Chapter 10: Photosynthesis

www.kathimitchell.comWord Roots:

auto- = self

-troph = food

chloro- = green

-phyll = leaf

hetero- = other

meso- = middle

photo- = light

Autotroph – self feeder – an organism that can make organic compounds from inorganic raw materials.

Heterotroph – other feeder – an organism that must obtain pre-made organic compounds from other living organisms.

Some introductory vocabulary•Autotroph: sustain life without eating; “photoautotrophs” include plants , algae, protists, cyanobacteria, sulfur bacteria

•Heterotroph consumers, must eat

•Photosynthesis: light energy of the sun is converted to the chemical energy of sugar and other organic nutrients

•6CO2 + 12H2O + Light Energy C6H12O6 + 6O2 + 6H2O

•6CO2 + 6H2O + Light Energy C6H12O6 + 6O2

Plant Structures•Chlorophyll: green pigment in chloroplasts

•Mesophyll: leaf tissue containing most chloroplasts

•Stomata: site of gas exchange

•Chloroplast: site of photosynthesis

•Stroma: dense fluid within chloroplast

•Thylokoid: membranous sacs containing chlorophyll

•Grana: stack of thylakoids

Where does the O2 come from?•Two possibilities

•CO2

•H2O

•C.B. van Niel

•Bacteria with H2S – create yellow sulfur globules

•Hypothesized O2 from H2O

•Supported with radioactive isotope labeling

•Oxygen-18 (18O)

•Plant given radiolabeled water did not have the labeled water in their tissues

2 Major Processes of Photosynthesis:•Light reactions (or Light Dependent Reaction)

•Calvin cycle (occurs in the Light Independent or Dark Reaction)

More helpful vocabulary…•NADP+ (nicotinomide adenine dinucleotide phosphate)

•E- acceptor, very similar to NAD+

•Reduces to NADPH

•Photophosphorylation: occurs in Light Dependent Reaction, generates ATP via chemiosmosis

•Carbon fixation: C of CO2 is assimilated into organic compounds by adding e- to them (reducing them)

•Part of the Light Independent Reaction

The Nature of Sunlight•Wavelength – the distance between two crests of an electromagnatic wave (λ).

•Electromagnetic Spectrum – entire range of electromagnetic radiation (Gamma rays – Radio waves).

•Visible Light – the electromagnetic waves that are detectible by the human eye, about 380 nm – 750 nm

•Photons – a fixed quantity of energy resulting from electromagnetic radiation.

•Photon energy is indirectly related to λ (wavelength)

•Decrease wavelength, increase energy

•Ex: a photon of violet light has 2x the energy of a photon of red. Violet light has a shorter wavelength.

Spectrophotometry•Spectrophotometer – measures the ability of a pigment to absorb wavelengths of light.

•Absorption Spectrum – a graph showing a pigment’s light absorption verses the wavelength.

•Action Spectrum – shows the relative effectiveness of a different wavelengths of light in photosynthesis.

Photosynthetic Pigments

•Chlorophyll a – used in light reaction, blue-green

•Chlorophyll b – olive green

•Photoabsorption

•Carotenoids – yellow orange, absorb violet and blue green

•Photoprotection: absorb and dissipate excessive light energy

•Has antioxidant properties

Chlorophyll•Amphipathic

•Porphyrin ring

•Mg center

•Hydrocarbon tail

Excitation of Chlorophyll by Light•Colors disappear when absorbed but energy does not.

•Photon absorbed – electron excited to higher energy level (increased PE)

•Very unstable

•Returns to ground state

•Energy is released as heat

•In chloroplasts

•Photon absorbed – electron excited to higher energy level.

•Captured by Primary Electron Acceptor

•Transferred down ETC

Photosystems: 2 Components

1.Light-harvesting complexes

•Chlorophylls and Proteins

2. Reaction center

•2 special chlorophyll a

•Primary electron acceptor

•Photosystem I (PS I)

•Chlorophyll a = P700

•Absorbs 700 nm best

•Photosystem II (PSII)

•Chlorophyll a = P680

Light drives synthesis of ATP and NADPH by energizing 2 photosystems embedded in the thylakoid membranes of chloroplasts

1. Photon excited e- in chlorophylls of P680 in PSII

2. e- captured by 1° e- acceptor (leaving P680 as P680+)

3. Photolysis – 2e -, 2H+, 1O produced

• 2e - go to P680+, O joins another O, leaving cell as O2

4. e- transferred from PS II to PS I by ETC

• Plastoquinone (Pq) - cytochrome

• Plastocyanin (Pc) – protein

5. ATP generated by exergonic fall of e- accompanied by H+ gradient in thylakoid and chemiosmosis. ATP synthesized in stroma.

6. Photon excited e- in P700, e- captured by 1° e- acceptor, leaving P700

7. e- transferred from PS I to NADP+ by ETC

Ferredoxin (Fd) - protein

8. NADP+ reductase transfers e- from Fd to NADP+ → NADPH

NADPH goes on to the Calvin Cycle…

Cyclic Electron Flow

•Only involves PSI to create ATP (no NADPH or O2) because Linear Electron Flow alone does not produce enough ATP to power the Calvin Cycle

•The number of ATP produced is not sufficient enough allow the Calvin cycle to create sugar

•NADPH is produced by Linear Electron Flow

•E- cycle back to Fd, then back to cytochrome complex and continue to P700 in PSI

Electron Flow During Light Dependent Reaction

• Water → PS II (P680 and primary e- acceptor)

→ Plastoquinone → Cytochrome Complex →

Plastocyanin → PS I (P700 and primary e-

acceptor) → Ferredoxin → NADP+ reductase

→ NADPH

Comparing Chemiosmosis in Mitochondria and Chloroplasts• Both use chemiosmosis to

generate ATP– Electrons are passed

through a series of carriers while protons are pumped through the membrane, creating an electrochemical gradient.

– This creates a proton motive force, driving the phosphorylation of ADP.

• Electrons carriers (cytochromes) and ATP synthases, are very similar in chloroplasts and mitochondria.

Key

Higher [H+]Lower [H+]

Mitochondrion Chloroplast

MITOCHONDRIONSTRUCTURE

Intermembrancespace

Membrance

Matrix

Electrontransportchain

H+ DiffusionThylakoidspace

Stroma

ATPH+

PADP+

ATPSynthase

CHLOROPLASTSTRUCTURE

Figure 10.16

Comparing Chemiosmosis in Mitochondria and Chloroplasts• Mitochondria: e- source is an

organic molecule• Chloroplast: e- source is water• Mitochondrial matrix is

analogous to chloroplast stroma• Proton gradient in mitochondria

is established in the intermembrane space

• Proton gradient in thylakoid is in the stroma

• However, ATP is generated on the inside of mitochondria (matrix) but the outside of the thylakoid

Key

Higher [H+]Lower [H+]

Mitochondrion Chloroplast

MITOCHONDRIONSTRUCTURE

Intermembrancespace

Inner Membrane

Matrix

Electrontransportchain

H+ Diffusion Thylakoidspace

Stroma

ATPH+

PADP+

ATPSynthase

CHLOROPLASTSTRUCTURE

Thylakoid Membrane

Path of Protons

•H+ pumped from stroma to Thylakoid

•ATP synthase generates ATP in stroma

•pH in stroma = 8

•pH in Thylakoid = 5

•NADPH also produced on stromal side

The Calvin Cycle•AKA:

•Light Independent Reation

•The Dark Reaction

•The Carbon Fixation Reaction

•Anabolic

•3-C sugar produced

•Glyceraldehyde-3-phosphate (G3P)

•Requires 3 turns of the Calvin Cycle

•Glucose would require 6 turns

The Calvin Cycle

Three Phases of the Calvin Cycle

Phase 1: Carbon Fixation

•3CO2 + 3 Ribulose bisphosphate (RuBP)

•CO2 through stomates

•Catalyzed by Ribulose-1,5-bisphosphate carboxylase oxygenase – Rubisco

•Creates an unstable 6-C intermediate

•6 molecules of 3-phosphoglycerate

Phase 2: Reduction

• 3-phosphoglycerate phosphorylated to 1,3-bisphosphoglycerate (ATP used)

• NADPH reduces 1,3-bisphosphoglycerate to G3P (carboxyl to aldehyde)

• G3P stores more potential energy

Phase 3: Regeneration

•5 G3P converted to 3 RuBP using 3 ATP

Recap of 3 Phases:

•3 CO2 fixed

•1 G3P synthesized

•9 ATP used

•6 NADPH used

Photorespiration• Transpiration: evaporative water

loss from leaves– On hot days, stomates will

close for water conservation• Therefore, CO2 concentration in

the leaf decreases as O2 increases

• A wasteful process called photorespiration is favored• Rubisco binds oxygen and then

creates 2-C molecule while wasting important resources

PhotorespirationOccurs because Rubisco has an affinity for both CO2 and O2

When CO2 is in low concerntrations, Rubisco will bind oxygen

1. Rubisco binds O2 producing a 2-C compound that leaves the chloroplast. This occurs when stomates are closed.

2. Peroxisomes and mitochondria rearrange the compound forming CO2.

• No ATP is generated but ATP is consumed

• No sugar is produced – decreases photosynthetic output

Usually occurs in C3 plants when stomata are partly closed

• Normally Fix CO2 to RuBP

• Examples – Rice, wheat, soy

Avoiding Photorespiration

• There are 2 alternate pathways of in which plants are better adapted for carbon fixation in hot, arid climates (when stomates are closed more often)

1.C4 plants

2.CAM plants (Crassulacean Acid Metabolism)

C4 Plants

•C4 Pathways uses carbon fixation to create an intermediate 4-Carbon molecule in the mesophyll cells

•Sugarcane and corn

•Leaf Anatomy

•Mesophyll cells contain C4 pathway, Bundle Sheath contain CalvinCycle

•Mesophyll cells

•Relies on Cyclic Electron Flow (PSI only)

•PEP carboxylase adds CO2 to phosphoenolpyruvate (PEP – 3C) to make oxaloacetate (4C)

•Transferred via plasmodesmata to Bundle Sheath Cells

•Bundle-sheath cells

•Found around the veins of the leaf

•4-C molecule release CO2

•Pyruvate regenerated, goes back to mesophyll and ATP is oxidized to generate PEP

C4 mechanism• Mesophyll of C4 pump CO2 into Bundle Sheath Cells

• Increasing the concentration of CO2 allows rubisco to bind carbon dioxide over oxygen

• Minimizes photorespiration and increases sugar production

CAM Plants•Occurs in succulents (water storing plants)

•Cacti, pineapples

•Stomata open at night and closed during the day

•CO2 incorporated into organic acids overnight and stored in vacuoles until daylight

•Crassulacean acid metabolism (CAM)

•Once light is present, ATP and NADPH are available to power the Calvin Cycle

•CO2 releases from organic acids (carboxyl groups)

•Sugar is made in the chloroplasts

Photosynthesis – Review•50% sugar produced used by plant

•Respiration

•Anabolism

•Sugar transported as sucrose through veins (vascular tissue)

•Used for anabolism of proteins, lipids and other molecules

•Glucose is synthesized into cellulose, especially in growing plants

•Sugar is stockpiled and stored as starch

•Starch is found in tubers, roots, seeds, fruits

•Creates atmospheric O2