Membrane Structure and Function

 

 

Fluid Mosaic Model of the Cell Membrane: The plasma membrane is a dynamic structure. It is composed of 2 layers of lipid material with protein molecules interspersed through out. The phospholipids move at a rate of 2um. /sec. while the proteins move at a much slower rate. The phospholipids have 2 definite ends , a hydrophilic (focusing to the outside and inside of the membrane) and a hydrophobic one (forming the center of the membrane). The proteins are of two types Integral (passing through the width of the membrane (act as transport channels) and peripheral (act as recognition sites). These peripheral proteins contain carbohydrates to help in cell to cell recognition. These carbohydrates are called oligosaccharides. I f they bind to proteins they become glycoproteins, if they bind to lipids , they become glycolipids. Cholesterol molecules are found in animal membranes to help add support to its structure. The majority of the phospholipids contain unsaturated fatty acids to keep it fluid. Looking at the over all picture of the membrane the term fluid mosaic should become evident.

 

Traffic of molecules across the membrane:

1. Hydrophobic molecules (Hydrocarbons and Oxygen) can cross with ease because they can dissolve in the lipid bilayer.

2. Small polar (uncharged) molecules will pass (water and carbon dioxide).

3. Large polar (uncharged) molecules will not pass (sugar).

4. Ions will not readily pass (H⁺, Na⁺, Cl^-).

 

Transport Proteins:

 

1. Uniport - one molecule moves in only one direction.

 

2. Symport - Two different molecules moving in only one direction.

 

3. Antiport - Two molecules moving in opposite directions.

 

These transport proteins work in the following ways: provide a tunnel to allow material to flow through, bind to the substance and physically move it, and they are specific for the substance they move.

Diffusion and Passive Transport:

 

Concentration gradient: is a regular concentration change over a distance in a particular direction. The net directional movement is away from the center of concentration. Diffusion is the net movement down the concentration gradient. Diffusion is caused by the following natural occurrences: thermal motion ( movement caused by the loss of heat), random molecular movement, and an increase in entropy of the system. In a solution of many different substances each substance diffuses down its own concentration gradient independent of the others. Diffusion is a passive type of transport, it requires no additional energy to make it work. Osmosis is diffusion of water across a biological membrane.

 

Solutions: A solution is a mixture of a solute ( thing being dissolved) and a solvent (thing doing the dissolving). Water is usually the solvent, hence the term aqueous solution.

Types of Aqueous Solutions:

 

1. Hyperosmotic: solution with the greater concentration of solute (salt water compared to fresh water). If a cell from a salt water fish is placed in a beaker of fresh water, the cell is said to be hyperosmotic to the water.

 

2. Hypoosmotic: solution with a lesser concentration of solute (fresh water compared to salt water). If a cell from a fresh water fish is placed into a beaker of salt water, the cell is said to be hypoosmotic to the water.

 

3. Isosmotic: solutions of equal solute concentrations are said to be isosmotic.

 

Water balance in a living cell:

 

1. Cells placed in a hyperosmotic environment (salt water) animal cells crenate (shrivel). Plant cells plasmolyze (shrivel).

 

2. Cells placed in a hypoosmotic environment (fresh water) animal cells lyse ( explode) and plant cell become turgid (firm).

 

3. Cells placed in an isosmotic solution animal cells are normal, plant cells are flaccid (limp).

 

Facilitated Diffusion: Passive form of transport caused by the help of carrier proteins specific for the molecules they transport. Movement is always down the concentration gradient. Polar molecules and ions are usually moved in this fashion.

 

Active Transport: Movement of molecules up the concentration gradient. The cell must expend its own energy to move the substances.

Sodium-Potassium Pump: Antiport type molecule. It takes 3 sodium ions out of the cell for every 2 potassium ions it takes into the cell. Since sodium is more abundant outside the cell and potassium inside the cell, the cell is working against the concentration gradient. In order for the protein to work, it must be energized by ATP. As this occurs the cytoplasm generates an over all negative charge 2K as opposed to 3Na on the outside, giving the extracellular fluid a positive charge. Another example is a Proton pump that transports Hydrogen ions. These are called electrogenic pumps, since they produce mild currents of electricity. Chloroplasts and mitochondria use this to perform energy creating products from sunlight and macromolecules.

 

Endocytosis and Exocytosis:

 

Exocytosis: Movement of materials out of the cell. These usually are trapped in vesicles that merge with the plasma membrane and release their contents. They are usually produced by the ER or Golgi Apparatus.

 

Endocytosis: Movement of materials into the cell. This is subdivided based on the type and size of material being transports.

 

1. Phagocytosis: Plasma membrane surrounding a large solid piece of material or cell, forming a vacuole and digesting it in conjunction with lysosomes.

 

2. Pinocytosis: plasma membrane forming a vacuole surrounding liquids and taking them into the cell.

 

3. Receptor mediated: special receptors embedded in the membrane will pick up specific materials and bring them into the cell.

 

To develop the concept of diffusion and osmosis complete the AP Lab 1: Diffusion and Osmosis.