Cell Membrane

entropy

tendency for systems to tend toward a state of greater randomness or disorder

system

a collection of physical objects

osmosis

diffusion of water across membranes

Water diffuses when the 3 conditions are met:

1. 2 solutions are separated by a lipid bilayer

2. the solutions have different concentrations of dissolved ions & molecules

3. the dissolved substances can’t move across the membrane 

The 3rd condition for osmosis is important because:

the dissolved ions & molecules can’t diffuse across the membrane. Only water can — it’s small enough to slip through tiny gaps between lipids in the bilayer.

What does it mean if one solution has a higher concentration of dissolved substances than the other?

the substance has a lower concentration of water molecules

Osmosis is the observation that water diffuses across membranes from the side with higher concentration of water (lower solutes) to the side with:

lower concentration of water (higher solutes).

concentration gradients

occur when dissolved substances, or solutes, differ in concentration in different areas (usually on either side of a cell membrane)

Concentration gradients describe the direction that:

diffusion will occur.

selective permeability

some substances can cross membranes more readily than others

The ability of a substance—an atom, ion, or molecule—to pass through a lipid bilayer depends on 2 things:

1. its size

2. its charge

All else equal, smaller substances pass through cell membranes much ____ than larger substances.

faster

All else equal, uncharged & nonpolar substances pass through cell membranes much ____ than charged or polar substances.

faster

Why is permeability dependent on size & charge?

It’s based on a substance’s ability to interact with both the polar heads of phospholipids on the outside of the membrane & their nonpolar hydrocarbon tails inside.

Can sodium & chloride ions pass through a lipid bilayer?

No; they cannot interact with the nonpolar tails at all.

diffusion

movement of ions or molecules from an area of higher concentration to an area of lower concentration

Entropy is the tendency of systems to:

reach a state of higher disorder or randomness

A concentration gradient is the difference in a substance’s concentration in space usually across a:

cell membrane.

potential energy

the energy of position

energy

the ability to do work

facilitated diffusion

diffusion of substances across cell membranes through integral membrane proteins

The phospholipid bilayer in a cell membrane exists because:

of the hydrogen bonding between hydrophilic heads on phospholipids & water molecules outside the cell, as well as hydrophobic interactions among the tails on phospholipids.

Transmembrane proteins are:

amphipathic

All organelles have a _____ membrane.

double

Diffusion is driven by ____. Diffusion does/does not require an input of energy.

entropy; does not