Photosynthesis

Equation for photosynthesis

Equation for photosynthesis

6CO2 + 6H2O = C6H12O6 + 6O2

Phosphorylation

An endogenic ( energy absorbing) reaction bonding a phosphate ion to a molecule of ADP using energy from light making ATP

Structure of a leaf and significance

• Large SA captures as much light as possible

• Thin so light penetrates through the leaf

• stomatal pore allow carbon dioxide to diffuse into the leaf

• air spaces in spongy mesophyll allow carbon dioxide to diffuse into photosynthesizing cells

• Spaces between Palisade cells allow carbon dioxide to diffuse to the photosynthesing cells

Structure of a leaf cell and significance

• Cuticle and epidermis are transparent , cellulose cell walls are thin so light penetrates through to the mesophyll

• Palisade cells have large vacuole so chloroplasts form a single layer at the periphery of each cell and don't shade each other

• Palisade cells are cylindrical, elongated at right angles to the surface so leaves can accommodate the large number of PC, light only passes through epidemic cell walls and one PCW before reaching chloroplast

Structure of chloroplasts and significance

• Large SA for maximum absorption

• Move within Palisade cells on top of the cell on dull days for maximum absorption. Bottom of the cell when light intensity is very high protecting pigments from bleaching

• rotate within Palisade cells so the thylacoids maximise the absorption of light

• Pigments in the thylacoids are in a single layer at the surface of the membrane so pigments maximise their absorption

• Five times as many in PC than spongy mesophyll as PC are at the top of the leaf are more exposed

Chloroplasts are transducers

They turn energy in the photons of light into chemical energy, made available through ATP and incorporated into molecules like glucose

Photosynthetic pigments

• Chlorophyll a: absorbs 435, 670 to 680. Pigment colour blue to green, occurs in all mosses ferns conifers flowering plants

• Chlorophyll B: 480, 650. Pigment colour yellow green, occurs in higher plants ( conifers flowering plants)

• Beta carotene: 425 to 480. Colour orange, all plants

• xantophyll: 400 to 500. Pigment colour yellow, occurs in most

Absorption spectrum and action spectrum

Absorption spectrum: a graph showing how much light is absorbed at different wavelengths

Action spectrum: a graph showing the rate of photosynthesis at different wavelengths

Correlation between action and absorption spectrum

There is a close correlation between the two suggesting the pigments are responsible for absorbing the light used in photosynthesis

Advantage of having many pigments in a leaf

Increases the range of wavelength light absorbed , increasing the efficiency of photosynthesis

Where is a photo system found

Lie in the plane of the thylacoid membrane

Antenna complex

An array of protein and pigment molecules in the thylakoid membranes of the granite that transfer energy from light of a range of wavelengths to chlorophyll a at the reaction centre

The reaction centre

Contains two molecules of chlorophyll a comma when chlorophyll a molecules absorb the light their excitation allows each one to emit an electron

• Photosystem 1 arranged around two Ca molecules 700nm

• Photosystem 2 680 peak nm

The process in the photo system

Some photons are absorbed by chlorophyll a directly but many are absorbed by chlorophyll B and carotenoids ; accessory pigments

The photons excite the accessory pigments and energy is passed through them to the reaction centre where electrons of chlorophyll a are excited and raised to a higher energy level

Chlorophyll a in the reaction centre

Is the most significant molecule as it passes energy to subsequent reactions of photosynthesis - referred as the primary pigment

Products of the light dependent stage

• ATP, providing chemical energy transduced from light energy to synthesise energy rich molecules like glucose

• Reduce the NADP providing reducing power to synthesise molecules like glucose from carbon dioxide

• Oxygen, a by product derived from water. And diffuses out of the chloroplasts out of the photosynthetic cells and out of the leaf through the stomata

Cyclical phosphorylation

ATP can be produced by electrons that take a cyclical pathway and are recycled back into the chlorophyll a in PSI

Non cyclical phosphorylation

ATP can be produced by electrons that take a linear pathway from water through PSI and PSII to NADP which they reduce

Cyclic phosphorylation process

• Light is absorbed by PSII. Electrons from the chlorophyll a are excited and picked up by the electronic sector

• This is passed down the electron transport chain to PSI

• Electrons are excited in chlorophyll a in PS1 and picked up by the electron acceptor

• The electron goes down the ETC back to PSI

Non cyclic phosphorylation

• The electron goes to NADP to help produce it

Photolysis of water

The Electrons lost from PSII must be replaced

In the thylakoid spaces water molecules absorb light which indirectly causes them to dissociate into hydrogen oxygen and electrons: H2O = 2H + 2e- +1/2O2

Photolysis is enhanced by protein complex in PSII

Passage of Protons and phosphorylation

• Photolysis of water also produces H + . concentration of H+ builds up in the thylakoid space producing an electrochemical gradient

• H+ is mopped up in the stroma by combining with electrons from PSI and reducing NADP to NADPH2

• H plus moves down the electrochemical gradient through ATP Synthetase, facilitating phosphorylation of ADP + Pi = ATP

Where does the light independence stage occur

in solution in the stroma of the chloroplast

Calvin cycle

• a five carbon acceptor molecule ribulose biphosphate combines with carbon dioxide, catalysed by enzyme ribulose biphosphate carboxylase (RUBISCO)

• Unstable 6 carbon compound is formed

• Six carbon compound splits into two molecules of three carbon compound: glycerate 3 phosphate

• GP is the first product made

• GP is reduced to triose phosphate by reduced NADP. Reducing a molecule requires energy and energy is provided by ATP made in LDS

• Triose phosphate is the first carb made in photosynthesis

• NADP is reformed

• Some of the triosphate is converted to glucose phosphate, then to starch by condensation

• Most of the trios phosphate goes through a series of rejections that regenerates RUBP so cycle continues, ATP provides energy for this

Product synthesis from the Calvin cycle: carbohydrates

First hexose made is fructose by phosphate. Compared to glucose and combined with fructose to make sucrose, for the transport around the plant

Alpha glucose converted to starch for storage, beta glucose polymerized into cellulose for cell walls

Product synthesis from Calvin cycle: fats

Acetyl coenzyme a can be synthesised from glycerate 3 phosphate and converted to fatty acids. Trios phosphate can be converted directly to glycerol. Fatty acids and glycerol undergo condensation reactions to form triglycerides

Product synthesis from carbon cycle: proteins

Glyceride three phosphate can be converted into amino acids for protein synthesis.**** is derived from ammonium ions made from nitrate ions (NO3-) taken in at the roots and transported throughout the plant

Limiting factors in photosynthesis

• Carbon dioxide

• Light intensity

• Temperature

Carbon dioxide concentration

• As the carbon dioxide concentration increases, the rate of light independent reactions increases and the rate of photosynthesis increases, therefore C02 concentration is a limiting factor

• If the concentration increased above nought 0.5%, the rate of photosynthesis remains constant implying C02 concentration is not affecting the rate and is not a limited factor at those concentrations

• The rate decreases above 1% as the stomata closes preventing carbon dioxide uptake

What is a limiting factor

A factor that limits the rate of physical process by being in short supply, an increase in the limiting factor increases the rate of the process

Light intensity

• in darkness, The light independent reactions of photosynthesis still occur but light dependent do not so not oxygen is evolved

• As the light intensity increases the light dependent reactions occur with increasing efficiency so rate of photosynthesis increases. Light intensity is controlling the rate and is a limited factor

• Higher light intensity doesn't produce fax reactions and so the rates of photosynthesis remains constant- isn't a limiting factor

• If the light intensity is even higher the rate will decrease because photosynthetic pigments are damaged and will not absorb light efficiently, light dependent stage fails

Light composition point

As light intensity decreases, rate of light dependent reactions decrease and so the rate of light independent reactions decrease and the rate of carbon dioxide uptake increase decreases

NAOH and KOH- Absorb carbon dioxide

NAHCO3- Release carbon dioxide

Light compensation point definition

The light intensity at which a plant has no net gas exchange as the volume of gas is used and produced in respiration and photosynthesis are equal

Temperature

Increase temperature increases rate as kinetic energy of the molecules involved increases. Particular temperature the enzymes progressively denature and the rate decreases so temperature doesn't control the rate at is a limited factor

When water is scarce a plant sells plasmolyse, stomata closes wilting occurs a many physical functions are affected

even slight water deprivation can reduce carbohydrates made so water availability isn't a limiting factor in vertices

Mineral nutrition

• Structural role: calcium in the middle lamella of cell walls

• Synthesis of compounds needed for the growth of the plant like nitrogen or as enzyme activators like magnesium requirements by ATPA's and DNA polymerase

• May form an integral part of a molecule: magnesium and chlorophyll iron in the carriers of theETC

Deprivation of magnesium and nitrogen in the soil

Causes chlorosis - the yellowing of leaves

Magnesium is used for chlorophyll production

Nitrogen is used for the growth of the plant