Chloroplasts and Light Energy

Chloroplasts Belong to the class of organelles known as the plastids. They are the sites for photosynthesis in plant cells and some forms of eukaryotic algae.

Chloroplast Parts Like mitochondria, they have two membranes. The interior space is filled with a protein-rich fluid called stroma. In the stroma are membrane-bound sacs called thylakoids. The inside of the thylakoid is known as the thylakoid lumen.

Chloroplast Parts The thylakoids stack on top of one another. This stack of thylakoids is called a granum. Grana are linked to one another by unstacked thylakoids, which are called lamellae.

Chloroplast Info The thylakoid membrane contains light gathering pigments and electron transport chains that are essential to photosynthesis. Chloroplasts contain their own DNA and ribosomes so they can reproduce independent of the cell. Excess glucose formed during photosynthesis is stored as starch inside the chloroplast.

Chloroplasts

Chloroplasts

Photosynthesis It has three linked stages: capturing light energy using the light energy to create ATP and NADPH (chemical energy) using the free energy from ATP and the reducing power of NADPH to synthesize organic compounds from atmospheric carbon. Stages 1 and 2 are better known as the light reactions while stage 3 is better known as the Calvin Cycle.

Light Of all the light that hits the surface of the earth only about 5% is converted to organic compounds by photosynthesis. Light travels in wave packets called photons and light from the sun is a mixture of photons of different energies.

Light Clusters of photosynthetic pigments, called photosystems, absorb particular wavelengths of light. Chlorophyll is located in these photosystems.

Photosystems Preview

Chlorophyll There are at least 4 different types of chlorophylls. Green plants contain chlorophyll a and chlorophyll b. Both these chlorophylls absorb blue light and red light the best and reflect green light, hence the green colour of chlorophyll.

Chlorophyll Therefore we can assume that it is blue light and red light that is used in photosynthesis. Chlorophyll a is the only pigment that can transfer energy of light to the carbon fixation portion of photosynthesis (Calvin Cycle). Chlorophyll b is an accessory pigment that absorbs photons that chlorophyll a misses.

Accessory Pigments There are other light absorbing pigments found in plants that can funnel light energy for photosynthesis.

Carotenoids They are natural, organic pigments found in many plants as well as some species of algae, fungi and bacteria. They absorb high energy blue light that would normally damage chlorophyll and make it able to be used in photosynthesis. They reflect orange-yellow light.

Carotenoids Some, like Beta-carotene, are important precursor molecules (Vitamin A) as well as anti-oxidants, so are good to have in one’s diet! Xanthophylls are a yellow reflecting pigment from this family. Xanthophylls make chicken yolks yellow!