A compound is called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet.

Vitamins are classified by their biological and chemical activity, not their structure.

Vitamins have diverse biochemical functions, including function as hormones (e.g. vitamin D), antioxidants (e.g. vitamin E), and mediators of cell signaling and regulators of cell and tissue growth and differentiation (e.g. vitamin A).

Vitamins A, D, E and K are lipid-soluble and accumulate within the fat stores of the body and within the liver.

The lipid-soluble vitamins are often associated with toxicity when taken in large amounts.

Vitamin A

› Also called trans-retinol› Vitamin A is either synthesized

from beta-carotene or absorbed in the diet

› It is primarily stored in the liver as an ester of palmitic acid

Vitamin A

› The form of Vitamin A active in the visual process is 11-cis-retinal, which combines with the protein opsin to form rhodopsin

› Rhodopsin is the primary light-gathering pigment in the retina

A deficiency of the Vitamin A may also lead to decreased resistance to infections, poor tooth development, and slower bone growth.

Vitamin D3

› Also known as cholecalciferol› Can be made in the skin from 7-

dehydrocholesterol in the presence of ultraviolet light

› Can also be acquired from the diet› Both Vitamin D3 made in the skin and

those absorbed from the small intestine are transported to the liver and hydroxylated at C-25 by a microsomal mixed-function oxidase.

Vitamin D3

› 25-Hydroxycholecalciferol appears to be biologically inactive until it is further hydroxylated at C-1 by a mixed-function oxidase in kidney mitochondria.

› 1,25-dihydroxycholecalciferol, the active compound, is delivered to target tissues for the regulation of calcium and phosphate metabolism.

› The function of Vitamin D is analogous to that of the steroid hormones.

Symptoms of vitamin D deficiency in growing children include rickets (long, soft bowed legs) and flattening of the back of the skull.

Vitamin D deficiency in adults is called osteomalacia, which leads to muscular weakness and weak bones.

Vitamin E› Also called alpha-tocopherol› first recognized in 1926 as an organic

compound that prevented sterility in rats› Function of Vitamin E still has not been

clearly established Favorite theory is that it is an antioxidant

that prevents peroxidation of polyunsaturated fatty acids

Taking antioxidant supplements, Vitamin E in particular, might help prevent heart disease and cancer.

Vitamin E is used in cosmetics and skin products to prevent cell damage by UV light.

Vitamin K› Discovered in Denmark in the 1920s as a

fat-soluble factor important in blood coagulation (“K” for “koagulation”)

› Vitamin K was shown to be needed for the formation of gamma-carboxyglutamic acid

› gamma-carboxyglutamic acid specifically binds calcium, which is important for blood coagulation

Without sufficient amounts of vitamin K, hemorrhaging can occur.

Newborn babies lack the intestinal bacteria to produce vitamin K and need a supplement for the first week.

Individuals on anticoagulant drugs (blood thinners) may become deficient in vitamin K as may those taking antibiotics, even if temporarily because intestinal bacteria populations are sometimes reduced by the long-term use of antibiotics.

Also, people with chronic diarrhea may be unable to absorb enough vitamin K through the intestine. These groups of people need to take additional Vitamin K to ensure a proper level in the body.

Phospholipids are ideal compounds for making membranes because of their amphipathic nature

The amphipathic nature of phospholipids has a great influence on mode of their biosynthesis.› Most of the reactions involved in lipid

synthesis occur on the surface of membrane structures catalyzed by enzymes that are themselves amphipathic.

Prokaryotic and eukaryotic cells both contain phospholipid bilayer on their membranes. However, the phospholipids they have are synthesized in different pathways.

Phospholipid synthesis in eukaryotes is more complex than in prokaryotes.› This relates to the other roles

phospholipids play in membranes aside from their structural role.

Escherichia coli, a prokaryote, contains three important classes of phospholipids:› Phosphatidylethanolamine (75%-85%)› Phosphatidylglycerol (10%-20%)› Diphosphatidylglycerol (5%-15%)

All three of these phospholipids share almost the same biosynthetic pathway.

Most of the enzymes for phospholipid synthesis are located on the inner plasma membrane of bacteria.

Phosphatidylglycerol and diphosphatidylglycerol are also synthesized in the mitochondria of eukaryotes by a pathway similar to prokaryotes.

The most important members of phospholipid family are phosphatidic acids, from which is derived the phosphatides and sphingomyelin.

Phosphatidic acid is the parent compound. Once it is synthesized, it is rapidly converted to diacylglycerol which is an intermediate for the synthesis of eukaryotic phospholipids.

Diacylglycerol can be metabolized to form phosphatidylcholine and phosphatidylethanolamine which are the two most important phospholipids in cells› As a rule:

phosphatidylcholine is the major phospholipid of animal cell membranes.

Phosphatidylethanolamine predominates in bacterial membranes

Sphingolipids are found in the membranes in animals and plants but are absent from most bacteria like E.coli

These lipid subgroup are particularly abundant in myelin sheath, a multilayered membranous structure that protects and insulates nerve fibers

Myelin is composed of about 80% lipid and about 20% protein.

Myelin is made up primarily of a glycopipid called galactocerebroside.

Glycosphingolipids are found largely in the plasma membrane and are oriented asymmetrically in the bilayer, with the carbohydrate moieties facing exclusively toward the outside of the cell

These lipid subgroup appear to have a structural role particularly in the myelin sheath and the membrane of the red blood cell.

The biosynthesis of glycosphingolipids is possible with the aid of enzymes called glycosyltransferases.

Glycosyltransferases are involved in the addition of carbohydrates to the acceptor lipid by transfer from a sugar nucleotide (e.g. UDP-glucose)

Some glycosphingolipids are blood group antigens› A person who displays the “B” blood has

the same “B” antigen oligosaccharide as a component of both a membrane-bound glycosphingolipid and glycoprotein

› Blood type “A” has antigen “A”› Blood Type “AB” has both “A” and “B”

antigens› Blood type “O” has no antigen on the RBC

membrane

The current model which is able to explain cell membrane mechanisms is the Fluid Mosaic Model.

Cell membranes exhibit asymmetry. This refers to differences in composition of the outer half and inner half of the membrane bilayer.

Asymmetry seen are:› A. Types of lipids found in the two halves of the

membrane› B. Types and localization of proteins› C. Carbohydrates which are mostly found on

the outer half of the membrane

The degree of fluidity of cell membranes is a function of both temperature and lipid composition› Melting transition (Tm) depends strongly

on the phospholipid composition in the bilayer. That is, increasing the length of the fatty acid chains increases the Tm.

› Cholesterol broadens the Tm of the phospholipid bilayer.

The major function of cell membranes is to maintain the status quo by preventing loss of vital materials and entry of harmful substances

Biological cells depend on an influx of phosphate and other ions, and of nutrients such as carbohydrates and amino acids

Mechanisms for the transport of organic and ionic substances through the membrane bilayer are varied and include processes for passive and active transport.

Proteins embedded on the cell membranes act as specific transporters, or permeases: they move substances from one side of a membrane to the other.

TRANSPORT OF NONPOLAR SUBSTANCES› 1. SIMPLE DIFFUSION

This is the simplest type of transport mechanism The substrate flows down a concentration gradient Nonpolar substances readily get into the cell via

simple diffusion process

› 2. FACILITATED DIFFUSION This involves carrier proteins as in the case of steroid

hormones Mobile receptors allow transport of substances into

the cell

TRANSPORT OF POLAR MOLECULES› 1. SIMPLE DIFFUSION

Water which is a polar compound can readily pass through the membrane bilayer

In areas where fluidity is probably higher, ions like K+ can move out of the cell

› 2. FACILITATED DIFFUSION Membrane-bound carrier protein molecules are used

for transport of substances Glucose, for example, is transported via a carrier

molecule Cotransported ion with glucose is Na+.

TRANSPORT OF POLAR MOLECULES› 3. ACTIVE TRANSPORT

Energy is required in this mechanism since transport of molecules is against the concentration gradient

Example is the Na+-K+ pump transport mechanism Sodium ions are transported with glucose into the cell

cytoplasm causing increase in Na concentration inside the cell.

Potassium ions are driven out of the cell through “leaky” parts of the membrane, causing decrease in K concentration inside the cell

To maintain the Na-K ratio of 3:2 ions, respectively, active transport should be continuous. ATP hydrolysis is used.

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