Cell Transport

• Cells require a steady state or consistent internal environment in order to carry out specific functions.

• Homeostasis is The maintenance of the internal environment at a steady state, regardless of external changes, to ensure survival of a living organism.

• Cell membrane is – Responsible for maintaining homeostasis and – Controls what passes through the membrane

A membrane can be

• 1. Permeable – allows particles to easily pass through the membrane

• 2. Selectively Permeable or Semi permeable – Allows only certain particles to pass the membrane

• 3. Impermeable- Does not allow particles to move across the membrane

The cell membrane uses three methods to allow materials to pass through:

• 1. Diffusion

 • Movement of molecules from a region of high

concentration to a region of low concentration

 • The Difference between these two regions is

called the Concentration Gradient.

 

• Water – is found in the extracellular fluid– Diffuses freely across the cell membrane– Thus, the concentration of water outside and

inside of the cell are equal

• Osmosis: Is the diffusion of a solvent across a semi-permeable (The Diffusion of Water)

• Recall,– Solvent: Fluid that can dissolve other

substance – Solute: Substance that dissolves in a

solvent to form a uniform mixture or solution

• Across the cell membrane,– The direction of osmosis depends on the

concentration of water molecules on each side.

 Iso-osmotic Conditions:– Concentration of water ( [H2O] ) inside =

concentration of water ( [H2O] ) outside the cell membrane

– (Thus, equal amounts of water move in and out)

 • Hypotonic Conditions OR HYPO-OSMOTIC:

– [H2O] inside is greater than [H2O] outside– (Thus, water moves out of the cell)**Total solutes is greater outside of the cell

• Hypertonic Conditions OR HYPER-OSMOTIC:– [H2O] inside is LESS than [H2O] outside– (Thus, water moves in cell)

• **Total solutes is greater inside the cell

2. Facilitated Diffusion

• Other substances than water, oxygen and carbon dioxide require assistance to pass through the cell membrane

• I.e. Glucose:– Too large to passively cross the membrane– Insoluble in lipids – cannot dissolve in bilayer

• Proteins in the membrane assist these molecules. They are called Transport Proteins.– They are specific to one type of molecule– depend on the shape, size and electron charge

of the particle moving across the membrane.

• There are two types of Transport proteins:

• 1. Carrier Protein– Glucose is carried across the membrane by a

carrier protein

Will only accept particles that are:– Not charged– With a specific shape

• 2. Channel Proteins– Transports charged particles or ions across the

membrane– These Proteins have a channel down the center to allow

these ions to pass the bilayer.

• Channel Proteins are similar to magnets:– A negatively charged channel Protein – Repels

Negatively charged ions– A Positively charged channel Protein – Repels

Positively charged ions

• Like diffusion, these particles are moving across these transport proteins according to their Concentration Gradient.

Active Transport