lecture 3: Cell Membranes

what are the general functions of cell membranes?

1. protective barriers b/n intracellular and extracellular environments

2. facilitate transport of substances in and out of cell

3. signal transduction - may house receptors IMP for cell signaling/initiation of cellular eventrs

4. energy storage - there’s a charge difference between the outside and inside of the cell

When and what is the fliud mosaic model?

• devised in 1972

• describes structure of cell membrane

• membrane is “fluid” - can bend and move

• membrane “mosaic” - composed of multiple macromolecules put together like a tile mosaic

What are the major parts of the cell membrane?

• membrane lipids

• membrane proteins

  • glycocalyx

What is involved with membrane lipids?

1. hydrophilic heads

   1. phosphates and glycerol

   2. negative charge

2. hydrophobic tails

   1. chains of fatty acids

   2. saturated = ||

   3. unsaturated = |/

3. cholesterol

   1. rigid (membrane structure/fluidity)

   2. temperature sensitive

Describe the arrangement of phospholipid molecules

• sheet-like arrangement

• a functional barrier between intra- and extracellular fuild

• permit flexibility/motion of the membrane

what is membrane fluidity? give examples

• ability of a cell to adapt its shape

• influenced by temperature, cholesterol, and type of fatty acid

temperature high/low

if high - more fluidity

if low - less fluidity

cholesterol high/low

if high - less fluidity

if low - more fluidity

saturated or unsaturated

if sat - less fluid

if unsat - more fluid

integral proteins

• embedded and span the membrane; ion channels, receptors

peripheral proteins

• NOT embedded, special functions

membrane protein functions

1. transport

   1. channels

   2. carriers

   3. vesicular transport

2. receptor

3. cell to cell adhesion

4. enzymes

5. communication

6. adhesion to extracellular matrix

types of integral proteins

• channels — selectively permit movement of certain substances into/out of cell

  • includes: aquaporins which transport water

• receptors — selectively bind specific molecultes, which initiaes a chemical reaction inside the cell (type of signaling)

• **** specificity ****

Glycocalyx

• either glycoprotein or glycolipid

• roles:

  • decrease cell water loss

  • antigenic function

    • immune system

    • blood type

    • — self recognition

What can move freely across the membrane?

small, non-polar molecules

How do substances get moved across the membrane?

• passive transport

• active transport

passive transport

• movement across the membrane without expedenture of ATP

  • simple diffusion

  • facilitated diffusion (protein needed)

  • osmosis

active transport

• movement across the membrane that requires ATP

• sodium/potassium pump; Big Pump

What is diffusion?

• the movement of particles from an area of high concentration to an area of lower concentration

  • concentration gradient (dif in conc of a substance across a space)

What is a solvent:?

liquid component of a solution

What is a solute?

a dissolved substance in a solution

dynamic equilibrium

solutes will become ~ evenly distributed over time

what is passive transport? - diffusion

where small, uncharged molecules diffuse across membrane

what is facilitated diffusion?

• substances are aided by a membrane protein that does NOT use ATP

  • larger/charged molecules (Na+)

  • proteins

What are aquaporins?

• special channels that allow water to move freely across the cell membrane

Osmosis

• the diffusion of water through a semipermeable membrane

Osmosis follows …

the concentration gradient

Na-K-ATPase

an important ion pump, found in most cells

passive transport =

diffusion

simple (diffusion through the membrane)

small, nonpolar, lipid soluble substances

facilitated (diffusion through channel/carrier)

above OR some larger/polar/H20 soluble substances

osmosis

diffusion of solvent with concentration gradient

active transport

requires ATP → large, polar, h2o soluble substances

What does membranes being differently permeable to solutes, including ions lead to?

• unequal distribution of solutes between intracellular fluid (ICF) and the extracellular fluid (ECF)

water is the main ____ and separated into multiple ____

solvent; compartments

total body weight is

• 60% of body weigh

  • varies by age, body composition, sex, etc

what is the total body weight make up?

• 2/3 intracellular fluid

• 1/3 extracellular

  • interstitial fluid (75%)

  • plasma (20%)

    • transcellular fluid (5%)

What is osmolarity?

the concentration of solute particles in a solvent (usually expressed as mOsm/kg)

What is normal ICF and ECF osmolarity?

• 300 mOsm/kg for both

• this is IMP to avoid big fluctuations of movement between intra and extra

While composition of ___ is different, ____ is heavily regulated by the body

solutes; osmolarity

Types of tonicity

• hypotonic

• hypertonic

• isotonic

Hypotonic

• [solutes]outside < [solutes]inside

• net movement of water IN

• cell swells

Isotonic

• solutes within and outisde cell =

• no net movement of water

Hypertonic

• [solutes]out > [solutes]inside

• shriveling