5.6.2 - Chloroplasts and Photosynthetic Pigments

• Chloroplasts are the organelles within plant cells where photosynthesis occurs. Algae have chloroplasts, but photosynthetic bacteria do not.
• Most plant chloroplasts are disc shaped and are around 2-10um long
• Each chloroplasts is surrounded by a double membrane, the envelope, with an intermembrane space of width 10-20nm between the inner and outer membrane. The outer membrane is highly permeable.
• There are two distinct regions, visible on electron micrographs inside a chloroplast:

  • The fluid filled matrix - the stroma
  • The grand - consists of stacks of thylakoid membranes
• The first stage of photosynthesis occurs in the grand, and is light-dependent.
• Chloroplasts have 3 distinct membranes, outer, inner and thylakoid, giving 3 separate internal compartments - intermembrane space, stroma and thylakoid space.
• Thylakoids in a geranium may be connected to thylakoids within another geranium by intergranal lamellae.
• The thylakoid membrane of each chloroplast is less permeable and is folded into flattened disc-like sacs called thylakoids.
• Thylakoids form stacks, and each stack is called a granum. One granum may contain up to 100 thylakoids.
• With many grana in every chloroplast, and many chloroplasts in each photosynthetic cell, there is a huge surface area for:

  • The distribution of the photosystems that contain the photo-synthetic pigments that trap sunlight energy
  • The electron carries and ATP synthase enzymes needed to convert that light energy into ATP
• Proteins embedded in the thylakoid membranes hold the photosystems in place.

  The grana are surrounded by the storm, so the products of the light-dependent stage can easily pass to the stroma for the light-independent stage.
• The stroma is the fluid-filled matrix:

  • It contains the enzymes needed to catalyse the reactions of the light-independent stage of photosynthesis
  • It also contains starch grains, oil droplets, small ribosomes similar to those in prokaryotic cells.
• The loop of DNA contains genes that code for some of the proteins needed for photosynthesis.

  These proteins are assembled at the chloroplast ribosomes.
• Within the thylakoid membranes of each chloroplast are funnel-shaped structures called photosystems.

  These photosystems contain photosynthetic pigments.

  Each pigment absorbs light of a particular wavelength and reflects other wavelengths of light.

  Each pigment appears to the eyes and the brain as a colour of the wavelength of light it is reflecting.
• The energy associated with the wavelengths of light captured is funnelled down to the primary pigment reaction centre, consisting of a type of chlorophyll, at the base of the photosystem that receives energy from photons trapped by other chlorophyll molecules.
• Chlorophylls are a mixture of pigments.

  All have a similar molecular structure consisting of a porphyrin group, containing a magnesium atom and hydrocarbon chain.
• Chlorophyll A is the most abundant. There are two forms but both appear blue-green and absorb red light.

  • P680 is in photosystem II and has peak absorption of 680nm.
  • P700 is in photosystem I and has peak absorption of 700nm.

  Chlorophyll A also absorbs some blue light at around 440nm.
• Chlorophyll B absorbs wavelength of 400-500nm and 640nm. It appears yellow-green.
• Carotenoids absorb blue light (400-500nm) and reflect yellow and orange light.

  Xanthophylls absorb blue and green light (375-550nm) and reflect yellow light.