Untitled Quiz

Membrane Structure and Function

• All cells (prokaryotic and eukaryotic) have a plasma membrane
• The membrane is dynamic and not fixed.
• It defines the boundary of the cell.
• It separates the cell's internal environment from the outside environment.
• It acts as a selective barrier for the passage of molecules.

Membrane Structure Outline

• Structure of the Plasma Membrane
  • Phospholipid bilayer
  • Cholesterol
  • Membrane proteins
  • Glycocalyx
• Mobility in Membrane
  • Mobility of phospholipids
  • Mobility of membrane proteins

Transport of Molecules Outline

• Transport of Molecules
  • Passive diffusion
  • Facilitated diffusion (carrier proteins and ion channels)
  • Active transport driven by ATP hydrolysis
  • Active transport driven by ion gradients
• Endocytosis
  • Phagocytosis
  • Receptor-mediated endocytosis
  • Protein trafficking in endocytosis

Membrane Components

• Phospholipid bilayer
• Cholesterol
• Proteins
• Glycocalyx

Phospholipid Bilayer

• The fundamental structure of the plasma membrane
• Phospholipids make up more than half of the lipids in most membranes
• Also contains glycolipids and cholesterol

Lipid Components of the Plasma Membrane

• Sphingomyelin
• Glycolipid
• Phosphatidylserine
• Phosphatidylinositol
• Phosphatidylcholine
• Cholesterol
• Phosphatidylethanolamine

Phospholipid Composition of Animal Cells

• Sphingomyelin
• Phosphatidylcholine
• Phosphatidylethanolamine
• Phosphatidylserine
• Phosphatidylinositol
• These phospholipids are asymmetrically distributed between the two halves of the membrane bilayer.

Cholesterol

• Found in eukaryotic cell membranes.
• Makes the membrane less fluid at warmer temperatures and more fluid at lower temperatures.
• Inserts into the phospholipid bilayer with its hydroxyl group close to the phospholipid head groups.
• Prevents membranes from freezing and maintains membrane fluidity.
• Forms discrete membrane domains known as lipid rafts with sphingomyelin and glycolipids.
• Moves laterally within the plasma membrane and associates with specific membrane proteins.

Membrane Proteins

• Responsible for carrying out specific membrane functions
• Membranes are viewed as fluid mosaics in which proteins are inserted.
• Peripheral proteins
• Integral proteins
  • Transmembrane
    • Single pass
    • Multipass
  • Lipid-anchored
  • GPI-anchored

The Glycocalyx

• The cell surface is coated with a carbohydrate coat.
• Formed by oligosaccharides of glycolipids and transmembrane glycoproteins.
• Protects the cell surface.
• Serves as markers for cell-cell recognition.

Structure of Lipid Rafts

• Lipid rafts are organized by interactions of sphingomyelin, glycolipids, and cholesterol.
• GPI-anchored proteins are preferentially found in lipid rafts.
• Other membrane proteins transiently reside in lipid rafts to mediate cell signaling or endocytosis.

Fluid Mosaic Model of the Plasma Membrane

• Shows the different components (carbohydrates, peripheral/ integral proteins,glycolipids incorporated)

Why Membrane Fluidity is Needed

• Need for various internal cellular functions and outside interactions.
• Maintaining membrane function in terms of fluidity.

Unique Properties of Cell Membrane

• Phospholipid mobility
• Lipid asymmetry
• Membrane fluidity
  • Fatty acid types
  • Temperature
  • Length of phospholipid tails
  • Presence of cholesterol
• Mobility of phospholipids (rotation, lateral movement, flip-flop)
  • Rotational movement (on its own axis)
  • Lateral movement (transition in same layer; very fast, common)
  • Flip-flop movement (requires energy or help from proteins/enzymes, rare)

Phospholipid Asymmetry in Membrane

• Normal conditions: no lipid flipping, no change in composition.
• Specific phospholipids are predominantly found in either the inner or outermost leaflet of the bilayer.

Membrane Fluidity in Membrane

• Fatty acid types (saturated & unsaturated):
  • Saturated → More rigid
  • Unsaturated → More fluid
• Temperature:
  • High temp. → More fluid/liquid
  • Low temp. → More rigid
• Length of phospholipid tail/fatty acid chains:
  • Long tails → More compact/rigid
  • Short tails → More fluid
• Cholesterol:
  • Present in lipid raft regions; acts like a buffer.
  • High temp → Makes more rigid
  • Low temp → Makes more fluid

Mobility of Membrane Proteins

• Proteins diffuse laterally through the phospholipid bilayer.
• Mobility of some proteins is restricted by associations with other proteins or specific lipids.
• Tight junctions prevent protein movement between distinct plasma domains of epithelial cells.

Functions of Membrane-Associated Proteins

• Transport (e.g., across plasma membrane, organelles)
• Enzymatic activity (e.g., within chloroplasts or mitochondria)
• Signal transduction (e.g., within plasma membrane)
• Intercellular joining (e.g., between cells)
• Cell-cell recognition (e.g., between cells)
• Attachment to the cytoskeleton or extracellular matrix (ECM).

Transport Across the Cell Membrane

• Passive transport
  • Simple diffusion (small uncharged molecules like O2, CO2, H2O)
  • Facilitated diffusion (carrier or channel proteins; larger molecules or ions)
  • Osmosis (water)
• Active transport
  • Driven by ATP hydrolysis
  • Driven by ion gradients

Permeability of Phospholipid Bilayer

• Small uncharged molecules diffuse easily.
• Bilayer is impermeable to larger polar molecules (like glucose, amino acids) and ions.

Passive Transport

• Diffusion: Movement of substances from high to low concentration.
• Facilitated diffusion: Use of transport proteins to aid in the movement of substances across the membrane.

Simple Diffusion

• Only small uncharged molecules diffuse freely.
• Small nonpolar molecules (like O2 and CO2) are soluble in the bilayer.
• Small uncharged polar molecules (like H2O) can also cross.
• Larger uncharged polar molecules (like glucose) cannot cross.

Facilitated Diffusion

• Charged molecules (like ions) cannot cross a lipid bilayer by free diffusion.
• Transmembrane proteins (transporters) facilitate transport.
• These transporters determine the selective permeability of cell membranes.

Channel and Carrier Proteins

• Channel proteins: Form open pores for specific molecules to pass through.
• Carrier proteins: Selectively bind molecules, undergo conformational changes to move them across the membrane.

Model for Facilitated Diffusion of Glucose

• Glucose binds to a transporter site on the outside of the membrane.
• Transporter changes shape, releasing glucose into the cytosol.
• Transporter returns to its original conformation.

Model of an Ion Channel

• Closed conformation: Flow of ions blocked by a gate
• Open conformation: Allows ions to flow rapidly through a pore.
• Channels contain narrow pore that restricts ions to appropriate size and charge.
• Well-studied in nerve and muscles for regulating electrical signals.

Three Properties of Ion Channels

• Transport is extremely rapid.
• Ion channels are highly selective.
• Most ion channels are not permanently open.

Active Transport

• Membrane proteins use ATP energy to control internal cell composition.
• Transport occurs against the concentration gradient.

Active Transport Driven by ATP Hydrolysis

• Energy from ATP hydrolysis drives the transport of molecules against electrochemical gradients.
• Conformational changes in transport proteins cause molecules to move across the membrane against their concentration gradients.

Model of Active Transport

• Energy from ATP hydrolysis moves molecules against the electrochemical gradient.
• This is achieved with a conformational change in transport proteins.

The Sodium-Potassium Pump

• Transports Na+ and K+ against their gradients.
• Uses ATP hydrolysis to maintain cell homeostasis and electrochemical gradients.

Active Transport Driven by Ion Gradients

• Ion gradients are used to drive the active transport of other molecules.

Active Transport of Glucose

• Na+ gradient drives glucose uptake against its concentration gradient from the intestinal lumen

Endocytosis

• Ingestion of large particles (e.g., bacteria) and fluids or macromolecules in small vesicles.
• Phagocytosis: (cell eating) - Ingestion of large particles.
• Pinocytosis: (cell drinking) - Ingestion of fluids or molecules.

Phagocytosis

• Formation of a phagosome (vesicle)
• Fusion with lysosome to create a phagolysosome.
• Digestion of ingested materials.

Receptor-Mediated Endocytosis

• Specific uptake of macromolecules via receptor binding and vesicle formation.

Protein Trafficking in Endocytosis

• Molecules are transported to endosomes
• Sorting for recycling to the plasma membrane or degradation in lysosomes.

Important Functions of the Cell Membrane

• Structural: Surrounds cytoplasm, gives form to organelles.
• Barrier: Regulates passage of necessary ions and compounds.
• Contact: Enables cell-cell interactions.
• Receptors: Detects signals from the surrounding environment.
• Transport: Allows active and passive transport of ions and other substances (including electron transport in mitochondria and chloroplasts).