Study Notes: Cells, Plasma Membrane, and Membrane Proteins (Chapters 3-5, Anatomy & Physiology)

Cells (Chapter 3)

General Structure and Function of Cells

Cells are the basic structural and functional units of all living organisms. Each cell is surrounded by a plasma membrane, contains a nucleus, and is bathed in extracellular fluid.

• Plasma Membrane: Separates the interior of the cell from the external environment, maintaining cellular integrity.

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

• Extracellular Fluid/Matrix: Surrounds the cell and provides nutrients, signaling molecules, and structural support.

Key Functions of the Plasma Membrane:

• Physical Barrier: Separates the cell's internal environment from the external environment.

• Communication: Transports and communicates with other cells, relaying information about environmental changes.

• Compartmentalization: Allows separate functions within the cell by forming distinct compartments.

• Biochemical Reactions: Participates in and regulates biochemical reactions.

• Selective Permeability: Controls the volume and internal composition of the cell's ions and molecules.

Plasma Membrane Structure

Components of the Plasma Membrane

The plasma membrane is a dynamic structure composed primarily of lipids and proteins, with carbohydrates attached to some proteins and lipids.

• Phospholipids: Consist of a glycerol backbone, two fatty acid tails (hydrophobic), and a phosphate group (hydrophilic). This amphipathic nature allows them to form bilayers.

• Cholesterol: Interspersed within the phospholipid bilayer, cholesterol stabilizes membrane fluidity and structure.

• Proteins: Integral and peripheral proteins serve various functions, including transport, signaling, and structural support.

• Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids), they play roles in cell recognition and adhesion.

Phospholipid Bilayer: Forms liquid bilayers with hydrophilic heads facing outward and hydrophobic tails facing inward, creating a semi-permeable barrier.

Fluid Mosaic Model: Proposed by Singer and Nicholson (1972), this model describes the membrane as a flexible, self-sealing structure with proteins embedded in or attached to a fluid lipid bilayer.

• Self-sealing: The membrane can reseal itself after minor damage.

• Dynamic: Components can move laterally within the layer, allowing for flexibility and repair.

Membrane Proteins

Types and Functions of Membrane Proteins

Membrane proteins are essential for the diverse functions of the plasma membrane. They can be classified based on their location and function.

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

• Peripheral Proteins: Attached to the membrane surface, often involved in structural support or signaling.

Major Functions of Membrane Proteins:

• Transport: Form channels or carriers for selective passage of materials (e.g., ions, nutrients).

• Enzymatic Activity: Catalyze reactions on the membrane surface.

• Cell Adhesion: Link cells together and anchor them to the extracellular matrix (e.g., integrins).

• Signal Transduction: Receptors bind with signal molecules (e.g., hormones), transmitting information into the cell.

• Cell Recognition: Glycoproteins act as identification tags (antigens), allowing cells to be recognized by the immune system.

• Intercellular Joining: Cell adhesion proteins attach membranes of adjacent cells, forming junctions.

Examples:

• Integrins: Anchor cells to the extracellular matrix and connect to microfilaments within the cell.

• Transport Proteins: Channels and pumps that regulate the movement of substances across the membrane.

• Receptors: Bind hormones and other signaling molecules, initiating cellular responses.

• Glycoproteins: Serve as antigens for immune recognition.

Additional info: The diversity of membrane proteins allows cells to interact with their environment, communicate with other cells, and maintain homeostasis.