Biology & Biomedical

The Cell: Smallest Living Level of Organization

THE CELL - Smallest living component of biological systems.

Two basic types of cells:

Prokaryotic (no nucleus or other membrane-bounded organelles)

Eukaryotic (nucleus and many membrane-bounded organelles).

Some facts and structures of import in prokaryotes (organisms composed of prokaryotic cells):

* prokaryotes are unicellular or colonial--there's no division of labor among cells.

* prokaryotic organisms are commonly called "bacteria," although not all prokaryotes are true bacteria. Close enough for our purposes, though.

* prokaryote cell wall is composed of a protein/sugar molecule called peptidoglycan, unique to prokaryotes.

* prokaryotic flagellum (whiplike "tail") is used for locomotion. (It superficially resembles the flagella we'll see in eukaryotic cells, but it is NOT the same at all.)

-composed of a protein called flagellin

-moves with rotation from the base

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And while we're on the subject of flagella that *look* superficially similar...

* ANALOGOUS STRUCTURE or CHARACTER: a structure found in two unrelated organisms that performs the same basic function in both, but is not derived from the same ancestral structure or source.

e.g. - wing of a bat and wing of a bird

* HOMOLOGOUS STRUCTURE or CHARACTER: a structure which may or may not perform the same function in the two organisms in which it is found, but it IS derived from the same ancestral source.

e.g. - middle fingernail of human, front hoof of a horse

bones of bat wing and bones of bird wing

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* PRIMITIVE STRUCTURE or CHARACTER: one that has changed very little from its form in an ancestral species.

e.g. - the vertebral column, found in all vertebrates, is primitive to all vertebrates.

* DERIVED STRUCTURE or CHARACTER: one that has significantly changed from its form in an ancestral species.

e.g. - the vertebral column, found ONLY in vertebrates, is derived with respect to skeletal structures in other organisms that do not have a vertebral column. (That is, it's an "evolutionary innovation" unique to the vertebrates. It set them apart from other organisms, yet unites them all together as one group descended from a common ancestor that also had a vertebral column.)

Note that the above four terms are RELATIVE. You can't call one structure or characteristic "analogous" or "primitive." You must be comparing at least two different things.

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Cellular structures: review in your text! Be familiar with the major organelles and with the general structure of the plasma membrane. Here are some helpful illustrations...

* Plasma membrane

* Nucleus

* Endoplasmic Reticulum

* Mitochondrion

* Chloroplast

* Flagellum

Plant cells, unlike animal cells...

1. lack lysosomes (little bags of digestive enzymes)

2. lack centrioles (involved in mitosis)

3. have chloroplasts (site of photosynthesis)

4. have large, central vacuole

5. have cell wall composed of a "jelly sandwich" of cellulose/pectin/cellulose.

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Cell reproduction: Asexual or Sexual

Mitosis: one parent cell divides to form two daughter cells, genetically identical to the parent and to each other. (Asexual)

Meiosis: one parent cell (called a "germ cell) divides to form daughter cells that have only half the genetic material of the original cell. (Sexual).

- the daughter cells are called gametes

- these are often sperm or ova

- when two complementary gametes fuse, they form a new cell which again has the same amount of genetic information as the original parent cell, but it is genetically different from the parent cell.

The fertilized ovum (fused sperm and egg) is called a ZYGOTE.

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Note of interest: all mitochondria in a zygote come from the female parent; the sperm mitochondria are lost after fusion of the gametes.

This is why mitochondrial DNA is sometimes fondly referred to as

"Eve's DNA" (think about it).

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Before we talk about cell division, we must mention...

The Wonders of the Chromosome

A chromosome is composed of a central strand of DNA, connected to a network of proteins and RNA.

DNA is the genetic "blueprint" that tells the cell how to build the organism out of proteins.

- structural proteins are those which form the actual, physical structure of the organism (e.g., muscle tissue, hair, etc.)

- enzymes are protein catalysts that construct proteins into their finished products. (enzymes are not usually structural)

DNA is composed of functional subunits called genes.

Each gene controls (all or part of) a physical trait in the organism!

For example, humans have a gene that controls such traits as...

- eye color

- Hitchhiker's Thumb

- widow's peak

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