Photosynthesis
Energy for Life Processes
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All organisms obtain and use energy to carry out the functions of life
o Energy comes from the sun
– (producers) make their own food
Heterotrophs – (
) obtain energy by eating autotrophs or other heterotrophs
Plants make their own food by converting
into chemical energy
o Use this energy for their own metabolic functions (respiration, growth and reproduction)
o Their stored energy becomes energy for consumers
Overview of Photosynthesis
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Who?
o Autotrophs (plants)
What?
o Photosynthesis – process by which
use light energy to produce ATP and
organic molecules (glucose) from inorganic molecules (CO2 and H2O)
Where does photosynthesis occur?
o Leaves  Leaf cell 
 Thylakoid Stroma (p. 103)
o Thylakoid – flat sac that contains chlorophyll (
that absorbs red and
blue light) where light-dependent reactions take place
 Light-Dependent reactions – where light energy is converted to chemical energy
o Stroma – fluid surrounding thylakoid where light-independent reactions take place
Why?
o To provide
(in form of organic molecules) upon which almost all life on Earth
depends
How?
o Three stages
1. Light Absorption – energy captured from sunlight
 LightReactions
2. Light energy converted to chemical energy (ATP and NADPH)
 LightReactions
3. Light-Independent Reactions – ATP and NADPH power synthesis of organic
molecules using CO2
What is in sunlight that plants can use?
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Light is radiation energy (electromagnetic spectrum)
Different wavelengths represent different amounts of energy
o Smaller the wavelength the
the energy
o Gamma rays (smallest, highest energy) all the way to radio waves (largest, lowest
energy)
o Visible spectrum – in between – contains the light plants absorb
o
has highest energy, red lowest
 Green in the middle
o Light is reflected (you see this),
(pass through, you don’t see)
(you don’t see this), or transmitted
First Stage of Photosynthesis  Light Absorption (Light-Dependent Reactions)
Capturing the sun’s energy
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Pigment – (found in thylakoid of chloroplast) molecule that absorbs light
o Like a sponge
Chlorophyll a – green pigment that is the primary light-absorbing pigment for photosynthesis
 Absorbs orange/red light
 Reflects
light
o Chlorophyll b – assists chlorophyll a
 Absorbs blue, reflects green
o Carotenoids – assists chlorophyll a
 Absorbs blue, reflects
and
o Electrons in chlorophyll now have energized electrons
 These energized electrons leave
and move on to next stage
Water – all the excitement causes water to be split into H+, O2 and electrons (e-)
o These electrons replace those lost from chlorophyll
o
is given off to atmosphere
Second Stage of Photosynthesis Conversion of Light Energy (Light Dependent Reactions)
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Follow the electrons!
Electrons leave chlorophyll and hop onto the electron transport chain
o Still in the
o Series of reactions result in the production of ATP (energy) and NADPH (electron
carrier)
 NADPH provides excited electrons for Stage 3
ATP and NADPH are released into stroma of chloroplast
Third Stage of Photosynthesis 
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of Energy (Light-Independent Reactions)
Carbon atoms from CO2 and hydrogen atoms (from water) go through Calvin Cycle to produce
organic compounds (glucose) which will be stored in plant for their own use/other use
Overall Photosynthesis Reaction
H2O +
CO2  C6H12O6 +
O2