Chapter 8:
Energy and
Photosynthesis
Energy and
Photosynthesis
• Energy is necessary for all life
–Plants and green organisms:
• Trap light energy in the form of
sunlight and store it for later use
–All other organisms:
• Eat green plants to obtain energy
Adenosine Triphosphate (ATP)
• Energy molecule
• Composed of:
– Adenosine molecule
– Ribose sugar
– 3 phosphate groups
• ATP becomes available for a cell to use when the bonds
between the phosphate groups are broken
– Releases energy when a phosphate group is removed
– Bond breaks between the 2nd and 3rd phosphate
groups
• Results in adenosine diphosphate (ADP)
• ATP is a small molecule
– Stored energy (ex: a battery)
• Cells use energy to maintain homeostasis
Photosynthesis
• Photo-: means light
• Synthesis: to put together
• Process that uses the sun’s
energy to make simple sugars
• 2 phases
– Light-dependent reactions
– Light-independent reactions
History
• Van Helmont (1600’s)
– Planted seedling in pot of soil, watering it
regularly
– Concluded plant mass comes from the water
• Priestley (1700’s)
– Candle with glass jar cover  flame died
– Mint sprig and candle with glass jar cover 
flame burned
– Conclusion: plants release oxygen
• Ingenhous
– Supported Priestley  will occur only when
mint plant is exposed to light
Photosynthesis Requirements
• Light (sun)
• Carbon Dioxide (from the air)
– Taken in to plants through little openings in
the leaves called stomata
• Water (from the ground)
– Taken in through the roots of plants
• Chlorophyll and enzymes in the plant
• Rate of photosynthesis depends on:
– The availability of sunlight, CO2, and H2O
– The intensity (brightness) of light
– Temperature (20-35º C is optimal)
• White light  mixture of different wavelengths of light
– Visible spectrum  ROYGBIV
• Pigments: molecules that absorb specific wavelengths
of sunlight
– Ex: Chlorophyll (in chloroplasts): a and b
• Absorbs well in blue and red light
• Absorbs most wavelengths except green (reflects
green light)
• Makes plants appear green
– Other plant pigments:
• Xanthophyll: yellow pigment
• Carotene: orange pigment
• Equation: 6CO2 + 6H2O + light  C6H12O6 + 6O2
Carbon Dioxide + Water  Sugar + Oxygen gas
• Chloroplast: cell organelle where
photosynthesis occurs
– Light-dependent reactions take place in the
thylakoid membrane (contains pigments)
• Picture of a chloroplast:
– Grana: stack of thylakoids
– Stroma: outside of the thylakoids
– Photosystem: clusters of pigment and protein
that absorb light energy (found in thylakoids)
Review:
1. Match the scientist with the appropriate
experiment or conclusion:
Priestley
van Helmont
Ingenhousz
_____ Plants need sunlight to produce oxygen
_____ Plants gain most of their mass by taking in water
_____ Using a candle and a jar, he observed that plants
produce a substance that kept the candle burning
_____ He measured the mass of the soil in which a plant grew
_____ He observed plants exposed to light
Light-Dependent
Reactions
• 1st phase
• Requires light
• Converts light energy into
chemical energy
• ATP produced fuels lightindependent reactions
• NADP+: electron carrier
– Holds 2 high-energy
electrons and H+
– Converts NADP+ to NADPH
• Light reactions use trapped light energy to
convert ADP to ATP and NADPH
– Occurs in the grana of chloroplasts
– Splits water (photolysis)
• H: needed to make glucose
• O: released as a waste product into the
atmosphere
Light
H2O in
Light
reactions
(in grana)
O2 as waste
CO2 in
ATP
H+
Dark
reactions
(in stroma)
C6H12O6
(glucose)
H2O as waste
Light-Independent Reactions
• Does not require light
• Called dark reactions or the Calvin cycle
• Process of actually making glucose (uses
CO2 to make simple sugars/starches)
– Uses energy from ATP and NADPH
• Can occur in the light or the dark
• Requires enzymes
• Occurs in the stroma of chloroplasts
• Any glucose not used immediately can be stored as
starch or sugars
• Chemosynthesis
– Process of making food from inorganic
compounds
– Does not use sunlight
• Uses Sulfur in deep sea vents
• Methanogens use Carbon Dioxide and Hydrogen
Autotrophs
• Use photosynthesis
or chemosynthesis
– Make their own
food
Bell Ringer:
1. The light-dependent reactions take place
within the __________ membranes.
2. The light-independent reactions are also
known as the ________.
3. In the light-dependent reactions, the gas ____
is produced.
4. High-energy sugars are produced during the
_______ reactions.
5. The light-independent reactions take place in
the ________.