Photosynthesis
Photosynthesis
• Photosynthesis is the process by which
organisms use the energy of the sun to
synthesize organic compounds (sugars) from
inorganic compounds (CO2 and water)
• Photosynthesis transforms the energy of the
sun into chemical energy (glucose)
• Provides the oxygen we breath, removes CO2,
and provides food, energy and shelter!
Photosynthesis
• Many organisms participate in photosynthesis:
– Nearly all plants
– Some Protists (Euglena, kelp)
– Some bacteria = cyanobacteria
Photosynthesis
Light
energy
6 CO2
+6
Carbon dioxide
H2 O
Water
C6H12O6
Photosynthesis
Glucose
+ 6
O2
Oxygen gas
• Carbon dioxide and water are waste products of
cellular respiration! Photosynthesis takes these
products and converts them to the glucose and
O2 necessary for cellular respiration
Photosynthesis
• Photosynthesis occurs on a cellular level
• Chloroplasts are organelles which carry out
photosynthesis
• Chloroplasts contain chlorophyll, a lightabsorbing pigment which give autotrophs
their distinctive color
The Chloroplast
• A chloroplast contains two membranes (as do
mitochondria)
• A thick fluid called the stroma fills the inner
compartment of the chloroplast
• Suspended in the stroma are the thylakoids, a
system of interconnected membranous sacs,
which enclose another compartment known
as the thylakoid space
Chloroplast
Outer and inner
membranes
Thylakoid
Stroma
Granum
Thylakoid
space
Intermembrane
space
The Chloroplast
• Built into the thylakoid membranes are the
chlorophyll molecules that capture light
energy
• Photosynthesis occurs throughout a plant (all
the green parts), but is concentrated in the
leaves
• A plant will invest much of its energy into the
production of its leaves in order to capture as
much light energy as possible
Make like a tree and…
• Leaves are designed to capture light and
increase the absorption of carbon dioxide
• Carbon dioxide enters the leaf (and oxygen
exits the leaf) via the stomata, tiny pores
protected by guard cells
• Water is supplied to the tree via its roots, but
may exit the leaves when the stomata are
open (a catch 22!); why stomata open at night
in many plants
Pigments
• Pigments are light-absorbing molecules built
into the thylakoid membranes
• Pigments absorb some wavelengths of light,
but reflect others
• We do not see the absorbed wavelengths
(because their energy has been absorbed by
the pigment molecules); we see the
wavelengths that the pigment reflects!
Pigments
• A leaf is green because chlorophyll absorbs
colors other than green; absorbs light most
strongly in the blue and red, but poorly in the
green
• Different pigments absorb different
wavelengths
• Chloroplasts contain different types of
pigments
Increasing energy
10–5 nm 10–3 nm
Gamma
rays
X-rays
103 nm
1 nm
UV
106 nm
Infrared
103 m
1m
Microwaves
Radio
waves
Visible light
380 400
600
500
Wavelength (nm)
700
650
nm
750
Pigments
• Chlorophyll a
(a type of
chlorophyll
pigment)
absorbs light
mainly in the
blue-violet (high
energy) and red
(low energy)
wavelengths
Light
Reflected
light
Chloroplast
Thylakoid
Absorbed
light
Transmitted
light
Autumn color change
• Autumn leaf color is a phenomenon that
affects the normally green leaves of deciduous
trees and shrubs, changing to reds and yellows
(and various shades in between)
• In late summer, the veins
that carry fluids into and
out of the leaf are
gradually closed off, and
chlorophyll decreases
Autumn color change
• In addition to chlorophyll, other pigments,
known as accessory pigments are present in
plants; these include carotene, and cyanins
• When chlorophyll concentrations decrease at
the end of summer, some of these other
pigments – which are usually masked by
chlorophyll – reveal their colors
• Carotene, for example, is especially longlasting
http://ohad.me/?showimage=53
Photosynthesis
• Photosynthesis occurs in 2 stages, each with
multiple steps
1. Light reactions convert light energy into
chemical energy, and produce O2.
2. Light-independent reactions (the Calvin
Cycle) assembles glucose molecules using CO2
(carbon fixation) and the energy-rich products
of the light reactions
1. Light (dependent) Reactions
• Light reactions occur in the thylakoid
membranes of the chloroplast
• Water is split, providing a source of electrons
and giving off O2 as a by-product
H2O  2H+ + 1/2 O2 + 2e-
• Light energy absorbed by chlorophyll
molecules is used to drive the transfer of
electrons and H+ from water to NADP+ and
generate ATP
1. Light (dependent) Reactions
• Light reactions absorb solar energy and
convert it to chemical energy (NADPH)
• Produces O2 as a by-product
• No sugar is produced in light reactions
2. Light-independent Reactions
• Light-independent reactions occur in the
stroma of chloroplasts
• Consist of the Calvin Cycle, which assembles
sugar molecules using CO2 and NADPH
• The incorporation of Carbon from CO2 into
organic compounds is called carbon fixation
2. Light-independent Reactions
• The Calvin Cycle does not require light, but
occurs during daylight hours when the light
reactions power the cycle by supplying NADPH
and ATP
• Often called “dark reactions”
• NADPH provides the electrons to reduce
Carbon from CO2 into glucose, and ATP powers
the cycle
CO2
H2O
Chloroplast
Light
NADP+
ADP
P
LIGHT
REACTIONS
CALVIN
CYCLE
(in stroma)
(in thylakoids)
ATP
NADPH
O2
Sugar
How do photosystems capture
solar energy?
• In the thylakoid membrane, chlorophyll
molecules are organized into clusters (with
other pigments and proteins) called
photosystems
• A photosystems consists of a number of lightharvesting pigments bound to proteins
surrounded by a reaction center complex
How do photosystems capture
solar energy?
• The pigments absorb light energy and pass the
electrons (energy) from molecule to molecule
until it reaches the reaction center
• The reaction center complex contains a pair
of chlorophyll a molecules and an electron
acceptor
• There are 2 photosystems: Photosystem II and
Photosystem I
Photosystem
Photon
Light-harvesting
complexes
Reaction
center complex
Thylakoid membrane
Primary electron
acceptor
e–
Transfer
of energy
Pair of
Chlorophyll a molecules
Pigment
molecules
Photosystems
• So how does this work?
• When light is absorbed by the pigments,
energy passes from pigment to pigment
molecules until it reaches the reaction center
of the photosystem where it excites an
electron of chlorophyll to a higher energy
state that is captured by the primary electron
acceptor
Photosystems
• Water is split, supplying its electrons to the
chlorophyll molecule which lost its electrons
to the primary electron acceptor, releasing O2
as a by-product
• Photosystems I and II were named in order of
their discovery, but Photosystem II functions
first in the sequence of steps that make up the
light reactions
A mechanical analogy of the light
reactions
e–
ATP
e–
e–
e–
e–
e–
NADPH
Mill
makes
ATP
e–
Photosystem II
Photosystem I
Photosynthesis: a review
• Light reactions occur in the thylakoid
membrane
– 2 photosystems capture solar energy which
energizes electrons
– Photosystems transfer these excited electrons
through electron transport chains which produces
ATP and NADPH
– Water is split and O2 is released
Photosynthesis: a review
• In the stroma of the chloroplast, sugars are
produced via the Calvin Cycle (lightindependent reactions)
• CO2 is reduced to form glucose
Photosynthesis: a review
• The sugar produced by plants during
photosynthesis provides the starting materials
to make structural components such as
cellulose
• 50% of this sugar
goes toward cellular
respiration (plants
respire!)
Photosynthesis: a review
• Most plants make considerably more food
each day than they need
• They stockpile this sugar as _____? storing it
in roots, tubers, and fruits (sound familiar?)
• Plants not only produce fuel for themselves,
but ultimately provide food for virtually all
other organisms (heterotrophs)
Global warming and the
greenhouse effect
Global warming and the
greenhouse effect
• CO2 is an important greenhouse gas
• Greenhouse gases are gases in the
atmosphere that absorb heat radiation
• When solar radiation reaches the atmosphere,
visible light passes is absorbed by the planet’s
surface warming it
Global warming and the
greenhouse effect
• This heat is radiated back as longer, infrared
wavelengths, which are absorbed by gases in
the atmosphere reflecting some of the heat
back to earth
• Very important! Without these greenhouse
gases, the Earth’s surface temperature would
be ~33 ̊C (59 ̊F) cooler than at present
• This process is called the greenhouse effect
Global warming and the
greenhouse effect
• CO2, water vapor, and methane are naturallyoccurring greenhouse gases
• Photosynthetic organisms absorb billions of
tons of CO2 every year!
• Most of this carbon returns to the atmosphere
via cellular respiration, but much of it remains
locked in large tracts of forests and in longterm storage as fossil fuels buried deep under
the Earth’s surface
Global warming
• Since 1850 (the start of the Industrial
Revolution), the concentration of CO2 in the
atmosphere has increased ~40%
• Increasing concentrations of CO has been
linked to global warming, a steady rise in
Earth’s surface temperature
• Earth’s average surface temperature has
increased by 1.2-1.4 ̊F in the last 100 years!
Annual Average Global Surface
Temperature Anomalies 1880-2006
400
2000
1800
Methane (CH4)
350
1600
Nitrous Oxide (N2O)
1400
1200
300
1000
800
250
600
0
500
1000
Year
1500
2000
CH4 (ppb)
CO2 (ppm), N2O (ppb)
Carbon Dioxide (CO2)
Concentration of CO2 1958-2006
Global warming
• Rapid warming is
changing the global
climate
• Sea level rise,
melting ice &
permafrost,
extreme weather
events, changing
weather patterns
Global climate change
• Global climate change affects biomes,
ecosystems, communities and populations
• The earlier arrival of warm temperatures in
the spring is disrupting ecological
communities
– What would happen if plants bloomed before
their pollinators arrived?
– What will happen to coral under increasing
temperature?