2 + 3. cell biology introduction + fluid mosaic model

How can we define life? Three general properties a. Metabolism b. Growth c. Reproduction Cells are the fundamental organizational units

of all living objects

There are two general classes of cells2. Prokaryocytes3. Eukaryocytes The evolution of prokaryocytes preceeded that of

eukaryocytes by 2 billion years Brief history of life on earth 4.5 billion years ago-------Earth formed 3.5 = = ------- First life (prokaryote bacteria)

1.5 billion years ago------Eukaryocyte (unicellular and nucleated)

0.5 billion years ago-------Eukaryocytes (multicellular cells )

Examples of prokaryocytes ..Bacteria (Strep hemolyticus ) and mycoplasma

Yeast.. Unicellular eukaryocytes Humans..Multicellular eukaryocytes

Characteristics in common between the pro and eukaryocytes

b. Both: Have DNA as the genetic materialc. Both – Membrane boundd. Both - Have ribosomese. Both – Have similar basic metabolismf. Both – amazingly diverse in forms

Major significant differences between the two;b. Eukaryocytes have a nucleus and membrane

bound organellesc. DNA of prokaryocytes floats freely while DNA of

eukaryocytes is held within its nucleus. The DNA of eukaryocytes is much more complex and much more extensive more than prokaryocytes. Prokaryocytes DNA is less than ONE megabase

a. Size: Eukaryocytes are on the average of X 10 the size of prokaryocytes.

b. Prokareyocytes have a cell wall composed of peptoglycan ( single large polymer of amino acids + polysaccharides ( polysaccharides are composed of alternating units of N-acetylglucosamine –NAG- and N-acetylmuramic acid-NAM)

a. Prokaryocytes---Rapid reproduction rate (once every 20 minutes)

b. Eukaryocytes contain more genes-In prokaryocytes about 1000 genes ( 2-3% 0f the no. of genes in human cells)

Membrane structure and composition Biologic membranes are bilipid layers which

create a spherical 3 dimensional lipid bilayer round the cell.

1. Phospholipids: Consist of PO4 group (head) and 2 tails (hydrophobic chains of hydrocarbons). The head is hydrophilic. Therefore the phospholipids are amphipathic

The tails orient towards each other creating a hydrophobic environment within the membrane. Therefore the PO4 head faces out or in into a hydrophilic environment

From time to time a given phopholipid will “flip flop” through the membrane to the opposite side, but this is uncommon. This requires the hydrophilic head of the phopholipids to pass fully through the highly hydrophobic interior and the

hydrophobic tails to be exposed to aqueous environment

Three classes of lipids are identified:3. Fats4. Cholesterol and steroids5. Phospholipids (discussed)

N.B.: Lipids share one chemical property. They have

little or no affinity for water. Lipids consist mainly of hydrocarbons.

a. Fats (triglycerides)- Fat is constructed from two kinds of smaller molecules; glycerol and fatty acids

+ Glycerol: Is an alcohol with three carbons each bearing a hydroxyl group

+ Fatty acid: Has long linear (unbranched) carbon skeleton 16 or 18 carbons

At one end of the fatty acid is the head consisting of carboxyl group.

To make fat, 3 fatty acids, each join to glycerol molecule by an ester linkage ( bond between OH and C=O group(carbonyl group)

The resulting fat is called triacylglycerol or triglyceride= 3 FA + 1 glycerol molecule

a. Steroids and cholesterol.. Amount of cholesterol varies with type of the

membrane. Plasma membrane has nearly one cholesterol/phospholipid molecule

.. Membranes of bacteria have no cholesterol

.. Cholesterol molecule inserts itself with the same orientation as the phopholipids

.. The polar head of cholesterol is aligned with the polar head of phopholipids

Functions of cholesterol1. Immobilizes the first hydrocarbon groups of

phopholipids and this makes lipid bilayer less deformable and decreases its permeability to small water soluble molecules.

1. Cholesterol prevents crystallization of hydrocarbons

C. Carbohydrates Membrane carbohydrates are usually

branched oligosaccharides with fewer than 15 sugar units. Some of these are covalently bonded to lipids forming glycolipids. Most are bonded to proteins (glycoproteins).

The oligosaccharides on the external side of the plasma membrane vary from species to species, among individuals of the same species and from cell to cell

The diversity of molecules and their location on cell’s surface enable oligosaccharides to function as markers that distinguish one cell from another.

Glycolipids may be;3. Protective4. Insulators5. Sites of receptor binding as for poisons of

cholera and tetanus toxin

D. Membrane proteins- They are amphipathic ( have hydrophilic and

phobic regions)- Some are attached to lipids in the bilayer- Others…Transmembrane proteins (polypeptide

chain usually traverses the lipid bilayer= integral proteins. The hydrophobic region spans the interior of the membrane while the hydrophilic regions face the aqueous media outside and inside the cell.

- Other types may be linked only at the cytoplasmic surface (by attachment with fatty acid chain) or at the external surface attached by oligosaccharides or they may be bound to other membrane proteins.

- These proteins are called peripheral membrane proteins (they are not embedded in the lipid bilayer but are loosely bound to the surface of the membrane, often to the exposed parts of the integral proteins.

- The transmembrane proteins inserted once through the membrane are called “ single pass transmembrane proteins”

- Those that pass through several times are called “ multi-pass transmembrane proteins”

Factors affecting fluidity of membrane1. Cholersterol embedded in the membrane

breaks up the Van der Waals interactions and close packing of the tails. This makes the membrane more fluid. For cell to control fluidity of its membrane is through regulating the levels of cholesterol

2. Another way through which cell can control fluidity of the membrane is to regulate the ratio of the saturated to unsaturated hydrocarbon chains of phospholipids

- Saturated hydrocarbon chain- if no double bonds between the carbon atoms composing the tail, as many hydrogen atoms as possible are attached to carbon atoms…Saturated F.As

- Unsaturated hydrocarbons.. Have 1-3 double bonds along the chain

- In saturated hydrocarbons, the tails pack tightly= HIGH MELTING POINT

- In unsaturated hydrocarbons, Loose packing= LOWER MELTING POINT

- These features play important role in relation between temperature and fluidity of cell membrane

The PM is approximately 5 nm thick Semipermeable Impermeable to large molecules Relatively impermeable to molecules as small as

charged ions Its permeability to water is not understood The rate of diffusion of molecules through the

membrane depends upon their ability to enter the hydrophobic interior of the bilayer