Bio Chapter 3

8/8/2019 Bio Chapter 3

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CHAPTER 3 :

MOVEMENT OF

SUBSTANCES ACROSS

THE PLASMAMEMBRANE

Athirah Badrulsham

4 Setia (2010)

5 Setia (2011)


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NECESSITY FOR MOVEMENT OF

SUBSTANCES ACROSS THE PLASMA

MEMBRANE

To provide nutrients for metabolism and

growth To supply oxygen for respiration;

To regulate solute concentration and suitable

pH for maintaining a stable internalenvironment for optimal enzymatic activities

To maintain an ion concentration gradient

required for nerve and muscle cell activities


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Substances can move into or out of a cell by :

Passive transport :a. Simple diffusion

b. Osmosis

c. Facilitated diffusion

Active transport


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Movement of substances across the plasma

membrane would depend on :

Selectivity of the partially permeable

membrane; The difference in concentration between the

cell & extracellular fluid


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Structure of plasma membrane


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Structure of the Plasma

Membrane

All cells are covered by a thin plasma

membrane.

It separates the cell contents from thesurrounding

1972, S.J. Singer & G.L. Nicolson proposed the

fluid-mosaic model of plasma membrane.


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The plasma membrane is dynamic and fluid.

The phospholipid molecules can move thus

giving the membrane its fluidity & flexibility

The proteins are scattered in the membrane

giving it a mosaic appearance

Thickness : 7.0 8.0 nm


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The membrane consists of a phospholipid bilayer(2 molecules thick)

The polar hydrophilic head

outer layer faceoutwards, chemicallyattracted to the waterysurrounding

The non-polar hydrophobic

hydrocarbon fatty acid tails face inwards, away fromwater.


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Membrane

There are proteins on the outer and inner

surfaces of the plasma membrane.

Some proteins penetrate partially through themembrane, others penetrate completely.

The phospholipid bilayer is permeable to

diffusion of small uncharged molecules such

as O and CO .


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Membrane

Two types of transport protein :

Pore proteins have pore to facilitate

diffusion of particular ions / molecules across the plasmamembrane.

Some carrier proteins have binding sites that bind to

specific molecules such as glucose or amino acids

alter their shape to facilitate the diffusion of solutes.Other carrier proteins function in active transport

an energized carrier protein actively pumps the soluteacross the cell membrane against the concentrationgradient.


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Cholesterol makes the bilayer stronger,more

flexible, and less permeable to water-soluble

substances such as ions.


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PASSIVE TRANSPORT

The movement of particles (molecules/ions)

within a gas or liquid across the plasma

membrane from a region of higher concentration

to a region of lower concentration.

Does not require expenditure of energy fromATP.

The substances move down their concentration

gradient through different ways :

-Phospholipid bilayer

-Pore protein

-Carrier protein


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PASSIVE TRANSPORT

SIMPLE DIFFUSION

OSMOSIS

FACILITATED DIFFUSION


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Simple diffusion

The net movement of molecules / ions from aregion of higher concentration to a region oflower concentration until an equilibrium is

reached. Substances :

-Small non-polar molecules O and CO

-Lipid-soluble substances vitamins ADEK,steroids & alcohols

-Water molecules


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The bigger the concentration gradient thefaster the rate of diffusion.

These substances will diffuse down the

concentration gradient if there is a

concentration gradient. (until an equilibrium is

reached).

Examples : gaseous exchange between the

alveolus & the blood capillaries, bloodcapillaries & body cells


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EXAMPLES:


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Osmosis

The diffusion of water molecules (solvent) from aregion of higher water concentration (dilutedsolution)to a region of lower water concentration(concentrated solution) through a semi-permeable membrane until an equilibrium isreached.

A special type of diffusion.

Examples :

-Absorption of water from soil solution by plantroot hairs

-Reabsorption of water by kidney tubules


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Facilitated Diffusion

The movement of molecules / ions down theirconcentration gradient assisted by transportproteins(pore protein) across the plasma membrane

withoutusing energy.

The transport proteins facilitate & increase the rate

of diffusion across the plasma membrane.

Not require energy


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The rate of facilitated diffusion depends on

the number of transport protein molecules in

the membrane & how fast they can move

their specific solute.

Only allows small charged molecules such asmineral ions to pass through the pore protein.

Carrier protein : allows larger uncharged polar

molecules glucose and amino acids to crossthe membrane


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THE MECHANISM

The solute moves to the binding site of thespecific carrier protein.

The solute binds to the carrier protein at the

binding site & triggers the carrier protein tochange its shape.

The carrier protein changes its shape & moves

the solute across the membrane. The carrier protein returns back to its original

shape


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Active transport

The movement of substances across the plasmamembrane from a region of low concentration to aregion of high concentration (against the concentration

gradient) by usingmetabolic energy.

The substances move across a membrane against theconcentration gradient, using metabolic energy

Perform by a specific protein embedded in the plasmamembrane.


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ACTIVE TRANSPORT

Require energy to change the shape of theprotein such that the substance can be pumpedacross the membrane.

Example : absorption of potassium ions frompond water by algae Nitella sp. against aconcentration gradient, the intake of mineral ionsby the plant roothairs, Na+/ K+ protein pumps in the plasmamembrane of neurones transport Na+ & K+against their concentration gradients


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Comparison between passive

transport and active transport

PASSIVE TRANSPORT ACTIVE TRANSPORT

The movement of substances follows the

concentration gradient

Substances move against the

concentration gradient

Cellular energy is not required Cellular energy (ATP) is required

The process continues until a dynamic

equilibrium is reached

The process results in the accumulation

and elimination of substances from the

cell.

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Bio Chapter 3