Cells: Plasma Membranes – Structure, Composition, and Function

The Cell

Basic Cell Structure and Function

Cells are the fundamental units of life, each specialized to perform distinct functions within the body. Despite their diversity, all cells share three essential components:

• Plasma membrane: The cell's outer boundary, which is selectively permeable and regulates the movement of substances in and out of the cell.

• Cytoplasm: The intracellular fluid containing water, salts, and organic molecules.

• Nucleus: The control center of the cell, responsible for regulating cellular activities.

Prefix: "cyto-" (e.g., cytoplasm) refers to cells. Suffix: "-cyte" (e.g., osteocyte) denotes a cell type.

Example: Muscle cells are specialized for contraction, while nerve cells are specialized for signal transmission.

Plasma Membrane

Fluid Mosaic Model

The Fluid Mosaic Model describes the dynamic structure of the plasma membrane. It consists of a phospholipid bilayer with proteins dispersed throughout, creating a flexible and selectively permeable barrier between the intracellular and extracellular environments.

• Intracellular fluid (ICF): Located inside the cell.

• Extracellular fluid (ECF): Located outside the cell.

Example: The plasma membrane separates the cytoplasm from the surrounding tissue fluid.

Chemical Composition of Membranes

Phospholipids and Cholesterol

The plasma membrane is primarily composed of phospholipids and cholesterol, which contribute to its structure and function.

• Phospholipids: Form the basic structure of the membrane. Each molecule has a hydrophilic (water-loving) phosphate head and two hydrophobic (water-fearing) fatty acid tails. The heads face the aqueous environments inside and outside the cell, while the tails form the interior of the membrane.

• Cholesterol: Interspersed within the bilayer, cholesterol molecules provide structural support and increase membrane stability by "stiffening" the membrane.

Effect on water-soluble molecules: The hydrophobic core prevents most water-soluble substances from freely crossing the membrane.

Composition of Cells: Proteins

Types of Membrane Proteins by Location

Proteins are crucial for the specialized functions of cells. Membrane proteins are classified by their location:

• Integral proteins: Embedded within the plasma membrane; some span the entire membrane (transmembrane proteins).

• Peripheral proteins: Loosely attached to integral proteins; not embedded in the lipid bilayer.

Example: Channel proteins are integral proteins that allow ions to pass through the membrane.

Types of Membrane Proteins by Function

• Transport proteins: Move substances into and out of the cell. They are selective, meaning only specific molecules are transported based on size, charge, or other properties.

• Receptor proteins: Relay messages to the cell interior when exposed to chemical messengers (e.g., hormones). The cell's response depends on the receptor's specificity.

• Enzymes: Catalyze chemical reactions, either alone or as part of a team for sequential steps.

• Cell-cell recognition proteins: Enable cells to identify each other, which is essential for immune response and tissue formation.

• Attachment proteins: Anchor membrane proteins to the cytoskeleton or extracellular matrix, maintaining cell shape.

• Intercellular junctions: Link cells together, allowing them to adhere temporarily or permanently.

Example: Insulin binds to its receptor protein, triggering a cascade of cellular responses.

Composition of Cells: Carbohydrates

Role of Carbohydrates and the Glycocalyx

Carbohydrates are present on the extracellular surface of the plasma membrane, often attached to lipids (glycolipids) or proteins (glycoproteins). These form the glycocalyx, a fuzzy coating that serves several functions:

• Cell identification: Different arrangements of carbohydrates allow cells to be recognized by other body cells.

• Barrier function: The glycocalyx can prevent very large substances from entering the cell.

Example: Immune cells use the glycocalyx to distinguish self from non-self cells.

Component	Location	Main Function
Phospholipids	Bilayer core	Structural barrier, selective permeability
Cholesterol	Interspersed in bilayer	Membrane stability
Integral proteins	Embedded in membrane	Transport, signaling
Peripheral proteins	Surface of membrane	Support, signaling
Carbohydrates	Extracellular surface	Cell recognition, barrier

Additional info: The notes above are expanded with academic context and examples for clarity and completeness.