BIOL #08: Cell Communication (I)

Question 1

Fluid Mosaic Model

Answer 1

• The Fluid Mosaic Model is a hypothesis that describes the membrane as a fluid structure with proteins embedded in or attached to the phospholipid bilayers
• Membranes are held together primarily by hydrophobic interactions, which are much weaker than covalent bonds
  + Individual phospholipids can move laterally throughout the lipid bilayer but rarely flip between layers.
• Proteins can also move throughout the membrane but much more slowly than the phospholipids
• Two major types of membrane proteins:Integral & Peripheral

Question 2

Integral Proteins

Answer 2

• Integral proteins penetrate the hydrophobic interior of the lipid bilayer
• Majority are transmembrane proteins that span the membrane
• Other integral proteins only extend part way into the hydrophobic region
• All integral proteins have a hydrophobic and hydrophilic region
• Some have hydrophilic channels through their center to allow passage of hydrophilic substances (ions and polar molecules)

Question 3

Peripheral Proteins

Answer 3

• Peripheral proteins are not embedded in the lipid bilayer at all
• These proteins are loosely bound to the surface of a membrane (interior or exterior)
• Often function to expose parts of integral proteins

Question 4

Major Functions of Membrane Proteins

Answer 4

1. Transport
2. Enzymatic Activity
3. Signal Transduction
4. Cell-Cell Recognition
5. Intercellular Joining
6. Attachment between the cytoskeleton and extracellular matrix (ECM)

Question 5

Membrane Proteins & Membrane Function

Answer 5

• Proteins associated with the cell membrane determine most of the membrane’s function
  + Different types of cells contain different sets of membrane proteins
  + Membranes within cells (i.e. organelles) have unique sets of proteins

Question 6

Many Factors Affect Membrane Behavior

Answer 6

• Many factors influence the behavior of the membrane:
  + Temperature
  + Number of double bonds between the carbons in the phospholipid’s hydrophobic tail
• Presence of kinks in the hydrocarbon tails
  + Length of the hydrophobic tails
  + Number of cholesterol molecules in the membrane

Question 7

Membrane Fluidity: Effects of Temperature and Lipid Structure

Answer 7

• How quickly molecules move within and across membranes is a function of temperature and the structure of the hydrocarbon tails in the bilayer.
• Membrane fluidity decreases with temperature because molecules in the bilayer move more slowly.
  + At some temperature, the membrane solidifies – the temperature at which this occurs depends on the type of lipids it is made out of
• Membranes can remain fluid at lower temperatures if they are rich in phospholipids with unsaturated hydrocarbon tails
• Kinks in hydrocarbon chains decrease packing

Question 8

Membrane Fluidity: Cholesterol

Answer 8

• Cholesterol (a steroid) is an important component of animal cell membranes
• Cholesterol buffers effects of temperature on membrane fluidity in animal cells (e.g. acts as a fluidity buffer)
  + At high temperatures (37°C in humans), cholesterol molecules make the membrane less fluid by restraining phospholipid movement
  + At low temperatures, cholesterol molecules can hinder the close packing of phospholipids

Question 9

Selective Permeability of Lipid Bilayers

Answer 9

• The permeability of a structure is its tendency to allow a given substance to pass across it.
• Phospholipid bilayers have selective permeability.
  + selective permeability of the membrane controls the flow of materials into and out of the cell
• Keep damaging materials out of the cell
• Allow entry of materials needed by the cell
• Facilitate the chemical reactions necessary for life

Question 10

Bond Saturation and Membrane Permeability

Answer 10

Membranes with unsaturated phospholipid tails are much more permeable than those formed by phospholipids with saturated tails.

Question 11

Selective Permeability of Lipid Bilayers PT.2

Answer 11

• Small or nonpolar molecules move across phospholipid bilayers quickly.
  + H2O is polar BUT small enough to pass through (but not very fast)
• Large polar or charged molecules (ions) cross slowly, if at all.

Question 12

Other Factors That Affect Permeability

Answer 12

• Hydrophobic interactions become stronger as saturated hydrocarbon tails increase in length.
  + Membranes containing phospholipids with longer tails have reduced permeability.
• Adding cholesterol to membranes increases the density of the hydrophobic section.
  + Cholesterol decreases membrane permeability.
• Membrane fluidity decreases with temperature because molecules in the bilayer move more slowly.
  + Decreased membrane fluidity causes decreased permeability

Question 13

Solute Movement across Lipid Bilayers

Answer 13

• Materials can move across the cell membrane in different ways.
  + Passive transport does not require an input of energy.
  + Active transport requires energy to move substances across the membrane.
• Small molecules and ions in solution are called solutes and are in constant, random motion.
  + The kinetic energy of these molecules can also be thought of as thermal energy (heat)
  + These molecules move randomly until they are spread out, a process called diffusion

Question 14

Diffusion along a Concentration Gradient

Answer 14

• A difference in solute concentrations creates a concentration gradient.
• Molecules and ions move randomly when a concentration gradient exists, but there is a net movement from high- concentration regions to low-concentration regions.
• Diffusion along a concentration gradient increases entropy (disorder) and is thus spontaneous (no energy input required).
• Equilibrium is established once the molecules or ions are randomly distributed throughout a solution.
  + Molecules are still moving randomly but there is no more net movement.

Question 15

Osmosis

Answer 15

• Water molecules can move passively across lipid bilayers.
  + The movement of water is a special case of diffusion called osmosis.
• Osmosis only occurs across a selectively permeable membrane
• Osmosis is a form of passive transport used by cells to maintain a balance of water between the internal and external environment
• Water moves from regions of low solute concentration to regions of high solute concentration.
  + This movement dilutes the higher solute concentration, thus equalizing the concentration on both sides of the bilayer.

Question 16

Osmosis in Hypertonic Solutions

Answer 16

In a hypertonic solution, water will move out of the cell by osmosis and the cell will shrink.

Question 17

Osmosis in Hypotonic Solutions

Answer 17

In a hypotonic solution, water will move into the cell by osmosis and the cell will swell.

Question 18

Osmosis in Isotonic Solutions

Answer 18

In an isotonic solution, there will be no net water movement and the cell size will remain the same.

Question 19

Membrane Proteins: Ions and Molecules Transport

Answer 19

• Integral or transmembrane proteins can be involved in the transport of selected ions and molecules across the plasma membrane.
  + Transmembrane proteins can therefore affect membrane permeability.
• The transmembrane proteins that transport molecules are called transport proteins. There are three broad classes of transport proteins, each of which affects membrane permeability:
  1) Channels
  2) Carrier proteins or transporters
  3) Pumps

Question 20

Channel Proteins

Answer 20

• Cells have many different types of channel proteins in their membranes, each featuring a structure that allows it to admit a particular type of ion or small molecule.
• These channels are responsible for facilitated diffusion: the passive transport of substances that would not otherwise cross the membrane.
• Channel proteins have hydrophilic passageways to allow water molecules or small ions to diffuse quickly from one side of the membrane to the other.

Question 21

Aquaporins

Answer 21

Aquaporins are water channel proteins that greatly enhance the passage of water molecules across the membrane. The presence of aquaporins is important in certain cell types which need to equilibrate quickly (e.g. kidney cells)

Question 22

Ion Channels

Answer 22

-Ion channels are specialized channel proteins.
+ Ion channels circumvent the plasma membrane’s impermeability to small, charged compounds.
+ Many ion channels are gated channels that open or close in response to a stimulus, e.g. electrical charge

Question 23

Electrochemical Gradient

Answer 23

When ions build up on one side of a plasma membrane, they establish both a concentration gradient and a charge gradient, collectively called the electrochemical gradient.

Question 24

Ion Channels and the Electrochemical Gradient

Answer 24

• Ions diffuse through channels down their electrochemical gradients.
• This passive transport decreases the charge and concentration differences between the cell’s exterior and interior.

Question 25

Carrier Proteins

Answer 25

• Facilitated diffusion can occur through channels or through carrier proteins, or transporters, which change shape during the transport process.
• Facilitated diffusion by transporters occurs only down a concentration gradient, reducing differences between external and internal solutions.
• Transporters change shape when in contact with a substance in such a way that it shuttles the substance across the membrane.
• Transporters are specific to the substances (or group of related substance) they translocate.

Question 26

Active Transport by Pumps

Answer 26

• Cells can transport molecules or ions against an electrochemical gradient.
  + This process requires energy in the form of ATP and is called active transport.
• Pumps are membrane proteins that provide active transport of molecules across the membrane.
• Active transport allows molecules to maintain internal concentrations of small solutes that differ from concentrations in the external environment.

Question 27

Secondary Active Transport

Answer 27

• In addition to moving materials against their concentration gradients, pumps set up electrochemical gradients.
• These gradients make it possible for cells to engage in secondary active transport, or cotransport.
  + The gradient provides the potential energy required to power the movement of a different molecule against its concentration gradient.
  + ATP indirectly provides the energy needed for cotransport.

Question 28

Extracellular Layer

Answer 28

Structure:
- Most cells possess a protective layer or wall that forms just beyond the membrane.
+ This layer generally consists of a “fiber composite” – a cross-linked network of long filaments surrounded by a stiff ground substance.

Function:
- Protection against tension and compression.

Question 29

Extracellular Matrix

Answer 29

• Most animal cells secrete a fiber composite called the extracellular matrix (ECM).
• One of the ECM’s most important functions is structural support.
• In addition to structural support:
  + Helps cells stick together
  + Forms protein-protein attachments that link the ECM directly to the cell’s cytoskeleton.
• The amount and composition of the ECM vary depending upon the cell type.
• ECM consists of a ground substance
  + gelatinous polysaccharide and a network of protein fibers.

Question 30

Collagen

Answer 30

The most common ECM protein fiber is collagen – more elastic than cellulose and forms a flexible extracellular layer.

Question 31

The ECM and Cytoskeleton Are Directly Linked

Answer 31

• The ECM is strengthened by connections to transmembrane proteins:
  + Actin protein (micro)filaments in the cytoskeleton bind to transmembrane integrin proteins
  + Integrins bind to ECM proteins such as fibronectins which then bind to collagen.
• Direct linkage between the cytoskeleton and ECM
  + keeps individual cells in place
  + helps adjacent cells adhere to each other.

Question 32

Collagen

Answer 32

The most common ECM protein fiber is collagen – more elastic than cellulose and forms a flexible extracellular layer.

Question 33

The ECM and Cytoskeleton Are Directly Linked

Answer 33

• The ECM is strengthened by connections to transmembrane proteins:
  + Actin protein (micro)filaments in the cytoskeleton bind to transmembrane integrin proteins
  + Integrins bind to ECM proteins such as fibronectins which then bind to collagen.
• Direct linkage between the cytoskeleton and ECM
  + keeps individual cells in place
  + helps adjacent cells adhere to each other.