Photosynthesis

Diatoms and photosynthesis

Diatoms are an extremely diverse and numerous group of microscopic plants living in the ocean.
They are at the base of the food pyramid because they can make themselves from carbon dioxide and sunlight;
they don't have to eat anyone.

All plants are able to use sunlight to convert carbon dioxide (from the air) plus water (diatoms are floating in water) into carbohydrate (sugar). This process is called photosynthesis. You can think of it as reducing carbon dioxide. In a formal way it is just the reverse of oxidizing carbohydrate, for example burning paper (paper is cellulose which is a carbohydrate). Organisms also need nitrogen to make amino acids and nucleotides, and a number of different minerals. In a typical seawater environment the limiting nutrients are usually phosphate or iron.

CO₂ + H₂O => HCOH + O₂

This just summarizes the end effect of many complex chemical reactions that take place inside the plant.

Photosynthesis occurs in chloroplasts

Chloroplasts are small organelles inside plant cells which are believed to have once been free living bacteria that could carry out photosynthesis. Several hundred million years ago these bacteria were engulfed by cells that we now call plant cells.

The orange box on the right is a cartoon of a plant cell which contains four oval, double membrane, chloroplasts. Inside the chloroplasts are stacks of membrane discs, thylakoids, that contain chlorophylls. The chlorophylls. are the molecules that actually adsorb the light and give the cell an overall green color.

There are many other organelles and membranes inside a plant cell. The sphere labeled "N" is the nucleus, which contains the genome (DNA) of the cell.

The core of the chlorophyll molecule

The molecule that adsorbs light, chlorophyll, contains four porphyrin rings. Each five member ring contains four carbon atoms (the carbons are not shown on the diagram, they occur at all unmarked corners) and one nitrogen atom (the "N"). The four nitrogens hold a magnesium atom, "Mg", in the middle of the molecule.

The chlorophyll molecule is very similar in structure to the heme molecule in hemoglobin. However, heme contains an iron molecule instead of magnesium, and its function is to carry oxygen in the blood.

The curve on the right is the absorption spectrum of chlorophyll a. Blue and red light are absorbed, and thus green is preferentially transmitted or reflected.

The Sea Star satellite obtains an image of the earth's surface in many wavelengths. The ratio of energy at 510 nm to 555 nm (the two arrows) is used as a measure of the concentration of chlorophyll.