Chapter 3

Chapter 3

Diffusion and

Osmosis

Diffusion and

Osmosis

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

OBJECTIVES

Introduction

• 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• By processes called _________, _________ and

_______________

diffusion osmosisactive transport

Diffusion

Diffusion 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

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region of low concentration of

molecule Xregi

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molecule moves from a region of high concentration to that of low concentration

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

A. B.

Diffusion

Diffusion in liquids

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

The particles continue to move until they are evenly distributed.

A. B.

Diffusion

Importance 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.

Diffusion

Importance 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.

oxygencarbon dioxide

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

Diffusion

1. Temperature: An increase in temperature results in a higher rate of diffusion.1. Temp Molecules move faster Rate of diffusion

2. Size of particles: Small molecules or ions diffuse faster than large ones.1. Particle size Energy to move particle Rate of diffusion

3. 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.1. Thinner membrane Higher rate of diffusion

4. Concentration gradient: The greater the concentration gradient, the higher the rate of diffusion1. Steeper concentration gradient Higher rate of diffusion

5. Surface area: The rate of diffusion into a cell depends on the total surface area of the cell membrane. 1. surface area rate of diffusion.

Factors which Affect the Rate of Diffusion

Lungs

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

Osmosis

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

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.

sucrose solutionwater

water molecules

sucrose molecule

selectively-permeable membrane

Osmosis

Water Potential

The 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

Osmosis

Osmosis and Plant Cells

Turgor pressure – The outward pressure

which the cell sap exerts against the

inside wall of the cell

In 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.

Importance of Turgor in Plants

2. Changes in turgor of the guard cell causes the opening of the stomata

Turgor (II)

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

Osmosis

Osmosis and Plant Cells

Turgid cells. The cytoplasm is pressed against the cell wall. The cell swells up and is prevented from bursting by the cellulose cell wall.

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

water enters by osmosis

cell sap more concentrated than outside solution

cellulose cell wall

cell sap less concentrated than outside solution

plasma membrane

water leaves by osmosis

cytoplasm

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 Cells

Changes observed under a microscope when blood is dropped into hypertonic and hypotonic solutions.

cell shrinks

in hypertonic solution

in hypotonic solution

cell swells

cell becomes crenated

cell bursts and cell is said to be haemolysed

normal red blood cell

Active Transport

Active 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.

substrate molecule

carrier protein

restorationcarrier protein changes shape

substrate released

energy from ATP used

OUTSIDE

INSIDE

binding site

cell membrane

binding

Active Transport

soil particle

high concentration of ions

low concentration of ions

Direction of active transport

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

Ion uptake by root hair is facilitated by active transport

Active Transport

Active transport enables absorption of glucose and amino acids through the inner surface of the small intestine.

microvillus

glucose molecule

direction of active transport

cell lining the villus

Uptake of glucose by cells in the villi

The presence of microvilli increases the surface area over which active transport occurs.