Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Chlorophyta

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Despite this popularity, chlorination is not without

drawbacks. Microorganisms such as Cryptosporidiumand

Giardiaform dormant structures called cysts that are resistant

to chlorination. The prevalence of these protozoans in world-

wide drinking water supplies is increasing. Thus, the effec-

tiveness of chlorination may be compromised in some water

systems. As well, adherent bacterial populations of bacteria

such as Escherichia colithat form in distribution pipelines are

extremely resistant to chlorine, and so can contaminate the

disinfected water that flows from the treatment plant to the

tap. A third concern with chlorination is the reaction between

chlorine and methane gas, which produces one or more chlo-

rinated derivatives. The best known are trichloromethane

(chloroform) and tetrachloromethane (carbon tetrachloride).

These chlorinated hydrocarbons have been shown to have

adverse health effects in humans when ingested in sufficient

quantity for a long time.

Furthermore, from an engineering point of view, excess

chlorine can be corrosive to pipelines. In older water treatment

systems in the United States, for example, the deterioration of

the water distribution pipelines is a significant problem to

water delivery and water quality.

See alsoInfection control; Water quality

CChlorophyllHLOROPHYLL

Chlorophyll is a green pigment contained in the foliage of

plants, giving them their notable coloration. This pigment is

responsible for absorbing sunlight required for the production of

sugar molecules, and ultimately of all biochemicals, in the plant.

Chlorophyll is found in the thylakoid sacs of the chloro-

plast. The chloroplast is a specialized part of the cell that func-

tions as an organelle. Once the appropriate wavelengths of

light are absorbed by the chlorophyll into the thylakoid sacs,

the important process of photosynthesisis able to begin. In

photosynthesis, the chloroplast absorbs light energy, and con-

verts it into the chemical energy of simple sugars.

Vascular plants, which can absorb and conduct moisture

and nutrients through specialized systems, have two different

types of chlorophyll. The two types of chlorophyll, designated

as chlorophyll a and b, differ slightly in chemical makeup and

in color. These chlorophyll molecules are associated with spe-

cialized proteins that are able to penetrate into or span the

membrane of the thylakoid sac.

When a chlorophyll molecule absorbs light energy, it

becomes an excited state, which allows the initial chain reac-

tion of photosynthesis to occur. The pigment molecules clus-

ter together in what is called a photosynthetic unit. Several

hundred chlorophyll a and chlorophyll b molecules are found

in one photosynthetic unit.

A photosynthetic unit absorbs light energy. Red and

blue wavelengths of light are absorbed. Green light cannot be

absorbed by the chlorophyll and the light is reflected, making

the plant appear green. Once the light energy penetrates these

pigment molecules, the energy is passed to one chlorophyll

molecule, called the reaction center chlorophyll. When this

molecule becomes excited, the light reactions of photosynthe-

sis can proceed. With carbon dioxide, water, and the help of

specialized enzymes, the light energy absorbed creates chem-

ical energy in a form the cell can use to carry on its processes.

In addition to chlorophyll, there are other pigments

known as accessory pigments that are able to absorb light

where the chlorophyll is unable to. Carotenoids, like B-

carotenoid, are also located in the thylakoid membrane.

Carotenoids give carrots and some autumn leaves their color.

Several different pigments are found in the chloroplasts of

algae, bacteria, and diatoms, coloring them varying shades of

red, orange, blue, and violet.

See alsoAutotrophic bacteria; Blue-green algae

CChlorophytaHLOROPHYTA

Chlorophyta are microorganismsthat are grouped in the king-

dom called Protista. The microbes are plant-like, in that they

are able to manufacture energy from sunlight. The microbes

are also commonly known as green algae

Depending on the species, Chlorophyta can be single-

celled, multicelled, and can associate together in colonies. The

environmental diversity of Chlorophyta is vast. Many types

live in marine and fresh water. Terrestrial habitats include tree

trunks, moist rocks, snowbanks, and creatures including turtles,

sloths and mollusks. There are some 8,000 species of chloro-

phytes, ranging in size from microscopic to visibly large.

There are three classes of Chlorophyta. The first class,

which contains the greatest number of organisms, is called

Chlorophyceae. A notable example of an organism from this

class is Chlorella, which is economically important as a dietary

supplement. Another member of the class is Volvox, a spherical

organized community containing upwards of 60,000 cells.

The second class is called Charophyceae. Members of

this class have existed since prehistoric times, as evidenced by

fossil finds. An example of this class is Spirogyra, which form

slimy filaments on the surface of freshwater.

The third class is called Ulvophyceae. These are marine

organisms. Some become associated with sea slugs where they

provide the slug with oxygen and are in turn provided with

protection and nutrients. Species of a calcium-rich green algae

called Halimedaform the blinding white sand beaches of the

Caribbean when they wash up onshore and become bleached

by the sun. Another example from this class is Ulvathat grows

on rocks and wharves as green, leafy-appearing clusters.

Chlorophyta contain structures that are called chloro-

plasts. Within the chloroplasts two pigments (chlorophylla

and chlorophyll b) are responsible for the conversion of sun-

light to chemical energy. The energy is typically stored as

starch, and in their cell walls, which are composed of a mate-

rial called cellulose. The stored material can be used for

energy as needed. This process of energy generation is similar

to that which occurs in plants. There is an evolutionary basis

for this similarity. Available evidence indicates that members

of Chlorophyta were the precursors of plants. Chlorophyte

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