Download Ch. 6 Bio PP

CHAPTER 6
PHOTOSYNTHESIS
OBJECTIVES
1. Explain how the structure of a chloroplast relates to
its function
2. Describe the job of pigments in photosynthesis
3. Summarize the events of the light reactions
4. Explain how ATP is made during the light reactions
5. Summarize the events of the dark reactions (Calvin
cycle)
6. Explain how some plants use alternative ways to fix
carbon (CAM and C4)
7. Know the chemical equation for photosynthesis
ENERGY FOR LIFE
PROCESSES
PHOTOSYNTHESIS- process by which light energy
is transferred to chemical energy
RECALL:
Autotrophs- organisms that manufacture their own
food>> PLANTS
- most use photosynthesis
Heterotrophs- cannot make their own food
- must eat autotrophs or other heterotrophs
Ex: A caterpillar (heterotroph) eats grass
(autotroph); a bird (heterotroph) eats the
caterpillar
- ALL LIFE DEPENDS ON AUTOTROPHS
Photosynthesis is an example of a BIOCHEMICAL
PATHWAY- a complex set of reactions in which
the product of one reaction is consumed (used)
in the next reaction
- Autotrophs use photosynthesis to
manufacture organic compounds from
CO2 and water
- during this reaction, O2 is released
While only autotrophs perform
photosynthesis, both autotrophs and
heterotrophs perform a process called
CELLULAR RESPIRATION
- in cellular respiration, organic
compounds are combined with O2 to
produce ATP
- CO2 and water are given off as wastes
- the products of photosynthesis
(organic compounds and O2) are
reactants in cellular respiration
LIGHT ABSORPTION AND
CHLOROPLASTS
LIGHT REACTIONS- initial reactions in photosynthesis
- begin with light absorption in chloroplasts
- a cell may have 50 or more chloroplasts
CHLOROPLAST STRUCTURE
2 Membranes: outer and inner
- inside the inner membrane is another system of
membranes, arranged in flattened sacs called
THYLAKOIDS
- thylakoids are interconnected
- GRANA- stacks of thylakoids
- STROMA- solution surrounding
thylakoids
DRAW A CHLOROPLAST!!!
LIGHT
Light from the sun appears white, but is made
up of a variety of colors
- you can separate light by passing it
through a prism
- the resulting colors ranging from red at one
end to violet at the other is called the
VISIBLE SPECTRUM
Light travels though space as waves of energy,
similar to waves that travel through a body of
water
- waves are measured in WAVELENGTHS- the
distance beween crests in a wave
- different colors in the spectrum have
different wavelengths
PIGMENT- a compound that absorbs light
- most pigments absorb some colors more strongly
than others
- when pigments absorb colors, they subtract
those colors from the visible spectrum
- the light that is reflected or transmitted by the
pigment no longer appears white
Ex: A shirt reflects or transmits the color yellow and
absorbs all other colors
WHAT COLOR IS THE SHIRT?
YELLOW, of course
visible spectrum
INTRO. TO PHOTOSYNTHESIS
Photosynthesis can be divided into 3 sets of
reactions:
1. Light absorption by chlorophyll
2. Light reactions (light-depended
reactions)
3. Dark reactions (light-independent
reactions)
PIGMENTS IN CHLOROPLASTS
The most important pigments in chloroplasts are
called CHLOROPHYLLS
- the most common types are chlorophyll a
and chlorophyll b
- these 2 types absorb different colors of light
CHLOROPHYLL a- absorbs less blue light but
more red light
CHLOROPHYLL b- absorbs more blue light but less
red light
- neither absorbs much green light; green
light is reflected or transmitted
- this is why most leaves are green
ONLY chlorophyll a is directly involved in the
light reactions
- Chlorophyll b is called an ACCESSORY
PIGMENT because it assists
chlorophyll a
Other accessory pigments are yellow, orange, and
blue CAROTENOIDS
- carotenoids absorb colors that chlorophyll a cannot,
and enables plants to capture more of the energy
in light
- during the fall, plants lose their chlorophylls
and take on the colors of their carotenoids,
which is why the leaves change colors
LIGHT REACTIONS/LIGHT
DEPENDENT REACTIONS & LIGHT
ABSORPTION BY CHLOROPHYLL
PHOTOSYSTEM- clusters of pigment molecules in
the thylakoid membrane
- there are 2 types: photosystem I and
photosystem II
Light reactions begin when accessory pigment
molecules in both photosystems absorb light
- the molecules acquire the energy that was
carried by the light waves
- the energy is passed to the other pigments until it
reaches a certain pair of chlorophyll a molecules
called the REACTION CENTER
STEPS OF PHOTOSYSTEM II REACTIONS
- photosystem II replaces electrons lost in
photosystem I
1. Light energy forces electrons to enter a higher
energy level in the reaction center (2 chlorophyll
a molecules)
- these energized electrons are said to be
“excited”
2. The electrons now have enough energy to leave
the reaction center
- since the reaction center has lost the
electrons, another substance must pick up
those electrons
3. Electrons are picked up a PRIMARY ELECTRON
ACCEPTOR
4. The primary electron acceptor gives the electrons
to a series of molecules in the thylakoid
membrane
- the series of molecules is called the
ELECTRON TRANSPORT CHAIN
***NOTE: Photosystem II loses electrons that MUST
BE REPLACED. The replacement comes from the
splitting of 2 water molecules.
(remember that water is a necessary ingredient for
photosynthesis)
***NOTE: When H2O is split, O2 is released
(remember that O2 is a product of
photosynthesis)
STEPS OF PHOTOSYSTEM I REACTIONS
- remember, occurs at the same time as
photosystem II reactions
1. At the same time light is absorbed by
photosystem II, light is also absorbed by
photosystem I
- electrons move from the reaction center in
photosystem I to a primary electron
acceptor
- the electrons that are lost by the reaction center
are replaced by the electrons that have gone
through the electron transport chain in
photosystem II
2. The primary electron acceptor of photosystem I
donates electrons to a different electron
transport chain
- the chain brings the electrons to the part of
the thylakoid membrane that faces the
stroma
- there electrons combine with a proton and
NADP+, another electron acceptor
- this causes NADP+ to be reduced to NADPH
THIS COMPOUND WILL BE USED IN THE DARK
REACTIONS
CHEMIOSMOSIS
CHEMIOSMOSIS- process by which chemicals pass
through a membrane resulting in ATP formation
STEPS IN CHEMIOSMOSIS:
1. As electrons are moved along the electron
transport chain, hydrogen ions (from the
splitting of H2O) are built up on one side of the
thylakoid membrane as the electrons are moved
to the other side of the membrane
- this is like a battery, with positive charges on one side
of the membrane and negative charges on the other
side of the membrane
- this creates a large amount of potential energy
(like a battery has a large amount of potential
energy)
2. As the H+ ions build up, they become higher in
concentration
- remember, electrons are in high concentrations
on the other side of the membrane
3. When the H+ concentration is at its highest, a
special protein called ATP synthase opens and
the H+ ions diffuse through, being pulled by the
very negative charge on the other side
4. This releases enough energy to make ATP from
ADP
- this ATP will also be used in the dark
reactions
DARK REACTIONS/LIGHT
INDEPENDENT REACTIONS
1. The dark reactions do not need light to occur,
however, they need the products of the light
reactions (ATP and NADPH)
- the light reactions must occur BEFORE the
dark reactions
2. The dark reactions occur in the STROMA
- What is the stroma again?
- solution inside chloroplast surrounding
thylakoid membrane
3. The dark reactions “fix” or put the carbon
from CO2 into organic carbohydrate
molecules
4. Carbon fixation is by the Calvin Cycle (dark
reactions; also called C3 cycle)
- the Calvin Cycle must make 6 complete
turns to produce a molecule of glucose
- named after American scientist Melvin Calvin
STEPS OF CALVIN CYCLE
STEP 1
CO2 diffuses into the stroma and it is bonded to a 5carbon compound called RuBP to form a 6-carbon
compound. In 3 turns, 3 CO2 molecules bind to 3
RuBP molecules.
STEP 2
The 6-carbon compounds immediately split to form 2
molecules of a compound called PGA. There is a total
of 6 PGAs formed here (2 for each of the 3 turns)
- PGA is a 3-carbon compound, hence the name C3
pathway
STEP 3
Next these PGA compounds are converted to
PGAL compounds. In order to perform this step
the ATP and NADPH from the light reactions are
needed.
- this step results in ADP (which can go back to the
light reactions to pick up more P and make ATP)
and NADP (which can also go back to light
reactions to pick up more H to make NADPH)
STEP 4
The remaining PGAL can do one of two things:
- 6 PGAL are used to make more RuBP to
keep the dark reactions going
- 2 PGAL can join together to make a glucose
molecule; glucose is then used by the plant to
make other carbohydrates (1 PGAL for every 3
turns)
Glucose is not the only organic compound formed
- some plants use PGAL to form a variety of
organic compounds such as amino acids,
lipids, and carbohydrates
DRAW THE CALVIN CYCLE
OTHER CARBON-FIXING
PATHWAYS
- The Calvin cycle is the most common method of
carbon fixation
- Some plant species living in hot, dry
climates have evolved different biochemical
pathways for carbon fixation
- Most water loss in plants occurs through small
pores called STOMATA, found on the underside of
leaves
- In hot, dry conditions, plants can partially close
their stomata and reduce loss of water
- However, when stomata are closed, CO2
cannot enter the plant and O2 cannot leave
the plant
- These conditions cause the Calvin Cycle to not
operate properly
- Plants need other methods of carbon
fixation
C4 PATHWAY
In the C4 pathway, CO2 is bound to an intermediate
compound before it enters the Calvin Cycle
- This produces a 4-carbon compound (hence
the name C4)
- Plants that use this pathway are called C4 plants
- The 4-carbon compound then undergoes a series
of reactions until the CO2 is finally transferred to
the RuBP
- It then enters the Calvin Cycle
ADVANTAGES OF C4:
- Allows plants to photosynthesize in very hot
and dry climates
- Allows plants to fix carbon 4 times faster and thus
grow faster in hot dry conditions
DISADVANTAGES OF C4:
- Requires more energy, but these plants grow
where there is always abundant sunlight
Ex: corn, sugarcane, crabgrass
CAM PATHWAY
In the CAM pathway, CO2 is brought in at night,
when conditions are cooler
- used by plants that live in very dry
conditions
- stomata are closed during the day to prevent
water loss
- the CO2 taken in at night is stored in a compound
called Crassulacean acid (giving it the name
Crassulacean Acid Metabolism)
- the acid then releases the CO2 into the stroma of
the leaves during the day where it then bonds
with RuBP and enters the Calvin Cycle.
ADVANTAGES OF CAM:
- growth without water loss
DISADVANTAGES OF CAM
- also uses extra energy; only helps plants in
dry climates
Ex: cactuses, pineapples
SUMMARY OF
PHOTOSYNTHESIS
Balanced equation for photosynthesis:
6 CO2 + 6 H2O + light energy  C6H12O6 + 6 O2
- the 6 CO2 are fixed during the dark reactions into
the resulting carbohydrates
- the 6 H2O are split in the light reactions
(remember, H+ are picked up by NADP and
the electrons restore photosystem I)
- the 6 O2 are released when water is split
- the C6H12O6 is a product, which can be used
directly by the plant or used to make more
advanced carbohydrates
THE END