Cell Membrane Functions and Transport Quiz

Cell Membrane

• A thin, elastic, semi-permeable membrane surrounding every animal cell
• 7.5 nm thick
• Controls what enters and exits the cell

Functions of the Cell Membrane

• Separates the cytoplasm from the extracellular fluid (ECF)
• Maintains the cell's internal environment
• Transports macromolecules into and out of the cell
• Controls the distribution of ions like sodium and potassium
• Generates transmembrane potentials
• Contains receptors for hormones and transmitter substances, important for signal transduction

Components of the Cell Membrane

• Composed of lipids, proteins, and carbohydrates
• 42% lipids, 55% proteins, 3% carbohydrates

Transport through the Cell Membrane

• Three main mechanisms: passive transport, active transport, and vesicular transport

Passive Transport (Diffusion)

• Movement of substances across the cell membrane down its electrochemical gradient
• Requires no energy
• Three types: simple diffusion, facilitated diffusion, and osmosis

Simple Diffusion

• Movement of substances across the cell membrane without the need for carrier proteins
• Occurs through the lipid bilayer or watery protein channels
• Characterized by:
  • Occurs down an electrochemical gradient
  • No energy is required
  • Not rate-limiting
  • Not saturable

Facilitated Diffusion

• Diffusion of substances across the cell membrane down an electrochemical gradient, requiring carrier proteins
• Characterized by:
  • Occurs down an electrochemical gradient
  • No energy is required
  • Requires the presence of a transport carrier protein
  • Rate-limiting and saturable

Osmosis (Diffusion of Water)

• Water diffuses through the lipid bilayer very rapidly due to its small size and high kinetic energy
• Water moves from high to low concentrations of water

Active Transport

• Movement of substances across the cell membrane against its electrochemical gradient
• Requires energy
• Two types: primary and secondary active transport

Primary Active Transport

• Obtains energy directly from the hydrolysis of ATP
• Example: Sodium-Potassium Pump

Secondary Active Transport

• Uses energy stored in a concentration gradient (like sodium)
• Example: Sodium-Glucose co-transport

Active Transport Characters

• Occurs against the electrochemical gradient
• Requires energy
• Requires the presence of a transport carrier protein
• Exhibits saturation and competition

Vesicular Transport

• Mechanism by which large sized substances can cross the cell membrane
• Two types: endocytosis and exocytosis

Endocytosis

• Extracellular material is trapped within invaginations of the cell membrane
• Forms vesicles
• Active process requiring energy

Exocytosis

• Intracellular material is trapped within vesicles
• Vesicles fuse with the cell membrane, releasing their contents into the ECF
• Active process requiring energy (e.g. release of hormones, digestive enzymes)