AP Biology
Lecture #16
Photosynthesis
CO 7
Photosynthesis
& Cellular
Respiration
Photosynthesis
• Method of converting sun energy into
chemical energy usable by cells
• Autotrophs: self feeders, organisms capable
of making their own food
– Photoautotrophs: use sun energy e.g. plants
photosynthesis-makes organic compounds
(glucose) from light
– Chemoautotrophs: use chemical energy e.g.
bacteria that use sulfide or methane
chemosynthesis-makes organic compounds from
chemical energy contained in sulfide or methane
energy
input
from sun
Photoautotrophs
(plants, other producers)
nutrient
cycling
Heterotrophs
(consumers, decomposers)
energy output
(mainly heat)
Photosynthesis
• Photosynthesis takes place in specialized
structures inside plant cells called chloroplasts
– Light absorbing pigment molecules e.g.
chlorophyll
http://www.youtube.com/watch?v=hj_WKgnL6MI&
feature=player_embedded
Figure 7.1a
7.1 Photosynthetic Organisms
A. Photosynthesis transforms
solar energy
B. Organic molecules built by
photosynthesis provide both the
building blocks and energy for
cells.
Figure 7.1b
C. Plants use the raw materials:
carbon dioxide and water
D. Chloroplasts carry out
photosynthesis
Figure 7.1c
E. Chlorophylls and other
pigments involved in absorption
of solar energy reside within
thylakoid membranes of
chloroplasts
Figure 7.2
7.2 Plants as Solar Energy Converters
A. Solar Radiation - Only 42% of solar radiation that hits the
earth’s atmosphere reaches surface; most is visible light.
B. Photosynthetic Pigments - Pigments found in
chlorophyll absorb various portions of visible light; absorption
spectrum.
1. Two major photosynthetic pigments are chlorophyll a and chlorophyll
b.
2. Both chlorophylls absorb violet, blue, and red wavelengths best.
3. Very little green light is absorbed; most is reflected back; this is why
leaves appear green.
4. Carotenoids are
yellow-orange
pigments which
absorb light in
violet, blue, and
green regions.
5. When
chlorophyll breaks
down in fall, the
yellow-orange
pigments in leaves
show through.
C. Absorption and action spectrum - A
spectrophotometer measures the amount of light that
passes through a sample of pigments.
1) As different wavelengths are passed through, some
are absorbed.
2) Graph of percent of light absorbed at each
wavelength is absorption spectrum
.
3) Photosynthesis produces
oxygen; production of oxygen is
used to measure the rate of
photosynthesis.
4) Oxygen production and,
therefore, photosynthetic activity
is measured for plants under
each specific wavelength; plotted
on a graph, this produces an
action spectrum.
5) Since the action spectrum
resembles absorption spectrum,
this indicates that chlorophylls
contribute to photosynthesis.
T.E. Englemann’s Experiment
• Observation: Photosynthesis produces oxygen.
• Question: What parts of sunlight do plants
favor?
• Hypothesis: If bacteria require oxygen, then
we can expect them to gather in places where
the most photosynthesis is occurring.
• Englemann showed that bacteria gathered
around red and violet light (see next slide),
indicating that these were the colors of light
T.E. Englemann’s Experiment
Bacteria gathered mostly where violet and red light fell
on the green alga because Photosynthesis was greatest
in those locations
Pigments
• Color you see is the wavelengths not absorbed (red
shirt reflects red light and absorbs all other colors)
• Light-catching part of molecule often has
alternating single and double bonds
• These bonds contain electrons that are capable of
being moved to higher energy levels by absorbing
light
Variety of Pigments
• Chlorophylls a (green, chloroplasts)
•
Chlorophyll b (bluish-green; plants, green algae, bacteria)
• Carotenoids: absorb blue-violet wavelengths but reflect yellow,
orange, and red (carrots,beta-carotene)
• Anthocyanins: pigments in flowers
• Phycobilins: red and blue pigments of red algae and cyanobacteria
Chlorophylls
Wavelength absorption (%)
Main pigments in most photoautotrophs
chlorophyll a
chlorophyll b
Wavelength (nanometers)
Accessory Pigments
percent of wavelengths absorbed
Carotenoids, Phycobilins
beta-carotene (carotenoid pigment)
phycoerythrin
(a phycobilin pigment)
wavelengths (nanometers)
D. Photosynthetic Reaction
1. In 1930 C. B. van Niel showed that O2 given off by
photosynthesis comes from water and not from CO2.
2. The net equation reads:
Overall Reaction
• 6CO2 + 12 H2O + light
energy → C6H12O6 + 6O2+ 6H2O
• Carbohydrate made is glucose
• Water appears on both sides because 12 H2O molecules are
required and 6 new H2O molecules are made
• Water is split as a source of electrons from hydrogen atoms
releasing O2 as a byproduct
• Electrons increase potential energy when moved from water
to sugar therefore energy is required
Overall Reaction
LIGHT ENERGY
12H2O + 6CO2
Water Carbon
Dioxide
6O2 + C6H12O 6 + 6H2O
Oxygen Glucose Water
E. Two Sets of Reactions in Photosynthesis
1. Light reactions cannot take place unless light is present. They
are the energy-capturing reactions.
b. Chlorophyl within thylakoid membranes absorbs solar energy
and energizes electrons.
c. Energized electrons move down the electron transport system;
energy is captures and used for ATP production.
d. Energized electrons are also taken up by NADP+, becoming
NADPH.
2. Calvin Cycle Reactions
a. These reactions take
place in the stroma; can
occur in either the light or
the dark.
b. These are synthesis
reactions that use
NADPH and ATP to
reduce CO2.
What you should know by now..
1. The equation for photosynthesis
2. The structure of a chloroplast
3. Which spectrum(s) of light is used by plants and the
pigments associated with light spectrum.
4. The two stages of photosynthesis and their products
**Things are about to get much more difficult**