Photosynthesis

Life from Light

Photosynthesis Overview Photosynthesis is the pathway by which energy

from the sun is captured, stored and used to convert CO2 and H20 into chemical energy and the carbon based organic compounds. The main products of photosynthesis are Glucose, Oxygen and ATP.

Photosynthesis is the pathway by which energy and carbon enter the web of life.

12H2O+6CO2 6O2 +C6H12O6 +6H2O

Oxygenic Photosynthesis (Oxygen Producing)Photosynthesis in Plants and Algae Cyanobacteria, Algae and Plants are self nourishing

organisms or Autototrophs. They make food from sunlight energy, water and carbon dioxide. They produce 220 billion tons of sugar each year and supply global oxygen.

Millions of photosynthesizers make up the pastures of the sea and provide the main source of food for the tiniest of crustaceans who in turn provide sustenance for the greatest and largest of mammals the Blue Whale.

Without these organisms life on Earth would cease to exist.

Sunlight As an Energy Source Photosynthesis is driven by the photon

energy from the sun identified as the visible light spectrum.

Photons in the visible light spectrum have wavelengths in the range of 380-750 nanometers. The shorter the wavelength the higher its energy content. (Fig. 6.2)

Sunlight As an Energy Source-Pigments Pigments are a class of molecules that absorb photons

at particular wavelengths Light that is not absorbed by the pigment molecules

bounces off (reflected) or passes through (transmitted) the molecule.

The most abundant pigments are Chlorophyll a and Chlorophyll b (Fig. 6.4)

Secondary pigments are the Carotenoids (Beta-carotene), Xanthophylls and Phycobilins.

(Fig. 6.4)

What is Photosynthesis and Where Does It Happen? Photosynthesis is a process that occurs in two main stages.

The Light Dependent Reaction-conversion of light energy into the chemical energy of ATP, liberation of Oxygen into the environment and the reduction of electron acceptor NADP+ to NADPH.

The Light Independent Reaction-Fixation of carbon into Glucose and other organic compounds using the energy from ATP and Hydrogen ions from the NADPH created during the Light Dependent Reaction. (Fig. 6.8)

Photosynthesis takes place in the Chloroplasts of the plant and algae cells. Chloroplasts are composed of stacks of membranes called the Thylakoid

which in turn, are made up of sacs called Granum, surrounded by a fluid called the Stroma. (Fig 6.7) It is here that the Photosystems perform the capture of light energy.

Light Dependent Reactions Pigment molecules capture the photons of light Energy from the photons is used to split water generating free O2 gas, which

diffuses out of the cell into the atmosphere, H+ions and excite electrons in chlorophyll a molecules embedded in the photosystems located in the thylakoid membrane.

The excited electrons are moved through a series of enzymes and proteins called the Electron Transfer Chain.

At the same time a H+ concentration builds up inside the thylakoid membrane space. Equilibrium is reestablished with the assistance of an enzyme called ATP synthase which carries the H+ back into the stroma space.

The movement of H+ back into the stroma space causes the attachment of inorganic phosphate to a molecule of ADP and ATP is formed.

An intermediary molecule NAD+ captures H+ as well and NADH is formed for the next stage of photosynthesis

Light Independent Reactions: The Sugar Factory-Calvin Cycle Carbon from atmospheric CO2 is fixed or bonded

in the form of sugar production, through what is called the Calvin-Benson cycle the occurs in the stroma inside the chloroplast.

Uses ATP and NADPH generated in the light reaction.

Process not dependent on the presence of light. Will occur as long as ATP and NADPH is present.ATP provides energy and NADPH donates hydrogen and electrons.

Light Independent Reactions:The Sugar Factory Calvin Cycle (Fig. 6.11) One carbon is transferred by Rubisco to RuBP (Ribulose

Biphosphate)-fixation point for carbon. RuBP splits into two stable molecules of PGA

(Phosphoglycerate) each will pick up a phosphate group from ATP and an H+ from NADPH generated during the light reaction, making PGAL (Phosphoglyceraldehyde).

2 PGAL combine to form glucose which can be converted to sucrose or starch and any other organic compound that is needed by the plant.

Additional PGAL combine with ATP to regenerate RuBP.

Different PlantsDifferent Carbon Fixation Pathways 3 variables control rate of photosynthesis

Temperature, Humidity and O2/CO2 conc. Plant Responses-Stomata Function

C3 Plants (Temperate Zone)-stomata closed sugar production declines in hot, dry weather.

C4 Plants (Tropical Zone)-sugar production occurs in hot dry weather and excess O2 present. Fixes carbon in an intermediary 4 carbon form until environmental conditions conducive to stomata opening.

CAM Plants (Desert Zones) sugar production occurs in hot dry climates by limiting the uptake of CO2 to periods when the sun is down, thereby preventing excess water loss through stomata opening.