Biochemistry - Chapter 12 - Membrane Structure and Function - Review

Liposomes, or lipid vesicle

Liposomes, or lipid vesicles, are aqueous compartments enclosed by a lipid membrane.

Lipid bilayers

Highly impermeable to ions and most polar molecules.

Cholesterol helps to maintain proper…

…membrane fluidity in membranes in animals.

Lipid soluble molecules such as __________ can cross the membrane but ______ and __________ and other cannot cross the membrane.

• O₂ and CO₂

• Ions (K⁺, Na⁺, Ca²⁺) and big molecules such as glucose

Although membrane lipids establish a permeability barrier, membrane proteins allow…

…transport of molecules and information across the membrane.

Integral membrane proteins

Proteins that are embedded in the hydrocarbon core of the membrane.

Peripheral membrane proteins

Proteins that are bound to the polar head groups of membrane lipids or to the exposed surfaces of integral membrane proteins.

________________ interact extensively with the hydrocarbon region of the bilayer.

Integral membrane proteins (a and b)

What are the 5 letters?

a) Integral membrane proteins

b) Integral membrane proteins

c) Peripheral membrane proteins (Interact with polar head groups of lipids

d) Peripheral membrane proteins (Bind to the surfaces of integral proteins)

e) Tightly anchored to the membrane (by a covalently attached lipid molecule.)

Transport Proteins

Proteins that function as pumps or channels or carriers to facilitate the flow of small molecules across the cell membrane.

Passive transport, or facilitate diffusion

It occurs when a molecule moves down its concentration gradient through a transport protein.

Active transport

Occurs when protein pumps use energy to move a molecule against its concentration gradient in the process.

The Na⁺–K⁺ ATPase or Na⁺–K⁺ pump uses the energy of ATP hydrolysis to simultaneously pump three Na⁺ ions out of the cell and two K⁺ ions into the cell against their concentration gradients.

• Pump three Na⁺ ions out.

• Pump two K⁺ ions into.

Because the reaction includes an intermediate in which the enzyme is phosphorylated, such pumps are called P-type ATPases.

This type of transport is called primary active transport

Membrane-spanning _________ are a common structural feature of integral memrbane proteins.

Membrane-spanning α-helices

What are examples of pores?

Porin

Nuclear Pore Complex

Aquaporin

What are examples of gated channels?

• Ligand-gated Channels

• Voltage-gated Channels

• Light-gated Channels

Ion channels

Passive transport systems that allow specific and rapid transport of ions down their concentration gradients.

Voltage-activated channels

Channels that can be activated by changes in the voltage across a membrane.

Ligand-activated channels

By binding specific molecules to the channels

Tetrodotoxin

, produced by the pufferfish, is a lethal inhibitor of the Na⁺ channel.

The potassium channel selectively and rapidly transports K⁺ across the cell membrane. Larger ions are not transported because… Smaller ions are excluded because

…because they are too big to enter the channel.

…because they cannot interact with the selectivity filter. Such ions are small enough that the nergy of desolvation cannot be compensated for by interactions with the selectivity filter.

• Lipids rapidly diffuse ______ in membranes, although _______ diffusion or flip-flopping is very rare without the assistance of enzymes.

• The prohibition of ______ diffusion accounts for the stability of membrane asymmetry.

• laterally

• transverse

• transverse

Secondary transporters

Use one concentration gradient to power the formation of another.

Examples: Symporters and Antiporters

Symporters

Power the transport of a molecule against its concentration gradient by coupling the movement to the movement of another molecule down its concentration gradient, with both molecules moving in the same direction.

Antiporters

Also, use one concentration gradient to power the formation of another, but the molecules move in opposite directions.

Glucose is moved into some animal cells against its concentration gradient by a…

…symporter that is powered by Na⁺ ions moving down a concentration gradient.