Advanced Biology: Membranes

Advanced BiologyAdvanced Biology

MembranesMembranes

Animal Animal CellCell

The plasma membraneThe plasma membraneIs the boundary that separates the living cell Is the boundary that separates the living cell from its nonliving surroundingsfrom its nonliving surroundings

It allows some substances to cross it more easily It allows some substances to cross it more easily than othersthan others

Membrane CompositionMembrane Composition

LipidsLipids PhospholipidsPhospholipids CholesterolCholesterol

ProteinsProteins TransportTransport SignalingSignaling StructuralStructural

PhospholipidsPhospholipids

The non-polar region The non-polar region forms a barrier from forms a barrier from the surrounding waterthe surrounding water

Polar Head Group

Non-polar Tail

8.1

Membrane ProteinsMembrane Proteins

8.2

Membrane ProteinsMembrane Proteins

8.2

Membrane Membrane FluidityFluidity

Both lipids and Both lipids and proteins can move proteins can move within a membranewithin a membrane

8.3

Enzymatic activityEnzymatic activity

Signal transductionSignal transduction

TransportTransport

ATP

Enzymes

Signal

Receptor

Functions of Functions of Membrane Membrane

ProteinsProteins

8.4

Functions of Functions of Membrane Membrane

ProteinsProteins

Cell-cell Cell-cell recognitionrecognition

Intercellular joiningIntercellular joining

Attachment to the Attachment to the cytoskeleton and cytoskeleton and extracellular matrixextracellular matrix

Glycoprotein

8.4

Membrane Membrane proteins and proteins and lipidslipids

Are synthesized Are synthesized in the ER and in the ER and Golgi apparatusGolgi apparatus

ER

Transmembraneglycoproteins

Secretoryprotein

Glycolipid

Golgiapparatus

Vesicle

Transmembraneglycoprotein

Membrane glycolipid

Plasma membrane:Cytoplasmic face

Extracellular face

Secretedprotein

4

1

2

3

The Permeability of the Lipid The Permeability of the Lipid BilayerBilayer

The membrane regulates transport of all The membrane regulates transport of all molecules in and out of the cellmolecules in and out of the cell Nutrients - amino acids, sugars, lipids Nutrients - amino acids, sugars, lipids Wastes Wastes Oxygen and carbon dioxideOxygen and carbon dioxide Ions - Na , K , Ca , ClIons - Na , K , Ca , Cl Many othersMany others

+ + +2 -

8.5

Two Types of Molecules to Two Types of Molecules to RegulateRegulate

Hydrophobic moleculesHydrophobic molecules Are lipid soluble and can pass through the Are lipid soluble and can pass through the

membrane rapidlymembrane rapidly

Hydrophilic (polar) moleculesHydrophilic (polar) molecules Do not cross the membrane rapidly without Do not cross the membrane rapidly without

the aid of transport proteinsthe aid of transport proteins

DiffusionDiffusion

Is the tendency for Is the tendency for molecules of any molecules of any substance to substance to spread out evenly spread out evenly into the available into the available spacespace

Molecules move Molecules move from areas of high from areas of high concentration to concentration to areas of low areas of low concentrationconcentration

8.6

TonicityTonicityIsotonic Isotonic Same concentration on both sides of a membraneSame concentration on both sides of a membrane

HypertonicHypertonic Higher concentrationHigher concentration

HypotonicHypotonic Lower concentrationLower concentration

8.7

OsmosisOsmosis

Is the movement Is the movement of water across a of water across a semi-permeable semi-permeable membranemembrane

Is caused by an Is caused by an imbalanced imbalanced concentration of concentration of dissolved dissolved substancessubstances

8.8/8.9

Passive and active transport comparedPassive and active transport comparedPassive transport Substances diffuse down their concentration gradients, crossing a membrane with no energy input required. The rate of diffusion can be greatly increased by transport proteins in the membrane.

Diffusion Hydrophobicmolecules can diffuse through the lipid bilayer.

Facilitated diffusion Many hydrophilic substances diffuse through membranes with the assistance of transport proteins,either channel or carrier proteins.

ATP

Active transport Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP.

1 2

3

One type of active One type of active transport systemtransport system

Maintains an Maintains an imbalance in of imbalance in of sodium and sodium and potassium inside potassium inside and outside the celland outside the cell

The Na -K The Na -K PumpPump

++

8.10

Membrane Potential and SignalingMembrane Potential and Signaling

1.1. The Na/K Pump creates a concentration The Na/K Pump creates a concentration difference across the cell membranedifference across the cell membrane

2.2. If a Na channel is then opened, Na is will If a Na channel is then opened, Na is will rush into the cellrush into the cell

3.3. This creates an electrical potential This creates an electrical potential measured in millivolts (mV)measured in millivolts (mV)

4.4. This potential is called an action potential This potential is called an action potential an is the basis for nerve impulsesan is the basis for nerve impulses

8.10

8.10

Bulk TransportBulk Transport

Exocytosis (exit)Exocytosis (exit) Transport vesicles migrate to the plasma Transport vesicles migrate to the plasma

membrane, fuse with it, and membrane, fuse with it, and releaserelease their their contentscontents

Endocytosis (enter)Endocytosis (enter) The cell The cell takes intakes in macromolecules by forming macromolecules by forming

new vesicles from the plasma membranenew vesicles from the plasma membrane

8.11

Three types of endocytosisThree types of endocytosis

8.11