Membranes 1
Chapter 5 - Lecture 1
(Intro to Membranes & Macromolecules)

Announcements
• Examination #2 Remember, it covers everything
since the beginning of the semester (including the Tt stuff). Same format!

Lecture
Let's take a look at a soccer ball. Does it share any characteristics with a living cell? Sure!
Similarities:
• an inside,
• a barrier, which is surrounded by an outside,
• the inside environment is different than the outside environment. In this
case, air pressure is much greater on the inside (otherwise it wouldn't
bounce).
But, in some ways, the soccer ball is much different than a living cell.
Dissimilarities:
• the ball does not undergo mitosis to make two soccer balls
• the ball does not include organelles, cytoskeleton, ...just air
• energy is not produced within the ball; any change in energy is due
energy transferred from the outside, kick ball, decrease volume
therefore air pressure increases, in response the ball goes away
from the foot. (draw)
• AND...things don't go through the skin of the ball (if they did, the ball
wouldn't remain inflated for very long!
• AND...the skin of the ball is stationary, this blue pentagon does not
move around relative to the others.

Very Important ->
If this ball were really a cell, the skin of the ball would be:
• composed of lipids, proteins, and sugars
• these components move around like icebergs in the Arctic Ocean (not like
an egg shell).
• semi-permeable (It would let some things move through the membrane, but
not others, pretty selective.) E.g., you want oxygen to come through
so you can use it to break down sugar to make ATPs; if the cell is
making lots of proteins (e.g., digestive enzymes) you want these to
leave the cell so they can be put to good use (amylase from cells in your
salivary glands).
• involved in many chemical reactions (proteins are very active in reactions)
• used to transmit signals (information) between the environment and the interior
of the cell.

Since we'll be talking about sugars, lipids and proteins, and how they function with respect to plasma membranes, let's go over what these macromolecules ("big molecules) are like. (If you want to review this material in more detail, you'll find it in your textbook pp. 48-65.) Please note: you'll be tested ONLY on what I say about these in lecture.

Definitions:
compound: A substance composed of two or more elements (e.g., N, C, O,...) combined in definite proportions (e.g., C6H12O6).
organic compound: A compound composed of a backbone made up of carbon atoms.
C-C-C-C-...
macromolecule: A very large organic compound.

FOUR Macromolecules:
1. Carbohydrates: Compounds containing carbon, hydrogen and oxygen in the approximate ratio of 1C:2H:1O (as with C6H12O6). Includes sugars, starch and cellulose.

2. Lipids: Any of a group of organic compounds which are insoluble in water, but soluble in organic solvents (e.g., alcohol, ether, benzene). (Explain "soluble".) Includes things like fats, oils, waxes, and phospholipids.

3. Proteins: Large, complex organic compounds composed of covalently (e.g. C-H) linked amino acid subunits. E.g., glycine-alanine-leucine-... Examples of proteins: lysozyme, restriction enzymes, proteases, nucleases, and much of the stuff that holds you together.

4. Nucleic Acids: Organic compounds consisting of a string of nucleotides. Nucleotides consist of a nitrogenous base (ATGCU), a sugar (e.g., deoxyribose) and a phosphate group. DNA and RNA are nucleic acids. (E.g., deoxyribonucleic acid)


Let's take a look at some general patterns with respect to macromolecules. Lipids tend to be in their own little world, so let's just ignore them at this point.
-> Macromolecules are polymers produced by linking small organic compounds called monomers.
sugar+sugar+sugar...+sugar -> carbohydrates
amino acid + amino acid + ... + amino acid -> proteins
nucleotide + nucleotide + ... + nucleotide -> nucleic acid
The process by which monomers are linked together to form polymers is called condensation (or sometimes: dehydration synthesis). Because...a water molecules is formed when two monomers are joined.

The process by which polymers are broken down to monomers is called hydrolysis. Because... the H and the OH are taken back up by the monomers when they're split.

ONE MORE THING before we get back to our plasma membranes...
Protein Structure: I want to briefly cover this because the structure of proteins is so important to our discussion of enzymes and structural proteins (e.g., the ones that float around in the plasma membrane.
There are four levels of structure. 1', 2', 3', and 4'
1': Primary structure: the sequence of amino acids in the protein
e.g., glycine-alanine-leucine-isoleucine-...
2': Secondary structure: a regular geometric structure formed by hydrogen bonding between adjacent amino acids (e.g., alpha helix, beta pleated sheets) draw
3': Tertiary structure: the overall 3-D shape of the protein which may include a great deal of folding due to interactions between many amino acids. E.g., some amino acids don't like water (hydrophobic) so they tend to hide in the middle, while those that like water (hydrophilic) move to the edges. Draw a globular protein.


4': Quaternary structure: Some proteins are made up more than one string of amino acids. Each string is technically called a polypeptide (dipeptide=2 a.a. combined; amino acids are joined by "peptide" bonds (covalent bonds of special design: CONH). Examples: hemoglobin and insulin are proteins with 4' structures.

***** Just want to mention it at this point. Transport of some molecules through the membranes are moved because there is a change in the 3' structure of a protein. Demonstrate with the soccer ball (molecule being moved) held above my head, something comes along and hits me in the gut (commonly a phosphate group), I change my 3' structure such that the ball is now close to the floor. I release the ball and return to my original shape. We will come back to proteins many times during this semester.

Let's move on to the general structure of a plasma membrane.
Fluid Mosaic Model of the Structure of a Plasma Membrane
The model: currently accepted model that describes a membrane as a double layer of phospholipids with proteins floating in it.
DRAW A MEMBRANE



Identify
1) phospholipid bilayer (two fatty acid chains linked to glycerol which is then linked to a phosphate group). Two layers (hydrophilic and hydrophobic regions).
2) transmembrane proteins (sometimes called integral proteins)
3) peripheral proteins (on surfaces
4) glycocalyx (mention the sugar coating that sometimes exists; cell recognition)

ITP: move on to the movement of these components, hence the fluid mosaic...