5.1 - 5.3 Flashcards
Fluid Mosaic Model
The currently accepted cell membrane and structure, depicting the membrane as a mosaic of protein molecules suspended in a fluid bilayer of phospholipid molecules.
Note 1 —-»
Biologists use the fluid mosaic model to describe a membrane’s structure-diverse protein molecules suspended in a fluid phospholipid bilayer. This module illustrates the structure and function of a plasma membrane, the boundary that encloses a living cell. Like all cellular membranes, the plasma membrane exhibits selective permeability; that is, it allows some substances to cross more easily than others. But the plasma membrane does more than just regulate the exchange of materials. This figure will help you visualize all the activity taking place in and across the membranes of two adjacent cells.
Selective Permeability
A property of biological membranes that allows them to regulate the passage of substances across them.
Can you identify six different types of functions of proteins in a plasma membrane?
Attachment to the cytoskeleton and ECM, signal reception and relay, enzymatic activity, cell-cell recognition, intercellular joining, and transport
Note 2 —-»
Phospholipids could spontaneously self-assemble into simple membranes. Indeed, this property can be demonstrated by shaking a mixture of phospholipids and water—the phospholipids organize into bilayers surrounding water-filled bubbles
Note 3 —-»
The formation of membrane-enclosed collections of molecules would have been a critical step in the evolution of the first cells. A membrane can enclose a solution that is different in composition from its surroundings. If that solution included self-replicating molecules such as RNA and a beneficial assortment of other molecules, these combinations could be passed on to daughter cells as new membrane-enclosed vesicles budded off. A membrane that not only encloses a successful assembly of molecules but also regulates chemical exchanges with the environment is a basic requirement for life. Indeed, all cells are enclosed by a membrane similar in structure and function—illustrating the evolutionary unity of life.
In the origin of a cell, why would the formation of a simple lipid bilayer membrane not be sufficient? What else would have to be part of such a membrane?
The membrane would need embedded proteins that could regulate the movement of substances into and out of the cell.
Diffusion
The random movement of particles, that results in the net movement of a substance down its concentration gradient from a region where it is more concentrated to a region where it is less concentrated.
Note 4 —-»
Randomly moving molecules will diffuse through air and water—or into and out of a cell.
Note 5 —-»
Although each molecule moves randomly, there will be a net movement from the side of the membrane where dye molecules are more concentrated to the side where they are less concentrated. Put another way, the dye diffuses down its concentration gradient. Eventually, the solutions on both sides will have equal concentrations of dye. At this dynamic equilibrium, molecules still move back and forth, but there is no net change in concentration on either side of the membrane.
Passive Transport
The diffusion of a substance across a biological membrane, with no expenditure of energy.
Note 6 —-»
Much of the traffic across membranes occurs by diffusion. For example, diffusion down concentration gradients is the sole means by which oxygen (O2), essential for the process of cellular respiration, enters your cells, and carbon dioxide (CO2), a metabolic waste, passes out of them.
Note 7 —-»
Both O2 and CO2 are small, nonpolar molecules that diffuse easily across the phospholipid bilayer of a membrane. But can ions and polar molecules also diffuse across the hydrophobic interior of a membrane? They can if they are moving down their concentration gradients and if they have transport proteins to help them cross.
Why is diffusion across a membrane called passive transport?
The cell does not expend energy to transport substances that are diffusing down their concentration gradients.
