Chloroplasts

The sketch of the chloroplast above was made from an electron micrograph of a chloroplast from a higher order plant (Levy). Plants use energy from the sun in tiny energy factories called chloroplasts. Using chlorophyll in the process called photosynthesis, they convert the sun's energy into storable form in ordered sugar molecules such as glucose. In this way, carbon dioxide from the air and water from the soil in a more disordered state are combined to form the more ordered sugar molecules.

Inside the chloroplasts are stacks of discs called thylakoids. They are compared to stacks of coins within the walls of the chloroplast, and they act to trap the energy from sunlight.

The stacks of thylakoids are called grana. They are connected with an extensive sytem of tubules. The thylakoid membranes contain chlorophyll and other pigments arranged in antenna arrays to capture light energy for two photosystems called Photosystem I and Photosystem II. In most plants, both photosystems are used in an electron transport process that yields energy in the form of ATP and reduced coenzymes to the stroma of the chloroplast to be used in the synthesis of carbohydrates. The energy is used in the Calvin cycle to fix carbon from atmospheric CO₂ and construct sugars.

Order can be produced with an expenditure of energy, and the order associated with life on the earth is produced with the aid of energy from the sun.

Qualitative statements of the Second Law of Thermodynamics

Order from disorder in biological systems

Energy cycle in living things

Chloroplast Structure

Plants use energy from the sun in tiny energy factories called chloroplasts. The green color of leaves is attributable largely to these chloroplasts because they contain chlorophyll for photosynthesis. Though obviously oversimplified, the illustration depicts the somewhat elongated sausage type shape with large dimension 5-10 μm and smaller dimension 3-4 μm. Moore suggests 40-200 chloroplasts per photosynthetic cell and about 500,000 per square millimeter of leaf area.

There are two membranes, and inside the inner membrane is the gelatanous matrix called the stroma. The stroma contains ribosomes, DNA, and is the location for biochemical synthesis. Membranous sacs called thylakoids are arranged in stacks called grana. The chlorophyll in the thylakoid membranes carries out photosynthesis.

The similarity of the thylakoid structures in the chloroplasts of plants to the photosynthetic structures in ancient cyanobacteria has led to the proposal that cyanobacteria were the origin of those chloroplasts by a process called endosymbiosis into the developing plant forms.

Index

Photosynthesis Concepts

Second law concepts

Reference
Levy
Ch 4

Moore, et al.
Ch 7

HyperPhysics***** Thermodynamics ***** Biology

R Nave

Go Back