Photosynthesis Basics

What organisms
are able to
photosynthesize?

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
Plants
Bacteria
Some protists
Photosynthesis Basics

Photosynthesis uses CO2, water, and
light to produce glucose and O2.
Photosynthesis Basics

Where do the reactants come from?



CO2
Water
Light Energy
Photosynthesis Basics

Where do the reactants come from?

CO2



Enters through pores in the leaf’s surface
called stomata.
Water
Light Energy
Photosynthesis Basics

Where do the reactants come from?


CO2
Water


Acquired by the roots and transported to
leaves
Light Energy
Photosynthesis Basics

Where do the reactants come from?

CO2

Water

Light Energy

From the sun!
Photosynthesis Basics

In plants, photosynthesis occurs in the
chloroplasts.



Thylakoid membrane
Grana
Stroma
Overview

Photosynthesis occurs in a twopathway process.

Light Dependent Reactions

Light Independent Reactions
Overview
1.
Light-dependent reactions



Chlorophyll and other molecules of the
thylakoids capture sunlight energy
Sunlight energy is converted to the energy
carrier molecules ATP and NADPH
Oxygen gas is released as a by-product
Overview
2.
Light-independent reactions
 Enzymes in the stroma synthesize glucose
and other organic molecules from CO2
using the chemical energy stored in ATP
and NADPH
Steps of the Light Dependent Rxns
Review Questions




What is a photosystem?
What is a pigment?
What is a pump?
What is a proton?
1. Light (average 680 nm
wavelength) is absorbed
by Photosystem II and
energy is passed between
pigment molecules
2.
At the reaction
center, when
energy arrives,
two electrons are
boosted out of
two chlorophyll
molecules…
3.
First electron carrier accepts the two
energized electrons. Electrons then passed
through an ETC.
4. Energy released from ETC is used to pump H+ into
thylakoid lumen from the stroma. The resultant H+ ion
concentration gradient used to drive ATP synthesis
(chemiosmosis).
5.
Light energy (average 700 nm wavelength)
absorbed by Photosystem I is passed to the
reaction center chlorophyll
6.
Two high energy electrons boosted and ejected
from reaction center. The electrons lost by PS I are
replaced using electrons from PS II.
7. Electrons passed down electron transport chain for
PS I…
8.
The two electrons, NADP+, and H+ ion are used to
form 1 NADPH molecule
9. The H+ ion is obtained from the splitting of H2O
into 2 H+ and ½O2.
•
•
O2 released as a byproduct
Electrons given to PS II ( no net electrons lost)
Summary: Light Dependent Rxns

Light is captured by pigments in the thylakoid
membrane in chloroplasts

Photosystem II Generates ATP
Photosystem I Generates NADPH
Splitting Water Maintains the Flow of
Electrons Through the Photosystems
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Animations


McGraw Hill
Another McGraw Hill
Assessment Statements

3.8.5 State that light energy is used to produce
ATP, and to split water molecules (photolysis)
to form oxygen and hydrogen.

8.2.3 Explain the light-dependent reactions.

8.2.4 Explain photophosphorylation in terms of
chemiosmosis.
Light-Independent Reactions



NADPH and ATP from light-dependent
reactions used to power glucose synthesis
Light not directly necessary for lightindependent reactions if ATP & NADPH
available
Light-independent reactions called the CalvinBenson Cycle or C3 Cycle
1.
Carbon fixation - CO2 is covalently linked
to a carbon skeleton (RuBP)

CO2 enters plants from air, goes into stroma

CO2 attaches to ribulose bisphosphate (RuBP),
product of which is immediately split into two
molecules of phosphoglycerate (PGA).

Reaction is catalysized by an enzyme called
rubisco.
2. Reduction - carbohydrate is formed at the
expense of ATP and NADPH

the PGA is energized and reduced by ATP and
NADPH from the light reactions to make
glyceraldehyde-3-phosphate (G3P).

G3P siphoned off from this part of the Calvin
cycle represents the carbohydrate product of
photosynthesis.

G3P is a three-carbon sugar-phosphate that can
be used to make a range of carbohydrates by
other pathways.
3. Regeneration - the CO2 acceptor RuBP
reforms at the expense of ATP

The remaining G3P is converted into more
ribulose-1,5-bisphosphate (RuBP) so that the
Calvin cycle can continue to go around again.

Requires an ATP and more than one G3P to
give the total of five carbons found in RuBP.
Animations

McGraw Hill
3.8.6 State that ATP and hydrogen
(derived from the photolysis of
water) are used to fix carbon
dioxide to make organic molecules.
 8.2.5 Explain the light-independent
reactions.
 8.2.6 Explain the relationship
between the structure of the
chloroplast and its function.

Limiting factors in photosynthesis
A limiting factor is something that
controls the rate of a process
Factors that limit photosynthesis:

Amount of light

As light intensity increases, the rate of the light-dependent
reaction, and therefore photosynthesis generally, increases
proportionately.
Factors that limit photosynthesis:

Wavelength of light


PSI absorbs energy most efficiently at 700 nm and
PSII at 680 nm.
Light with a high proportion of energy concentrated
in these wavelengths will produce a high rate of
photosynthesis.
Factors that limit photosynthesis:

Amount of water


Plants shut stomata
to avoid loosing
water
However, shutting
the stomata will
also deprive the
plant of CO²
Factors that limit photosynthesis:

Amount of CO2

An increase in the carbon
dioxide concentration
increases the rate at
which carbon is
incorporated into
carbohydrate in the lightindependent reaction and
so the rate of
photosynthesis generally
increases until limited by
another factor.
Factors that limit photosynthesis:

Temperature




At low temperatures the enzymes
responsible for photosynthesis
have very little energy so the rate
of photosynthesis is very slow.
As the temperature increases, the
enzymes get more energy so the
rate of photosynthesis increases.
If it gets too hot the enzymes
begin to lose their shape
(denature). They are unable to
function properly and the rate of
photosynthesis decreases again.
At higher temperatures the
stomata close to prevent water
loss, this also stops gas exchange
which slows photosynthesis even
further.



3.8.7 Explain that the rate of photosynthesis
can be measured directly by the production of
oxygen or the uptake of carbon dioxide, or
indirectly by an increase in biomass.
3.8.8 Outline the effects of temperature, light
intensity and carbon dioxide concentration on
the rate of photosynthesis.
8.2.8 Explain the concept of limiting factors in
photosynthesis, with reference to light
intensity, temperature and concentration of
carbon dioxide.