chapter 3-biology form 4-part 1
TRANSCRIPT
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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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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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