© 2014 Pearson Education, Inc. 5.1 How Is the Structure of the Cell Membrane Related to Its Function? All the membranes of a cell have a similar basic

© 2014 Pearson Education, Inc.

5.1 How Is the Structure of the Cell Membrane Related to Its Function?

All the membranes of a cell have a similar basic structure

– Proteins suspended in a double layer of phospholipids

Phospholipids are responsible for the isolating function of membranes

Proteins are responsible for selectively exchanging substances and communicating with the environment, controlling biochemical reactions, and forming attachments



Functions of the plasma membrane

– It isolates the cell’s contents from the external environment

– It regulates the exchange of essential substances

– It allows communication between cells

– It creates attachments within and between cells

– It regulates biochemical reactions



Membranes are “fluid mosaics” in which proteins move within layers of lipids

– The “fluid mosaic” model of a membrane was proposed in 1972 by S.J. Singer and G.L. Nicolson

– This model indicates that each membrane consists of a mosaic, or “patchwork,” of different proteins that constantly shift and flow within a viscous fluid formed by a double layer of phospholipids

– A fluid is any substance whose molecules can flow past one another and includes gases, liquids, and cell membranes


Figure 5-1 The plasma membrane

cholesterol

interstitial fluid (outside)

cytosol (fluid inside cell)

enzyme

cytoskeleton

recognitionprotein

binding site

connectionprotein

phospholipid

receptorprotein

glycoprotein

phospholipidbilayer

pore

transportprotein

carbohydrate

proteinextra-cellularmatrix



The fluid phospholipid bilayer helps to isolate the cell’s contents

– Phospholipids are the basis of membrane structure and consist of two very different parts

– A polar, hydrophilic head

– Two nonpolar, hydrophobic tails

– The outer surfaces of animal plasma membranes are bathed in watery interstitial fluid, a weakly salty liquid resembling blood without its cells or proteins


Figure 5-2 A phospholipid

tails (hydrophobic)

head (hydrophilic)



The fluid phospholipid bilayer helps to isolate the cell’s contents (continued)

– The phospholipid bilayer is the fluid portion of the membrane

– Plasma membranes face both exterior and interior watery environments




– Hydrophobic and hydrophilic interactions drive phospholipids into bilayers

– Hydrogen bonds between water and the hydrophilic heads cause the heads of the outer layer to orient outward toward the watery exterior, while the heads of the inner layer face the watery interior

– The nonpolar tails, being hydrophobic, face the inside of the membrane, away from the watery environment around the bilayer

© 2014 Pearson Education, Inc. Animation: Plasma Membrane Structure


Figure 5-3 The phospholipid bilayer of the cell membrane

interstitial fluid (watery environment)

cytosol (watery environment)

hydrophilic head

hydrophilic head

hydrophobic tails

phospholipid

bilayer




– The phospholipid bilayer’s flexible, fluid membrane allows for cellular shape changes

– Individual phospholipid molecules are not bonded to one another

– Some of the phospholipids have unsaturated fatty acids, whose double bonds introduce “kinks” into their “tails”

– The above features make the membrane fluid


Figure E5-1 Tail kinks in phospholipids increase membrane fluidity

more unsaturatedgreater fluidity

more saturatedless fluidity




– Water-soluble substances such as salts, amino acids, and sugars cannot easily cross phospholipid bilayers

– However, the isolation provided by the plasma membrane is not complete, and very small molecules such as water, oxygen, and carbon dioxide as well as larger, lipid-soluble molecules can pass through this selective barrier




– Membranes become more fluid at high temperatures (more movement) and less fluid at low temperatures (less movement)

– Cell membranes of organisms living in low temperatures tend to be more unsaturated (more kinks help them maintain fluidity)

– Cholesterol stabilizes membranes, affecting fluidity and reducing permeability



A variety of proteins form a mosaic within the membrane

– Proteins are embedded within, or attached to, the phospholipid bilayer

– Many proteins have attached carbohydrates (glycoproteins) on their outer membrane surface



A variety of proteins form a mosaic within the membrane (continued)

– Membrane proteins may be grouped into five major categories

– Enzymes

– Recognition proteins

– Receptor proteins

– Connection proteins

– Transport proteins




– Enzymes are proteins that promote chemical reactions that synthesize or break apart biological molecules

– Some plasma membrane enzymes are used to synthesize the extracellular matrix, a web of protein and glycoprotein fibers that fills spaces between animal cells




– Receptor proteins trigger cellular responses upon binding of specific molecules, such as hormones, sent by other cells

– Recognition proteins are glycoproteins that serve as identification tags on the surface of a cell


Figure 5-4 Receptor protein activation

messengermolecule

receptorprotein

(interstitial fluid)

(cytosol)

A messenger moleculebinds to the receptor protein

Messenger moleculebinding activates thereceptor protein,changing its shape

The activated receptor protein stimulates aresponse in the cell




– Connection proteins anchor cell membranes in various ways

– Maintain cell shape by linking the plasma membrane to the cell’s cytoskeleton

– Link the cytoskeleton inside the cell with the extracellular matrix outside, anchoring the cell in place within a tissue

– Form connections between adjacent cells




– Transport proteins regulate the movement of hydrophilic molecules through the plasma membrane

– Channel proteins are pores that can be opened or closed to allow specific substances to pass across the membrane

– Carrier proteins bind substances and carry them through the membrane, sometimes using cellular energy

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© 2014 Pearson Education, Inc. 5.1 How Is the Structure of the Cell Membrane Related to Its Function? All the membranes of a cell have a similar basic