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Chapter 7 - Photosynthesis

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What Is Photosynthesis?

The ability to capture sunlight energy and convert it to chemical energy.

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The Photosynthetic Equation

6CO2 carbondioxide

+ 6H2O water

+ light energy sunlight

C6H12O6

glucose (sugar)

+ 6O2

oxygen

Photosynthetic Organisms

Plants, algae, and some prokaryotes Are autotrophs (“self- feeders”)

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Photosynthesis and Cellular Respiration

Are interconnected Water, CO2, sugar, and O2 are used or

produced as byproducts in both processes

Adaptations for Photosynthesis

Leaves

Chloroplasts

Leaves

Flattened leaf shape exposes large surface area to catch sunlight

Epidermis upper and lower leaf surfaces

Cuticle waxy, waterproof outer surface reduces water evaporation

Leaf Anatomy

Stomata adjustable pores allow for entry of air with CO2

Mesophyll inner cell layers that contain majority of

chloroplasts

Vascular bundles (veins) supply water and minerals to the leaf while

carrying sugars away from the leaf

Anatomy of a Chloroplast

Chloroplasts bounded by a double membrane composed of

inner and outer membranes

Stroma semi-fluid medium within the inner membrane

Thylakoids disk-shaped sacs found within the stroma in stacks called grana

Fig. 7-5b, p. 111

two outer membranes of chloroplast

stroma

part of thylakoid membrane system:

thylakoid compartment, cutaway view

B Chloroplast structure. No matter how highly folded, its thylakoid membrane system forms a single, continuous compartment in the stroma.

Location of Photosynthetic Reactions

2 sets of chemical reactions occur in the:

1. Thylakoid membranes

2. Stroma

Light-dependent Reactions

Pigment molecules (e.g. chlorophyll) of the thylakoids capture sunlight energy

Sunlight energy is converted to the energy carrier molecules ATP and NADPH

Oxygen is released as a by-product

Fig. 7-5c, p. 111

sunlight O2 H2O CO2

CHLOROPLAST

light-dependent reactions

NADPH, ATP

NADP+, ADP

light-independent

reactions

sugars

CYTOPLASM

C In chloroplasts, ATP and NADPH form in the light-dependent stage of photosynthesis, which occurs at the thylakoid membrane. The second stage, which produces sugars and other carbohydrates, proceeds in the stroma.

Light-independent Reactions

Enzymes in stroma synthesize glucose and other organic molecules using the chemical energy stored in ATP and NADPH

The Energy in Visible Light

Sun radiates electromagnetic energy

Photons (basic unit of light) packets of energy with different energy levels

short-wavelength photons are very energetic longer-wavelength photons have lower

energies

Visible light is radiation falling between 400-750 nanometers of wavelength

Absorption of certain wavelengths light is “trapped”

Reflection of certain wavelengths light bounces back

Transmission of certain wavelengths light passes through

Light Captured by Pigments

Light Captured by Pigments

Absorbed light drives biological processes when it is converted to chemical energy

Pigments absorb visible light

Common pigments:Chlorophyll a and b

absorb violet, blue, and red light but reflect green light (hence they appear green)

Carotenoids absorb blue and green light but reflect yellow, orange,

or red (hence they appear yellow-orange) Are accessory pigments

Electromagnetic Spectrum of Radiant Energy

Why Do Autumn Leaves Change Colors?

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Why Autumn Leaves Turn Color

Chlorophyll breaks down before carotenoids in dying autumn leaves revealing yellow colors

Red fall colors (anthocyanin pigments) are synthesized by some autumn leaves, producing red colors

Light-Dependent Reactions

Photosystems within thylakoidsAssemblies of proteins, chlorophyll,

& accessory pigments

Two Photosystems PSII (comes 1st) and PSI (comes 2nd)

Each Photosystem is associated with a chain of electron carriers

The Thylakoid Membrane

Light-Dependent Reactions

Steps of the light reactions:

1. Accessory pigments in Photosystems absorb light and pass energy to reaction centers containing chlorophyll

2. Reaction centers receive energized electrons…

3. Energized electrons then passed down a series of electron carrier molecules (Electron Transport Chain)

4. Energy released from passed electrons used to synthesize ATP from ADP and phosphate

5. Energized electrons also used to make NADPH from (NADP+) + (H+)

Fig. 7-8, p. 113

to second stage of reactionsThe Light-Dependent Reactions of Photosynthesis

ATP synthase

light energylight energy NADPH ATP

ADP + Pi

photosystem IIelectron transfer chain photosystem I

thylakoid compartment

stroma

A Light energy drives electrons out of photosystem II.

C Electrons from photosystem II enter an electron transfer chain.

E Light energy drives electrons out of photosystem I, which accepts replacement electrons from electron transfer chains.

G Hydrogen ions in the thylakoid compartment are propelled through the interior of ATP synthases by their gradient across the thylakoid membrane.B Photosystem II pulls

replacement electrons from water molecules, which dissociate into oxygen and hydrogen ions (photolysis). The oxygen leaves the cell as O2.

D Energy lost by the electrons as they move through the chain causes H+ to be pumped from the stroma into the thylakoid compartment. An H+ gradient forms across the membrane.

F Electrons from photosystem I move through a second electron transfer chain, then combine with NADP+ and H+. NADPH forms.

H H+ flow causes the ATP synthases to attach phosphate to ADP, so ATP forms in the stroma.

NADP+

Maintaining Electron Flow

Electrons from PSII flow one-way into PS I

PSII – produces ATP

PSI – produces NADPH

Oxygen

May be used by plant or released into atmosphere

Light-Independent Reactions

NADPH and ATP from light-dependent rxns used to power glucose synthesis

Light not directly necessary for light-independent rxns if ATP & NADPH available

Light-independent rxns called the Calvin-Benson Cycle or C3 Cycle

The C3 Cycle

6 CO2 molecules used to synthesize 1 glucose (C6H12O6)

CO2 is captured and linked to a sugar called ribulose bisphosphate (RuBP)

ATP and NADPH from light dependent rxns used to power C3 reactions

Relationship Between Rxns

“Photo” capture of light energy (light dependent rxns)

“Synthesis” glucose synthesis (light-independent rxns)

Light dependent rxns produce ATP and NADPH which is used to drive light-independent rxns

Depleted carriers (ADP and NADP+) return to light-dependent rxns for recharging

The Ideal Leaf

The ideal leaf:

Large surface area to intercept sunlight

Very porous to allow for CO2 entry from air

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Leaves

Problem: Substantial leaf porosity leads to substantial

water evaporation, causing dehydration stress on the plant

Plants evolved waterproof coating and adjustable pores (stomata) for CO2 entry

When Stomata Are Closed

When stomata close, CO2 levels drop and O2 levels rise

Photorespiration occurs Carbon fixing enzyme combines O2 instead of CO2

with RuBP

When Stomata Are Closed

Photorespiration:O2 is used up as CO2 is generatedNo useful cellular energy madeNo glucose producedPhotorespiration is unproductive and

wasteful

When Stomata Are Closed

Hot, dry weather causes stomata to stay closed

O2 levels rise as CO2 levels fall inside leaf

Photorespiration very common under such conditions

Plants may die from lack of glucose synthesis

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C4 Plants Reduce Photorespiration

“C4 plants” have chloroplasts in bundle sheath cells and mesophyll cells

Bundle sheath cells surround vascular bundles deep within mesophyll

C3 plants lack bundle sheath cell chloroplasts

C4 Plants Reduce Photorespiration

C4 plants utilize the C4 pathway

Two-stage carbon fixation pathway

Takes CO2 to chloroplasts in bundle sheath cells

Environmental Conditions C4 pathway uses up more energy than C3

pathway

C3 plants thrive where water is abundant or if light levels are low (cool, wet, and cloudy climates) Ex. : most trees, wheat, oats, rice, Kentucky

bluegrass

C4 plants thrive when light is abundant but water is scarce (deserts and hot climates) Ex. : corn, sugarcane, sorghum, crabgrass,

some thistles

The End