
[CH2O]
(sugar)
6 carbon
Sugars
6 carbon
molecules
TWO PHOTOSYSTEMS
Photosystems: are complexes of chlorophyll
(other pigments) and accessory proteins that act
as light-harvesting centers
 Each photosystem absorbs light and transfers its
energy to a reactive chlorophyll molecule
 There it excites and electron to increase energy
levels

A mechanical analogy for the light
reactions
In this "construction analogy" for the light reactions, the input of
light energy is represented by the large yellow mallets. The light
energy boosts the electrons up to their excited states atop the
platform in each photosystem. The energy released as the
electrons move down the electron transport chain between the
photosystems is used to pump hydrogen ions across a membrane
and produce ATP.
PHOTOSYSTEM
A photosystem

Is composed of a reaction center surrounded by a number of
light-harvesting complexes
Thylakoid
Photosystem
Photon
Light-harvesting
complexes
Thylakoid membrane

STROMA
Primary election
acceptor
e–
Transfer
of energy
Figure 10.12
Reaction
center
Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
WHERE THOSE ELECTRONS COME
FROM
Water
 Electrons from the splitting of water (photolysis)
supply the chlorophyll molecules with the
electrons they need
 The left over oxygen is given off as gas

The Splitting of Water
• Chloroplasts split water into
– Hydrogen and oxygen, incorporating the
electrons of hydrogen into sugar molecules
Reactants:
Products:
Figure 10.4
12 H2O
6 CO2
C6H12O6
6
H2O
6
O2
HIGH QUALITY H2O

Photolysis – Splitting of water with light energy

Hydrogen ions (H+) from water are used to power
ATP formation with the electrons

Hydrogen ions (charged particle) actually move from
one side of the thylakoid membrane to the other

Chemiosmosis – Coupling the movement of Hydrogen
Ions to ATP production
VOCABULARY REVIEW

Light-Dependent Reaction



Converts light into chemical energy (ATP & NADPH
are the chemical products).
Oxygen is a by-product
Chlorophyll
Green pigment in plants
 Traps sun’s energy
 Sunlight energizes electron in chlorophyll


Electron Transport Chain

Series of Proteins embedded in a membrane that
transports electrons to an electron carrier
VOCABULARY REVIEW

ATP (Adenosine Triphosphate)


Stores energy in high energy bonds between
phosphates
NADPH



Made from NADP+; electrons and hydrogen ions
Made during light reaction
Stores high energy electrons for use during lightIndependent reaction (Calvin Cycle)
SO WHERE DOES THE SUGARS COME
FROM?
•
The light reactions convert light energy to the
chemical energy of ATP and NADPH.
•
But recall that photosynthesis also produces
sugar.
•
So far no sugar has been produced.
•
That is the job of the Calvin cycle, which uses the
ATP and NADPH produced by the light reactions.
PART II

LIGHT INDEPENDENT REACTION






Also called the Calvin Cycle or “Dark Reaction”
No Light Required (as long as ATP and NADH are
available)
Takes place in the stroma of the chloroplast
Takes carbon dioxide & converts into sugar
It is a cycle because it ends with a chemical used in
the first step
Vital to organisms with chloroplasts, the calvin cycle
makes it possible to produce energy in the form of
glucose.

The Calvin cycle
Light
H2 O
Carbon
Input Dioxide
CO2
3 (Entering one
CO2 at a time)
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTION
ATP
Phase 1: Carbon fixation
NADPH
O2
6 carbon
molecules
Rubisco
[CH2O] (sugar)
3 P
3 P
P
Short-lived
intermediate
P
Ribulose bisphosphate
(RuBP)
P
6
3-Phosphoglycerate
6
ATP
6 ADP
CALVIN
CYCLE
3 ADP
3
ATP
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 P
P
1,3-Bisphoglycerate
6 NADPH
6 NADPH+
6 P
P
5
(G3P)
6
P
Glyceraldehyde-3-phosphate
(G3P)
6 carbon
Sugars
P
1
Figure 10.18
G3P
(a sugar)
Output
Glucose and
other organic
compounds
Phase 2:
Reduction
Chloroplast – Where the Magic
Happens!
+
H2 O
CO2
Energy
Which splits
water
ATP and
NADPH2
Light is Adsorbed
By
Chlorophyll
ADP
NADP
Chloroplast
O2
Light Reaction
Calvin Cycle
Used Energy and is
recycled.
+
C6H12O6
Dark Reaction
6 CO2 + 12 H2O + Light Energy→ C6H12O6 + 6 O2 + 6 H2O