Lecture 4:
PHOTOSYNTHESIS
Life’s grand device
By
Edgar Moctezuma
TODAY…
•
Photosynthesis
I.
II.
III.
IV.
V.
VI.
•
Intro
Properties of light and pigments
Chloroplast structure and function
Light reactions
“Dark” or Carbon reactions
Summary and conclusions
Respiration
I. Energy and food chains
II. Carbon Cycle
I. Introduction to photosynthesis
• From the Greek
PHOTO = produced by light
SYNTHESIS = a whole made of parts put
together.
Definition: PHOTOSYNTHESIS is the process
whereby plants, algae, some bacteria,
use the energy of the sun to synthesize organic
compounds (sugars) from inorganic compounds
(CO2 and water).
WHY IS PHOTOSYNTHESIS SO
IMPORTANT?
PHOTOSYNTHESIS is one of the most
important biological process on earth!
• Provides the oxygen we breathe
• Consumes much of the CO2
• Food
• Energy
• Fibers and materials
GENERAL FORMULA FOR
PHOTOSYNTHESIS
light
*
*
6 CO2 + 12 H2O ---------> C6H12O6 + 6 O2 + 6 H2O
pigments, enzymes
• Oxygen on earth allowed for the evolution of aerobic
respiration and higher life-forms.
• Respiration: extracting energy from compounds (sugars)
C6H12O6 + O2  6 CO2 + ATP
II.
PROPERTIES OF LIGHT
Virtually all life depends on it!
• Light moves in waves, in energy units
called PHOTONS
• Energy of a PHOTON inversely proportional
to its wavelength
• Visible light (between UV and IR) occurs in
a spectrum of colors
Visible light contains just the right amount of
energy for biological reactions
Light is absorbed by pigments
• The primary pigment for photosynthesis is
chlorophyll a
• It absorbs blue and red light, not green (green
light is reflected back!)
Absorption spectrum
of chlorophyll a
• Absorption spectrum of chlorophyll a:
BLUE & RED
• Action spectrum of photosynthesis closely
matches absorption spectrum of
chlorophyll a, but not perfectly (due to
accessory pigments)
Accessory pigments like chlorophyll b and
carotenoids (beta-carotene, lycopene):
• absorb light at different wavelengths,
(extending the absorption range)
• help transfer some energy to chlorophyll a
• protect cell from harmful byproducts
Chlorophyll a is the primary
photosynthetic pigment that drives
photosynthesis.
Accessory pigments absorb at
different wavelengths,
extending the range of light
useful for photosynthesis.
Where does photosynthesis occur?
The plant cell
III. Chloroplast structure and
function: solar chemical factory
Chloroplast structure
•
•
•
•
Football shaped
Double membrane
Stroma
Thylakoid
membrane
• Grana (stacks)
• Lumen
(inside thylakoid)
stroma
Grana
lumen
thylakoids
Inside a Chloroplast
• Remember: Structure correlates to function!
Overview of photosynthesis:
Note: The Light and “Dark”or Carbon
reactions happen at different sites in the
chloroplast
H2O
light
CO2
LIGHT
REACTIONS
(Thylakoids)
ATP
“DARK” or CARBON
REACTIONS
NADPH
(Stroma)
(ENERGY)
O2
C6H12O6
(OXYGEN GAS)
(GLUCOSE)
IV. The Light Reactions
1. Light dependent
2. Occur in the thylakoid membrane of
chloroplast
3. Water is split into oxygen gas (O2) and H+
4. Use light energy (photons) to generate two
chemical energy compounds: ATP &
NADPH
Chemical energy compounds
made in the light reactions
ADP + Pi + Energy  ATP
adenosine
diphosphate
inorganic
phosphate
adenosine
triphosphate
NADP+ + 2e- + H+  NADPH
Nicotinamide adenin dinucleotide phosphate
Sequence of events in the Light Reactions
STROMA
NADP+ + H+
NADPH
ADP + Pi
ATP
PS II
e-
PS I
ATPS
H+
2 H2O
O2 + 4 H+
(gas) (protons)
LUMEN (inside thylakoid)
Summary of the Light reactions
2 H2O + 2 NADP+ + 3 ADP + 3 Pi
O2 + 2 NADPH + 3 ATP + 4 e- + 2 H+
(gas)
Light reactions: Chemical energy compounds
are made from light energy, water is split into
O2 and protons
V. The“Dark” or Carbon Reactions
1. Light independent (can occur in light or dark;
some enzymes require activation by light)
2. Occur in the stroma of chloroplasts
3. Use the chemical energy produced in Light
Reactions (ATP; NADPH) to reduce CO2 to
carbohydrate (sugar).
4. CO2 is converted to sugar by entering the
Calvin Cycle
• Named for M. Calvin
The Calvin Cycle
RuBP
ADP
CO2
Ribulose bisphosphate
rubisco
• 3 phases, 13 steps
• CO2 goes 6 cycles
to produce 1 glucose
ATP
carboxylation
regeneration
reduction
3-PGA
3-phosphoglycerate
ATP
ADP
GAP
NADPH
Glyceraldehyde 3-phos.
sugars
NADP+
Pi
The Calvin Cycle
• CO2 enters the Calvin Cycle
• First product is a 3-carbon molecule: 3-PGA
(phosphoglyceric acid). That’s why it’s also
called C-3 cycle.
• Enzyme RUBISCO (ribulose bisphosphate
carboxylase/oxygenase) is the main enzyme that
catalyzes the first reactions of the Calvin Cycle.
• RUBISCO: Is the most abundant protein on
earth!
Most plants use the Calvin Cycle to
Convert CO2 into sugars.
These plants are called C-3 plants
Summary of Carbon Reactions
6 CO2 + 18 ATP + 12 NADPH + 12 H2O
C6H12O6 + 18 ADP + 18 Pi + 12 NADP+
glucose
+ 6 H2O + 6 O2
Carbon reactions: Use CO2 and chemical energy (ATP &
NADPH) to produce sugars by means of the Calvin Cycle
Limitations on Photosynthesis
• Photosynthesis is not perfect in C-3 plants,
it is only 1 - 4 % efficient
• Low efficiency due to photorespiration
• Photorespiration occurs when internal CO2
concentration becomes too low (drought);
rubisco begins fixing oxygen.
C-4 plants are more efficient
• C-4 plants first product is a 4-carbon molecule
• The C-4 plants (sugar cane, corn, etc.), are more
efficient than C-3 plants – they grow in
hotter climates with more light.
• For example, sugar cane’s
photosynthetic efficiency is
7%
• C-4 plants have a different
leaf anatomy
C-3 vs. C-4 leaf anatomy
Net venation
Parallel venation
VI. Summary of Photosynthesis:
1. Light energy absorbed by chlorophyll a
drives the reactions of photosynthesis.
2. Converts light energy into chemical
energy to make organic compounds.
3. CO2 and H2O used to produce
C6H12O6 (glucose) and O2 (gas).
4. Light Reactions occur in thylakoids of the
chloroplasts; ATP and NADPH are formed;
water is split to O2 (gas) and protons.
5. Carbon Reactions occur in stroma – Calvin
Cycle fixes CO2 to produce C6H12O6
(glucose).
6. Low efficiency, about 1- 4% in C-3 plants.
7. Nevertheless, PHOTOSYNTHESIS is still the
most important biological process on earth!
Importance of photosynthesis and the
impact that it has in all our lives.
Without photosynthesis, virtually all
plants and animals would become
extinct.
Respiration, Energy & Carbon Cycle
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Energy
Virtually all organisms require energy of food for:
Making chemicals
(proteins, carbs, etc.)
Movement
Cell division
Heat, electricity and light production
The way living organisms obtain energy is through
Cell respiration
RESPIRATION
• Process of making energy of food available in
the cell…
• Involves breaking down
• Complicated molecules  into simple molecules
(C6H12O6, sugars)
(CO2, water)
RESPIRATION
The energy held by complicated molecules is held
temporarily as ATP (energy currency)
C6H12O6 + 6 O2  6CO2 + 6 H2O + 36 ATP
(glucose)
(energy)
Respiration occurs mainly in
Mitochondria and Cytoplasm
Stages of Respiration
Cellular Respiration has three main stages:
• Glycolysis
• Krebs Cycle
• Electron transport system
3 Stages of cellular respiration
• Glycolysis: Splitting of glucose – 2 net ATP generated
• Krebs Cycle: Energy of glucose molecule is harvested
as ATP (2) – it occurs in the mitochondria (matrix)
• Electron Transport System: also happens in the
mitochondria, more ATP are generated (32).
• For each glucose molecule, total ATP = 36
• Only 39% efficient, rest is lost as heat.
Chapter 4: Table 4.1, p. 63
• Reaction:
Photosynthesis
Respiration
CO2+H2O+sunC6H12O6+O2+H2O
C6H12O6+O2CO2+H2O+36ATP
• Reactants: Carbon dioxide, water, sun
Glucose, oxygen
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Energy
Products: Glucose
By-products: Oxygen
Cellular location: Chloroplasts
Energetics: Requires energy
• Chemical paths: Light reactions &
Calvin cycle
• Summary: Sugar synthesized using
energy from the sun
Carbon dioxide, water
Cytoplasm, mitochondria
Releases energy
Glycolysis, Krebs cycle
& Electron Transport Syst.
Energy released from
sugar breakdown
Photosynthesis and respiration
• Photosynthesis and respiration are
complimentary reactions…
PHOTOSYNTHESIS
CO2 + H2O  O2 + SUGARS
O2
H2O
RESPIRATION
SUGARS + O2  H2O + CO2
CO2
CO2
PLANTS,
ALGAE,
BACTERIA
Sunlight
energy
SUGARS
O2
MOST LIVING
ORGANISMS
USEFUL CHEMICAL
ENERGY (ATP)
H2O
ENERGY: ability to do work
Newton’s First Law of Thermodynamics:
(1701)
“Energy cannot be created or destroyed, it
can only be transformed from one form
to another”
• Once a cell has used energy to do work, it cannot
be used again by any organism.
ENERGY
ENERGY FLOW IS LINEAR
Sun  Earth Producers 1o consumers  2o consum
heat
resp, heat
resp, heat
resp, heat
Energy flows into ecosystem from the sun
Energy travels in a straight line by way of
food chains.
ENERGY
However, much energy is lost as heat along the
way – as a result of respiration.
Approximately 90% energy is lost on each step!
• Newton’s Second Law of
Thermodynamics:
“In any transfer of energy there is always a
loss of useful energy to the system, usually in
the form of heat”
Food Chains
• (Not referring to SHOPPERS, SAFEWAY or GIANT !!!)
• Food chains demonstrate linear nature of energy
• Producers are the base of the food chain, they
include photosynthetic organisms like:
• Plants
• Algae
• Certain bacteria
Food chains
• Primary consumers – all plant
eaters (herbivores).
• Secondary consumers –
Eat primary consumers,
(carnivores)
Food chains
• Decomposers – obtain energy by breaking
down remaining organic material of the
other members of the food chain.
• Fungi and bacteria.
Matter
• All important elements move in cycles;
Environment
Organisms
Cycles called biogeochemical cycles:
Water cycle
Carbon cycle
Nitrogen cycle
The Carbon Cycle
• Carbon from the atmosphere (CO2) enters the
biosphere by way of plants!
– CO2 used in photosynthesis
– Carbon moves into food chain
• Carbon is released to the physical environment by
respiration
– Release CO2 during respiration
– Amount CO2 fixed in photosynthesis = the amount
released by respiration
Carbon Cycle
• Carbon moves from atmosphere to plants to
animals and back to atmosphere.
“Look deep into nature,
and then you will
understand everything
better.”
Albert Einstein