Microbiology and Immunology

Chloroplast WORLD OF MICROBIOLOGY AND IMMUNOLOGY

120

•

fossils date from over one billion years ago, before the devel-

opment of plants.

See alsoPhotosynthesis

CChloroplastHLOROPLAST

Chloroplasts are organelles—specialized parts of a cell that

function in an organ-like fashion. They are found in vascular

plants, mosses, liverworts, and algae. Chloroplast organelles

are responsible for photosynthesis, the process by which sun-

light is absorbed and converted into fixed chemical energy in

the form of simple sugars synthesized from carbon dioxide

and water.

Chloroplasts are located in the mesophyll, a green tissue

area in plant leaves. Four layers or zones define the structure

of a chloroplast. The chloroplast is a small lens-shaped

organelle that is enclosed by two membranes with a narrow

intermembrane space, known as the chloroplast envelope.

Raw material and products for photosynthesis enter in and

pass out through this double membrane, the first layer of the

structure.

Inside the chloroplast envelope is the second layer,

which is an area filled with a fluid called stroma. A series of

chemical reactions involving enzymesand the incorporation of

carbon dioxide into organic compounds occur in this region.

The third layer is a membrane-like structure of thy-

lakoid sacs. Stacked like poker chips, the thylakoid sacs form

a grana. These grana stacks are connected by membranous

structures. Thylakoid sacs contain a green pigment called

chlorophyll. In this region the thylakoid sacs, or grana, absorb

light energy using this pigment. Chlorophyll absorbs light

between the red and blue spectrums and reflects green light,

making leaves appear green. Once the light energy is absorbed

into the final layer, the intrathylakoid sac, the important

process of photosynthesis can begin.

Scientists have attempted to discover how chloroplasts

convert light energy to the chemical energy stored in organic

molecules for a long time. It has only been since the beginning

of this century that scientists have begun to understand this

process. The following equation is a simple formula for pho-

tosynthesis:

6CO 2 + 6H 2O →C 6H 12O 6 + 6O 2.

Carbon dioxide plus water produce a carbohydrate plus

oxygen. Simply, this means that the chloroplast is able to split

water into hydrogen and oxygen.

Many questions still remain unanswered about the com-

plete process and role of the chloroplast. Researchers continue

to study the chloroplast and its evolution. Based on studies of

the evolution of early complex cells, scientists have devised

the serial endosymbiosis theory. It is suspected that primitive

microbes were able to evolve into more complex microbes by

incorporating other photosynthetic microbes into their cellular

structures and allowing them to continue functioning as

organelles. As molecular biologybecomes more sophisticated,

the origin and genetic makeup of the chloroplast will be more

clearly understood.

See alsoAutotrophic bacteria; Blue-green algae; Evolution

and evolutionary mechanisms; Evolutionary origin of bacteria

and viruses

CChromosomes, eukaryoticHROMOSOMES, EUKARYOTIC

Chromosomes are microscopic units containing organized

genetic information, eukaryotic chromosomes are located in

the nuclei of diploid and haploid cells (e.g., human somatic

and sex cells). Prokaryotic chromosomes are also present in

one-cell non-nucleated (unicellular microorganisms) prokary-

otic cells (e.g., bacteria). The sum-total of genetic information

contained in different chromosomes of a given individual or

species are generically referred to as the genome.

In humans, eukaryotic chromosomes are structurally

made of roughly equal amounts of proteins and DNA. Each

chromosome contains a double-strand DNA molecule,

arranged as a double helix, and tightly coiled and neatly

packed by a family of proteins called histones. DNA strands

are comprised of linked nucleotides. Each nucleotide has a

sugar (deoxyribose), a nitrogenous base, plus one to three

phosphate groups. Each nucleotide is linked to adjacent

nucleotides in the same DNA strand by phosphodiester bonds.

Phosphodiester is another sugar, made of sugar-phosphate.

Nucleotides of one DNA strand link to their complementary

nucleotide on the opposite DNA strand by hydrogen bonds,

thus forming a pair of nucleotides, known as a base pair, or

nucleotide base.

Chromosomes contain the genes, or segments of DNA,

that encode for proteins of an individual. Genes contain up to

Thin section electron micrograph showing the stacked arrangement of

chloroplast membranes.

womi_C 5/6/03 2:04 PM Page 120