Biology Chapter 5: Homeostasis and Cell Transport

Chapter 5: Homeostasis and Cell Transport

• This chapter covers homeostasis and cell transport, specifically passive and active transport mechanisms.

Section 1: Passive Transport

• Objectives:

  • Explain how equilibrium is established through diffusion.
  • Differentiate between diffusion and osmosis.
  • Detail how substances cross cell membranes through facilitated diffusion.
  • Explain the role of ion channels in ion diffusion across cell membranes.

• Passive Transport:

  • Molecules move across the cell membrane without energy input from the cell.
  • Examples of substances that move passively include water, lipids, and lipid-soluble substances.
  • No energy required.

• Types of Passive Transport:

  • Diffusion
  • Osmosis
  • Facilitated Diffusion
  • Filtration

Diffusion

• Movement of molecules from an area of higher concentration to an area of lower concentration.
• Driven by the kinetic energy of the molecules.
• Continues until a dynamic equilibrium is reached.
• Equilibrium is when there is no net change in concentration across space. It is influenced by random movement of particles (Brownian motion).

Osmosis

• Specific type of diffusion.

• Movement of water across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration.

• Direction of Osmosis:

  • Determined by relative solute concentrations on either side of the membrane.
  • Hypertonic: Higher solute concentration outside the cell; water moves out of the cell (plasmolysis).
  • Hypotonic: Lower solute concentration outside the cell; water moves into the cell (cytolysis).
  • Isotonic: Equal solute concentrations inside and outside the cell; no net water movement.

• How Cells Deal with Osmosis:

  • Cells in multicellular organisms often use mechanisms to regulate water levels, including pumping solutes out or using contractile vacuoles.
  • Red blood cells (RBCs) cannot pump solutes to regulate water.

Facilitated Diffusion

• Movement of molecules across a membrane that aren't lipid-soluble or are too large to pass through membrane pores.
• Molecules bind to carrier proteins on one side of the membrane.
• The carrier protein changes shape, transporting the molecule down its concentration gradient to the other side of the membrane.

Diffusion Through Ion Channels

• Ion channels are proteins (or groups of proteins) that create small passageways across the cell membrane.
• Special channels allow particular ions to diffuse across the membrane, crucial for cellular function.
• Important ions include sodium, potassium, calcium, and chloride.

Section 2: Active Transport

• Objectives:

  • Compare and Contrast passive and active transport.
  • Explain how the sodium-potassium pump functions.
  • Contrast endocytosis and exocytosis processes.

• Active Transport: Movement of molecules across the membrane from an area of lower concentration to an area of higher concentration.

• Energy is required for this process in order to move against the concentration gradient.

• Example: Sodium-Potassium Pump

• Sodium-Potassium Pump:

  • Moves 3 sodium ions (Na+) out of the cell and 2 potassium ions (K+) into the cell.
  • Uses ATP (energy) for its process.
  • Creates an electrical gradient across the membrane (outside becomes positively charged, inside becomes negative).
  • Essential for nerve cell function and other cellular processes.

• Movement in Vesicles:

  • Endocytosis: Cells take in external fluids, molecules, and large particles by folding their membranes to form pouches that pinches off (becoming vesicles).
    • Pinocytosis: Liquid.
    • Phagocytosis: large particles or whole cells.
    • Receptor-mediated endocytosis: Molecules bind to receptors, resulting in vesicles forming in coated pits (proteins).
  • Exocytosis: Vesicles fuse with the cell membrane, releasing their contents into the external environment. Used to release proteins, waste products, or toxins that could be harmful if released within the cell.