Biology 101 Computer Laboratory
Photosynthesis


Overview

Photosynthesis is the process by which plants, some bacteria and protists, use the energy from sunlight to produce sugar. In turn, the sugar is utilized to make ATP through the process of cellular respiration. ATP is commonly known as the "fuel" for all living things. The conversion of sunlight energy into chemical energy, is associated with the actions of the green pigment chlorophyll. For the most part, this photosynthetic process consumes water and carbon dioxide, releases the oxygen, and produces sugar.
The Plant

Plants are the only photosynthetic organisms to have leaves (and not all plants have leaves). Imagine a leaf as a solar collector crammed full of photosynthetic cells where the raw materials of photosynthesis (water and carbon dioxide) enter the cells of the leaf, and the products of photosynthesis (sugar and oxygen) leave the leaf.

Water enters the root and is transported up to the leaves through specialized plant cells known as xylem. Due to the cuticle (a waxy layer covering the leaf), carbon dioxide enters the leaf through an opening known as the stoma (stomata, pl.). Likewise, oxygen can only pass out of the leaf through the stomata. The opening of the stomata are regulated by two, flanking guard cells.

Light

Light behaves both as a wave and a particle. Wave properties of light include the bending of the wave path when passing from one material (medium) into another (i.e., prisms, rainbows, water, etc...). In addition, another particle property is the photoelectric effect. The photoelectric effect is a phenomenon in which electrons are emitted from matter (metals and non-metallic solids, liquids, or gases) after the absorption of energy from electromagnetic radiation such as visible light. One example of the photoelectric effect of light can be seen with zinc. When zinc is exposed to ultraviolet light, it becomes positively charged because light energy forces electrons from the zinc. Subsequently, these electrons can create an electrical current. Other elements, such as sodium, potassium and selenium, exhibit a photoelectric effect with visible light range. The maximum wavelength of light (visible or invisible) that creates a photoelectric effect is known as the critical wavelength.
White light can be separated into the different colors by passing it through a prism. Each color consists of a specific wavelength. The longer the wavelength of visible light, the more red the color. Likewise the shorter wavelengths are towards the violet side of the spectrum. Wavelengths longer than red are referred to as infrared, while those shorter than violet are ultraviolet. In the electromagnetic spectrum, visible light is only a small part.
Chlorophyll and Accessory Pigments

A pigment is any substance that absorbs light. The color of the pigment comes from the wavelengths of light reflected (not absorbed). Chlorophyll, the green pigment common to all photosynthetic cells, absorbs all wavelengths of visible light except green, which it reflects to be detected by our eyes. Black pigments absorb all of the wavelengths that strike them. White pigments/lighter colors reflect all or almost all of the energy striking them.
There are different types of chlorophyll (a, b, c, d, and e). All photosynthetic organisms (plants, certain protistans, prochlorobacteria, and cyanobacteria) have chlorophyll a. Accessory pigments absorb energy that chlorophyll a does not absorb. Accessory pigments include xanthophylls and carotenoids (such as beta-carotene). Chlorophyll a absorbs its energy from the Violet-Blue and Reddish orange-Red wavelengths, and little from the intermediate (Green-Yellow-Orange) wavelengths.

The action spectrum of photosynthesis is the relative effectiveness of different wavelengths of light at generating electrons. If a pigment absorbs light energy, one of three things will occur. Energy is dissipated as heat. The energy may be emitted immediately as a longer wavelength, a phenomenon known as fluorescence. Energy may trigger a chemical reaction, as in photosynthesis. Chlorophyll only triggers a chemical reaction when it is associated with proteins embedded in a membrane (as in a chloroplast) or the membrane infoldings found in photosynthetic prokaryotes such as cyanobacteria and prochlorobacteria.


The Structure of the Chloroplast

The chloroplast is the site of photosynthesis in eukaryotic organisms. It consists of an outer membrane, inner membrane, and grana (granum, pl.). Grana consists of stacks of flattened sacs/vesicles called thylakoids. The thylakoid is the structural unit of photosynthesis and contains the photosynthetic components. The space between the outer and inner membrane is the inner-membrane space. The space between the inner membrane and the granum is the stroma.

© 2009 Wilhelm S. Cruz, All Rights Reserved