AP® Biology - Cell Structure Part 3

Plasma membrane Specifically, cholesterol (shown here) is a lipid that confers rigidity to the membrane. Cholesterol can either be acquired through the diet or synthesized by cells.

• Peripheral proteins are loosely attached to either the inner or outer face of the plasma membrane. These proteins can be easily removed from the lipid bilayer.

• Integral proteins are firmly attached to the lipid bilayer and are very difficult to remove. These proteins extend at least partially into the hydrophobic interior region of the bilayer.

A transmembrane protein is a specific type of integral protein that spans, or extends through, the entire lipid bilayer.

• function in cell-to-cell adhesion

• function in cell-to-cell communication and recognition

• serve as receptors for hormones and other ligands

• assist in transport across the membrane

In a solution, it is the substance whose phase is lost after solvation (or the substance of which less is present). For example, dissolving a small amount of solid NaCl into liquid water produces a liquid solution. In this case, NaCl is the solute.

On the AP Biology exam, you will often see references to solutes dissolved in plasma or in other bodily fluids.

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.

• Passive transport involves the movement of a substance down its concentration gradient without the use of energy.

• Active transport requires energy, usually in the form of ATP, and moves a substance against its gradient.

Substances that require membrane channels include large molecules, polar molecules, and ions.

When these materials move down their concentration gradient, the process is known as facilitated diffusion. Small, nonpolar molecules do not require membrane channels and can travel via simple diffusion.

• 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.

Itis one with the same solute concentration as a solution to which it is compared.

For example, fluids that are administered in an IV should be isotonic with human cells. In other words, they should have the same osmolality, or solute concentration.

In comparison to a reference solution, a hypertonic solution has a greater solute concentration, while a hypotonic solution has a smaller solute concentration.

For example, say that a certain compartment contains 4 grams of NaCl per liter of solution. A solution of 2 g NaCl per L solution would be hypotonic, while a solution of 8 g NaCl per L solution would be hypertonic.

osmosis

Water will always move from areas of low solute to areas with a higher solute concentration.

Osmosis is generally tested in cases where two compartments are separated by a semipermeable membrane. The membrane allows water, but not solute, to pass through; osmosis is thus required to promote similar solute concentrations on both sides.

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.

Shark cells are likely hypertonic to human cells.

To avoid excessive water loss or gain, shark cells must be specialized to be isotonic, or at least close, to their surroundings. Since the ocean is composed of salt water, shark cells likely contain more solute than the cells of land-dwelling species.

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 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.