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6
Photosynthesis and Respiration
Chemical Energy
• Energy is stored in chemical bonds.
• Chemical energy available to do work is
termed free energy (G).
• Adenosine triphosphate (ATP) is a kind
of “energy currency” in cells.
• When ATP is hydrolyzed, free energy is
released to drive endergonic reactions.
Redox Reactions
• Energy can also be transferred by the
transfer of electrons in oxidation–
reduction, or redox reactions.
• Reduction is the gain of one or more
electrons.
• • Oxidation is the loss of one or more
electrons.
Redox Reactions
• Transfers of hydrogen atoms involve
transfers of electrons (H = H+ + e–).
• When a molecule loses a hydrogen
atom, it becomes oxidized.
• The more reduced a molecule is, the
more energy is stored in its bonds.
Coenzyme NAD+
• Coenzyme NAD+ is a key electron
carrier in redox reactions.
•
NAD+ (oxidized form)
•
NADH (reduced form)
Oxidative Phosphorylation
• In cells, energy is released in catabolism
by oxidation and trapped by reduction of
coenzymes such as NADH.
• • Energy for anabolic processes is
supplied by ATP.
• Oxidative phosphorylation transfers
energy from NADH to ATP.
Chemiosmosis
• The coupling is chemiosmosis—
diffusion of protons across a membrane,
which drives the synthesis of ATP.
• Chemiosmosis converts potential energy
of a proton gradient across a membrane
into the chemical energy in ATP.
Chemiosmosis
• ATP synthase—membrane protein with
two subunits:
•
F0 is the H+ channel; potential energy
of the proton gradient drives the H+
through.
•
F1 has active sites for ATP synthesis.
Cellular Respiration
• Cellular Respiration
• A lot of energy is released when reduced molecules with many
C—C and C—H bonds are fully oxidized to CO2.
• 3 main steps:
•
1. glycolysis (Anaerobic- in cytoplasm)
•
•
2. pyruvate oxidation (Aerobicmitochondria)
3. citric acid cycle (Aerobic- mitochondria)
Step 1: Glycolysis
• Takes place in the cytosol.
• Begins with glucose and 2 ATP.
• Final products:
•
2 molecules of pyruvate (pyruvic
acid)
•
2 molecules of ATP (net)
•
2 molecules of NADH
Step 2: Pyruvate Oxidation
• Products: CO2 and acetate; acetate is
then bound to coenzyme A (CoA)
Step 3: Citric Acid Cycle
• Starts with Acetyl CoA; acetyl group is
oxidized to two CO2.
• Oxaloacetate is regenerated in the last
step.
Electron Transport Chain- ATP!!
• NADH and FADH2 are oxidized and O2
is reduced to H2O in a series of steps.
• Series of redox carrier proteins
embedded in the inner mitochondrial
membrane.
Chemiosmosis
• The oxidation reactions are exergonic;
the energy is used to actively transport
H+ ions out of the mitochondrial matrix,
setting up a proton gradient.
• ATP synthase in the membrane uses the
H+ gradient to synthesize ATP by
chemiosmosis.
Aerobic Respiration
• About 32 molecules of ATP are produced
for each fully oxidized glucose.
• The role of O2: most of the ATP produced
is formed by oxidative phosphorylation,
which is due to the reoxidation of NADH.
Anaerobic Respiration
• Under anaerobic conditions, NADH is
reoxidized by fermentation.
• The overall yield of ATP is only two—the
ATP made in glycolysis.
• 1. Lactic acid fermentation:
•
NADH is used to reduce pyruvate to
lactic acid, thus regenerating NAD+.
Anaerobic Respiration
• 2. Alcoholic fermentation:
• Pyruvate is converted to acetaldehyde,
and CO2 is released. NADH is used to
reduce acetaldehyde to ethanol,
regenerating NAD+ for glycolysis.
Photosynthesis
• Light reactions convert light energy and
water into chemical energy (in ATP and
the reduced electron carrier NADPH)
and oxygen.
• Carbon-fixation reactions use the ATP
and NADPH, along with CO2, to produce
glucose.
Light (Dependent) Reactions
• Light is a form of electromagnetic
radiation, which travels as a wave but
also behaves as particles (photons).
• Photons can be absorbed by a molecule,
adding energy to the molecule—it moves
to an excited state.
Pigments
• Pigments: molecules that absorb
wavelengths in the visible spectrum.
• Chlorophyll absorbs blue and red light;
the remaining light is mostly green.
• In plants, two chlorophylls absorb light
energy chlorophyll a and chlorophyll
b.
Chlorophyll
• When chlorophyll (Chl) absorbs light, it
enters an excited state (Chl*), then
rapidly returns to ground state, releasing
an excited electron.
• Chl* gives the excited electron to an
acceptor and becomes oxidized to Chl+.
• The acceptor molecule is reduced.
Electron Transport Chain (yes
also in photosynthesis)
• The electron acceptor is first in an
electron transport system in the thylakoid
membrane.
• Final electron acceptor is NADP+, which
gets reduced:
• ATP is produced chemiosmotically
during electron transport
(photophosphorylation).
Light Reactions
• Photosystem I absorbs light energy at
700 nm, passes an excited electron to
NADP+, reducing it to NADPH.
• • Photosystem II absorbs light energy at
680 nm, produces ATP, and oxidizes
water molecules.
Cyclic Electron Transport
• ATP is needed.
• Cyclic electron transport uses only
photosystem I and produces ATP; an
electron is passed from an excited
chlorophyll and recycles back to the
same chlorophyll.
Carbon Fixation Reactions
(Calvin Cycle)
• The Calvin cycle: CO2 fixation. It occurs
in the stroma of the chloroplast.