Download Photosynthesis: How Do Organisms Get Energy From the Sun?

Photosynthesis: How Do
Organisms Get Energy From
the Sun?
Chapter 7
Johann Baptista van Helmont
• Christian, Chemist, Physician, Philosopher
• Recognized gas as a form of matter
• Aristotle claimed plants extract materials
from the soil
• Planted a 5 lb. willow tree in 200 lbs. of soil
• Gave only water for 5 years
• Tree weighed 169 lbs; soil lost 2 oz.
• But, he thought the weight came from the
water.
• Hydroponics
• Microscopists discover stomata in plant
leaves
Joseph Priestly
Put a candle in a bell jar →
Candle goes out
Put a mouse in a bell jar →
Mouse dies
Put a plant and a mouse in a bell jar →
Mouse lives
Photosynthesis
6 CO2 + 6 H2O → C6H12O6 + 6 O2
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O
Cellular respiration
Plants, algae and many photosynthetic
bacteria can harness the sun’s energy and
turn it into sugar.
Autotrophs
What about proteins?
Proteins contain C,H,O, and N.
Nitrogen and other trace elements are taken
in from the soil.
Fertilizer - 10:5:5
Nitrogen : Phosphorus : Potassium
Do plants always “breathe out “
oxygen?
• When plants break down glucose for
energy they do it in the same manner as
other organisms, and then they give off
CO2.
• They cannot give off oxygen if there is no
light.
Photosynthesis is a two “step” process
one set of reactions requires light
the second set of reactions does not –
light independent (“dark”) reactions
The light reactions are called
photophosphorylation because they use
light to add a phosphate group to ADP to
make ATP.
Light
It’s a particle
discrete units – photons
travels in a straight line
No, it’s a wave
wavelength – can we see it?
What color is it?
ROY G BIV
Why do plants appear green?
Pigments – have electrons that can be
more easily excited.
Chlorophylls – alpha and beta absorb most
of the photons in plants.
Carotenoids – yellow and orange pigments
that transfer energy to chlorophyll
Phycobilins – red and blue pigments in red
algae and cyanobacteria
Very few chlorophyll molecules actually
photosynthesize.
Most chlorophyll molecules along with the
carotenoids form an energy gathering
system called an antenna complex.
Energy is transferred to a photochemical
reaction center.
So, in a nutshell:
1. Chlorophyll and carotenoids absorb light.
2. The energy is transferred to the reaction
center.
3. The energy splits oxygen from water and
forms chemical bonds.
Plants have two distinct sets of reactions:
Photosystem I
Photosystem II
Photosystem I absorbs light of 700nm best
(P700)
Photosystem II absorbs light of 680 nm
(also into the blue and violet range)
(P680)
What does this have to do with
me?
Plants grown indoors need a range of light
colors to grow well.
Fluorescent bulbs have very little 700 nm
light.
Incandescent bulbs have very little 680 nm
light.
Use both for maximum growth.
The action takes place across the thylakoid
membrane.
Chlorophyll is found in the thylakoid
membrane in association with proteins.
When P700 absorbs light, electrons are excited,
move to outer orbitals, and P700 becomes a
good electron donor.
When it gives up its electron, it becomes
oxidized.
The electron can then travel one of two paths:
Cyclic photophosphorylation – electron is
given to an electron transport chain, ATP is
formed and the electron is given back to P700
Noncyclic Photophosphorylation – electron
is transferred to NADP+ → NADPH.
This provides the reducing power for the
formation of glucose.
But now P700 is left oxidized. Where do we get
the electrons to reduce it again?
From Photosystem II !
Photosystem II uses P680
The electrons from P 680 go into their own
electron transport chain where ATP is
produced by noncyclic photophosphorylation.
The final electron acceptor is P700.
But now P680 is oxidized. Where can it get
electrons?
It gets them by splitting water and releasing
oxygen.
Water H+, electrons and O2
Until cyanobacteria figured out how to do
this, there was very little oxygen in the
Earth’s atmosphere.
Now we have ATP and NADPH formed →
The light-independent reactions which make
glucose.
Glucose is a more stable energy storage
molecule than ATP.
In the stroma, the Calvin cycle takes the
hydrogens from water, the carbon and
oxygen from carbon dioxide and makes
glucose.
Calvin Cycle
1. Capturing carbon
CO2 + ribulose biphosphate → 6 carbon
compound → 2 3-carbon molecules
This is catalyzed by an enzyme called ribulose
biphosphate carboxylase or Rubisco
This is a very slow reaction, so plants produce a
lot of it.
It is probably the most abundant protein on the
planet!!
2. Making sugar
The end product of the Calvin Cycle is a three
carbon compound called glyceraldehyde
phosphate.
Some enters the cytosol of the cell,
where it can be turned into glucose, fats or
amino acids.
3. Regenerating ribulose biphosphate
Some remains in the Calvin cycle and is
used to make more ribulose biphosphate.
It takes 18 molecules of ATP and 12 molecules
of NADPH to make one molecule of glucose.
What factors affect photosynthesis?
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Wavelength of light
Intensity of the light
Length of the day
Length of the growing season
Pollution
Other taller vegetation
Availability of carbon dioxide and water
Balance between CO2 and H2O
• If the stomata are open all the time, the
plant could dehydrate.
• If the stomata are closed all the time, CO2
can’t get in and O2 can’t leave.
• Hot, dry conditions → photorespiration,
wastes about half of carbohydrate
produced.
Some plants make a 4 carbon molecule
instead of the 3 carbon phosphoglycerate.
This is oxaloacetic acid → 3-carbon
molecule → glucose production and CO2
which goes back to the Calvin Cycle.
These plants are called C4 plants.
Corn and sugar cane survive well in hot, dry
climates.
The other plants are called C3 plants.
Desert plants
Cacti use a modified C4 pathway callled
crassulacean acid metabolism or CAM.
These plants open the stomata only at
night and store CO2 in a four carbon
molecule for use the next day.
Requires a lot of energy, so growth rates are
very slow.