Fotosíntesis: Reacciones de la Luz y Ciclo de Calvin - Prof. Atienza, Diapositivas de Biología

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Lesson 10 (I)

Photosynthesis I. Light reactions

Calvin Cycle. Photorrespiration

C4 and CAM plants

References

Chapter 10. Biological science. Freeman et al. Chapter 9. Biology. Campbell & Reece Chapter 7. Plant physiology. Taiz & Zeiger

Index

1. Overview of photosynthesis
2. Light and pigments I. Light spectrum: visible light II. When a pigment absorbs light...
3. Elements of the chloroplast electron transport chain I. Photosystem II II. Antenna complex III. Cytochrome b6f IV. Photosystem I V. FNR VI. ATP synthase
4. "Z scheme" and photophosphorylation

Photosynthesis is a process in

which green plants (autotrophs)

use energy from the sun to

transform water, carbon dioxide,

and minerals into oxygen and

organic compounds.

Photosynthesis: global reaction

Potential energy increases 6 CO (^2) (carbon dioxide)

6 O 2

(oxygen)

(Input)

Glucose (C 6 H 12 O 6 )

6 H 2 O

(water)

Photosynthesis: global reaction

6 CO 2 + 12 H 2 O + light energy → C 6 H 12 O 6 + 6 O 2 + 6 H 2 O

• Photosynthesis takes place in the

chloroplasts of epidermal cells and

photosynthetic parenchyma cells

• In plants, cells that photosynthesize

typically have 40–50 chloroplasts

• Two compartments:
  • Lumen (inside the thylakoid)

-Stroma ("matrix" of the

chloroplast, outside of the

thylakoid)

Outer membrane Inner membrane

0.5 μm

10 μm

Chloroplast

Thylakoids (flattened sacs) Granum (stack of thylakoids)

Anatomy of photosynthesis

CHLOROPLASTS

DNA RNA (bacterial) Double membrane Vesicles inside

H 2 O

LIGHT REACTIONS

Chloroplast

LIGHT

ATP NADPH

O (^2)

NADP+

CO 2

ADP

• Pi CALVIN CYCLE

[CH 2 O] (SUGAR)

plant cell energetic needs

export to other tissues as sucrose (phloem)

THYLAKOIDS STROMA

Photosynthesis: 2 stages

Visible light comprises the colors we can see, including the wavelengths necessary for photosynthesis (PAR) 400-700nm

I. Light spectrum: visible light

Photosynthetic pigments. Light absorbance spectrum

Photosynthetic pigments are molecules able to absorb part of the energy of the light spectrum

Chlorophylls

ABSORB: violet-to-blue & red light TRANSMIT: green light

Carotenoids

ABSORB: blue & green light TRANSMIT: yellow, orange, or red light

Action spectrum

of photosynthesis

Wavelength of light (nm)

Light absorbed

Oxygen produced

a b

EXCITATION & RELAXATION

(ENERGY ABSORPTION)

The more energetic the wavelength, the more electrons it is able to eject

Photosynthetic pigments. Electron excitation

Photons

Blue photons excite electrons to

an even higher energy state

Red photons excite electrons

to a high-energy state

Energy state of electrons in chlorophyll

II. When a pigment absorbs light...

Electrons jump to higher energy levels

I) In most cases, the electrons return to their initial state almost immediately because it is the thermodynamically stable configuration.

The energy released when returning to the original energy level may:

1. Dissipate as heat (internal conversion)
2. Be emitted immediately as a longer wavelength (fluorescence)
3. Be absorbed by a neighboring molecule, which releases its electrons to higher energy levels (transfer of excitation by inductive resonance)

Chlorophyll only triggers a chemical reaction when it is associated with proteins embedded in a membrane (as in a chloroplast). Separation of charged molecules to avoid formation of an ionic bond

When a pigment absorbs light...

II) In other cases, the energy absorbed triggers a chemical reaction (redox). The energy absorbed by the pigment releases an electron from its molecule. This high-energy electron is captured by another molecule, which, therefore, is reduced

SEPARATION OF CHARGED MOLECULES

to avoid formation of an ionic bond

If there is a free electron shell next to the excited chlorophyll molecule, the

electron drops in it

Transfer of charges: redox reactions

Chloroplasts Thylakoids Photosystems

Stroma Grana

Thylakoid membrane CITOCROME C Photosystem II

Photosystem I (^) ATP Synthase

Abundant in Grana Membranes (^) Abundant in membranes facing stroma

Photosystem II

Photosystem I

Where is energy absorbed and transformed?