BISC1111 - Chapter 7 - Membrane Structure and Functions

Membranes are composed of...

phospholipids

Most membranes are amphipathic, meaning they have...

Hydrophilic heads and hydrophobic tails

Where do the hydrophilic parts of the phospholipid bilayer point?

Toward the cytosol and extracellular fluid in and outside of the cell

Where do the hydrophobic parts of the phospholipid bilayer point?

Toward the bilayer's interior

Most lipids and proteins move ___ within the plasma membrane

Sideways

What does cholesterol do in the cell membrane?

Reduces membrane fluidity at moderate temperatures, but hinders solidification at low temperatures by disrupting packing

Membranes switch from a ___ state to a ___ state at low temperatures

Liquid to a solid

What are membranes held together by?

weak hydrophobic interactions

fluid mosaic model

Structural model of the plasma membrane where molecules are free to move sideways within a lipid bilayer. Every protein in it has a purpose and is not randomly assigned

Membranes must be what to function properly?

Fluid. They must be fluid.

What happens if a membrane is too fluid?

It cannot support protein function

Does every organism have the same phospholipid bilayer composition?

No. Organisms in extreme temperatures have adapted differences in their membrane lipid concentration

How do phospholipid bilayers vary depending on the temperature?

By increasing in production of unsaturated phospholipid tails in order to prevent membrane solidification in colder temperatures

Which is more permeable: a membrane rich in unsaturated fatty acids, or a membrane rich in saturated fatty acids?

Unsaturated fatty acids, as the kinds in their double bonds allow more molecules to flow in

peripheral proteins

The proteins of a membrane that are not embedded in the lipid bilayer; they are appendages loosely bound to the surface of the membrane.

Integral proteins

penetrate the hydrophobic interior of the lipid bilayer

Transmembrane proteins

Integral proteins spanning the entire membrane.

What do the hydrophobic regions of an integral protein consist of?

Nonpolar amino acids in alpha helices

What functions do cell surface membrane proteins carry out?

Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the extracellular matrix

How do cells recognize each other?

By binding to surface molecules, often carbohydrates, on the plasma membrane

Glycolipids

Membrane carbohydrates that are covalently bonded to lipids.

Glycoproteins

Membrane carbohydrates that are covalently bonded to proteins.

Selective permeability

A property of a plasma membrane that allows some substances to cross more easily than others.

What types of molecules pass through the lipid bilayers more easily?

Hydrophobic ones like O2, CO2, and hydrocarbons

dynamic equilibrium

Result of diffusion where there is continuous movement of particles but no overall change in concentration

Diffusion

Movement of molecules from an area of higher concentration to an area of lower concentration.

Osmosis

Diffusion of water through a selectively permeable membrane from the lower solute concentrated region to the higher solute concentrated region

Tonicity

the ability of a surrounding solution to cause a cell to gain or lose water

Isotonic

when the solute concentration within the cell is the same as the volute concentration outside the cell

Hypertonic

Having a higher concentration of solute OUTSIDE the cell than INSIDE the cell. Leads to the net loss of water

Hypotonic

Having a higher concentration INSIDE the cell than OUTSIDE. Leads to excessive water coming inside the cell

concentration gradient

The region along which the density of a chemical substance increases or decreases.

Turgidity

the pressure created by water pushing up against the cell wall in a hypotonic solution. Makes the plant stand firm.

Flaccidity

State where plant cells are surrounded by isotonic solutions with no net movement, causing them to limp

Plasmolysis (plant cells)

the shrinkage of cytoplasm & the pulling away of the plasma membrane from the cell wall when a plant loses water. Happens in HYPERTONIC solutions

facilitated diffusion

Movement of specific molecules across cell membranes through protein channels that do NOT require ATP

Channel proteins

Have hydrophilic channels where molecules and ions use as a tunnel

Aquaporins

A transport protein in the plasma membrane of a plant or animal cell that specifically facilitates the diffusion of water across the membrane

Ion channels

A transmembrane protein channel that allows a specific ion to diffuse across the membrane down its concentration or electrochemical gradient.

Gated channels

A protein channel in a cell membrane that opens or closes in response to a particular stimulus, chemical or electrical. (THINK NERVE CELLS! AP PSYCH!)

Carrier proteins

bind to molecules and change shape to shuttle them across the membrane

Active transport

requires energy (ATP) to move substances against their concentration gradients

All proteins involved in active transport are...

... carrier proteins

cotransport

The coupling of the "downhill" diffusion of one substance to the "uphill" transport of another against its own concentration gradient.

Sodium-potassium pump

a carrier protein that uses ATP to actively transport sodium ions out of a cell and potassium ions into the cell. Example of cotransport

Active transport helps cells do what?

Maintain solute concentrations that

differ from the environment