Diffusion Notes

In Support of the Diffusion Lab

I. Definitions: solute: a substance that gets dissolved (ex: salt) solvent: a substance in which a solute is dissolved ���(ex: water) concentration: the percent represented by either the solute or solvent concentration gradient: the division between two distinctly different concentrations diffusion: the movement of molecules, down a concentration gradient, from an area of higher concentration to an area of lower concentration

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solution: a mixture in which a solute is dissolved evenly throughout a solvent osmosis: the diffusion of water molecules down a concentration gradient semi-permeable membrane: a membrane which allows the passage of some molecules (ex. water), while preventing the passage of other substances - some molecules are prevented from passing through due to their size and/or charge passive transport: the movement of molecules down a concentration gradient without assistance or without the expenditure of energy (ex. osmosis and diffusion)

water

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II.  Osmosis and Cells - 3 Examples A. Hypotonic Solution - a solution with a lesser concentration of solute outside the cell compared to the concentration of solute inside the cell

Ex: a single-cell salt water���organism is placed in a brackish water solution

The cell membrane is semi-permeable membrane

Identify the solute and the solvent.

What do you predict will happen? Be thorough.

brackish water

answer: salt is the solute, water is the solvent - water moves from an area of high concentration in the brackish water through the cell’s membrane (causing the cell to swell) and will continue to do so until the concentration of water inside the cell equals the concentration outside the cell - or - until the cell explodes

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B. Hypertonic Solution - a solution with a greater concentration of solute outside the cell compared to the concentration of solute inside the cell

Ex: a single-cell fresh water organism is placed in a sea water solution

The cell membrane is semi-permeable membrane

Identify the solute and the solvent.

What do you predict will happen? Be thorough.

sea water

answer: salt is the solute, water is the solvent - water moves from an area of high concentration in the cell through the cell’s membrane (causing the cell to shrink) and will continue to do so until the concentration of water inside the cell equals the concentration outside the cell - or - until the cell collapses

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C. Isotonic Solution - a solution with an equal concentration of solute outside the cell compared to the concentration of solute inside the cell

Ex: a single human red blood cell is placed in a 0.9% saline (salt) solution.

The cell membrane is a semi-permeable membrane

Identify the solute and the solvent.

What do you predict will happen? Be thorough.

0.9% saline

answer: salt is the solute, water is the solvent - since the concentration of salt and water is the same inside and outside the cell - there is no concentration gradient - water can still pass through the cell’s membrane, however equal amounts of water pass in and out of the cell - so that there is no net gain or loss of water inside the cell

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Osmosis

Animation

Osmosis Overview

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Answer:

Cell membranes are semi-permeable. Only certain molecules can pass

through freely. The salt molecules are blocked by the cell membrane due to

their size and/or charge.

Question:

Consider the hypotonic solution example. Why is it not true that the salt molecules, which are higher in

concentration inside the cell, move through the cell membrane into the brackish water where the salt

concentration is lower?

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Answer:

The organism must have one or more adaptations allowing the movement

of molecules up a concentration gradient, and the movement of

molecules too large to diffuse across its cell membrane.

Question:

How is it possible that an aquatic organism can survive in an environment that is not isotonic to itself.

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III. Facilitated Diffusion A. an example of passive transport B. an adaptation solving the problem of moving molecules that are too large, or having a charge, that prevents them from moving across the cell membrane even when the concentration gradient favors their movement C. recall: no energy required D. molecules called carrier proteins imbedded in the cell membrane facilitate the movement

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IV. Active Transport A. the transport of molecules across a cell membrane and up a concentration gradient (from a area of lower concentration to an area of higher concentration) B. an adaptation requiring both carrier proteins and energy C. carrier proteins in conjunction with high energy molecules pump molecules up a concentration gradient

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ATP ATP

Energy Providing Molecule

Moving Either Large or Charged Molecules up a Concentration Gradient

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V. Summary Questions

1. Why do supermarkets regularly spray vegetables with water?

- answer: water is in a much higher concentration inside a vegetable than outside - due to osmosis, water moves out of the vegetable causing the vegetable to dry out - regular spraying of water creates an unnaturally high concentration of water immediately outside the vegetable, dramatically slowing down osmosis

2. What process does a single-cell salt water organism use to survive in a brackish water environment where excess water is flowing into the cell? Explain your answer.

a) diffusion b) facilitated diffusion c) active transport

- answer: (c) as water flows into the cell due to osmosis, the cell must pump water up the concentration gradient - the concentration of water inside the salt water organism needs to be less than the water concentration in the brackish environment

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