Biochemistry: Introduction to Lipids

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Questions and Answers

What is the primary source of calories for infants that gastric lipase primarily digests?

Seed oils

Milk fat (correct)

Vegetable oils

Alcohol

Which enzyme is acid-stable and acts in the stomach of infants?

Gastric lipase

Lingual lipase (correct)

Bile salts

Pancreatic lipase

What is the optimal pH range for lipid digestion in the stomach of infants?

pH 6

pH 8-9

pH 4-6 (correct)

pH 1.5-2.5

What role do bile salts play in lipid digestion in the small intestine?

They act as emulsifying agents for fats. Signup and view all the answers

What is required for the proper functioning of pancreatic lipase in lipid digestion?

Bile salts and co-lipase Signup and view all the answers

What is the primary function of gastric lipase?

Hydrolyzing triglycerides in milk fat Signup and view all the answers

In which part of the digestive system does emulsification of fats primarily occur?

Small intestine Signup and view all the answers

What is the function of pancreatic juice in lipid digestion?

It contains lipases to hydrolyze different lipids. Signup and view all the answers

What is the primary action of pancreatic enzymes during lipid digestion?

Hydrolysis of dietary lipids Signup and view all the answers

Which of the following products results from the action of pancreatic lipase on triacylglycerols?

2-Monoacylglycerol and fatty acids Signup and view all the answers

What is the role of bile salts in lipid digestion?

They aid in micelle formation Signup and view all the answers

What happens during the second stage of lipid absorption?

Re-esterification and assembly of chylomicrons Signup and view all the answers

How are resynthesized triglycerides transported after lipid absorption?

As chylomicrons via lymphatics Signup and view all the answers

Chylomicrons primarily consist of which structural components?

Triglyceride core with protein outer layer Signup and view all the answers

Which enzyme is responsible for converting cholesterol esters into cholesterol and free fatty acids?

Cholesterolesterase Signup and view all the answers

What is the first stage of lipid absorption?

Passive diffusion of micelles Signup and view all the answers

What condition is characterized by an increased fat content of feces, exceeding 6 gm/day?

Steatorrhea Signup and view all the answers

What is the primary function of lipoproteins?

To transport lipids Signup and view all the answers

Which lipoprotein is responsible for transporting dietary lipids from the intestine?

Chylomicrons Signup and view all the answers

Where does the endogenous pathway of lipid transport primarily operate?

Liver Signup and view all the answers

What is the structure of lipoproteins characterized by?

Hydrophobic core and hydrophilic surface Signup and view all the answers

What is the fate of chylomicron remnants in the body?

Removed from the blood by the liver Signup and view all the answers

What is the hydrophobic component found in the core of lipoproteins primarily composed of?

Triglycerides Signup and view all the answers

Which lipoprotein is involved in transporting lipids from the liver to peripheral tissues?

VLDL Signup and view all the answers

What is the primary reason lipids (TAG) are utilized as a fuel reserve during growth over carbohydrates?

Lipids can sustain physiological functions for longer durations than carbohydrates. Signup and view all the answers

Which of the following correctly describes a saturated fatty acid?

It contains only single bonds. Signup and view all the answers

How does the degree of unsaturation in fatty acids affect their physical state?

Increased double bonds result in a liquid state at room temperature. Signup and view all the answers

What role do lipids play in the absorption of vitamins?

They serve as a transport mechanism for fat-soluble vitamins. Signup and view all the answers

Which statement about the configuration of double bonds in fatty acids is true?

Double bonds are always separated by a single bond. Signup and view all the answers

What is one of the primary structural roles of lipids?

They are crucial constituents of cell membranes. Signup and view all the answers

In the context of fatty acids, how is the term 'amphipathic' best defined?

Having both hydrophobic and hydrophilic properties. Signup and view all the answers

What is the significance of lipid’s role as a metabolic regulator?

They act as steroid hormones regulating various physiological processes. Signup and view all the answers

What is the primary role of Very Low Density Lipoprotein (VLDL) in the body?

Transport synthesized TAG from the liver to peripheral tissues Signup and view all the answers

What distinguishes High Density Lipoprotein (HDL) from Low Density Lipoprotein (LDL)?

HDL removes cholesterol from the blood, LDL deposits it Signup and view all the answers

What components are typically measured in a lipid profile blood test?

Total cholesterol, LDL, HDL, TAG Signup and view all the answers

How is the atherogenic index of plasma (AIP) calculated?

LDL cholesterol / HDL cholesterol or TAG concentration / HDL cholesterol Signup and view all the answers

Elevated cholesterol levels are associated with which of the following conditions?

Atherosclerosis and gallstones Signup and view all the answers

Study Notes

Lipids: Introduction

Lipids provide 9 calories per gram.
They are important constituents of cell membranes.
Lipids aid in the absorption of fat-soluble vitamins (A, D, E, and K).
Cholesterol is a precursor for vitamin D synthesis.
Lipids are a crucial component of nervous tissue.
They act as a thermal insulator in subcutaneous tissues.
Lipids maintain cellular integrity.
Lipids act as metabolic regulators (steroid hormones) and emulsifying agents (amphipathic lipids).

Lipids Vs. Carbohydrates

Lipids, specifically triacylglycerol (TAG), are chosen as a fuel reserve over carbohydrates (glycogen) during growth.
TAG stores allow survival for several weeks.
Glycogen sustains physiological function for about 24 hours.

Fatty Acids

Fatty acids are the building blocks of lipids.
They are composed of a carboxylic group (-COOH) and a hydrocarbon chain (-R).

Classification of Fatty Acids

Fatty acids are categorized into:
- Saturated fatty acids (with only single bonds).
- Unsaturated fatty acids (with at least one double bond).
  - Further classified into mono- and polyunsaturated fatty acids based on the number of double bonds.

Double Bonds in Fatty Acids

The position of a double bond is indicated by Δn, where n represents the lower-numbered carbon in the double-bonded pair.
Double bonds in polyunsaturated fatty acids are separated by a CH2 (methylene) group, indicating they are not conjugated.
The double bonds in fatty acids have a cis- configuration (same direction).

Melting Point of Fatty Acids

The length of the hydrocarbon chain and the degree of unsaturation affect the melting point of fatty acids.
- As the number of carbons increases, the melting point increases (solid at room temperature).
- As the number of double bonds increases, the melting point decreases (liquid at room temperature).

Digestion of Lipids - Stage 1: Mouth

Lingual lipase, present in infants, begins digestion in the stomach as it is an acid-stable enzyme.

Digestion of Lipids - Stage 2: Stomach

Gastric lipase is secreted by the stomach.
Lipids require a slightly acidic pH (pH 6) for digestion.
Gastric lipase hydrolyzes TAG molecules, primarily those found in milk fat.
This is particularly important for infants, as milk fat is their primary calorie source.
Note: Stomach pH in adults ranges from 1.5 to 2.5, while it is 4 to 6 in infants.

Digestion of Lipids - Stage 3: Small Intestine

Acidic chyme containing dietary fat moves from the stomach to the duodenum, stimulating the secretion of pancreatic juice.
Digestion of lipids in the upper small intestine (duodenum) occurs in two main steps:
- Emulsification: Bile salts serve as effective emulsifying agents for fats.
- Hydrolysis: Pancreatic juice contains lipolytic enzymes (lipases) for breaking down different lipids.

Emulsification of Lipids

Bile salts play a crucial role in the effective functioning of pancreatic lipase.
They aid in the combination of lipase with co-lipase (a small protein) in the intestinal lumen.
Bile salts emulsify fats into fine droplets by lowering surface tension. This increases the surface area for pancreatic lipase activity.

Hydrolysis of Lipids

Pancreatic enzymes from pancreatic juice act on dietary lipids in the small intestine.
Pancreatic lipase breaks down TAGs into 2-monoacylglycerol (MAG) and two free fatty acids (F.F.As).
Phospholipase A2 breaks down phospholipids into lysophospholipids.
Cholesterolesterase breaks down cholesterol ester into cholesterol and a free fatty acid.

Absorption of Lipids

The products of fat digestion are absorbed along with the help of bile salts and acids.
Micelles are formed.
Micelles reach the brush border of intestinal mucosal cells.
Lipids from micelles passively diffuse through mucosal cells and are absorbed.

Re-esterification and Assembly of Chylomicrons

Resynthesis of triglycerides occurs using partial glycerides and liberated free acids.
The resynthesized triglycerides enter the lymphatics of the abdominal cavity.
They travel through the thoracic duct and ultimately appear in the blood as lipoprotein particles called chylomicrons.
Chylomicrons have a triglyceride core surrounded by a phospholipid and protein outer layer.

Lipid Malabsorption (Steatorrhea)

Steatorrhea is a clinical condition characterized by increased fat content in feces (daily excretion exceeding 6 gm/day).
This is caused by defective digestion and absorption of fat.

Lipoproteins

Lipoproteins are large, water-soluble spherical complexes.
They transport lipids from various tissues for storage and utilization.
Lipoproteins have a hydrophobic core containing TAGs and cholesterol esters.
Their hydrophilic surface is composed of phospholipids, unesterified cholesterol, and apoproteins.

Exogenous & Endogenous Pathways of Lipid Transport

All lipids in plasma are transported as lipoproteins.
The transport process is generally divided into two pathways:
- Exogenous Pathway: Transports dietary lipids from the intestine to the liver.
  - Involves chylomicrons.
- Endogenous Pathway: Transports lipids synthesized in the liver to peripheral tissues.
  - Involves very low-density lipoproteins (VLDL).

Transport Function of Lipoproteins & Their Fates:

Chylomicrons (CM)
- Source: Small intestine
- Transported Lipids: Dietary exogenous TAGs
- Fate: Chylomicron remnants containing cholesterol and fat are removed from the blood by the liver.
Very Low Density Lipoprotein (VLDL)
- Source: Liver
- Transported Lipids: Endogenously synthesized TAGs in the liver
- Fate: Circulate and deposit fats in all tissues and become LDL.
Low Density Lipoprotein (LDL)
- Source: Derived from VLDL in circulation
- Transported Lipids: Cholesterol synthesized in the liver to peripheral tissues
- Fate: Eventually removed from the blood by the liver and other tissues.
High Density Lipoprotein (HDL)
- Source: Liver
- Transported Lipids: Excess tissue cholesterol to the liver for disposal in feces
- Fate: Eventually removed from the blood primarily by the liver.

Clinical Significance of High Cholesterol

Elevated cholesterol levels in the body can be associated with pathogenic conditions:
- Heart diseases
- Atherosclerosis
- Gallstones

Lipid Profile

A lipid profile is a blood test that measures cholesterol and other fats like TAGs in the blood.
It helps assess the risk of heart disease and stroke.
Typically, a fast of 8 to 12 hours is required before the test.
A lipid profile measures:
- Total cholesterol
- LDL ("bad") cholesterol
- HDL ("good") cholesterol
- TAGs
The atherogenic index of plasma (AIP) can be calculated to predict atherosclerosis and cardiovascular disease:
- Atherogenic index = LDL-cholesterol / HDL-cholesterol
- Atherogenic index = TAG concentration / HDL-cholesterol

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Explore the essential roles of lipids in biochemistry, from energy provision to cell membrane structure. This quiz covers the classifications, functionalities, and comparisons of lipids and carbohydrates, as well as the significance of fatty acids. Perfect for anyone studying biochemistry or health sciences.