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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; autotrophs– Produce their organic energy from CO2 and

other inorganic raw materials

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Biology – Kevin Dees

Autotrophs

• These are the producers in the biosphere

• Heterotrophs are known as consumers– 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

Photosystem 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 an 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