Photosynthesis
• Review the following terms:
– Autotrophs and heterotrophs
– The structure of chloroplasts and cell
membrane
– Electron transport chain
– The functions of ATP and NADH
– Potential and kinetic energy
– Organic and inorganic molecules
Photosynthesis in Overview
• Process by which plants and other autotrophs
store the energy of sunlight and use this energy
to make sugars and other organic molecules.
• Requires sunlight, water, and carbon dioxide.
• The main purpose of photosynthesis is to make
organic molecules (carbohydrates).
• Overall equation:
6 CO2 + 6 H20  C6H12O6 + 6 O2
• Occurs in the leaves of plants in the chloroplasts.
• Oxygen is also produced in this process. This is
the oxygen that we use to breath in for cellular
respiration.
Leaf Structure
• Most photosynthesis occurs in the mesophyll layer of the
leaf.
• Gas exchange of CO2 and O2 occurs at openings called
stomata surrounded by guard cells on the lower leaf
surface.
• Stomata are able to open and close because water is also
evaporated through them into the atmosphere from the
plant.
• When stomata are open, CO2 can freely enter the leaf but
water also leaves at the same time.
http://croptechnology.unl.edu/animation/transpiration.swf
Chloroplast Structure
• Inner membrane
called the thylakoid
membrane.
• Thickened regions
called thylakoids. A
stack of thylakoids
is called a granum.
(Plural – grana)
• Stroma is a liquid
surrounding the
thylakoids.
Pigments
• Pigment is any molecule that is able to absorb
light . Only light that is absorbed by pigments is
useful for photosynthesis.
• Chlorophyll a is the most important photosynthetic
pigment.
• Other pigments called antenna or accessory
pigments are also present in the leaf.
– Chlorophyll b
– Carotenoids (orange / red)
– Xanthophylls (yellow / brown)
• These pigments are embedded in the membranes
of the chloroplast in groups called photosystems.
Photosynthesis: The Chemical Process
• Occurs in two main phases.
– Light reactions
– The Calvin Cycle
• Light reactions are the “photo” part of photosynthesis.
Light is absorbed by pigments and the energy of light is
used to form ATP and NADPH molecules. This takes
place in the grana of the chloroplast.
• The Calvin cycle is the “synthesis” part of
photosynthesis. Trapped energy from the sun is
converted to the chemical energy of sugars by using the
ATP and NADPH that was produced in the first process.
This process takes place in the stroma of the chloroplast.
Light Reactions (aka lightdependent process)
• We are going to go over the steps of this
process together in class.
• The products of the light reaction are ATP,
NADPH and oxygen (O2).
• ATP and NADPH move to the Calvin cycle
to help to synthesize carbohydrates.
• Oxygen is released as a waste product or
used up in cellular respiration
• The amount of ATP and NADPH molecules
depends on the concentration of water in the
plant and on the intensity of sunlight. The
rate of the reaction is determined by these
environmental factors
•
•
http://vcell.ndsu.nodak.edu/animations/photosynthesis/movie.htm and
http://vcell.ndsu.nodak.edu/animations/photosystemII/movie.htm
The Calvin Cycle
• During this process, carbohydrates are formed.
This is the only process on the earth that can form
organic molecules from inorganic ones. All other
organic molecules (big 4) forms from
carbohydrates.
• This cycle requires ATP, NADPH and CO2 to take
place in the stroma of the chloroplast.
• ATP, NADPH are from the light reaction,
while CO2 has to be taken in from the
atmosphere through the stomata of the
leaves.
• We will discuss this process in steps in class
•
http://www.science.smith.edu/departments/Biology/Bio231/calvin.html
Relationship Between the Two Stages of
Photosynthesis
Factors Affecting Photosynthesis
• Shortage of water can slow down or stop
photosynthesis.
• Enzymes that are necessary for
photosynthesis function only within a
certain temperature range.
• Light intensity is also a factor that can
speed up or slow down photosynthesis.
• Carbon dioxide concentration is crutial
Alternative Pathways of
Photosynthesis
• These pathways adapt to perform
photosynthesis in dry and hot environment
• They are more efficient than the traditional
C3 pathway (what we learned earlier)
• Plants with alternative pathways have a
slightly different Calvin cycle.
• In C4 plants the location of the cycle is
different, in CAM plants the timing is
different.
C4 Pathway
• CO2 trapping (fixation) and the Calvin cycle
take place in two separate location.
• CO2 fixation is in the mesophyll cells of the
leaf, the Calvin cycle takes place in the
bundle sheath cells (around the veins of the
leaf) that are low in oxygen.
• Examples of C4 plants: corn, sugar cane
CAM pathway
• Occurs in succulent plants (cacti)
• Carbon fixation (trapping CO2) takes place
at night when the stomata are open
• Calvin cycle takes place during the day,
when stomata are closed
• This way plants do not lose much water
during hot and dry days.