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Photosynthesis:
Using Light to
make Food
Bio Honors
Ch. 7
Plants have to create their
own food to use for energy.
 PHOTOSYNTHESIS
Plants convert the
energy of sunlight into
energy stored in
chemical bonds of
carbohydrates:
 Simple sugars- in the
form of glucose
 Larger molecules of
starches- in the form
of cellulose
REQUIREMENTS FOR
PHOTOSYNTHESIS
Reactants
6 CO2 + 6 H2O
Light
Products
C6H12O6 + 6 O2
(glucose)
REQUIREMENTS FOR
PHOTOSYNTHESIS
1) Light – used as a source of energy by plants and
some other autotrophs to make food molecules from
simple organic molecules in the environment.
2) Pigments – colored substances that absorb or reflect
light
 Chlorophyll is the main pigment used in photosynthesis.
This pigment gives plants their green color, by reflecting
green light!
 Photons of light hits the chlorophyll pigments (P680 and
P700) and excites the electrons in chlorophyll.
Electromagnetic
Spectrum
REQUIREMENTS FOR
PHOTOSYNTHESIS continued…
3) Energy storing compounds –
 ATP (adenosine triphosphate)- Energy
storage molecule used by all living things,
consisting of adenine (a 5-carbon sugar)
and 3 phosphate groups.
4) Carbon dioxide (CO2) – gas from the
surrounding environment are brought into
the plant cells through STOMATA, small
pores in the leaves.
5) Water – mostly absorbed through the roots
of the plant and transported to the cells.
Structure of the Chloroplast
(site of photosynthesis)
THE LIGHT AND DARK
REACTIONS of Photosynthesis
Stage 1: Light Reactions
 Takes place in the thylakoid membranes, inside the
chloroplasts.
 Involves an electron transport chain, moving particles
across protein channels in the membrane.
Stage 1: Four basic step of LIGHT
REACTIONS
1)
Light absorption clusters of pigment molecules (Chlorophyll P680
and P700) absorb photons of light, which excite
electrons in the chemical structure of the
chlorophyll molecules with energy.
 Electrons move from Photosystem II to
Photosystem I.
2) Electron transport – high energy electrons
transported along electron carriers, and
then to NADP+ to form NADPH.
(Show e- in your foldable diagrams!)
Stage 1: Light Reactions continued…
3) oxygen production – splitting 2 water
molecules, leaving 4 H+ and O2 .
* REMEMBER: PLANTS PRODUCE THE
OXYGEN THAT WE BREATHE!
4) ATP formation- energy is transferred into an
ATP molecule. ATP is a stored energy
source that plants use to carry out everyday
functions.
 Move H+ ions through ATP synthase (same as
seen in cellular respiration!)
 Movement of H+ through the ATP synthase
moves energy used to bond a P to ADP, forming
ATP
Light reactions:
1) Quick
Animation)
2) Detailed
Animation of
Light
reactions
STAGE 2: Calvin Cycle
(Dark Reactions)
Called “dark” because sunlight does not play a role!

Takes place in the STROMA, area around the thylakoids in
the chloroplast.
Basic Steps:
1. Starts with CO2 absorb from atmosphere and ATP and
NADPH produced in the light reactions.
2. CO2 enters and attaches to RuBP with the help of rubisco
(enzyme)
3. As it moves around the cycle, RuBP breaks down into 3PGA
4. ATP and NADPH get oxidized and form G3P
5. 2 G3P’s come together to form glucose or other sugars.
6. RuBP gets recycled Animation of Calvin Cycle (Dark Reactions)
Comparing and Contrasting Photosynthesis
and Cellular Respiration
Synthesizing energy molecules:


Plants go through photosynthesis to create their own glucose.
Synthesis = Photosynthesis = Chloroplast
Breaking down molecules to release energy:

Plant obtain energy from the glucose and sugar
molecules they produce during photosynthesis.

Animals, Fungi and some Protists obtain food from
food they consume and break it down into glucose.
 Breakdown = Cellular Respiration = in mitochondria
C3 plants
Stomata video
 Uses CO2 directly from air
 Called C3 because the is first incorporated into
a 3-carbon compound.
 Stomata in the leaves are open during the
day to acquire CO2, but can lose water
through stomata as well.
 RUBISCO, the enzyme involved in
photosynthesis, is also the enzyme involved in
the uptake of CO2.
 PHOTORESPIRATION- happens when CO2
levels decrease and make plant less efficient
because it doesn’t produce sugar or ATP.
Adaptive Value: more efficient than C4 and
CAM plants under cool and moist conditions and
under normal light because requires less
machinery (fewer enzymes and no specialized
anatomy)
 Most plants are C3- soybean, oats, agricultural
plants, etc.
C4 plants



Called C4 because the CO2 is first
incorporated into a 4-carbon
compound.
When weather is hot and dry, keep
stomata closed during the day, to
prevent photorespiration.
Photosynthesis takes place in inner
cells (requires special anatomy)
Adaptive Value:
 Photosynthesizes faster
than C3 plants under high
light intensity and high
temperatures
 Better for hot, dry climates

C4 plants include several thousand
species in at least 19 plant
families. Example: fourwing
saltbush pictured here, corn,
sugarcane.
CAM plants
(Crassulacean Acid Metabolism)
 The CO2 is stored in the form of an acid before use in
photosynthesis.
Adaptive Value:
 Better water efficiency than C3 plants under dry conditions
due to opening stomata at night when transpiration rates are
lower
(no sunlight, lower temperatures,
lower wind speeds, etc.).
 When conditions are extremely arid, CAM plants can just
leave their stomata closed night and day. Oxygen given off in
photosynthesis is used for respiration and CO2 given off in
respiration is used for photosynthesis. But this does allow the
plant to survive dry spells, and it allows the plant to recover
very quickly when water is available again (unlike plants that
drop their leaves and twigs and go dormant during dry spells).
 CAM plants include many succulents such as pineapples,
cacti, agaves and also some orchids.
Photosynthesis and
Global Warming
How does
photosynthesis help
regulate global
temperatures?
ANSWER:
 Photosynthesis absorbs CO2 from the
atmosphere.
 DECREASES the amount of CO2 in the
atmosphere, which can trap radiation
and increase temperatures.
So how could we help reduce
global warming???
PLANT MORE TREES
AND OTHER PLANTS!!!
Reminder!
Quiz on Ch.
7 and
Foldable
due Monday
12/6!