Osmosis
• Water is a special case. It can move down its
own concentration gradient.
– High H2O concentration = pure water.
– Low H2O concentration = salt water, sugar water.
• This causes water levels to change across a
membrane.
• Osmosis: the movement of water molecules
down their concentration gradient.
Osmosis
Osmosis
• Importance:
• The cell membrane is usually permeable to
water, but not to many solutes.
• Water, instead, moves into or out of the cell in
order to make the concentrations equal.
Osmosis
• The movement of water due to osmosis
creates pressure.
– This is called “osmotic pressure”.
– The greater the osmotic pressure, the more likely
it is water will move in a given direction.
Osmosis - Scenarios
• The direction of osmosis depends on how
concentrated the surrounding environment is.
• There are three possible “types” of
environments (next slide – make way for a
table!)
Osmosis - Scenarios
Environment
Isotonic solution
Hypotonic solution
Hypertonic solution
Description
The solution and the cell have
the same concentrations of
solutes.
The solution has a lower
concentration than the cell.
The solution has a higher
concentration than the cell.
Osmosis - Scenarios
Osmosis - Scenarios
Environment
Isotonic solution
Effects on Cells
No change.
Hypotonic solution
Cells swell. They may burst.
Hypertonic solution
Cells shrivel up.
Large and Charged Particles
• (Recall: Only some particles can pass through
the cell membrane on their own.)
• Particles that are large or are charged cannot
cross on their own.
– Ex: proteins, salts, and polar molecules like sugars
(glucose, fructose, etc.)
Channel Proteins
• (Recall: The cell membrane is made out of not
only lipids, but also embedded proteins.)
• Channel proteins in the cell membrane act as
passages for ions and molecules.
– They allow free movement for these particles.
Channel Proteins
• (Side-note slide)
• A malfunctioning chloride (Cl-) channel is
responsible for cystic fibrosis.
Normally, water and
sodium will “follow”
chloride when it leaves the
cell.
In CF, no water leaves
because barely any
chloride leaves.
Carrier Proteins
• Some membrane proteins act as carriers for
large, charged, or polar particles.
– These are NOT open channels.
Carrier Proteins
• Carrier proteins are membrane proteins that
allow a specific molecule to pass, and no
others. (eg: glucose carriers only transport
glucose, even if other sugars “look” similar).
• These carrier proteins act by one of two
methods:
1. Facilitated Transport (aka facilitated
diffusion)
2. Active Transport
Facilitated Transport
• (Remember that molecules have
concentration gradients. They want to move
along these gradients, but the cell membrane
blocks them.)
• Carrier proteins let molecules move down
their concentration gradients across the
membrane through diffusion (they act like a
turnstile).
Facilitated Transport
Facilitated Transport
• Note: these carrier proteins can be controlled
– they can be turned on or off to manage the
amount of a molecule that crosses the cell
membrane.
• (REMEMBER: Facilitated transport does NOT
require energy – the molecules want to move
down their concentration gradient already –
the carriers just provide a way to do that.)
Active Transport
• Sometimes, molecules must be moved
against their concentration gradients (from an
area of low concentration to an area of high
concentration).
• This requires energy.
• This process is called active transport.
Active Transport
• The energy provided is usually chemical
energy in the form of a molecule called “ATP”
(we’ll return to this).
• Cells that rely on a lot of active transport
usually have many mitochondria.
Example – Na+/K+ Pump