THE CELL MEMBRANE

1. The cell membrane is selectively permeable; it controls which substances pass into and out of the cell.

2. The cell membrane is composed primarily of a phospholipid bilayer with proteins embedded within the membrane.

SELECTIVELY PERMEABLE (SEMI PERMEABLE)

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MEMBRANE

3.FLUID MOSAIC MODELThe cell membrane is composed of lipids, proteins & carbohydrate tails (a “mosaic” of molecules). It is also a dynamic structure, with the molecules constantly shifting & moving (it is “fluid” (like buoys on the ocean) ).

Scientists therefore describe the structure of the cell membrane as a Fluid Mosaic Model.

4. The Cell Membrane

Composed of a lipid bilayer. And two general types of proteins:

peripheral proteins – proteins located on both the interior surface and the exterior surface of the cell membrane.

integral proteins – proteins that are embedded in the membrane bilayer.

fig. 4-5 p. 74

marker

receptor

channel

MEMBRANE PROTEINSMembrane proteins can further be classified by their

particular function.

1. Carrier proteins – proteins that transport specific substances across the membrane.

2. Channel proteins – proteins that provide a passageway across the cell membrane through which small molecules or ions can diffuse.

3. Marker/Recognition proteins – Marker/Recognition proteins extend across the cell membrane and serve to identify the cell. The immune system uses these proteins to tell friendly cells from foreign invaders. They are as unique as fingerprints. They play an important role in organ transplants. If the marker proteins on a transplanted organ are different from those of the original organ the body will reject it as a foreign invader.

4. Receptor proteins – proteins within the cell membrane that are able to receive signals from the outside of the cell which can cause the cell to respond.

(5-1) 1. PASSIVE TRANSPORT

All molecules have movement due to vibrations caused by heat (all except those at Absolute Zero (theoretically)). The molecules do not need to be physically moved; they move because they have their own energy. Therefore a cell can transport molecules using this natural movement. It is called “Passive Transport” because the cell does not have to use additional energy in the process.

Different forms of passive transport include:A. DiffusionB. OsmosisC. Facilitated DiffusionD. Diffusion Through Ion Channels

DIFFUSION

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A. DIFFUSIONThe simplest type of passive transport is diffusion.

1. The difference in the concentration of molecules across a space is called a concentration gradient.

2. “ Molecules tend to move "down" their concentration gradient, from areas where they are more concentrated to areas where they are less concentrated.”

3. When the concentration of the molecules of a substance is the same throughout a space, a state of equilibrium exists.

4. The ability of a molecule to diffuse across a cell membrane depends on:

1. the size of the molecule

2. the type of molecule

3. the chemical nature of the membrane

What types of molecules can diffuse through the cell membrane?Small molecules (water) and molecules that can dissolve in lipids

(oxygen & carbon dioxide).

B. OSMOSIS

The process by which water molecules diffuse across a cell membrane from an area of high concentration to an area of lower concentration is called osmosis.

Because water moves down its concentration gradient, osmosis does not require cells to expend energy. Osmosis, therefore, is a type of passive transport.

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OSMOSISWATER MOLECULES

b.1 HYPOTONIC/HYPERTONIC/ISOTONIC

Solute – a substance dissolved in a solution.

Solution – a mixture in which one or more substances are uniformly dissolved in another substance.

Solvent- in a solution, the substance in which a solute is dissolved.

Hypotonic environment (“Fresh water”) a solution whose solute concentration is lower than that inside the cell.Hypertonic environment (“salty”) a solution whose solute concentration is higher than that inside the cell.Isotonic environment (“same”) a solution whose solute concentration equals that inside a cell.

B.2.Contractile Vacuole

Paramecium live in fresh water which is Hypotonic. That means water is constantly diffusing in through osmosis. It will pop like a water balloon if the water is not removed by the contractile vacuole.

Click here to see a video of a Paramecium using its contractile vacuole

b.3 Turgor Pressure and Plasmolysis

In a hypotonic (“fresh”) environment the plant cells fill with water. This creates pressure called turgor pressure.

In a hypertonic (“salty”) environment the plant cells lose water and plasmolysis results.

b.4. Cytolysis

In a hypotonic solution, animal cells (which do not have a cell wall) can fill up too much and burst! (Remember the paramecium?)

Also remember the “Hold your wee for a Wii tradgedy”?

Hitler’s doctors apparently killed people in hospitals by giving high doses of pure water in ivs!

CHANNEL & CARRIER PROTEINS

C. FACILITATED DIFFUSION 1. This process is used for molecules that cannot diffuse rapidly through cell membranes, even when there is a concentration gradient across the membrane.

2. The movement of these kinds of molecules across the cell membrane is assisted by proteins known as carrier proteins. Molecule binds to carrier protein. Carrier protein changes shape. Molecule can then pass through hydrophobic interior of membrane. Carrier protein returns to original shape.

3. Glucose as an example shows that:

>Carrier proteins are specific to the type of molecule.

>Facilitated diffusion speeds up the diffusion process.

D. DIFFUSION THROUGH ION CHANNELS Membrane proteins that allow for passive transport

of ions. 1. Ions such as sodium (Na+), potassium (K+), calcium (Ca2+),

and chloride (Cl-) are important for a variety of cell functions. Because they are not soluble in lipids, ions cannot diffuse across the lipid bilayer without assistance. Ion channels provide small passageways across the cell membrane through which ions can diffuse.

2. Stimuli that can open ion channels: Stretching of membrane, electrical signals & chemicals in the cytosol or external environment.

(5-2) ACTIVE TRANSPORT1,2 & 4. Phospholipids

The cell membrane is like a oreo cookie

The inside is hydrophobic which means it doesn’t soak up milk (water). That is because it is made of “fatty acid tails”.

The outside soaks up the milk (water) and so we call it hydrophilic. It is not made of fat but of a water-loving phosphate group.

3. Channel Marker & Receptor ProteinsMEMBRANE PROTEINSMembrane proteins can further be classified by their particular function.Carrier proteins – proteins that transport specific substances across the membrane. Channel proteins – proteins that provide a passageway across the cell membrane through

which small molecules or ions can diffuse.Marker/Recognition proteins – Marker/Recognition proteins extend across the cell

membrane and serve to identify the cell. The immune system uses these proteins to tell friendly cells from foreign invaders. They are as unique as fingerprints. They play an important role in organ transplants. If the marker proteins on a transplanted organ are different from those of the original organ the body will reject it as a foreign invader.

Receptor proteins – proteins within the cell membrane that are able to receive signals from the outside of the cell which can cause the cell to respond.

II. ACTIVE TRANSPORT1. In many cases, cells must move materials up their concentration gradient, from an area of lower concentration to an area of higher concentration. Such movement of materials is known as active transport. Unlike the passive transport of facilitated diffusion, active transport requires a cell to expend energy.

2. Carrier proteins that go against the concentration gradient are called “pumps” and they are similar to channels because they both shield molecules from the phospholipids inside the membrane.

Require Energy!!! (ATP)

B. Sodium(Na)/ Potassium(K) Pumps:

1. Pump against the concentration gradient.

2. 3 Na are pumped out

3. 2 K are pumped in.

4. ATP is used as energy.

5. This creates an electrical potential which means it has the potential to send messages (nerves).

Some substances such as macromolecules and food particles, are too large to pass through the cell membrane by the transport processes mentioned thus far. Cells employ two other mechanisms – endocytosis and exocytosis – to move such substances across their membranes.

Endocytosis & Exocytosis

C. ENDOCYTOSIS1.Endocytosis is the process by which cells ingest external fluid, macromolecules, and large particles, including other cells.

2. External materials are enclosed by a portion of the cell, which folds into itself and forms a pouch. The pouch then pinches off from the cell membrane and becomes a membrane-bound organelle called a vesicle.

3. Pinocytosis – a type of endocytosis in which the cell engulfs solutes orfluids.

Phagocytosis – a type of endocytosis in which the cell engulfs large particles or whole cells.

D. EXOCYTOSISExocytosis is essentially the reverse of endocytosis.

During exocytosis, vesicles in the cytoplasm fuse with the cell membrane, releasing their contents into the cell’s external environment.

Nerves for instance send messages from one cell to the next by chemicals called neurotransmitters that are released & send the message to the next

nerve cell.

Drugs and alcohol interfere with this and

make it difficult to react appropriately!