Chapter 10 Photosynthesis 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 CO 2 and other inorganic raw materials 1
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 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 2
Most of the autotrophic producers we are familiar with are photoautotrpohs Large, multicellular algae like kelp Most of the autotrophic producers we are familiar with are photoautotrpohs Unicellular algae - Euglena 3
Most of the autotrophic producers we are familiar with are photoautotrpohs Prokaryotic blue-green algae - cyanobacteria 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: 6CO 2 + 6H 2 O + light energy C 6 H 12 O 6 + 6O 2 4
Form: Leaf anatomy Epidermis Vein Guard cells Stoma Mesophyll Chloroplast Thylakoid Grana Stroma Leaves are typically thin for light transmission 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 5
Why are leaves green? Certain wavelengths of the visible spectrum are absorbed by the pigments in the chloroplasts Primary photosynthetic pigment - chlorophyll Photosynthetic pigments in plants Chlorophyll a Chlorophyll b Carotenoids 6
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? Losing electrons!!! 7
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 Light reactions Occur on the thylakoid membranes Requires light Calvin cycle Occurs in the stroma 8
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??? 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 9
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 Why is water needed?? 10
Calvin Cycle carbon fixation CO 2 enters the stroma ATPs and NADPHs from the light reactions are used to create sugar Important enzyme Ribulose biphosphate (RuBP) Accepts CO 2 and is recreated at end of cycle 11
Special problems with photosynthesis So for a plant to photosynthesize: Water Light CO 2 How does the water get into the leaf? How does the CO 2 get into the leaf? Does this present a problem??? 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 12
Plants adapted to hot dry climates may use: C4 photosynthesis More efficient at capturing and using CO 2 This means the stomata do not have to be open as long during the day CAM photosynthesis (crassulacean acid metabolism) CO 2 is fixed at night Pineapples, aloe vera,succulents from deserts 13