Science Starter #3 – 11/2/16
Draw a chloroplast and labels
its parts
(stroma, granum, thylakoid)
Notes 4.2 –
The Light Reaction
(Stage 1 of Photosynthesis)
Chapter 6
pg. 116-124
Overview of
Photosynthesis
 Steps:
The Light
Reactions
 Absorb Light
 Electron Transport
Chain
 Chemiosmosis
 Goals
 Absorb light and
water
 Create ATP and
NADPH
 Produce Oxygen
 Light: Visible light includes the colors of
the rainbow.
 Pigments: Compounds that absorb
specific colors of light.
 Colors that we see are the colors that are
NOT absorbed (they are reflected)
How is light
absorbed?
 Chloroplast Pigments
 Chlorophyll a: absorbs some blues and reds
reflecting green
 Located in thylakoids
 Located in the thylakoid membrane
 Thylakoid: flattened sacs within the chloroplasts
 Granum: Stack of thylakoids
 Stroma: area surrounding thylakoid inside chloroplast
 Clusters of pigments called photosystems absorb
light energy
Step 1. Light
Absorption
 Located within thylakoid membrane
 There are two photosystems: Photosystem I and
Photosystem II
 Absorbed light excites (energizes) electrons to
leave photosystem
 Water molecules are split into H+
(protons), electrons, and oxygen
 Oxygen gas diffuses out of the cell
and into the environment
Replacing
the used
electrons
 Step 2 - Called Electron Transport Chain
 Peripheral and Integral proteins pass
excited electrons through the thylakoid
membrane
 Electrons (e-) from Photosystems move
along ETC
Step 2.
Electron
Transport
Chain (E.T.C.)
 The energy released here helps to move H+
ions INSIDE the thylakoid
 The e- attach to an e- carrier called NADP+
to form NADPH at the end of the ETC
 Many protons (H+) on
INSIDE of thylakoid
 H+ move from HIGH to
LOW and pass through
ATP synthase
 ATP synthase converts
ADP + P  ATP
Step 3.
Chemiosmosis
 ATP is used later on too!
Light + H2O + ADP + NADP+  ATP + NADPH + O2
1. Chlorophyll absorbs light and excites e2. e- are replaced by splitting H20 into H+, e-,
and O2
Summary of
Light
Reaction
3. Electrons move down the ETC
 Moves H+ into thylakoid
 e- attaches with NADP+ to create NADPH
4. H+ (protons) move through the enzyme
ATP synthase from HIGH to LOW
concentration to convert ADP into ATP
Reactants: light, H20, NADP+, ADP
Products: NADPH, ATP, O2