Photosynthesis

Chloroplasts,  Absoption and action spectra, light harvesting, light-dependent and light independent stage, product synthesis.

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An overview

6CO2 +6H2O > C6H12O6 +6O2

It involves two stages

  • Light dependent stage: Converts light energy to chemical enegy using the photolysis of water to create protons and electrons. This reduces NADP, and phosphorylates to make ATP.
  • Calvin cycle e.g. light independent: This uses NADP and ATP to reduce CO2 and make glucose.

MAIN SITE:

PALISADE TISSUE

Importance:

  • fixes CO2
  • and O2
  • into a useable form/ organic compunds
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Englemann's experiment

Englemann found the site of photosynthesis.

He did this by

  • suspending filamentous green alga (each cell contains a large chloroplast) in a suspension with dilute aerobic, motile bacteria,
  • then exposed different algae cells to different wavelengths of light.
  • Blue and red wavelengths had the most clustered bacteria because they were the most efficient in photosynthesis, so more oxygen was produced.
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Chloroplasts: Structure and Role

Chloroplasts have a double membrane, and inside this there are:

  • Grana: Stacks of up to 100 disc like structures called thykaloids. This is where photosynthetic pigments are kept and light dependent reactions take place.
  • Stroma: Fluid filled interior where light independent reactions take place. 

Photosynthetic pigments

  • These trap light energy. Flowering plants have two main types: chlorophylls and carotenoids.

Chlorophylls absorb light energy in the red and blue-violet spectrums, and carotenoids absorb them mainly in the blue-violet. The two types of carotenoids (the carotenes and the xanthophylls) are accessory pigments.

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Absorption and action spectra

The shorter the wavelength, the more energy the wave contains.

Pigments absorb different wavelengths; this can be demonstrated by making solutions of each pigment and shining light through them

An absorption spectrum

  • can then be displayed as a graph,
  • but only shows whether the pigments absorb the wavelengths, and not if they are used in photosynthesis.
  • Chlorophyll absorbs red and blue-violet region light energy.

An action spectrum

  • shows the rate of photosynthesis with different wavelengths of light.
  • This shows the amount of carbohydrate synthesised by plants. 

The two graphs, when superimposed, show similar correlations, suggesting that pigments are responsible, or important, in photosynthesis.

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Light Harvesting in antenna complexes

The chlorophylls pigments in the membrane of thykaloids clump together in clusters of several hundred molecules. These clust are called antenna complexes.

These are intrinsic in the thylakoid membrane.

They have proteins associated with them which funnel photons of light energy, so that the pigments are moving the energy from molecule to molecule. The photons are moved until they reach chlorophyll a, which is the primary pigment molecule and so called the reaction centre.

There are two types of reaction centre:

  • Photosystem I (PSI) is arranged around a chlorophyll a molecule with an absorption peak of 700nm.
  • Photosystem II (PSII) same as above but with absorption peak of 680nm.
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Light dependent stage

Reactions involve:

  • Photolysis- splittling of water by light to make protons and electrons
  • Photophosphorylation of ADP and Pi to make ATP
  • NADP+ protons to make reduced NADP

When light strikes the reaction centre of an antenna complex, an electron is raised to a high energy level, creating a flow of electrons, which we call the Z scheme.

The loss of electrons is OXIDATION.

CHEMIOSMOSIS:

The light dependent stage take place in the thylakoid membranes. ATP produced from the flow of electrons from PII to PI provides energy to pump protons from the stroma into the thylakoid space. A concentration gradient builds up, and H+ ions move out by facillitated diffusion, by ATP synthetase, synthesising ATP.

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Non-cyclic photosphosphorylation

ATP is synthesised in 2 ways:

  • Non cyclic photophosphorylation

This involves both Photosystem I and II.

  • Photons of light are absorbed by PII.
  • 2 Electrons become excited and are lost (and then replaced by electrons from the photolysis of water)
  • The electrons are picked up by an electron acceptor which then passes them to PI, which absorbs them.
  • Whilst moving to PI, energy lost by the electrons is used to phosphorylate ADP ino ATP.
  • PI then loses two excited electrons, which are absorbed by a electron acceptor.
  • These electrons then combine with NADP and protons (from photolysis of water) to make reduced NADP.
  • The key product is reduced NADP, and a waste product is O2.

It is non-cyclic because water is constantly feeding in electrons and protons.

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Photolysis of water

Occurs in thylakoid space, so raising H+ concentration here.

When light hits a reaction centre, the photosystem loses electrons which must be replaced. These are provided by water molecules splitting using light energy.

Water molecules split into hydrogen ions, electrons and oxygen. Light is responsible for this indirectly.

  • Electrons replenish photosystem II.
  • Protons reduced NADP, (along with electrons) This takes place in the stroma, so H+ conc decreases- this maintains the concentration gradient between space and stroma.
  • NADP is the FINAL ELECTRON ACCEPTOR.
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Cyclic photophosphorylation

This involves only photosystem I.

  • Light energy is absorbed by PSI and passed to chlorophyll a at the reaction centre.
  • The electrons lost are at a higher energy level and accepted by the second electron acceptor.
  • As electrons pass along electron carriers, energy is generate to make ATP.
  • Electrons which are not used to make reduced NADP, return to PI before going round again = CYCLE.
  • No NADP is made, just ATP.

The Electron Transport Chain

  • H+ is high in thylakoid space
  • H+is low in stroma of cholroplast
  • H+ diffuse down stalked particle into stroma, mechanically turning the particle.
  • To Generate ATP.
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Light independent stage

Occurs in the stroma of the chloroplast, and each reaction has a specific enzyme.

It uses the products, ATP and rNADP, from the LDS.

They are used as follows:

  • a 5 carbon acceptor molecule, ribulose bisphosphate (RuBP) combines with CO2  to form an unstable 6 carbon compound. This is catalysed by enzyme Rubisco.
  • The six-carbon compound immediately splits into two molecules of three carbon compund called glycerate-3-phosphate (GP).
  • GP is phosphorylated by ATP then reduced by NADP to make triose phosphate.
  • 1/6 of these three carbon compounds can be built up to make glucose phosphate and then into starch by condensation.
  • Most (5/6) triose phosphate enters a series of ATP-driven reactions which then regenerates RuBP.
  • NADH is oxidised and goes back to the light dependent stage to be reduced again.
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Product Synthesis: inorganic ions

Following the metabolism of triose phosphate into carbohydrates, proteins and lipids, plus the absorption of mineral ions from the soil, a plant can synthesis all prodicts needed for life.

Inorganic nutrients can be a limiting factor on metabolism if in short supply.

Macronutrients: sodium, potassium, nitrate, calcium, magnesium and phosphate.

(Smart people never combine MD + pepper)

Micronutrients: manganese and copper.

Phosphate is absorbed and used in cell membranes and ATP.

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Nitrates and Magnesium

Nitrogen

  • is taken up as nitrates through the roots of a plant.
  • It is transported at nitrates in the xylem, and amino acids in the phloem.
  • Used for the synthesis of proteins and nucleic acids. Nitrate deficiency is characterised by reduced growth, and chlorosis (yellow leaves.)

Magnesium

  • is absorbed as Mg2+ and is used for chlorophyll production and activation of ATPase.
  • It is transpoted as Mg2+ in the xylem
  • Magnesium forms part of the chlorophyll molecule.
  • Pronounced chlorosis in the veins of older leaves is a key symptom of magnesium deficiency. Any magensium is moved to the leaves,for photosynthesis, so the veins will appear yellow first. 
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