Membrane Transport and Cell Signaling: Chapter 5 Study Notes

Chapter 5: Membrane Transport

Overview

This chapter explores the structure and function of biological membranes, mechanisms of membrane transport, and the processes of cell signaling. Understanding these concepts is essential for grasping how cells interact with their environment and maintain homeostasis.

Plasma Membrane Structure and Function

Plasma Membrane Function

The plasma membrane is a critical cellular structure that:

• Surrounds every cell, physically separating the cell from its external environment.

• Maintains homeostasis by regulating the internal environment.

• Controls passage of materials into and out of the cell through selective permeability.

Selective permeability means only certain substances can cross the membrane, allowing the cell to regulate its internal composition.

Plasma Membrane Structure

• The plasma membrane is primarily a phospholipid bilayer.

• Phospholipids are amphipathic molecules, with hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.

• In water, phospholipids naturally arrange into two layers, with heads facing outward and tails inward.

Fluid Mosaic Model

The Fluid Mosaic Model describes the plasma membrane as:

• A mosaic of protein molecules floating in a fluid bilayer of phospholipids.

• Proteins and lipids can move laterally within the layer, contributing to membrane fluidity.

Membrane Proteins

• Integral proteins: Firmly embedded in the membrane, often spanning the entire bilayer (transmembrane proteins).

• Peripheral proteins: Loosely attached to the membrane surface.

• Proteins are responsible for most of the membrane's specific functions, such as transport and signaling.

Fluidity of Plasma Membranes

• Phospholipids and proteins can move within the membrane, making it fluid.

• Cholesterol acts as a "fluidity buffer":

  • Reduces fluidity at moderate temperatures by restricting phospholipid movement.

  • Prevents solidification at low temperatures by disrupting regular packing of phospholipids.

Movement Through Membranes

Selective Permeability

The plasma membrane allows only certain substances to pass, based on size and charge. This property enables the cell to regulate its internal environment.

Types of Transport Proteins

• Channel proteins: Form channels for specific molecules to pass through.

• Carrier proteins: Bind to molecules and change shape to shuttle them across the membrane.

Types of Membrane Transport

• Passive transport: No energy required; substances move down their concentration gradient.

• Active transport: Requires energy (usually ATP); substances move against their concentration gradient.

Passive Transport

Diffusion

Diffusion is the movement of molecules from an area of high concentration to an area of low concentration, driven by kinetic energy.

• Continues until dynamic equilibrium is reached (no net change in concentration).

Simple Diffusion

• Small, nonpolar molecules (e.g., O2, CO2) move directly through the membrane.

Facilitated Diffusion

• Transport proteins help larger or polar molecules cross the membrane.

• Molecules still move down their concentration gradient.

• Examples: Glucose transport via carrier proteins.

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane.

• Water moves from areas of high water concentration (low solute) to low water concentration (high solute).

Water Balance in Cells

Tonicity describes the ability of a solution to cause a cell to gain or lose water.

Solution Type	Solute Concentration	Effect on Animal Cell	Effect on Plant Cell
Isotonic	Equal inside and outside	No net water movement; cell remains normal	Cell becomes flaccid (wilts)
Hypotonic	Lower outside than inside	Water enters; cell swells and may burst	Cell becomes turgid (firm)
Hypertonic	Higher outside than inside	Water leaves; cell shrinks (crenates)	Cell undergoes plasmolysis (membrane pulls away from wall)

Active Transport

Overview

Active transport uses energy (usually ATP) to move substances against their concentration gradient (from low to high concentration).

Direct Active Transport

• Example: Sodium-potassium pump ($3 \text{Na}^+$ out, $2 \text{K}^+$ in per ATP used).

• Maintains electrochemical gradients essential for nerve and muscle function.

Cotransport

• Transport protein couples the "downhill" movement of one solute to the "uphill" movement of another.

• Example: H+/Glucose cotransport—H+ diffuses down its gradient, driving glucose uptake against its gradient.

Bulk Transport: Endocytosis and Exocytosis

Exocytosis

• Vesicles fuse with the plasma membrane to release materials outside the cell.

• Example: Secretion of neurotransmitters, hormones.

Endocytosis

• Cell takes in materials by forming vesicles from the plasma membrane.

• Types include phagocytosis ("cell eating"), pinocytosis ("cell drinking"), and receptor-mediated endocytosis.

Cell Signaling

Overview

Cell signaling refers to the mechanisms by which cells communicate with each other to coordinate activities.

• Three main steps: (1) Signal reception, (2) Signal transduction, (3) Cellular response.

Step One: Signal Transmission

• Signaling molecules (e.g., neurotransmitters, hormones) are synthesized, released, and transported to target cells.

Step Two: Reception

• Target cells have receptors (proteins) that bind specific signaling molecules (ligands).

• Reception is highly selective—only matching ligands bind to their receptors (Lock and Key Model).

Types of Receptors

• Ion channel-linked receptors: Transmembrane proteins that open or close in response to ligand binding.

• G protein-linked receptors: Activate intracellular G proteins upon ligand binding.

• Intracellular receptors: Located in the cytosol or nucleus; bind small, hydrophobic ligands that diffuse across the membrane.

Step Three: Signal Transduction

• Conversion of an extracellular signal into an intracellular signal via a cascade of molecular interactions.

• Often involves multiple steps and molecular switches (proteins that toggle between active and inactive states).

Step Four: Cellular Response

• The cell responds by altering its activity, such as opening/closing membrane channels, changing metabolic processes, or modifying gene expression.

Summary Table: Types of Membrane Transport

Transport Type	Energy Required?	Direction Relative to Gradient	Example
Simple Diffusion	No	Down	O2, CO2
Facilitated Diffusion	No	Down	Glucose via carrier protein
Osmosis	No	Down	Water
Active Transport	Yes (ATP)	Up	Sodium-potassium pump
Bulk Transport	Yes	Varies	Endocytosis, Exocytosis

Additional info: Academic context and terminology have been expanded for clarity and completeness. All key processes and terms are explained for exam preparation.