Lecture 9: lipids and membranes

Question 1

Know the structure of a phospholipid, the structures phospholipids form with one
another, and the role van Der Waals forces and fatty acid chain flexibility play in the
composition of a plasma membrane

Answer 1

Amphipathic: hydrophobic and
hydrophilic regions

Arrange themselves spontaneously
in an aqueous environment:
dependent on head group size
a. Large head group with one
hydrophobic tail: form spheres called
micelles
b. Small head group with two
hydrophobic tails: form a bilayer

In a test tube: phospholipids
spontaneously form enclosed bilayers
called liposomes

Question 2

Compare saturated vs unsaturated fatty acid chains and describe how each type
contributes to the properties of a plasma membrane

Answer 2

Saturated has no double bonds.
Unsaturated have one or more carbon double bonds forming kinks.
The strength of the interactions between tails depends on
i. presence or absence of double bonds between neighboring carbon atoms
Ii. length of the tails

Question 3

Know the properties of cholesterol and describe its effect on membrane fluidity at
normal and cooler temperatures

Answer 3

component of animal cell membranes
• Cholesterol is amphipathic: hydrophilic hydroxyl group and a
hydrophobic region of four interconnected carbon rings

• Cholesterol can increase or decrease membrane fluidity depending on
temperature
• At lower temperatures: cholesterol
prevents the phospholipids from packing
tightly and increases membrane fluidity
• Normal cell temperature: interaction
between cholesterol and the fatty acid
tails reduces the mobility of the
phospholipids and the fluidity of the
membrane

Question 4

Know and describe the four classes of proteins that associate with the plasma
membrane

Answer 4

transporters, anchors, enzymes and receptors

Transporters: move ions or
molecules across the membrane

Enzymes: catalyze chemical
reactions

Receptors: allow the cell to receive
signals from the environment

Anchors: attach to proteins that
maintain cell structure and shape

Question 5

Know and compare integral and peripheral membrane proteins

Answer 5

Integral membrane proteins: permanently associated with cell membranes; span the lipid bilayer

Peripheral membrane proteins: temporarily associate with the lipid
bilayer or with integral membrane proteins through weak noncovalent
interactions; can be on the internal or external side of the membrane

Question 6

Describe the fluid mosaic model of plasma membranes

Answer 6

Lipids and proteins coexist in the membrane, forming a
“mosaic”
• Molecules move laterally in the membrane, so the membrane
is “fluid”

Question 7

Describe the functions of the plasma membrane and its role as a selective barrier

Answer 7

• Main functions:
i. Separate internal contents from the surrounding environment
ii. Maintain homeostasis: keeps the internal environment within a persistent
and narrow window of conditions

• The cell membrane is also a selective barrier: allows some molecules to
move in and out freely while others move in and out under certain
conditions (or not at all)

Question 8

Describe and compare the processes of diffusion, facilitated diffusion, osmosis,
primary active transport, secondary active transport

Answer 8

PASSIVE TRANSPORT:
Diffusion:
movement of molecules (solute) from high to low concentration
Osmosis:
Diffusion of Water
Facilitated diffusion:
molecules move through a membrane protein channel or carrier
ACTIVE TRANSPORT:
Primary active transport:
movement of substances against the concentration gradient. ie) sodium/potassium pump.
Secondary Active Transport:
• Active transporters drive the transport of molecules through a different
transporter through the creation of an electrochemical gradient

Question 9

Describe the effects of osmosis on cells (hypertonic vs isotonic vs hypotonic
solutions)

Answer 9

Hypertonic:

• concentration of solutes is greater outside the cell than inside it.
• shrunk cell

Isotonic:

• concentration of solutes is the same both inside and outside of the cell
• normal cell

Hypotonic:

• concentration of solutes is greater inside the cell than outside of it
• swollen cell
• if cell is very hypotonic it is lysed.

Question 10

what’s an electrochemical gradient?

Answer 10

concentration gradient = chemical gradient

difference in charge = electrical gradient

Question 11

Know the function of plant cell walls, turgor pressure, and vacuoles in relationship to
the water content of plant cells.

Answer 11

• Cell wall: rigid structure that surrounds the plasma membrane and resists
cell expansion when the cell takes in water
• Turgor pressure: force exerted by water pressing against the cell wall
• Vacuoles: absorb water and
contribute to turgor pressure