CHAPTER 18: LIPIDS AND MEMBRANES

Types of Lipids
Fatty Acids
- Saturated
- Unsaturated
Glycerides
- Neutral
- Phosphoglycerides
Complex Lipids
- Lipoproteins
- Glycolipids
Nonglycerides
- Sphingolipids
- Steroids
- Waxes

Lipid Functions
Cell membrane structure
- Creates a barrier for the cell.
- Controls flow of materials.
Energy storage
- Fats stored in adipose tissue.
Hormones and Vitamins
- Hormones - communication between cells.
- Vitamins - assist in the regulation of biological processes.

Fatty Acid Structure
Long chain monocarboxylic acids
CH3(CH2)nCOOH
Size Range: C12 - C24
Always an even number of carbon.
Saturated - no double bonds.
Unsaturated - one or more double bonds.

Reactions of Fatty Acids
React like any other carboxylic acid.
Esterification
Hydrolysis
Acid-Base

Reactions of Unsaturated Fatty Acids
They can undergo the same reactions.
Will also undergo addition.
Most common addition is hydrogenation.
Neutral Glycerides
Ester of glycerol and a fatty acid.
Principal function is energy storage - fat.
May have 1-3 fatty acids which can each be different.

1 - monoglyceride
2 - diglyceride
3 - triglyceride

Phosphoglycerides
Lipids that contain a phosphate group.
Modified fat
- Phosphate replaces one fatty acid chain.
Uses
- Production of cell membranes.
- Emulsifying agents.
Example: Lecithin
polar head, nonpolar tail

Non-glycerol Lipids
Sphingolipids
- A type of phospholipid NOT derived from fat.
Used primarily in nerve tissue - myelin sheath.
In people, 25% of all lipids are sphingolipids.
Example: Spingomyelin
fatty acid, phosphate group, choline

Biological Membranes
Cell and organelle membranes are composed of two layers - lipid bilayers.
Fluid structure of membranes
- Membranes are not static.
- Layers move over each other based on percent of unsaturated fatty acids.
Mosaic structure of membranes
- Components
- Peripheral membrane proteins bound only to one side of membrane.
- Integral membrane proteins imbedded within the membrane.
- Both types of proteins can move around on surface of cell.
- Proteins don't flip in and out or act like revolving door.

Membrane Transport
Cell membrane controls passage of materials in and out of cell.
Most transport is controlled by integral membrane transport proteins.
Small molecules, like water, pass through membrane on their own by passive transport
Passive Transport
Diffusion
- Small molecules (CO2, O2, H2O) will simply pass through cell membrane.
Entropy is driving force - wants equal concentrations of both sides of membrane.
Membrane is considered selectively permeable to these molecules.

Facilitated Diffusion
Permeases - Specific protein transports materials across a membrane.

Osmosis
The diffusion of a solvent from a dilute solution through a semipermeable membrane to a more concentrated one.
Semipermeable membranes
- only allow small molecules to go through
- cell walls are semipermeable membranes

Osmotic Pressure
Three conditions can exist for cells
- the concentration is the same on both sides - isotonic
A red blood cell and plasma have the same osmotic pressure
- the concentration is greater on the inside - hypertonic
- the concentration is greater on the outside - hypotonic
Cells in 'high salt' solutions
If the level of salt in the plasma is too high, the cell collapses.
Hypotonic cell, hypertonic solution.
crenation - water is drawn out of the cell.
Cells in 'low salt' solutions
If the level of salt in the plasma is too low, the cell swells and ruptures. Hypertonic cell, hypotonic solution.
hemolysis - water is drawn into the cell.

Active Transport
When a cell must expend energy to move needed materials across the cell membrane.

Unsaturated Fatty Acids (eicosanoids)
Eicosanoids
All are unsaturated.
All have twenty carbons.
Some are Essential Fatty Acids.
Can't be produced by the body.
Examples: linolenic and linoleic acids.
Three groups
- Prostoglandins, leukotrienes, thromboxanes

Prostaglandins
Originally isolated from seminal fluid.
All are derived from arachidonic acid.
Biological effects
- Stimulation of smooth muscles
- Regulation of steroid production
- Inhibition of gastric secretion
- Inhibition of hormone-sensitive lipases
- Inhibition / stimulation of plate aggregation
- Regulation of nerve transmission
- Sensitization to pain
Mediation of inflammatory response

Inflammatory response
Protective mechanism when tissue is damaged.
Results in swelling, redness, fever, and pain.
Prostaglandins promote this response.
Drugs like aspirin and Ibuprofen.
- Anti-inflammatories.
- Block prostaglandin synthesis.
- Cause reduction in this response.
- Tylenol - analgesic, not an anti-inflammatory.

Smooth Muscle Contractions
Prostaglandins stimulate contractions in the reproduction system - uterine contractions.
Dysmenorrhea
- Painful menstruation.
- Evidence shows that this may result from an excess of prostaglandins.
- Physicians often order Motrin (Ibuprofen) for this.

Gastrointestinal Tract
Prostoglandins will:
- Inhibit the secretion of hydrochloric acid in the stomach.
- Increase secretion of mucus layer.
- Protects mucosa from acid invasion.
Aspirin inhibits prostaglandin production.
- Extended use can result in ulceration of the stomach lining. Why?

Other Uses
In the kidneys
- Cause renal blood vessels to dilate.
- Aid in excretion of water and electrolytes.
In the respiratory tract
- Produced by WBC in lungs - leukotrienes.
- Cause constriction of bronchi - asthma.
- Other prostaglandins act as bronchodilators.

Steroids
Nonsaponifiable lipids based on a common four-ring skeleton
Broad class of compounds that all have the same base structure.
Cholesterol
- Principal membrane lipid for fluidity.
- Associated with hardening of the arteries.
- Appears to coat the arteries - plaque formation.
* Results in
* Increased blood pressure from: Narrowing of arteries & Reduced ability to stretch
* Clot formation leading to: Myocardial Infarction & Stroke
Some reproductive hormones.
- progesterone
- testosterone
Cortisone
- Associated with a many biological processes
- metabolism of carbohydrates, treatment for rheumatoid arthritis, asthma,
- GI disorders, rashes...

Complex Lipids
Lipids bound to other molecules.
Combination results in a structure.
Four major classes of plasma lipoproteins.
- Chylomicrons
- Very low-density lypoproteins (VLDL)
- Low-density lypoproteins (LDL)
- High-density lypoproteins (HDL)
Each is composed of several types of lipids.

Function of Lipoproteins
Chylomicrons Transport triglycerides from intestines to other tissue except kidneys.
VLDL - Bind triglycerides in liver and carry them to fat tissue.
LDL - Carry cholesterol to peripheral tissues.
HDL - Bound to plasma cholesterol. Transport cholesterol to liver.

Transport of Lipoproteins
VLDL moves triglycerides from liver to tissues.
LDL transfers cholesterol to tissues from liver.
HDL carry cholesterol from tissues to liver.
Monoglycerides and fatty acids are absorbed by intestines - transported as chylomicrons in lymph system to blood.