Cell Membrane Transport Overview

Membrane Transport Overview

• Membrane transport encompasses various mechanisms cells use to move substances across their membranes.

Cell Membrane Structure

• The cell membrane is composed of a lipid bilayer with embedded proteins.
• The lipid bilayer primarily consists of phospholipids.
• A diagram showing the different membrane proteins/components may help here.

Membrane Proteins

• Integral membrane proteins are embedded within the lipid bilayer.
• Peripheral membrane proteins are loosely associated with the membrane's surface.
• Lipid-linked membrane proteins are attached to the lipid bilayer.
• These proteins participate in various cellular processes including transport.

Fluid Mosaic Model

• Developed to explain membrane structure and function.
• The model describes the membrane as a fluid structure with proteins embedded within the lipid bilayer (Not a rigid structure).
• The Davison-Danielli model (sandwich model) preceded current understanding.

Membrane Permeability

• Membranes are semi-permeable, meaning they allow certain substances to pass through while restricting others based on size, charge, and solubility.
• Lipid-soluble, small, and non-polar substances can freely diffuse across the membrane.
• Larger or polar substances typically require assistance like proteins to cross.

Passive Transport

• Passive transport occurs down a concentration gradient and doesn't require energy.

• Includes simple diffusion, facilitated diffusion, and osmosis.

• Simple diffusion- Direct movement through the lipid bilayer (high concentration to low concentration).

• Facilitated diffusion- Requires protein channels/carriers to cross (high concentration to low concentration).

• Osmosis - The passive movement of water across a semi-permeable membrane from a region of high water concentration to a region of low water concentration; important in maintaining cell balance.

Active Transport

• Active transport requires energy (ATP) to move substances against their concentration gradient.
• Includes primary active transport and secondary active transport.
• Primary active transport directly utilizes ATP to move molecules.
• Secondary active transport uses the energy from an ion gradient created by a primary active transport process to move other molecules.

Vesicle Transport (Bulk Transport)

• Involves larger molecules or bulk quantities of substances, transported in bubble-like sacs called vesicles across a membrane.
• Includes endocytosis and exocytosis.
• Endocytosis is the moving of substances into the cell.
• Exocytosis is the moving of substances out of the cell.

Types of Endocytosis

• Phagocytosis (cell eating): Ingestion of large solid particles.
• Pinocytosis (cell drinking): Ingestion of dissolved substances.
• Receptor-mediated endocytosis: a specific type used in to ingest substances (hormones, cholesterol) through specific receptors.
• Phagocytosis, pinocytosis, and receptor-mediated endocytosis are examples of endocytosis.

Ion Channels

• Ion channels are integral membrane proteins that allow ions to cross the membrane.
• Different types of ion channels: leak channels, gated channels (ligand-gated, mechanically-gated, voltage-gated).

Carrier Proteins

• Carrier proteins are integral membrane proteins that facilitate the passive or active transport of molecules across the plasma membrane.
• Illustrate how solutes bind to receptors and facilitate movement.

Primary Active Transport

• Examples include Na+/K+ pumps, H+ pumps, Ca++ pumps, H+/K+ pumps
• Shows a detailed step-by-step process.

Secondary Active Transport

• The use of an ion gradient is utilized to drive molecule movement (usually an ion) across the membrane.

Specific Transport Systems

• Summarize and illustrates how specific transport systems, including facilitated diffusion, are used.

Explore the mechanisms of membrane transport, focusing on the structure of cell membranes and the roles of various membrane proteins. This quiz also covers the Fluid Mosaic Model which describes the dynamic nature of the membrane. Test your understanding of how substances move across cellular membranes.