Photosynthesis

Chapter 7

Photosynthesis

• Occurs in green plants, algae, and some bacteria

• Transforms light E into chemical E– Stored in sugar bonds

• Also produces O2

Anatomy of a Leaf

• Exterior of stoma (pl. stomata) for gas exchange• Veins carry water from roots and sugars from leaves• Mesophyll is interior leaf tissue– Filled with chloroplasts, site of photosynthesis

• Stroma, thylakoids with chlorophyll, grana

– All green areas of plants, concentrated in leaves

The Electromagnetic Spectrum• Electromagnetic (light)

energy travels in waves– Wavelength from crest to crest

• Energy levels vary– Low energy = long waves

• Microwaves, radio waves– High energy = short waves

• Gamma rays, x-rays– Visible spectrum

• Colors seen are those that are reflected, not absorbed– White reflects all light– Black absorbs all light– Green plants reflect green

Light Energy• Exists as photons, discrete units of

energy• Pigments absorb specific photons – Energy elevates e-s of pigments to

higher orbital ( PE)

• Unstable in upper orbital so ‘fall’ back quickly– Releases energy as heat

• White vs black cars/clothing in the South

Plant Pigments• Leaves contain a variety of pigments– Chlorophyll a: reflects green

• Direct photosynthetic reaction– Chlorophyll b: reflects green

• Absorbs E and transfers to chlorophyll a– Carotenoids: reflects yellow and orange

• Behaves like chlorophyll b• Expand system capabilities

– Photoprotection: absorbs excess E to prevent damage– Similar mechanism in retina “Carrots for good eyes”

– Anthocyanins: reflects red and purple• Cherries and flowers

• Fall foliage change b/c other pigments outlast chlorophyll’s

Overview of Photosynthesis• Light reaction

– In the thylakoids– Light E to ATP and NADPH

• Fuels the dark cycle– Oxygen atoms escape

• Dark reaction– In the stroma– Assembles sugars

• Carbon fixation: incorporate CO2 to organic compounds

– NADPH reduces carbon• Electron mov’t?

– ATP provides chemical E to run the cycle

– No direct need for light

Light Capturing• In thylakoids• Electron transfer chain in photosystem– Clusters of chlorophyll, pigments, and proteins– Light-gathering “antennae”– Photosystem I (P680)—absorbs red light at 680nm– Photosystem II (P700)—absorbs far-red light at 700nm

Light-Dependent (Light) Reaction

• Starts with:– Sunlight – Water

• Ends with:– ATP and NADPH

• Important products in this process:– Oxygen

• e-’s from water to PS II/PS I to NADP+

– ETC between photosystems set up [H +] gradient in thylakoid space

– H + out to stroma through ATP synthase• Photophosphorylation

• Where do you recognize this process from?

Light-Independent (Dark) Reaction

• Starts with:– 6 CO2

– ATP and NADPH• Ends with:– 2 G3P (3C) = sugar

precursor• Important products in this

process:– NADP+ and ADP + Pi

– 3 RuBP

Review of Photosynthesis