biology chapter 10 - wcjc
TRANSCRIPT
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Biology – Kevin Dees
Chapter 10Photosynthesis
Biology – Kevin Dees
Photosynthesis
• The process which feeds the Biosphere!• Recall that all forms of life require energy• Some forms of life have the ability to
sustain themselves without eating anything derived from other organisms; autotrophsautotrophs– Produce their organic energy from CO2 and
other inorganic raw materials
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Biology – Kevin Dees
Autotrophs
• These are the producersproducers in the biosphere
•• HeterotrophsHeterotrophs are known as consumersconsumers– Unable to make their own organic energy
molecules for food, heterotrophs must ingest portions of other organisms
Biology – Kevin Dees
Most of the autotrophic producers we are familiar with are photoautotrpohs
– Use light as the energy source fro the synthesis of these organic compounds
– Examples • Plants
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Biology – Kevin Dees
Most of the autotrophic producers we are familiar with are photoautotrpohs
• Large, multicellular algae like kelp
Biology – Kevin Dees
Most of the autotrophic producers we are familiar with are photoautotrpohs
• Unicellular algae - Euglena
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Biology – Kevin Dees
Most of the autotrophic producers we are familiar with are photoautotrpohs
• Prokaryotic blue-green algae - cyanobacteria
Biology – Kevin Dees
Each of these organisms accomplishes photosynthesis in a similar fashion, but our
focus will be on plants
• All green parts of plants have chloroplasts– Small stems– Unripened fruit– Leaves
• The leaf is the major photosynthetic organ in plants• The leaf is a prime example of ‘form=function’• Function:
– 6CO2 + 6H2O + light energy C6H12O6 + 6O2
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Biology – Kevin Dees
• Form:Leaf anatomy
– Epidermis– Vein – Guard cells– Stoma– Mesophyll– Chloroplast
• Thylakoid• Grana• Stroma
• Leaves are typically thin for light transmission
Biology – Kevin Dees
Physical properties of light energy• Light is part of the electromagnetic spectrum• Wavelength – distance between crests of waves – energy relationship• Photons – packets of light energy
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Biology – Kevin Dees
Why are leaves green?• Certain wavelengths of
the visible spectrum are absorbed by the pigments in the chloroplasts
• Primary photosynthetic pigment - chlorophyll
Biology – Kevin Dees
Photosynthetic pigments in plants• Chlorophyll a• Chlorophyll b• Carotenoids
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Biology – Kevin Dees
When molecules like chlorophyll absorb photons of light, they absorb energy
• They become excited!!!• They are pushed to a higher energy level• This high energy level is not stable• How is stability reached?
Biology – Kevin Dees
Losing electrons!!!
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Biology – Kevin Dees
Photosynthesis:a two stage process
• Light reactions – the photo portion of the reactions
• Calvin Cycle – AKA dark reactions or carbon fixation reactions– Light not needed, only products of light
reactions are required and carbon will be ‘fixed’ – put into a usable form
– Named for Melvin Calvin in the 1940s
Biology – Kevin Dees
• Light reactions– Occur on the thylakoid membranes– Requires light
• Calvin cycle– Occurs in the stroma
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Biology – Kevin Dees
Light reactions – occur on the thylakoid membranes
Phototsystem I and photosystem II• Photosystem II occurs first!!!
• (I know, it is backwards!!)
• A photon of light strikes the chlorophyll in the Photosystem II on the thylakoid membrane exciting it – losing electrons!!!– Oxidation right???
Biology – Kevin Dees
• These electrons flow down an energy gradient and are picked up by chlorophyll in photosystem I – reducing it!!
• The electrons lost from photosystem II are replaced by the splitting of water– Forming
• electrons, H ions and releasing OXYGEN
• The H ions concentration builds up in the thylakoid space and, using ATP synthase, are used as bullets a proton pistol!!!– ATP synthesis
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Biology – Kevin Dees
• Meanwhile, our reduced chlorophyll in photosystem I is now ready to absorb a photon of light– Gets excited and loses electrons (oxidized)– Electrons travel down and energy gradient along the
thylakoid membrane again and reduce a molecule of NADP+
• Forming and energy carrier called NADPH
• The ATP from photosystem II and the NADPH from photosystem I are routed to the stroma of the chloroplast – to the Calvin cycle
Biology – Kevin Dees
Why is water needed??
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Biology – Kevin Dees
Calvin Cycle – carbon fixation• CO2 enters the
stroma • ATPs and NADPHs
from the light reactions are used to create sugar
• Important enzyme– Ribulose
biphosphate(RuBP)
– Accepts CO2 and is recreated at end of cycle
Biology – Kevin Dees
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Biology – Kevin Dees
Special problems with photosynthesis
• So for a plant to photosynthesize:– Water– Light– CO2
• How does the water get into the leaf?• How does the CO2 get into the leaf?• Does this present a problem???
Biology – Kevin Dees
• For a plant to photosynthesize by the methods we just described, on hot dry days sugar production declines? WHY??
• These plants are commonly called C3 plants (first intermediate has 3C) and typically grow during cool seasons and do not do as well in hot dry climates
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Biology – Kevin Dees
• Plants adapted to hot dry climates may use:– C4 photosynthesis
• More efficient at capturing and using CO2
• This means the stomata do not have to be open as long during the day
– CAM photosynthesis (crassulacean acid metabolism)
• CO2 is fixed at night• Pineapples, aloe vera ,succulents from deserts