Chapter 6 WHERE IT STARTS – PHOTOSYNTHESIS

Introduction




Before photosynthesis evolved, Earth’s atmosphere had little free oxygen




Oxygen released during photosynthesis changed the atmosphere
Favored

evolution of new metabolic pathways, including aerobic respiration

Sunlight as an Energy Source Visible light
A

small part of a spectrum of electromagnetic energy radiating from the sun
Travels in waves
Wavelength – The distance between the crest of two successive waves of the light is called wavelength
Measured in nanometer (nm)
It is organized as photons or packets of electromagnetic energy

Electromagnetic Spectrum

Photosynthetic Pigments Photosynthesis begins when photons are absorbed by photosynthetic pigment molecules
Pigments absorb nearly all wavelengths of visible light
The wavelength of light that are not absorbed are reflected as its characteristic color


Major Photosynthetic Pigments Chlorophyll a Main photosynthetic pigment Absorbs violet and red light (appears green) Chlorophyll b, carotenoids, phycobilins Absorb additional wavelengths

Overview of Photosynthesis Chloroplasts are organelles of photosynthesis in plants
They have three membranes
Two enclose a semifluid matrix called stroma
Folded thylakoid membrane
Chloroplasts contain two kinds of photosystems, type I & type II

Sites of Photosynthesis

6.3 Overview of Photosynthesis


Photosynthesis proceeds in two stages
Light-dependent

reactions
Light-independent reactions

Summary equation: energy 6H2O + 6CO2 Light 6O2 + C6H12O6 enzymes

Sites of Photosynthesis: Chloroplasts




Light-dependent reactions occur at a much-folded thylakoid membrane
Forms

a single, continuous compartment inside the stroma (chloroplast’s semifluid interior)




Light-independent reactions occur in the stroma

Products of Light-Dependent and Light –Independent Reactions Light - Dependent
Typically, sunlight energy drives the formation of ATP and NADPH
Oxygen is released from the chloroplast (and the cell) Light – Independent
Synthesis of sugar and other carbohydrates

sunlight O2

CO2

H2O

CHLOROPLAST

lightdependent reactions

NADPH, ATP NADP+, ADP

lightindependent reactions

sugars CYTOPLASM

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-Dependent Reactions


Two types of photosystems
In thylakoid membrane




Light-harvesting complexes
Absorb light energy and pass it to photosystems which then release electrons


Photosynthesis begins when photon energy is captured by light harvesting complexes and transferred to photosystem II




P700 – Photosystem I, absorb energy of 700nm P680– Photosystem II, absorb energy of 680nm




Noncyclic Photophosphorylation


Electrons released from photosystem II flow through an electron transfer chain
At

end of chain, they enter photosystem I

Photon energy causes photosystem I to release electrons, which end up in NADPH
Photosystem II replaces lost electrons by pulling them from water
Photolysis – The process by which the energy of light breaks down a molecule


ATP Formation In both pathways, electron flow through electron transfer chains causes H+ to accumulate in the thylakoid compartment
A hydrogen ion gradient builds up across the thylakoid membrane
H+ flows back across the membrane through ATP synthases into stroma
The ATP and NADPH formed are used in sugar-synthesizing, light-independent reactions in the stroma

Noncyclic Photophosphorylation

Cyclic Photophosphorylation




Electrons released from photosystem I enter an electron transfer chain, then cycle back to photosystem I




It yields only ATP




NADPH does not form, oxygen is not released

Energy Flow in Light-Dependent Reactions

Light Independent Reactions: The Sugar Factory


Calvin – Benson cycle build sugars in the stroma of chloroplast

Light-independent reactions proceed in the stroma
Carbon fixation: Enzyme rubisco attaches carbon from CO2 to RuBP (a 5 carbon molecule) to start the Calvin–Benson cycle


Calvin–Benson Cycle


Cyclic pathway makes phosphorylated glucose
Uses energy from ATP, carbon and oxygen from CO2, and hydrogen and electrons from NADPH




Reactions use glucose to form photosynthetic products (sucrose, starch, cellulose)




Six turns of Calvin–Benson cycle fix six carbons required to build a glucose molecule from CO2

Light-Independent Reactions

Adaptations: Different Carbon-Fixing Pathways


Environments differ
Plants

have different details of sugar production in light-independent reactions
On dry days, plants conserve water by closing their stomata
Stomata is a small opening across the surface of leaves
O2 from photosynthesis cannot escape

Plant Adaptations to Environment C3 Plant
At high O2,rubisco attaches oxygen (not carbon) to RuBP in a pathway called photorespiration
This reduces the efficiency of sugar production

CO2

O2 glycolate

RuBP

CalvinBenson cycle

PGA

sugar ATP

NADPH

C3 plants. On dry days, stomata close and oxygen accumulates in air spaces inside leaves. The high concentration of oxygen makes rubisco attach oxygen instead of carbon to RuBP. Cells lose carbon and energy as they make sugars.

Plant Adaptations to Environment


C4 plants
Carbon

fixation occurs twice
First reactions release CO2 near rubisco, limit photorespiration when stomata are closed

CO2 from inside plant

C4 oxaloacetate cycle

CO2 RuBP CalvinBenson PGA cycle sugar

C4 plants. Oxygen also builds up in the air spaces inside the leaves when stomata close. An additional pathway in these plants keeps the CO2 concentration high enough to prevent rubisco from using oxygen.

Plant Adaptations to Environment


CAM plants (Crassulacean Acid Metabolism)
Open

stomata and fix carbon at night