Photosynthesis
LA Charter School Science Partnership
3 March 2012
Nick Klein
Today’s Talk
• Intro
• Part 1: Photosynthesis – the big picture
• Part 2: Chemistry of photosynthesis
• Part 3: Biology and ocean context
Today’s Talk cont’d
• I’ve split this talk into three (roughly) even
parts. We will take a brief (1-2min) break
between sections. Please feel free to take
notes, handle the props, think of questions
for after the talk, etc.
Part 1: The big picture
• What is photosynthesis? Let’s define it!
– Photosynthesis is the process by which
organisms use the energy in sunlight to
chemically transform carbon dioxide (CO2)
into organic carbon compounds such as
sugars
• Some organisms can use energy sources
other than sunlight, this is called
chemosynthesis (Jason’s talk!)
Part 1: The big picture
• Organisms that make their own food are
called autotrophs. Organisms that make
food using photosynthesis are
photoautotrophs
• All animals, including humans, are
heterotrophs—we have to consume other
organisms as food
Part 1: The big picture
• Why is photosynthesis important?
– Most life on Earth depends on it
– Responsible for us having oxygen to breathe!
– Fossil fuels are derived from ancient
photosynthesis
– Photosynthesis removes CO2 from the
atmosphere
Part 1: The big picture
Part 1: The big picture
• Photosynthesis uses sun energy to
transform inorganic carbon dioxide (CO2)
into organic carbon compounds such as
sugars, fats, and amino acids… the
building blocks of life!
• Scientists refer to this conversion as
“fixing” carbon. So, photosynthesis is
sun-driven carbon fixation.
Part 1: The big picture
• Photosynthesis involves
reduction/oxidation (redox) reactions—
chemical reactions that involve the
movement of electrons from one molecule
to another
• In photosynthesis, when carbon dioxide is
fixed, it is reduced (electrons are added to
it) which produces organic carbon
compounds
Part 1: The big picture
Loss of
Electrons is
Oxidation
goes
Gain of
Electrons is
Reduction
Part 1: The big picture
• Example: nitrogen fixation
2x
nitrogen gas
ammonia
Part 1: Bonding on an atomic level
• How many electrons in nitrogen gas?
N N
Part 1: Bonding on an atomic level
• How many electrons in two molecules
of ammonia?
H N H
H
Part 1: The big picture
• In photosynthesis, photoautotrophs use
the energy of sunlight to fix carbon dioxide
into glucose
• Carbon dioxide is reduced to make the
basic chemical building blocks of all life
• Photosynthesis is one of the most
important biological processes!
Break!
Part 2: Chemistry of photosynthesis
• What (chemically) is required for
photosynthesis?
– Light
– Water
– Carbon dioxide
12H2O + 6CO2  C6H12O6 + 6O2 + 6H2O
Part 2: Chemistry of photosynthesis
• There are two distinct parts of
photosynthesis
• Light-dependent (light) reactions: sun
energy is absorbed and transformed into
chemical energy
• Light-independent (dark) reactions: also
called the Calvin cycle, chemical energy
from the light reactions is used to fix CO2
into glucose
Part 2: Chemistry of photosynthesis
• Light-dependent (light) reactions can ONLY
happen when there is sunlight available
• Of course, the first step in transforming sun
energy into chemical energy is to soak up
some rays!
Part 2: Chemistry of photosynthesis
• Plants use pigments to absorb light
energy from the sun
• Main pigment is chlorophyll a (shown
next slide), this is responsible for plants
being green
• There are many other accessory
pigments which help plants harvest
different colors of light
Part 2: Chemistry of photosynthesis
Part 2: Chemistry of photosynthesis
anthocyanins
Part 2: Chemistry of photosynthesis
• The pigments, along with a large number
of enzymes and other chemical
machinery, form photosystems
• Light energy is absorbed by the pigments
inside the photosystem
• This energy is used to split water
molecules into electrons, protons, and
oxygen gas
Part 2: Chemistry of photosynthesis
Pigments
2H2O
Photosystem
4e- + 4H+ + O2
Part 2: Chemistry of photosynthesis
• The electrons we get from splitting water
are then excited by more sun energy
captured by the pigments and enter the
electron transport chain
• The electron transport chain is a series of
different molecules each electron is
passed along to. The electron loses some
of its energy with each “step” it makes
Part 2: Chemistry of photosynthesis
Electron Transport
Chain
Pigments
Photosystem
4e-
Part 2: Chemistry of photosynthesis
• After passing through the electron
transport chain, the photosystem attaches
the electrons to a molecule called NADPH
• NADPH is a reductant—it can reduce
(give electrons) to other compounds
• Where do you think the electrons from
NADPH will go?
Part 2: Chemistry of photosynthesis
NADPH
Part 2: Chemistry of photosynthesis
Electron Transport
Chain
Pigments
Photosystem
4e-
NADPH
Part 2: Chemistry of photosynthesis
• The energy from the electron transport
chain is used to pump protons (the H+ we
made from splitting water) through
chemical machinery that makes ATP
• The high-energy phosphate bonds in ATP
are broken to provide the energy for most
chemical reactions that occur in biology
Part 2: Chemistry of photosynthesis
ATP
Part 2: Chemistry of photosynthesis
• Light-dependent (light reactions)
summary: light is captured by pigments
and used to split water, ultimately
producing oxygen gas, NADPH (a
reductant) and ATP (energy)
Part 2: Chemistry of photosynthesis
Part 2: Chemistry of photosynthesis
• Light-independent (dark) reactions can
occur with or without light, they don’t
require it
• NADPH and ATP from the light-dependent
reactions are used to reduce CO2 into
sugars and other organic compounds
Part 2: Chemistry of photosynthesis
ATP + NADPH
CO2
C6H12O6
The Calvin Cycle (light independent reactions)
Part 2: Chemistry of photosynthesis
Part 2: Chemistry of photosynthesis
• Light-dependent (light reactions)
summary: light is captured by pigments
and used to split water, ultimately
producing oxygen gas, NADPH (a
reductant) and ATP (energy)
• Light-independent reactions: ATP and
NADPH are used to fix CO2 into organic
carbon (sugars)
Part 2: Chemistry of photosynthesis
12H2O + 6CO2  C6H12O6 + 6O2 + 6H2O
C6H12O6 + 6O2  6H2O + 6CO2
Respiration is photosynthesis backwards!
Break!
Part 3: Broader context
• We’ve gone over the basics and the
chemistry of photosynthesis, let’s explore
some of the biology and think about it in
the context of the ocean and the Earth as
a whole!
Part 3: Broader context
Part 3: Broader context
• The overall trend of atmospheric CO2 is
upward (it is increasing). Why is that?
• Within each year, what is happening to the
CO2? Why might this be?
• The “wave” pattern is the Earth “breathing”
in and out—lower CO2 in the summer
when all the plants are growing and using
it for photosynthesis!
Part 3: Broader context
• Land plants have
stomata on the
undersides of their
leaves
• They open their
stomata to breathe in
CO2. When might
they want to open
them? When would
they close them?
Part 3: Broader context
Part 3: Broader context
Part 3: Broader context
• The oceans account for about half of all
carbon fixation
• However, the oceans only have 0.2% of
the biomass
• Fast turnover—what is the lifespan of a
phytoplankton compared to a tree?
• What about these facts might make algae
interesting to scientists?