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Flashcards in Ch. 5 Deck (23)
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composed of phospholipids & proteins fluid mosaic "mosaic" - diverse protein molecules embedded in a framework of phospholipids "fluid" - most of these molecules can drift abt in the membrane



double bonds in the unsaturated fatty acid tails of many phospholipids produce kinks that prevent phospholipids from packing tightly together



steroid wedged into the bilayer helps stabilize the membrane at warm temp's but also keeps the membrane fluid at lower temp's



refers not only to the position of proteins in the phosph. bilayer but also to the varied functions of these proteins diff types of cells contain diff membrane proteins, & the various membranes w/in a cell each have unique proteins


functions of membrane proteins

stronger framework - integrins

cell-cell recognition - glycoproteins

junctions - plasma membrane proteins



integrins span the membrane & attach to the cytoskeleton on the inside & the extracellular matrix (ECM) on the outside


cell-cell recognition


the outside surface of the membrane has carbs (chains of sugars, blue in Figure 5.1A) bonded to proteins or lipids in the membrane

the carbs vary among species, among individuals, & even among cell types

many function as identificiation tags that are recognized by other cells; this cell-cell recognition allows cells in an embryo to sort themselves into tissues & organs; also enables cells of immune system to recognize & reject foreign cells, such as infectious bacteria


forming junctions

specific to plasma membrane proteins

b/w cells



many membrane proteins are enzymes

enzymes may work as a team to carry out sequential steps in a pathway as shown below

other proteins function as receptors



other proteins function as receptors for chemical messengers from other cells

a receptor protein has a shape that fits a specific messenger, such as a hormone

often the binding of the messenger to the receptor triggers a chain reaction involving other proteins, which relay the message to molecules that perform specific functions inside the cell

this message-transfer process is called signal transduction



membranes exhibit selective permeability; they allow some substances to cross more easily than others

their hydrophobic interior is 1 reason why - nonpolar, hydrophobic molecules can easily pass thru membranes. in contrast, polar molecules & ions rn't soluble in lipids

many essential molecules, such as glucose & ions, require transport proteins to enter or leave the cell


the 6 different types of functions that membrane proteins can perform

support by attaching to the cytoskeleton & ECM

cell-cell recognition

intercellular junctions


signal transduction



membranes form spontaneously, a critical step in the origin of life

phospholipids, the key ingredients of biological membranes, were prob among the 1st organic molecules that formed from chemical reactions on early Earth

these lipids could spontaneously self-assemble into simple membranes, as we can demonstrate in a test tube (and below)

when a mixture of phospholipids & water = shaken, the phospholipids organize into bilayers surrounding water-filled bubbles. requires neither genes nor other info beyond the prop's of the phospholipids themselves


the importance of membranes

the formation of membrane-enclosed collections of molecules was a critical step in the evolution of the 1st cells

a membrane can enclose a solution that is diff in composition from its surroundings

a plasma membrane that allows cells to regulate their chemical exchanges with the environ is a basic requirement for life - all cells r enclosed by a plasma membrane that is similar in structure & function - one of the features that illustrates the evolutionary unity of life


describe the structure of a membrane

the phospholipids form a bilayer

the hydrophobic fatty acid tails cluster in the center and the hydrophilic phosphate heads face the water on both sides



the tendency for particles of any kind to spread out evenly in an available space, moving from where they are more concentrated to regions where they r less concentrated

molecules vibrate & move randomly as a result of a type of energy called thermal motion (heat)

requires no work; results from the thermal motion of atoms & molecules


concentration gradient

  1. Figure 5.3A shows a solution of green dye sep-ed from pure water by a membrane permeable to the dye

although each molecule moves randomly, there will be a net movement from the side of the membrane where dye molecules r more concentrated to the side where they're less concentrated

put another way, the dye diffuses down its concentration gradient

eventually, the solutions on both sides have = concentrations of dye; at this dynamic equilibriummolecules still move back & forth, but there is no net change in concentration on either side of the membrane

  1. Figure 5.3B illustrates the important pt that 2+ substances diffuse independently of each other; that is, each diffuses down its own concentration gradient


passive transport

diffusion across a membrane w/ no energy investment

much of the traffic across cell membranes occurs by diffusion

in our lungs, diffusion down concentration gradients is the sole means by which oxygen (O2), essential for metabolism, enters red blood cells & carbon dioxide (CO2), a metabolic waste, passes out of them

both O2 and CO2 r small, nonpolar molecules that diffuse easily across the phospholipid bilayer of a membrane, but ions & polar molecules can only move by passive transport  if they're moving down their concentration gradients and they have transport proteins to provide a pathway across the membrane


Why is diffusion across a membrane called passive transport?

The cell doesn't expend energy to trasnport substances that  are diffusing down their concentration gradients



the diffusion of water across a membrane


a selectively permeable osmosis example

Figure 5.4A shows what happens if a membrane permeable to water but not to a solute (such as glucose) separates 2 solutions w/ diff concentrations of solute

the solution on the right side initially has a higher concentration of solute than that on the left

as u can see, water crosses the membrane until the solute concentrations (molecules/mL of solution) r equal on both sides


a close-up view

below, u can see what happens @ the molecular level

clusters of polar water molecules form weak bonds w/ solute molecules, so that fewer water molecules r free to diffuse across the membrane

the less concentrated solution on the left, w/ fewer solute molecules, has more free water molecules

there is a net movement of water down its own concentration gradient, from the solution w/ the lower solute concentration & more free water molecules to that w/ the higher solute concentration & fewer free water molecules

the direction of osmosis is determined by the diff in total solute concentration, not by thenature of the solutes