Photosynthesis & Cellular Respiration. What is Photosynthesis?
Photosynthesis: Overview Chapter 8 -...
Transcript of Photosynthesis: Overview Chapter 8 -...
Chapter 8
Photosynthesis
Photosynthesis: Overview
• Photosynthesis converts solar energy into chemical energy
• Organisms that carry on photosynthesis are called autotrophs
• Heterotrophs are organisms that feed on other organisms
Light energy
PHOTOSYNTHESIS
6 CO2 6 + H2O
Carbon dioxide Water
C6H12O6 6 + O2
Glucose Oxygen gas
• Photosynthesis occurs in chloroplasts – In plants, photosynthesis occurs primarily in the
leaves, in the chloroplasts, which contain stroma, and stacks of thylakoids called grana
Leaf Cross Section
Leaf
Mesophyll Cell
Mesophyll
Vein Stoma
CO2 O2
Chloroplast
Chloroplast
Grana Stroma
TEM
9,7
50 ×
Stroma
Granum Thylakoid Thylakoid space
Outer membrane
Inner membrane
Intermembrane space
LM 2
,600
×
Grana
cuticle
stomata
stroma stroma
Leaf cross section upper epidermis
mesophyll
lower epidermis
leaf vein
CO2
O2
inner membrane outer membrane
Chloroplast
thylakoid space
thylakoid membrane
granum
channel between thylakoids
© Dr. George Chapman/Visuals Unlimited
Chloroplast, micrograph 37,000x
Overview of Photosynthesis
• Glucose and oxygen are the products of photosynthesis
• The oxygen given off comes from water • CO2 gains hydrogen atoms and becomes
a carbohydrate
CO2 + 6 H2O C6H12O6 + 6 O2
solar energy
• Photosynthesis is a redox process, as is cellular respiration
– In photosynthesis • H2O is oxidized and CO2 is reduced
Reduction
Oxidation
6 O2 6 H2O
Reduction
Oxidation
6 O2 6 CO2 + 6 H2O C6H12O6 +
C6H12O6 + 6 CO2 +
Photosynthesis
Cellular Respiration
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CH2O
H2O CO2
ADP + P NADP+
ATP
O2
thylakoid membrane
stroma
Calvin cycle
reactions Light reactions
solar energy
NADPH
• Photosynthesis consists of two sets of reactions – Light Reactions (light-dependent) in Thylakoid
Membrane – Calvin Cycle Reactions (light-independent) in
Stroma
– The Light Reactions • Convert light energy to chemical energy and produce
O2 – The Calvin Cycle assembles sugar molecules from CO2
• Using ATP and NADPH from the light reactions
Light
CO2 H2O Chloroplast
LIGHT REACTIONS (in thylakoids)
CALVIN CYCLE
(in stroma)
NADP+
ADP + P
ATP
NADPH
O Sugar
Solar Energy Capture
• Solar energy can be described in terms of its wavelength and its energy content
• White or visible light is only a small portion of the spectrum
Increasing wavelength
Increasing energy
X rays UV Infrared
visible light
500 600 750
Gamma rays
Micro- waves
Radio waves
Wavelengths (nm)
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380
Solar Energy Capture
• The photosynthetic pigments in chlorophylls a and b and the carotenoids can absorb specific portions of visible light
• Green light is reflected and only minimally absorbed Wavelengths (nm)
380 500 600 750
Chlorophyll a Chlorophyll b carotenoids
Rel
ativ
e A
bsor
ptio
n
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Ene
rgy
of e
lect
ron
Photon
Excited state
Heat
Photon (fluorescence)
Ground state
Chlorophyll molecule
e–
Photosystem
Light-harvesting complexes
Reaction center
Primary electron acceptor
e–
To electron transport chain
Pigment molecules
Chlorophyll a molecule Transfer of energy
Photon
Thyl
akoi
d m
embr
ane
Photosystems capture solar power • Thylakoid membranes contain multiple photosystems
– That absorb light energy, which excites electrons
– The excited electrons • Are passed from the primary electron acceptor to
electron transport chains
Thylakoid space
Photon
Stroma Th
ylak
oid
mem
bran
e 1
Photosystem II
e–
P680
2
H2O 1 2
+ 2 O2 H+
3 ATP Electron transport chain
Provides energy for synthesis of by chemiosmosis
4
Photosystem I
Photon
P700
e–
5
+ NADP+ H+ NADPH
6
Chloroplast
Stroma (low H+ concentration)
Light Light
NADP+ + H+ NADPH
H+
H+
H+ H+
ATP P ADP +
Thylakoid membrane
H2O 1 2
O2 2 H+ H+ H+
H+ H+ H+
H+
H+ H+
H+
H+
Photosystem II Electron transport chain
Photosystem I ATP synthase
Thylakoid space (high H+ concentration)
+
– The diffusion of H+ back across the membrane through ATP synthase • Powers the phosphorylation of ADP to produce ATP
(photophosphorylation) photosystem II
Stroma
Pq
H+
A T P synthase complex
chemiosmosis
photosystem I
granum thylakoid membrane thylakoid space
stroma
H2O +
A TP + ADP P
2 NADPH
O2 1 2
thylakoid
e–
electron transport chain
e–
e– e–
e–
thylakoid space
NADP reductase
H+
H+
H+
H+
H+
H+
H+
H+
NADP+
Light Reactions
• ATP Production – Thylakoid space acts as a reservoir for
hydrogen ions (H+) • H+ from water being split • Pumped in by electron transport chain
– More H+ in thylakoid space than stroma • Electrochemical gradient
– H+ can only flow through ATP synthase – Energy powers making ATP by chemiosmosis
Calvin Cycle Reactions
• The Calvin Cycle – Series of reactions that use CO2 from the
atmosphere to produce carbohydrate – Includes
• Carbon dioxide fixation • Carbon dioxide reduction • Ribulose-1,5-bisphosphate (RuBP) regeneration
THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS • ATP and NADPH power sugar synthesis in the
Calvin cycle – The Calvin cycle
• Occurs in the chloroplast’s stroma • Consists of carbon fixation, reduction, release of
G3P, and regeneration of RuBP
Input CO2 ATP NADPH
CALVIN CYCLE
G3P Output:
• Photosynthesis uses light energy to make food molecules
Light
H2O CO2
NADP+
Photosystem II
Photosystem I
Electron transport chains
ADP P +
RUBP
CALVIN CYCLE
(in stroma) 3-PGA
Stroma
G3P NADPH
ATP
O2
LIGHT REACTIONS CALVIN CYCLE
Sugars
Cellular respiration
Cellulose
Starch Other organic compounds
Thylakoid membranes
Chloroplast
Photosynthesis: Summary
Calvin Cycle Reactions
• Fixation of Carbon Dioxide – CO2 is attached to 5-carbon RuBP molecule
• This results in a 6-carbon molecule that splits into two 3-carbon molecules (3PG)
– RuBP Carboxylase is the enzyme that makes this happen (RuBisCo)
Calvin Cycle Reactions
• Reduction of Carbon Dioxide – Each 3PG molecules undergoes reduction to
G3P in two steps – Energy and electrons needed for this reaction
are supplied by ATP and NADPH (from light reaction)
Calvin Cycle Reactions
• Importance of the Calvin Cycle – G3P (glyceraldehyde-3-phosphate) can be converted
to many other molecules • The hydrocarbon skeleton of G3P can form:
– Fatty acids and glycerol to make plant oil – Glucose phosphate (simple sugar) – Fructose (+ glucose = sucrose) – Starch and cellulose – Amino acids
Alternate Pathways for Photosynthesis
• C3 photosynthesis – First detectable molecule following fixation is
a 3-carbon molecule – Wheat, rice, oats – Mesophyll layers of leaves in parallel layers – Bundle sheath cells around the plant veins do
not contain chloroplasts
Alternate Pathways for Photosynthesis
• C3 photosynthesis – RuBP can also bind with oxygen
• Photorespiration • Wasteful reaction because it uses oxygen and
releases carbon dioxide, decreasing the overall efficiency of the enzyme
• Oxygen concentration in leaf rises when weather is hot and dry, because plant keeps stomata closed to conserve water
vein stoma
mesophyll cells
bundle sheath cell
a. C3 Plant
Calvin cycle
CO2
G3P
RuBP
mesophyll cell
3PG (C3)
a. CO2 fixation in a C3 plant, tulip
C3 Pathway
Alternate Pathways for Photosynthesis
• C4 Pathway – Sugarcane and corn – Mesophyll cells are arranged in concentric
rings around the bundle sheath cells, which also contain chloroplasts
– CO2 is initially fixed into a four-carbon molecule
– The four-carbon molecules is later broken down into a three-carbon molecule and CO2
– CO2 enters the Calvin cycle
Alternate Pathways for Photosynthesis
• C4 Pathway – C4 plants tend to be found in hot, dry climates – In these climates, stomata tend to close to conserve
water – Oxygen then builds-up in the leaves – But, RuBP carboxylase is not exposed to this oxygen
in C4 plants and photorespiration does not occur – Instead in C4 plants, the carbon dioxide is delivered to
the Calvin cycle, which is located in bundle sheath cells that are sheltered from the leaf air spaces
vein stoma
mesophyll cells
bundle sheath cell
b. C4 Plant
CO2
CO2 C4
G3P
Calvin cycle
b. CO2 fixation in a C4 plant, corn
mesophyll cell
bundle sheath cell
C4 Pathway Alternate Pathways for Photosynthesis
• When the weather is moderate, C3 plants ordinarily have the advantage
• But when the weather becomes hot and dry, C4 plants have the advantage, and we can expect them to predominate
• In the early summer, C3 plants such as Kentucky bluegrass predominate in lawns in the cooler parts of the United States, but by midsummer, crabgrass, a C4 plant, begins to take over
Alternate Pathways for Photosynthesis
• CAM Pathway – Prevalent among most succulent plants that
grow in deserts, including the cacti – CAM plants partition carbon fixation by time – During the night CAM plants fix CO2 forming
C4 molecules, – The C4 molecules are stored in large vacuoles – During daylight – C4 molecules release CO2 to Calvin cycle
CO2
CO2
C4
G3P
night
day
c. CO2 fixation in a CAM plant, pineapple
Calvin cycle
CAM Pathway
Photosynthesis Versus Cellular Respiration
• Both plant and animal cells carry out cell respiration – In mitochondria – Breaks glucose down – Utilizes O2 and gives off CO2
• Only plant cells photosynthesize – In chloropalsts – Builds glucose – Utilizes CO2 and gives off O2
• Both processes utilize an electron transport chain and chemiosmosis for ATP production
Thylakoid membrane H2O O2 Cristae O2 H2O
ADP ATP ADP ATP
solar energy
H2O CO2
NADH+H+
e–
e–
NADH+H+ e–
e–
e–
e–
Preparatory reaction Citric acid
cycle
NADH + H+
and FADH2
Electron transport chain
2 ATP 2 ADP
4 ADP 4 ATP total 2 ATP
net gair 2 ADP 2 32 ADP 32 or 34 or 34
ATP ATP
Matrix NAD+
CH2O CO2
Cellular Respiration
Light reactions
ADP + P
Calvin cycle
reactions
NADP+
NADPH
thylakoid membrane O2 CH2O
stroma
Stroma NADPH NADP+
CO2 CH2O
Photosynthesis
Glycolysis
glucose pyruvate
ATP
NADH
Photosynthesis Versus Cellular Respiration