Membranes

Chapter 5

Adapted by G. Cornwall, Ph.D. From Raven’s Biology, McGraw Hill Publishing

2

Membrane Structure• Phospholipids arranged in a bilayer• Globular proteins inserted in the lipid bilayer

– Integral membrane proteins– Peripheral membrane proteins

• Fluid mosaic model – mosaic of proteins floats in or on the fluid lipid bilayer like boats on a pond

3

• Cellular membranes have 4 components1. Phospholipid bilayer

• Flexible matrix, barrier to permeability

2. Transmembrane proteins• Integral membrane proteins

3. Interior protein network• Peripheral membrane proteins

4. Cell surface markers• Glycoproteins and glycolipids

4

• Both transmission electron microscope (TEM) and scanning (SEM) used to study membranes

• One method to embed specimen in resin– 1µm shavings– TEM shows layers

• Freeze-fracture visualizes inside of membrane

5

6

Phospholipids• Structure consists of

– Glycerol – a 3-carbon polyalcohol

– 2 fatty acids attached to the glycerol

• Nonpolar and hydrophobic (“water-fearing”)

– Phosphate group attached to the glycerol

• Polar and hydrophilic (“water-loving”)

7

• Spontaneously forms a bilayer– Fatty acids are on the inside– Phosphate groups are on both surfaces

• Bilayers are fluid• Hydrogen bonding of

water holds the 2 layers together

• Individual phospholipids and unanchored proteins can move through the membrane

8

• Environmental influences– Saturated fatty acids make the membrane

less fluid than unsaturated fatty acids• “Kinks” introduced by the double bonds keep them

from packing tightly• Most membranes also contain sterols such as

cholesterol, which can either increase or decrease membrane fluidity, depending on the temperature

– Warm temperatures make the membrane more fluid than cold temperatures

• Cold tolerance in bacteria due to fatty acid desaturases

9

10

Membrane Proteins• Various functions:

1. Transporters

2. Enzymes

3. Cell-surface receptors

4. Cell-surface identity markers

5. Cell-to-cell adhesion proteins

6. Attachments to the cytoskeleton

11

Structure relates to function

• Diverse functions arise from the diverse structures of membrane proteins

• Have common structural features related to their role as membrane proteins

• Peripheral proteins– Anchoring molecules attach membrane

protein to surface

• Anchoring molecules are modified lipids with1. Nonpolar regions that insert into the internal portion

of the lipid bilayer

2. Chemical bonding domains that link directly to proteins

12

13

• Integral membrane proteins– Span the lipid bilayer (transmembrane

proteins)• Nonpolar regions of the protein are embedded in

the interior of the bilayer• Polar regions of the protein protrude from both

sides of the bilayer

– Transmembrane domain• Spans the lipid bilayer• Hydrophobic amino acids arranged in α helice

• Proteins need only a single transmembrane domain to be anchored in the membrane, but they often have more than one such domain

14

• Bacteriorhodopsin has 7 transmembrane domains forming a structure within the membrane through which protons pass during the light-driven pumping of protons

15

16

• Pores– Extensive nonpolar regions within a

transmembrane protein can create a pore through the membrane

– Cylinder of sheets in the protein secondary structure called a -barrel

• Interior is polar and allows water and small polar molecules to pass through the membrane

Card Quiz A

Which part of the phospholipid is responsible for H bonding with water?

Fatty acid chains Phosphate head Glycerol molecule Non polar tails

Card Quiz A

What type of amino acids would you expect to find in the transmembrane portion of a membrane protein?

Charged amino acids Polar amino acids Non-polar amino acids Hydrophilic amino acids

Card Quiz A

How are transmembrane proteins held in the correct position in the membrane?

Covalent bonding to the phosphate group Hydrophobic domain is held in place by

hydrophobic exclusion Cytoskeleton filaments keep them in place The cell wall keeps the proteins in place

Card Quiz Answers

YellowRedBlue

21

Passive Transport• Passive transport is movement of molecules

through the membrane in which– No energy is required– Molecules move in response to a concentration

gradient

• Simple Diffusion is movement of molecules from high concentration to low concentration across a semipermeable membrane– A form of Passive Transport– Will continue until the concentration is the same in all

regions

Annimation

22

• Major barrier to crossing a biological membrane is the hydrophobic interior that repels polar molecules but not nonpolar molecules– Nonpolar molecules will move until the concentration

is equal on both sides– Limited permeability to small polar molecules– Very limited permeability to larger polar molecules

and ions

• Facilitated diffusion– Molecules that cannot cross membrane easily

may move through proteins– Move from higher to lower concentration– Channel proteins

• Hydrophilic channel when open

– Carrier proteins• Bind specifically to molecules they assist

• Membrane is selectively permeable

23

24

Channel proteins• Ion channels

– Allow the passage of ions– Gated channels – open or close in response

to stimulus (chemical or electrical)– 3 conditions determine direction

• Relative concentration on either side of membrane• Voltage differences across membrane• Gated channels – channel open or closed

25

Carrier proteins• Can help transport both ions and other solutes,

such as some sugars and amino acids• Requires a concentration difference across the

membrane• Must bind to the molecule they transport

– Saturation – rate of transport limited by number of transporters

26

Osmosis• Cytoplasm of the cell is an

aqueous solution– Water is solvent– Dissolved substances are solutes

• Osmosis – net diffusion of water across a membrane toward a higher solute concentration

27

Osmotic concentration• When 2 solutions have different osmotic

concentrations– Hypertonic solution has a higher solute concentration– Hypotonic solution has a lower solute concentration

• When two solutions have the same osmotic concentration, the solutions are isotonic

• Aquaporins facilitate osmosis

Osmotic pressure

• The amount of water that enters a cell depends on the difference in solute concentration between the cell and the extracellular fluid

• Cell in a hypotonic solution gains water causing cell to swell – creates pressure

28

• If membrane strong enough, cell reaches counterbalance of osmotic pressure driving water in with hydrostatic pressure driving water out– Cell wall of prokaryotes,

fungi, plants, protists

• If membrane is not strong, may burst– Animal cells must be in

isotonic environments

29

30

Maintaining osmotic balance

• Some cells use extrusion in which water is ejected through contractile vacuoles

31

• Isosmotic regulation involves keeping cells isotonic with their environment– Marine organisms adjust

internal concentration to match sea water

– Terrestrial animals circulate isotonic fluid

• Plant cells use turgor pressure to push the cell membrane against the cell wall and keep the cell rigid

Card Quiz B

Permeability refers to – the movement of molecules from an area of

greater concentration to an area of lower concentration

the amount of solute in a solution the extent a membrane allows a substance to

pass through the state of being permanent

Card Quiz B

The movement of molecules from an area of high concentration to an area of low concentration is –

Osmosis Active transport Solubility Diffusion

Card Quiz B

A cell is placed in an hypotonic solution. Which way will the water move?

Into the cell Out of the cell No net movement

Card Quiz B

If a blood cell (0.9% NaCl) shrivels, what type of solution was it placed in?

0.8% 0.1% 0.9% 4%

Card Quiz Answers

BlueBlueBlue

Yellow

37

Active Transport

• Some molecules need to be moved against their concentration gradient

• Requires energy – ATP is used directly or indirectly to fuel active transport

• Moves substances from low to high concentration (against the gradient)

• Requires the use of highly selective carrier proteins

38

• Carrier proteins used in active transport – Uniporters – move one molecule at a time– Symporters – move two molecules in the

same direction– Antiporters – move two molecules in opposite

directions

39

Sodium–potassium (Na+–K+) pump• Direct use of ATP for active transport• Uses an antiporter to move 3 Na+ out of the cell

and 2 K+ into the cell– Against their concentration gradient

40

• ATP energy is used to change the conformation of the carrier protein

• Affinity of the carrier protein for either Na+ or K+

changes so the ions can be carried across the membrane

41

Coupled transport• Uses ATP indirectly (2° Active Transport)• Uses the energy released when a molecule

moves by diffusion to supply energy to active transport of a different molecule

42

• Symporter is used• Glucose–Na+ symporter captures the energy

from Na+ diffusion to move glucose against a concentration gradient

Bulk Transport• Endocytosis

– Movement of substances into the cell– Phagocytosis – cell takes in particulate matter– Pinocytosis – cell takes in only fluid

43

• In the human genetic disease familial hypercholesterolemia, the LDL receptors lack tails, so they are never fastened in the clathrin-coated pits and as a result, do not trigger vesicle formation. The cholesterol stays in the bloodstream of affected individuals, accumulating as plaques inside arteries and leading to heart attacks.

44

Receptor-mediated endocytosis – specific molecules are taken in after they bind to a receptor

• Exocytosis – Movement of materials out of the cell– Used in plants to export cell wall material– Used in animals to secrete hormones,

neurotransmitters, digestive enzymes

45

Card Quiz C

A macrophage engulfing a bacterium is an example of – Exocytosis Pinocytosis Chemocytosis Phagocytosis

Card Quiz C

What type of transport protein can move 2 different molecules in the same direction?

Uniporter Antiporter Symporter Multiporter

Card Quiz C

How is the sodium-potassium pump able to move ions against their concentration gradients?

Facilitated diffusion Expending ATP Ion channels Bulk transport

Card Quiz Answers

GreenBlueRed