CHAPTER 3 : MOVEMENT OFSUBSTANCES ACROSS THE PLASMA MEMBRANE
SUBSTOPICS3.1 - Movement of Substances Across the Plasma Membrane3.2 – Understanding the Movement of Substances Across the Plasma Membrane in Everyday Life3.3 – 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 OFSUBSTANCES ACROSS THE PLASMA MEMBRANE To 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 transport Simple diffusion Osmosis Facilitated diffusionActive 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 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 ACROSSTHE PLASMA MEMBRANEPERMEABILITY A 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 ACROSSTHE PLASMA MEMBRANE A 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 OUTCOMES To 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 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 from ATP. The substances move down their concentration gradient through different ways : Phospholipid bilayer Pore protein/ channel protein Carrier protein
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 & CO2 Lipid-soluble substances – vitamins ADEK, steroids & alcohols Water molecules
SIMPLE DIFFUSION 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.
osmosis 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 hairs Reabsorption of water by kidney tubules
FACILITATED DIFFUSION 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.
THE MECHANISM The solute moves to the binding site of the specific carrier protein. The solute binds to the carrier protein at the binding site & triggers the carrier protein to change its shape. The carrier protein changes its shape & moves the solute across the membrane. The carrier protein returns back to its original shape.
FACILITATED DIFFUSION 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.
ACTIVE TRANSPORT 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 Require 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.
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 PASSIVE SIMILARITIES ACTIVETRANSPORT TRANSPORT DIFFERENCES Concentration gradient Cellular energy Outcome of the process Occurs in Name of process Examples
COMPARISON BETWEEN PASSIVE & ACTIVE TRANSPORT PASSIVE SIMILARITIES ACTIVE TRANSPORT TRANSPORT Transport of substances across the plasma membraneNeed a difference of concentration gradient between extracellular environment & the cell DIFFERENCES Follow Concentration Against gradientDoes not expend energy Cellular energy Need to expend energy Until an equilibrium is Outcome of the Depends on the cells reached process requirement (no need to reach an equilibrium) Non-living & living Occurs in Living organisms only organismsSimple diffusion, osmosis, Name of process Active transport facilitated diffusion Examples