Topic 4) Cell Membranes and Transport (AS)

🧬 Topic 4: Cell Membranes and Transport

4.1 Fluid Mosaic Membranes

• Fluid Mosaic Model

  • Describes the structure of cell membranes as a dynamic and flexible arrangement of phospholipids and proteins.

  • The term "fluid" refers to the lateral movement of components within the membrane, while "mosaic" indicates the patchwork of proteins embedded in or associated with the lipid bilayer. Wikipedia

• Phospholipid Bilayer

  • Composed of two layers of phospholipids, each with a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails.

  • Hydrophilic heads face outward toward the aqueous environment, while hydrophobic tails face inward, away from water.

  • This arrangement creates a semi-permeable barrier that regulates the entry and exit of substances. Study Rocket

• Proteins in the Membrane

  • Integral Proteins: Span the entire membrane and are involved in transport, acting as channels or carriers for molecules.

  • Peripheral Proteins: Attached to the inner or outer surface of the membrane and play roles in signaling, cell recognition, and maintaining cell shape. Study Rocket

• Cholesterol

  • Interspersed among phospholipids, cholesterol molecules help to stabilize membrane fluidity.

  • At high temperatures, cholesterol stabilizes the membrane by preventing excessive movement of phospholipids.

  • At low temperatures, it prevents the membrane from becoming too rigid by disrupting the packing of phospholipids. Study Rocket

• Glycoproteins and Glycolipids

  • Carbohydrate chains attached to proteins (glycoproteins) or lipids (glycolipids) on the extracellular surface.

  • Involved in cell recognition, signaling, and forming tissue structures.

  • Play a role in immune responses and determining blood types. Google Sites

4.2 Movement of Substances into and out of Cells

• Passive Transport

  • Movement of substances across the membrane without the expenditure of energy (ATP).

  • Occurs down a concentration gradient (from high to low concentration).Quizgecko | AI Quiz Maker

  • Diffusion

    • The net movement of molecules from an area of higher concentration to an area of lower concentration.

    • Occurs directly through the phospholipid bilayer.

    • Examples: Oxygen and carbon dioxide.

  • Facilitated Diffusion

    • Utilizes transport proteins (channels or carriers) to move substances across the membrane.

    • Occurs when molecules are too large or polar to pass through the lipid bilayer directly.

    • Examples: Glucose and amino acids.

  • Osmosis

    • A specific type of facilitated diffusion involving water molecules.

    • Water moves through the membrane via specialized water channels called aquaporins.

    • Direction of water movement depends on the relative concentrations of solute on either side of the membrane.

• Active Transport

  • Movement of substances against their concentration gradient (from low to high concentration), requiring energy in the form of ATP.

  • Involves specific transport proteins or pumps.Quizgecko | AI Quiz Maker+1Free Exam Academy+1

  • Sodium-Potassium Pump

    • A carrier protein that transports sodium ions out of the cell and potassium ions into the cell.

    • Maintains essential gradients for nerve impulse transmission and muscle contraction.

• Bulk Transport

  • Involves the movement of large quantities of substances into or out of the cell via vesicles.

  • Requires energy expenditure.Quizgecko | AI Quiz Maker

  • Endocytosis

    • The process by which cells engulf substances from their external environment.

    • Includes phagocytosis (cellular eating) and pinocytosis (cellular drinking).

    • Substances are enclosed in vesicles and brought into the cell.

  • Exocytosis

    • The process by which cells expel substances.

    • Vesicles containing the substances fuse with the plasma membrane, releasing their contents outside the cell.