# chapter 3 diffusion & osmosis - lecture notes

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Jurongville Secondary School Secondary 3 Express Biology Notes Syllabus 5100 Name: _________________________ ( Class: Sec 3___ Chapter 3: 3.1 ) Date: ____________

Diffusion, Osmosis & Surface Area: Volume Ratio

Diffusion Definition: The net movement of ions or molecules of a substance from a region where they are in a higher concentration to a region where they are in lower concentration, down a diffusion gradient Diffusion gradient: Difference in concentrations of a substance between the two regions o The steeper the diffusion gradient, the faster will be the rate of diffusion for the substance

3.1.1

Diffusion may occur with or without the presence of a membrane Types of membranes Permeable membrane: Allows all molecules (solute or solvent) to pass thorough it o Example of permeable membrane: Cellulose cell wall in plants Partially permeable (or selectively permeable) membrane: Allows certain molecules to pass through it but not others o Example of selectively permeable membrane: Cell membrane in all cells

3.2

Osmosis

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Definition: The net movement of water molecules across a selectively-permeable membrane from a region of higher water potential to a region of lower water potential, down a water potential gradient Water potential: A measure of how much water is present in comparison with solute molecules in the same volume of solution

o Low water potential High solute concentration (solution is concentrated) o High water potential Low solute concentration (solution is dilute) Osmosis is therefore a special case of diffusion! Take note: Osmosis occurs only when a selectively permeable membrane is used!

Example of Diffusion (Permeable membrane separates dilute and concentrated solutions)

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Example of Osmosis (Selectively permeable membrane separates dilute and concentrated solutions)

Special Definitions of Solutions with respect to Fig 2.4 above o Note: This definitions only apply to ANIMAL systems!! Term State of solution Hypotonic Solution A (higher water potential) is hypotonic with respect to solution B (lower water potential) Hypertonic Solution B (lower water potential) is hypertonic with respect to solution A (higher water potential) Isotonic Solutions A and B have the same water potential and are said to be isotonic

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3.3

Osmosis in living systems (Cells) Cell surface membrane is selectively permeable In plant cells, remember also that the cell wall is tough, slightly-elastic and permeable o Its cell sap in the vacuole is highly concentrated as it contains a mixture of substances

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3.4

Active Transport Definition: Process where energy is used to move the particles of a substance against a concentration gradient from a region of lower solute concentration to a region of higher solute concentration

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Cells carrying out active transport usually o Contains numerous mitochondria o Have high cellular respiratory rate to provide energy needed

3.5

Transport across the cell surface membrane Materials move in and out of cells across their cell membranes by the following methods: o o o Diffusion: eg. Uptake of oxygen and excretion of carbon dioxide by all living cells during respiration Osmosis: eg. Uptake of water molecules from the soil by root hair cells and the flow of water from here to the xylem vessel of the root Active transport: eg. Uptake of (a) Mineral salts (ions) by root hair cells (b) Glucose and amino acids by the epithelial cells in the villi of the small intestine

3.6

Surface Area: Volume Ratio It is very important that the rates of movement of substances (oxygen, carbon dioxide and nutrients) across cell membranes must be high enough to meet the metabolic needs of the cells How fast these 3 processes is taking place depends strongly on the ratio of the surface area of the cells to its volume

o Mathematically, is it defined as

SurfaceAre a Volume

o Normally expressed in ratio form The greater the surface area: volume ratio The faster the rate at which these processes (diffusion, osmosis, active transport) take place A smaller cell has a larger surface area: volume ratio than a large cell of the same shape

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o

Thus, a large organism is made up of many small cells (multicellular)

Having multifold of cell surface membrane can increase surface area: volume ratio of a cell. o This modification is very important to cells that are concerned with absorption of substances. Eg. Microvilli of cells lining the small intestine

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