Photosynthesis

Chapter 10

Chloroplasts

•Three Stages of Photosynthesis

–Capturing energy from sunlight.

–Making ATP & NADPH.

–Carbon fixation.

•Chloroplast Structure

–Internal membranes (thylakoids and grana)

•Semiliquid fluid (stroma) surrounds membranes

Leaf Organization

Chloroplast Structure

–Pigments clustered in photosystem in membranes of thylakoids.

•Light strikes pigment, electron passed from one pigment molecule to another.

Photosynthesis

Biophysics of Light

•Light = oscillating electric and magnetic fields.

–Energy content of photon is inversely proportional to wavelength of light.

•Short wavelengths = higher energy

•long wavelengths = lower energy

•Absorption Spectra

–light strikes a molecule = lost as heat or absorbed by electrons

•specific range of wavelengths it absorbs.

Photosynthetic pigments

•Pigments - Light-absorbing molecules.

–Photosynthesis:

•Carotenoids and Chlorophyll

–Chlorophyll a (main pigment) = violet and red light

–Chlorophyll b (accessory pigment) = blue light

–Carotenoids (accessory pigments) = violet to blue-green light

Chlorophylls and Carotenoids

•Plants, algae & cyanobacteria use chlorophyll a as primary pigment.

–higher overall photon capture rates with chlorophyll than with other pigments.

•Carotenoids absorb other photons

–not as efficient as chlorophyll

–allow for broader range of absorption

Photosystems

•Photosystem:

–Antenna Complex - Captures energy from sunlight

–Reaction Center - Pair of chlorophyll a transfer excited electron to an acceptor.

•Plants have 2 photosystems that work together

Photosystems I and II

Two Photosystems Work Together

•Two-stage photosystem

•Photosystem II acts first.

–H₂O split into H⁺ and O₂

–electrons transferred by photosystem II synthesize ATP

–electrons then passed to photosystem I to drive NADPH production.

Products of Photosystems

•ATP

•NADPH

•H⁺

•O₂

Calvin Cycle

•Carbon fixation = attachment of 3CO₂ to 3RuBP (5C molecule)

•Split into 6PGA (3C molecule)

•6ATP added

•6NADPH added

•Produces 6G3P (3C molecules w/ 2P)

•1G3P exits cycle, used to make glucose (1/2)

•5G3P recycled into 3RuBP (5C molecule)

–3ATP required

Calvin Cycle

•2 turns of cycle = 1 glucose (C₆H₁₂O₆)

•Each turn requires

_–3CO₂

–9ATP total

–6NADPH