1

Cell Biology - Part 2Membranes

The organization of cells is made possible by membranes.Membranes isolate, partition, and compartmentalize cells.

2

Membranes

• isolate the inside of the cell from the outside - maintains theorder of the cell

• regulate the movement of materials into and out of the cell

• compartmentalize the interior of cells and allows the creation ofspecialized regions within the cell

• provide a site for many chemical reactions - on or within membranes

• serve as the site of many chemical receptors - provide information about the outside environment

• serve as the site for many chemical identifiers - provide information about the cell to other cells

3

Cell and plasma membranes have a "fluid-mosaic” structureIt has nonrigid arrangement of mobile lipids and proteins.

4

Human-Mouse Cell-Hybridization provides evidence for thefluid nature of membranes.

5

The basis of the behavior of cell membrane is largely due to thebehavior of phospholipids in water.

Phospholipids are called amphipathic molecules - they have bothpolar and nonpolar regions within the same molecule.

Polar head - charged -hydrophilic - hydrogenbonds with water

Nonpolar tail - nocharge - hydrophobic -does not hydrogenbond with water

6

Phospholipids in water display much of the behavior of cellmembranes - they form lipid bilayers

Water

Water

7

Within the phospholipid bilayer are other molecules.Cholesterol helps to stabilize membrane structure and restrictmovement of water soluble substances through the membrane.

8

Proteins with nonpolarand polar domains fitwithin the membrane andassociate withphospholipids tails in themembrane and water oneither side of themembrane

9

Imbedded proteins in the plasma membrane have many functions

Imbedded proteins found in internal membranes have similarfunctions

10

A primary function of cell membranes is to restrict or facilitatemovement of materials into or out of the cell. Movement ofmaterials across membranes occurs in several ways.

Simple diffusionRandom movement of molecules in solution ultimately results inan even distribution of molecules. The net result is thatmolecules move from areas of higher concentration to areas oflower concentration.

11

Diffusion occurs through cell membranes. Substances that aremore concentrated on the outside of the cell than inside the celltend to move into the cell and vice versa.

Some substances move through cell membranes more readilythan others. Very small molecules move readily. Nonpolarmolecules move more readily than polar molecules.

Diffusion of substances into or out of cells is passive - it requiresno energy use by the cell to make it happen.

12

Facilitated diffusionProteins can aid the passage of molecules across the cellmembrane.Membrane proteins may serve as pores or channels for themovement of specific chemicals.

Facilitated diffusion requires no direct energy input, but itdoes require the synthesis of proteins.

13

Channel proteins allow specific chemicals to move in responseto specific conditions. The chemicals are often ions: H+, Cl-,Na+, Ca++ etc.

14

Some large molecules (such as sugars and amino acids) that donot diffuse readily through cell membranes can move throughpores in the cell membrane made of protein.Pores and channels canbe specific for only asingle molecule, or fora very narrow class ofmolecules.

Cells producemembrane proteins forsome chemicals thatthey need to have enteror exit the cell morequickly than theywould otherwise.

15

Because facilitated diffusion is mediated by a limited number ofmembrane proteins, the proteins can be saturated.

The rate ofmovement reachesa maximum whenall of the proteinsare being used attheir maximumcapacity.

Simple diffusion isdoes not exhibitsaturation. Greaterconcentrationdifferences result infaster diffusion.

16

Bulk Transport of materials through the formation of vesiclesEndocytosis - intake by vesicle formation

pinocytosis - intake of fluids by vesicle formation (e.g. yolk)phagocytosis - intake of solids by vesicle formation

17

receptor mediated endocytosisspecific receptors line pits in cell membrane,when many molecules of the substance attach to the receptor, avesicle is formed (e.g. cholesterol)

18

Exocytosis - the expulsion of the contents of a vesicle - thereverse of endocytosis (e.g. digestive proteins, mucus)

Endocytosis and exocytosis are active processes. Energy isrequired for the formation and movement of vesicles.

19

Osmosis - the passage of water through a selectively permeablemembrane. Water moves from solution with a lower soluteconcentration to the solution with a higher solute concentration.Cell membranes are selectively permeable membranes. Theyallow only some chemicals to move through them

20

The force of water moving through a membrane can result in anincrease in pressure, or water moving can oppose other forcesthat would move the water, like gravity. Solute differences oneither side of a semipermeable membrane creates “osmoticpressure.”

21

Osmosis is an important phenomenon in cells. Water will moveinto cells if the surrounding solution is hypoosmotic orhypotonic. This will inflate the cell. Animal cells can be overinflated and can burst. Water will leave cells if the surroundingsolution is hyperosmotic or hypertonic. Cells collapse andbecome flaccid.

22

Plant cells also inflate in hypoosmotic solutions, but they don’tburst because they are contained within cell walls. In a leafyplant, turgid cells make leaves stand out straight. Plant cellsdeflate in hyperosmotic solutions. In leafy plants, deflation ofcells results in “wilting.”

23

Active transport - themovement of substancesthrough a membrane againstthe concentration gradient -from low to high concentration- not passive - requires amembrane protein and energyin the form of ATP.

Active transport proteins arevery specific for certaincompounds and will movethem into or out of the cellwhen chemical energy isavailable.

24

Active transport is very important for maintaining theconcentration of certain solutes inside cells and in theextracellular environment - especially Na+ and K+.Most cells maintain the cytoplasm with a relatively highconcentration of K+ and low concentration of Na+ relative to thethe external medium - they do this with the use of a protein calledthe Na/K pump.

25

The difference in concentration of Na+ and K+ between theinside and outside is involved with many cellular phenomena

• the conduction of nervous impulses• the sensitivity of sense organs• the response of many cells to cues in their environment

Stimuli trigger opening of gated channels for Na+ and K+ whichtriggers further cellular change.

One pump cancycle over 300times persecond.

26

Active transport can be coupled to the transport of othersubstances.A large concentration of Na+ on the outside of cells can be usedto carry other substances into or out of the cell.

Cotransport orsymport is coupledtransport in the samedirection (both in orboth out).

Countertransport iscoupled transport inopposite directions (onein the other out).

27

One very important example of the use of a concentration gradientis found in mitochondria. During the breakdown of foodstuffs, H+

ions are pumped out of the mitochondrion using energy extractedfrom REDOX reactions.

This creates a largeconcentration gradient,with more H+ outsidethe matrix than in. TheH+ gradient is used todrive the production ofATP. This idea is calledthe Theory ofChemiosmosis.

28

END OF

CELL BIOLOGY

TIME TO WORKPROBLEM SETS