Photosynthesis

Chapter 10

Autotrophs

• Chemoautotroph - An organism, that obtains its nourishment through the oxidation of inorganic chemical compounds as opposed to photosynthesis.

• Photoautotroph – An organism that uses radiant energy (light) as a source of energy to synthesize carbohydrates

Chloroplast structure

Inner and Outer membranes

Granum

Stroma (Fluid interior)Thylakoid membrane

Cross-section of leaf

Chloroplasts can be found in the mesophyll cells

Bundle sheath cell

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Chlorophyll is a Photoreceptor

• Chlorophyll is found in the chloroplasts of green plants, and is what makes green plants, green.

• The basic structure of a chlorophyll molecule is a porphyrin ring, with a central atom. This is very similar in structure to the heme group found in hemoglobin, except that in heme the central atom is iron, whereas in porphyrin it is magnesium.

• There are actually 2 types of chlorophyll, named a and b. They differ only slightly, in the composition of a sidechain (in a it is -CH3, in b it is CHO). Both of these two chlorophylls are very effective photoreceptors.

What is Electromagnetic Radiation?• Electromagnetic radiation can be described in terms of a

stream of photons, which are massless particles (they actually have EXTREMELY LOW MASS) each traveling in a wave-like pattern and moving at the speed of light (only in a vacuum).

• So, all electromagnetic radiation travels at the speed of light (c) which is 299,792,458 meters per second (1,079,252,848.8 km/h).

• Each photon contains a certain amount (or bundle) of energy, and all electromagnetic radiation consists of these photons.

• The only difference between the various types of electromagnetic radiation is the amount of energy found in the photons.

• Radio waves have photons with low energies, microwaves have a little more energy than radio waves, infrared has still more, then visible, ultraviolet, X-rays, and ... the most energetic of all ... gamma-rays.

How can Electromagnetic Radiation affect us?

• EMR comes from all kinds of different sources – the primary source being outer space. EMR also comes from the sun (all heavenly bodies), man-made objects such as radios, etc.

• EM radiation carries energy and momentum, which may be transferred to any matter when it interacts with the matter.

– For example, Ultraviolet radiation (from the sun) can cause damage to our DNA – resulting in skin cancer.

– Microwave radiation agitates molecules of water, producing heat and eventually “cooking” the matter (hopefully food)

The Electromagnetic Spectrum

Are all the photons different?• All the photons are the same, they just

contain different amounts of energy and therefore travel at different wavelengths

• Radio waves, visible light, X-rays, and all the other parts of the electromagnetic spectrum are fundamentally the same thing. They are all electromagnetic radiation

Rainbows

• In empty space (vacuum) all photons travel with the same speed or velocity.

• Photons are slowed down when they pass through different media such as water, glass or even air. This slowing down accounts for the refraction or bending of light by optical lenses.

• The energy of the photon is not changed, but the wavelength is.

• Different energy optical photons are slowed by different amounts in glass or water; this leads to the dispersion of light and the appearance of rainbows.

The Visible Spectrum

• Some photons have energy and wavelengths that allow us to see them

• This is visible light – what is light?

• Zillions of photons moving in a wave-like pattern with energy levels that make them visible to the human eye (between 400 and 700 nanometers).

• Some animals like fish and snakes, can see photons (wavelengths) that we cannot

The Visible Light Spectrum

Visible Light

• So visible light appears white, but it is a collection of photons traveling at different wavelengths and frequencies.

• A greater intensity of light means more photons present.

What happens to photons once they hit matter?

• Photons interact with electrons in all matter- one photon interacts with one electron.

• The collision causes the electrons to be ejected from their shells (they enter an excited state). They eventually may return to ground state.

• If the energy of the photon is high enough, the electron leaves the matter

• The photons are destroyed and others are created during these collisions with electrons

• this is called the photoelectric effect.

Chlorophyll absorbs blue light

As the chlorophyll in leaves decays in the autumn, the green colour fades and reveals the oranges and reds of carotenoids.

There are three major pigment types: 1. Chlorophylls 2. Caretonoids and 3. Phycobilins (phycoerythrin, phycocyanin)

These main pigment types each have a characteristic absorption spectrum.

Chlorophylls absorb blue/red and caretonoids absorb blue/green light. Photosynthetic pigments in plants do not effectively absorb green and yellow light, which is why plants appear green or yellow-green.

Phycobilins are found (in addition to the first two pigments) in cyanobacteria (also termed ‘blue-green’ algae) that absorb the portions of the spectrum not effectively absorbed by plants. Cyanobacteria (which are Prokaryotic - i.e. do not possess sub cellular components) were probably the first photosynthetic organisms.

Absorption of light wavelengths

Spectrophotometry

• Using photons of visible light to measure and analyze materials.

• You can use it to determine how much light a particular solution absorbs

• You can also use it to determine how much light a particular solution transmits (allows to pass through)

• You can also determine the wavelength of light the sample absorbed or transmitted.

Light Reactions- Occur in the thylakoid membrane- They capture the energy from light- Photons of light excite electrons in chlorophyll which jump to a primary electron acceptor in the photosystem- The e- leaves am “electron hole” in its place, which is replaced by the photolysis of water- The electrons travel down an electron transport chain in the thylakoid membrane- The proteins of the e- transport chain get reduced and oxidized. When reduced, they are able to pump H+ from the stroma, into the thylakoid space, creating a proton motive force- The H+ then diffuse back into the stroma, via ATP synthase- ATP synthase is energized by the H+ force to phosphorylate ADP into ATP- The final electron recipient is NADP+ (nicotinamide dinucleotide phosphate) a cousin of NAD+

Dark Reactions

- Do not really happen in the dark- Do not require light- Carbon is fixed in this reaction (From atmospheric CO2)- Energy from the ATP and electrons from NADPH (generated in the light reactions), is used to reduce the CO2 into G3P, a 3-

carbon sugar

What happens to when light “hits” chlorophyll molecules

• Photons of light excite certain electrons in the pigments

• These electrons “jump” to a higher state, leaving an “electron hole” behind in the ground state

• If the electron is not “captured”, it falls back to ground state, releasing the energy from the photon as heat and fluorescent light

Chlorophyll excited by UV light fluoresces red

LIGHT REACTIONS

The thylakoid membranes of plant chloroplasts have two different kinds of photosystems each with its own set of light harvesting chlorophyll and carotenoid molecules and the photochemical reaction centre.

Photosystem I - is maximally excited by light at longer wavelengths. (P700)Photosystems II - is maximally excited by shorter wavelengths. (P680)

The purpose of light reactions1) To build the chemiosmotic or proton gradient.2) Generate ATP.3) Reduce NADP+ to NADPH.

Non-cyclic Electron flow – starts at photosystem II and ends at NADPH

Light Reaction Facts

• O2 is created as a by-product of the Splitting of water (Photolysis)

• Proton Motive Force created for ATP Synthase (Some H+ come from photolysis of water and some pumped into lumen by electron transport chain)

• ATP produced for Dark Reactions• NADPH produced to reduce CO2 in Dark

Reactions (e- deliverer)

CALVIN CYCLE (Dark Reactions)

CALVIN CYCLE FACTS• The fixation of the CO2 is carried out by a giant enzyme

ribulose biphosphate carboxylase/oxidase (RUBISCO) which is the most abundant enzyme on earth. This enzyme is very sluggish it works much slower than most other enzymes. (i.e. ~ 3 molecules of substrate per sec. compared with ~1000/sec for others). Therefore, there are many copies of this enzyme in the stroma ~ 50% of chloroplast protein and most abundant protein in the world!

• The Facts on G3P (Glyceraldehyde - 3 - Phosphate)Plants also make other sorts of molecules from the products of the Calvin cycle. For example G3P is used by most seed plants to fashion a number of lipids and amino acids as well as Nitrogen bases (DNA and RNA).

Calvin Cycle Facts, cont’d.

• The cycle spends ATP as an energy source and consumes NADPH as reducing power for adding high energy electrons to make the sugar

• There are three phases of the cycle

1. CO2 fixation 2. Reduction (NADPH2 with energy from ATP) 3. Regeneration of CO2 acceptor (RuBP)

• For every 3 molecules of CO2 that enter the cycle one G3P is made and released from the cycle 6 NADPH are used for reduction 9 ATP are used for energy

Cyclic Light Reactions

Cyclic Light Reactions

• In cyclic electron flow, the electron begins in a pigment complex called photosystem I, passes from the primary acceptor to the rest of the electron transport chain, before returning to chlorophyll in photosystem I.

• This pathway is known as cyclic photophosphorylation, and it produces neither O2 nor NADPH. ATP is produced. In bacterial photosynthesis, a single photosystem is used, and therefore is involved in cyclic photophosphorylation.

Why do plants switch from non-cyclic to cyclic?

• Non-cyclic (linear) produces equal amounts of ATP and NADPH

• But the Dark reactions need more ATP than NADPH

• So when there is plenty of NADPH but not enough ATP, the plant switches-off half of the electron transport chain.

Photorespiration?Qu’est-ce que c’est?

As shown in the following graph, in the presence of elevated O2 levels, photosynthesis rates are lower.

• O2 has an inhibitory effect upon photosynthesis.

C3 Plants

• Most plants fix carbon via Rubisco, to make a 3-carbon compound called 3-phosphoglycerate

• These plants are called C3 plants because the first compound made upon CO2 fixation is this 3-carbon compound.

• Plants found in temperate biomes are all C3 plants. Also agricultural plants such as rice, wheat, soy, are C3 plants.

• In C3 plants, all photosynthesis steps (light as well as dark reactions) take place in the chloroplasts of the mesophyll cells

Cross-section of leaf

Chloroplasts can be found in the mesophyll cells

Bundle sheath cell

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Photorespiration – A problem with C3 plants

• On hot, dry days, plants partially close their stomata, thus reducing CO2 concentrations in the leaves.

• Since O2 is still being made from the light reactions, O2 concentrations increase in the leaves.

• Rubisco’s active site can bind both O2 and CO2.

• So when CO2 levels drop and O2 levels increase, O2 competes for the Rubisco active site.

• Rubisco now fixes O2 instead of CO2 in the Calvin cycle

• The final product is no G3P (sugar), but an intermediate product that peroxisomes and mitochondria in the plant cells split to release CO2.

• Because O2 is used and CO2 is released in the presence of light, this reaction is called Photorespiration. Unlike cellular respiration however, no ATP is generated.

• Photorespiration is wasteful! No sugar is made and the ATP generated from light reactions is used.

Photorespiration – A problem with C3 plants

Photorespiration, cont’d.

• This is because there is competition between O2 and CO2 for the active site on the Rubisco enzyme of the Calvin cycle.

• In other words, Rubisco can bind to both, CO2 and O2 equally well

In "normal" photosynthesis, CO2 is joined with RUBP to form 2 molecules of 3PGA

In the process called photorespiration, O2

replaces CO2 in a non-productive (less sugar and no ATP made), wasteful reaction.O2 is taken in and CO2 is released, hence Photorespiration

Does Photorespiration have ANY benefits?

• Researcher believe that photorespiration, as wasteful as it is, may protect the plant from damage caused by too much light.

• In other words, if light reactions are still taking place, NADPH and O2 are building up, but the Calvin cycle is not progressing due to low CO2 levels – this can harm a plant.

• So photorespiration alleviates this buildup by using some of the O2 to push forward the Calvin cycle.

Hence, the evolution of C4 plants…

• C4 plants are so named because the first compound made upon CO2 fixation is a 4-carbon compound such as oxaloacetate or malate

• In C4 plants, the photosynthesis steps are split between the chloroplasts of two different types of cells: The mesophyll cells and the bundle-sheath cells

• The mesophyll cells fix carbon dioxide via an enzyme called PEPc (phosphenolpyruvate carboxylase) to make the 4-carbon oxaloacetate

• Examples of C4 plants : various grasses, corn, sugarcane, many hardy weeds, etc.

PEPc vs. Rubisco. Hungry anyone?

• PEPc as a much higher affinity to CO2 than O2, unlike rubisco which swings both ways

• Therefore, PEPc binds to CO2 even when CO2 levels are depleted.

• The mesophyll then exports its 4-carbon malate to the bundle-sheath cells via plasmodesmata

• The 4-carbon malate releases CO2 in the bundle-sheath cells, which rubisco fixes into 3-phosphoglycerate.

• The Calvin cycle can proceed as usual now.

C4 Plants - Synopsis

• In C4 plants, the mesophyll cells can keep the CO2 concentrations in the bundle-sheath cells high enough to keep rubisco from binding O2.

• Therefore, C4 photosynthesis minimizes photorespiration and increases sugar production.

• Found in plants belonging to hot, biomes with intense sunlight.

What is a bundle-sheath cell?

• Thick-walled plant cell surrounding veins that functions in a different version of photosynthesis called C4 photosynthesis.

Parenchyma or

CAM PlantsCAM Plants• Another evolutionary adaptations to hot and arid Another evolutionary adaptations to hot and arid

conditions is the Crassulacean metabolism.conditions is the Crassulacean metabolism.

• These plants perform C4 photosynthesis, but they keep These plants perform C4 photosynthesis, but they keep their stomata completely closed during the day and open their stomata completely closed during the day and open them at night.them at night.

• At night, the mesophyll cells use PEPc to fix COAt night, the mesophyll cells use PEPc to fix CO22 and and store it as 4-carbon organic compounds such as malatestore it as 4-carbon organic compounds such as malate

• In the daytime, when their stomata close, the organic In the daytime, when their stomata close, the organic compounds release COcompounds release CO22 for Rubisco to fix into 3- for Rubisco to fix into 3-phosphoglycerate and the Calvin cycle proceeds as phosphoglycerate and the Calvin cycle proceeds as usual.usual.

Crassulacean Metabolism or CA plants

Aloe vera

• Succulents such as various cacti and pineapple, perform CAM photosynthesis

C4 vs. CAM

Comparison of ALL three types of photosynthesizing plants

Why does Rubisco bind O2?

• Possible evolutionary remnant – Earth’s early atmosphere had low O2 if any at all.

• When Rubisco evolved in the first Photoautotrophs, it did not matter that its active site was not specific to CO2 only, since no O2 was present