plant physiology

• Green plants synthesise the food they need, by photosynthesis and all other organisms depend on them for their needs
• Photosynthesis

  A physico-chemical process by which plants use light energy to drive the synthesis of organic compounds
• The use of energy from sunlight by plants doing photosynthesis is the basis of life on earth
• Importance of photosynthesis

  • It is the primary source of all food on earth
  • It is also responsible for the release of oxygen into the atmosphere
• Photosynthesis occurred only in green part of leaves in the presence of light
• CO₂ is needed for photosynthesis
• Joseph Priestley's experiment

  Using a burning candle, a mouse, mint plant and a bell jar for closed space, hypothesised that plants restore to the air whatever burning candles or breathing animals remove
• Jan Ingenhousz's experiment

  In an elegant experiment with an aquatic plant, showed that in bright sunlight plants produce oxygen
• Julius von Sachs' finding

  Glucose is made in green plant parts and stored as starch
• T. W. Engelmann's experiment

  Using a prism, green alga Cladophora and aerobic bacteria, described the action spectrum of photosynthesis, which roughly resembles the absorption spectrum of chlorophyll- a and b
• Cornelius van Niel's demonstration

  1. Photosynthesis is essentially a light dependent reaction in which hydrogen from suitable oxidisable compound reduces CO2 to carbohydrates

  2. H₂S is hydrogen donor for purple & green sulphur bacteria. H₂O, the hydrogen donor in green plants is oxidised to 02

  3. The oxidation product is sulphur or sulphate in purple & green sulphur bacteria and not O2. Hence it was inferred that O₂ evolved by green plants comes from H2O and not from CO2
• Where photosynthesis takes place

  In green parts of the plants, mainly in the mesophyll cells in the leaves, which have large number of chloroplasts
• Chloroplast alignment

  • Perpendicular to the incident light in low or optimum light intensity to get maximum light
  • Parallel to the incident light in extremely high light intensity to avoid photo-oxidation
• Division of labour within the chloroplast

  • Membranous system (Grana+Stroma lamellae) responsible for trapping light & synthesis of ATP and NADPH
  • Stroma where enzymatic reactions to synthesise sugar, which in turn forms starch, takes place
• The dark reactions are not dark reactions, they are light-dependent
• Pigments involved in photosynthesis

  • Chlorophyll-a
  • Chlorophyll-b
  • Xanthophyll
  • Carotenoids
• Chlorophyll-a


  The chief pigment associated with photosynthesis
• Chlorophyll-b, carotenoids and xanthophyll

  Accessory pigments that absorb light and transfer the energy to Chl-a, enabling a wider range of wavelength of incoming light to be utilised for photosynthesis and also protect chlorophyll-a from photo-oxidation
• Light reaction processes

  • Light absorption
  • Water splitting
  • Oxygen release
  • ATP and NADPH formation
• Photosystems

  • PS-I with absorption peak at 700 nm (P700)
  • PS-II with absorption peak at 680 nm (P680)
• Photosynthesis


  Takes place in the green parts of plants, mainly in the mesophyll cells in the leaves, which have a large number of chloroplasts
• Chloroplast alignment

  • Chloroplasts align themselves along the walls of mesophyll cells to get optimum quantity of the incident light
  • In low or optimum light intensity to get maximum incident light
  • In extremely high light intensity to avoid photo-oxidation
• There is a clear division of labour within the chloroplast
• Chloroplast membranous system (Grana+Stroma lamellae)

  • Responsible for trapping light & synthesis of ATP and NADPH
  • Directly light driven, called LIGHT REACTION (photochemical reactions)
• Stroma

  • Enzymatic reactions to synthesise sugar, which in turn forms starch, takes place
  • Dependent on products of light reactions (ATP & NADPH)
  • By convention called DARK REACTIONS (Carbon reactions)
• The dark reactions occur in the light and are light-dependent
• Accessory pigments (Chl-b, carotenoids and xanthophyll)

  • Absorb light and transfer the energy to Chl-a
  • Enable a wider range of wavelength of incoming light to be utilised for photosynthesis
  • Protect chlorophyll-a from photo-oxidation
• Light reactions

  • Light absorption
  • Water splitting
  • Oxygen release
  • ATP and NADPH formation
• Photosystems

  • PS-I
  • PS-II
• Water splitting

  1. Water split into 2H, [O] & electrons

  2. Creates oxygen, one of the net products of photosynthesis
• Cyclic and non-cyclic photo-phosphorylation

  • Non-cyclic (both PS-I and PS-II involved): Produces ATP, NADPH+H and oxygen
  • Cyclic (only PS-I functional): Produces only ATP
• Cyclic photo-phosphorylation also occurs when only light of wavelengths beyond 680 nm are available for excitation
• Chemiosmotic hypothesis

  • ATP synthesis in photosynthesis is linked to the development of a proton gradient across the membranes of thylakoid and protons accumulate in the lumen of thylakoids
  • Protons or hydrogen ions produced by splitting of water, accumulate in the lumen of the thylakoids
  • The primary acceptor of electron located towards outer side of membrane transfers its electron to an H carrier, which removes a proton from stroma while transporting an electron to thylakoid lumen
  • Breakdown of this gradient leads to synthesis of ATP, when protons move across the membrane to the stroma through transmembrane channel of the CF1 of the ATP synthase
• Pigments involved in photosynthesis

  • Chlorophyll-a
  • Chlorophyll-b
  • Xanthophyll
  • Carotenoids
• Chlorophyll-a is the chief pigment associated with photosynthesis
• Products of light reaction

  • ATP
  • NADPH
  • O2
• Calvin cycle

  • Carboxylation: RuBP + CO2 -> 2 3-PGA
  • Reduction: Series of reactions that lead to formation of glucose, utilising 2 ATP and 2 NADPH per CO2
  • Regeneration: Regeneration of RuBP, requires 1 ATP
• C4 plants

  • Have special type of leaf anatomy (Kranz anatomy)
  • Tolerate higher temperatures
  • Show response to high light intensities
  • Lack photorespiration
  • Have greater biomass productivity
• Photorespiration

  • RuBisCO has the active site that can bind to both CO2 and O2
  • In C3 plants, some O2 does bind to RuBisCO, and hence CO2 fixation is decreased
  • In C4 plants, they have a mechanism that increases the concentration of CO2 at the enzyme site, minimising the oxygenase activity
• Factors affecting photosynthesis

  • Internal factors: Number, size, age & orientation of leaves, mesophyll cells and chloroplasts, internal CO2 concentration & the amount of chlorophyll
  • External factors: Availability of sunlight, temperature, CO2 concentration and water