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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


  • 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 phospholipids move rapidly. Protein movement

    depends on size and attachment to cytoskeleton.

  • Membrane fluidity also depends on temperature.

Hydrophilic region

of protein

Phospholipid

bilayer

Hydrophobic region of protein


Extracellular

layer

Proteins

Knife

Plasma

membrane

Cytoplasmic

layer

Extracellular layer

Cytoplasmic layer


  • Cholesterol

    maintains

    membrane fluidity.

Lateral movement

(~107 times per second)

Flip-flop

(~ once per month)

Movement of phospholipids

Viscous

Fluid

Saturated hydro-

carbon tails

Unsaturated hydrocarbon

tails with kinks

Membrane fluidity

Cholesterol

Cholesterol within the animal cell membrane


  • Proteins determine the function of the

  • PM.

  • There are two types of membrane protein:


N-terminus

EXTRACELLULAR

SIDE

C-terminus

CYTOPLASMIC

SIDE

a Helix



Glyco-

protein

Attachment to the

cytoskeleton and extra-

cellular matrix (ECM)

Cell-cell recognition

Intercellular joining


  • 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.


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.


  • 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).


  • Diffusion is the tendency of molecules

  • of any substance to spread out in the

  • available space

Molecules of dye

Membrane (cross section)

WATER

Net diffusion

Net diffusion

Equilibrium

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

Equilibrium

Equilibrium

Net diffusion

Net diffusion

Diffusion of two solutes


  • 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.


  • 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.


  • 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.


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


  • 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

diffusion:

  • Channel proteins: allows for fast

  • transport.

Ex.

Aquaporins



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 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. 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

  • expend its 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 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.

This pump maintains higher

concentrations of K+ inside the cell and

lower concentrations of Na+ inside the

cell.

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 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.

  • 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- 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.

  • 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 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.

  • 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 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.

  • 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

protein.


  • Exocytosis 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. 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: 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.

  • Phagocytosis:

  • Pinocytosis:


  • Receptor-mediated endocytosis: 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.

This type of endocytosis is very

specific. Special receptors bind to

ligands that are in the extracellular

space.

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 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.

endocytosis is cholesterol. Cholesterol

is taken in for the synthesis of

membranes and as a precursor for

steroids.

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 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.

NO

NO

NO

YES

YES

YES

YES

YES

YES

NO

NO

Polar – Yes

Nonpolar - No


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