TEST 6 - MEMBRANE STRUCTURE AND FUNCTION

Question 1

Cell Membrane

Answer 1

• Definition: A dynamic, semi-permeable structure that surrounds the cytoplasm of a cell. Composed mainly of a phospholipid bilayer embedded with proteins, cholesterol, glycoproteins, and glycolipids.
• Functions:
  >Maintains structural integrity of the cell.
  >Regulates material exchange (nutrients, waste, ions).
  >Facilitates cell signaling and recognition via receptors and glycoproteins.
  >Serves as a platform for biochemical reactions.
  >Enables cellular adhesion and communication.

Question 2

Phospholipid Bilayer

Answer 2

• Definition: The fundamental structure of the cell membrane formed by two layers of phospholipids. Each phospholipid has:
  > A hydrophilic phosphate head: Attracted to water.
  > Two hydrophobic fatty acid tails: Repelled by water.
• Properties:
  > Amphipathic nature creates a semi-permeable barrier.
  > Hydrophobic core prevents the free passage of hydrophilic molecules and ions, making selective transport mechanisms necessary.

Question 3

Fluid Mosaic Model

Answer 3

• Definition: A model describing the cell membrane as a fluid structure where proteins and lipids move laterally. Proposed by Singer and Nicolson in 1972.
• Key Components:
  > Lipids: Provide the flexible matrix (phospholipids and cholesterol).
  > Proteins: Integral and peripheral proteins serve in transport, signaling, and structural support.
  > Cholesterol: Stabilizes membrane fluidity, especially at varying temperatures.
• Significance:
  > Explains how membranes maintain flexibility while being robust enough to form barriers.

Question 4

Glycocalyx

Answer 4

• Definition: A carbohydrate-rich coat on the cell membrane’s outer surface, made of glycoproteins and glycolipids.
• Functions:
  > Facilitates cell-to-cell recognition (immune system role).
  > Protects the cell from mechanical and chemical damage.
  > Aids in cellular adhesion.
  > Contributes to the membrane’s hydration layer, improving interaction with the extracellular environment.

Question 5

Integral Proteins

Answer 5

• Definition: Proteins embedded within the phospholipid bilayer, spanning across the membrane (transmembrane).
• Functions:
  > Serve as channels and transporters for molecules and ions.
  > Act as receptors for signaling molecules (e.g., hormones).
  > Facilitate enzymatic activity and electron transfer in processes like photosynthesis and respiration.

Question 6

Peripheral Proteins

Answer 6

• Definition: Proteins loosely attached to the membrane surface or to integral proteins.
• Functions:
  > Support structural scaffolding.
  > Participate in signaling pathways by interacting with membrane-bound receptors.
  > Mediate the anchoring of the cytoskeleton to the membrane.

Question 7

Selectively Permeable

Answer 7

• Definition: A property of the cell membrane that permits selective passage of certain molecules based on size, polarity, and charge.
• Examples:
  > Freely passes: Small nonpolar molecules (O2, CO2) and small polar molecules (H2O via aquaporins).
  > Blocked: Large polar molecules (glucose) and charged ions (Na+, Cl-), requiring specific transport mechanisms.

Question 8

Osmosis

Answer 8

• Definition: The passive diffusion of water molecules across a semi-permeable membrane from an area of lower solute concentration to one of higher solute concentration.
• Key Points:
  > Water moves to balance solute concentrations.
  > Requires no energy input (passive transport).
  > Influences cell tonicity (hypertonic, hypotonic, isotonic).

Question 9

Diffusion

Answer 9

• Definition: The movement of molecules from an area of higher concentration to one of lower concentration, driven by the concentration gradient.
• Characteristics:
  > Passive process; no energy required.
  > Rate influenced by the steepness of the gradient, temperature, and molecular size.

Question 10

Facilitated Diffusion

Answer 10

• Definition: Passive movement of larger or charged molecules across the membrane through specific transport proteins.
• Mechanism:
  > Carrier proteins bind to the molecule and change shape to transport it across.
  > Channel proteins provide a hydrophilic pathway for ions and polar molecules.
• Examples:
  > Glucose transport via GLUT proteins.
  > Ion movement through gated channels (e.g., potassium channels)

Question 11

Active Transport

Answer 11

• Definition: An energy-dependent process that moves molecules against their concentration gradient (low to high concentration) using ATP.
• Examples:
  > Sodium-Potassium Pump: Exchanges 3 Na+ out of the cell for 2 K+ into the cell, critical for nerve impulses.
  > Proton Pumps: Pump H+ ions in mitochondria during cellular respiration.

Question 12

Endocytosis

Answer 12

• Definition: Active transport where the cell engulfs external materials into vesicles.
• Types:
  > Phagocytosis: “Cell eating” of solids (e.g., engulfing bacteria).
  > Pinocytosis: “Cell drinking” of liquids.
  > Receptor-mediated: Specific uptake of molecules via receptor-ligand interaction.

Question 13

Exocytosis

Answer 13

• Definition: Active process where vesicles fuse with the plasma membrane to release contents outside the cell.
• Examples:
  > Secretion of hormones (e.g., insulin).
  > Release of neurotransmitters at synapses.
  > Elimination of cellular waste.

Question 14

Aquaporins

Answer 14

• Definition: Specialized channel proteins that facilitate rapid water movement across the cell membrane.
• Importance:
  > Found in high abundance in kidney cells and plant root cells.
  > Enhance water permeability significantly compared to diffusion.

Question 15

Tonicity

Answer 15

• Definition: Describes the effect of osmotic pressure on a cell due to solute concentration differences across a membrane.
• Types:
  > Hypertonic: Higher solute concentration outside the cell, causing water to leave (cell shrinks).
  > Hypotonic: Lower solute concentration outside the cell, causing water to enter (cell swells or bursts).
  > Isotonic: Equal solute concentrations; no net water movement.

Question 16

Glycoproteins

Answer 16

• Definition: Proteins with carbohydrate chains attached, extending from the cell membrane.
• Functions:
  > Critical for cell recognition and immune response.
  > Act as receptors for hormones and other signaling molecules.

Question 17

Glycolipids

Answer 17

• Definition: Lipids with carbohydrate chains attached, present on the cell membrane’s outer layer.
• Functions:
  > Assist in cell recognition.
  > Provide stability to the membrane.

Question 18

Vesicle-Mediated Transport

Answer 18

• Definition: Transport of materials into or out of the cell via vesicles.
• Examples:
  > Exocytosis: Vesicles fuse to release contents.
  > Endocytosis: Vesicles form to bring substances in (e.g., receptor-mediated endocytosis).

Question 19

Sodium-Potassium Pump

Answer 19

• Definition: A membrane-bound protein that uses ATP to pump 3 sodium ions out and 2 potassium ions into the cell.
• Function:
  > Maintains the electrochemical gradient essential for nerve impulses.
  > Regulates cell volume and osmotic balance.

Question 20

Cholesterol

Answer 20

• Definition: A lipid molecule interspersed within the phospholipid bilayer.
• Functions:
  > Enhances membrane fluidity at low temperatures.
  > Prevents excessive membrane fluidity at high temperatures.
  > Contributes to membrane stability.

Question 21

Membrane Proteins

Answer 21

• Definition: Proteins that are integral to the structure and function of the membrane.
• Types:
  > Integral Proteins: Span the bilayer (e.g., channels and transporters).
  > Peripheral Proteins: Loosely attached to the membrane surface.
• Functions:
  > Transport (channels and pumps).
  > Cell adhesion and communication.
  > Hormone binding and enzymatic activity.

Question 22

Types of Membrane Transport

Answer 22

• Passive Transport: Movement of substances without energy input.
  > Includes diffusion, osmosis, and facilitated diffusion.
• Active Transport: Requires ATP to move substances against a gradient.
  > Examples: Sodium-potassium pump, endocytosis, and exocytosis.

Question 23

Vesicle-Mediated Transport

Answer 23

Endocytosis & Exocytosis

Question 24

Sodium-Potassium Pump

Answer 24

• Definition: A primary active transport system that exchanges 3 sodium ions out of the cell for 2 potassium ions in.
• Functions:
  > Maintains resting potential in nerve and muscle cells.
  > Regulates cell volume and osmotic balance.