Lecture Cell

Chapters 5 and 6 Biological Membranes and

Cell Communication

Membrane structure, I

Selective permeability Amphipathic~

hydrophobic & hydrophilic regions

Singer-Nicolson: fluid mosaic model

Fig. 5-6, p. 111

Glycoprotein

Glycolipid

Cytosol

Hydrophilic

Carbohydratechain

Hydrophilic

HydrophobicExtracellular fluid

Cholesterol

α-helix Integralproteins

Peripheralprotein

Carbohydratechains

Membrane structure, II

Phospholipids~ membrane fluidity Cholesterol~ membrane stabilization “Mosaic” Structure~ Integral proteins~ transmembrane

proteins Peripheral proteins~ surface of

membrane Membrane carbohydrates ~ cell to

cell recognition; oligosaccharides (cell markers); glycolipids; glycoproteins

Fig. 5-2b, p. 108

Integral(transmembrane)

protein

(b) Fluid mosaic model. According to this model, a cellmembrane is a fluid lipid bilayer with a constantly changing“mosaic pattern“ of associated proteins.

Phospholipidbilayer

Peripheralprotein

Hydrophilicregion of protein

Hydrophobicregion of protein

The Lipid Bilayer

Lipids of the bilayer fluid or liquid-crystalline state

Proteins move within the membrane

Membrane Proteins

Functions of Membrane Proteins

Membrane traffic Diffusion~ tendency of any

molecule to spread out into available space

Concentration gradient Passive transport~ diffusion

of a substance across a biological membrane without using energy

Osmosis~ the diffusion of water across a selectively permeable membrane

Diffusion

Osmosis

Fig. 5-12, p. 117

Pressure appliedto piston to resistupward movement

Watermolecule

Selectivelypermeablemembrane

Pure water

Moleculeof solute

Waterplussolute

Water balance Osmoregulation~ control

of water balance

Hypertonic~ higher concentration of solutes

Hypotonic~ lower concentration of solutes

Isotonic~ equal concentrations of solutes

Cells with Walls: Turgid (very firm)

Flaccid (limp)

Plasmolysis~ plasma membrane pulls away from cell wall

Osmotic Pressure

Fig. 5-13a, p. 118

SolutemoleculesNo net water

movement

(a) Isotonic solution.When a cell is placed in anisotonic solution, watermolecules pass in and outof the cell, but the netmovement of watermolecules is zero.

Outsidecell

Insidecell

H20molecules

Fig. 5-13b, p. 118

Outsidecell

(b) Hypertonic solution.When a cell is placed in ahypertonic solution, thereis a net movement ofwater molecules out of thecell (blue arrow). The cellbecomes dehydrated andshrunken.

Net water movementout of the cell

Insidecell

Fig. 5-13c, p. 118

10 μ

m

(c) Hypotonic solution.When a cell is placed in ahypotonic solution, the netmovement of watermolecules into the cell(blue arrow) causes the cellto swell or even burst.

Insidecell

Outsidecell

Net water movementinto the cell

Turgor and Plasmolysis

Fig. 5-14, p. 119

Plasmamembrane

Vacuole

Vacuolarmembrane(tonoplast)

Vacuole

Cytoplasm

Plasmamembrane Nucleus

Specialized Transport

Transport proteins Facilitated diffusion~

passage of molecules and ions with transport proteins across a membrane down the concentration gradient

Active transport~ movement of a substance against its concentration gradient with the help of cellular energy

Facilitated Diffusion

Fig. 5-16a, p. 120

1 Glucose binds to GLUT 1.

Cytosol

Lowconcentrationof glucose

Glucosetransporter(GLUT 1)

Highconcentrationof glucose

Glucose

Outside cell

Fig. 5-16b, p. 120

GLUT 1 changes shape andglucose is released inside cell.

2

Fig. 5-16c, p. 120

GLUT 1 returns to its originalshape.

3

Types of Active Transport Sodium-potassium pump Exocytosis~ secretion of

macromolecules by the fusion of vesicles with the plasma membrane

Endocytosis~ import of macromolecules by forming new

vesicles with the plasma membrane

•phagocytosis•pinocytosis•receptor-mediated endocytosis (ligands)

Fig. 5-17a, p. 121

(a) The sodium–potassium pump is a carrier protein that requiresenergy from ATP. In each complete pumping cycle, the energy ofone molecule of ATP is used to export three sodium ions (Na+)and import two potassium ions (K+).

So

diu

mco

nce

ntr

atio

n g

rad

ien

t

Lower

Higher

Cytosol

Outsidecell

Activetransportchannel

Lower

Higher

Po

tass

ium

c on

c en

trat

ion

gr a

die

nt

Fig. 5-17b, p. 121

1. Three Na+ bindto transport protein.

2. Phosphate group istransferred from ATPto transport protein.

3. Phosphorylationcauses carrier proteinto change shape, releasing3 Na+ outside cell.

4. Two K+ bind totransport protein.

5. Phosphate is released.6. Phosphate release causescarrier protein to returnto its original shape. TwoK+ ions are released insidecell.

Phagocytosis

Large particles enter cell

Receptor-MediatedEndocytosis

Intracellular junctions PLANTS: Plasmodesmata: cell wall

perforations; water and solute passage in plants

ANIMALS: Tight junctions~ fusion of

neighboring cells; prevents leakage between cells

Desmosomes~ riveted, anchoring junction; strong sheets of cells

Gap junctions~ cytoplasmic channels; allows passage of materials or current between cells

Tight Junctions

Gap Junctions

Cell Signaling

1. Synthesis, release, transport of signaling molecules

- neurotransmitters, hormones, etc- ligand binds to a specific receptor

2. Reception of information by target cells

Cell Signaling

3. Signal transduction- receptor converts extracellular signal into

intracellular signal- causes change in the cell

4. Response by the cell

Cell Signaling

Fig. 6-2, p. 136

Signalingprotein

Protein thatregulates a gene

Alteredgene activity

EnzymeProtein

Alteredmembrane

permeability

Alteredmetabolism

Response

Signaltransduction

Cell sendssignal

Reception

Receptor

Signalingmolecules

1

2

3

4

Local Regulators

Paracrine regulation diffuse through interstitial fluid act on nearby cells

Local regulators histamine growth factors prostaglandins nitric oxide

Local Signals

Neurotransmitters

Chemical signals released by neurons (nerve cells)

Hormones

Chemical messengers in plants and animals

Secreted by endocrine glands in animals

Transported by blood to target cells

Hormones