Download - Subtopic 3 1[1] FORM 5
CHAPTER 3 : MOVEMENT OF
SUBSTANCES ACROSS THE PLASMA MEMBRANE
SUBSTOPICS3.1 - Movement of Substances Across the
Plasma Membrane
3.2 – Understanding the Movement of
Substances Across the Plasma Membrane in Everyday Life
3.3 – Appreciating the Movement of
Substances Across the Plasma Membrane
LEARNING OUTCOMES
To state the substances required by living cells
To state the substances that have to be eliminated from cells
To explain the necessity for movement of substances across the plasma membrane
To describe the structure of the plasma membrane
To describe the permeability of the plasma membrane
NECESSITY FOR MOVEMENT OF SUBSTANCES ACROSS THE PLASMA
MEMBRANETo provide nutrients for metabolism
& growth;To supply oxygen for respiration;To regulate solute concentration &
suitable pH for maintaining a stable internal environment for optimal enzymatic activities
To maintain an ion concentration gradient required for nerve & muscle cell activities;
To secrete useful substances, for example, digestive enzymes & hormones;
To eliminate toxic waste products such as urea & carbon dioxide
Substances can move into or out of a cell by :
Passive transportSimple diffusionOsmosisFacilitated diffusion
Active transport
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
Structure of the Plasma Membrane
Structure of the Plasma Membrane
All cells are covered by a thin plasma membrane.
It separates the cell contents from the surrounding
1972, S.J. Singer & G.L. Nicolson proposed the fluid-mosaic model of plasma membrane.
The plasma membrane is dynamic & 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.
Structure of the Plasma Membrane
The membrane consists of a phospholipid bilayer (2 molecules thick)
The polar hydrophilic heads – outer layer face outwards, chemically attracted to the watery surrounding
The non-polar hydrophobic hydrocarbon fatty acid tails – face inwards, away from water.
Structure of the Plasma Membrane
There are proteins on the outer & inner surfaces of the plasma membrane.
Some proteins penetrate partially through the membrane, others penetrate completely.
The phospholipid bilayer is permeable to diffusion of small uncharged molecules such as O2 & CO2.
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 solute across the cell membrane against the concentration gradient.
Structure of the Plasma Membrane
Cholesterol molecules stabilise the structure of PM.
MECHANISM OF MOVEMENT OF SUBSTANCES ACROSS THE
PLASMA MEMBRANEPERMEABILITYA semipermeable @ partially permeable
membrane = selectively permeable to small molecules such as water & glucose.
Does not permit large molecule to move through it.
Examples : egg membrane, plasma membrane of living cells & cellaphone membrane of the Visking tubing.
MECHANISM OF MOVEMENT OF SUBSTANCES ACROSS THE
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 polythene membrane.
LEARNING OUTCOMESTo explain the movement of substances across
the plasma membrane through the process of passive transport
To explain the movement of water molecules across the plasma membrane by osmosis,
To explain the movement of substances across the plasma membrane through the process of active transport,
To explain the process of passive transport in living organisms using examples
PASSIVE TRANSPORTThe 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 from ATP.
The substances move down their concentration gradient through different ways :
Phospholipid bilayerPore protein/ channel proteinCarrier protein
PASSIVE TRANSPORT
SIMPLEDIFFUSION
OSMOSIS
FACILITATEDDIFFUSION
SIMPLE DIFFUSION
The net movement of molecules / ions from a region of higher concentration to a region of lower concentration until an equilibrium is reached.
Substances :Small non-polar molecules – O2 & CO2Lipid-soluble substances – vitamins ADEK,
steroids & alcoholsWater molecules
The bigger the concentration gradient the faster 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, blood capillaries & body cells.
SIMPLE DIFFUSION
The diffusion of water molecules (solvent) from a region of higher water concentration (diluted solution) to a region of lower water concentration (concentrated solution) through a semi-permeable membrane until an equilibrium is reached.
A special type of diffusion.
Examples :Absorption of water from soil solution by plant root
hairsReabsorption of water by kidney tubules
osmosis
The movement of molecules / ions down their concentration gradient assisted by transport proteins (channel protein / pore protein) across the plasma membrane without using energy.
The transport proteins facilitate & increase the rate of diffusion across the plasma membrane.
Not require energy
FACILITATED DIFFUSION
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 as mineral ions to pass through the pore protein.
Carrier protein : allows larger uncharged polar molecules – glucose & amino acids to cross the membrane.
FACILITATED DIFFUSION
THE MECHANISM
1. The solute moves to the binding site of the specific carrier protein.
2. The solute binds to the carrier protein at the binding site & triggers the carrier protein to change its shape.
3. The carrier protein changes its shape & moves the solute across the membrane.
4. The carrier protein returns back to its original shape.
The solutes can be transported by carrier proteins in either direction but the net movement is always down the concentration gradient.
Examples : the transportation of glucose, amino acids & mineral ions across the membrane of the vilus at the ileum & body cells.
FACILITATED DIFFUSION
The movement of substances across the plasma membrane from a region of low concentration to a region of high concentration (against the concentration gradient) by using metabolic energy.
The substances move across a membrane against the concentration gradient, using metabolic energy
Perform by a specific protein embedded in the plasma membrane.
ACTIVE TRANSPORT
ACTIVE TRANSPORTRequire energy to change the shape of the
protein such that the substance can be pumped across the membrane.
Example : absorption of potassium ions from pond water by algae Nitella sp. against a concentration gradient, the intake of mineral ions by the plant root hairs, Na+/ K+ protein pumps in the plasma membrane of neurones transport Na+ & K+ against their concentration gradients.
ACTIVE TRANSPORT
ACTIVE TRANSPORT
LEARNING OUTCOMES
To explain the process of active transport in living organisms using examples,
To compare and contrast passive transport & active transport.
COMPARISON BETWEEN PASSIVE & ACTIVE TRANSPORT
PASSIVETRANSPORT
SIMILARITIES ACTIVETRANSPORT
DIFFERENCES
Concentration gradient
Cellular energy
Outcome of the process
Occurs in
Name of process
Examples
COMPARISON BETWEEN PASSIVE & ACTIVE TRANSPORT
PASSIVETRANSPORT
SIMILARITIES ACTIVETRANSPORT
Transport of substances across the plasma membraneNeed a difference of concentration gradient between extracellular
environment & the cell
DIFFERENCES
Follow Concentration gradient Against
Does not expend energy Cellular energy Need to expend energy
Until an equilibrium is reached
Outcome of the process Depends on the cells requirement (no need to
reach an equilibrium)
Non-living & living organisms
Occurs in Living organisms only
Simple diffusion, osmosis, facilitated diffusion
Name of process Active transport
Examples