Chapter 5: Membrane Transport & Cell Signaling

Overview: Life at the Edge

The plasma membrane is a fundamental structure that separates the cell from its external environment. It is selectively permeable, meaning it allows certain substances to cross more easily than others, thereby maintaining the internal conditions necessary for life.

• Plasma membrane: The boundary that defines the cell and regulates the movement of materials in and out.

• Selectively permeable: Only specific molecules can pass through freely; others require assistance.

• Function: Maintains homeostasis by controlling the exchange of substances.

• Example: Oxygen and carbon dioxide can diffuse across the membrane easily, while ions and large molecules require transport proteins.

Cell Membranes as Fluid Mosaics of Lipids & Proteins

Structure of Membranes

Cell membranes are primarily composed of amphipathic lipids and proteins, with some carbohydrates present. The fluid mosaic model describes the membrane as a dynamic structure with proteins and lipids moving within the bilayer.

• Amphipathic lipids: Molecules with both hydrophobic (water-repelling) and hydrophilic (water-attracting) regions. Phospholipids are the main type.

• Proteins (amphipathic): Integral and peripheral proteins are embedded or attached to the membrane, contributing to its function.

• Carbohydrates: Usually found on the extracellular surface, attached to lipids (glycolipids) or proteins (glycoproteins).

• Fluid mosaic model: The membrane is a mosaic of proteins floating in or on the fluid lipid bilayer, similar to icebergs in the sea.

Fluidity of Membranes

Membrane components are not static; they move laterally within the bilayer. Fluidity is essential for membrane function and is influenced by lipid composition and temperature.

• Lateral movement: Phospholipids and proteins can shift sideways rapidly.

• Flip-flop movement: Rare movement of lipids from one leaflet to the other.

• Anchoring: Some proteins are anchored by the cytoskeleton or extracellular matrix, restricting their movement.

• Fluidity factors: Unsaturated fatty acid tails increase fluidity; saturated tails decrease it. Cholesterol modulates fluidity depending on temperature.

Key Terms and Concepts

• Phospholipid bilayer: The fundamental structure of the membrane, consisting of two layers of phospholipids with hydrophobic tails facing inward and hydrophilic heads facing outward.

• Integral proteins: Span the membrane and are involved in transport and signaling.

• Peripheral proteins: Attached to the surface of the membrane, often involved in signaling or maintaining cell shape.

• Glycoproteins and glycolipids: Play roles in cell recognition and communication.

Examples and Applications

• Example: The sodium-potassium pump is an integral protein that maintains ion gradients essential for nerve impulse transmission.

• Application: Membrane fluidity affects the ability of cells to adapt to temperature changes and to fuse with other membranes during processes like endocytosis and exocytosis.

Table: Comparison of Membrane Components

Additional info: The fluid mosaic model is supported by experimental evidence such as freeze-fracture electron microscopy, which shows the distribution of proteins within the membrane.