Chapter 3-Biology Form 4-Part 1

7/28/2019 Chapter 3-Biology Form 4-Part 1

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Chapter 3: The movement ofSubstances across the PlasmaMembrane


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The necessity for the movement of

substances across a plasma membrane

are:

a) Cells need water,nutrients and oxygen.

b) Cells produce waste product which exitthrough the plasma membrane.

c) The plasma membrane control the typesand the amounts of substances needed bythe cell at any one time.


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Cytoplasm

Nucleus

Plasmamembrane

External environment

Cell

Movement of substances outof the cell

Movement of substances into

the cell

Figure 3.1: Movement of substances in and out of the cell


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Plasma Membrane Function

Protect and separate the interior

part (protoplasm) from externalenvironment.

Maintain specific shape of the

cell.

Act as semipermeable or selective

barrier ,does not allow entry of allmolecules inside the cell.

Help to adhere with adjacent cells

to form tissue.

Maintains connection with

adjacent cells via pores on

membrane known as

plasmodesmata(in plants)and

desmosome(in animals)

Helps in transport across the

membrane via osmosis anddiffusion.

Allow transport ofmolecules(mainly protein) againstconcentration gradient via activetransport.

Contains ion channels which helpin active transport.

exocytosis (excretion of wasteoutside the cell) and endocytosis(intake of large particles insidethe cell).

(phagocytes) include importantrole in immunity

Receptor proteins present on cellsurface(in plasma membrane)plays important role in cellsignalling.


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The structure of the plasma

membrane


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Singer and Nicholson : fluid mosaic model(1972)

The basic unit : phospholipid molecule,

proteins


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Structure of the Plasma Membrane

There are proteins on the outer & innersurfaces of the plasma membrane.

Some proteins penetrate partiallythrough the membrane, otherspenetrate completely.

The phospholipid bilayer is permeableto diffusion of small unchargedmolecules such as O2 & CO2.


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Structure of the Plasma Membrane Two types of transport protein :

Channel / pore proteins have pore to facilitate

diffusion of particular ions / molecules across the PM.

Some carrier proteins have binding sites that bind to

specific molecules such as glucose @ 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 concentration

gradient.


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The phospholipid molecule consists of:

a) A polar head gives Hydrophilic propertymeans water-loving or attracted to watermolecule

b) A pair of non-polar fatty acid tails givesHydrophobicwater-hating, or repellingwater molecules.


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In this phospholipid bilayer the:

a) Hydrophilic heads points outwards facingwater molecule on both sides.

b) Hydrophobictails points inwards, away from

water molecules.

h l l h l


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Other molecule present in the plasma

membrane are:a) Cholesterol -rigidand stable.

b) Carrier protein and channel protein -movement of water-soluble ions

c) Glycolipids -lipids and polysaccharides, cells

recognisationd) Glycoprotein -protein and polysaccharides,

cells recognisation


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[email protected]

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[email protected]
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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The Permeability of PlasmaMembrane


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Membrane permeability

A quality of a cellsplasma membrane that

allows substances to pass in and out of it, so

that the cell can expel waste products and ship out the

chemicals it assembles for the body nutrients that the cell needs can pass through

the membrane to the inside.

Cell membranes have selective permeability, meaning thatthey will allow certain substances to pass while forming a

barrier against others.
http://www.wisegeek.com/what-is-plasma.htmhttp://www.wisegeek.com/what-is-plasma.htm


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MECHANISM OF MOVEMENTOF SUBSTANCES ACROSSTHE PLASMA MEMBRANEPERMEABILITY

A semipermeable @ partially permeablemembrane = selectively permeable to small

molecules such as water & glucose.

Does not permit large molecule to move throughit.

Examples : egg membrane, plasma membraneof living cells & cellaphone membrane of theVisking tubing.


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MECHANISM OF MOVEMENTOF SUBSTANCES ACROSSTHE PLASMA MEMBRANEA permeable membrane permeable to the

many solvent (water) & solute molecules diffusion can occur.

Example : cellulose cell wall of plant cell

An impermeable membrane not allow

substances to diffuse through it.

Example : the impermeable polythenemembrane.


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The phospholipid

bilayer is permeable to

Non-polar molecules(lipid soluble)

Non-polar

molecules (lipidsoluble)


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The phospholipid bilayer is permeable to:

a)Small non-polar (hydrophobic) molecules that

are lipid-soluble, such as fatty acids, glycerol,

steroid, vitamin A, D, E and K.b)Small uncharged molecules, such as water,

oxygen and carbon dioxide. These molecules are

small enough to squeeze through between the

phospholipid gaps by simple diffusion or osmosis

down their respective concentration gradients.

Substances that are non-polar and lipid-soluble candiffuse in and out a plasma membrane. Lipid soluble

means able to dissolve in fats.


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The phospholipid bilayer is

not permeable to

Polar molecules (lipid-insoluble) Charged ions


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The phospholipid bilayer is not permeable to:a)Large polar molecule, that are not soluble in lipid, such

as glucose, amino acids, nucleic acids and

polysaccharides.

b)Ions (charged), regardless of size, such as: H+, Na+,

HCO3-, K+, Ca+, and Mg+

Polar substances are not able to pass throughthe cell membrane because the heads will

repel them; the charged substances repel

other charged substances, much like two

magnets. The cell membrane is also

impermeable to substances that are not lipid-

soluble, as they are unable to pass through

the lipids of the membrane.


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Chapter 3-Biology Form 4-Part 1

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