Visible Light Where It Starts – Photosynthesis Chapter 6
Electromagnetic Spectrum Shortest wavelength
Longest wavelength
Gamma rays X-rays UV radiation Visible light Infrared radiation Microwaves Radio waves
• Wavelengths humans perceive as different colors • Violet (380 nm) to red (750 nm) • Longer wavelengths, lower energy
Pigments • Light-absorbing molecules • Absorb some wavelengths and transmit others • Color you see are the wavelengths not absorbed
1
Light-Dependent Reactions
Main pigments in most photoautotrophs
• Pigments absorb light energy, give up ewhich enter electron transfer chains
Wavelength absorption (%)
Chlorophylls
chlorophyll a
chlorophyll b
Wavelength (nanometers)
Light-Independent Reactions
• Water molecules are split, ATP and NADH are formed, and oxygen is released • Pigments that gave up electrons get replacements
Photosynthesis Equation
• Synthesis part of photosynthesis • Can proceed in the dark • Take place in the stroma • Calvin-Benson cycle
2
Chloroplasts
Inside the Chloroplast
Organelles of photosynthesis leaf’s upper surface
photosynthetic cells
central vacuole
chloroplast one photosynthetic cell inside the leaf
vein
stoma (gap) in lower epidermis
• Two outer membranes enclose a semifluid interior, the stroma • Thylakoid membrane inside the stroma
two outer membranes
thylakoid membrane system
chloroplasts
see next slide
stroma
section from the leaf, showing its internal organization
Linked Processes Photosynthesis • Energy-storing pathway
Aerobic Respiration • Energy-releasing pathway
Two Stages of Photosynthesis sunlight energy
ATP lightdependent reactions
• Releases oxygen
ADP + Pi
lightindependent reactions
NADPH
• Requires oxygen • Requires carbon dioxide
CO2 (carbon dioxide)
H2O (water)
NADPH+ glucose
• Releases carbon dioxide O2
H2O (metabolic water)
3
Photosystem Function: Reaction Center
Inside the Chloroplast • Photosystems are embedded in thylakoids, containing 200 to 300 pigments and other molecules that trap sun’s energy • Two types of photosystems: I and II
light harvesting complex
electron transfer chain
PHOTOSYSTEM II
thylakoid membrane
• Molecule of chlorophyll a (P700 or P680) is the reaction center of a photosystem
PHOTOSYSTEM I
• Reaction center accepts energy and donates electron to acceptor molecule
thylakoid compartment
Electron Transfer Chains
ATP and NADPH Formation
• Adjacent to photosystem
LIGHTHARVESTING COMPLEX
photon
• Acceptor molecule donates electrons from reaction center
PHOTOSYSTEM II
PHOTOSYSTEM I
NADPH
NADPH + H+ H+
H+ H+
• As electrons flow through chain, energy they release is used to produce ATP and, in some cases, NADPH
sunlight
a light-harvesting complex has a ring of pigment molecules
A photosystem is surrounded by densely packed light harvesting complexes.
H+ H+
H+ H+ H+
H+
H+ H+
thylakoid compartment thylakoid membrane
ADP + Pi
ATP stroma
4
ATP Formation • When water is split during photolysis, hydrogen ions are released into thylakoid compartment • More hydrogen ions are pumped into the thylakoid compartment when the electron transfer chain operates
Calvin-Benson Cycle
ATP Formation • Electrical and H+ concentration gradient exists between thylakoid compartment and stroma • H+ flows down gradients into stroma through ATP synthesis • Flow of ions drives formation of ATP
Calvin-Benson Cycle 6CO2
• Overall reactants
• Overall products
– Carbon dioxide
– Glucose
– ATP
– ADP
– NADPH
– NADP+
ATP 6 RuBP
12 PGA
12
6 ADP Calvin-Benson cycle
ATP
12 ADP + 12 Pi 12 NADPH
4 Pi
Reaction pathway is cyclic and RuBP (ribulose bisphosphate) is regenerated
12 NADP+ 10 PGAL
12 PGAL
1 Pi 1
glucose-6-1-phosphate
5
Building Glucose
Using the Products of Photosynthesis • Phosphorylated glucose is the building block for:
• PGA accepts – phosphate from ATP – hydrogen and electrons from NADPH
• PGAL (phosphoglyceraldehyde) forms • When 12 PGAL have formed – 10 are used to regenerate RuBP
– Sucrose • The most easily transported plant carbohydrate
– Starch • The most common storage form
– 2 combine to form phosphorylated glucose
Summary of Photosynthesis sunlight LightDependent Reactions
12H2O
6O2
ADP + Pi
ATP
6CO2 6 RuBP LightIndependent Reactions
NADPH
CalvinBenson cycle
NADP+
The evolution of oxygen About 3.8 billion years ago, the first organisms appeared on the young planet Earth. They were able to use the water vapor, nitrogen, methane and ammonia that made up Earth's atmosphere for food and energy, probably through a process facilitated or catalyzed by metals such as iron and magnesium. Between 3.3 and 3.5 billion years ago, cyanobacteria (blue-green algae) appeared. These single-celled organisms had the ability to convert energy from the sun into chemical energy through photosynthesis using hydrogen sulfide (H2S). Between 1 and 2 billion years ago, some bacteria adapted to use water (H2O) in photosynthesis. Oxygen, which is released as a byproduct of photosynthesis, appeared
12 PGAL
in Earth's atmosphere. About 500 million years ago, hemoglobin and myoglobin proteins evolved.
6H2O phosphorylated glucose http://www.hawaii.edu/ur/heme.html
end products (e.g., sucrose, starch, cellulose)
6
Making ATP
Glucose metabolism
• Plants make ATP during photosynthesis • Cells of all organisms make ATP by breaking down carbohydrates, fats, and protein
• Cellular respiration – Aerobic – Produces 36 ATP – Takes place within mitochondrion
http://staff.jccc.net/PDECELL/cellresp/respintro.html#stages
Overview of Aerobic Respiration
Main Pathways Start with Glycolysis • Glycolysis occurs in cytoplasm • Reactions are catalyzed by enzymes
C6H1206 + 6O2
6CO2 + 6H20
glucose
carbon
oxygen
dioxide
water
Glucose (six carbons)
2 Pyruvate (three carbons)
7
Overview of Aerobic Respiration glucose
cytoplasm 2
Glucose metabolism
ATP
ATP
GLYCOLYSIS
energy input to start reactions
e- + H+
(2 ATP net) 2 pyruvate
2 NADH mitochondrion 2 NADH 8 NADH 2 FADH2 e-
e- + H+
2 CO2
e- + H+
4 CO2
e- + H+
Krebs Cycle
2
ELECTRON TRANSPORT PHOSPHORYLATION
H+
32
ATP
ATP
water
e- + oxygen
• Glycolysis – Converts one molecule of glucose to two molecules of pyruvate – Anaerobic – Produces 2 molecules ATP (net) – Cytoplasmic
TYPICAL ENERGY YIELD: 36 ATP
www.sirinet.net/jgjohnso/respiration.html
Net Energy Yield from Glycolysis Energy requiring steps: 2 ATP invested
Energy releasing steps: 2 NADH formed 4 ATP formed
Second-Stage Reactions • Occur in the mitochondria • Pyruvate is broken down to carbon dioxide • More ATP is formed • More coenzymes are reduced
inner mitochondrial membrane
outer mitochondrial membrane
inner outer compartment compartment
Net yield is 2 ATP and 2 NADH
8
Results of the Second Stage • All of the carbon molecules in pyruvate end up in carbon dioxide • Coenzymes are reduced (they pick up electrons and hydrogen) • One molecule of ATP is formed • Four-carbon oxaloacetate is regenerated
Second Stage of Aerobic Respiration Acetyl-CoA Formation
pyruvate coenzyme A (CO2)
NADH CoA acetyl-CoA
Krebs Cycle
CoA
oxaloacetate
• Occurs in the mitochondria • Coenzymes deliver electrons to electron transfer chains • Electron transfer sets up H+ ion gradients • Flow of H+ down gradients powers ATP formation
citrate
NAD+
NADH
NADH
NAD+ FADH2
FAD
NAD+ NADH
ATP
Electron Transfer Phosphorylation
NAD+
ADP + phosphate group
Electron Transfer Phosphorylation
glucose
GLYCOLYSIS
pyruvate
• Electron transfer chains are embedded in inner mitochondrial compartment • NADH and FADH2 give up electrons that they picked up in earlier stages to electron transfer chain
KREBS CYCLE
ELECTRON TRANSFER PHOSPHORYLATION
• Electrons are transferred through the chain • The final electron acceptor is oxygen
9
ATP Formation
Summary of Transfers glucose ATP 2 PGAL
ATP
2 NADH 2 pyruvate
glycolysis
ATP INNER COMPARTMENT ADP + Pi
2 CO2
2 FADH2
e–
2 acetyl-CoA
2 NADH
H+ H+
2
ATP
6 NADH
Krebs Cycle
ATP 2 FADH2 4 CO2
KREBS CYCLE
H+ H+
ATP 36 ATP
H+ H+
ADP
electron + Pi transfer phosphorylation
H+
H+
H+
Importance of Oxygen
Summary of Energy Harvest (per molecule of glucose)
• Electron transfer phosphorylation requires the presence of oxygen
• Glycolysis
• Oxygen withdraws spent electrons from the electron transfer chain, then combines with H+ to form water
• Krebs cycle and preparatory reactions
– 2 ATP formed by substrate-level phosphorylation – 2 ATP formed by substrate-level phosphorylation
• Electron transfer phosphorylation – 32 ATP formed
10
Anaerobic Pathways
Fermentation Pathways
• Do not use oxygen
• Begin with glycolysis
• Produce less ATP than aerobic pathways
• Do not break glucose down completely
• Two types of fermentation pathways – Alcoholic fermentation
to carbon dioxide and water • Yield only the 2 ATP from glycolysis
– Lactate fermentation
Yeasts • Single-celled fungi • Carry out alcoholic fermentation • Saccharomyces cerevisiae – Baker’s yeast – Carbon dioxide makes bread dough rise
• Saccharomyces ellipsoideus – Used to make beer and wine
Evolution of Metabolic Pathways • When life originated, atmosphere had little oxygen • Earliest organisms used anaerobic pathways • Later, noncyclic pathway of photosynthesis increased atmospheric oxygen • Cells arose that used oxygen as final acceptor in electron transfer
11
Processes Are Linked Aerobic Respiration • Reactants
Summary Aerobic Respiration Photosynthesis • Reactants
2
ATP
– Carbon dioxide
– Oxygen
– Water
e- + H+
(2 ATP net) 2 pyruvate
2 NADH
2 NADH 8 NADH 2 FADH2
• Products
ATP
GLYCOLYSIS
energy input to start reactions
mitochondrion
– Sugar
• Products
glucose
cytoplasm
e-
e- + H+
2 CO2
e- + H+
4 CO2
e- + H+
Krebs Cycle
2
ELECTRON TRANSPORT PHOSPHORYLATION
H+
32
ATP
ATP
water
e- + oxygen TYPICAL ENERGY YIELD: 36 ATP
– Carbon dioxide
– Sugar
– Water
– Oxygen
Why do animals inhale oxygen and exhale carbon dioxide? • Aerobic cellular respiration – Oxygen acts as electron acceptor – O2 combines with hydrogen ions to form water – Carbon dioxide is waste product – Produces 36 ATP
Why is ATP important? • High energy bonds hydrolyzed by ATPases to produce ADP + Pi + energy • Kinases phosphorylate (add Pi) to other enzymes to activate them • Facilitates muscle contraction, active transport, etc.
12
13
