Ch. 5- Membrane Structure and Function

Components of the Plasma Membrane

Phospholipid bilayer Protein Molecules that are either partially or

wholly embedded Cholesterol Steroids Only animal cells include an Extracellular

membrane Each membrane is different by the type of

proteins embedded

Fluid- Mosaic Model

The phospholipid bilayer is fluid and the fluidity prevents the membrane from solidifying when external temperature drops.

At body temp, the bilayer has the consistency of olive oil.

Cholesterol prevents the membrane from becoming too fluid at higher temperatures.

Carbohydrate Chains

Phospholipids and proteins have sugar chains attached called glycolipids and glycoproteins.

These chains are on the cells exterior and have various functions including cell-to-cell adhesion, reception of signaling molecules, and cell-to-cell recognition.

Proteins

Channel Proteins- allows particular molecules or ions to cross the membrane freely

Carrier Proteins- selectively interact with specific molecules or ions to cross the membrane

Cell recognition membranes- glycoproteins, and help the body recognize when being invaded by pathogens

Proteins

Receptor Proteins- have a specific shape so only specific molecules can bind to it

Enzymatic proteins- carry out metabolic reactions directly

Junction proteins- tight junctions join cells so that a tissue can fulfill its function.

Permeability of the Plasma Membrane

The plasma membrane is selectively permeable, meaning it only allows certain substances into the cell while keeping others out.

Substances that are hydrophobic can diffuse across the membrane with no energy cost. Substances that are polar require and expenditure of energy.

Passive Transport

Diffusion is the movement of molecules from high concentration to low concentration- down their concentration gradient- until equilibrium is reached.

A solution contains a solute and a solvent. The solute is the substance being dissolved

and the solvent is the substance doing the dissolving.

Passive Transport Osmosis is the diffusion of water across a semi

permeable membrane from high to low concentration. Isotonic solution- the concentration of both solute and water

inside and outside the cell are equal, so there is no net water loss or gain.

Hypotonic Solution- The concentration of solute is lower outside the cell than the inside. Therefore water will move into the cell, potentially causing it burst.

Hypertonic Solution- The concentration of solute is higher inside the cell. This causes water to move out of the cell, potentially causing the cell to shrink.

Active Transport

Active Transport requires the use of proteins, usually called pumps, and energy to move molecules against their concentration gradient.

Example is the sodium-potassium pump

Active TransportBulk Transport moves large molecules like

proteins, polysaccharides and nucleic acids across the cell membrane. Exocytosis:

The secretion of hormones, neurotransmitters, and digestive enzymes out of the cell.

Intracellular vesicles fuse with the plasma membrane to secrete these.

Pancreatic cells specialize in exocytosis.

Active TransportEndocytosis:

Cells take in substances by forming vesicles around the material.

There are 3 ways endocytosis occurs:Phagocytosis

Cell takes in large materials and is common in unicellular organisms.

Pinocytosis Vesicles form around a liquid or a very small particle

Receptor-Mediated Endocytosis A form of pinocytosis that is specific because it uses receptors to

recognize compatible molecules and bring them into the cell.