Photosynthesis Using light energy to build sugars.
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Transcript of Photosynthesis Using light energy to build sugars.
Photosynthesis
Using light energy to build sugars
Leaf structures for photosynthesis
Chloroplast
• Thylakoid membrane contains photopigments.
• Thylakoid arranged in stacked sacs called grana.
• Surrounding semi-fluid region is called stroma.
• Light reactions take place in grana, Calvin cycle in stroma.
Photosynthesis
includes
of
take place intakes place in uses
to produce to produce
use
Light-dependentreactions
Calvin cycle
Thylakoidmembranes Stroma NADPHATPEnergy from
sunlight
ATP NADPH O2 Chloroplasts High-energysugars
Section 8-3
Concept Map
Go to Section:
Role of Photopigments
• Capture light energy and release electrons
• Each photopigment absorbs only part of the spectrum (range of wavelengths)
• Unabsorbed wavelengths are reflected
• Chlorophylls a and b (reflect green) are most important
Role of Photopigments
• Chlorophyll b and other photopigments capture light energy and transfer it to chlorophyll a using electrons.
• Chlorophyll a also captures more light energy and releases more electrons.
• Replace electrons lost from chlorophyll b by taking them from water.
Absorption Spectra of Chlorophylls
Chlorophyll b
Chlorophyll a
How does water fit in?
Energy from light also used to split water:
– O2 is released as a byproduct.
– Electrons replace those lost from chlorophyll b.
– H+ ions are used to power construction of ATP from ADP and Phosphate.
Role of energy carrier molecules
• ATP and NADPH receive energy in light dependent reactions; can only hold the energy briefly.
• Carry the energy to the Calvin cycle to power carbon fixation.
• Build these by using H+ ions released from water and electrons from chlorophyll a.
PHOTOSYNTHESIS
6CO2 + 6H2O C6H12O6 + 6O2
Chloroplast
Sugars + O2CO2 + H2O
H2O
O2
Sugars
CO2
Light-Dependent Reactions
CalvinCycle
NADPH
ATP
ADP + PNADP+Chloroplast
The light dependent reactions: Energy from sunlight is used to:
– Excite chlorophyll to release electrons
– Split H2O:• Electrons go to chlorophyll• O2 is released as a byproduct.• H+ ions are used to build ATP using
chemiosmosis.
– Electrons and H+ ions are used to build ATP and NADPH using the electron transport chain.
The light reactions at work!
The light-dependent reactions
HydrogenIon Movement
Photosystem II
InnerThylakoidSpace
ThylakoidMembrane
Stroma
ATP synthase
Electron Transport Chain Photosystem I ATP Formation
Chloroplast
The light reactions and chemiosmosis
The Calvin Cycle
• Reactions that actually produce high energy molecules, like glucose.
• Use ATP and NADPH from the light reactions to power “fixation” of carbon from CO2 into an organic molecule.
• Also returns low energy ADP plus inorganic phosphate ions and NADP+ to be used by the light reactions.
The Calvin Cycle
ChloropIast
CO2 Enters the Cycle
Energy Input
5-CarbonMoleculesRegenerated
6-Carbon SugarProduced
The Calvin cycle
Photosynthesis summary
So what do we need for photosynthesis?
• Think about the equation: what reactants are required?
• How about light? – Quantity (intensity)? – Quality (color)?
• What else???
• Environmental conditions can affect the rate of photosynthesis.
Environmental conditions: temperature
• When it gets too hot, plants lose too much water from their leaves.– What does this cause?
• Solution: prevent water loss by closing the stomata.
• Resulting problem: no CO2 enters the leaf, and O2 can accumulate.
Stomates
Spider plant Tobacco leaf
Special cases: CAM metabolism
In very high heat, stomates close to conserve water. No CO2 can enter.
Take in CO2 at night and store it as a stable molecule (crassulacean acid). Use that molecule to release CO2 when light is available but gas CO2 is not.
• All done in mesophyll cells. Think: different time, same place
Special cases: C4 metabolism
When there is constant bright sunlight, store CO2 as a 4 carbon molecule to supplement available gas. Uses ATP Increases carbon fixation above normal Avoids photorespiration
• Original carbon fixation in mesophyll cells. • Calvin cycle in bundle sheath cells.
Think: same time, different place
Figure 10.18 C4 leaf anatomy and the C4 pathway
Figure 10.19 C4 and CAM photosynthesis compared