Transcript Photosynthesis - Leavell Science Home

Photosynthesis
Warm-Up
 Why do leaves
change colors in
the fall?
 What does this
have to do with
photosynthesis?
What is Photosynthesis?
 Photosynthesis converts light energy into
chemical energy through complex series
reactions known as biochemical pathways
 Autotrophs use photosynthesis to make organic
compounds from carbon dioxide and water
 6CO2 + 6H20 = 6O2 + C6H12O6
Where does Photosynthesis occur?
In plants and algae, photosynthesis
occurs inside the chloroplasts
Chloroplasts
Roy G. Biv
 White light from the sun is
composed of an array of colors
called the visible spectrum
 Different colors in the visible
spectrum have different
wavelengths
 Pigments absorb certain colors
of light and reflect or transmit
the other colors
Stages of Photosynthesis
 STAGE 1 - LIGHT DEPENDENT REACTIONS
 Energy is Capture from Sunlight. Water is Split into
Hydrogen Ions, Electrons, and Oxygen (O2). The O2
Diffuses out of the Chloroplasts (Byproduct).
 The Light Energy is Converted to Chemical Energy,
which is Temporarily Stored in ATP and NADPH.
 STAGE 2 - CALVIN CYCLE
 The Chemical Energy Stored in ATP and NADPH powers
the formation of Organic Compounds (Sugars), Using
Carbon Dioxide, CO2.
Stages of Photosynthesis
Stages of Photosynthesis
Photosystems I & II
 Photosystem II - photons split water molecules
(producing O2), electrons produced from the split
travel down electron transport chain. Energy
provided in the transport chain is used to make ATP
from ADP
 Photosystem I - photons boost electrons to a higher
energy state, electrons travel down electron
transport chain, energy is used to make NADPH from
NADP.
 The ATP and the NADPH are used for the next main
step, the Calvin cycle,or "Light independent
reactions" - also called the Dark Reaction
Photosystems
Light Reactions
 The light reactions of photosynthesis
begin with the absorption of light by
chlorophyll a and accessory pigments in
the thylakoids
 Accessory pigments absorb colors of
light that aren’t absorbed by chlorophyll
a, and they transfer some of the energy
in this light to chlorophyll a
Electron Transport
 Excited electrons that leave chlorophyll a travel along
two electron transport chains, resulting in the
production of NADPH
 The electrons are replaced when water is split into
electrons, protons, and oxygen in the thylakoid
 Oxygen is released as a byproduct of photosynthesis
Electron Transport continued…
 As electrons travel along the electron
transport chains, a concentration gradient
of protons builds up across the thylakoid
membrane
 The movement of protons down this
gradient results in the synthesis of ATP
through chemiosmosis
Calvin Cycle
 The ATP and NADPH produced in the
light reactions drive the second part of
photosynthesis, the Calvin cycle
 In the Calvin cycle, CO2 is incorporated
into organic compounds, a process
referred to as carbon fixation
Calvin Cycle continued…
 The Calvin cycle produces a compound
called PGAL
 Three turns of the Calvin cycle are
needed to produce one PGAL molecule
Calvin Cycle continued…
 Most PGAL molecules are converted into
another molecule that keeps the Calvin
cycle operating
 However, some PGAL molecules are
used to make other organic compounds,
including amino acids, lipids, and
carbohydrates
 During this phase, carbon dioxide is "fixed" or
attached to a 5-carbon sugar called RUBP,
producing a 6-carbon sugar similar to the
sugar glucose. Each (unstable) 6 carbon
molecules splits into 2 (3 Carbon compounds)
known as phosphoglyceric acid (PGA)
 ATP and NADPH used to convert PGA to PGAL
 PGAL can be removed from the cycle and used
for the formation of larger carbohydrates such
as glucose, sucrose and starch.
Photosynthesis Reaction Summary
 In the overall equation for photosynthesis,
CO2 and water are the reactants, and
carbohydrate and O2 are the products
6CO2 + 6H20 = 6O2 + C6H12O6
Alternate Pathways
 Most plants fall into the C3 category
 Some plants living in hot, dry climates
supplement the Calvin cycle with the C4
or CAM pathways
 These plants carry out carbon fixation
and the Calvin cycle either in different
cells or at different times
Alternate Pathways
 CAM
 Cactus, pineapples have different adaptations
to Hot, Dry Climates. They Fix Carbon through
a pathway called CAM. Plants that use the CAM
Pathway Open their Stomata at NIGHT and
Close during the DAY, the opposite of what
other plants do. At NIGHT, CAM Plants take in
CO2 and fix into Organic Compounds. During
the DAY, CO2 is released from these
Compounds and enters the Calvin Cycle.
Because CAM Plants have their Stomata open
at night, they grow very Slowly, But they lose
LESS Water than C3 or C4 Plants.
Alternate Pathways
 C4
 Allows certain plants to fix CO2 into
FOUR-Carbon Compounds. During the
hottest part of the day, C4 plants have
their Stomata Partially Closed. C4 plants
include corn, sugar cane and crabgrass.
Such plants lose only about half as much
water as C3 plants when producing the
same amount of Carbohydrate.
Rate of Photosynthesis
 The rate of photosynthesis increases and then
reaches a plateau as light intensity or CO2
concentration increases
 Below a certain temperature, the rate of
photosynthesis increases as temperature increases
 Above that temperature, the rate of photosynthesis
decreases as temperature increases