2. Purpose of Photosynthesis
⢠Photosynthesis converts electromagnetic
energy in light to potential energy (sometimes
called chemical energy) stored in chemical
bonds of glucose
â The potential energy in glucose can be stored and
moved to allow it to be used when and where it is
needed
3. Two Stages of Photosynthesis
⢠Convert electromagnetic energy in light to
potential energy in ATP and NADPH
â Light Dependent Reactions
⢠Convert potential energy in ATP and NADPH to
potential energy stored in glucose
â Light Independent Reactions
4. Where Does Photosynthesis Occur?
⢠Most
photosynthesis
takes places in
leaves
âPalisade and
spongy
mesophyll cells
⢠Organelles
known as
chloroplasts
7. Component of a Photosystem
⢠Antennae Pigments
â Chlorophyll a, b and carotene
â 200 â 300 per photosystem
⢠Reaction Center
â Chlorophyll a
⢠Primary Electron Acceptor
9. What Happens in a Photosystem?
⢠Antennae pigment absorbs a photon of light
energy
â Electron excited to higher energy level
⢠Potential energy in excited electron
â When electron falls back to resting stage, the
energy that is released is used to excited an
electron in an adjacent antennae pigment
⢠resonance
10. What Happens in a Photosystem?
⢠Eventually energy released by one excited
electron is used to excite an electron in the
reaction center
â Chlorophyll a
⢠This excited electron does not fall back to resting
stage
⢠Instead, the excited electron (and its potential
energy) moves to an adjacent molecule know as
the Primary Electron acceptor
â 1o electron acceptor has âextraâ electron
â Reaction center is missing one electron
11. What Happens in a Photosystem?
Summary
⢠Light energy is converted into potential energy
that is stored in an excited electron that is
passed form the reaction center to the
primary electron acceptor
⢠Energetic results
â Light energy converted into potential energy in
excited electron
12. Why Is This Important
⢠Potential energy in excited electron can be
used to do work!!!
13. Electron Flow
⢠Two patterns of electron flow
â Cyclic electron flow
â Non-cyclic electron flow
15. Electron Flow
⢠Excited electron in 1o electron acceptor moves
to adjacent molecule
⢠Electron drops to lower energy level
⢠Energy released
⢠Energy used to actively transport H+ from
stroma into the thylakoid space
â Causes a H+ concentration gradient
16. Cyclic Electron Flow
⢠As the name suggests, the excited electron is
eventually returned to the reaction center
chlorophyll that originally lost it.
17. Importance of the H+ Concentration
Gradient
⢠The active transport of H+ inside of the
thylakoid space produces a H+ concentration
gradient
⢠This concentration gradient powers a process
known as Chemiosmosis that converts ADP to
ATP
â Potential energy stored in the chemical bonds of
ATP
19. Cyclic Electron Flow
Summary
⢠Excited electron from 1o electron acceptor is
moves from molecule to molecule and
eventually returns to the reaction center that
lost it
⢠Energetic Result
â Potential energy stored in excited electron is
converted to potential energy stored in ATP
20. Photosystems
⢠Turns out there are two types of photosystems
â Photosystem I and Photosystem II
â Photosystems are named based on the order that
they were discovered
⢠PS I discovered before PS II
⢠PS I and PS II differ slightly in the absorption
spectra of the Chlorophyll a molecule
â PS I P700
â PS II P680
23. Non-cyclic Flow
⢠Starts in PS II
â Electron from P700 passed to 1o electron acceptor
â excited electron undergoes electron flow
⢠Similar to in cyclic flow
⢠H+ concentration gradient powers chemiosmosis-> ATP
â excited electron does not return to P700 that lost it
⢠Instead, that electron and its remaining potential energy is
transferred to P680 in the reaction center of PS I
24. Non-cyclic Flow
⢠In PS I, this electron that originated in PS II is
re-excited to an even higher energy level
â Excited electron undergoes a different pattern of
electron flow
â Result of this flow is that energy in excited
electron is converted into potential energy in
NADPH
25. Non-cyclic Flow
Do You See the Problem??
⢠The electron from P700 in PS II used to help
produce NADPH
â Thus P700 is missing an electron
⢠P700 recovers its missing electron by taking an
electron from water (H20)
â H+ and O2 released
26. Non-cyclic Flow Summary
⢠Energy from two photons of light energy are
converted into potential energy in ATP and
NADPH
⢠Water is broken down to release H+ and O2
28. Light Dependent Reactions
Summary
⢠Both cyclic and non-cyclic flow are occurring
simultaneously in the same chloroplasts
⢠Energetic result- light energy is converted into
potential energy in ATP and NADPH
⢠Chemical result- H20 => H+ + O2
29. The Light Dependent Reactions
⢠Light dependent reactions
involves molecules
imbedded in the thylakoid
membrane
⢠Allows the cell to precisely
control the spatial
organization of molecules
â Electron transport and
resonance can not occur if
molecules are floating in
cytoplasm
30. Whatâs Next!
⢠The ATP and NADPH produced during the light
dependent reactions will be used to power
the light dependent reactions of
photosynthesis