Photosynthesis
Ch. 7
Photosynthetic Organisms

Photosynthesis coverts solar energy into the
chemical energy of a carbohydrate.

Photosynthetic organisms are called autotrophs
because they produce their own food.

With a few exceptions, it is possible to trace any
food chain back to plants and algae.

Photosynthesis takes place in the green portions
of plants.

The raw materials for photosynthesis are water
and carbon dioxide. The roots absorb water,
while carbon dioxide enters a leaf through
stromata.

Then CO2 and H2O diffuse into chloroplasts, which
carries out photosynthesis.

Carbon Dioxide and Water then diffuse into chloroplasts,
which are the organelles that carry out photosynthesis.

A double membrane surrounds a chloroplast, and its
semifluid interior called the stroma.

A different membrane system within the stroma forms
flattened sacs called thylakoids.

The thylakoid membrane contains chlorophyll and other
pigments capable of absorbing solar energy.
The Photosynthesis process

The overall process of photosynthesis can be
represented by the following equation:

In the reaction, CH2O represents carbohydrates.

Photosynthesis is a redox reaction
 Lose
of electrons is oxidation and gain of electrons is
reduction.

It takes a lot of energy to reduce CO2. Solar
energy is not used directly to provide the energy,
but instead is used to generate the ATP needed.
Two Sets of Reactions

Light Reactions
 These
reaction only occur when solar energy is present.
 The
solar energy energizes electrons to move down the
electron transport chain.
 As
the electrons move, energy is released and captured
to produce ATP molecules.
Solar Energy  Chemical Energy (ATP, NADPH)
Two Sets of Reactions

Calvin Cycle Reactions
 Named
after Melvin Calvin.
 He
discovered the enzymatic reactions that reduce
carbon dioxide to a carbohydrate in the stroma of
chloroplasts.
 These
reactions can occur during both day and night.
 During
the Calvin Cycle, CO2 is taken up and then
reduced to a carbohydrate that can later be converted
to glucose.
Chemical Energy (ATP, NADPH)  Chemical Energy
(carbohydrate)
Pigments and Photosystems

Pigment molecules absorb wavelengths of light. Most pigments
absorb only some wavelengths and reflect or transmit the
others.

The pigments in chloroplasts are capable of absorbing portions
of visible light called their absorption spectrum.
Pigments and Photosystems

Photosynthetic organisms differ in the type of chlorophyll they
contain.

Chlorophyll a and chlorophyll b play a prominent role in
photosynthesis.


Chlorophyll a and b absorb violet, blue, and red light better than others.

Green light is reflected by chlorophyll giving plants their green color.
Carotenoids play an accessory role.


Carotenoids, which are shades of yellow and orange, absorb the violetblue-green range. These pigments become noticeable in fall when
chlorophyll breaks down.
A photosystem consists of a pigment complex and electron acceptor
molecules within the thylakoid membrane. It’s the “antenna” for
gathering solar energy.
Electron Flow in Light Reactions

The light reactions utilize 2 photosystems, called
photosystem I (PS I) and photosystem II (PSII).

During light reactions, electrons usually follow a
noncyclic pathway that begins with PS II.
Electron Flow in Light Reactions
1.
PS II, which consists of pigment complexes and electronacceptor molecules, receives electrons from water as
water splits, releasing oxygen.
2.
The electron transport chain (ETC), consisting of
plastoquione and cytochrome complexes, carries electrons
from PS II to PS I via redox reactions.
3.
PS I, which also consists of a pigment complex and
electron acceptor molecules, is adjacent to NADP
reductase.
4.
The ATP synthase complex which has a channel and a
protruding ATP synthase, is an enzyme that joins ADP + P.
Plants are Carbon Dioxide Fixers

Step 1: Fixation of Carbon Dioxide
 Carbon
dioxide fixation is the first step in the calvin
cycle. During this step, CO2 is attached to RuBP
(ribulose-1,5-bisphosphate) to form a 6 carbon
molecule. The resulting 6 carbon molecule is then split
into two 3 carbon molecules.
 RuBP
carboxylase is the enzyme that speeds this
reaction and makes up 20-50% of the protein content in
chloroplasts.
Plants as Carbon Dioxide Fixers

Step 2: Reduction of Carbon Dioxide

3PG (3-phosphoglycerate is reduced to G3P (glyceraldehyde-3phosphate) in 2 steps:
Plants of Carbon Dioxide Fixers

Step 3: Regeneration of RuBP

For every 3 turns of the Calvin cycle, 5 molecules of G3P are used
to reform 3 molecules of RuBP.