11-Lipids and the Plasma Membrane: Structure, Function, and Transport

Lipids: Structure and Function

Definition and Types of Lipids

Lipids are a class of organic molecules that are insoluble in polar solvents such as water. They play essential roles in cellular structure and metabolism.

• Types of lipids:

  • Triglycerides

  • Phospholipids

  • Steroids

  • Waxes

  • Carotenoids

Functions of Lipids in the Body

• Storage of energy: Triglycerides serve as long-term energy reserves.

• Hormonal roles: Steroid hormones regulate physiological processes.

• Insulation: Sphingolipids help insulate nerve cells.

• Protection: Fats and waxes protect internal cavities.

• Structural components: Phospholipids are key elements of cell membranes.

Mnemonic: SHIPS (Storage, Hormonal, Insulation, Protection, Structural)

Triglycerides

Triglycerides (fats and oils) function primarily as a long-term energy source. They are formed via condensation reactions between glycerol and three fatty acid chains:

• Formation equation: $ext{Glycerol} + 3 imes ext{Fatty Acid} ightarrow ext{Triglyceride} + 3 imes ext{Water}$

Fatty Acids

Fatty acids are long hydrocarbon chains found in certain types of lipids (e.g., triglycerides, phospholipids).

• Saturated fatty acids:

  • No double bonds

  • Linear structure, often solid at room temperature

• Unsaturated fatty acids:

  • One or more double bonds

  • Bent structure, usually liquid at room temperature

  • Monounsaturated: single double bond

  • Polyunsaturated: multiple double bonds

Unsaturated Fatty Acids: Isomers

Unsaturated fatty acids can exist as cis or trans isomers, which differ in their chemical properties and health effects.

Cis Isomer	Trans Isomer
H atoms on same side	H atoms on different sides
Double bond creates kink	No kink in chain
Loosely packed (liquid)	Tightly packed (solid)
Common in nature	Common in processed food
Generally good for health	Generally bad for health

Lipid Transport and Lipoproteins

Lipid Transport in the Body

Lipids are insoluble in water and must be complexed with proteins for transport in the blood.

• Lipid globules are broken down by the digestive system.

• Components are combined with protein to form chylomicrons (via Golgi complex in intestinal cells).

• Chylomicrons are released into the lacteals for transport to the liver.

• The liver converts chylomicrons into soluble lipoproteins.

Lipoproteins

Lipoproteins transport lipids in the blood and regulate cholesterol levels. High cholesterol is an indicator of coronary heart disease (CHD).

Low Density Lipoproteins (LDL)	High Density Lipoproteins (HDL)
Carries cholesterol from liver to body (for use by cells)	Carries excess cholesterol back to liver (for disposal)
Raises blood cholesterol levels	Lowers blood cholesterol levels
"BAD" – increases risk of CHD	"GOOD" – reduces risk of CHD

Coronary Heart Disease

High blood cholesterol leads to hardening and narrowing of arteries (atherosclerosis).

• Fatty deposits form plaques in vessel walls.

• Rupturing of plaques triggers blood clot formation (thrombus).

• Clotting in coronary arteries (coronary thrombosis) leads to heart disease.

Health Risks of Fatty Acids

• Cis fats: Increase HDL, lower blood cholesterol, reduce CHD risk.

• Saturated fats: Increase LDL, raise blood cholesterol, increase CHD risk.

• Trans fats: Raise LDL, lower HDL, greatly raise cholesterol, highest CHD risk.

Evidence of Health Risks

• Diets rich in cis fats reduce CHD risk.

• Diets rich in saturated and trans fats increase CHD risk.

• Other factors: dietary components, genetics, cohort size, study duration, correlation vs. causation.

Plasma Membrane: Structure and Function

Overview of the Plasma Membrane

The plasma membrane is a selective barrier that allows passage of oxygen, nutrients, and wastes to service the cell. It is also called the cell membrane, plasmalemma, or cytoplasmic membrane.

• Organelles also have extensive internal membranes.

Hydrophilic vs. Hydrophobic

• Hydrophilic: Water-loving

• Hydrophobic: Water-fearing

• Amphipathic: Molecules with both hydrophilic and hydrophobic parts (e.g., phospholipids)

Structure of the Plasma Membrane

• Composed of two layers of phospholipids (lipid bilayer).

• Hydrophobic molecules pass easily; hydrophilic molecules do not.

Functions of the Plasma Membrane

1. Boundary and permeability barrier

2. Transport processes

3. Signal detection

4. Cell-to-cell communication

5. Organisation and localisation of function

Fluid-Mosaic Model

Concept and Features

The fluid-mosaic model describes the plasma membrane as a dynamic structure with proteins and lipids capable of lateral movement.

• Mosaic: Proteins are embedded in the lipid bilayer, not forming a continuous sheet.

• Fluid: Lipid components are in constant motion, allowing lateral diffusion.

• Phospholipids have hydrophilic heads and hydrophobic tails, forming a bilayer in aqueous environments.

• Cholesterol and other molecules stabilize the membrane and prevent solidification at low temperatures.

Membrane Components

• Phospholipids

• Cholesterol

• Proteins (peripheral and integral)

• Carbohydrates (glucose, glycoproteins, glycolipids)

Component	Function
Phospholipids	Form bilayer, provide barrier
Cholesterol	Stabilizes membrane, maintains fluidity
Integral proteins	Transport, signaling
Peripheral proteins	Support, cell recognition
Glycolipids/Glycoproteins	Cell recognition, communication

Structural Components of the Plasma Membrane

Lipids

1. Phospholipids: Amphipathic molecules forming the bilayer.

2. Cholesterol: Amphipathic, stiffens the membrane.

3. Glycolipids: Fatty acid tail and carbohydrate head, involved in cell recognition.

Proteins

1. Integral proteins: Embedded in the bilayer, involved in transport.

2. Peripheral proteins: Associated with membrane surfaces, involved in support and recognition.

Phospholipids

Structure and Arrangement

• Phospholipids have a hydrophilic head (phosphate group) and two hydrophobic tails (fatty acid chains).

• Hydrophobic tails avoid water, hydrophilic heads face water.

• Bilayer arrangement keeps hydrophobic regions away from water.

Cholesterol

Role in the Membrane

• Composed of four rings of hydrogen and carbon atoms.

• Hydrophobic, found among phospholipid tails.

• Maintains membrane consistency and fluidity.

• Prevents phospholipid tails from solidifying.

Glycolipids

Function and Importance

• Short carbohydrate chain attached, exposed on cell surface.

• Role in cell recognition and communication.

• Contribute to blood type variation.

Cell Membrane Proteins

Types and Functions

1. Integral proteins: Embedded in the bilayer, generally transport proteins.

2. Peripheral proteins: Not embedded, associate with membrane surfaces via weak electrostatic forces.

Membrane Transport Proteins

Classes of Transport Proteins

• Carrier proteins: Bind solute on one side, deliver to other side via conformational change.

• Channel proteins (Ion channels): Form hydrophilic pores for diffusion of ions.

Plasma Membrane Transport

Types of Transport

Transport across the lipid bilayer is classified by energy requirement:

1. Passive transport

2. Active transport

Passive Transport

• Does not require energy

• Driven by diffusion from higher to lower concentration

• Types:

  • Simple diffusion: Molecules move directly across membrane

  • Facilitated diffusion: Molecules move via specialized membrane proteins

Active Transport

• Requires energy (ATP)

• Moves molecules against concentration gradient

• Involves transport proteins (e.g., pumps)

Summary Table: Membrane Transport

Transport Type	Energy Required	Direction	Example
Simple Diffusion	No	High to low concentration	O2, CO2
Facilitated Diffusion	No	High to low concentration	Glucose, ions
Active Transport	Yes (ATP)	Low to high concentration	Na+/K+ pump

Key Equations

• Diffusion rate: $J = -D \frac{dC}{dx}$ where J is flux, D is diffusion coefficient, C is concentration, x is position.

• Active transport (Na+/K+ ATPase): $3 \text{Na}^+_{\text{out}} + 2 \text{K}^+_{\text{in}} + \text{ATP} \rightarrow 3 \text{Na}^+_{\text{in}} + 2 \text{K}^+_{\text{out}} + \text{ADP} + \text{P}_i$

Additional info:

• Membrane fluidity is essential for cell signaling, transport, and adaptation to temperature changes.

• Glycolipids and glycoproteins are crucial for immune recognition and cell-cell interactions.