Topic 2a: plasma membrane

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Plasma membrane:

Selectively permeable

Composed of phospholipid bilayer—nonpolar molecules pass more easily than most other molecules—and other molecules (proteins, carbs, cholesterol, etc)

Unique molecules that are amphiphiles; have hydrophobic (head—glycerol and phosphate group) and hydrophobic (tail—fatty acids) region

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Proteins are scattered within the bilayer, serve multiple purposes: transport, enzymatic activity, cell adhesion, communication, receptors for specific substances, etc.

Integral proteins: integrated completely into the membrane, interact with all or part of the hydrophobic regions

Channel proteins: span the entirety of the membrane, provide a hydrophilic channel for ions to pass through

Aquaporins: water channel, allows water to pass through quickly and is useful in particular cells (RBCs, plant cells, etc).

Gated channel proteins: have a "gate" that must be open for solutes to pass

Mechanically-gated

Ligand-gated

Voltage-gated

Open: no gate, solutes can freely go through

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Glycolipids and glycoproteins: Lipids or proteins in the membrane with carb groups on them; serve as cell antigens (markers).

Fluid Mosaic model

Cholesterol regulates the fluidity of the membrane in animals. It also keeps unwanted substances from entering the cell.

In the heat, cholesterol keeps the phospholipids together and fills gaps. In the cold, cholesterol keeps phospholipids from being too rigid and allows for fluidity.

Waxes are the equivalent of cholesterol in plants.

The composition of the phospholipids can also impact fluidity--> the more unsaturated lipids, the greater the fluidity due the kinks from double bonds. This is helpful in maintaining fluidity in cooler temperatures.

Temperature can impact membrane fluidity (in the heat, it is more fluid, in the cold, it is more rigid).

Lipid rafts: microdomains of glycoproteins and molecules in the membrane that serve as organizing sites for functions like signal transduction, transport, etc.

Membrane dynamics

Lipids move within the membrane

Catalyzed: use a protein catalyst to facilitate movement; may or may not require ATP.

Flippase: brings a phospholipid from the outer layer to the inner layer; requires ATP

Floppase: brings a phospholipid from the inner layer to the outer layer; requires ATP

Scramblase: coupled movement-->phospholipids from outer and from inner layers moved together; does not require ATP.

Uncatalyzed: movement including transbilayer diffusion (where a phospholipid moves from one layer to another) and lateral diffusion (where phospholipids move from side to side but stay on the same layer of the membrane).

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Movement of solutes across the membrane

Concentration gradient: Relative comparison of solutes and overall [fluids] inside and outside the cells. Solutes tend to move from high to low concentration. Moving from low to high [] requires energy and doesn’t occur spontaneously. Direction of movement is regulated by Gibbs free energy.

Passive transport: Movement from high to low concentration that occurs without requiring input of energy (spontaneously)

Diffusion: Movement of small particles down their concentration gradient; generally, a slow process but the rate can be influenced by temperature, size of solute, density of solvent, and size of the gradient/distance traveled. Continues until equilibrium is reached.

Facilitated diffusion: Diffusion of hydrophilic solutes through protein carriers that separate them from hydrophobic membrane; this is done because it is thermodynamically unfavorable for charged ions to pass through the hydrophilic region.

Osmosis: Movement of water down its concentration gradient; occurs when a solute is too large to move across the membrane; then water moves to where there is more solute and less water.

Osmotic pressure: the pressure exerted by the solvent via osmosis; reverse pressure is required to prevent osmosis. The greater the [solute] difference, the greater the pressure. It is determined by osmolarity (the total [solute] of the solution). High osmolarity is when the [water] is less than than the [solute] and low osmolarity is when the [water] is greater than [solute].

Hypotonic solution: [solute] < [ water] in the extracellular solution (low osmolarity). When cells are placed in this solution, they swell up because water enters the cell.

Hypertonic solution: [solute] > [water] in the extracellular solution (high osmolarity). When cells are placed in this solution, they shrivel up because water leaves the cells.

Isotonic solutions: No net movement of water, equal concentrations of solutes on both sides, goal for cells.

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Active transport: Requires input of energy

Primary: uses chemical energy source like ATP

ex: sodium-potassium pumps in neurons--> 2 K+ ions enter the cell and 3 Na+ ions leave the cell, resulting in greater [Na+] outside the cell and [K+] inside the cell. Ultimately the inside of the cell is more negative than the outside and the action potential is transmitted through the depolarization (make the membrane more positive/less negative) of the membrane.

Steps for functioning of the pump

Pump faces the inside of cell, with a high affinity for Na+ ions. 3 of them bind to the pump.

Pump hydrolyzes ATP, leaving ADP attached to it.

As a result of this hydrolysis reaction, the pump undergoes a conformational change with decreased affinity for Na+ ions. It faces towards the exterior of cell and the 3 Na+ ions that were attached leave the pump.

While facing outside, the pump has a greater affinity for K+ ions. 2 K+ ions attach, and ADP detaches.

The pump reorients to the inside of cell again after the ADP detaches and the 2 K+ ions detach inside the cell.

The cycle begins again once the pump faces inside and regains the Na+ affinity.

ABC (ATP-binding cassette) transporters: use ATP to transport substrates, typically out of the cell

Secondary: uses an electrochemical gradient as a source of energy; also known as cotransport

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Bulk transport: Movement of large items or large quantities of substance via specific mechanisms

endocytosis: brings items into the cell by forming a vesicle from the membrane around the item being engulfed

pinocytosis: liquids moved into the cell

phagocytosis: other large items, like cells. Can be receptor mediated (molecule must bind to a receptor on the membrane before it is brought in).

receptor mediated endocytosis: uses specific receptor proteins to mediate endocytosis, and the vesicle is coated with ligand

exocytosis: transport large items out of the cell. Vesicles carrying the item fuse with the membrane.

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Membrane Potential: voltage across the membrane of some cells, causing it to be polarized. Resting potential is determined by membrane permeability to each ion, and the concentration gradients of ions across the membrane. Movement of ions causes change in potential, resulting in depolarization.

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Membrane receptors: serve many purposes, such as facilitating change in cell's internal environment, cell communication, and molecular transport.

G-Protein Linked Receptors: binding of a ligand activates the G protein (bound to the receptor intracellularly), which results in activation of a signaling cascade. All G protein receptors have 7 transmembrane domains.

Ion-channel linked receptors: bind a ligand to open the channel via a conformational change and allow ions to pass through. The receptor has both hydrophobic (on the exterior) and hydrophilic regions (in the interior where the ions will pass) as it spans both membrane layers.

Enzyme linked receptors: receptor is linked to an enzyme or has an enzyme domain, that sets of a cascade when a ligand binds to the receptor.

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Intercellular junctions: regions of contact between plasma membranes of 2 or more cells, essential for multicellular organisms to be held together

gap junctions: channels between neighboring cells connecting the cytoplasm of 2 cells, allowing for rapid movement of ions and other substances

in vertebrates, 6 connexins (membrane proteins) come together to form a connexon (a pore). When they align, they form a channel between the cells

ex: found in heart cells to sync electrical activity

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tight junctions: adjacent cells are held flush together, creating a watertight seal; ideal in cells where leakage of fluids is not desirable.

in animal cells, claudins arranged in a network of strands form a barrier from which not even liquid can escape.

actin microfilaments of neighboring cells are coupled together

ex: cells lining an organ.

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Adherens junctions: attach cells in area that need to stretch

ex: skin and bladder, epithelia, muscle

desmosomes: hold cells together in tissues

hemidesmosomes: hold cells to the basal lamina

in animal cells, cadherins (specialized adhesion proteins) on both membranes interact in the space between the cells to hold them together. Within the cell, they interact with cytoplasmic plaque which is bound to intermediate filaments to anchor the cells.

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