DIFFUSION, OSMOSIS, AND

THE CELL MEMBRANE

Structure and Function of Cell Membrane

Structure – phospholipid bilayer embedded with protein

pumps

cells want to maintain equilibrium

selectively regulates passage of substances in and out of

cell

How do things pass through the cell membrane?

Through passive transport: no additional

energy required

Three types

small molecules such easily pass through via

simple diffusion

large molecules (starch) require use of protein

pumps for facilitated diffusion

Water passes through the cell membrane via

osmosis

Simple Diffusion

Simple diffusion is process in which molecules intermingle as a

result of kinetic energy

all molecules are in constant motion and continually spreading apart

Simple diffusion occurs because of a concentration gradient

A concentration gradient exists when one area is more or less

concentrated than another.

molecules move from an area of high to low concentration, aka

simple diffusion

Example:

Tea Bag or sugar cube in water

intracellular circulation –

movement of materials

within a cell occurs through diffusion

Let's see diffusion in action

(I am a link!)

Facilitated Diffusion

Transport of substances across the cell membrane

from higher concentrations to areas of lower

concentration through means protein pump

No energy required

Let’s see facilitated diffusion in action (I am a link!)

OSMOSIS

Diffusion of water is known as osmosis

water moves via kinetic energy from a less salty/sugary solution (lower solute concentration) to a more salty/sugary solution (higher solute concentration) in order to dilute out the salt/sugar

if too much water builds up inside a membrane, osmotic pressure results

if osmotic pressure is too high, the cell can burst/lyse

Let’s see osmosis in action (I am a link!)

Osmosis and Cells

Whether there is net movement of water into or out of the cell and which direction it moves depends on whether the cell’s environment is isotonic, hypotonic, or hypertonic.

Tonicity: is a measure of the osmotic (water) pressure gradient of two solutions separated by a semipermeable membrane.

Hypertonic refers to a higher concentration

Hypotonic refers to a lower concentration

Isotonic refers to equal concentrations

Tonicity and cells: Cells in

an Isotonic Solution

When two environments are isotonic, the concentration of dissolved solutes is the same in both of them.

When cells are in isotonic solution, movement of water out of the cell is exactly balanced by movement of water into the cell.

Tonicity and cells: Cells in a

Hypertonic Solution

Greek "hyper," = over, and "tonos," = stretching.

In a hypertonic solution the concentration of

solutes is greater than that of the concentration

in a cell.

If concentrations of solutes are greater outside

the cell, the concentration of water outside is

correspondingly lower.

As a result, water inside the cell will flow

outwards to attain equilibrium, causing the cell

to shrink.

As cells lose water, they lose the ability to

function or divide.

Tonicity and cells: Cells in a

Hypotonic Solution

Greek "hypo,"=under, and "tonos,"=stretching.

In a hypotonic solution the concentration of

solutes is less than that of a cell.

When concentrations are less outside the cell

than inside, the concentration of water outside

is correspondingly greater outside the cell.

When a cell is in hypotonic conditions, there is

net water movement into the cell.

Cells without walls will swell and may burst

(lyse) if excess water is not removed from the

cell. Cells with walls often benefit from the

turgor pressure that develops in hypotonic

environments.

Active Transport

movement of particles with the use of

energy other than kinetic

sometimes active transport can move

molecules from an area of low to high

concentration (against the gradient)

Two Types of Active Transport

Often uses ATP (the

cell’s energy)

involves protein

molecules in the cell

membrane that act as

carriers aiding in

transportation

First Type: Primary Active Transport

Second type:

ENDOCYTOSIS Phagocytosis (cell eating) – white blood

cells that keep the body healthy by engulfing foreign pathogens

Membrane folds in around large molecules so they can be taken in by the cell resulting in a vacuole containing the molecule

Pinocytosis (cell drinking) – fluids containing large molecules such as DNA or proteins are taken into the cell in the same manner

Two Types of Active Transport

Putting it all together!