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Photosynthesis!
Plant questions?
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How do plants obtain energy?
Why do plants need light?
Why do plants need water?
Why are plants green?
http://fig.cox.miami.edu/~cmallery/150/phts/phts.htm
• The best known form of
photosynthesis is the one carried
out by higher plants and algae,
as well as by cyanobacteria and
their relatives, which are
responsible for a major part of
photosynthesis in oceans.
The very basics:
• Energy from light is used to convert CO2, H2O
into sugar
• O2 is a byproduct or “leftover” and is released
Photosynthesis:
• In: 6CO2+6H2O+Energy
• Out: C6H12O6+6O2
Anatomy of a leaf (photosynthetic organ)
Leaf functions:
• “Trap” sunlight
• Perform photosynthesis
• Move food to storage
areas in the plant
• Absorb CO2 from the air
• Minimize water loss
from evaporation
Leaf structure is related to
function…
Why are leaves green?
Because….
• Their cells are full of chloroplasts…which contain
chlorophyll…a pigment that reflects green
light…which we see….so leaves are green
Summary of chloroplast structure
• Enclosed in a double membrane
• Contain Thylakoid disks called grana
(pigments embedded in Thylakoid
membrane)
• The liquid part of chloroplasts is the
Stroma
– Stroma contains
• Electron Transport System
• “Loose” enzymes
Photosynthesis takes place in 2 stages –
each made up of a set of reactions:
• Light Dependent Reactions
– Convert light energy to chemical energy in the
form of ATP & NADPH
• Light Independent Reactions (CalvinBenson Cycle)
– Convert chemical energy from ATP & NADPH
to long term storage form: glucose/starch
Where does the light come from?
• What are photons?
– Light particles
– No mass
– Travel as a wave
• Photons & Energy
– The smaller the
wavelength, the
higher the energy per
photon
Sunlight is a mixture of the colors
of the rainbow
Photosynthetic Pigments: Light energy captors
• Photosynthetic plant
pigments
• Embedded in
membranes of
thylakoid disks
• Consist of different
varieties of
chlorophylls and other
accessory pigments
such as betacarotene
Pigments
• Pigments absorb light energy and convert it
to chemical energy.
• Chlorophyll: GREEN (most important in
photosynthesis)
• Carotenoids: YELLOW, ORANGE & RED.
• Photosynthesis only takes place when
chlorophyll is present.
Photosynthetic pigments absorb
the energy in specific colors of light
What colors are
absorbed?
What colors
reflected?
Different pigments
absorb different
colors of light
This allows plants to
use most of the
available light
1. Light Reaction
• needs light energy
• takes place in the
thylakoids
• needs water (H2O),
which is divided into
electrons and oxygen
(which is liberated)
• uses 2 photosystems to
capture the sun’s energy
• produces ATP which is
used in the dark rx
(Calvin-Benson cycle)
The Light Reaction
Two Steps:
1. Trapping Energy from the Sun.
2. Splitting of Water
Animation of the Light Reaction:
http://www.biology4all.com/resources_library/sour
ce/61a.swf
LIGHT REACTION PRODUCTS
• Energy
– ATP
– NADPH
• Oxygen (which is released into the
atmosphere)
2.The Dark Reaction (Calvin cycle)
- uses CO2 to make glucose
(“fixation” of carbon dioxide)
- takes place in the stroma of the
chloroplast
- also known as C3 bc of the 3 carbon
molecule (PGA) that is formed
- there are 2 other methods that plants
may use to make glucose… C4 and
CAM
Calvin Cycle : the synthesis
The rate of photosynthesis
- which factors affect the rate of “fixed” glucose?
1) The rate of photosynthesis
can be limited by physical
factors such as temperature.
The lower the temperature,
the slower the photosynthesis.
2) The availability of light is a factor
that limits the rate of photosynthesis.
When the amount of light decreases,
the rate of photosynthesis also
decreases
The C4 & CAM
plants
The Calvin cycle and the “C4 Plants”
- in certain climates, the sun is too abundant, and it never
limits photosynthesis
- however, these climates are also very try and hot
- here, it’s the CO2 that limits photosynthesis
- we can think of it as the availability and the loss of water
(H2O)
- when the plant photosynthesises in the sun, the CO2 must
enter the leaves through the stomatas (little holes under the
leaves)
- but, when these holes are open, H2O is lost and the plant
dehydrates
- if you close the stomatas, CO2 cannot enter so this limits
photosynthesis
- in C4 plants, the stomatas are only partially opened
during the day
- these plants have an enzyme that can transform the CO2
into a 4-carbon molecule
- these molecules are stored in the different cells and the
CO2 can be released when the plant lacks any
Examples of C4 plants are corn, sugar canes
Crassulacean Acid Metabolism (CAM)
- the ultimate prevention of CO2 loss is found in desert plants
like the cactus
- in these plants, the stomatas are only opened at night
- the plants fixes the CO2 into 4-carbon molecules during the
night and transfer the carbon to the Calvin cycle during the
day
- in the day, the
stomatas are
completely
closed and
there is no
water loss
Comparison between photosynthesis and aerobic resp.
Global rx of photosynthesis:
6CO2 + 6H2O + light energy → C6H12O6 + 6O2
Global rx of aerobic cellular resp:
C6H12O6 + 6O2 + 38 ADP + 38 P → 6CO2 + 6H2O + 38 ATP
- in a way, photosynthesis is the opposite of aerobic resp.
- the products of photosynthesis are the raw materials
(reactants) of resp.
Where?
PHOTOSYNTHESIS
In cholorophyll-bearing cells
RESPIRATION
In all cells
When?
In the presence of light
All the time
Input?
Carbon dioxide and water
Reduced carbon
compounds and
oxygen
Output?
Reduced carbon compounds,
oxygen, and water
Energy sources? Light
Carbon dioxide and water
Energy result?
Energy stored
Energy released
Reaction?
Reduction of carbon
compounds
Oxidation of carbon
compounds
Energy carrier(s) NADP
Chemical bonds
NAD and FAD