Cell Membranes

Major Biological Functions

The cell membrane is a dynamic structure essential for maintaining cellular integrity and function. Its primary roles include:

• Selective permeability: Controls the entry and exit of substances.

• Compartmentalization: Separates cellular processes into distinct regions.

• Transport: Facilitates movement of molecules across the membrane.

• Signal transduction: Transmits signals from the environment to the cell interior.

• Cell recognition: Enables cells to identify and interact with each other.

• Structural anchoring: Provides attachment points for cytoskeletal elements.

Fluid Mosaic Model

The fluid mosaic model describes the structure of the cell membrane as a flexible bilayer of lipids with embedded proteins. Key points include:

• Lipid bilayer: Formed due to the amphipathic nature of lipids.

• Proteins: Embedded within the bilayer, contributing to the mosaic aspect.

• Lateral movement: Demonstrated by FRAP (Fluorescence Recovery After Photobleaching).

• Membrane asymmetry: Different composition between inner and outer leaflets.

Lipids provide fluidity, while proteins create the mosaic pattern.

Membrane Components

• Phospholipids

• Glycolipids

• Sterols: Cholesterol (animal), ergosterol (fungal)

• Integral proteins: Span the membrane

• Peripheral proteins: Attached to membrane surface

• Glycoproteins: Proteins with carbohydrate chains

Phospholipids

Phospholipids are the main structural component of membranes:

• Structure: Glycerol backbone, two fatty acid tails (hydrophobic), phosphate head (hydrophilic)

• Amphipathic: Contains both hydrophobic and hydrophilic regions

• Bilayer formation: Hydrophobic tails face inward, hydrophilic heads face outward

Cholesterol

Cholesterol is an amphipathic sterol that modulates membrane fluidity:

• Inserts between phospholipids

• Buffers fluidity: prevents rigidity at low temperatures and excess fluidity at high temperatures

Membrane Fluidity Factors

• Temperature: Higher temperature increases fluidity

• Fatty acid length: Shorter chains increase fluidity

• Saturation: Unsaturated (double bonds) increase fluidity

• Cholesterol: Buffers fluidity

Lipid Rafts

Lipid rafts are microdomains enriched in cholesterol and sphingolipids:

• More ordered and less fluid

• Serve as platforms for cell signaling

Glycoproteins & Glycolipids

Carbohydrate chains attached to proteins and lipids face the exoplasmic side:

• Involved in cell recognition

• Determine ABO blood type

Membrane Asymmetry

• Lipid distribution differs between leaflets

• Proteins maintain orientation

• Flip-flop movement is rare and requires flippases

Detergents

Detergents solubilize membrane proteins by surrounding hydrophobic regions.

SDS-PAGE

• Separates proteins by size

• SDS denatures proteins and imparts a uniform negative charge

FRAP (Fluorescence Recovery After Photobleaching)

• Measures lateral mobility of membrane components

• Photobleaching followed by recovery rate indicates fluidity

Transport Across Membranes

Diffusion

Diffusion is the passive movement of molecules down their concentration gradient.

• Simple diffusion: No protein required

• Facilitated diffusion: Requires membrane protein

• Both are passive processes

Passive vs Active Transport

• Passive: Down gradient, no ATP required

• Active: Against gradient, requires energy

Primary Active Transport

• Direct ATP usage

• Example: Na+/K+ pump

Secondary Active Transport

• Uses ion gradient established by primary active transport

• Can be symport (same direction) or antiport (opposite direction)

Symport vs Antiport

Carrier vs Channel Proteins

• Carrier: Undergoes conformational change, saturable, can be uniport, symport, or antiport

• Channel: Forms a pore, faster, often gated

Osmosis

Osmosis is the movement of water across a membrane, often facilitated by aquaporins.

Tonicity

RBC Transport Examples

• O2 via simple diffusion

• Cl-/HCO3- antiporter

• Water via aquaporins

Endomembrane System and Protein Sorting

Components of the Endomembrane System

The endomembrane system is a network of membranes within eukaryotic cells, responsible for protein and lipid synthesis, modification, and transport.

• Nuclear envelope

• Rough ER

• Smooth ER

• Golgi apparatus

• Lysosomes

• Vesicles

• Note: Chloroplasts are not part of the endomembrane system.

Rough ER

• Synthesizes membrane, secreted, and lysosomal proteins

Smooth ER

• Lipid synthesis

• Detoxification

• Ca2+ storage

Cotranslational Translocation

• Protein translation and ER insertion occur simultaneously

• Occurs at rough ER

Protein Synthesis Locations

Protein Topology

• Signal-anchor (SA) and stop-transfer (STA) sequences are hydrophobic

• Orientation is preserved during trafficking

• N-terminus facing cytosol in ER remains cytosolic at plasma membrane

Golgi Apparatus

• Cis: receiving side

• Medial: modifying side

• Trans: shipping side

Vesicle Budding Components

• GTP-binding protein

• Coat protein (selects cargo)

• v-SNARE

• Cargo protein

• Cargo receptor

• Note: t-SNARE is not part of budding

Vesicle Targeting

• Rab GTPases identify target

• v-SNARE binds t-SNARE

• Membrane fusion follows

Anterograde vs Retrograde Transport

KDEL Sequence

• ER retention signal

• If protein reaches Golgi, KDEL sequence ensures return to ER

Receptor-Mediated Endocytosis

• Specific ligand binding

• Clathrin coat formation

• Endosome formation

Lysosome

• Acidic environment

• Contains hydrolytic enzymes

• Responsible for recycling and degradation

Cytoskeletal Systems

Cytoskeleton Definition

The cytoskeleton is a network of protein filaments that provides structural support, facilitates transport, enables cell division, and drives cellular movement.

• Structure

• Transport

• Division

• Movement

Microfilaments (Actin)

• Smallest cytoskeletal element

• Double helix structure

• Polar (plus and minus ends)

• Treadmilling: simultaneous growth at one end and shrinkage at the other

• Functions: muscle contraction, contractile ring in cytokinesis

Myosin

• Motor protein for actin

• Myosin II is bipolar and involved in muscle contraction

• Moves toward the plus end of actin filaments

Muscle Contraction Mechanism

1. Ca2+ binds to troponin

2. Tropomyosin shifts position

3. Myosin binds to actin

4. Power stroke occurs

5. Actin filaments slide past myosin

Microtubules

• Largest cytoskeletal element

• Composed of α and β tubulin dimers

• Protofilament: linear chain of tubulin dimers

• 13 protofilaments form a hollow tube

• Polar structure

• Exhibit dynamic instability

• Originate from MTOC (Microtubule Organizing Center)

• Form mitotic spindle during cell division

γ-TuRC (Gamma Tubulin Ring Complex)

• Nucleates microtubules

• Anchors minus end

Mitotic Spindle Types

• Kinetochore microtubules: attach to chromosomes

• Polar microtubules: interact with other spindle microtubules

• Astral microtubules: anchor spindle to cell cortex

Motor Proteins on Microtubules

• Kinesin: moves toward plus end

• Dynein: moves toward minus end

• Both require ATP

Treadmilling

• Growth at plus end, shrinkage at minus end

• Powers intracellular movement

Dynamic Instability

• Rapid switching between growth and shrinkage

• Unique to microtubules

Intermediate Filaments

• Rope-like structure

• No polarity

• No motor proteins

• Provide tensile strength

Centrioles

• Centrioles are not doublet structures