Important structures in a leaf Two structures important for photosynthesis: Stoma (pl. Stomata): Pores in a plant’s

cuticle through which water and gases are exchanged between the plant and the atmosphere.

Mesophyll cells: Contain a lot of chloroplasts (between 40-200) arranged to receive maximum amount of light.

Plastids• Chloroplasts -photosynthetic; green due to

chlorophyll content• Chromoplasts: contain pigments other than

chlorophyll (in fruits, leaves, flowers)• Leucoplasts: involved in lipid biosynthesis• Amyloplasts: store starch (colorless)• Etioplasts: intermediate state in production of

chloroplasts, in tissue exposed to light for the first time

The Chloroplast• Usually lens-shaped, an organelle needed for

photosynthesis

• Has internal membrane system arranged into flattened sacs (thylakoids) 2 compartments: thylakoidspace and stroma

• Thylakoids stacked forming grana (1 granum)

• Contains the green pigment chlorophyll & pigments of other colors (red, blue, yellow/brown)

• Depending on light conditions, chloroplasts can move within the cells e.g. to the surface to catch more light in low light conditions

ChlorophyllsThe principle photoreceptor in

photosynthesis is Chlorophyll Chlorophyll a& b in plant, bacteriochlorophyll

a& b in bacteriaChlorophyll is similar to the heme group of

globins and cytochromes, but with very significant differences

• Mg2+ is in the center, not Fe2+• Ring V is fused to pyrrolering III• Hydrocarbon tail• Ring IV is partially reduced

• Ratio of Chlorophyll a:b in plant (3:1)

• Only chl-a is a constituent of the photosynthetic reaction centers, hence central photosynthesis pigment

• Chlorophyll molecules are bound to chlorophyll-binding proteins. • In a complex with proteins the absorption spectrum of the bound chlorophyll may differ from the absorption spectrum of the free chlorophyll

•The same applies for other light-absorbing substances, (carotenoids, xanthophylls etc)

Chlorophyll , Pigments & Light

• Free absorbing substances are called chromophore and the chromophore-protein complexes are called pigments.• Pigments are often named after the wavelength of their absorption maximum. • Chlorophyll-a 700 means a pigment of chl-a with an absorption maximum of 700 nm.

• All photosynthetic organisms have Chlorophyll a

• Chlorophyll a absorbs Light in Red (660 nm) and Blue (450 nm) Wavelengths

• Leaves are green because chlorophyll reflect the Green light (which is detected by our eyes)

• The Color of the pigment comes from the wavelengths of light reflected NOT absorbed

Accessory Pigments (Light Antenna)Light Harvesting complex are membrane

proteins containing pigments to absorb light energy outside the range of chlorophyll.

The most common pigments are Chlorophyll b, Carotenoids, Xanthophylls& Pilins

(in water-dwelling algae & Bacteria).

Fall Colors• Leaves contain chlorophyll and other pigments, but

they appear green because chlorophyll is the major component.

• During the fall, the green chlorophyll pigments are greatly reduced revealing the other pigments: Carotenoids and/or Xanthophylls

When a pigment absorbs light, It goes from a ground state to an excited state, which is unstable

Ground state

Excited state

Heat

Fluorescence

Photon

e-

Chlorophyll molecule

If a Pigment absorbs light, it must release its energy to return to its ground state,this can be accomplished via four common mechanisms:1.Dissipated as Heat (the most common route in general)

2.Transferred to another molecules (required special arrangements)

3.Emitted as Fluorescence (required special molecules)

4.Trigger a Chemical Reaction (special molecules)

Efficiency of photosynthesis is nearly 100% due to special arrangement of proteins in the thylakoids membrane(Such arrangement prevents dissipation of energy as heat)only the other three mechanisms are important for photosynthesis

How light is harvested

PhotosystemI and PhotosystemIIPhotosystemI (PS I) It needs light of longer wave lengths

(lambda > 700 nm)PhotosystemII (PS II) It becomes active when exposed to

shorter wave lengths (lambda < 680 nm)

Electron TransferWhen Light Strikes

Magnesium (Mg) Atom in Center of Chlorophyll Molecule, the Light Energy Excites a Mg Electron

The Electron Can Be Converted to Useful Chemical Energy

PhotophosphorylationThe Excited

Electron eventually Provides Energy so a Phosphate Group Can Be Added to (ADP), Yielding (ATP)

Photosynthesis: The Main Players

In eukaryotes, photosynthesis is carried out by four protein complexes, all located in the thylakoid membrane:

1. Photosystem II or P680: pass electrons & splits H2O2. Cytochromeb6f complex : the electric acceptors (plastoquinone& plastocyanin) 3. Photosystem I or P700 generates NADPH4. Proton translocating ATP synthase complex