Properties of the plasma membrane that allow molecules to diffuse across the phospholipid bilayer

1. Selectively Permeable

1.1. Selectively permeable means that the plasma membrane only allows certain molecules to enter and exit the cell

1.2. This property helps the cell maintain its internal order despite changes in the environment

1.3. This property makes it so that transport proteins and channels are needed for specific substances to move across the membrane

1.3.1. Transport proteins can be broken down into two categories: Transporters and channels

1.3.1.1. A transporter transfers only the molecules that fit into specific binding sites on the protein

1.3.1.2. Channels discriminate based mainly on size and charge

1.3.1.2.1. When a channel is open only substances of a certain size and charge may pass through

2. Has the capacity for growth and movement

3. Has the capability to deform and repair itself

4. Presence of membrane proteins

4.1. Most membrane functions are carried out by membrane proteins

4.1.1. Membrane proteins account for around 50 percent of the mass of plasma membranes

4.2. There are five different functional classes of membrane proteins

4.2.1. 1. Transporters:

4.2.1.1. An example of a transporter is the Na+ puimp

4.2.1.1.1. This actively pumps NA+ out of cells and K+ in

4.2.2. 2. Ion channels:

4.2.2.1. An example of an ion channel is the K+ leak channel

4.2.2.1.1. This allows K+ ions to leave cells, thereby influencing cell excitability

4.2.3. 3. Anchors:

4.2.3.1. An example of an anchor are integrins

4.2.3.1.1. Integrins link intracellular actin filaments to extracellular matrix proteins

4.2.4. 4. Receptors:

4.2.4.1. An example of a receptor is the platelet-derived growth factor (PDGF) receptor

4.2.4.1.1. This binds extracellular PDGF and, as a consequence, generates intracellular signals that direct the cell to grow and divide

4.2.5. 5. Enzymes:

4.2.5.1. An example of a type of enzyme is adenylyl cyclase

4.2.5.1.1. This enzyme catalyzes the production of teh small intracellular signaling molecule cycle AMP (cAMP) in response to extracellular signals

4.3. Membrane proteins associate with the lipid bilayer in different ways

4.3.1. They can be transmembrane proteins

4.3.2. The associate with the cytosolic half of the membrane

4.3.3. The proteins that are directly attached to the membrane are not easily removed

4.3.3.1. This classifies them as integral membrane proteins

4.3.3.2. Peripheral membrane proteins asre more easily released because of this

5. Phospholipid Bilayer

5.1. Fluidity

5.1.1. Hydrocarbon tails play a large role in the fluidity of the membrane

5.1.1.1. Unsaturated tails have one or more double bond and this causes the tails to "kink"- making them more difficult to pack oild

5.1.1.2. Saturated fatty acid tails have no double bonds and are easily packed together (hard fats)

5.1.1.3. The closer and more regular the packing of the hydrocarbon tails the less fluid the bilayer is

5.1.2. This fluidity of the lipid bilayer depends on its composition

5.2. Flexibilty

5.2.1. The bilayer is a flexible two-demensional fluid

5.2.1.1. The phospholipids in the layer rarely flip-flop (switch spots in the membrane)

5.2.1.2. This flexibility allows the phospholipids to move horizontally and laterally

5.2.1.2.1. This direction of movement is due to the phospholipid molecules not being covalently bonded to each other

5.3. Amphipathic nature

5.3.1. Amphipathic means having both hydrophobic and hydrophilic parts- which helps drive lipids to form a bilayer

5.3.2. The forces that make the bilayer amphipathic help the bilayer be self-sealing

5.3.2.1. Self-sealing is when free edges are energetically unfavorable and quickly spontaneously re-arranged

5.3.2.1.1. This causes curling and the formation of a bilayer around in an enclosed aqueous space

5.4. Hydrophilic head

5.4.1. Hydrophilic molecules can interact with water due to polar or charged groups

5.5. Hydrophobic tails

5.5.1. Purely hydrophobic molecules, such as fats found in oils of plant seeds, coalesce (fuse) into large fat droplets when in water

5.5.2. Hydrophobic molecules form no favorable interactions because they are not polar or charged

5.5.2.1. This is why fats coalesce (fuse) into large fat droplets when in water

5.5.2.2. Costs free energy but that is minimized by clustering the hydrophobic molecules closer together

5.5.2.3. They force water to form a cage-like structure around them