Membrane Structure and Function Quiz

Membrane Structure and Function

• All cells (prokaryotic and eukaryotic) are surrounded by a plasma membrane
• This membrane is dynamic and constantly changing, not fixed
• It defines the cell boundary
• It separates internal cell contents from the external environment
• It acts as a selective barrier to specific molecules

Membrane Outlines

• Structure of the Plasma Membrane
  • Phospholipid bilayer
  • Cholesterol
  • Membrane proteins
  • Glycocalyx
• Mobility in Membrane
  • Mobility of phospholipids
  • Mobility of membrane proteins
• Transport of Molecules
  • Passive diffusion
  • Facilitated diffusion (carrier proteins & 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
  • Phospholipids
  • Cholesterol
  • Proteins
  • Glycocalyx
• Phospholipids form a bilayer in most membranes, accounting for more than half of the lipid
• Glycolipids and cholesterol are also components of the bilayer

Lipid Components of the Plasma Membrane

• Sphingomyelin
• Glycolipids
• Phosphatidylserine
• Phosphatidylinositol
• Phosphatidylcholine
• Cholesterol
• Phosphatidylethanolamine

Phospholipid Composition

• Animal cells contain five types of phospholipids:
  • Sphingomyelin
  • Phosphatidylcholine
  • Phosphatidylethanolamine
  • Phosphatidylserine
  • Phosphatidylinositol
• These phospholipids are asymmetrically distributed between the two halves of the bilayer

Cholesterol

• Cholesterol is a component of eukaryotic cell membranes
• It is crucial to membrane fluidity
• Affects fluidity by being less fluid at warmer temperatures and more fluid at lower temperatures
• It inserts into the phospholipid bilayer and interacts with the polar head groups of the phospholipids
• In the plasma membrane, cholesterol, sphingomyelin, and glycolipids cluster together to create distinct regions known as lipid rafts
• Lipid rafts are dynamic and are crucial for cell signaling and endocytosis
• Cholesterol helps prevent membrane freezing and maintains fluidity

Membrane Proteins

• Responsible for specific membrane functions
• Membranes are described as fluid mosaics where proteins are inserted into phospholipid bilayers
• Membrane proteins can be peripheral, integral (transmembrane), anchored
  • Peripheral proteins are loosely associated and might only be on one side of the membrane
  • Integral proteins cross the entire membrane
    • These can be single-pass, multi-pass, lipid-anchored, or GPI-anchored

The Glycocalyx

• A carbohydrate coat on the cell surface
• Formed by oligosaccharides of glycolipids and transmembrane glycoproteins
• Functions to protect the cell surface and as markers for cell-cell recognition

Lipid Rafts

• Organized by sphingomyelin, glycolipids, and cholesterol
• GPI-anchored proteins often present in them
• Can transiently contain other membrane proteins involved in signaling or endocytosis

Fluid Mosaic Model

• A model of the cell membrane depicting the components as a dynamic fluid mosaic
• The components are constantly moving and changing.

Membrane Fluidity

• Factors like
  • Fatty acid types (saturated and unsaturated)
  • Temperature
  • Length of phospholipid tails
  • Presence of cholesterol affect membrane fluidity.
  • Saturated = more rigid, unsaturated = more fluid, longer tails = more rigid, shorter tails = more fluid.
• Higher temperatures mean more fluidity; lower temperatures mean more rigidity

Mobility of Membrane Proteins

• Membrane proteins move laterally through the phospholipid bilayer but are restricted by other proteins or specific lipids
• Tight junctions in epithelial cells restrict protein movement between different parts of the cell membrane

Transport Across the Cell Membrane

• Passive transport (no energy)
  • Simple Diffusion
  • Facilitated Diffusion
• Active transport (energy used)
  • Vesicle mediated (endocytosis, exocytosis)
  • Pump mediated (primary & secondary)

Permeability of Phospholipid Bilayers

• Small, uncharged molecules readily pass through the bilayer
• Larger polar molecules and ions cannot pass freely because they do not dissolve well in the lipid portions of the bilayer

Simple Diffusion

• Small, uncharged molecules and substances that can dissolve in the lipid bilayer passively move across the membrane due to the concentration gradient

Facilitated Diffusion

• Some larger, polar molecules or ions move passively across the membrane with the aid of specific transmembrane proteins (transporters)

Channel and Carrier Proteins

• Channel proteins are pores that allow specific ions to pass through
• Carrier proteins bind to molecules, undergo a change in shape, and release them to the other side of the membrane

Model for the Facilitated Diffusion of Glucose

• Glucose binds to the transporter protein on one side
• The transporter changes shape and releases the glucose on the other side

Model of an Ion Channel

• Ion channels have gates to control the passage of ions
• The gate opens and closes to regulate ion flow
• Ion channel size and shape govern which types of ions can pass through

Active Transport

• Membranes use free energy (often ATP) to maintain concentration gradients against the flow of passive movement.

Active Transport Driven by ATP Hydrolysis

• ATP hydrolysis directly powers molecules against their electrochemical gradient.

Model of Active Transport

• ATP binding to the protein triggers a shape change, moving the molecule against its gradient

The Sodium-Potassium Pump

• A crucial active transport mechanism for concentration gradients of sodium and potassium ions across the plasma membrane
• Uses ATP to move sodium out and potassium into the cell.

Active Transport Driven by Ion Gradients

• The concentration gradient of ions (e.g., sodium) can be used to actively move other molecules against their gradient using cotransport or other transport mechanisms
• Often used for glucose uptake

Endocytosis

• Ingestion of large particles (bacteria) or fluids/macromolecules in vesicles
  • Phagocytosis (cell eating)
  • Pinocytosis (cell drinking)
• Receptor-Mediated Endocytosis
• Protein trafficking in endocytosis
• Materials are transported to endosomes then sorted for recycling to the plasma membrane or for degradation in lysosomes.

Important Functions of the Cell Membrane

• Structural: Surrounds cytoplasm, supports organelles, provides shape
• Barrier: Regulates passage of molecules and ions
• Contact: Allows cell interaction via contacts points
• Receptors: Detecting signals from the cellular environment
• Transport: Facilitates active and passive transport of molecules and ions, including electrons in organelles.

Test your knowledge on the structure and functions of the plasma membrane. This quiz covers key components, transport mechanisms, and dynamic properties of cell membranes. Perfect for students studying cell biology or related fields.