Chapter 8
Photosynthesis
8–1 Energy and Life
Energy – the ability to do work No energy = no life
A. Autotrophs and Heterotrophs
1. Autotrophs – energy from sun or other sources 2. Heterotroph –consume
3. All organisms have to release the energy
in sugars and other compounds to live.
B.Chemical Energy and ATP
ATP – Adenosine triphosphate – energy for cell
Made of adenine, ribose, and three
phosphates.
1. Storing Energy – When bonds are formed, energy is stored.
a. ADP – Adenosine diphosphate is similar to ATP, but with two phosphates instead of
three.
b. Energy is stored when a phosphate is added to ADP
ADP ATP
Energy Energy
Adenosine triphosphate (ATP)
Partially charged battery
Fully charged battery Adenosine diphosphate (ADP) + Phosphate
2. Releasing Energy
a. Energy is released when bonds are broken.
b. When a phosphate is removed from ATP, energy is released
c. As many as two phosphates can be removed from ATP.
remove one phosphate = ADP (adenosine diphosphate) remove two phosphates = AMP
(adenosine monophosphate)
C.Using Biochemical Energy
1.Cells use ATP for active transport, to move organelles in the cell, and to synthesize
proteins and nucleic acids
2.Cells do not keep large amounts of ATP in the cell. The cell can regenerate ATP from glucose, as needed.
3. ATP is great for transferring energy, but
not for storing it.
8–2 Photosynthesis: An Overview
Photosynthesis – the process by which plants use sunlight to convert water and carbon
dioxide into sugar and starches A.Investigating Photosynthesis
1. Van Helmont’s Experiment – concluded
that trees gain most of their mass from
water.
2.Priestley’s Experiment – finds that plants release a substance that keeps a candle burning - oxygen
3.Jan Ingenhousz’s Experiment–
concludes that plants need sunlight
to produce oxygen
4. These early investigations and the work
of other scientists led to the discovery that in the presence of light, plants transform
carbon dioxide and water into
carbohydrates and release oxygen in the process.
B.The Photosynthesis Equation
6 CO
2+ 6 H
2O ----> C
6H
12O
6+ 6 O
2carbon dioxide + water sugar and oxygen
C. Light Absorption and pigments
1. Photosynthesis requires light - mixture of wavelengths
2. Pigments – light absorbing molecules in the chloroplast that are organized into
photosystems
a. chlorophyll – principal pigment that absorbs light in the blue-violet and red
regions of the visible spectrum and not the green region
1). chlorophyll a
2). chlorophyll b
b. carotenoids – accessory pigments such as carotene that absorbs other wavelengths of light
c. Energy absorbed by the chlorophyll molecules is transferred directly to the electrons in the chlorophyll raising their energy levels
d. It is these high energy electrons that make
photosynthesis work.
8–3 The Reactions of Photosynthesis
A. Inside a Chloroplast
1 Photosynthesis takes place inside the organelle the chloroplast
a. thylakoid – saclike photosynthetic membranes where chlorophyll and other pigments are found
Site of light dependent reactions
b. photosystems – light collecting units in the thylakoid membrane
c. granum – a stack of thylakoids
d stroma – space outside the thylakoid membrane
Site of Calvin cycle or Light Independent Reactions
http://upload.wikimedia.org/wikipedia/commons/d/da/Photosystems.png
3. The two sets of photosynthesis reactions work together- the light-dependent reactions trap the energy of sunlight in chemical form, and the light-independent reactions use that chemical energy to produce stable, high energy
sugars from carbon dioxide and water
4. Oxygen is released in the process for us to use
2. Scientists describe photosynthesis in two parts:
the light–dependent reactions and the light-independent reactions or
Calvin cycle.
B. Electron Carriers
1.Sunlight excites the electrons in chlorophyll
causing them to gain energy and become high energy electrons.
2.An electron carrier molecule is a compound that can accept a pair of these high energy electrons and then pass energy to another molecule
3.This process is called electron transport and the carrier molecules are known as the electron
transport chain or ETC.
4. NADP+ (nicotinamide adenine dinucleotide
phosphate) is such an electron carrier that is
converted into NADPH when it accepts its
electrons and a proton(H+).
C. Light-Dependent Reactions
1. Require light and take place in the thylakoid
2. These reactions produce oxygen gas and
convert NADP+ and ADP into NADPH and
ATP
3. Light –dependent reactions consist of a series of steps a. Light excites the electrons of the chlorophyll
molecules in photosystem II (PS II) causing them to become high energy electrons.
These electrons are replaced by the splitting of a water molecule
Low energy High energy
H2O ---- 2H+ + O + 2e-
b.These high energy electrons are accepted by an electron carrier molecule and move down an ETC until they reach photosystem I (PS I).
As the electrons move down the chain they lose energy. This energy is used to transport protons (H+) from the stroma into the thylakoid space.
Carrier
Carrier
Carrier Photosystem I
H+ H+
H+
H+
c. Light strikes the electrons in PS I causing them to become high energy electrons that move down a second ETC until they reach NADP+
and form NADPH.
Photosystem I
(NADP+ + 2e- + H+ -> NADPH)
NADPH WILL THEN GET USED IN THE CALVIN CYCLE This accumulation of H+ in the thylakoid space
causes a difference in charge across the
membrane. It is this difference that provides the energy needed to make ATP.
Photosystem I
Carrier
e. ATP synthase is a protein that moves the H+
ions back to the stroma and uses their energy to convert ADP into ATP.
Hydrogen Ion Movement Photosystem II
Inner Thylakoid Space
Thylakoid Membrane
Stroma
ATP synthase
Electron
Transport Chain Photosystem I ATP Formation
