Cell Membrane and Membrane Transport

Chapter 5.1-5.5: Cell Membrane and Membrane Transport

Learning Objectives (LO6)

  • Biological Membrane:

    • Cell membranes are selectively permeable barriers.

    • Correlate the structure of different cell membrane components (e.g. phospholipids, cholesterol, oligosaccharides, integral and peripheral membrane proteins) with their respective functions:

    • Phospholipids: Form the basic structure of the cell membrane, creating a bilayer due to their amphipathic nature (hydrophilic heads and hydrophobic tails).

    • Cholesterol: Modulates membrane fluidity and stability.

    • Oligosaccharides: Participate in cell recognition and signaling.

    • Integral Proteins: Span the membrane, functioning as channels or transporters.

    • Peripheral Proteins: Attach to the membrane's surface, assisting in signaling and structural roles.

  • Predict how changes in membrane composition and temperature can affect membrane structure and fluidity.

  • Modes of transport: Compare different modes of transport across cell membranes concerning pathway and energy source:

    • Simple Diffusion: Movement of molecules from high to low concentration without energy use.

    • Facilitated Diffusion: Movement via protein channels; does not require energy.

    • Active Transport: Movement against concentration gradient; requires energy (ATP).

    • Coupled Transport: One solute's diffusion boosts the transport of another against its gradient.

    • Bulk Transport: Includes endocytosis and exocytosis for large particles.

  • Describe how transport of ions across cell membranes generates electrochemical gradients for cell work:

    • Example: Movement of Na+ and K+ in nerve cells contributes to action potential.

  • Osmosis: Define osmosis, explaining occurrences via aquaporins and phospholipid bilayers:

    • Example: Predict net water movement based on solute concentrations across membranes in plant and animal cells.

Content Coverage (C5.1-5.5)

  • Concept 5.1: Cellular Membranes Are Fluid Mosaics of Lipids and Proteins

  • Concept 5.2: Membrane Structure Results in Selective Permeability

  • Concept 5.3: Passive Transport Is the Diffusion of a Substance Across a Membrane with No Energy Investment

  • Concept 5.4: Active Transport Uses Energy to Move Solutes Against Their Gradients

  • Concept 5.5: Bulk Transport Across the Plasma Membrane Occurs by Exocytosis and Endocytosis

Overview: Life at the Edge

  • The plasma membrane isolates the cell from its environment.

  • Exhibits selective permeability.

Selective Permeability Discussion

  • Worksheet Activity: Categorize compounds based on permeability speed.

    • Fast vs Slow permeability considerations.

Rules of Membrane Permeability

  • Factors affecting membrane permeability:

    • Composition of lipids, presence of cholesterol, temperature, and external environment.

The Fluid Mosaic Model

  • Description:

    • The membrane comprises a mosaic of protein molecules drifting in the phospholipid bilayer.

    • Phospholipid Bilayer:

    • Hydrophilic phosphate heads facing outwards and hydrophobic fatty acid tails facing inwards (

    • Mosaic Nature: Movement of proteins in the bilayer not fixed.

The Fluidity of Membranes

  • Mechanisms behind fluidity:

    • Proteins can move within the lipid bilayer. Factors influencing fluidity include:

    • Temperature.

    • Fatty acid composition (saturation vs unsaturation).

    • Cholesterol presence: Stabilizes and modulates membrane fluidity.

      • Cholesterol reduces permeability to water-soluble substances.

Membrane Proteins and Their Functions

  • Types of Membrane Proteins:

    • Integral Proteins: Span the membrane, involved in transport.

    • Peripheral Proteins: Attached externally, assisting with signaling or structural roles.

  • Functions:

    • Transport

    • Enzymatic activity

    • Signal transduction

    • Cell-cell recognition

    • Intercellular joining

    • Attachment to cytoskeleton and extracellular matrix (ECM).

The Role of Membrane Carbohydrates in Cell-Cell Recognition

  • Recognition via binding to carbohydrates on the extracellular surface. \n- Membrane carbohydrates form bonds:

    • Glycolipids: Carbohydrates attached to lipids.

    • Glycoproteins: Carbohydrates attached to proteins.

Synthesis and Sidedness of Membranes

  • Cell membranes exhibit distinct inside and outside faces.

  • The asymmetrical arrangement is established during construction in the ER and Golgi apparatus.

Roles of Membrane Proteins in the Medical Field

  • Example: HIV infection requires the CD4 immune cell surface protein and co-receptor CCR5. Resistance occurs in individuals lacking CCR5, steering drug development.

Transport Proteins

  • Transport Proteins: Facilitate passage of hydrophilic substances.

    • Channel Proteins: Form pores in the membrane for specific ions/molecules.

    • Carrier Proteins: Change shape to transport solutes across the membrane.

Effects of Osmosis on Water Balance

  • Osmosis: Diffusion of water across selectively permeable membranes, influenced by tonicity:

    • Isotonic Solution: Equal concentrations, no net water movement.

    • Hypertonic Solution: Higher solute concentration outside; causes water loss from cells.

    • Hypotonic Solution: Higher solute concentration inside; causes water influx, potentially leading to turgor pressure in plant cell walls.

Water Balance Considerations

  • Osmoregulation: Control of solute concentration and water balance vital across environments.

    • Example: Contractile vacuole in Paramecium maintains water balance.

Case Study: NEWater and Osmosis

  • Discussion regarding Reverse Osmosis (RO) in converting seawater to drinkable water: Consider costs and safety for agricultural use.

Aquaporins

  • Aquaporins: Facilitate rapid water movement; highly selective for water.

  • Important in kidney cells for water reabsorption; critical in conditions like nephrogenic diabetes insipidus (NDI), affecting kidney function.

Na+-K+ Pump

  • Mechanism:

    • Moves 3 sodium ions out and 2 potassium ions in; necessary for establishing membrane potential.

    • Uses ATP for energy, operates against the concentration gradient, termed an electrogenic pump.

Coupled Transport by Membrane Protein (Cotransport)

  • Uses the diffusion of one solute to drive the active transport of another against its gradient.

  • Example: In plants, the gradient of H+ ions can transport nutrients into cells.

Oral Rehydration Therapy (ORT)

  • Mechanism utilizes sodium-glucose cotransport, enhancing sodium entry into intestinal cells.

    • Effective in treating dehydration, particularly in cases of diarrhea.

Endocytosis

  • Mechanisms for capturing molecules/particles:

    • Phagocytosis: Cellular eating.

    • Pinocytosis: Cellular drinking.

    • Receptor-mediated Endocytosis: Specific uptake via receptors.

Conclusions

  • End of Chapter 5.6 leads to Learning Objectives 7 (LO7).