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Ch. 7: Membrane Structure and Function. Introduction. The plasma membrane is selectively permeable . The macromolecules that make up the PM are lipids, proteins, and carbohydrates. Phospholipids make up most of the PM. Phospholipids are amphipathic molecules.

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Ch. 7: Membrane Structure and Function

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Ch. 7:


Structure and Function

  • Introduction

  • The plasma membrane is selectively

  • permeable.

  • The macromolecules that make up the PM

  • are lipids, proteins, and carbohydrates.

  • Phospholipids make up most of the PM.

  • Phospholipids are amphipathic

  • molecules.

  • A model of the PM is known as the

  • “Fluid Mosaic Model.”

  • This model shows that the PM is a fluid

  • structure, with proteins embedded or

  • attached to a double layer of

  • phospholipids.

  • The PM is not rigid. There is lateral


  • The phospholipids move rapidly. Protein movement

    depends on size and attachment to cytoskeleton.

  • Membrane fluidity also depends on temperature.

Hydrophilic region

of protein



Hydrophobic region of protein









Extracellular layer

Cytoplasmic layer

  • Phospholipids with

    unsaturated fatty

    acids are more fluid.

  • Cholesterol


    membrane fluidity.

Lateral movement

(~107 times per second)


(~ once per month)

Movement of phospholipids



Saturated hydro-

carbon tails

Unsaturated hydrocarbon

tails with kinks

Membrane fluidity


Cholesterol within the animal cell membrane

  • Membranes are mosaics of structure and

  • function

  • Proteins determine the function of the

  • PM.

  • There are two types of membrane protein:

  • Integral proteins: spans the entire

  • membrane.







a Helix

B. The six major functions of membrane




Attachment to the

cytoskeleton and extra-

cellular matrix (ECM)

Cell-cell recognition

Intercellular joining

  • Peripheral proteins: They are not

  • embedded into the bilayer at all.

  • They are loosely bounded to the PM,

  • often to the exposed part of an

  • integral protein.

C. The cytoplasmic and exterior sides of the

PM differ. This difference is determined

as the ER builds the PM. Vesicles fuse

with the PM enlarging the PM.

  • Membrane carbohydrates are important

  • for cell-cell recognition.

1. Cells recognize other cells by keying on

surface molecules, often carbohydrates,

on the plasma membrane.

  • Membrane carbohydrates are usually

  • branched oligosaccharides with fewer

  • than 15 sugar units.

  • They may be covalently bonded either

  • to lipids, forming glycolipids, or, more

  • commonly, to proteins, forming

  • glycoproteins.

  • The oligosaccharides on the external

  • side of the plasma membrane vary from

  • species to species, individual to

  • individual, and even from cell type to

  • cell type within the same individual.

Example: Four human blood types

(A, B, AB, O)

  • Traffic Across Membranes

  • The membrane’s organization results in

  • selective permeability.

  • Sugars, amino acids, and other

  • nutrients enter a muscle cell and

  • metabolic waste products leave.

  • The cell absorbs oxygen and expels

  • carbon dioxide.

  • It also regulates concentrations of

  • inorganic ions, like Na+, K+, Ca2+, and

  • Cl-, by shuttling them across the

  • membrane.

  • Permeability of a molecule through a

  • membrane depends on its interactions

  • with the hydrophobic core of the membrane.

  • Hydrophobic molecules can easily

  • dissolve through the bilayer (hydro-

  • carbons, CO2, and O2).

2. Ions and polar molecules pass through

with difficulty.

Ex. Water & glucose

  • Proteins assist and regulate the

    transport of ions and polar molecules.

  • Polar molecules are transported across

  • the membrane are called transport

  • proteins.

  • Some transport proteins have hydro-

  • philic channels that are used as tunnels.

  • Each transport protein is specific as to

  • the substances that it will translocate

  • (move).

  • Passive transport is diffusion across a

  • membrane.

  • Diffusion is the tendency of molecules

  • of any substance to spread out in the

  • available space

Molecules of dye

Membrane (cross section)


Net diffusion

Net diffusion


Diffusion of one solute

Diffusion is random, until equilibrium

is reached.

  • A substance will diffuse down its

  • concentration gradient. Substance will

  • diffuse from where it is more

  • concentrated to where it is less

  • concentrated, down its concentration

  • gradient.

Net diffusion

Net diffusion



Net diffusion

Net diffusion

Diffusion of two solutes

  • The diffusion of a substance across a

  • biological membrane is passive

  • transport because it requires no

  • energy from the cell to make it happen.

  • Osmosis is the passive transport of water.

1. A solution with the higher concentration

of solutes is hypertonic.

2. A solution with the lower concentration

of solutes is hypotonic.

3. Solutions with equal solute

concentrations are isotonic.

Water molecules will move from a hypotonic

solution to a more hypertonic solution. It

will move across a selectively permeable

membrane until the solutions are isotonic.

  • Cell survival depends on balancing water

  • uptake and loss.

  • An animal cell immersed in an isotonic

  • environment experiences no net

  • movement of water across its plasma

  • membrane. Water flows across the

  • membrane, but at the same rate in both

  • directions.

  • An animal cell in a hypertonic environ-

  • ment will loose water, shrivel, and

  • probably die.

  • A cell in a hypotonic solution will gain

  • water, swell, and burst.

Plant cells have a cell wall that contribute to its water balance.

If a cell is isotonic to it surroundings, there is no movement of water into the cell and the cell is flaccid and the plant may wilt.

In a hypertonic solution, a cell wall has no

advantages. Plasmolysis will take place.

  • For a cell living in an isotonic environ-

  • ment (for example, many marine

  • invertebrates) osmosis is not a problem.

  • The cells of most land animals are bathed

  • in an extracellular fluid that is isotonic to

  • the cells.

  • Organisms without rigid walls have

  • osmotic problems in either a hypertonic

  • or hypotonic environment and must have

  • adaptations for osmoregulation to

  • maintain their internal environment.

Example: Paramecium are hypertonic to

their environment.

  • Water will continually enter the


Paramecium have a specialized organelle, the contractile vacuole, that functions as a pump to force water out of the cell.

  • Specific proteins facilitate the passive

  • transport of water and hydrophilic solutes.

  • When polar molecules pass through the

  • membrane via proteins, this is called

  • facilitated diffusion.

  • Transport proteins are specialized for the

  • solute it transports. It is solute-specific.

3. There are two models of facilitated


  • Channel proteins: allows for fast

  • transport.



  • Gated channels: a stimulus causes

  • them to open or close (chemical or

  • electrical).

Fig. 8.16 Both diffusion and facilitated diffusion are forms of passive transport of molecules down their concentration gradient, while active transport requires an investment of energy to move molecules against their concentration gradient.

  • Active transport is the pumping of solutes

  • against their concentration gradients.

  • Some transport proteins can move

  • solutes against their concentration

  • gradient.

  • Active transport requires the cell to

  • expendits own metabolic energy.

  •  ATP supplies the energy for active

  • transport.

  • Active transport is performed by specific

  • proteins embedded in the membranes.

  • Active transport is a way in which cells

  • can maintain differing concentrations of

  • solutes inside the cell and outside the

  • cell.

  • Example: Sodium-Potassium pump

This pump maintains higher

concentrations of K+ inside the cell and

lower concentrations of Na+ inside the


The sodium-potassium pump uses the

energy of one ATP to pump three Na+

ions out and two K+ ions in.

  • Some ion pumps generate voltage

  • All cells have voltages across the

  • membrane. Voltage is electrical potential

  • energy – a separation of opposite

  • charges.

  • The inside of a cell is negative

  • compared to the outside of the cell.

  • There are more anions inside than

  • outside the cell.

  • This voltage across the membrane is

    called membrane potential.

  • The membrane potential and chemical

  • concentration gradient together form

  • an electrochemical gradient.

  • An ion will diffuse down its electro-

  • chemical gradient:

  • When gated channels open, Na+ ions

    will diffuse into the cell down the

    electrochemical gradient.

  • The sodium-potassium pump helps

  • maintain an electrochemical gradient

  • (3 Na+ pumped out for every 2 P+

  • pumped into the cell).

  • Transport proteins that generate

    voltage across the membrane are

    called electrogenic pumps.

  • Another example of the electrogenic

  • pump is the proton pump. It actively

  • transports hydrogen ions (protons) out

  • of the cell.

Proton pumps are found in mitochondria

and chloroplasts. These pumps store

energy that is later used for cellular work.

  • Cotransport: a membrane protein couples

  • the transport of two solutes.

A substance that has been pumped across

a membrane can do work; as it diffuses

back, it can power another transport


  • Exocytosis and endocytosis transport

  • large molecules

  • Large molecules like proteins and poly-

  • saccharides cannot be transported via

  • proteins. They are exported out of the

  • cell via exocytosis.

  • A transport vesicle budded from the

  • Golgi apparatus is moved by the cyto-

  • skeleton to the plasma membrane. The vesicle will fuse with the PM and

  • expel the contents of the vesicle.

  • A cell takes in macromolecules via

  • endocytosis.

  • There are three types of endocytosis:

  • Phagocytosis:

  • Pinocytosis:

  • Receptor-mediated endocytosis:

This type of endocytosis is very

specific. Special receptors bind to

ligands that are in the extracellular


Receptor proteins are usually clustered

in regions of the membrane called

coated pits, which are lined on their

cytoplasmic side with protein.

An example of receptor-mediated

endocytosis is cholesterol. Cholesterol

is taken in for the synthesis of

membranes and as a precursor for


Cholesterol travels in blood in particles

called low-density lipoproteins (LDLs).

These particles bind to LDL receptors

and then enter the cells by endocytosis.

  • Hypercholesterolemia: defective

    LDL receptors

Passive Transport Active Transport












Polar – Yes

Nonpolar - No

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