Chapter 10

Photosynthesis

Modes of Nutrition• Heterotrophs – obtain organic

compounds by consuming other organisms (animals)

• Photoautotrophs – build organic compounds with light energy (plants)

• Chemoautotrophs – use inorganic substances (sulfur, ammonia) as an energy source to make organic compounds (some bacteria)

Photosynthesis

NET:

But really..

Opposite of Cellular Respiration!

• Photosynthetic prokaryotes do not have chloroplasts. Chlorophyll is in their plasma membrane.

Contains chlorophyll

Light Dependent: Closer Look

• Sunlight is electromagnetic energy– Wave-like properties and Particle-like

properties– Waves Wavelengths– Visible wavelengths = 380nm-750nm

Pigments: Substances that absorb visible light

Chlorophyll a is main pigment

Accessory Pigments:-Chlorophyll b (yellow-green-Carotenoids (oranges, yellows)

So, why do leaves look green?

Lab #4 – Plant Pigments

What wavelengths are absorbed

Effectiveness of wavelengths = activity

• Doesn’t exactly match due to accessory pigments (transfer energy to Chlor. a)

Light Dependent: Photosystems Light harvesting units of the thylakoid membrane Composed mainly of protein and pigment antenna

complexes Antenna pigment molecules are struck by photons Energy is passed to reaction centers Excited e- from chlorophyll is trapped by a primary e-

acceptor

Light Dependent: Photosystems• 2 photosystems in thylakoid membrane– Photosystem II• chlorophyll a• P680 = absorbs 680nm ~ red light

– Photosystem I• chlorophyll b• P700 = absorbs 700nm ~ red light

Light Dependent = The ETC

• ETC uses light energy to produce– ATP & NADPH

• go to Calvin cycle

• PS II absorbs light– excited electron passes from chlorophyll to “primary

electron acceptor”– need to replace electron in chlorophyll– enzyme extracts electrons from H2O & supplies

them to chlorophyll• splits H2O• O combines with another O to form O2

• O2 released to atmosphere• and we breathe easier!

Fill inStroma (fluid)

Thylakoid (disk)

Lab #4 - Photosynthesis

• DPIP was used to replace NADP+• DPIP accepted e- (reduced = RIG)• Turned from Blue Clear• More light could pass through

cuvette• Transmittance % increased

Light Dependent: Photophosphorylation

• Using light energy to add a P to ADP• Two types:– Non-Cyclic– Cyclic

Non-Cyclic Photophosphorylation

• Light reactions elevate electrons in 2 steps (PS II & PS I) – PS II generates

energy as ATP

– PS I generates reducing power as NADPH

– NADPH used in Calvin

Cycle (light independent)

Cyclic Photophosphorylation

• Involves only PS1

• Generates ATP but no NADPH or O2

• Supplements the ATP supply required for the Calvin Cycle

Light Independent: A Closer Look

• AKA The Calvin Cycle• In stroma• Uses ATP and NADPH to convert CO2 to

sugar• Makes a 3-C sugar• Needs 3 CO2 to make 1 Glucose

(C6H12O6)

• Uses 18 ATP (endergonic) and 12 NADPH to make 1 Glucose

G3P

• Glyceraldehyde-3-P – end product of Calvin cycle– energy rich 3 carbon sugar

“C3 photosynthesis”

• G3P is an important intermediate• G3P glucose carbohydrates

lipids phospholipids, fats, waxes

amino acids proteins nucleic acids DNA, RNA

RuBisCo• Enzyme which fixes carbon from air– ribulose bisphosphate carboxylase – CO2 + 5-C sugar (RuBP) is broken down by

RuBisCo into 3-C sugars–most abundant enzyme

Types of Plants• C3 - most plants, produce G3P – Ex: rice, wheat, soybeans– Occurs in a single chloroplast– CO2 + RuBp (5-C) = 6-C split into G3Ps

Glucose

Ruh-Roh…

• Hot, dry days– Stomata close to conserve water– CO2 is depleted

– O2 builds up from light reactions

• RuBisCo– when O2 concentration is high• RuBisCo bonds O to RuBP• O2 is a competitive substrate

• oxidation of RuBP• breakdown sugars

CALLED PHOTORESPIRATION

!

Photorespiration

• Consumes oxygen• Makes carbon dioxide• Produces no ATP• Decreases photosynthetic output

(decreases organic molecules used in Calvin Cycle)

SOLUTIONS!

• Plants living in hot, arid climates have evolved different modes of carbon fixation– C4 and CAM plants

• C4 – turn CO2 into a 4-C compound

– Ex: corn, sugarcane– Favored in hot, arid

environments– 2 chloroplasts– Mesophyll = CO2 is

fixed into a 4-C– Bundle Sheath Cells

(cells surrounding veins) = Calvin Cycle

– Facilitates production of CO2 to combat photorespiration

FYI: the PEP Carboxylase has a much greater affinity for CO2 than RuBisCo at higher temps

• CAM Plants– Ex: Cacti,

Pineapple– Very arid

environments– At night: Stomata

open, take in CO2

– Store a 4-C compound in vacuole

– During Day: Light Rxns supply ATP and NADPH, Uses stored CO2 to complete Calvin Cycle

Photosynthesis Summary

• Light reactions– produced ATP– produced NADPH– consumed H2O– produced O2 as byproduct

• Calvin cycle– consumed CO2

– produced G3P (sugar)– regenerated ADP– regenerated NADP

Interdependence in Nature

REDOX rxns

Cell Respiration• Exergonic

– Energy is RELEASED from the oxidation of sugar

• Electrons are transported to OXYGEN = Water– Oxygen pulls e -s down to

produce concentration gradient of H +

– H+ is pumped through ATP Synthase ADP + Pi = ATP

Photosynthesis• Endergonic

– Energy is REQUIRED– Light is the energy

source that moves e-

• Water is split, electrons are moved from water to carbon dioxide– CO2 is reduced

– Makes sugar (glucose)

Photosynthesis Animation

• http://dendro.cnre.vt.edu/forestbiology/photosynthesis.swf