1 Review Summarize what happens during the light
dependant reactions
2 Explain How do each of the three primary factors
affect the rate of photosynthesis
3 Review What are the different ways that plants
cope with water shortages
CH 8 PHOTOSYNTHESIS
8.3 The Process of Photosynthesis


Why do chloroplasts contain so many membranes?
Membranes are the key to capturing light energy.
Light Dependant Reactions: Generating
ATP and NADPH

Photosystems
 Series
of proteins in the thylakoid that absorb sunlight
and make high-energy electrons.
Electron Transport Chain


Series of electron carriers that shuttle high-energy
electrons during ATP-generating reactions
Electron Transport Chain (ETC).
Photosystem II


Light energy is
absorbed by electrons
High energy electrons
move through the ETC.
Photosystem II


Thylakoid provides new
electrons from water
Water breaks up into 2
electrons, 2 H+ ions,
and 1 oxygen atom.
Photosystem II



2 electrons replace the
high-energy electrons
that have been lost to
ETC
H+ ions released inside
thylakoid
Oxygen is released into
the air.
Photosystem II

Energy from electrons
is used by proteins in
ETC to pump H+ ions
from the stroma into
the thylakoid space.
Photosystem II

The ETC gives electrons
to Photosystem I.
Photosystem I


Electrons used energy to pump H+ ions so have less
energy
Photosystem I uses energy from light to reenergize
the electrons.
Photosystem I

After 2nd ETC, NADP+ molecules in the stroma pick
up high-energy electrons and H+ ions at the outer
surface of the thylakoid membrane to become
NADPH.
Hydrogen Ion Movement and ATP
Formation

H+ ions accumulate within the thylakoid space from
splitting water and pumping them across.
Hydrogen Ion Movement and ATP
Formation

Gradient of H+ ions
 Charge
and concentrations.
Hydrogen Ion Movement and ATP
Formation


H+ ions cannot directly cross membane
ATP synthase
that allows H+ ions to pass through
 Causes it to rotate and bind ADP and a phosphate.
 Protein
Hydrogen Ion Movement and ATP
Formation

Produces NADPH and ATP.
The Light-Independent Reactions:
Producing Sugars


Called the Calvin Cycle
Plants use ATP and NADPH to build stable highenergy carbohydrate compounds that can be stored
for a long time.
Carbon Dioxide Enters the Cycle



Carbon dioxide molecules enter the Calvin cycle
from the atmosphere
Combine CO2 with 5-carbon compounds
6 CO2 enter, twelve 3-carbon compounds are
produced.

Enzymes convert the 3carbon compounds
into higher-energy
forms using ATP and
NADPH.
Sugar Production

Two of the 3-carbon
molecules are removed
to make sugar.
Sugar Production


Remaining ten 3carbon molecules are
converted back into six
5-carbon molecules
Combine with the new
CO2 to start the next
cycle.
ADP
ATP
Summary of the Calvin Cycle

Uses 6 molecules of
carbon dioxide to
produce a single 6carbon sugar.
ADP
ATP

Energy for the
reactions is supplied by
compounds produced
in the light-dependent
reactions.
ADP
ATP

Sugars are used by the
plant for energy needs
and to build other
macromolecules for
growth and
development.
ADP
ATP
Factors Affecting Photosynthesis



Temperature
Light intensity
Water availability.
Temperature



Enzymes work best between 0°C and 35°C
Lower temps slow enzymes
Higher temps denature enzymes.
Light Intensity


High light intensity increases the rate of
photosynthesis
There is a maximum to light intensity increasing
photosynthesis.
Water Availability



Water is needed in the reaction
Water shortage can stop reaction
Certain plants have special adaptations
 C4
plants
 CAM plants.
C4 Photosynthesis

Very good and getting CO2

Uses a 4 carbon molecule
Requires added energy
Corn, sugar cane, and sorghum.


CAM Plants

Open their leaves at night and trap CO2
Seal leaves shut during day to conserve water and
use trapped CO2

Pineapple trees and many desert cacti.
