1) Photosynthesis converts light energy to chemical energy stored in glucose through two stages: the light-dependent reactions and light-independent reactions. 2) In the light-dependent reactions, light energy is absorbed by antennae pigments and transferred through photosystems I and II to excite electrons, which is used to generate ATP and NADPH via electron transport. Water is also split to release protons and oxygen. 3) The ATP and NADPH produced in the light-dependent reactions will then be used in the light-independent reactions to convert carbon dioxide into glucose.

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