Energy needs of life
• All life needs a constant input of energy
– Heterotrophs (Consume)
• get their energy from “eating others”
– eat food = other organisms = organic molecules
• make energy through respiration
– Autotrophs (Producers)
•
•
•
•
produce their own energy (from “self”)
Photo-autotrophs convert energy of sunlight
build organic molecules (CHO) from CO2
make energy & synthesize sugars through
photosynthesis
PHOTOSYNTHESIS
The process that occurs in the chloroplasts of
plant cells in which the sun’s energy is used to
make simple sugars (food energy).
These sugars are then converted to complex
carbohydrates, starches, and stored.
• Obtaining raw materials
– sunlight
• leaves = solar collectors
– CO2
• stomata = gas exchange
– H2O
• uptake from roots
– nutrients
• N, P, K, S, Mg, Fe…
• uptake from roots
The chloroplasts, where photosynthesis occurs,
are found within the mesophyll cells in leaves.
Two kinds of mesophyll
cells in a typical leaf:
Palisade- close
together, most
photosynthesis occurs
Spongy- here the cells
aren't so close.
LEAVES
• Carbon Dioxide moves in and Oxygen and water
move out of a leaf through the stomata, which are
located on the top and bottom of the leaf.
LEAVES
• Guard cells are tiny cells that surround and
control whether the stomata are open or shut.
• The loss of water through the stomata is called
transpiration.
Photosynthesis takes place inside the
chloroplasts in the leaf of a plant.
There can be from 1 to 50 or more chloroplasts in a
single mesophyll cell. The number varies with the plant
species, age, and health of the cell.
The pigments (most commonly, chlorophyll) are in
the membrane of the thylakoid discs.
These are what absorb the sunlight.
Photons (light particles) from sunlight hit the pigments, electrons
are "knocked" loose, and off they go to energize the complicated
process of photosynthesis.
Pigments are organized in the
Thylakoid membrane in clusters
known as Photosystems
(Reaction centers)
• Photosynthesis gets energy by absorbing wavelengths of light
– chlorophyll a
• absorbs best in red & violet-blue wavelengths & reflects green
– accessory pigments with different structures absorb light of
different wavelengths
• chlorophyll b (absorbs blue and orange),
• Carotenoids (absorbs blue-violet and blue green)
• Phycobilins (absorbs blue)
Why are
plants green?
A Look at Light
• The spectrum of color
V
I
B
G
Y
O
R
Photosystems of photosynthesis
• 2 photosystems in thylakoid membrane
– collections of chlorophyll molecules
– act as light-gathering molecules
– Photosystem II
reaction
• chlorophyll a
• P680 = absorbs 680nm
wavelength red light
center
– Photosystem I
• chlorophyll b
• P700 = absorbs 700nm
wavelength red light
antenna
pigments
chlorophyll a
ETC of Photosynthesis
Photosystem II
chlorophyll b
Photosystem I
PHOTOSYNTHESIS
LIGHT-DEPENDENT PHASE
(Requires Sunlight)
LIGHT-INDEPENDENT PHASE
(Does Not Require Sunlight)
LIGHT-DEPENDENT
REACTIONS
ATP Production- FIRST PHASE
The Light-DEPENDENT reactions convert water and
energy from sunlight into chemical energy
(ATP and NADPH)- which is used later (in light independent)to make
food energy….
Light Dependent Reaction
Reactants
Products
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Light energy is transferred into the
electrons within the atoms of the pigment
molecules, and the electrons become
highly energized.
• PS II (with chlorophyll a) absorbs light
– excited electrons pass from chlorophyll to
“primary electron acceptor”
Light Dependent Reaction
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Once the energized electrons hit the primary
electron accepter, they then go through an
electron transport chain.
Passing from one protein to the next,
losing a little bit of energy along the way.
(still pretty energized at the end)
The electron transport chain is a series of
proteins embedded in the thylakoid membrane.
A little energy is lost at
each protein.
The energy that is lost goes and bonds a phosphate to ADP
(Adenosine Di-phosphate), creating ATP.
This process is called Photophosphorylation
Now, Photosystem I comes into play. The electron transport chain terminates
with PS I (P700). Here the electrons again become energized by sunlight,
passing to yet another electron acceptor. (different from PS II)
From the second electron transport chain in the
thylakoid memebrane, the electrons leave and
go to the stroma, the fluid in the chloroplasts
that surrounds the stacks of thylakoid (grana.
STROMA
(Fluid interior)
Electrons leave the electron transport chain
and enter the stroma by attaching to an
electron carrier molecule called NADP.
It is important to replace the lost electrons from
the chlorophyll molecules because then the
chlorophyll wouldn’t be able to absorb more
light energy.
• To replace the lost electrons, molecules
of water are split. This reaction is called
photolysis.
ATP
18 ATP +
12 NADPH
The second phase of photosynthesis is called the
Calvin Cycle (Light-Independent phase)
LIGHT-DEPENDENT PHASE
(Requires Sunlight)
LIGHT-INDEPENDENT PHASE
(Does Not Require Sunlight)
REMEMBER: This is when the chemical energy produced in the
first phase is combined with Carbon Dioxide to create sugar.