Cell Transport: Diffusion, Osmosis, Active Transport

Cell Transport

Diffusion

• Process of movement of molecules from an area of higher concentration to an area of lower concentration.
• Does not require energy (passive transport).
• Occurs until equilibrium is reached.
• Factors influencing diffusion:
  • Concentration gradient
  • Temperature
  • Surface area
  • Size of molecules

Osmosis

• Special type of diffusion specifically for water molecules.
• Water moves through a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration.
• Can be isotonic (equal concentration), hypertonic (higher solute concentration outside), or hypotonic (lower solute concentration outside).
• Essential for maintaining cell turgor pressure in plants.

Active Transport

• Movement of molecules against their concentration gradient (from low to high concentration).
• Requires energy (ATP).
• Types of active transport:
  • Primary active transport: Direct use of ATP (e.g., sodium-potassium pump).
  • Secondary active transport: Uses the energy from the electrochemical gradient created by primary transport (e.g., glucose transport).

Endocytosis and Exocytosis

• Endocytosis: Process by which cells engulf external substances, packaging them into vesicles.

  • Types:
    • Phagocytosis: "Cell eating" – engulfing large particles.
    • Pinocytosis: "Cell drinking" – engulfing liquids.
    • Receptor-mediated endocytosis: Specific uptake of molecules based on receptor binding.

• Exocytosis: Process of vesicles fusing with the cell membrane to release their contents outside the cell.

  • Used for secretion of hormones, neurotransmitters, and waste removal.

Transport Proteins

• Integral membrane proteins that facilitate the movement of substances across the cell membrane.
• Types:
  • Channel proteins: Form pores that allow specific ions or water to pass through.
  • Carrier proteins: Bind to specific molecules and change shape to shuttle them across the membrane.
• Important in facilitated diffusion and active transport processes.

Cell Transport

Diffusion

• Movement of molecules from high to low concentration areas without energy input.
• Occurs until a state of equilibrium is achieved.
• Influencing factors include:
  • Concentration gradient: The difference in concentration across distances.
  • Temperature: Higher temperatures increase molecular movement.
  • Surface area: Greater surface enhances diffusion rates.
  • Size of molecules: Smaller molecules diffuse more easily.

Osmosis

• Specialized diffusion for water molecules across a selectively permeable membrane.
• Water moves from lower solute concentration to higher solute concentration.
• Can occur in three conditions:
  • Isotonic: Solute concentrations are equal inside and outside the cell.
  • Hypertonic: Higher solute concentration outside the cell, causing it to shrivel.
  • Hypotonic: Lower solute concentration outside the cell, leading to swelling and potential bursting.
• Vital for maintaining turgor pressure in plant cells, providing structural support.

Active Transport

• Involves movement of molecules against their concentration gradient (low to high).
• Requires energy in the form of ATP for operation.
• Two main types:
  • Primary active transport: Direct ATP usage to move ions (e.g., sodium-potassium pump).
  • Secondary active transport: Utilizes energy from the electrochemical gradient formed by primary transport for other substances (e.g., glucose transport).

Endocytosis and Exocytosis

• Endocytosis: Cellular process of engulfing external substances and encapsulating them in vesicles.
  • Types include:
    • Phagocytosis: "Cell eating" for large particles.
    • Pinocytosis: "Cell drinking" for liquid uptake.
    • Receptor-mediated endocytosis: Specific absorption based on receptor interactions.
• Exocytosis: Vesicles fuse with cell membrane to release substances outside the cell.
  • Crucial for the secretion of hormones, neurotransmitters, and waste elimination.

Transport Proteins

• Integral membrane proteins that assist in moving substances across the cell membrane.
• Two primary types:
  • Channel proteins: Create pores allowing selective passage of ions or water.
  • Carrier proteins: Bind to specific molecules, changing shape to transport them across the membrane.
• Essential for processes in facilitated diffusion and active transport.

Cell Transport Study Notes

Diffusion

• Movement of molecules from high concentration to low concentration.
• Passive process that requires no energy expenditure.
• Occurs until equilibrium is achieved in the system.
• Influenced by concentration gradient, temperature, surface area, and distance.

Osmosis

• A specific diffusion focused on water molecules through a selectively permeable membrane.
• Water naturally relocates from areas of low solute concentration to high solute concentration.
• Tonicity classifications:
  • Isotonic: solutions with equal solute concentrations.
  • Hypertonic: solutions with higher solute concentrations.
  • Hypotonic: solutions with lower solute concentrations.
• Osmosis is crucial for maintaining turgor pressure in plant cells.

Active Transport

• Involves movement of molecules against their concentration gradient, from low to high concentration.
• Requires energy, specifically in the form of ATP.
• Includes:
  • Primary active transport, which directly utilizes ATP.
  • Secondary active transport, which derives energy from primary transport processes.
• Notable examples include the sodium-potassium pump and proton pumps, critical for cellular function.

Transport Proteins

• Specialized proteins that aid in the movement of substances across cell membranes.
• Types of transport proteins include:
  • Channel proteins, which create pores for specific molecules to pass through.
  • Carrier proteins, which bind to molecules and alter shape to facilitate transport.
• Function in both passive (facilitated diffusion) and active transport mechanisms.

Endocytosis

• Refers to the process where cells engulf materials from their surroundings.
• Types of endocytosis include:
  • Phagocytosis: the engulfing of large particles, often called "cell eating."
  • Pinocytosis: the absorption of liquids, commonly referred to as "cell drinking."
  • Receptor-mediated endocytosis: involves selective uptake through specific receptors.
• This process is energy-dependent, utilizing cellular resources for transport.

Exocytosis

• Mechanism by which cells expel materials into the extracellular environment.
• Involves vesicles that fuse with the plasma membrane, releasing their contents outside the cell.
• Plays a critical role in processes such as neurotransmitter release, hormone secretion, and the elimination of waste products.
• Like endocytosis, exocytosis is also an energy-dependent process.