Membrane structure and Function

Question 1

What is a plasma membrane and how do we idnetify it?

Answer 1

• The plasma membrane is the boundary that separates the living cell from its surroundings
• The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others

Question 2

What is the main component of membranes?

Answer 2

Phospholipids which are amphipathic molecules

Question 3

What is a fluid mosiac model?

Answer 3

a membrane is a fluid structure with a “mosaic” of various proteins embedded in it – integral proteins

Question 4

Major characteristics of integral proteins

Answer 4

Permanently anchored within the cellular membrane

Question 5

What are freeze fractures and why do we use them?

Answer 5

Is a specialized preparation technique that
splits a membrane along the middle of the phospholipid bilayer

Question 6

What determines the the asymmetrical distribution of proteins, lipids, and
associated carbohydrates in the plasma membrane?

Answer 6

Determined when the membrane is built by the ER and
Golgi apparatus

Question 7

How fluid are membranes?

Answer 7

• Phospholipids in the plasma membrane can move within the bilayer
• Most of the lipids, and some proteins, drift laterally (rare for it to flip transversely)

Question 8

How often do they move?

Answer 8

Lateral: 10^7 per second
Transvere: one per month

Question 9

Do membrane proteins move?

Answer 9

Proteins are larger than lipids - move more slowly

Question 10

WHat factors influence the fluidity of membranes?

Answer 10

• As temperatures cool, membranes switch from a fluid state to a more solid state
• The temperature at which a membrane solidifies depends on the types of lipids in the bilayer
• Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids
• Membranes must be fluid to work properly; they are usually about as fluid as salad oil

Question 11

What is the function of cholesterol in cell membranes in different temperatures?

Answer 11

• At warm temperatures (such as 37°C), cholesterol
  restrains/reduces movement of phospholipids
• At cool temperatures, it maintains fluidity by preventing tight packing

acts as a fluidity buffer

Question 12

Peripheral vs integral proteins

Answer 12

• Peripheral proteins are bound to the surface of the
  membrane
• Integral proteins penetrate the hydrophobic core

Question 13

what are integral proteins that span the membrane called?

Answer 13

Transmembrane proteins

Question 14

The hydrophobic regions of an integral protein consist of?

Answer 14

one or more stretches of nonpolar amino acids, often
coiled into alpha helices

Question 15

Six major functions of membrane proteins

Answer 15

• Transport – form channels; pumps for active transport; carriers for passive transport
• Enzymatic activity – on inner mitochondrial membrane
• Signal transduction – receptor proteins
• Cell-cell recognition
• Intercellular joining – desmosomes
• Attachment to the cytoskeleton and extracellular matrix (ECM) – via proteins like integrins

Question 16

What is the role of carbohydrates in cell-cell recognition?

Answer 16

Complex carbohydrates coat the surfaces of cells and have the potential to carry the information necessary for cell-cell recognition
-markers for blood types

Question 17

Describe the permeability of the lipid bilayer

Answer 17

Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly
* Polar molecules, such as sugars, do not cross the membrane easily

Question 18

What are transport proteins?

Answer 18

• Transport proteins allow passage of hydrophilic
  substances across the membrane

Question 19

How do transport proteins work?

Answer 19

• Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel
• Channel proteins called aquaporins facilitate the
  passage of water

Question 20

carrier proteins?

Answer 20

Bind to molecules and change shape to shuttle them across the membrane

Question 21

What are transport proteins?

Answer 21

Passive transport is diffusion (the tendency for molecules to spread out evenly into the available space) of a substance across a membrane with no
energy investment

Question 22

Passive transport of a substance across s biological membrane

Answer 22

Substances diffuse down their concentration gradient- the difference in concentration of a substance from one area to another
Its considered passive transport as no energy from the cell to make it happen.

Question 23

What is osmosis?

Answer 23

The diffusion of water across a selectively
permeable membrane

Question 24

What are 4 things related to water balance in a cell. Name and explain them

Answer 24

• Tonicity is the ability of a solution to cause a cell to gain or lose water
• Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane
• Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water
• Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water

Question 25

How do cell walls help maintain water balance ?

Answer 25

• In a hypotonic solution - swells until the wall
  opposes uptake; the cell is now turgid (firm)
• If a plant cell and its surroundings are isotonic, the cell becomes flaccid (limp), and the plant may wilt
  *In a hypertonic environment, plant cells lose water;
  eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis

Question 26

What occurs in facilitated transport?

Answer 26

• Transport proteins speed the passive
  movement of molecules across the plasma membrane
• Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane

Question 27

What makes active transport different from passive transport ?

Answer 27

• Active transport moves substances against their
  concentration gradient
• Active transport requires energy, usually in the form of ATP
• Active transport is performed by specific proteins embedded in the membranes

Question 28

What is a type of active transport?

Answer 28

sodium-potassium pump

Question 29

What is membrane potential and how ion pumps maintain it?

Answer 29

• Membrane potential is the voltage
  difference across a membrane
• Voltage is created by differences in
  the distribution of positive and
  negative ions

Question 30

What drives the diffusion of ions across the membrane?

Answer 30

*Two combined forces, collectively called the
electrochemical gradient
-A chemical force (the ion’s concentration gradient)
- An electrical force (the effect of the membrane potential on the ion’s movement)

Question 31

What is an electrogenic pump?

Answer 31

An electrogenic pump is a transport protein that
generates voltage across a membrane , e.g. sodium-potassium pump.

main electrogenic pump of plants, fungi, and
bacteria is a proton pump

Question 32

What is cotransport?

Answer 32

when active transport of a solute
indirectly drives transport of another solute

Question 33

Exocytosis

Answer 33

transport vesicles migrate to the
membrane, fuse with it, and release their contents

Many secretory cells use exocytosis to export their
products

Question 34

Endocytosis

Answer 34

The cell takes in macromolecules by
forming vesicles from the plasma membrane
* Endocytosis is a reversal of exocytosis

Question 35

What are the 3 types of endocytosis?

Answer 35

• Phagocytosis (“cellular eating”): cell engulfs a particle in a vacuole which fuses with a lysosome to digest the particle
• Pinocytosis (“cellular drinking”): molecules are taken up when extracellular fluid is “gulped” into tiny vesicles
• Receptor-mediated endocytosis: binding of ligands (any molecule that binds specifically to a
  receptor site of another molecule) to receptors triggers vesicle formation