lipids, lipid membranes; lipid metabolism and transport

Question 1

what are lipids?

Answer 1

Lipids are fats, waxes, and similar molecules that do not dissolve well in water.

Question 2

what are fatty acids composed of

Answer 2

Fats are composed of glycerol and fatty acids. Glycerol always has three carbon atoms and three hydroxyl (OH) groups, but there are several dozen kinds of fatty acids, ranging in size from 4 carbon atoms to 24. On one end of a fatty acid we find a carbon atom with a double bond to an oxygen atom and a single bond to a hydroxyl group. This entire group of four atoms, often written as COOH, is called a carboxyl group and is able to ionize to release a hydrogen ion into solution, thus acting as an acid. (The ionized carboxyl group is symbolized as COO-.)

Question 3

what are the acidic properties of fatty acids?

Answer 3

the carboxyl group at the end of a fatty acid is able to ionize and release a hydrogen ion into solution, thus acting as an acid. (The ionized carboxyl group is symbolized as COO-.). In any group of such molecules, only a few are ionized at any one time, so fatty acids are all weak acids. All the rest of a fatty acid molecule is pure hydro- carbon (hydrogen and carbon). Fatty acids are designated as saturated or unsaturated according to whether they are filled to capacity with hydrogen atoms or not.

Question 4

how are the carbon atoms in saturated fatty acids bonded?

Answer 4

In a saturated fatty acid, all of the carbon atoms are joined to one another by single bonds, and each one (other than the carboxyl carbon) is bonded to at least two hydrogen atoms. (The one on the end has three.) In an unsaturated fatty acid, at least one pair of carbon atoms is joined by a double bond, so that each of those carbon atoms is bonded to only one hydrogen atom, leaving the fatty acid with at least two fewer hydrogen atoms than it would have if it were saturated. The double bond often throws a kink in the hydrocarbon chain as shown in the space-filling model here.

Question 5

what is a triglyceride and what is its structure?

Answer 5

A fat–chemically known as a triglyceride–consists of a molecule of glycerol joined to three fatty acid molecules by the same kind of dehydration condensation we saw in the formation of disaccharides and polysaccha- rides. The three fatty acids may be all the same or any combination of different ones.

Question 6

what is a monounsaturated fatty acid?

Answer 6

if two of the fatty acids are saturated and one is unsaturated. This would be called a monounsaturated fat, because it is unsaturated (has a carbon double bond) at only one point in the entire triglyceride molecule. If it were unsaturated at two or more points, it would be called a polyunsaturated fat.

Question 7

why are triglycerides non-polar?

Answer 7

Since hydrocarbons are nonpolar the entire triglyceride molecule is nonpolar except for a slight polarity around the oxygen atoms. For this reason, triglycerides (fats) are not attracted much to water molecules. If you have ever tried to wash butter or other animal fat off of your hands with just water, you will have noticed that.

Question 8

what are phospholipids?

Answer 8

In molecular structure phospholipids are like triglycerides except that in place of the third fatty acid they have a phosphate group and some other polar group. This results in a molecule with a dual nature. The hydrocarbon chains of the fatty acids are not attracted to water and are called hydrophobic (water-fearing”). The phosphate and the other group are attracted to water and are called hydrophilic (water-loving”). It is precisely this dual nature that allows phospholipids to form membranes.

Question 9

what is the structure of a steroid nucleus?

Answer 9

The steroid nucleus consists of four interlocking rings of carbon atoms with numerous hydrogen atoms attached It forms the core of a wide variety of important molecules including many hormones, which differ in the groups of atoms substituted for the hydrogen atoms at various points on the rings

Question 10

why are waxes important for plants?

Answer 10

Waxes provide protective coatings for various plant and animal tissues and for bees to make honeycombs. They are formed by the dehydration condensation of a long-chain alcohol (hydrocarbon with a hydroxyl group at one end) and a long-chain fatty acid. Other, less common lipids (not illustrated) combine) fatty acids with various other groups, such as sugars and amino acids.

Question 11

how are Body fats divided into fuel & what are their differing structural types

Answer 11

Fuel fats are stored in fat depots of the adipose tissue, which consists of fat cells with large cytoplasmic stores of fat. Adipose tissue is active, continuously forming and degrading fats, It occurs in the abdominal cavity, within or around organs (muscle, heart), and under the skin. The subcutaneous fat helps in thermal insulation. Brown fat is a form of subcutaneous fat with many mitochondria that liberate mainly heat (not ATP) upon oxidation, to protect the body against cold temperatures. Brown fat location, structure, and physiology are discussed in plates 140 and 141. Structural fats (phospholipids, cholesterol) are not utilized for energy; phospholipids occur in cell membranes, and cholesterol functions in synthesis of steroid hormones, vitamin D and neural myelin tissue

Question 12

basic chemistry of fats

Answer 12

Triglycerides (TG, triacylglycerols, neutral fats) are the storage fats (fat depots). They are esters of glycerol and three fatty acids (FA). FA are long hydrocarbon chains with a single carboxylic acid group at one end. The longer the chain and the smaller the number of double bonds, the lower the fluidity state of the FA and the associated TG. The most commonly occurring body FA are palmitic, stearic, and oleic acids with chains between 14 and 16 carbon atoms long. TG are broken down either completely to glycerol and FA or incompletely to FA and mono- or diglycerides. The breakdown of TG (lipolysis) is catalyzed by various lipase enzymes in the intestine, liver, and adipose tissue.

Question 13

fats as an energy source

Answer 13

Fats take small space & liberate much energy- Fats are ideal for fuel storage because per unit weight they occupy less volume and produce more energy (ATP) than carbohydrates or proteins. When oxidated, 1 g of fat produces 9.3 kcalories- 2.3 times more than 1 g of carbohydrate or protein. Some tissues easily utilize FA for energy; 60% of the heart’s basal energy requirement is derived from fats, chiefly FA. Skeletal muscle also utilizes FA to obtain energy –especially during recovery from strenuous exercise – to replenish the exhausted supply of ATP, creatine phosphate, and glycogen (plate 27).

FA undergo ß-oxidation to form ATP or for conversion to amino acids –To liberate their energy, the FA are degraded to acetate (acetyl CoA) by a process called ß-oxidation. The acetyl CoA is then oxidized to CO2 and H20 in the mitochondria to produce ATP

Glycerol can be oxidized through glycolysis or used to form glucose–Both of the products of triglyceride lipolysis- glycerol and FA–can be utilized for energy production. Glycerol can be converted to intermediates of glycolysis and then to pyruvate, which then enters the Krebs cycle to form ATP (plate 6). Alternatively, glycerol can be converted to glucose in the liver (gluconeogenesis); glucose is used by tissues such as the brain for fuel

Question 14

fat metabolism in adipose tissue

Answer 14

Glycerol and FA are esterified to make storage fats (lipogenesis)- After a carbohydrate meal, the adipose tissue fat cells, stimulated by insulin, take up the abundant plasma glucose and convert it to glycerol and FA. The glycerol (alcohol) and FA (acids) are then esterified to form TG (lipogenesis). Fatty meals increase the blood chylomicrons -very large size lipoprotein particles transporting the absorbed TG and cholesterol in the blood. Within the capillaries of adipose tissue and liver, an enzyme called lipoprotein lipase hydrolyzes the glycerides, freeing glycerol and FA. These are taken up by fat cells and re-esterified to form storage TG. TG with
sufficiently long chains tend to solidify and are therefore easily stored. Increased storage of solid fats in the cytoplasm increases the size of fat cells, which accumulate in the thick fat pads of the adipose tissue. If excessive, this condition leads to obesity

Question 15

Lipase enzymes degrade stored fats to glycerol and FA (lipolysis)

Answer 15

When stimulated by catecholamines and other hormones, the TG are lipolyzed by lipase enzymes, mobilizing the glycerol and FA into the blood. The mobilized FA are then used by the heart, muscles, and liver for energy. Glycerol is usually taken up by the liver to make new glucose.

Question 16

FAT METABOLISM IN THE LIVER

Answer 16

Liver can convert fats into proteins & glucose and vice versa
The liver, like the adipose tissue, is capable of forming, degrading, and storing fats, although the fat granules in the liver hepatocytes are not intended for long-term storage. The particular importance of the liver lies in its capability for metabolic interconversion between fats, carbohydrates, and proteins. The liver hepatocytes contain all the enzymes required for these chemical transformations. For example, excess glucose can be metabolized into fatty acids, which are then either incorporated into TG or mobilized for consumption by tissues. Glycerol can be converted to glucose by reverse glycolysis and then to glycogen, FA can be converted to some amino acids and vice versa. These amino acids can then make proteins. The only reaction that the liver, and animal cells in general, cannot do in this regard is convert FA into glucose.

Question 17

Liver can form cholesterol & ketone bodies

Answer 17

A major liver role in fat metabolism is to make cholesterol and ketone bodies. Cholesterol metabolism is detailed in plate 135. When carbohydrates are low in the diet or in cells (as in diabetes), the liver degrades FA to acetate (acety] CoA). When the available pool of acetyl CoA exceeds the loading capacity of the mitochondria, the acetate molecules are instead condensed together to form compounds such as acetoacetic acıd, acetone, and other keto acids, collectively called ketone bodies. Ketone bodies leak out from the liver into the blood, where they are excreted ìn the kidneys. Excessive amounts of ketone bodies in blood lead to ketosis and metabolic acidosis, conditions that may be fatal, such as untreated insulin-deficiency diabetes (Type I).

Question 18

Ketones are normally excreted but may be used as fuel in certain conditions

Answer 18

In normal adults, ketone bodies are little utilized for energy. However, in newborns, pregnant women, and individuals subjected to prolonged starvation, many tissues, particularly the brain, undergo metabolic adaptation, increasing their rate of uptake and utilization of the ketone bodies for energy. This ability accounts not only for the continued function of the brain (an organ that usually uses only glucose) in starvation, but also for the lack of ketone toxicity in children and starved individuals.

Question 19

Cholesterol has many functions but is not a fuel

Answer 19

CholesteroÌ is a structural fat of animal origin. It is a sterol syn- thesized from acetate, mainly in the liver; the adrenal cortex and gonads also synthesize cholesterol. In the liver it is used to make bile salts to facilitate intestinal fat digestion. It is the precursor for gonadal and adrenocortical steroids and vitamin Da formation in the skin. Cholesterol is a component of the neural-tissue myelin sheath and in the corneum of the skin (the outer keratinized layer), which helps make the skin waterproof and prevents water evaporation. In the membranes of cells and some cellular organelles, cholesterol helps stabilize the movement of phospholipid chains.

Question 20

Dietary vs. endogenous cholesterol

Answer 20

In the body, choles- terol may be exogenous (dietary) or endogenous –i.e., synthe- sized in the tissues, chiefly the liver. Dietary cholesterol comes solely from foods of animal origin (egg yolk, liver, fatty meats, cheese). Cholesterol is absorbed in the intestine with other fats inside chylomicrons, large lipoprotein particles (LPPs). Chylomicrons are digested by the enzyme lipoprotein lipase in the capillaries of liver and adipose tissue. Triglycerides are delivered to the adipose tissue, and the remaining cholesterol and phospholipids are delivered to the liver

Question 21

Cholesterol can be made in the liver from acetate

Answer 21

To make cholesterol, liver acetate (acetyl-CoA) undergoes several reactions to form mevalonic acid, which is converted first to squalane and then to cholesterol. Cholesterol regulates its own synthesis by substrate inhibition of the enzyme that forms mevalonic acid. Dietary cholesterol inhibits the liver’s synthesis of cholesterol in the same manner. Most of the liver cholesterol is used to form bile salts (e.g. cholate) that help digest fat by emulsification

Question 22

Liver cholesterol is exported to tissues packed in lipo-protein particles

Answer 22

The liver supplies cholesterol to most tis- sues through the blood. Cholesterol is packed in LPPs that resemble chylomicrons. LPPs have different fat and protein composition and are of different size and density. The higher the fat content of the LPPs, the lower their density. Each LPP has a core of hydrophobic fats (triglycerides and cholesterol as cholesteryl esters) engulfed in a coat of hydrophilic proteins and phospholipids. The protein in the coat is called apoprotein (apolipoprotein), a very important protein since it binds with the tissue receptors that bind LDL.

Question 23

Lipoprotein particles vary in size, density, & lipid content

Answer 23

LPPs vary in size between 10 to 80 um. Cholesterol for export is transported in the largest of plasma LPPs-very low density lipoproteins (VLDL). In the plasma, these are transformed to smaller lipoproteins –low density lipoproteins (LDL) and intermediate density lipoproteins (IDL)-by the actions of enzymes. Cholesterol delivered directly to tissues is in LDL particles. Once inside the tissue cells, cholesterol is utilized for the variety of functions named above. The excess cholesterol is packed in the smallest of LPP- high density lipoproteins (HDL)–and transported back to the liver for processing.

Question 24

Thyroid & sex hormones influence cholesterol levels

Answer 24

Thyroid hormones, decrease plasma cholesterol by increasing its uptake by the liver and tissues. Estrogen, the female sex steroid hormone, decreases cholesterol level, while androgen, the male sex steroid hormone, increases it. These sex steroid effects may relate to the higher incidence of atherosclerosis (see below) in men

Question 25

Cholesterol, Atherosclerosis, & HEART DISEASE Cholesterol contributes to atherosclerosis & heart disease

Answer 25

This disease is responsible for nearly half of all deaths, mostly in men and in the elderly. When the inner wall of an artery is damaged, platelets adhere to the site of damage, stimulating fibrosis. Plasma cholesterol is deposited on these lesions, along with calcium ions, forming hard, calcified cholesterol plaques. Atherosclerosis is involved in many arterial diseases, such as arteriosclerosis (hardening of the arteries).

Buildup of plaques in the lumen of coronary arteries (coronary occlusion disease) reduces blood flow to various cardiac regions, causing coronary ischemia. Plaques also facilitate blood clot (thrombus) formation, which blocks blood flow to a heart region (thrombosis), leading to heart attacks. Similar events can occur in brain arteries, leading to strokes (brain attacks).

Question 26

LDL gives up cholesterol to the vascular plaques

Answer 26

It is now believed that high levels of plasma cholesterol, particularly the cholesterol in LDL (“bad cholesterol), favours plaque formation. Presumably, LDL particles, coming from the liver to the tissues in high numbers and with specific receptors for tissue binding, are more likely to attach to damaged arterial walls and deposit their cholesterol at these sites. In contrast, cholesterol in HDL particles (“good cholesterol”) travelling from tissues to the liver is not deposited in the lesions.

Question 27

Low plasma cholesterol & high HDL:LDL ratios reduce plaques & heart disease

Answer 27

The normal range of plasma cho- lesterol is 120-220 mg/dl (average 170 mg/dl). High dietary intake of cholesterol contributes to the disease by increasing plasma cholesterol. Lowering of plasma cholesterol by diet and drugs (which inhibit cholesterol synthesis) reduces plaque formation and may even
reverse it. The recommended
maximum plasma cholesterol level for men with a family history of heart disease is 180 mg/dl, since the incidence of arterial plaques and heart attacks increases above this limit; for women at risk, this value is 200 mg/cl.

In addition to decreased total plasma cholesterol, a high ratio of HDL to LDL cholesterol appears to ward off plaque formation and heart disease. Ideal values for HDL cholesterol (“g00d cholesterol”) are > 35 mg/dl and for LDL cholesterol (“bad cholesterol”) are < 130 mg/dl. Women have lower LDL and higher HDL levels than men, probably because of their higher estrogen levels, which may account for their lower incidence of heart disease in their forties.

Question 28

Role of dietary fatty acids

Answer 28

Dietary fatty acids have been shown to play a role in plasma cholesterol and atherosclerosis through influencing the cholesterol content of LPP. Monounsaturated fats present in olives, almonds, avocados, and canola are highly recommended since they increase the HDL and decrease the LDL cholesterol (i.e., increase their ratios); polyunsaturated fats (vegetable oils) are also good, but they mainly increase HDL cholesterol. Saturated fats (butter, animal fats, and hydrogenated vegetable oils) should be kept to a minimum since they increase the LDL cholesterol.

Question 29

what are membranes composed of?

Answer 29

When we isolate membranes from the rest of the cell and analyze them chemically, we find that they are composed primarily of protein and lipid with small amounts of carbohydrates. The kinds and amounts of protein vary considerably from one type of cell to another. The kinds of lipids also vary, but most of them are phospholipids, and the amount is just about enough to make a double layer around the cell if the molecules are lined up in a particular way.

Question 30

how do we devise the way in which lipids are lined up in membranes?

Answer 30

Our hypotheses of how the lipids are lined up in membranes are largely derived from what we know about the structure and the properties of lipids and their behaviour when mixed with water since cells are largely made of water and exist in watery environments. If you have ever shaken a bottle of oil-and-vinegar salad dressing and watched it for a few minutes, you know that even before the oil floats to the top, it gathers together in droplets. The water molecules (which make up most of the vinegar) are polar and have such a strong attraction for one another and so little attraction for the oil, which is largely nonpolar, that the oil molecules are excluded from the water and gather together into droplets with other oil molecules. Sophisticated experiments have shown that once the oil molecules are closely packed together, a weak but effective attractive force develops between them and tends to keep them together.

Question 31

what happens to phospholipids if they are put in water

Answer 31

If placed in water, phospholipids will become spontaneously arranged in such a way that the hydrophobic portions of the molecules pack closely together and avoid contact with the water molecules while the hydrophilic portions face outward to form hydrogen bonds with the water molecules. Depending on the conditions, this results either in ultramicroscopic spheres called micelles or a double layer of phospholipid, generally called a bilayer, which is the way the lipids are believed to be arranged in the membranes of living cells.

Whenever you use a detergent to clean something oily or greasy, you form molecules similar to phospholipids, since one end of the detergent molecule attaches to the hydrophobic oil or grease molecule while the other end is polar and forms hydrogen bonds with water. The result is the formation of micelles that are easily washed away with water.

Question 32

what is the peculiarity of cells infected by the sendai virus?

Answer 32

Cells infected with a peculiar virus called Sendai virus will fuse with other cells they contact, even if they are from a different species. Mouse cells can be fused with human cells, for example. (No, this does not produce a two-legged mouse 6 feet tall; the cell dies in short order.)

Biologists have been able to follow what happens to the membrane proteins after cell fusion by using stains that fluoresce under ultraviolet light. The proteins of the mouse membrane are stained with a substance that fluoresces
in one color, and the proteins of the human membrane are stained with a substance that fluoresces in a different color. The cells are then caused to fuse and observed with a fluorescence microscope, which uses an ultraviolet light source. Immediately after fusion, the proteins are observed to be still confined to the portion of the membrane they originally came with. But in 30 to 60 minutes they migrate around until each kind of protein is evenly distributed over all of the newly formed cell- Some proteins do not migrate like this, but most do.