Chap 7 BIOL135 Lecture Slides

Chapter 7 Overview

• Overview of plasma membranes and their essential functions.

• Topics to be discussed:

  • Structure of the membrane: lipids, proteins, carbohydrates

  • Membrane permeability and transport methods

• Relevant textbook sections: 7.1-7.5

Plasma Membrane Functions

• Functions as a gatekeeper, regulating material movement in and out of the cell.

• Comparison made to the bridge keeper from Monty Python.

Types of Membrane Transport

Passive Transport

• Small molecules cross the membrane without energy.

• May involve transport proteins.

• Includes diffusion and osmosis.

Active Transport

• Requires energy to move small molecules across the membrane.

• Utilizes transport proteins.

Bulk Transport

• Involves larger molecules crossing the membrane.

• Includes:

  • Exocytosis: Large molecules exit the cell.

  • Endocytosis: Large molecules enter the cell.

Structure of the Plasma Membrane

• Described as a "fluid mosaic model":

  • Composed of a bilayer of phospholipids with embedded proteins.

  • Carbohydrates attached to lipids and proteins.

• Fluid nature allows for the movement of lipids and proteins within the membrane.

• Components:

  • Lipids: Phospholipids with hydrophilic heads and hydrophobic tails.

  • Proteins: Integral (spanning the membrane) and peripheral (on the surface).

  • Carbohydrates: Play a role in cell recognition and signaling.

Importance of Membrane Fluidity

• Essential for permeability and movement of proteins.

• Influenced by:

  • Temperature: Cooler temperatures lead to a solid state.

  • Types of fatty acids:

    • Unsaturated fatty acids increase fluidity due to bending caused by double bonds.

    • Cholesterol maintains fluidity by preventing tight packing at varying temperatures.

Membrane Proteins

• Key to membrane functionality, varying by cell type.

• Types of proteins:

  • Integral Proteins: Penetrate the membrane, often as transmembrane proteins.

  • Peripheral Proteins: Bound to the surface, affiliated with cytoskeleton or extracellular matrix.

Functions of Membrane Proteins

• Transport: Movement of substances across the membrane.

• Enzymatic Activity: Catalyzing biochemical reactions.

• Signal Transduction: Receiving and transmitting signals from the outside to inside.

• Cell-Cell Recognition: Identifying neighboring cells.

• Intercellular Joining: Forming connections with other cells.

• Attachment to Cytoskeleton and ECM: Maintaining structure and communication.

Role of Carbohydrates in the Plasma Membrane

• Functions primarily in cell-cell recognition.

• Formed when bonded to lipids (glycolipids) or proteins (glycoproteins).

• Diversity of carbohydrate chains aids in individual cell identification.

Membrane Asymmetry

• Differences between inside and outside membrane

  • Distinct proteomic and lipid composition.

• Asymmetry established during synthesis.

Selective Permeability of Membranes

• Control over what crosses the membrane easily:

  • Hydrophobic molecules (e.g., O2, CO2) cross easily.

  • Hydrophilic molecules (e.g., sugars, ions) do not cross easily.

  • Require transport proteins for passage.

Types of Transport Proteins

• Channel Proteins: Allow specific substances (e.g., ions, water) to pass through a hydrophilic tunnel.

• Carrier Proteins: Change shape to shuttle substances across the membrane.

Types of Passive Transport

• Diffusion: Movement from high to low concentration.

• Osmosis: Specific diffusion of water across a membrane.

• Tonicity: The ability of a solution to affect cell volume based on solute concentrations:

  • Isotonic: Equal concentration, stable volume.

  • Hypertonic: Causes cells to lose water and shrink.

  • Hypotonic: Causes cells to gain water and potentially burst.

Osmoregulation in Cells Without Cell Walls

• Require mechanisms for balance in hypotonic or hypertonic conditions.

• Examples include contractile vacuoles in paramecium and osmotic adaptations in bacteria.

The Role of Cell Walls in Various Environments

• Plant Cells:

  • In hypotonic solutions: Turgor pressure is maintained.

  • In isotonic solutions: Cells become flaccid.

  • In hypertonic solutions: Cells undergo plasmolysis.

Bulk Transport Overview

• Involves movement of large molecules in vesicles:

  • Endocytosis: Encompasses phagocytosis and pinocytosis.

  • Exocytosis: Secretion of molecules from the cell.

Endocytosis Types

• Phagocytosis: Engulfing particles by extending pseudopodia.

• Pinocytosis: Non-specific uptake of extracellular fluid.

• Receptor-Mediated Endocytosis: Specific uptake via receptor binding.

Conclusion

• The structure of the plasma membrane, defined by the fluid mosaic model, impacts permeability and transport efficiency.

• Understanding these membranes is critical for comprehending cell functionality and interaction with their environment.