Anatomy & Physiology

Anatomy & Physiology Study Guide

Plasma Membrane The outer boundary of the cell is the plasma membrane , also called the cell membrane or plasmalemma. It is extremely thin and delicate and ranges from 0.6 to 1 nm in thickness. This membrane contains lipids, proteins, and carbohydrates. Its general functions include the following: • Physical isolation: The plasma membrane is a physical barrier that separates the inside of the cell from its surroundings, primarily extracellular fluid. The difference in conditions inside and outside the cell must be maintained to preserve homeostasis. For example, the plasma membrane keeps organelles, enzymes, and certain proteins inside the cell while it maintains certain levels of ions intracellularly. • Regulation of exchange with the environment: The plasma membrane controls the movement of ions and nutrients, such as sodium, potassium, glucose, and the release of secretions. • Sensitivity to the environment: The plasma membrane is sensitive to changes in the extracellular fluid. It can recognize and react to differences in composition and pH through receptors that allow the cell to respond to specific molecules. • Structural support: Tissue stability is maintained through specialized connections between plasma membranes (or between membranes and extracellular materials). For example, the epidermal cells at the surface of the skin are bound together to protect underlying structures. 3.2 Material Transport The plasma membrane is a barrier that isolates the cytoplasm from the extracellular fluid. Because the plasma membrane is an effective barrier, conditions inside the cell can be considerably different from conditions outside the cell. However, the barrier is not impenetrable because cells are not self- sufficient. Each day they require nutrients to provide the energy they need to stay alive and function normally. They also generate waste products that must be eliminated. Permeability is the property of the plasma membrane that determines which substances can enter or leave the cytoplasm. A membrane through which nothing can pass is described as impermeable. Amembrane through which any substance can pass without difficulty is freely permeable. The permeability of plasma membranes lies somewhere between those extremes; thus, plasma membranes are said to be selectively permeable . A selectively permeable membrane permits the free passage of some materials and restricts the passage of others. The distinction may be based on size, electrical charge, molecular shape, lipid solubility, or other factors. Passage across the membrane is either passive or active. Passive processes move ions or molecules across the plasma membrane with no output of energy by the cell. Active processes require that the cell expends energy, generally in the formof ATP. Transport processes are also categorized by the mechanism involved. The three major categories are diffusion, carrier-mediated transport, and vesicular transport. Diffusion Over time, the molecules in any given space will tend to become evenly distributed. This distribution process is called diffusion. As molecules move around, they will move from areas of higher concentration to areas of lower concentration. The difference between the high and low

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