2. The Light Reactions
• Almost all of the energy in living systems
comes from the sun
• Sunlight energy enters living systems when
plants and some other organisms absorb light
and use it to make sugars
3. Obtaining Energy
• Organisms can be classified by how they get
energy
• Autotrophs use energy from sunlight to make
sugars (their own food)
– Most use photosynthesis to convert sunlight
energy into chemical energy
• Heterotrophs get energy from food
5. Overview of Photosynthesis
• The products of
photosynthesis are the
reactants for cellular
respiration
• The products of cellular
respiration are the
reactants for
photosynthesis
6. Overview of Photosynthesis
• Photosynthesis can be divided into 2 stages
– Light Reactions – light energy is converted to
chemical energy, which is temporarily stored in
ATP and NADPH
– Calvin Cycle – Sugars are formed using CO2 and
the chemical energy stored in ATP and NADPH
7. Overview of Photosynthesis
• Overall photosynthesis equation:
Light energy
6CO2 + 6H2O C6H12O6 + 6O2
Light energy
Carbon dioxide + water sugars + oxygen
8. Capturing Light Energy
• The first stage of photosynthesis is called the
light reactions because they require light
• Photosynthesis begins with the absorption of
light in the chloroplast
9. Parts of the chloroplast
• Chloroplasts –
organelles found in the
cells of plants and algae
• Thylakoids –
membranes arranged as
flattened sacs
• Grana – stacks of
thylakoids
• Stroma – solution
surrounding the grana
10. Light and Pigments
• Light from the sun
appears white, but it is
made of a variety of
colors
11. Light and Pigments
• Pigments are compounds that absorb light
• Many objects contain pigments that absorb
some colors of light and reflect others
• The colors that are reflected are the ones you
see
12. Chloroplast Pigments
• There are several pigments in the thylakoid
membranes
– Most important are chlorophylls
• Chlorophyll a absorbs mostly red and blue light and
reflects mostly green light
– Accessory pigments
• Chlorophyll b assists chlorophyll a in capturing light
energy
• Carotenoids are the yellow, orange, and brown
pigments
14. Chloroplast Pigments
• In plant leaves, chlorophylls are the most
abundant pigments and therefore mask the
colors of the other pigments
• During the fall, many plants lose their
chlorophylls and become the color of the
carotenoids
15. Light Reactions
• The first set of reactions – the light reactions
take place in the thylakoid membranes
16. Light dependent reactions
1. Light energy is absorbed by chlorophyll in
photosystem II
2. Electrons in chlorophyll are energized, or
“excited” and leave the chlorophyll
3. These electrons are passed to the primary
electron acceptor
17. Light Dependent Reactions
4. The electrons move down the electron
transport chain (series of molecules that
transport electrons)
a. As the electrons move down the chain, the
energy is used to move H+ ions into the
thylakoid
18. Light-Dependent Reactions
5. Light is absorbed by chlorophyll in
Photosystem I
6. Electrons are energized and passed on to a
different electron transport chain
7. NADP+ accepts these electrons and is
converted to NADPH
19. Replacing Electrons in Light Reactions
• To replace the electrons lost from chlorophyll
in photosystem II, an enzyme splits water
molecules
• 2H2O 4H+ + 4e- + O2
20. Making ATP in Light Reactions
• Hydrogen ions move from the inside of the
thylakoid to the outside through ATP
Synthase, an enzyme
• As this happens, ATP is produced
22. The Calvin Cycle
• Second set of reactions in photosynthesis
• Uses the energy stored in ATP and NADPH to
produce sugars
23. The Calvin Cycle
1. Carbon atoms from carbon dioxide in the air
enter the plant
2. This carbon joins with sugars in the plant
3. Energy from ATP and NADPH converts these
compounds into sugar for the plants to use
4. Using 3 ATP the remaining sugars are
converted back to the original carbon
compounds to begin the cycle again
24. A Summary of Photosynthesis
• Happens in two stages
1. The light reactions – Energy is absorbed from
sunlight and converted into chemical energy,
which is temporarily stored in ATP and NADPH
2. The Calvin cycle – Carbon dioxide and the
chemical energy stored in ATP and NADPH are
used to form sugars
