BIOL #08: Cell Communication (I) Flashcards Preview

Biology > BIOL #08: Cell Communication (I) > Flashcards

Flashcards in BIOL #08: Cell Communication (I) Deck (33)
Loading flashcards...
1
Q

Fluid Mosaic Model

A
  • 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
2
Q

Integral Proteins

A
  • 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)
3
Q

Peripheral Proteins

A
  • 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
4
Q

Major Functions of Membrane Proteins

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

Membrane Proteins & Membrane Function

A
  • 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
6
Q

Many Factors Affect Membrane Behavior

A
  • 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
7
Q

Membrane Fluidity: Effects of Temperature and Lipid Structure

A
  • 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
8
Q

Membrane Fluidity: Cholesterol

A
  • 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
9
Q

Selective Permeability of Lipid Bilayers

A
  • 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
10
Q

Bond Saturation and Membrane Permeability

A

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

11
Q

Selective Permeability of Lipid Bilayers PT.2

A
  • 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.
12
Q

Other Factors That Affect Permeability

A
  • 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
13
Q

Solute Movement across Lipid Bilayers

A
  • 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
14
Q

Diffusion along a Concentration Gradient

A
  • 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.
15
Q

Osmosis

A
  • 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.
16
Q

Osmosis in Hypertonic Solutions

A

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

17
Q

Osmosis in Hypotonic Solutions

A

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

18
Q

Osmosis in Isotonic Solutions

A

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

19
Q

Membrane Proteins: Ions and Molecules Transport

A
  • 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
20
Q

Channel Proteins

A
  • 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.
21
Q

Aquaporins

A

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)

22
Q

Ion Channels

A

-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

23
Q

Electrochemical Gradient

A

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.

24
Q

Ion Channels and the Electrochemical Gradient

A
  • Ions diffuse through channels down their electrochemical gradients.
  • This passive transport decreases the charge and concentration differences between the cell’s exterior and interior.
25
Q

Carrier Proteins

A
  • 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.
26
Q

Active Transport by Pumps

A
  • 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.
27
Q

Secondary Active Transport

A
  • 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.
28
Q

Extracellular Layer

A

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.

29
Q

Extracellular Matrix

A
  • 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.
30
Q

Collagen

A

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

31
Q

The ECM and Cytoskeleton Are Directly Linked

A
  • 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.
32
Q

Collagen

A

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

33
Q

The ECM and Cytoskeleton Are Directly Linked

A
  • 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.