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Diploma In Microbiology Diploma In Microbiology

MIC102MIC102

CHAPTER 2CHAPTER 2 Movement In And Out Of Movement In And Out Of

CellCell

Lecturer: Pn Aslizah Binti Mohd Aris06-4832100 / 016-7377621

Chapter OutlineChapter OutlineDiffusionOsmosis & Active TransportEndocytosis

- Phagocytosis - Pinocytosis

Excocytosis

DIFFUSIONDIFFUSIONDiffusion is the tendency for molecules to

spread out evenly into the available space.Although each molecule moves randomly,

diffusion of a population of molecules may exhibit a net movement in one direction.

At dynamic equilibrium, as many molecules cross one way as cross in the other direction

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Molecules of dye Membrane (cross section)

WATER

Net diffusion Net diffusion Equilibrium

(a) Diffusion of one solute

Net diffusion

Net diffusion

Net diffusion

Net diffusion

Equilibrium

Equilibrium

(b) Diffusion of two solutes

The

diff

usio

n of

solu

tes a

cros

s a m

embr

ane

Two mode of diffusions:Two mode of diffusions:1) Simple diffusion2) Facilitated diffusion

Figure 4.17a

Movement of Materials Movement of Materials across Membranesacross Membranes

Simple diffusion: Movement of a solute from an area of high concentration to an area of low concentration

Figure 4.17b-c


Facilitated diffusion: Solute combines with a transporter protein in the membrane

Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another

No work must be done to move substances down the concentration gradient

The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen


Figure 4.18a


Osmosis: The movement of water across a selectively permeable membrane from an area of high water to an area of lower water concentration

Osmotic pressure: The pressure needed to stop the movement of water across the membrane

Figure 4.17d


Through lipid layer

Aquaporins (water channels)

Figure 4.18a–b

The Principle of OsmosisThe Principle of Osmosis

Figure 4.18c–e

The Principle of OsmosisThe Principle of Osmosis

Effects of Osmosis on WaterEffects of Osmosis on Water BalanceBalance

Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration


Lowerconcentration

of solute (sugar)

H2O

Higher concentration

of sugar

Selectivelypermeablemembrane

Same concentrationof sugar

Osmosis

Water Balance of Cells Without Water Balance of Cells Without WallsWalls

Tonicity is the ability of a solution to cause a cell to gain or lose water

Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane

Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water

Hypotonic solution: Solute concentration is less than that inside the cell; cell gains waterCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Hypotonic solution

(a) Animal cell

Lysed Normal

Isotonic solution

Shriveled

Hypertonic solution

H2O H2O H2O H2O

Hypertonic or hypotonic environments create osmotic problems for organisms

Osmoregulation, the control of water balance, is a necessary adaptation for life in such environments

The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump

Video: Video: ChlamydomonasChlamydomonas Video: Video: ParameciumParamecium Vacuole Vacuole


Filling vacuole 50 µm

(a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm.

Contracting vacuole

(b) When full, the vacuole and canals contract, expelling fluid from the cell.

Water Balance of Cells with Water Balance of Cells with WallsWalls

Cell walls help maintain water balance

A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm)

If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt


Hypotonic solution

(b) Plant cell

Turgid (normal)

Isotonic solution

Flaccid Plasmolyzed

Hypertonic solution

H2OH2OH2O H2O

PlasmolysisPlasmolysisIn a hypertonic environment, plant

cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis


Facilitated Diffusion: Passive Transport Facilitated Diffusion: Passive Transport Aided by ProteinsAided by Proteins

In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane

Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane

Channel proteins include◦ Aquaporins, for facilitated diffusion of water◦ Ion channels that open or close in response to a

stimulus (gated channels)


EXTRACELLULAR FLUID

Channel protein

(a) A channel protein

Solute CYTOPLAS

M

Solute Carrier protein

(b) A carrier protein

Active transport uses energy to Active transport uses energy to move solutes against their move solutes against their

gradientsgradients

Facilitated diffusion is still passive because the solute moves down its concentration gradient

Some transport proteins, however, can move solutes against their concentration gradients


The Need for The Need for EnergyEnergy in in Active TransportActive Transport

Active transport moves substances against their concentration gradient

Active transport requires energy, usually in the form of ATP

Active transport is performed by specific proteins embedded in the membranes

Active transport allows cells to maintain concentration gradients that differ from their surroundings

The sodium-potassium pump is one type of active transport system


2

EXTRACELLULARFLUID [Na+] high

[K+] low

[Na+] low [K+] high

Na+ Na+

Na+

Na+ Na+

Na+ CYTOPLASM

ATP

ADP P

Na+ Na+

Na+

P

3

K+

K+ 6

K+

K+

5 4

K+

K+

P P

1

The

sodi

um-p

otas

sium

pum

p: a

spec

ific

case

of a

ctiv

e tra

nspo

rt

Passive transport

Diffusion Facilitated diffusion

Active transport

ATP

How Ion Pumps Maintain How Ion Pumps Maintain Membrane PotentialMembrane Potential

Membrane potential is the voltage difference across a membrane

Voltage is created by differences in the distribution of positive and negative ions


Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane:

◦A chemical force (the ion’s concentration gradient)

◦An electrical force (the effect of the membrane potential on the ion’s movement)


An electrogenic pump is a transport protein that generates voltage across a membrane

The sodium-potassium pump is the major electrogenic pump of animal cells

The main electrogenic pump of plants, fungi, and bacteria is a proton pump


Fig. 7-18

EXTRACELLULAR

FLUID H+

H+

H+

H+

Proton pump

+

+

+

H+

H+

+

+

H+

–

–

–

–

ATP

CYTOPLASM

–

Cotransport: Coupled Cotransport: Coupled Transport by a Membrane Transport by a Membrane

ProteinProteinCotransport occurs when active transport of

a solute indirectly drives transport of another solute

Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell


Fig. 7-19

Proton pump

–

–

–

–

–

–

+

+

+

+

+

+

ATP

H+

H+

H+

H+

H+

H+

H+

H+

Diffusionof H+

Sucrose-H+

cotransporter

Sucrose

Sucrose

Bulk transport across the plasma membrane Bulk transport across the plasma membrane occurs by exocytosis and endocytosisoccurs by exocytosis and endocytosis

Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins

Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles

Bulk transport requires energy


ExocytosisExocytosis

In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents

Many secretory cells use exocytosis to export their products


EndocytosisEndocytosisIn endocytosis, the cell takes in

macromolecules by forming vesicles from the plasma membrane

Endocytosis is a reversal of exocytosis, involving different proteins

There are three types of endocytosis:◦Phagocytosis (“cellular eating”)◦Pinocytosis (“cellular drinking”)◦Receptor-mediated endocytosis


Phagocytosis & PinocytosisPhagocytosis & PinocytosisIn phagocytosis a cell engulfs a

particle in a vacuoleThe vacuole fuses with a lysosome to digest the particleIn, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles


In receptor-mediated endocytosis, binding

of ligands to receptors triggers vesicle formation

A ligand is any molecule that binds specifically to a receptor site of another molecule


PHAGOCYTOSIS

EXTRACELLULARFLUID

CYTOPLASM

Pseudopodium

“Food”orother particle

Foodvacuole

PINOCYTOSIS

1 µm

Pseudopodiumof amoeba

Bacterium

Food vacuole

An amoeba engulfing a bacteriumvia phagocytosis (TEM)

Plasmamembrane

Vesicle

0.5 µm

Pinocytosis vesiclesforming (arrows) ina cell lining a small

blood vessel (TEM)

RECEPTOR-MEDIATED ENDOCYTOSIS

Receptor Coat protein

Coatedvesicle

Coatedpit

Ligand

Coatprotein

Plasmamembrane

A coated pitand a coated

vesicle formedduring

receptor-mediated

endocytosis(TEMs)

0.25 µm

Movement of Substances Across Movement of Substances Across MembranesMembranes Passive Transport: Cell expends no

energy to move substances down a concentration gradient (high to low concentration)

1. Simple Diffusion2. Facilitated Diffusion3. Osmosis

Active Transport: Cell expends energy from ATP, enabling it to transport substances against a concentration gradient

Simple Diffusion: The random movements of molecules cause them to spread out (diffuse) from an area of high concentration

to area of low concentration

Facilitated Diffusion: Carrier protein molecules aid in the

movement of substances through cell membrane from

high to low concentration

Osmosis: The diffusion of water from an area of

high water concentration to an area of low water

concentration

Experiments that examine the effects of tonicity on osmosis

Active Transport: Carrier Protein molecules aid in movement of molecules against a concentration gradient

Endocytosis and ExocytosisEndocytosis and Exocytosis Eukaryotic cells move substances

by forming membrane-enclosed vesicles

1. Endocytosis: Form by invagination (poking in) and surrounding substances from outside the cell

2. Exocytosis: Vesicles inside the cell fuse with the plasma membrane and extrude contents from the cell

To be continued…..To be continued…..

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