Photosynthesis

Photosynthesis

Photosynthesis & the Cycle of Life

• Captures the energy of light– Chlorophyll traps the light of the sun

• Overall reaction:6 CO2 + 12 H2O C6H12O6 + 6 O2

• End product is glucose, a 6 carbon sugar – the primary energy molecule for many

living organisms

• Uses CO2 & produces O2

In the Beginning• Life on Earth originated 3.5 to 4 billion years

ago.• The atmosphere was composed of methane,

carbon dioxide, and water vapor. • The cooling water collected in pools,

assimilating the nutrients from the rocks. • As water evaporated, the nutrients

concentrated, forming a rich soup. • The first organisms used these molecules for

food, breaking them down into water and carbon dioxide through respiration.

Evolution of Photosynthesis

• Eventually, these food molecules grew scarce

• Some organisms were able (through random mutation) to use the sun's energy to synthesize large molecules from small molecules.– This is the process of photosynthesis

• Organisms who create complex molecules this way are called autotrophs– Autotrophs are found in the bacterial and in the

plant kingdom.

• Produced O2 as a byproduct, changing the atmosphere

Discovery of Photosynthesis

• Joseph Priestly - chemist and minister– discovered that under an inverted jar, a candle would burn

out quickly– found that a mouse could similarly "injure" air. – showed that the "injured" air could be restored by a plant.

• Jan Ingenhousz – 1778 - Austrian court physician – repeated Priestly's experiments – discovered that it was the effect of sunlight on the plant

that caused it to rescue the mouse• Jean Senebier – 1796- a French pastor

– showed that CO2 was the "fixed" or "injured" air and that it was taken up by plants in photosynthesis.

• Theodore de Saussure – showed that the increased mass of a plant as it grows is

not due only to CO2 uptake, but also to water.

Photosynthesis Reactions

6 CO2 + 12 H2O C6H12O6 + 6 O2

• This is really a series of complex reactions

• Involves 2 phases:1. The Light Reactions – use energy from sunlight

2. The Dark Reactions– fix carbon

Overview• Light energy entering the plant splits the water

into hydrogen and oxygen: • H2O + light energy ½ O2 + 2H+ + 2 electrons• Electrons travel through the membrane much

like the electrons in oxidative phosphorylation• Their energy pumps protons through the

membrane. – This proton gradient can be used to synthesize ATP.

• The same electron reduces NADP+ to NADPH. – This molecule plays the same role in synthesis as

does NAD+ in respiration– a carrier of reductive power.

• This reductive power converts CO2 to glucose

The Nature of Light• Light behaves as both a wave and a

particle– particles of energy are called photons.

• As a wave. light has a wavelength (the distance from one peak of the wave to the next) and an amplitude (the distance the wave oscillates from its centerline).– Different wavelengths of light have different

characteristic energies and properties. • Light travels at various speeds in

different media, producing a frequency at which the wave travels.

Visible Light• Visible light is only part of the electromagnetic

spectrum – Only 1% of light that reaches planet

• Visible white light is actually made of different colors

Light Energy• The energy in a light wave is related

to frequency and wavelength• Different colors have different

wavelengths – different wavelengths have different

amounts of energy• Short wavelengths have high

energies and long wavelengths have lower energies.– Violet light has 2x energy of red

Pigments• How is light captured by living things?• Molecules, when struck by a wave or

photon of light, reflect some energy back

• Absorb some of the energy, and thus enter into a higher energy or excited state.

• Each molecule absorbs or reflects its own characteristic wavelengths of light.

• Pigments = molecules that absorb wavelengths in the visible region of the spectrum

Absorption of Light

• Light energy comes in "packets" called photons

• Plants can utilize energy only from absorbed wavelengths

• The color we see is the color reflected; the rest is absorbed

Absorption & Action Spectra

• An absorption spectrum for a pigment describes the wavelengths at which it can absorb light and enter into an excited state.

• An action spectrum describes the efficiency of a particular molecule at absorbing light– Shows what wavelengths of light the molecule can

trap to conduct photosynthesis.

• The action spectrum closely follows the absorption spectrum for a particular pigment – the molecule has to be able to absorb light to

enter into its excited state and pass the energy on.

Absorption & Action Spectra

Chlorophyll

• Chlorophylls are the green pigments in plants – Chlorophyll a – directly involved in the light reactions of

photosynthesis– Chlorophyll b – assists chlorophyll a; an accessory pigment

• Located in the membranes of the thylakoids inside chloroplasts

Chlorophyll & Light• When a photon strikes

chlorophyll, the photon's energy is transferred to an electron in the chlorophyll molecule– energized electrons

can't remain in this state– as the electron returns

to its original energy level, it releases absorbed energy.

Structure of Chlorophyll

Photosystems• Clusters of several hundred

pigment molecules in the thylakoid membranes

• Two types:– Photosystem I– Photosystem II

• Both are involved in the light reactions

Accessory Pigments

• Accessory pigments absorb light in other parts of spectrum & pass the energy to chlorophyll:– Xanthophylls - yellow pigments – Carotenoids - orange pigments– Anthocyanins – red pigments

The Chloroplast• The chloroplast is the organelle of

photosynthesis. • Resembles the mitochondrion

– Both are surrounded by a double membrane with an intermembrane space.

– Both have their own DNA. – Both are involved in energy metabolism. – Both have membrane reticulations filling their inner

space to increase the surface area on which reactions with membrane-bound proteins can take place.

• Has three membranes: inner, outer, and thylakoid

• Has three compartments: stroma, thylakoid space, and inter-membrane space.

Structures of Photosynthesis• The compartments and membranes isolate

different aspects of photosynthesis. • Chloroplasts have a highly organized array of

internal membranes called thylakoids• These form stacks of flattened structures = grana

– Photosynthesis begins in the grana– Pigments are embedded here

• Stroma are dark fluid-filled spaces between grana and the outer membrane – contain enzymes, DNA, RNA, ribosomes

• Light reactions take place on the thylakoid membranes.

• Dark reactions take place in the stroma.

Materials for Photosynthesis• CO2 is the source of C & O used to make

glucose • H2O is the source of H • Oxygen from H2O is released into the air

and produces O2 in the atmosphere– O2 drives cellular respiration in living

organisms

The Light Reactions

• Use trapped energy to convert ADP to ATP, which stores energy for later use

• Uses energy to split H2O to H and O

• The reactions leading to the production of ATP and reduction of NADP+ are called the light reactions because they are initiated by splitting water by light energy.

Photosystems• Non-cyclic photophosphorylation

Involves two sets of pigments:– Photosystem 1 (PS1)– Photosystem 2 (PS2)

• PS1 is better excited by light at about 700 nm– sometimes called P-700

• PS2 can’t use wavelengths longer than 680 nm– sometimes called P-680.

Non-cyclic Photophosphorylation

• Energy enters the system when PS2 becomes excited by light.

• Electrons are shed by the excited PS2 (oxidation), which grabs electrons from water– This produces a molecule of oxygen gas

for every two waters split. • PS2 thus returns it to its unexcited

state (reduction) . • The electrons are passed through a

chain of oxidation-reduction reactions.

The Redox Chain• Each element in the pathway is reduced by the

electrons• Each element then reduces its neighbor in the

pathway by giving it the electrons • Thus each element is reoxidized and ready for the

next electrons to pass through the photosystem.• PS2 passes on the energy to move the electron

through the redox chain– this pumps protons through the membrane to generate

ATP.

• PS1 passes on the energy required to reduce NADPH.

Cyclic Photophosphorylation• Sometimes an organism has all the

reductive power (NADPH) needed to synthesize new carbon skeletons – still needs ATP to power other activities in the

chloroplast.

• Many bacteria can shut off PS2, allowing the production of ATP in the absence of glucose

• A proton gradient is generated across the membrane using the mechanisms of photosynthesis.

• This type of energy generation is called cyclic photophosphorylation.

The Role of PS1• The role of PS1 seems contradictory

– In noncyclic phototphosphorylation PS1 was responsible for NADPH production

– In cyclic photophosphorylation it is needed for ATP production.

• PS1 is a good candidate for noncyclic photophosphorylation and for NADPH production.– PS1 is good at transferring an electron, whether to

NADP or to ferredoxin (fd). – It is a powerful reductant.

The Role of PS2• PS2 is better at grabbing electrons

from water and transferring them to quinone (Q). – It is a good oxidant.

• The electron transferred is not derived from water, but from PS1 itself.

• It therefore must be recycled to PS1.

Steps of the Light Reactions - 1

Chlorophyll in the grana absorb photons of light– energy from the photons boosts electrons from the

chlorophyll a molecules of Photosystem II to a higher energy level

Steps of the Light ReactionsThe excited electrons leave chlorophyll a

– They are transferred to a molecule in the thylakoid membrane: the primary electron acceptor


• Electrons lost from the chlorophyll are replaced by electrons from water molecules. – This splits H2O into

H ions & O2 gas


The primary electron acceptor donates the electrons to the first of a series of molecules called the electron transport chain.– energized electrons

move from one molecule to another in the electron transport chain

– each time a transfer is made, energy is released


• Chemiosmosis– Energy from the proton

gradient created in the thylakoid membrane fuel ATP synthase, an enzyme

– Energy released from electrons as they move down the electron transport chain is used to form ATP, combining ADP & phosphates in the stroma of chloroplasts

• Energy from the light reactions in the form of NADPH and ATP will fuel the dark reactions that follow


• Light is also absorbed by Photosystem I– Electrons move from chlorophyll a molecules to

another primary electron acceptor– These electrons are replaced by the electrons that

passed through the electron transport chain in photosystem II


• The primary electron acceptor of Photosystem I donates electrons to a 2nd electron transport chain. – These electrons reduce NADP+ to NADPH

Summarizing the Light Reactions

The Dark Reactions• The reduction of carbon dioxide to glucose,

using the NADPH produced by the light reactions, is governed by the dark reactions

• Also known as the Calvin Cycle for Melvin Calvin, described in 1950’s– Requires several enzymes & produces several

byproducts– Takes place in the stroma of the chloroplasts

• Fixes carbon from CO2 to form glucose

• Begins and ends with a five carbon sugar, RDP (ribulose diphosphate)

Products of The Calvin-Benson Cycle

• The cycle runs 6 times, each time incorporating a new carbon.

• Ribulose is a five-carbon sugar and the gylceraldehydes are three-carbon sugars

• Running through the cycle six times generates: 6(5-carbon sugars) + 6(incorporated carbon dioxides)

• Those six CO2 are reduced to glucose by the conversion of NADPH to NADP+.

• Glucose serves as a building block to make polysaccharides, other monosaccharides, fats, amino acids, nucleotides, and all the molecules living things require.

Steps of the Dark Reactions - 1

• CO2 from the atmosphere combines with RDP in series of reactions which use ATP as an energy source.

• This forms PGA (phosphoglyceric acid), a molecule with 3 carbons


• PGA reacts with hydrogen from the light reactions to form PGAL (phosphoglyceraldehyde)– Most of the PGAL

formed is used to make more RDP.

– RDP combines with more CO2 and the cycle repeats.


• Some PGAL is combined to form glucose: – 2 PGALs form one

glucose C6H12O6

• Excess glucose is stored as starch which can be used as needed

Rubisco• The key enzyme in the Calvin Cycle

catalyzes the transformation of the 5-C sugar, ribulose-5-phosphate and the single-C CO2 to two 3-C 3-phosphoglycerates.

• This reaction has a very high G of -12.4 kcal/mol.

• The enzyme is called ribulose-1-5-biphosphote carboxylase, or Rubisco .

Abundance of Rubisco

• Rubisco accounts for 16% of the protein content of the chloroplast

• The most abundant protein on Earth. • Why is this protein so abundant?

– It is crucial to all life to have a source of carbon fixation

– The enzyme is very inefficient

Light Regulation of the Calvin Cycle

• The energy required for the Calvin Cycle, in the form of ATP and NADPH, comes from the light reactions.

• The plant or photosynthesizing bacterium needs to tightly regulate the Calvin Cycle with photosynthesis. – It would be wasteful to run the process

using ATP generated for other plant metabolism.

Linkage of Late & Dark Reactions

• The pH of the stroma increases as protons are pumped out of it through the membrane – The enzymes of the Calvin Cycle

function better at this higher pH.

• As the reduction potential of ferredoxin (fd) increases, it reduces a protein called thioredoxin.

Linkage of Late & Dark Reactions(continued)

• This reduction breaks a disulphide bridge in thioredoxin. – The enzyme now has free cysteines that

can compete for the the disulphide bonds in other enzymes.

– Several enzymes of the Calvin Cycle are activated by the breaking of disulphide bridges.

– So the activity of the light reactions is communicated to the dark reactions by an enzyme intermediate.

Linkage of Late & Dark Reactions(continued)

• The reactions of the Calvin cycle stop when they run out of substrate– As photosynthesis stops, there is no more

ATP or NADPH in the stroma for the dark reactions to take place.

• The light reactions increase the permeability of the stromal membrane to cofactors such as Mg++ which are required for the Calvin

Photosynthesis Summary

Alternative Pathways• Many diverse environments• Led to different approaches to

photosynthesis• Some organisms forgo the use of light for

energy production• Others modify photosynthetic pathways to

make then more compatible with environmental conditions

• All of these adaptations are variations on the same basic pathways of photosynthesis and respiration

Adaptations to Hot Climates

• In hot, dry climates plants need special adaptations– Water loss through stomata which must open

to exchange CO2 for O2 would be damaging

• Plants that fix carbon through the Calvin Cycle are C3 plants

• 2 alternative pathways:– C4 pathway

– CAM pathway

Rubisco• Rubisco is the most abundant

enzyme on Earth. • It is a very important • It is believed that Rubisco is so

abundant because of its inefficiencies.

• Rubisco will sometimes recognize oxygen as a substrate instead of carbon dioxide.

The Inefficiency of Rubisco• When Rubisco uses oxygen as a

substrate instead of carbon dioxide, it does not fix CO2 into sugar

• Instead, it creates phosphoglycolate, a nearly useless compound.

• This wastes energy• This reaction, directly competes with

the regular reaction at the same site on the enzyme.

• The result is very detrimental to photosynthesis

Alternate Fate of Rubisco

The Effect of Temperature• At 25°C, the rate of the carboxylase

reaction is 4x that of the oxygenase reaction– the plant is only about 20% inefficient.

• As temperature rises, the balance between O2 and CO2 in the air changes (due to changing solubility in the ocean)

• The carboxylase reaction is less and less dominant.

• Plants living in warm climates have to overcome this handicap

Balancing CO2 Input & Water Loss

• Plants in arid climates close the stomata (pores) in their leaves when it is very dry

• This creates a closed environment • As CO2 is used up in

photosynthesis, the relative concentration of O2 increases

• The oxygenase reaction begins to dominate.

The C4 Solution

• Plants living in these dry conditions have evolved a mechanism to make the CO2 concentration very high in the immediate environment of Rubisco

• This prevents the oxygenase reaction• This is called the C4 pathway

because it involves a 4 carbon intermediate in the outer cells.

The C4 Pathway• The conventional pathway is called the C3

pathway – it involves only the 3-carbon sugars.

• In the C4 pathway, a 4-carbon intermediate brings a molecule of CO2 into the bundle sheath cells– it is dropped right next to the location of the Calvin

Cycle.

• This ensures that the concentration of CO2 at the site of Rubisco is very high,

• Only the carboxylase reaction can take place. • The C4 pathway still uses the Calvin Cycle with

its 3-carbon sugar intermediates– it makes use of 4-carbon sugars to bring the carbon

dioxide closer to the site of fixation.

Picturing the C4 Pathway

Chemistry of the C4 Pathway

Why Aren’t All Plants C4?• Why don't the C4 plants out-compete

the C3 plants, which are inefficient?

• It takes ATP to bring the CO2 to the Rubisco.

• In moderate temperatures, this energy burden outweighs the advantage of eliminating the 1 in 5 times that Rubisco binds O2 instead of CO2.

• In warmer climates the C4 plants win

CAM Plants• Another strategy used in hot or dry

climates– Prevents water loss

• Plants open their stomata at night and close them during the day

• Take in CO2 at night and fix it in organic compounds

• Later, release carbon from these compounds to enter the Calvin cycle

• Steps of the photosynthetic pathway are separated in time

Comparing C4 & CAM Strategies

Lithotrophs• Some autotrophs don’t use energy from

sunlight• These bacteria derive reductive power

by oxidizing compounds such as hydrogen gas, carbon monoxide, ammonia, nitrite, hydrosulphuric acid, sulphur, sulphate, or iron.

• These organisms are called lithotrophs or “rock-eaters”

• This process = Chemosynthesis – oxidizing an inorganic substance and

transporting electrons through the membrane

Chemosynthesis• Chemosynthesis oxidizes inorganic substances &

transports electrons through the membrane– like in oxidative phosphorylation and photosynthesis– (remember - oxidation is a loss of electrons, so this

inorganic substance is the electron donor).

• This electron transport pumps protons through the membrane generating a proton gradient which can be used to form ATP.

• These organisms make so much ATP that they can drive the electron transport chain backwards to generate NADH.

• This NADH provides the reducing power needed to synthesize carbon structures from carbon dioxide.

Methanogens• The methanogens are a class of anaerobic bacteria. • They derive energy by reducing CO2 to methane• They use CO2 as an energy source rather than

treating it as an energy-depleted waste product. • The methanogens can oxidize hydrogen gas to

directly reduce NAD+ to NADH, • Don’t have to waste energy making ATP through

chemosynthesis and then driving it backwards through the electron transport chain.

• These organisms still incorporate their carbon into the Krebs Cycle for processing into amino acids, nucleic acids, and sugars.

Education

Photosynthesis