Download Photosynthesis - kestrelteambiology

Capturing Solar
Energy:
Photosynthesis
(a)
(b) internal leaf structure
mesophyll
cells
(c) chloroplast in
mesophyll cell
outer membrane
inner membrane
thylakoid
stroma
vein
channel
interconnecting
thylakoids
stoma
chloroplasts
Photosynthesis

Complex series of chemical
reactions involving a transition in
forms of energy
Photosynthesis



Uses light energy to make food
process by which some organisms can
make organic compounds from
inorganic compounds
uses energy from the sun
Photosynthesis

Light energy captured and stored as
chemical potential energy in the
covalent bonds of carbohydrate
molecules
6 CO2 + 6 H2O + light  C6H12O6 + 6 O2
Life depends on
photosynthesis



A. Foundation of energy for most
ecosystems
B. Source of oxygen
C. Key component of the carbon
cycle
The mechanism of
photosynthesis


Solar energy and light
The electromagnetic spectrum

Pigment molecules absorb some
wavelengths of light and reflect others
•
Chlorophyll—a green photosynthetic
pigment associated with the thylakoid
membranes of chloroplasts
(a)
(b)
chlorophyll
carotenoids
phycocyanin
(c)
The mechanism of
photosynthesis

Chloroplasts are the sites of photosynthesis


Have a membrane system within internal space
(stroma)
Arranged in disk-shaped sacks (thylakoids)


The thylakoids contain light-harvesting
photosynthetic pigments & enzymes
Internal membranes define space (lumen) that is
separate from the rest of the stroma
chloroplast
thylakoids
reaction
center
electron transport system
light- harvesting
complex
The mechanism of
photosynthesis
Photosynthesis occurs in two steps
1. Light-dependent reactions
•
a. Provides the energy necessary to fix carbon
•
b. Occurs in the thylakoid membranes
•
c. Generates ATP
•
d. Photolysis—light, electrons and water
7
8
electron transport
system
3
6
5
reaction
center
2
4
energy to drive
1
synthesis
reaction
center
photosystem I
9
photosystem II
ELECTRON TRANSPORT STEPS
Step 1: Light energy excites electrons in chlorophyll “a” molecules of photosystem 2 (ps)
Step 2: These electrons move to a primary electron acceptor.
Step 3: Electrons are transferred along an electron transport chain in thylakoid membrane,
losing energy as they move.
Step 4: Light excites electrons in chlorophyll “a” molecules in photosystem 1. As these
electrons move to another primary electron acceptor, they are replaced by electrons from
photosystem 2.
Step 5: The electrons from photsystem 1 are transferred along a second electron transport
chain. At the end of this chain they combine with NADP and H to make NADPH
FYI: PHOTOSYSTEM 1 WAS DISCOVERED BEFORE 2, BUT IT GOES AFTER 2 IN THIS
PROCESS!
Light-Dependent Reactions

What happens during light reactions?

Chlorophyll in plants (Photosystem I [PS I])

Absorbs blue & red light and reflects back the green light



Blue & red light energy boosts electrons to higher energy
levels
Electrons are passed to electron transport molecule in the
thylakoid
Can only be passed when highly energized
Light-Dependent Reactions

What happens during light reactions?

During transport of electrons from PS II to
PS I


Some energy is harnessed to produce ATP
Eventually, chlorophyll from PS II is
oxidized, in other words:

Gets replacement electrons from water.
Light-Dependent Reactions

Energy of light has thus been captured
in two forms:


The synthesis of NADPH from NADP+
Proton gradient across the thylakoid
membrane


Cannot be used directly to make food
Must first be converted to ATP by chloroplast ATP
synthase
The mechanism of
photosynthesis
Energy carriers ATP and NADPH
transport energy from the light
reactions to the dark reactions.
energy from
sunlight
Light-dependent
reactions occur
in thylakoids.
Light-independent
reactions (C3cycle)
occur in stroma.
The mechanism of
photosynthesis
2. Light-independent reactions
a. Uses energy of the light-dependent
reaction to make sugar from CO2
b. Occurs in the stroma
Light-Independent Reactions

Steps in Light-Independent Reactions:


CO2 joins with RuBP forming an unstable 6-C
molecule
Breaks into two 3-C PGA molecules


This first step in Calvin cycle is catalyzed by enzyme
Called ribulose biphosphate carboxylase (Rubisco)
1
3
Carbon fixation
combines CO2
with RuBP.
RuBP
regeneration
uses energy
and 10 G3P.
2
G3P synthesis
uses energy.
2 G3P available
for synthesis of
organic molecules.
Keep track of the carbons as you follow the illustration around.
Six molecules of RuBP ( react with 6 molecules of CO2 and 6
molecules of H2O to form 12 molecules of PGA.
The energy of 12 ATPs and the electrons and hydrogens of 12
NADPHs are used to convert the 12 PGA molecules to 12
G3Ps. (G3P: glyceraldehyde 3-phosphate)
Energy from 6 ATPs is used to rearrange 10 G3Ps into 6
RuBPs, completing one turn of the C3 cycle.
The remaining 2 G3P molecules are further processed into
glucose.
Photosynthesis
Summary
•Light Dependent Reaction
–Light + chlorophyll --> ATP + NADPH + (O2
as waste)
•Light Independent Reaction (Calvin Cycle)
–CO2 + ATP + NADPH --> glucose
Alternative pathways in Photosynthesis
Calvin cycle is most common pathway for carbon fixation
Plant species that fix carbon this way are known as C3 plants.
Other methods of carbon fixation
-Generally found in plants that evolved in hot dry climates.
-water loss occurs when stomata open for gas exchange.
- Closed stomata means LOW CO2 and high O2. This inhibits
carbon fixation.
C4 Pathway
- enables plants to fix carbon into a four carbon compound.
- C4 plants have an enzyme that can fix carbon even when )2 is
high and CO2 is low.
- species include corn, sugar cane, and crab grass.
C4 Pathway
- enables plants to fix carbon into a four carbon compound.
- C4 plants have an enzyme that can fix carbon even when O2 is
high and CO2 is low.
- species include corn, sugar cane, and crab grass.
CAM pathway
- CAM plants open stomata at night and close them during the
day - Opposite of what other plants do).
- CO2 is fixed at night into organic compounds. During the day
CO2 is released from these compounds and enters the Calvin cycle.
- grow more slowly than other plants due to stomata being open
at night (temps are lower).
- species include cactuses and pineapples.
CAM pathway
- CAM plants open stomata at night and close them during
the day
-Opposite of what other plants do).
- CO2 is fixed at night into organic compounds. During the
day CO2 is released from these compounds and enters
the Calvin cycle.
- grow more slowly than other plants due to stomata being
open at night (temps are lower).
- species include cactuses and pineapples.