Chapter 8 Photosynthesis:Energy from the Sun

Biology 101

Tri-County Technical College

Pendleton, SC

The Chemical Equation

6CO2 + 6H2OC6H12O6 + 6O2

– Not quite correct, but it will do Light and chlorophyll omitted from

equation but absolutely essential Conversion of light energy into chemical

energy Conversion of inorganic substances into an

organic substance

The Two Pathways

Photosynthesis occurs in chloroplasts of photosynthetic eukaryotic cells

Consists of LIGHT and DARK reactions– Light reactions aptly named but not so the dark

**Key: Dark reactions require products of light reaction

Light reactions produces ATP and NADPH Dark reactions produces sugar

More Redox…hot damn!!

Light reactions composed of 2 photosystems PS I and PS II Can run either cyclic or noncyclic electron flow Light reactions are series of redox reactions using

electron carriers in thylakoid membrane and chemiosmosis

Depending on “what’s” running, can make ATP and NADPH

Photon Power Light is form of electromagnetic radiation Wave-theory explains most of what we know

about light– Waves composed of discrete packets of energy called

photons Wavelength is distance from peak of one wave to

peak of the next wave Humans see in range of 400-700 nm Below 400 is ultraviolet, above 700 is infrared

Light, cont.

**Shorter the wavelength, the greater the energy (energy is inversely proportional to wavelength)

Three things can happen when photon meets a molecule– Bounce off (be reflected)– May pass through molecule (transmitted)– May be absorbed by molecule

If absorbed, it disappears but not its energy—energy acquired by molecule absorbing photon

Light III

Molecule is raised from ground state to excited state

An electron is boosted into a higher orbital– May fall back and emit that energy as light– May be so excited it is lost to the molecule

Molecules that absorb wavelengths in visible spectrum called pigments

Let’s talk about reflection and absorption

Chlorophyll Absorption In plants, 2 chlorophylls predominate

– a and b Absorb in red and blue wavelengths (their

absorption spectrum) If only chlorophyll pigments were active in

photosynthesis, MOST of visible spectrum would be unused

Accessory pigments absorb photons between red and blue wavelengths and transfer that energy to the chlorophylls (action spectrum for photosynthesis)

Absorption Visual

Action Spectrum Visual

Photosynthesis and Pigments

Chlorophyll a and b Carotenoids (B-carotene & others) absorb in

blue/blue-green and appear deep yellow Phycobolins (phycoerythrin &

phycocyanin) absorb in yellow-green, yellow, and orange

Warms my heart…

Pigment molecule absorbs photon and gets excited

Can return to ground state by emitting energy as fluorescence or pass energy along to another pigment molecule

Pigments arranged into antenna systems– Excitation passed from one pigment to another

until reaches reaction center– Center is always chlorophyll a molecule that

absorbs longest wavelengths

Fluorescence Visual

Excitation, cont.

Light energy absorbed is passed along as an electron

Ground state chlorophyll not much of reducing agent, but excited chlorophyll (Chl+) is– Chl+ can react with an oxidizing agent

Electron(s) can be boosted out of PS beginning series of redox reactions

E & E Visual

Photosystem

Accumulation of pigment molecules held together by proteins in right orientation for light absorption

Pigment molecules located so they can pass excitation to next pigment molecule in system

Eventually winds up at reaction center which (in plants) is always a chlorophyll a molecule– Absorbs at highest wavelength of all pigments in

system

Photosystem, cont.

Only reaction center can “boost” electron out of PS to electron carriers in ETC in thylakoid membrane

Depending on PS being used and pathway, electron(s) can “wind up” in an electron carrier or be used in ETC (chemiosmosis) to eventually manufacture ATP

Photophosphorylation Chalk talk time on PS I and cyclic electron

flow Chalk talk time on noncyclic electron flow

utilizing PS I and PS II One can bet the farm they will need this

info: Product(s) of cyclic electron flow using PSI, product(s) of noncyclic electron flow using PS I and II, and why is PS II needed in the first place?

Light Reactions Data

6CO2 + 6H2OC6H12O6 + 6O2

Water molecules split in noncyclic electron flow to replace electrons lost by PS II

Diatomic oxygen produced (2 waters split = 1 O2)

NOT shown by balanced formula are NADPHs and ATPs needed by dark reactions to “fix” a sugar

Comparing Chemiosmosis Works in both oxidative and

photophosphorylation Uses an ETC and redox reactions This active proton transport = proton motive

force (DOES 3 THINGS!!!!!) In mitochondrion, H+s pumped OUT of matrix

into intramembranous space In chloroplast, H+s pumped into interior of

thylakoid (from stroma to inside of thylakoid) ATPase and synthesis of ATP

Calvin-Benson Cycle Often called the “dark reactions” Incorporate CO2 into a sugar Occurs in stroma of chloroplast, why? Is a cycle, so something must already be

“there” Uses high-energy compounds made during

light reactions to reduce carbon dioxide to carbohydrate

Three processes make up the cycle

Fixation of CO2

RuBP (ribulose bisphosphate) already present in stroma; starts the “cycle”

Is a 5-carbon compound with two phosphates Rubisco “fixes” carbon from CO2 into RuBP Forms unstable 6-carbon biphosphate

moleculesplits into two 3-carbon phosphated molecules called PGA (phosphoglyceric acid)

Fixed CO2 into Carb Series of reactions involves phosphorylation using

ATP and a reduction using NADPH Product of interest is G3P (PGAL) Two G3P can be combined into one glucose In typical leaf, about 1/3 winds up in starch

– 2/3s winds up converted to sucrose Transported out to other organs where hydrolyzed

to glucose/fructose for various uses Carbons from glucose can be used to make AAs,

lipids, and precursor of nucleic acids How important is this process?

Regeneration of RuBP

Most of G3P (10 of them) end up as regenerated RuBP so cycle can continue

For every turn of the Calvin-Benson cycle, with one CO2 “fixed”, the acceptor (RuBP) gets regenerated

**Takes 6 turns of the Calvin-Benson cycle to make ONE glucose molecule

Calvin-Benson Visual

What’s the Cost? 12 ATP required to change 12 3-phosphoglyceric

acid to 12 1,3 diphosphoglyceric acid 12 NADPH required to finish conversion of 1,3

diphosphoglyceric acid to G3P 2 of the 12 G3P converted to one glucose Other 10 are converted to 6 ribulose 5-phosphate Requires another 6 ATP to convert ribulose 5-

phosphate into RuBP so cycle can continue Just have to understand the question..NOW and

later

Fickle finger of fate..so to speak

Balanced formula already provided and should be KNOWN

CO2 is “fixed” into G3P (takes 2 to make a glucose)

H2O split to provide electrons for PS II during noncyclic electron flow

Some protons (H+s) from splitting water wind up in carbohydrate

O2 generated by splitting of water and is released to environment (stomata)

Part and Parcel… Glucose produced used by plants to make other

compounds beside sugars– AAs, lipids, precursors of nucleic acids

Most of stored energy in these products released by glycolysis and cellular respiration during plant growth, development, and reproduction

Much plant matter ends up being consumed by animals– Glycolysis and cellular respiration in animals releases

free energy from plant matter for use in animal cells

See 3, See 4, See AM

Calvin-Benson cycle is the C3 Cycle Photorespiration is a big problem for C3 plants Some plants have a C4 cycle and others use CAM C4 plants fix oxaloacetate and cycle it into Calvin-

Benson CAM plants fix oxaloacetate, convert it into malic

acid and use it to provide carbons for Calvin-Benson