Photosynthesis OverviewPhotosynthesis Overview6 CO6 CO2 2 (g) + 6 H(g) + 6 H22O(I) + light energy O(I) + light energy C C66 H H1212 O O66 (aq) + 6 O(aq) + 6 O22 (g)(g)

LightLight Only 5% of the light that hits earth's surface is converted to Only 5% of the light that hits earth's surface is converted to

organic compounds by photosynthesis.organic compounds by photosynthesis. Light travels in Light travels in wave packets called photonswave packets called photons Light from the sun is a Light from the sun is a mixture of photons of different mixture of photons of different

energiesenergies (called (called wavelengthswavelengths, measured in , measured in nanometersnanometers).). Photons are captured in plants by Photons are captured in plants by plastids (ex. plastids (ex.

Chloroplasts)Chloroplasts)..

Functions:Functions: Photosynthesis factories for plants and algaePhotosynthesis factories for plants and algae Store excess glucose as starchStore excess glucose as starch

ChloroplastsChloroplasts

ChloroplastsChloroplastsStructure:Structure: 2 membranes2 membranes (like mitochondria) (like mitochondria) Interior space filled with Interior space filled with stroma (stroma (protein-rich fluid) protein-rich fluid) In the stroma areIn the stroma are thylakoids thylakoids (membrane bound sacs) (membrane bound sacs) The thylakoid membrane contains light gathering The thylakoid membrane contains light gathering

(photosynthetic) pigments(photosynthetic) pigments

ThylakoidsThylakoids Grana: Grana: stack of thylakoids (~60 grana per stack of thylakoids (~60 grana per

chloroplast with 30-50 thylakoids each)chloroplast with 30-50 thylakoids each) Lamellae: Lamellae: unstacked thylakoids that link grana unstacked thylakoids that link grana

together.together. Lumen: Lumen: interior of the thylakoid, water- filledinterior of the thylakoid, water- filled

PigmentsPigments Green plants contain chloroplasts with the Green plants contain chloroplasts with the

pigments pigments chlorophyll chlorophyll aa and and bb both both absorbabsorb blue and red light and blue and red light and reflectreflect

green light, so we assume blue and red light is green light, so we assume blue and red light is used in photosynthesisused in photosynthesis

Chloropyhll Chloropyhll aa is used to is used to transfer energy from transfer energy from light into chemical energy.light into chemical energy.

Chlorophyll Chlorophyll bb is an is an accessory pigmentaccessory pigment. . It It absorbs photons that absorbs photons that aa misses. misses.

Accessory PigmentsAccessory Pigments In addition to chlorophyll In addition to chlorophyll b b there are other there are other

accessory pigments. Their job is to accessory pigments. Their job is to absorb light absorb light that can damage chlorophyll and lose it as that can damage chlorophyll and lose it as heat instead.heat instead.

CarotenoidsCarotenoids: appear yellow/orange (ie. Carrots).: appear yellow/orange (ie. Carrots). BetacaroteneBetacarotene protects our eyes protects our eyes

XanthophyllsXanthophylls: reflect yellow light, also found in : reflect yellow light, also found in the thylakoidthe thylakoid

PhotosystemsPhotosystems

Photosynthetic pigments are embedded in the Photosynthetic pigments are embedded in the thylakoid membrane clustered in groups called thylakoid membrane clustered in groups called photosystems.photosystems.

A photosystem contains a chlorophyll A photosystem contains a chlorophyll a a molecule molecule called a called a reaction centre,reaction centre, surrounded by accessory surrounded by accessory pigments called the pigments called the antenna complex.antenna complex.

The antenna complex The antenna complex harvests the lightharvests the light and and passes itpasses it to the reaction centre. to the reaction centre.

Photosystem Diagram

PPII (P700) and P (P700) and PIIII (P680) (P680) Two types of photosystems exist: Two types of photosystems exist:

Photosystem Photosystem II (P (PII) and Photosystem ) and Photosystem IIII (P (PII)II).. They have They have identicalidentical reaction centres, but they reaction centres, but they

absorb light at slightly different wavelengthsabsorb light at slightly different wavelengths because of the proteins the reaction centre is because of the proteins the reaction centre is associated with. associated with.

The reaction center in PThe reaction center in PII is called is called P700P700; ; absorption peaks at absorption peaks at 700nm.700nm.

The reaction centre in PThe reaction centre in PIIII isis calledcalled P680 P680, , absorption peaks at absorption peaks at 680nm.680nm.

Photosystem Diagram