Biology 101: Photosynthesis

Biologists define us humans, as well as almost all other animals, fungi, and even many bacteria as heterotrophs--which means we can't make our own food. Plants, algae, and cyanobacteria, on the other hand, are defined as autotrophs, or more specifically, photoautotrophs, which means that they make their own food from carbon dioxide, water, and sunlight.

We get our nutrition from plants. Even extreme carnivores who might eat nothing but meat really get their nutrition from plants--it's just that they get it by eating animals who eat plants, or even animals who eat animals who eat plants. This is what's meant by eating lower on the food chain. At the base of that chain is plants (and algae, etc).

In my last post (Cellular Respiration, 5/10/12), I talked about why we need oxygen and ended with the question: 'where does the oxygen come from?' From plants, of course. Plants and algae and cyanobacteria.

It's believed that Earth's original atmosphere was mostly methane, and the first organisms were anaerobic. Cynobacteria came along and began giving off oxygen, which triggered what is sometimes called the 'Great Oxygenation Event' or the 'Oxygen Catastrophe'--where some of the oxygen caused rust and mineral formation, some of the oxygen combined with the methane to form carbon dioxide, and some of the oxygen stayed in free form. This killed off much of the aerobic population and allowed the formation of oxygen breathing creatures.

Plants (etc) keep oxygen in the atmosphere and, as I said, feed us as well. In my last post I gave the formula: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O, which means we take in glucose (and other carbohydrates) and oxygen and break them down into carbon dioxide and water. Photosynthesis allows plants to do the opposite, to take carbon dioxide and water and using the energy from sunlight, make them into glucose and oxygen. The formula for photosynthesis is the reverse: 6 CO2 + 6 H2O → C6H12O6 + 6 O2.

Photosynthesis is a two part operation. First the plant collects light using the 'light-dependent reactions', then it feeds the generated energy into what's called the Calvin cycle or even the 'dark reactions' (because light isn't needed for this).

So here's another question, similar to why do we need oxygen and where does the oxygen come from: Why are plants green?

Light, as you may know, is made of a spectrum of colors. We see black when something absorbs the entire spectrum of light, and we see white when something rejects (or reflects) the full spectrum. The spectrum goes from red to blue and purple--and apparently the colors at either end have the most useful energy for the plants. The plants look green to us because that's what's in the middle and what the plants can't use. The chloroplasts of the plants are filled with pigments that pull in the red and the blue and reflect back green (chlorophyll), as well as pigments that pull in other parts of the spectrum and reflect back yellow (xanthophyll) or orange (carotene)--this is why, when the chlorophyll disappears in the fall, leaves turn yellow or orange.

All of these pigments collect photons from the light and pass them to a 'Reaction Center' which uses the energy to excite electrons from the hydrogen in water (H2O) to send along an electron transport chain (similar to what I talked about in my last post). The protons are sent through various pumps and the oxygen is given off as a waste product. (Yes, this is the oxygen we breathe.) It also uses the energy from the electron transport and the proton powered pumps to create ATP and NADPH--which, as I explained in my last post, are just ways of storing energy.
The Calvin Cycle is sort of like the Krebs cycle in reverse. It takes in carbon dioxide and the reconstituted hydrogen from the electron transport system (and the energy from the ATP and NADPH created by the system) and uses them all to form sugars.

All this happens in the chloroplasts of plants. It also happens in the chloroplasts of algae. Something similar happens in cyanobacteria, but they don't have chloroplasts--in fact, the theory is that cyanobacteria were absorbed by the ancestors of plants and algae and became the chloroplasts.

Again, plants take in carbon dioxide and water and give off oxygen as well as creating sugars, starches, proteins, etc. We breathe in the oxygen and eat the nutrients from plants and breathe out carbon dioxide and pee out water--which is what the plants can use. Photosynthesis and cellular respiration are connected in a cycle that basically keeps the whole planet alive. This is why I think learning this stuff is important. As I heard someone once say, every time you breathe, you should thank a plant.

Quote of the Day: "On a global scale, the collective productivity of the minute chloroplasts is prodigious; it is estimated that photosynthesis makes about 160 billion metric tons of carbohydrates per year... No other chemical process on the planet can match the output of photosynthesis. And no other process is more important than photosynthesis for the welfare of life on Earth." - Neil Campbell and Jane Reece