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Inputs and Architecture of Photosynthesis

Organic molecules, also known as molecules that contain carbon and hydrogen, sustain life on this planet. Animals—that's us, by the way—that depend completely on taking in organic compounds from our environment are called heterotrophs. No organic compounds = no living animals. Bad-o-mundo. Many lucky plants, algae, and certain prokaryotes can instead make their own organic compounds from inorganic molecules like carbon dioxide (CO2) using a process called photosynthesis. During photosynthesis, light energy is converted into chemical energy to make organic compounds. Organisms that do the photosynthesis dance (not like the safety dance) are called autotrophs, and without them, life on this planet would not exist in the way it does today. As in, we would have nothing to eat and no air to breathe.

Light is the driving force behind photosynthesis. Before we discuss light, though, it is useful to think about kicking waves in a pool of water. Waves form in the pool because you have put energy into the water, and this energy travels as a wave. While it may seem like the water itself is moving, it is actually the energy that is moving as a wave. The measurement of the distance between two peaks of a wave is called the wavelength. Waves with less energy have a longer wavelength, like the waves you make when you kick slowly, while waves of a higher energy have a shorter wavelength, like the waves you make when you kick rapidly. While this principle may seem backward, it actually makes a lot of sense. Think of it in terms of frequency. If you were standing in one place watching waves go by in the water for several minutes, you would only see a wave that had a long wavelength go by a few times. However, if the distances between the waves were close together because the waves had a lot of energy, then you would see many more waves go by in the same period.

Light also consists of waves of energy. Light is a tricky sucker in that it behaves as though it is made of both particles and waves. Visible light is a type of electromagnetic radiation that falls into a specific region of the electromagnetic spectrum.

The electromagnetic spectrum consists of all kinds of electromagnetic radiation with different energy levels. From most energy to least, the most important types of radiation are as follows:
  • Gamma-rays—Nuclear disaster-type radiation
  • X-rays—Dental checkup or CAT scan radiation
  • Ultra-violet rays—Lobster fun in the sun with zero SPF radiation
  • Visible light—What allows you to see stuff when the Sun is out
  • Infrared light—Military goggle see-in-the-dark radiation
  • Microwaves—Delicious burrito-warming radiation
  • Radio waves—Kind of obvious, but cell phones are also in this range
  • TV waves—Duh
Presenting, the electromagnetic spectrum:

In photosynthesis, we are most concerned with visible light, which includes your classic ROYGBIV color scheme:
  • Red
  • Orange
  • Yellow
  • Green
  • Blue
  • Indigo
  • Violet
Rainbows always have this color order, or its exact opposite. Yes, we have entered the land of physics momentarily. Hush it. We measure visible light wavelengths in nanometers (nm), and the human eye can pick up a wavelength range from about 390 to 750 nm. A wavelength of 390 nm is toward the violet end of things and higher energy. Another way to remember this is that higher energy wavelengths are generally more dangerous to you. You know that you need to watch sun exposure due to ultraviolet light and the burning of your skin. Ultraviolet light burns skin because it is a higher energy wavelength. A wavelength of 750 nm is on the red end of things and a lower energy.

Light is the opposite of paint. When you mix all the major colors of paint together, you get black paint, right? However, only in the absence of light do we see black. When you have no color added to your paint, it is clear, or white. However, when we mix all the colors of light, we get white light. White light, when shown through a prism, will break into all of the colors of the visible light spectrum. Rainbows, anyone?

We know that during photosynthesis, light hits leaves, which are green to the human eye. Why? Spectrophotometry is the fancy word that biologists and chemists use to talk about the way that light interacts with matter. Light can be reflected, transmitted, diffracted, scattered, or absorbed by matter. The absorption of light is most obvious when you put a piece of paper in front of a lit lamp. The paper absorbs some of the light, creating a degree of darkness. Depending on the paper's thickness, it may also allow some light to pass through, which is called transmittance.

When we see colors, we are actually seeing the changes in light of different wavelengths, or energy levels. If we are seeing a green leaf, that wavelength of light is bouncing back at us, reflected from the leaf. So, the color that an object appears is the result of the wavelengths of light that are not absorbed by that object and are instead reflected. When an object appears green, it is because that object absorbs light of all the different visible light wavelengths except green. Tricky, but it makes sense. How else would you see the color if it wasn't shining back into your eyes? If you can see it, it is not absorbed, and it is therefore reflected from the object.

Certain molecules, called pigments, have the ability to absorb specific wavelengths of light. There are about six types of pigments in photosynthetic plants that are each better at absorbing wavelength ranges of light. These pigments include chlorophyll A, chlorophyll B, and carotenoids, the latter of which are accessory pigments. It's not too important to remember which pigment absorbs which wavelengths of light best so don't worry too much about these pigments.

The important fact is that when a chlorophyll molecule absorbs light energy, its electrons are excited to a high-energy state. Think about how you feel after too many caramel Frappuccinos. Just like for you, that superficial energy boost to the chlorophyll is startling, and all that energy needs to go somewhere. While you might use your newfound energy to study for your biology exam, the pigments let go of that energy in the form of heat or light. In the light reactions, also known as the light-dependent reactions, this energy is captured and transferred from one molecule to another, in turn creating the cell’s energy staples adenosine triphosphate (ATP) or nicotinamide adenine dinucleotide phosphate (NADPH). That's right, two important molecules with two even longer names.

Photosynthesis occurs in both single-celled organisms and in single cells within multicellular organisms. In plants, photosynthesis occurs mainly in the mesophyll cells (meso = the area under the upper epidermis of leaves), also known as the palisade cells, of the leaves. Gases such as CO2 and O2 enter and exit the leaves through special openings called stomata. Cellular organelles called chloroplasts, localized in the mesophyll cells’ cytoplasm, are the hubbub for photosynthesis. Like mitochondria, the chloroplasts are unique organelles because they contain their own DNA. This DNA encodes for many of the proteins that are used in photosynthesis; however, the cell's nuclear DNA also encodes for many other photosynthetic proteins. The chloroplast’s DNA is excellent evidence for the chloroplast's evolutionary origin as a single-celled photosynthetic organism. To learn more, see our Big Themes section for details.

An envelope consisting of two membranes surrounds the chloroplast. Inside the chloroplast are internal membrane-bound structures called thylakoids, which form sac-like structures. The thylakoid membrane is distinct from the chloroplast’s outer membranes. The thylakoids appear stacked on top of each other, and inside of each of these sacs is called the lumen. The inside, fluid-filled part of the chloroplast that is not a part of the thylakoid sacs is called the stroma, and it contains all of the necessary enzymes needed to convert carbon dioxide (CO2) to complex carbohydrates.

Photosynthesis consists of two stages:
  1. The light reactions, or the light-dependent reactions (Photosystems I and II)
  2. The light-independent reactions
In the light reactions, water (H2O) and light are inputs, and the gas O2 and the chemical energy molecules ATP and NADPH are produced. The ATP and NADPH are then used in the light-independent reactions with CO2 to build carbohydrates. These two processes are spatially separated within the chloroplast, with the light reactions occurring in the thylakoid membrane and the light-independent reactions occurring in the stroma.

Here is what the chloroplast busies himself with:

Brain Snack

The applications of photosynthesis for our well-being were understood as early as a few hundred years ago. In 1779, Jan Ingen-Housz published in his book Experiments Upon Vegetables… to suggest that plants might be used to "purify" air for respiratory patients.1

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