Biology 101 - Cell Biology Part 3

They are linear, thin filaments composed of many actin monomers. Microfilaments are thinner than either microtubules or intermediate filaments.

As actin polymers, microfilaments play a crucial role in muscle contraction and also facilitate cytokinesis.

They are fibers composed of various cell-specific proteins; many are composed of keratin. Intermediate filaments are thicker than microfilaments but thinner than microtubules.

These fibers contribute to the rigidity of the cell and also form desmosomes, a type of cellular junction.

They are thick, hollow tubulin polymers. Specifically, units of alpha- and beta-tubulin dimerize, and many of these dimers come together to form a microtubule.

Microtubules form the spindle apparatus, an essential component of cell division. They also comprise cilia and flagella and perform various other functions.

• Cilia are short, found in large numbers, and beat rhythmically in a back-and-forth pattern.

• Flagella are longer, with only a few found per cell at the most, and beat more slowly but powerfully to propel the cell in a certain direction.

In eukaryotes, both cilia and flagella have a 9+2 method of organization. This consists of 9 microtubule pairs arranged in a circle surrounding 2 single microtubules.

It consists of a phospholipid bilayer, with polar heads on the exterior (pointing toward the extracellular fluid and cytoplasm) and nonpolar tails on the interior.

The membrane also contains cholesterol, associated large proteins, and sphingolipids, among other components.

Water will exit the cell, causing it to shrivel.

A hypertonic solution is one that contains a comparatively high amount of solute (salt). Due to osmosis, water will flow from a low-solute to a high-solute environment in an attempt to equalize the solute concentrations.

It is a difference in the amount of solute molecules per unit volume between one region and another.

Molecules are prone to moving down their concentration gradient. For example, if the outside of a cell contains a much higher concentration of glucose than the inside, glucose will tend to move into the cell (if possible). If the membrane is impermeable to glucose, water will tend to move out instead.

The cell will swell, possibly to the point of lysing (rupturing).

Since pure water contains no solute, it is hypotonic in comparison to the cell. Water will travel down its concentration gradient from the exterior of the cell to the interior.

• It is the movement of a substance down its concentration gradient with the help of transmembrane protein channels.

• Since facilitated diffusion does not require energy, it is a form of passive transport.

It is a form of transport in which a material is engulfed by the cell membrane, then enters the cell in a vesicle. Endocytosis requires the expenditure of energy.

Phagocytosis and pinocytosis are both subtypes of endocytosis.

It is a form of transport in which a material is packaged into a vesicle which then fuses with the cell membrane. This process, which requires energy, allows the material to be exported from the cell.

Waste products and secreted hormones often leave a cell via exocytosis.

It is a type of endocytosis. In this process, the cell membrane engulfs the extracellular fluid, as well as the small particles it contains, in a vesicle.

Other types of endocytosis include phagocytosis and receptor-mediated endocytosis.

They are composed of multiple proteins, including claudins, and form an impermeable seal between adjacent cells. This prevents fluid and solutes from going "around" the cell to enter a cavity.

Tight junctions are found between cells in the same epithelial layer.

They are composed of intermediate filaments and are found at localized regions throughout a cell's membrane. They generally attach one epithelial or cell layer to another.

Desmosomes are found in epithelial and muscle tissue.

They are small channels formed from connexin proteins. They allow small solutes and fluid to pass from one cell to another.

While gap junctions are found between most cells, they are generally associated with cells that must communicate or function together, like neurons.

Unlike eukaryotes, prokaryotes lack a nucleus, as well as all membrane-bound organelles.

Note that membrane-bound organelles include mitochondria, lysosomes, the ER, and the Golgi apparatus, but not ribosomes. Prokaryotes do contain ribosomes, a fact that may appear on the AP Biology exam.

Prokaryotes are always unicellular, while eukaryotes can be either unicellular or multicellular.

One common example of a unicellular eukaryote is yeast, a fungus. Most other single-celled eukaryotes are classified as protists.

eukaryote

Only a eukaryote would possess mitochondria, since prokaryotes lack membrane-bound organelles. Eukaryotes also have linear, not circular, chromosomes. Note that both eukaryotes and prokaryotes can be unicellular.

The nucleus.

It is also the site of DNA replication and transcription.

While the mitochondria also include DNA, mitochondrial DNA is found in small circular chromosomes, not linear ones.

The ribosome.

Ribosomes are small organelles found in both eukaryotic and prokaryotic cells. At these organelles, proteins are synthesized (translated). A typical ribosome includes a small and a large subunit, although the sizes of these subunits vary depending on the type of cell.