Chapter 10-Photosynthesis Photosynthesis is the process that ...

Chapter 10-Photosynthesis
Goals for today’s lecture:
Review chemical cycling between photosynthesis and
cellular respiration
What are the characteristics of light energy
How is light energy absorbed by pigments and
converted to chemical energy
How is this energy used to make ATP in photosynthesis
What are the two parts of photosynthesis
Photosynthesis is the process that converts solar
energy into chemical energy
Directly or indirectly, photosynthesis nourishes
almost the entire living world
Autotrophs are the producers of the biosphere,
producing organic molecules from CO2 and other
inorganic molecules
Plants
Almost all plants are photoautotrophs using the
energy of sunlight to make organic molecules
from water and carbon dioxide
Unicellular protist
Heterotrophs obtain their organic material from other
organisms
Heterotrophs are the consumers of the biosphere
Almost all heterotrophs, including humans, depend
on photoautotrophs for food and oxygen
Multicellular algae
Cyanobacteria40 µm
Purple sulfur 1.5 µm
bacteria
•Photosynthesis occurs in plants, algae, certain other
protists, and some prokaryotes
Chloroplasts: The Sites of Photosynthesis in Plants
•Their green color is from chlorophyll, the green pigment
within chloroplasts
The Overall Equation for Photosynthesis
•Photosynthesis can be summarized as the following equation:
We use glucose to simplify the relationship
between photosynthesis and respiration,
6 CO 2
+ 12 H 2
O + Light energy → C 6
H 12
O 6
+ 6 O 2
+ 6 H 2
O

Photosynthesis as a Redox Process
•We can simplify the equation by indicating only
the the net consumption of water.
Photosynthesis is not a single process, but two
processes,each with many steps.
The two stages of photosynthesis are called:
The light reactions convert solar energy to chemical energy.
NADPH is an electron carrier, similar to NADH.
The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules
The light reactions convert solar energy to the
chemical energy of ATP and NADPH
Chloroplasts are solar-powered chemical factories
Their thylakoids transform light energy into the chemical
energy of ATP and NADPH

The Nature of Sunlight
•Light is a form of electromagnetic energy, also
called electromagnetic radiation
Photosynthetic Pigments: The Light Receptors
Pigments are substances that absorb visible light
Different pigments absorb different wavelengths
Chloroplast
Light
Reflected
light
Absorbed
light
Granum
spectrophotometer
Transmitted
light
White
light
Refracting
prism
Chlorophyll
solution
Photoelectric
tube
Galvanometer
•This machine sends light through pigments and
measures the fraction of light transmitted at each
wavelength
Slit moves to
pass light
of selected
wavelength
Green
light
0 100
The high transmittance (low
absorption) reading
indicates that chlorophyll
absorbs very little green
light.
Absorption of light by
chloroplast pigments
Chlorophyll a
Chlorophyll b
Carotenoids
400 500 600 700
Wavelength of light (nm)
Absorption spectra
absorption spectrum

Photosynthetic Pigments: The Light Receptors
Energy of electron
Rate of photosynthesis
(measured
by O 2 release)
•The action spectrum of photosynthesis was first
demonstrated in 1883 by Thomas Engelmann
Action spectrum
action spectrum
Filament
of algae
•He used aerobic bacteria clustered along the alga
as a measure of O 2
production
400 500 600 700
Engelmann’s experiment
Absorption of light by
chloroplast pigments
Chlorophyll a
Chlorophyll b
Carotenoids
400 500 600 700
Wavelength of light (nm)
Absorption spectra
CH 3 in chlorophyll a
CHOin chlorophyll b
Porphyrin ring:
light-absorbing
“head” of molecule;
note magnesium
atom at center
Hydrocarbon tail:
interacts with
hydrophobic
regions of proteins
inside
thylakoid membranes
of chloroplasts; H
atoms not shown
e –
Excited
state
Heat
Photon
Chlorophyll
molecule
Photon
(fluorescence)
Ground
state
Excitation of isolated chlorophyll molecule

A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes
A photosystem consists of a reaction center
surrounded by light-harvesting complexes
Thylakoid
Solar-powered transfer of an electron from a chlorophyll a
molecule to the primary electron acceptor is the first step of
the light reactions
Thylakoid membrane
Photon
Photosystem
STROMA
Light-harvestingReaction
Primary electron
complexes center
acceptor
e –
Transfer
of energy
Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
How the Light Reactions Generate ATP and NADPH
Two types of photosystems cooperate in the light
reactions:
1.
2.
An electron transport chain connects the two photosystems and releases energy.

Study outline-Chapter 10- Photosynthesis
Review – Chemical cycling between photosynthesis and cellular respiration
Autotrophs vs. heterotrophs
Producers vs. consumers
Photoautotrophs – plants, algae, certain protists, and some prokaryotes
Be able to recognize the sites of photosynthesis and function- leaf tissue (mesophyll), stomata, stroma, thylakoids
Know the equation for photosynthesis (understand what is oxidized and what is reduced).
General overview of photosynthesis
(What happens in each process Where do the processes take place in the chloroplast)
1) Light reactions
2) Calvin cycle
Understand general principles of light energy- wave and particle (photon)
Know and understand the following terms: wavelength, absorption spectra, photon, action spectra, pigment,
spectrophotometer, accessory pigment
Be able to recognize chlorophyll a and b
Understand the components of a reaction center and how light energy is converted to chemical energy
Know the two types of photosystems that participate in the light reactions
(What happens in each photosystem)
1) Photosystem II
2) Photosystem I