Dual Enrollment Biology - Chapter 15

Cell Membranes

• Fluid Mosaic Model

• Selectively permeable

• Made of two layers of phospholipids, with the following embedded components - integral proteins, peripheral proteins, glycoproteins, glycolipids, carbohydrates, cholesterol

• Bordered on one side by extracellular matrix, and on the other side by the cytoskeleton

• Aquaporins - membrane proteins that transport water only

  • One aquaporin enables billions of water molecules to pass through the membrane per second!

Phospholipids

• hydrophilic phosphate heads and hydrophobic fatty acid tails

Amphipathic

• having both hydrophilic and hydrophobic parts.

• Phospholipids are amphipathic.

• Most membrane proteins are also amphipathic.

Lateral Movement

• Lateral movement is what provides the membrane with a fluid structure

• Most of the lipids and some of the proteins can shift about laterally.

• Lateral movement of phospholipids within the membrane is rapid.

• Proteins are much larger than lipids and move more slowly, but some membrane proteins do drift.

• Some membrane proteins seem to move in a highly directed manner, maybe driven along cytoskeletal fibers by motor proteins.

• Many other membrane proteins seem to be held immobile by their attachment to the cytoskeleton or to the extracellular matrix.

Membrane Fluidity

• Membrane remains fluid as temperatures decrease, until phospholipids pack into a closely-packed arrangement and the membrane solidifies.

• The temperature at which the membrane solidifies depends on the types of lipids it is made of. 

• If the membrane is rich in phospholipids with unsaturated fatty acid tails, remains fluid to a lower temperature (due to kinks).

Cholesterol

• Has different effects on membrane fluidity at different temperatures

• Makes membranes less fluid by restraining phospholipid movement

• Also hinders close packing of phospholipids, so it lowers the temperature required for membrane to solidify

• Helps membrane resist changes in fluidity, when the temperature changes

• When temperatures increase, prevents membrane from flowing too fast. (speed bump)

• When temperatures decrease, prevents membrane from solidifying. (wedge)

Membrane Fluidity

• Membranes must be fluid to work (salad oil!)

• When a membrane solidifies, permeability changes, and enzymatic proteins in the membrane may become inactive.

• Membranes that are too fluid cannot support protein function, either.

• Extreme environments pose a challenge.

• Evolutionary adaptations that include differences in membrane lipid composition:

  • Some organisms can change the proportion of unsaturated phospholipids in their membranes in response to winter cold or extreme heat.

  • Natural selection has favored organisms whose mix of membrane lipids has ensured an appropriate level of membrane fluidity for their environment.

Membrane Proteins - Integral proteins

• Most membrane functions are carried out by proteins.

  • Penetrate the hydrophobic interior of the lipid bilayer

  • Majority are transmembrane proteins, which span the membrane

  • Others extend only partway into the hydrophobic interior

  • Hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, usually coiled into alpha helices

  • Hydrophilic parts of the molecule are exposed to the aqueous solutions on either side of the membrane

  • Some proteins also have one or more hydrophilic channels that allow passage of hydrophilic substances.

Membrane Proteins - Peripheral proteins

• Most membrane functions are carried out by proteins.

  • Not embedded in the lipid bilayer at all

  • Loosely bound to the surface of the membrane, often to exposed parts of integral proteins

  • On the cytoplasmic side of the membrane, some membrane proteins are held in place by attachment to the cytoskeleton

  • On the extracellular side, some membrane proteins are attached to fibers of the extracellular matrix

Functions of Membrane Proteins, 1-3

1. Transport - may provide a hydrophilic channel (channel proteins) across the membrane that is selective for a particular solute - may shuttle a substance from one side to the other by changing shape (ATP) (carrier proteins)

2. Enzymatic activity

3. Signal transduction - membrane protein (receptor) may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone - the external messenger (signaling molecule) may cause the protein to change shape, allowing it to relay the message to the inside of the cell, usually by binding to cytoplasmic protein

Functions of Membrane Proteins, 4-6

4. Cell-to-Cell Recognition - some glycoproteins serve as ID tags that  are specifically recognized by membrane proteins of other cells (may be short-lived)

5. Intercellular joining - membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions (long-lasting)

6. Attachment to cytoskeleton and extracellular matrix (ECM)- microfilaments or other elements of the cytoskeleton may be noncovalently bound to membrane proteins - helps maintain cell shape - stabilizes location of certain membrane proteins

Role of Membrane Carbs in Cell-Cell Recognition

• Membrane carbs are usually short, branched chains of fewer than 15 sugar units

• Some are covalently bonded to lipids - glycolipids

• Most are covalently bonded to proteins - glycoproteins

• Carbs on the extracellular side of the membrane vary from species to species, among individuals of the same species, even from one cell type to another in an individual

• Diversity of the molecules and their locations on the cell’s surface enable membrane carbs to function as markers that distinguish one cell from another

A, B, AB, O blood types - arise from variation in the carb part of glycoproteins on surface of red blood cells

Synthesis and Sidedness of Membranes

• Membrane proteins and lipids are made in the ER.

• In ER, carbs are added to transmembrane proteins, making them glycoproteins.

• Inside Golgi bodies, glycoproteins undergo further carb modification, and lipids acquire carbs, becoming glycolipids.

• Glycoproteins, glycolipids, secretory proteins are transported in vesicles to membrane

• Vesicles fuse with membrane

• Outside face of vesicle becomes continuous with cytoplasmic face of membrane

• Releases secretory proteins from cell (exocytosis)

• Positions the carbs of glycoproteins and glycolipids on outside face of membrane

Membrane Structure Results in Selective Permeability

• Nonpolar molecules like hydrocarbons, CO2, O2 are hydrophobic.

• They can dissolve in the lipid bilayer and cross it easily, without aid of membrane proteins.

• Ions and polar molecules cannot pass through hydrophobic interior of membrane.

• Hydrophilic molecules cross through transport proteins - ex. channel proteins, aquaporins, carrier proteins (glucose transporter is so specific it rejects fructose!)

Movement of Molecules Across Membranes

1. Diffusion

2. Osmosis

3. Passive transport (facilitated diffusion)

4. Active transport

Diffusion

• Molecules move HIGH to LOW

• Down a concentration gradient

• Lots of molecules → fewer molecules

• Can occur in air, liquid, or across membranes

• Ex. Oxygen, Carbon Dioxide

• Molecules travel through phospholipids, down a concentration gradient, from high concentration to low concentration.

Osmosis

• Diffusion of WATER only

• Aquaporins

• Only occurs through membranes

• High water to low water

• Hypertonic solutions - solutions that have a higher solute concentration than the cells found in that solution - water always LEAVES the cells (plasmolysis)

• Hypotonic solutions - solutions that have a lower solute concentration than the cells found in that solution - water always ENTERS the cells

• Isotonic solutions - solutions that have an EQUAL solute concentration with the cells found in that solution - NO NET MOVEMENT of water

Passive Transport (Facilitated Diffusion)

• Large or charged molecules

• Through a protein (channel or carrier)

• High to Low - down a concentration gradient

• NO ATP required

• Ex. sugar, salt, ions

• Channel proteins that transport ions are called ion channels.

• Many ion channels are gated channels, which open or close in response to a stimulus.

Active Transport

• Large or charged molecules

• Through a protein ( ALL carriers)

• Low to High - AGAINST the concentration gradient

• ATP required

• Ex. sugar, salt, ions