CHAPTER 7Where it Starts--Photosynthesis

SUSTAIN LIFE Obtain energy

AutotrophsHeterotrophs

Metabolism—biochemical processes release energyPhotosynthesisCellular Respiration

SUSTAIN LIFE Food energy stored in chemical bonds

Exergonic (cellular respiration)Endergonic (photosynthesis)

Energy transfers from endergonic to exergonic through ATP

PHOTOSYNTHESIS Chlorophyll

PlantsAlgaeSome bacteria

Transfer sun’s energy into chemical bonds

PHOTOSYNTHESIS Three stages

Light-capturingLight-dependentLight-independent

CO2 + H2O => C6H12O6 (glucose) + O2

PROPERTIES OF LIGHT Wavelength

Spectrum

PROPERTIES OF LIGHT Photons

Packets of particle-like lightFixed energy

Energy levelLow energy = long wavelength

Microwaves, radio wavesHigh energy = short wavelength

Gamma rays, x-rays

PROPERTIES OF LIGHT The light that you see is REFLECTED, not

absorbed.

Therefore, a green plant is reflecting the green part of the spectrum (and photons of that energy), not absorbing them.

PIGMENTS Molecules that absorb photons of only a

particular wavelength Chlorophyll a

Absorbs red, blue, violet lightReflects green, yellow lightMajor pigment in almost all photoautotrophs

Chlorophyll bAbsorbs red-orange, some blueReflects green, some blue

PIGMENTS Carotenoids

Absorb blue-violet, blue-green lightReflect red, orange, yellow lightGive color to many flowers, fruits,

vegetablesColor leaves in Autumn

PIGMENTS Anthocyanins

Absorb green, yellow, some orange lightReflect red, purple lightCherries, many flowersColor leaves in Autumn

PhycobilinsAbsorb green, yellow, orange lightReflect red, blue-green lightSome algae & bacteria

ELECTRON ENERGY Pigment absorbs light of specific

wavelentghCorresponds to energy of photon

Electron absorbs energy from photon Energy boosts electron to higher level Electron then returns to original level When it returns, emits some energy

(heat or photon)

OVERVIEW Stage 1 (Light-Dependent)

Light energy converted to bond energy of ATP

Water molecules split, helping to form NADPH

Oxygen atoms escape Stage 2 (Light-Independent)

ATP energy used to synthesize glucose & other carbohydrates

CHLOROPLASTS

LIGHT-DEPENDENT REACTIONS Occurs in thylakoids Electrons transfer light energy in

electron transport chain

Electron transfers pump H+ into inner thylakoid compartment

Repeats, building up concentration and electric gradients

LIGHT-DEPENDENT REACTIONS

H+ can only pass through channels inside ATP Synthase

Ion flow through channel makes protein turn, forcing Phosphate onto ADP

LIGHT-DEPENDENT REACTIONS

Electrons continue until bonding NADP+ to form NADPH

NADPH used in next part of cycle

LIGHT-DEPENDENT REACTIONS

LIGHT-INDEPENDENT REACTIONS

CO2 in air attaches to rubisco (RuBP) Splits to form PGA PGA gets phosphate from ATP, then H+

and electrons from NADPH Forms PGAL Two PGAL combine to form glucose plus

phosphate group

LIGHT-INDEPENDENT REACTIONS

Some PGAL recycles to form more RuBP Takes 6 “turns” of cycle to form one

glucose molecule 6 CO2 must be fixed and 12 PGAL must

form to produce one glucose molecule and keep the cycle running

LIGHT-INDEPENDENT REACTIONS

*(G3P = PGAL)

StomataClose when hot & dryKeeps water insidePrevents CO2 & O2 exchange

Basswood, beans, peas, evergreens 3-Carbon PGA is first stable intermediate

in Calvin-Benson cycle Stomata close, O2 builds up Increased O2 levels compete w/ CO2 in

cycle Rubisco attaches oxygen, NOT carbon to

RuBP This yields 1 PGA rather than 2 Lowers sugar production & growth of

plant 12 “turns” rather than 6 to make sugars

Better adapted to cold & wet

C3 PLANTS

Corn, tropical plants Also close stomata on hot, dry days Pumps carbon through cycles in 2 cells

Mesophyll cells: create 4-carbon molecule (oxaloacetate)

Bundle-sheath cells: take 4-carbon molecule (malate), releases CO2 to Calvin-Benson cycle

This allows CO2 to remain high for C-B cycle

Requires 1 more ATP than C3, but less water lost & more sugar produced

Adapted to higher light & temp, lower water

C4 PLANTS

C4 PLANTS

Desert plants (cactus) Crassulcean Acid Metabolism Opens stomata at night, uses C4 cycle Cells store malate & organic acids During day when stomata close, malate

releases CO2 for C-B cycle

CAM PLANTS