cell membranes and signaling - mrs....

Cell Membranes and

Signaling

5

Concept 5.1 Biological Membranes Have a Common Structure

and Are Fluid

A membrane’s structure and functions are determined by its constituents: lipids, proteins, and carbohydrates.

The general structure of membranes is known as the fluid mosaic model.

Phospholipids form a bilayer which is like a “lake” in which a variety of proteins “float.”

Figure 5.1 Membrane Molecular Structure


and Are Fluid

Lipids form the hydrophobic core of the membrane.

Most lipid molecules are phospholipids with two regions:

• Hydrophilic regions—electrically charged “heads” that associate with water molecules

• Hydrophobic regions—nonpolar fatty acid “tails” that do not dissolve in water


and Are Fluid

A bilayer is formed when the fatty acid “tails”

associate with each other and the polar

“heads” face the aqueous environment.

Bilayer organization helps membranes fuse

during vesicle formation and phagocytosis.


and Are Fluid

Membranes may differ in lipid composition as there are many types of phospholipids.

Phospholipids may differ in:

• Fatty acid chain length

• Degree of saturation

• Kinds of polar groups present


and Are Fluid

Two important factors in membrane fluidity:

• Lipid composition—types of fatty acids can

increase or decrease fluidity

• Temperature—membrane fluidity decreases

in colder conditions


and Are Fluid

Biological membranes contain proteins, with

varying ratios of phospholipids.

• Peripheral membrane proteins lack

hydrophobic groups and are not embedded

in the bilayer.

• Integral membrane proteins are partly

embedded in the phospholipid bilayer.


and Are Fluid

Anchored membrane proteins have lipid components that anchor them in the bilayer.

Proteins are asymmetrically distributed on the inner and outer membrane surfaces.

A transmembrane protein extends through the bilayer on both sides, and may have different functions in its external and transmembrane domains.


and Are Fluid

Some membrane proteins can move within

the phosopholipid bilayer, while others are

restricted.

Proteins inside the cell can restrict movement

of membrane proteins, as can attachments

to the cytoskeleton.

Figure 5.2 Rapid Diffusion of Membrane Proteins


and Are Fluid

Plasma membrane carbohydrates are

located on the outer membrane and can

serve as recognition sites.

• Glycolipid—a carbohydrate bonded to a

lipid

• Glycoprotein—a carbohydrate bonded

to a protein


and Are Fluid

Membranes are constantly changing by

forming, transforming into other types,

fusing, and breaking down.

Though membranes appear similar, there

are major chemical differences among the

membranes of even a single cell.

Concept 5.2 Some Substances Can Cross the Membrane by

Diffusion

Biological membranes allow some

substances, and not others, to pass.

This is known as selective

permeability.

Two processes of transport:

• Passive transport does not require

metabolic energy.

• Active transport requires input of

metabolic energy.


Diffusion

Passive transport of a substance can

occur through two types of diffusion:

• Simple diffusion through the

phospholipid bilayer

• Facilitated diffusion through channel

proteins or aided by carrier proteins


Diffusion

Diffusion is the process of random

movement toward equilibrium.

Speed of diffusion depends on three

factors:

• Diameter of the molecules—smaller

molecules diffuse faster

• Temperature of the solution—higher

temperatures lead to faster diffusion


Diffusion

• The concentration gradient in the

system—the greater the concentration

gradient in a system, the faster a

substance will diffuse

A higher concentration inside the cell

causes the solute to diffuse out, and a

higher concentration outside causes the

solute to diffuse in, for many molecules.


Diffusion

Simple diffusion takes place through the phospholipid bilayer.

A molecule that is hydrophobic and soluble in lipids can pass through the membrane.

Polar molecules do not pass through—they are not soluble in the hydrophilic interior and form bonds instead in the aqueous environment near the membrane.


Diffusion

Osmosis is the diffusion of water across

membranes.

It depends on the concentration of solute

molecules on either side of the

membrane.

Water passes through special membrane

channels.


Diffusion

When comparing two solutions separated

by a membrane:

• A hypertonic solution has a higher

solute concentration.

• Isotonic solutions have equal solute

concentrations.

• A hypotonic solution has a lower solute

concentration.

Figure 5.3A Osmosis Can Modify the Shapes of Cells

Figure 5.3B Osmosis Can Modify the Shapes of Cells

Figure 5.3C Osmosis Can Modify the Shapes of Cells


Diffusion

The concentration of solutes in the

environment determines the direction of

osmosis in all animal cells.

In other organisms, cell walls limit the

volume that can be taken up.

Turgor pressure is the internal pressure

against the cell wall—as it builds up, it

prevents more water from entering.


Diffusion

Diffusion may be aided by channel

proteins.

Channel proteins are integral membrane

proteins that form channels across the

membrane.

Substances can also bind to carrier

proteins to speed up diffusion.

Both are forms of facilitated diffusion.


Diffusion

Ion channels are a type of channel

protein—most are gated, and can be

opened or closed to ion passage.

A gated channel opens when a stimulus

causes the channel to change shape.

The stimulus may be a ligand, a

chemical signal.


Diffusion

A ligand-gated channel responds to its

ligand.

A voltage-gated channel opens or closes

in response to a change in the voltage

across the membrane.

Figure 5.4 A Ligand-Gated Channel Protein Opens in Response to a Stimulus


Diffusion

Water crosses membranes at a faster

rate than simple diffusion.

It may “hitchhike” with ions such as Na+

as they pass through channels.

Aquaporins are specific channels that

allow large amounts of water to move

along its concentration gradient.

Figure 5.5 Aquaporins Increase Membrane Permeability to Water (Part 1)

Figure 5.5 Aquaporins Increase Membrane Permeability to Water (Part 2)


Diffusion

Carrier proteins in the membrane

facilitate diffusion by binding

substances.

Glucose transporters are carrier proteins

in mammalian cells.

Glucose molecules bind to the carrier

protein and cause the protein to change

shape—it releases glucose on the other

side of the membrane.

Figure 5.6 A Carrier Protein Facilitates Diffusion (Part 1)

Figure 5.6 A Carrier Protein Facilitates Diffusion (Part 2)


Diffusion

Transport by carrier proteins differs from

simple diffusion, though both are driven

by the concentration gradient.

The facilitated diffusion system can

become saturated—when all of the

carrier molecules are bound, the rate of

diffusion reaches its maximum.

Concept 5.3 Some Substances Require Energy to Cross the

Membrane

Active transport requires the input of

energy to move substances against their

concentration gradients.

Active transport is used to overcome

concentration imbalances that are

maintained by proteins in the

membrane.

Table 5.1 Membrane Transport Mechanisms


Membrane

The energy source for active transport is often ATP.

Active transport is directional and moves a substance against its concentration gradient.

A substance moves in the direction of the cell’s needs, usually by means of a specific carrier protein.


Membrane

Two types of active transport:

• Primary active transport involves

hydrolysis of ATP for energy.

• Secondary active transport uses the

energy from an ion concentration

gradient, or an electrical gradient.


Membrane

The sodium–potassium (Na+–K+) pump

is an integral membrane protein that

pumps Na+ out of a cell and K+ in.

One molecule of ATP moves two K+ and

three Na+ ions.

Figure 5.7 Primary Active Transport: The Sodium–Potassium Pump


Membrane

Secondary active transport uses energy

that is “regained,” by letting ions move

across the membrane with their

concentration gradients.

Secondary active transport may begin

with passive diffusion of a few ions, or

may involve a carrier protein that

transports both a substance and ions.

Concept 5.4 Large Molecules Cross the Membrane via Vesicles

Macromolecules are too large or too

charged to pass through biological

membranes and instead pass through

vesicles.

To take up or to secrete macromolecules,

cells must use endocytosis or

exocytosis.

Figure 5.8 Endocytosis and Exocytosis (Part 1)

Figure 5.8 Endocytosis and Exocytosis (Part 2)


Three types of endocytosis brings

molecules into the cell: phagocytosis,

pinocytosis, and receptor–mediated

endocytosis.

In all three, the membrane invaginates, or

folds around the molecules and forms a

vesicle.

The vesicle then separates from the

membrane.


In phagocytosis (“cellular eating”), part

of the membrane engulfs a large particle

or cell.

A food vacuole (phagosome) forms and

usually fuses with a lysosome, where

contents are digested.


In pinocytosis (“cellular drinking”),

vesicles also form.

The vesicles are smaller and bring in

fluids and dissolved substances, as in

the endothelium near blood vessels.


Receptor–mediated endocytosisdepends on receptors to bind to specific molecules (their ligands).

The receptors are integral membrane proteins located in regions called coated pits.

The cytoplasmic surface is coated by another protein (often clathrin).


When receptors bind to their ligands, the coated pit invaginates and forms a coated vesicle.

The clathrin stabilizes the vesicle as it carries the macromolecules into the cytoplasm.

Once inside, the vesicle loses its clathrin coat and the substance is digested.

Figure 5.9 Receptor-Mediated Endocytosis (Part 1)

Figure 5.9 Receptor-Mediated Endocytosis (Part 2)


Exocytosis moves materials out of the

cell in vesicles.

The vesicle membrane fuses with the

plasma membrane and the contents are

released into the cellular environment.

Exocytosis is important in the secretion of

substances made in the cell.

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