chapter 3 - diffusion & osmosis

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Chapte r 3Diffusion and Osmosis

OBJECTIVES By the end of the lesson, you should be able to define and describe diffusion, osmosis and active transport to describe the idea of a partially permeable membrane assess the importance of water potential gradient in water uptake by plants explain the effects of osmosis in plant and animal tissues define and discuss importance of active transport in ion-uptake by root hairs and glucose uptake by cells in the villi

Int roduc tion Recall: ~ A cell is the basic unit of life ~ Many complex reactions take place inside the cell ~This means that many raw materials must be brought into the cell ~ And new products formed by the chemical reactions, and waste materials, must exit the cell.

How ? How do the raw materials get into the cell? How do the new products leave the cell? How are the waste materials removed from the cell? Answer: Materials move across the partially permeable cell membrane diffusion osmosis active transport By processes called _________, _________ and _______________

DiffusionDiffusion is the net movement of ions or solute molecules from a region of higher concentration to a region of lower concentration, down a concentration gradient molecule moves from a region of highregion of high concentration of molecule X

concentration to that of low concentration

concentration gradient region of low concentration of molecule X

Diffusion Diffusion in gases


Gas molecules move from a region of higher concentration to that of a lower concentration


The movement continues until they are evenly spread

Diffusion Diffusion in liquids

A. Dissolved particles of a

B. The particles

substance in a liquid move from a region where they are more concentrated to a region where they are less concentrated.

continue to move until they are evenly distributed.

DiffusionImportance of Diffusion

Initially, the concentration of a substance outside the cell is higher than inside the cell. The particles diffuse across the cell membrane.

Later, an equal concentration of the substance inside and outside the cell is attained.

DiffusionImportance of Diffusion Living cells continuously use up oxygen during aerobic respiration. The concentration of oxygen inside the cell falls. Oxygen molecules diffuse into the cell until the oxygen concentration is raised again. During respiration, carbon dioxide is produced. The concentration of carbon dioxide rises and creates a concentration gradient.carbon dioxide oxygen

Oxygen is supplied to Amoeba by diffusion. Diffusion enables metabolic wastes such as carbon dioxide to be excreted out.

DiffusionFactors which Affect the Rate of Diffusion Temperature: An increase in temperature results in a higher rate of diffusion. Temp Molecules move faster Rate of diffusion

Size of particles: Small molecules or ions diffuse faster than large ones. Particle size Energy to move particle Rate of diffusion

Thickness of the barrier: Membranes of plant and animal cells are of about the same in thickness and permeability. Plant cell walls vary in their thickness. The thicker the cell walls, the slower the rate of diffusion. Thinner membrane Higher rate of diffusion

Concentration gradient: The greater the concentration gradient, the higher the rate of diffusion Steeper concentration gradient Higher rate of diffusion

Surface area: The rate of diffusion into a cell depends on the total surface area of the cell membrane. surface area rate of diffusion.


Diffusion in our body Recall: Our cells have a partially permeable membrane Partially permeable membrane Allows some substance through and not others How do these substances pass through? via DIFFUSION

Practical Application Kidney dialysis Waste products are removed from the blood by diffusing across artificial membrane in the machine Blood cells and other large protein are retained

OsmosisOsmosis is the diffusion of water across a selectively permeable membrane, from a high water potential to a low water potential.selectively-permeable membrane

The selectively permeable membrane on the side of the water is being continuously bombarded by water molecules which pass through. This causes an increase in the volume of sucrose solution and a decrease in the volume of water.

water molecules

sucrose molecule


sucrose solution

OsmosisWater PotentialThe water potential of a solution is a measure of the tendency of water molecules to move from one place to another A dilute solution has a higher water potential than a concentrated solution The difference in water potential between two solutions is known as the water potential gradient

OsmosisOsmosis and Plant CellsIn plant cells, the cell sap contains dissolved salts and sugar. If the cell sap has a lower water potential than that of the surrounding fluids, water from the outside enters by osmosis. Turgor A state when a cell is firm or turgid due to water entering the cell, causing it to swell. Turgor pressure The outward pressure which the cell sap exerts against the inside wall of the cell

Importance of Turgor in Plants1. Changes in turgor of the guard cell causes the opening of the stomata

Turgor (II) Changes in turgor of the pulvinus (small swelling at the base of the leaflets) causes the folding of leaflets in the Mimosa

OsmosisOsmosis and Plant Cellscell sap more concentrated than outside solution water enters by osmosis

Turgid cells. The cytoplasm is pressed against the cell wall. The cell swells up and is prevented from bursting by the cellulose cell wall.cell sap less concentrated than outside solution water leaves by osmosis

cellulose cell wall

plasma membrane

Plasmolysed cells. Water is lost from its vacuole, the cytoplasm shrinks and is withdrawn from the cell wall.


Plasmolysis leads to wilting

Fertilizer - Good or Bad? Too much fertilizer added will cause the plant to wilt Soil solution becomes very concentrated Water moves out of root Plant will wilt Sufficient water must be added to ensure plant survival

OsmosisOsmosis and Animal CellsChanges observed under a microscope when blood is dropped into hypertonic and hypotonic hypertonic solutionnormal red blood cell

in hypotonic solution

cell shrinks

cell swells

cell becomes crenated

cell bursts and cell is said to be haemolysed

Active TransportActive transport is an energy-consuming process by which substances are transported from regions of low concentration to regions of high concentration against a concentration gradient. binding sitesubstrate molecule carrier protein OUTSIDE cell membrane

carrier protein changes shape




substrate released

energy from ATP used

Active TransportPlants need to absorb mineral salts from the soil. They are present as charged ions which cannot diffuse across partially-permeable membranes.

soil particle high concentration of ions low concentration of ions

Direction of active transport

Ion uptake by root hair is facilitated by active transport

Active TransportActive transport enables absorption of glucose and amino acids through the inner surface of the small intestine. The presence of microvilli increases the surface area over which active transport occurs.cell lining the villus


Uptake of glucose by cells in the villiglucose molecule direction of active transport