1. Photosynthesis
Light Dependent Reactions
Dr. Mark A. McGinley
Honors College and Department of
Biological Sciences
Texas Tech University

2. Purpose of Photosynthesis
• Photosynthesis converts electromagnetic
energy in light to potential energy (sometimes
called chemical energy) stored in chemical
bonds of glucose
– 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
– Light Dependent Reactions
• Convert potential energy in ATP and NADPH to
potential energy stored in glucose
– Light Independent Reactions

4. Where Does Photosynthesis Occur?
• Most
photosynthesis
takes places in
leaves
–Palisade and
spongy
mesophyll cells
• Organelles
known as
chloroplasts

5. Cloroplast

6. Photosystem

7. Component of a Photosystem
• Antennae Pigments
– Chlorophyll a, b and carotene
– 200 – 300 per photosystem
• Reaction Center
– Chlorophyll a
• Primary Electron Acceptor

8. Chlorophyll

9. What Happens in a Photosystem?
• Antennae pigment absorbs a photon of light
energy
– Electron excited to higher energy level
• Potential energy in excited electron
– 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
– 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
– 1o electron acceptor has “extra” electron
– 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
– 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
– Cyclic electron flow
– Non-cyclic electron flow

14. 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
– 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
– Potential energy stored in the chemical bonds of
ATP

18. Chemiosmosis

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
– Potential energy stored in excited electron is
converted to potential energy stored in ATP

20. Photosystems
• Turns out there are two types of photosystems
– Photosystem I and Photosystem II
– 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
– PS I P700
– PS II P680

21. Photosystems
• Cyclic Flow
– PS I
• Non-cyclic Flow
– Both PS I and PS II

22. Non-cyclic Flow
Involves both photosystems. Begins in PS II
and then involves PS I

23. Non-cyclic Flow
• Starts in PS II
– Electron from P700 passed to 1o electron acceptor
– excited electron undergoes electron flow
• Similar to in cyclic flow
• H+ concentration gradient powers chemiosmosis-> ATP
– 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
– Excited electron undergoes a different pattern of
electron flow
– 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
– Thus P700 is missing an electron
• P700 recovers its missing electron by taking an
electron from water (H20)
– 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

27. Non-cyclic Flow-

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
– Electron transport and
resonance can not occur if
molecules are floating in
cytoplasm

30. What’s Next!
• The ATP and NADPH produced during the light
dependent reactions will be used to power
the light dependent reactions of
photosynthesis