cell membrane structure and function u · cell membrane structure and function • activity 9...

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9 cell membrane structure and function

Understanding the cell membrane is the key to understanding many dis-

eases, and is of great value in developing treatments. Some diseases dis-

able the ability of the cell membrane to reseal after it has been penetrated, and

some destroy the function of the cell’s transport channels. The immune system

recognizes foreign cells through receptors on the membrane of both the foreign

and immune cells. Many disease-causing pathogens, such as the HIV/AIDS virus

and the polio virus, infect humans by recognizing and binding to the cell mem-

brane, and then entering the cell. Preventing and treating such diseases as HIV/

AIDS can improve the quality and length of people’s lives and improve their social

and economic well-being.

Challengehow do the structures of the cell membrane help it function?00

Procedure 1. When reading, answer the Stopping to Think Questions in your mind.

ReadingMolecular Building Blocks of Cells and the Cell MembraneFour types of large molecules, called macromolecules, are essential building

blocks for all of the structures in a cell. These macromolecules are carbohydrates,

lipids, proteins, and nucleic acids. Each type of macromolecule is made of one or

more chains of simpler compounds, or subunits. The table below shows the kinds

of subunits that make up each type of macromolecule.

Macromolecule Subunits MaCroMoleCule SuBunitS

Carbohydrates sugars

lipids fatty acids

Proteins amino acids

nucleic acids nucleotides

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The cell membrane is made mostly of lipids and proteins. The

lipids give the membrane its strength, yet allow it to remain flex-

ible and fluid. The bubble film in Activity 7, “A Model Mem-

brane,” modeled the lipid layer that is the basis for the structure

of the cell membrane. Most of the lipids in the cell membranes of

animals, plants, protists, and bacteria are members of a special

class of lipids, called phospho lipids. A phospholipid

is a lipid with a phosphate head attached to two long

lipid tails.

The structure of the membrane results from the

properties of the phospholipids. The heads of phos-

pholipid molecules are attracted to water, so they face the watery

environment inside and outside the cell. The tails line up and point

toward the middle of the membrane. This results in a double-layered

structure called a phospholipid (lipid) bilayer, as shown at right.

Proteins are embedded in the membrane’s phospholipid bilayer, also

shown at right. Some of these proteins are linked to carbohydrate chains.

Membrane proteins are able to move sideways through the membrane,

just as you were able to move tubes through the bubble membrane. The

complex arrangement of proteins in the membrane reminded scientists

of the tiles in a mosaic. This led scientists to refer to the structure of the

cell membrane as a fluid mosaic model, shown below.

There are many proteins in the membrane of a typical cell. One kind of pro tein acts

as channels or pumps, controlling what enters and leaves the cell. You modeled these

channels when you inserted straws and tubes through the bubble membrane. You

also modeled the glucose channel with the dialysis membrane.

cell membrane sTrucTure and funcTion • acTiviTy 9

A phospholipid molecule

The phospholipid bilayer of the cell membrane with an embedded protein

The fluid mosaic model for the cell membrane

Science & Global iSSueS/bioloGy • cell bioloGy

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Other membrane molecules made up of proteins or proteins linked to carbohy-

drates act as signaling or receptor proteins. These molecules are involved in cell-

to-cell communication. An example of a receptor is the membrane protein that

binds to insulin hormone. The body needs insulin to regulate blood glucose levels.

When blood glucose levels are high, insulin attaches to receptors in the mem-

branes of liver and muscle cells. This stimulates the cells to take up glucose from

the blood and store it as glycogen or fat.

Stopping to think 1

What are the molecular building blocks that make up the cell membrane?

How are these molecules arranged in the cell membrane?

What are the functions of these molecules in the membrane?

the Cell Membrane Controls What enters and leaves the Celldiffusion and osmosis

The cell membrane is semipermeable, or selectively permeable, which means

not all substances can cross it. This property helps the cell maintain homeostasis—

stable internal conditions—by controlling what enters and leaves. You modeled this

property of the membrane with the dialysis tubing in the last activity: small mole-

cules, like water and glucose, could cross the dialysis membrane, while large mol-

ecules, like sucrose and starch, could not. Size is one factor that determines whether

a molecule can cross the cell membrane. Other factors determining whether cer-

tain molecules can cross a cell membrane are the molecules’ shape and electrical

charge.

For the cell to function properly, it must allow desirable substances, such as nutri-

ents, to enter, and allow wastes to leave. Many of these substances enter and leave

the cell by diffusing from an area of high concentration to an area of low con-

centration. When a substance moves naturally from high to low concentration,

the cell does not have to expend energy. For example, oxygen diffuses from a high

concentration outside the cell across the membrane into the cell. As the oxygen

is used up and the concentration in the cell drops, more oxygen enters the cell.

Similarly, as carbon dioxide builds up inside the cells of the body it diffuses from

high concentration across the cell membrane to lower concentration outside

of the cell.

Water also diffuses naturally in both directions across the cell membrane. The

overall direction of movement of water depends on the concentration of dissolved

substances inside and outside the cell. Diffusion will continue until the two solutions

have an equal concentration of the dissolved substance. When a cell is placed in a

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solution with a higher concentration of dissolved sub-

stances, water will move out of the cell, and the cell will

shrink, as shown in the diagram at right. When a cell is

placed in a solution with a lower concentration of dis-

solved substances, water will enter the cell and the cell

will swell. This diffusion of liquid water across a semi-

permeable membrane is called osmosis.

passive transport

Not all substances used or produced by the cell can freely diffuse through its mem-

brane, especially substances that are electrically charged or are very large in size.

Proteins in the cell membrane must transport the substances in or out. When a

mem brane protein moves a substance from higher to lower concentration through

a protein channel, the process is called facilitated diffusion, or passive transport,

because it does not require energy. An example of facilitated diffusion is the move-

ment of glucose from high concentrations outside the cell into the cell through a

glucose transport protein.

active transport

In some cases, proteins transport substances into or out of a cell against the

normal direction of diffusion. The molecules of the substance are moving from

low to high concentration. This process is called active transport (shown below),

and it requires the cell to expend energy. An example of active transport is the

pumping of substances like calcium and sodium out of the cell.

The active transport of a substance from low to high concentration requires energy.


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Exocytosis

Endocytosis

transport by vesicles

Sometimes materials are transported across the cell membrane by vesicles, which are

small compartments surrounded by a membrane. To transport materials into the cell,

the membrane forms a pocket that then pinches into the cell, forming a vesicle. This

formation and movement of vesicles into the cell is called endocytosis. To transport

materials out of the cell, a vesicle in the cytoplasm fuses with the cell membrane, and

re-forms a smooth outer membrane, releasing its contents to the outside. This pro-

cess of releasing material in vesicles to the outside of the cell is called exocytosis. Both

of these processes are shown below. The fluid nature of the membrane is key to its

ability to form and release vesicles. The movement of the vesicles within the cell

involves the cytoskeleton and requires energy.

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Stopping to think 2

What types of movement happen across the cell membrane?

Which type of movement(s) across the cell membrane need(s) energy? Why?

Viruses and the Cell MembraneThe causes of many human infectious diseases are viruses that enter our cells.

HIV/AIDS is a well-known example. Like all viruses, HIV/AIDS is not made of

cells, and is not capable of producing offspring on its own. Instead, a virus must

invade a cell in order to reproduce. Viruses can infect cells in plants and animals,

and can even infect bacteria. A virus is mainly composed of its genetic material and

a few proteins, surrounded by a protein coat, and sometimes a lipid membrane. To

make copies of itself, the virus invades a cell and uses the cell’s structures. Because

of this, the invaded cell is called a host cell. Most viruses don’t infect every type of

cell in the body. Instead, each virus infects one or a few types of cells. HIV/AIDS, for

example, only infects cells of the immune system. When the virus comes into con-

tact with an immune system cell, it must first cross the cell membrane. If the virus

does not cross that first barrier, it does not infect the cell, cannot produce more

viruses, and will eventually be destroyed.

Stopping to think 3

What makes a virus different from a bacterial or animal cell?

Why do viruses need host cells?

Viruses can infect many sorts of organisms, including animals, plants, and bac-

teria. They attach to a cell by interacting with the lipids, proteins, or carbohydrates

of the cell membrane. Some viruses, such as those that infect bacteria, invade a cell

by making a hole in the membrane and injecting themselves into the cell through

the hole. Other viruses, like HIV, have an external membrane similar to the cell’s

membrane. The virus membrane fuses with the cell membrane, allowing the virus

to enter the cell. Still other viruses, such as flu viruses, get a “free ride” into the cell

through the cell’s endocytosis mechanism. In endocytosis the part of the cell mem-

brane in contact with the virus surrounds the virus and pinches into the cell, in a

process similar to the one shown on the previous page. Once the virus is inside the

cell it will use others of the cell’s structures to make more viruses. These new viruses

then exit the cell in one of two ways: 1) by pinching off, or budding, from the host

cell without destroying the host cell, as is the case with HIV/AIDS; or 2) by breaking

the host cell open, killing it in the process, as is the case with polio virus.


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Analysis 1. What functions and properties of the cell membrane depend on each of the

following?

a. Phospholipids

b. Proteins

c. Carbohydrates

2. Explain why it is important for the cell membrane to be fluid.

3 a. What are the functions of the cell membrane?

b. Explain which parts of the cell membrane allow it to perform these

functions.

4. What determines whether or not a substance can cross the cell membrane?

5. Explain how the cell membrane helps the cell maintain homeostasis—a stable

internal environment. Name specific structures of the cell membrane and

describe their functions in your explanation.

6. Imagine a single-celled organism living in a pond. What would happen to the

organism if runoff from irrigation caused the pond to become significantly

salty? Use evidence to support your explanation.

Key vocabulary

active transport macromolecule

cellmembrane,membrane osmosis

diffusion passive transport

endocytosis phospholipid

exocytosis phospholipid(lipid)bilayer

facilitated diffusion protein

fluid mosaic model semi-permeable, selectively permeable

lipid vesicle

lipidbilayer virus

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