agar jelly experiment

Practical Biology

A collection of experiments that demonstrate biological concepts and processes.

Observing earthworm locomotion

Practical Work for Learning

Published experiments

Effect of size on uptake by diffusion, class practical.

Set up cubes of agar jelly and see how far liquid penetrates them by diffusion over five minutes. Calculate surface area to volume ratio for cubes of different sizes and consider the problems faced by large organisms .

Lesson organisation

While the cubes are soaking, students can try the calculation of surface area to volume ratio for themselves. Many students struggle with these calculations and their implications, so it’s worth taking the time to go through the calculations methodically.

Apparatus and Chemicals

For each group of students: :.

Beaker, 100 cm 3 , 1

White tile,1

Paper towel, 1

Stopclock/ stopwatch

For the class – set up by technician/ teacher

Agar cubes 2 cm x 2 cm, 1 per group

Agar cubes 1 cm x 1 cm, 1 per group

Agar cubes 0.5 cm x 0.5 cm, 1 per group ( Note 1 )

Hydrochloric acid, 0.1 M, 20 cm 3 per group

Health & Safety and Technical notes

The low concentrations of sodium hydroxide and hydrochloric acid are below the concentration that needs to be labelled IRRITANT. Wear eye protection and rinse splashes off the skin.

Universal indicator is dissolved in ethanol, so it is highly flammable in the stock bottle, but not once added to agar. See CLEAPSS Hazcard and Recipe card.

Read our standard health & safety guidance

1 To make good solid agar, stir 2 g of plain (technical) agar powder into 100 cm 3 of water. Heat, in an aluminium water bath filled with boiling water, with stirring, until the agar solution boils, then allow to cool. Make the agar blocks in straight-sided dishes or ice cube trays. If students cut cubes themselves, each group will need a block 2 cm x 3 cm x 2 cm. If you have chosen to use hydrochloric acid to soak the cubes, make up the agar with 0.01 M sodium hydroxide and colour the agar with universal indicator (Hazcard 31, Recipe card 36) or phenolphthalein ( Note 2 ).

2 Phenolphthalein is described as LOW HAZARD on the CLEAPSS Hazcard. Refer to the CLEAPSS Recipe card (acid-base indicators): Dissolve 1 g in 600 cm 3 of IDA then make up to 1 litre with water.

Ethical issues

There are no ethical issues associated with this procedure.

SAFETY: Take care with the solutions used: wear eye protection and rinse splashes off the skin.

Preparation

a Make up plain (technical) agar with sodium hydroxide and universal indicator, or with sodium hydroxide and phenolphthalein. ( Notes 1 and 2 .)

b Cut agar cubes for the students, or provide them with a larger block to cut up. ( Note 1 .)

Investigation

a Collect agar cubes of different sizes – 2 cm x 2 cm, 1 cm x 1 cm and 0.5 cm x 0.5 cm – or cut cubes from the larger cube provided.

b Place the cubes in a beaker and cover with the diffusing solution.

c Start the stopclock.

d Leave the cubes for 5 minutes

e While you are waiting, complete the table with calculations for each cube. (See student sheet.)

f Pour off the solution. Rinse the cubes in a little water and blot the surfaces of each cube dry with a paper towel.

g Time how long it takes for the acid to change the colour of the indicator in each agar block. If the acid does not penetrate the largest block in the time available, cut the block and measure how far it has penetrated in the time.

Teaching notes

If this investigation is familiar to the students, some groups could investigate the effect of different shapes.

Health and safety checked, September 2008

Related experiments

Evaluating Visking tubing as a model for a gut

Agar Blocks Practical ( AQA A Level Biology )

Revision note.

Biology Lead

Practical Skill: Agar Blocks

• The effect of changing surface area to volume ratio on diffusion can be investigated by timing the diffusion of ions through cubes of agar of different sizes
• Purple agar can be created if it is made up with very dilute sodium hydroxide solution and Universal Indicator
• Alternatively, the agar can be made up with Universal Indicator only
• The surface area, volume and surface area to volume ratio of these cubes is calculated and recorded
• The same volume of dilute hydrochloric acid should be carefully measured out into each boiling tube
• The acid should have higher molarity than the sodium hydroxide so that its diffusion can be monitored by a change in colour of the indicator in the agar blocks
• The time taken for the acid to completely change the colour of the indicator in the agar blocks
• The distance travelled into the block by the acid (shown by the change in colour of the indicator) in a given time (eg. 5 minutes)

The steps used to investigate the effect of changing the surface area to volume ratio on diffusion

• If the time taken for the acid to completely change the colour of the indicator in the agar blocks is recorded, these times can be converted to rates
• A graph could be drawn showing how the rate of diffusion (rate of colour change) changes with the surface area : volume ratio of the agar cubes

To analyse the results of the investigation, calculate the rates of diffusion before drawing a graph for rate of diffusion against surface area : volume ratio

When an agar cube (or for example a biological cell or organism) increases in size, the volume increases faster than the surface area, because the volume is cubed whereas the surface area is squared. When an agar cube (or biological cell/organism) has more volume but proportionately less surface area, diffusion takes longer and is less effective. In more precise scientific terms, the greater the surface area to volume ratio , the faster the rate of diffusion !

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Investigating Transport Across Membranes (A-level Biology)

Investigating transport across membranes, investigating diffusion.

We can investigate how diffusion occurs in biological cells by using cubes of agar jelly. The basic concept of this experiment is outlined below:

• The agar jelly contains a pH indicator. We can make up agar jelly with an alkaline solution (e.g. sodium hydroxide) and add a few drops of phenolphthalein to it before the jelly sets. Phenolphthalein is a pH indicator which turns pink in the presence of alkaline solutions, thus, the jelly will have a bright pink colour.
• The agar jelly is placed in an acidic solution. Once the jelly has set, we can cut it up into cubes and place it in an acidic solution, such as dilute hydrochloric acid.
• The agar jelly is neutralised by the diffusion of the acid. The acidic solution will slowly diffuse into the agar jelly and neutralise the alkaline solution. As it does, the jelly will lose its pink colour and become colourless, as phenolphthalein turns colourless in non-alkaline environments.

We can alter different parts of this experiment to model how different factors affect the rate of diffusion.

Investigating the effects of surface area on diffusion

• Cut the agar jelly into different sized cubes to investigate the effects of surface area . Cut the jelly into cubes of different sizes and work out each cube’s surface area to volume ratio . For example, a cube with 2cm edges will have a surface area to volume ratio of 3:1.
• Place the cubes in the same volume and concentration of acid. Put the cubes into containers which hold the same volume and concentration of hydrochloric acid. Then measure the time it takes for the different cubes to go colourless.
• The cube with the largest surface area: volume ratio will go colourless the quickest. The cube with the largest surface area: volume ratio has the greatest amount of space available for the hydrochloric acid to diffuse into the jelly so it will be neutralised the fastest.

Investigating the effects of concentration on diffusion

• Place the agar jelly cubes in different concentrations of acid. Cut the agar jelly into equal sized cubes and put them in different containers, each with a different concentration of hydrochloric acid. Measure the time it takes for the different cubes to go colourless.
• The cube placed in the highest concentration of acid will go colourless the quickest. The cube placed in the container with the highest concentration will have the greatest concentration of acid being diffused into the jelly per minute. As such, it will go colourless the quickest.

Investigating the effects of temperature on diffusion

• Place the agar jelly cubes in different temperatures. Cut the agar jelly into equal sized cubes and put them in different containers, each with the same concentration of hydrochloric acid. Put the containers in water baths heated to different temperatures. Be careful not to heat the water baths over 65° as the agar jelly will melt.
• The cube placed in the highest temperature of acid will go colourless the quickest. As high temperatures speed up the rate of diffusion, the cube in the hottest container will be neutralised the quickest.

Investigating Osmosis

Osmosis is the movement of water molecules from an area of high water potential to an of low water potential by osmosis. Water potential is determined by the concentration of solutes in the solutions on either side of the cell membrane.

Investigations using plant tissue

This experiment involves placing plant tissue, e.g. potato cylinders, in varying concentrations of sucrose solutions to determine the water potential of the plant tissues.

• Prepare the different concentrations of sucrose solutions . Using distilled water and 1M sucrose solution, prepare a series of dilutions such that you now have 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0M sucrose. Place 5cm 3 of each dilution into separate beakers.
• Prepare equal sized pieces of potato chips. Using a cork borer, cut out 18 pieces of potato chips, all of equal sizes.
• Weigh the mass of the potato chips. Dry the potato chips gently with a paper towel. Divide them into groups of three and weigh each group.
• Place each group of potato chips in each solution . The potato chips should be left in the solutions for a minimum of 20 minutes.   All groups should be left in the solution for the same amount of time.

• Weigh the mass of the potato chips again. Once your desired amount of time has passed, remove the chips from the solutions, and dry them gently using a paper towel. Reweigh each group again.
• Calculate % change in mass. Using the mass of the potato chips before and after being placed in the solution, calculate the % change in its mass.
• Plot the % change in mass on a calibration curve. The calibration curve helps us determine the water potential in the potato sample. Plot the % change in mass against concentration of sucrose solution.   The point at which the curve crosses the x axis is when the sucrose solution is isotonic with the potato samples i.e. the water potential of the sucrose solution is the same as the water potential of the potatoes. At this point, there is no movement of water in or out of the potato. Overall:
• The potato samples in the dilute solutions will have a net increase in mass – the water potential is greater in the potato than in the sucrose solution, so water moves into the potato samples via osmosis.
• The potato samples in the concentrated solutions will have a net decrease in mass – the water potential is lower in the potato than in the sucrose solution, so water moves out of the potato samples via osmosis.

Investigations using Visking tubing

Visking tubing is an artificial membrane that is selectively permeable as it has many microscopic pores. This allows smaller molecules such as water and glucose to pass through it, while larger molecules such as starch and sucrose are unable to cross the membrane.

• Prepare three equal-sized pieces of Visking tubings. Run the tubing under tap water to soften it and knot each tubing on one end to create a bag.
• Place a rubber bung at the open end of the Visking tubing. Find rubber bungs with an opening in the centre that will fit the open end of the Visking tubing. Then seal the tubing using the bung and fix it in place using a rubber band.
• Prepare sucrose solutions with concentrations of 0.5M and 1.0M. You may wish to add a food dye to the 0.5M solution so that it is easier to see later on.
• Pipette in the 0.5M sucrose solution. Using a pipette or a syringe, fill each tubing through the opening of the rubber bung with the 0.5M sucrose solution. Make sure it is filled completely to the brim with no air bubbles.
• Insert capillary tubes into each of the tubings . Insert a capillary tube through the rubber bung’s opening. Mark the level at which the sucrose solution has risen to in the capillary tube.
• Place each Visking tubing into containers of different solutions. Prepare three beakers, each containing distilled water, 0.5M sucrose, and 1.0M sucrose. Place each Visking tubing into each of the beakers and leave them in for the same amount of time.
• Measure the change in liquid level. Mark the new liquid level on the capillary tube before removing the Visking tubing from its beaker. Measure the change in the liquid level. Overall:
• The liquid level of the Visking tubing placed in distilled water will have risen as the sucrose solution in the tubing is hypertonic to the water i.e. the sucrose is more concentrated. Thus, there is net movement of water into the Visking tubing via osmosis.
• The liquid level of the Visking tubing placed in 0.5M sucrose will remain the same as the solution inside the tubing and outside the tubing are isotonic i.e. the solutions are the same concentration.
• The liquid level of the Visking tubing placed in 1.0M sucrose will have decreased as the solution inside the tubing is hypotonic to the solution outside the tubing i.e. the solution inside the tubing is less concentrated.

Transport across membranes is the movement of substances such as ions, molecules, and fluids from one side of a biological membrane to the other. This process is crucial for maintaining cellular homeostasis and allowing cells to exchange materials with their environment.

Investigating transport across membranes is important because it helps us understand the mechanisms by which cells regulate the flow of substances in and out of the cell. This is essential for understanding cellular processes such as metabolic reactions, waste removal, and communication between cells.

There are several methods used to investigate transport across membranes, including: Diffusion experiments to study the movement of substances through the lipid bilayer Osmosis experiments to study the movement of water across a semi-permeable membrane Active transport experiments to study the movement of substances against a concentration gradient with the use of energy Electrochemical experiments to study the movement of ions across the membrane

Factors that can affect transport across membranes include the size of the substance being transported, the charge of the substance, the concentration gradient, and the presence of specific transport proteins.

Transport across membranes can be measured in a variety of ways, including measuring changes in substance concentration, changes in electrical potential, and changes in fluid movement.

The limitations of investigating transport across membranes include the difficulty of obtaining pure and intact biological membranes, the potential for damage to the membrane during experimentation, and the limitations of experimental techniques.

In A-Level Biology, knowledge of transport across membranes can be applied to understand cellular processes such as the movement of nutrients and waste, the regulation of cell volume, and the communication between cells. This knowledge is also important for understanding diseases and disorders related to the malfunction of transport processes, such as cystic fibrosis and diabetes.

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CIE 1 Cell structure

Roles of atp (a-level biology), atp as an energy source (a-level biology), the synthesis and hydrolysis of atp (a-level biology), the structure of atp (a-level biology), magnification and resolution (a-level biology), calculating cell size (a-level biology), studying cells: confocal microscopes (a-level biology), studying cells: electron microscopes (a-level biology), studying cells: light microscopes (a-level biology), life cycle and replication of viruses (a-level biology), cie 10 infectious disease, bacteria, antibiotics, and other medicines (a-level biology), pathogens and infectious diseases (a-level biology), cie 11 immunity, types of immunity and vaccinations (a-level biology), structure and function of antibodies (a-level biology), the adaptive immune response (a-level biology), introduction to the immune system (a-level biology), primary defences against pathogens (a-level biology), cie 12 energy and respiration, anaerobic respiration in mammals, plants and fungi (a-level 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Cell Diffusion Lab using Gelatin or Agar

This is a remix of  https://goopenva.org/courses/agar-cell-diffusion  from the Exploratorium Teacher Institute. The original uses agar blocks to demonstrate the effect of cell size on cell transport., and it includes great instructions on how to incorporate the lab into your classroom However, plain gelatin is often easier to obtain than agarose and can be used instead. Standard gelatin recipes are too soft to carve though, so my coworkers and I have tweaked the recipe over the years until we found one that is fool proof and comes out perfectly every time. This recipe will yield gelatin that cuts easily and has almost the consistency of the agar. It is firmer and easier to carve than standard gelatin. If you can spare the expense, agar is still the best option. However, this gelatin recipe is a great substitute. I hope other teachers will find the gelatin variation useful! Simply make the gelatin as noted below and then follow the instructions on the original resource found here  https://www.exploratorium.edu/snacks/agar-cell-diffusion . 

Gelatin Cell Blocks Recipe

Heat 1.75 cups (414 mL) of water in a microwave or hot plate until boiling or close to boiling. Carefully pour the hot water into a glass baking dish. Sprinkle three (3) packs of plain gelatin on top of the hot water and allow it to bloom for a few seconds. Whisk the gelatin into the water until dissolved. Allow the mixture to cool for just a bit. You want it to be still liquid, but not so hot that it evaporates your base. Once the mixture is cooler, add a few drops of phenolphthalein indicator and just enough weak base to turn the gelatin a dark pink/purple. We don't have an exact measurement for how much base we add. Generally we just add splashes of base and stir until we get the correct color. It's quite a bit of base to add (probably around 1/2 cup total), but go slow and monitor the color to decide when to stop. Be aware that the gelatin will most likely lighten up over night a bit. Also using too strong of a base or too much base can denature the gelatin and prevent it from setting up. We have used weak ammonia and 0.1M NaOH successfully in the past. Other bases may work as well, but you want something that will be safe for the students and not overly strong. Just strong enough to turn your indicator a nice dark pink. Cover and place your dark pink gelatin into a fridge overnight to gel. If the gelatin lightens up too much the next day, you can add more base on top and give it about an hour to soak back in. It will deepen the color more. The gelatin can be carved up and soaked in white vinegar for the experiment the next day. The vinegar will turn the gelatin clear as it diffuses into the blocks. 

Safety concerns: some of these chemicals can be irritating. Students must wear goggles and could even use gloves if available. If gloves are not available, have them wash their hands very well after they are done carving the gelatin blocks. 

Virginia Science Standards of Learning 2018

Learning Domain: Biology

Standard: The student will demonstrate an understanding of scientific and engineering practices by: planning and carrying out investigations; individually and collaboratively plan and conduct observational and experimental investigations ; plan and conduct investigations or test design solutions in a safe and ethical manner including considerations of environmental, social, and personal effects; determine appropriate sample size and techniques; select and use appropriate tools and technology to collect, record, analyze, and evaluate data.

Degree of Alignment: Not Rated (0 users)

Standard: The student will investigate and understand that cells have structure and function. Key ideas include the structure and function of the cell membrane support cell transport.

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Agar Jelly Experiment Report

Aim:  

To find out the speed of diffusion in agar jelly, when there are three     blocks of jelly of different sizes.

Material:  

A container with agar jelly, safety goggle, knife, ruler, stop watch, a glass plate, sodium hydroxide, an indicator.

First, we took the block of agar jelly from the container. We placed it onto the glass plate, and took our knife and ruler. We measured and cut the block, making three cubes of different measurements. The smallest one is 1cm by 1cm by 1cm, the second one is 2cm by 2cm by 2cm, and the biggest one is 3cm by 3cm by 3cm. We then dropped a few drops of the indicator. Next, we put our safety goggles on, and took the beaker full of sodium hydroxide, and poured it into the glass plate, which the cubes are in. At the same time, we started the stop watch. We saw everything, the cubes, sodium hydroxide turn pink (that’s because sodium hydroxide reacted with the indicator). When the stop watch showed 5 minutes, we took the cubes, and cut all of them in half. Then we measured the distance from the surface, to the line where the pink and the white color separate, like on the diagram, using our ruler. We recorded what we saw, and the results.

We saw the cubes turn pink, and the sodium hydroxide turn pink on the plate. We also saw, when we cut the cubes that the pink color travelled into the cube, and we saw that at a certain point, there was the separation of pink from the surface, to white, the color of agar jelly. Like I said, we measured the distance between the surface to where the separation line of pink and white color was. This is what we got:

1cm*1cm*1cm                4mm

2cm*2cm*2cm                4mm

3cm*3cm*3cm                4mm

This is a preview of the whole essay

Although they were all 4mm, we saw that the smallest cube was almost completely diffused, almost everything was pink. And we saw that the biggest cube was not at all completely diffused; most of it was white inside the cube.

Speed of Diffusion=Distance/Time

4mm/5minutes=0.8mm/minute

Conclusions:

From this experiment, I learned these things:

Firstly, I learned that the pink color traveled into the cube, because of diffusion. Diffusion is when particles move from an area of high concentration to an area of low concentration. So in this experiment, the sodium hydroxide diffused from the high concentrated area (which is the surface of the cube, as sodium hydroxide was poured onto the cube) to low concentrated area (inside the cube). I also learned that even if you change the volume, or the surface area of the agar jelly, the speed in diffusion doesn’t change. As you can see from the results, no matter the size of the cube, the sodium hydroxide diffused at the same speed. The speed of diffusion can be calculated by distance/time. In this experiment, we saw that the distance which the sodium hydroxide traveled was 4mm, and the time we used was 5 minutes. So, 4mm/5min= 0.8mm/minute.

Because the speed doesn’t change, the smaller the cube, the faster it’ll get completely diffused.

From this experiment, I can expect that 1cm*1cm*1cm cube jelly will be completely diffused in 6minutes 15seconds, because 1cm/0.8mm*2=6.25min. (times 2 because it diffused from the top and the bottom). In the same way, I calculated that 2cm*2cm*2cm cubed jelly will be completely diffused in 12.5minutes and 3cm*3cm*3cm jelly in 18.75minutes. You can see that 2cm*2cm*2cm jelly will take twice as much time as 1cm cubed one and 3cm*3cm*3cm jelly will take three times as much time as the 1cm cubed block.

Teacher Reviews

Here's what a teacher thought of this essay.

Cornelia Bruce

3* This is a standard biology laboratory report on diffusion. The layout is standard, however does leave out some key information including an introduction and evaluation, where the biological significance of the experiment should be noted. The method should be written more clearly and include a labelled diagram. The experiment was straight forward with a variety of volumes used, to improve this more agar jelly cubes could have been tested using repeats to more clearly establish the rate of diffusion.

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Diffusion in Agar Block

Investigation: Why Are Cells So Small?

Essential Question: How does the size and shape of a cell influence the speed at which materials can move into and out of the the cell?

Process: Create cell models using agar molds to compare rates of diffusion.

Agar Mold with Bromothymol Blue (made in advance) Tweezers, Scalpel (or plastic knife) Ruler, Beaker with white vinegar

1. You will receive a small tray filled with an agar mold. *See below for directions* Avoid handling the agar with your bare hands and use a scalpel and tweezers to cut three agar cubes with the following approximate dimensions. Save your agar, you will need it later!

1 cm x 1 cm x 1 cm (small) 2 cm x 2 cm x 2 cm (medium) 1 cm x 1 cm x 6 cm (large)

2. Measure your cubes (the actual dimensions may not be perfect, depending on how you cut it) and determine the surface area, the volume, and the SA:V ratio. Record on data table.

3. Drop each block into a separate beaker (or container) of vinegar. The agar has been infused with a chemical called bromothymol blue, the blue will turn to a yellow in the presence of acid. You will be able to observe this change with your cubes. Record the time it takes for the blue to completely disappear.

Part 2: How Does Shape Influence Rates of Diffusion?

With what remains of your agar, design a cell that maximizes volume and mass, but minimizes diffusion time .

Your "cell" will compete with other cells in the class to see which one has the fastest diffusion time.

• No donut-like holes through the agar cell - cell membranes cannot sustain this shape
• No poking or agitating the beaker when the cell is submerged
• Instructor determines when 100% diffusion has occurred
• Agar cell will be massed at the end of the race
• Winner = highest ratio of mass divided by time

Sketch your design below.

1. Which of the initial cubes had the fastest diffusion time? Which had the slowest?

2. Which of the three variables you tested seemed to have the biggest impact on the rate of diffusion? Explain how you know this.

3. How does the agar cube model the cell and the cell membrane?

4. What designs (Part 2) seemed to have the fastest diffusion rate?

5. How do these experiments model the cell and the cell membrane?

Agar Recipe:

15 g of agar in 1 liter water (or follow directions on packaging). You do want the agar to be thick so that it can be handled, so reduce water amounts. Agar is boiled in DI water and then allowed to cool. Knox gelatin can also be subsituted, but you may need to play around with the measurements.

While it is cooling, add .1 g of bromothymol blue (or about 10 ml aqueous solution, you just need to ensure that the agar turns a dark blue.) If the mixture is green/yellow, then add NaOH until it turns blue.

Pour agar into trays for students. You can be creative with the trays (ziploc tupperware containers should work, or even metal dissecting trays.) The molds must be at least 2 cm deep. Molds can be covered and refrigerated.

These are the molds I created using specimen containers and the lids from a box of micropipettes. You can be creative!

Alternatively, you can add phenolphthalein to the agar and then submerge cubes in sodium hydroxide. <-- this tends to be more expensive than white vinegar, and NaOH is dangerous to handle.

Image below shows saturation of agar by vinegar. The yellow area started as blue. A ruler placed under the flask can be used as another way to measure the rate of diffusion.

Other Activities on Cell Transport

Observe Diffusion in a Bag  – model diffusion using a plastic baggie, iodine and a beaker.   This  article  explains what happens.

Transport Across the Cell Membrane  – simple diagram shows how molecules enter the cell through diffusion, facilitated diffusion, and osmosis

Cell Membrane Images  – work in groups to create captions and titles for images depicting the cell membrane and transport across it.

Case Study: Cystic Fibrosis  – for AP Biology, examines the role of cell membrane proteins in clearing mucus from the lungs.

Observing Osmosis  – use an egg, vinegar, corn syrup, will take a few days Salt and Elodea  – quick lab to observe the effects of salt water on elodea cells

Cheese Rolling – Gravitational Potential Energy to Kinetic

You mean you’re not on holiday yet?

Jun 05 2014

Diffusion in Solids, Liquids, Gases and Jelly

• By Daniel Powell in Miscellaneous

June 5, 2014

Background information Diffusion in liquids:  When substances dissolve in liquids (like salt dissolving in water) the substances spread out. We call this spreading out of dissolved particles diffusion . The end result of this is that the solute particles that have dissolved in water will spread out evenly. This movement or spreading out is due to the fact that in liquids the particles are moving randomly.

Diffusion in gases:  When two or more different gases, like oxygen and nitrogen are mixed they will mix themselves evenly. We call this mixing of particles diffusion.  This is due to the random movement of particles.

Diffusion in solids:  Diffusion does not happen in solids because the particles are not free to move around and so they cannot inter-mix.

Diffusion in jelly:  Jelly is a liquid before it has set and looks like a solid when it has set.  However the truth is a little more interesting. After it has set jelly is not really a solid or a liquid, it is in fact a mixture of both of them. As shown in the diagram below there are long fibres of protein or carbohydrate which form the solid part of the jelly and between these fibres there are spaces where water molecules are free to move around. This is why substances can diffuse through jelly.

Using Jelly in science experiments…  Because it allows diffusion through it, jelly is very useful as it allows us to track the movement substances through the jelly for example in Bioassay experiments testing the effectiveness of antibiotics as shown in the photograph below.    In this photo it is easy to see which antibiotic is the best at killing bacteria (the biggest clear area). The antibiotic has diffused through the jelly.

The rate of Diffusion is affected by a number of factors:

• Temperature
• Concentration
• The surface area of the exchange surface
• The size of the particles

Experiment to determine how temperature affects the rate of diffusion through jelly

• Petri dishes
• Agar jelly with Universal indicator mixed into it
• Hydrochloric acid of the following concentrations: 1.0M, 0.8M, 0.6M, 0.4M 0.2M.
• Ruler with mm graduation

1)    Pour equal volumes of the hot Agar/Indicator mix into a different petri dish, leave to cool and set overnight.

2)   Use the cork borer to cut 5 wells into the jelly making sure all the jelly is removed from the well.

3)   Use a pipette to carefully fill the first well with 1.0 M Hydrochloric acid and start stop-clock.

4)   As the acid diffuses through the agar the indicator will turn red. After 5 minutes use the ruler to measure how far the red colour has moved.

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How to teach osmosis and diffusion with agar cubes – a free activity

High School

Why did the oxygen molecule cross the cell membrane? To get to the other side, of course! Ok, that's a little corny, but how cells work is one of the many fascinating natural phenomena that your students will begin to understand when they study diffusion and osmosis.

The cell membrane is the world's greatest border guard. Despite being only 6 to 10 nanometers thick and visible only through an electron microscope, the cell membrane keeps a close watch over what goes in and out of the cell. The cell membrane allows nonpolar molecules (those that don't readily bond with water) to flow from an area where they're highly concentrated to an area where they're less concentrated.

Your students will be fascinated by how many other ways diffusion and osmosis can be found throughout nature. The size of elephant ears, flatworm survival, swollen water-soaked raisins, and plant root systems are just a few examples. Once your students complete this activity, they can have fun trying to think of other examples of where diffusion and osmosis play a role in the real world.

Students can use cubes of agar in this activity to model how diffusion occurs in cells and investigate how size impacts diffusion. Then they'll finally be able to answer that nagging question we all have in the back of our minds "why are cells so darn small?" 

Download lesson

Recommended Products:

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Ward's® Prepared Agar Cubes  

Making agar cubes can be a time-consuming and messy process. Use these prepared agar cubes that make this activity a lot faster and easier! These cubes are exclusive to Ward's Science and are also included in our AP Biology Investigation kit on osmosis and diffusion.

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Prepared Phenolphthalein Agar

Convenient: no mixing or sterilizing needed.

Ward's® Timer

Great for classroom activity. Three-button operation: start/stop, lap, and reset. Lap button allows you to stop, take a reading, then press again to continue. Includes battery. Includes a storage case.

Subscribe to get the latest content from Ward's World.

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How to Create Agar

Last Updated: May 17, 2023 Fact Checked

This article was reviewed by Anne Schmidt and by wikiHow staff writer, Jessica Gibson . Anne Schmidt is a Chemistry Instructor in Wisconsin. Anne has been teaching high school chemistry for over 20 years and is passionate about providing accessible and educational chemistry content. She has over 9,000 subscribers to her educational chemistry YouTube channel. She has presented at the American Association of Chemistry Teachers (AATC) and was an Adjunct General Chemistry Instructor at Northeast Wisconsin Technical College. Anne was published in the Journal of Chemical Education as a Co-Author, has an article in ChemEdX, and has presented twice and was published with the AACT. Anne has a BS in Chemistry from the University of Wisconsin, Oshkosh, and an MA in Secondary Education and Teaching from Viterbo University. There are 8 references cited in this article, which can be found at the bottom of the page. This article has been fact-checked, ensuring the accuracy of any cited facts and confirming the authority of its sources. This article has been viewed 63,334 times.

Whether you're running your own science experiments at home or you want to save a little money in your lab, start mixing your own agar medium. Agar is a gel-like material that's made from algae or animal proteins and you can use it to grow bacteria for your experiments. Although it doesn't take a lot to create agar, your simple agar powder or homemade mixture has all the nutrients that microbes need to grow. Once you've made either type of agar medium, pour it into Petri dishes with your samples and grow some germs!

Ingredients

Basic nutrient agar.

• 2 1/2 teaspoons (4.6 g) of agar powder
• 3 ⁄ 4 cup (180 ml) plus 1   1 ⁄ 2 tablespoons (22 ml) of distilled water

Makes enough medium for 10 Petri dishes

Homemade Agar

• 1 beef bouillon cube
• 2 teaspoons (8 g) of sugar
• 1 cup (240 ml) of distilled water
• 1 packet (2 1/2 teaspoons or 7 g) of unflavored gelatin

Makes enough medium for 6 Petri dishes

Making and Sterilizing an Agar Medium

• If you don't have Petri dishes, use glass baby food jars with the labels removed. Cover them with foil and sterilize them in the oven along with the beaker.

Did You Know: Although you can use the homemade medium for many experiments, it may not work if the bacteria in your experiment eat protein. This is because the gelatin in the homemade medium contains protein so the medium may liquify.

• If you don't have a hot plate, put the beaker into the microwave and heat it for 10-second increments until the mixture boils.
• Ensure that the agar powder or bouillon mixture dissolves completely.

• If you don't have a thermometer to check the temperature, wait about 10 to 15 minutes before using the medium.

Pouring Agar Plates

• Spend a minute washing your hands after you've sanitized your space.

• For example, you might write the bacteria, the date, and your initials.

• Repeat this for each Petri dish that you have.

Tip: Try to use homemade agar medium faster than medium made from agar powder since the ingredients may break down quicker.

Expert Q&A

• Buy bouillon that doesn't contain preservatives or flavorings. These can make it harder for your medium to grow and it might be harder to see growth if your bouillon has specks of seasonings. [11] X Research source Thanks Helpful 2 Not Helpful 0
• If you don't have a lab beaker, put the medium ingredients into a saucepan. Heat the mixture on the stove over medium-high heat while you stir it constantly. Thanks Helpful 1 Not Helpful 0

• Always use caution and wear oven mitts when handling hot materials. Thanks Helpful 5 Not Helpful 0

Things You'll Need

• 1,000 ml (34  fl oz) laboratory beaker
• Petri dishes
• Aluminum foil
• Measuring cups or jugs
• Measuring spoons
• Oven mitts or heat-resistant gloves
• Scientific hot plate
• Thermometer
• Isopropyl alcohol or bleach
• Alcohol wipe
• Saucepan, optional

You Might Also Like

• ↑ https://www.umsl.edu/microbes/files/pdfs/sterilizing.pdf
• ↑ https://teach.genetics.utah.edu/content/microbiology/plates/
• ↑ https://bitesizebio.com/6938/how-to-make-the-perfect-agar-plate-every-time/
• ↑ https://youtu.be/h9ViwEW7Xss?t=104
• ↑ https://youtu.be/aK9nDAzjyv0?t=64
• ↑ https://youtu.be/h9ViwEW7Xss?t=134
• ↑ https://sciencing.com/make-agar-gel-powder-10070363.html
• ↑ https://sciencing.com/homemade-agar-plates-6132952.html

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Phenolphthalein agar: size and diffusion:

The original prac is in Biological Science, The Web of Life Teacher’s Guide Part 2, Exercise 10b.4 1. Approx 25g agar (increase from 18g) 4g NaOH Dissolve NaOH in 100ml water. Heat 900ml water in a beaker almost to boiling Add agar and continue boiling while stirring till dissolved. Allow to cool until temperature drops below 60C then add NaOH, still stirring. Add phenolphthalein until very deep pink – 6 droppers full. Allow to set then cut unto 1cm, 2cm and 3cm cubes, leaving some for cylinders. Do NOT refrigerate (it fades). Can be made 2-3 days in advance if you take it out of container and leave loosely covered with plastic wrap on glass Petri dish and store in chemical store shelf. 2. Calculate amount of agar required to make gel at 1 cm, 2 cm & 3 cm deep Recipe: Dissolve 18 g agar per litre in distilled water. Microwave in 1L beakers, with caution Add 4 g NaOH per litre after agar solution has cooled below 60C then add – phenolphthalein and pour into containers. Can last for 2 weeks in the fridge 3. We provided kits for students:- Kit set up in large plastic tray containing – 500 ml 0.1M Sulphuric Acid 2 pairs gloves plastic ruler plastic spoon paper towel large crystallizing dishes. These make removing the agar blocks easier than using beakers which are much deeper. knife small white plastic dish for cubes of phenolphthalein agar -1 cm3, 2 cm3 & 3 cm3 To make up 500mL of jelly for the cubes, weigh out 9g of agar and 2g solid NaOH. Dissolve the NaOH in 50mL of water. Heat 400mL of water in a beaker almost to boiling, add the agar and stir until dissolved. Allow to cool then, when the temperature falls below 60C, add the NaOH solution, stirring continuously, until the jelly is deep pink. Allow to set. The agar can then be cut into blocks of the appropriate sizes. 4. 36gm agar 1600ml water, enough for 24 groups with 1 x 3cm,1 x2 cm,1 x 1cm block each 9gm NaOH in 100mls water Phenolphthalein in dropper bottles Make agar with water & boil till dissolved until clear solution Dissolve NaOH in the water stirring constantly as this is exothermic Cool agar to 60 deg C then add NaOH soln – this is important Add phenolphthalein – a lot at first then drop by drop till agar is a deep pink colour Pour into containers to set. You can use the rectangular “Unitray” which has square corners. 5. 1800mL boiling water + 40g Agar, stir until dissolved Turn hot plate off, but leave stirrer on and wait until it drops to less than 60oC. Mix 6g NaOH in 150mL water and add to above Squirt in 200mL Phenolphthalein. Yes, 200mLs! Pour it into plastic dropper bottle trays, the “Unitray”. Leave overnight to set and the students then cut them to sizes 6. We changed the prac because the agar tends to fade. Make the cubes ahead of time. Students place them in 0.5% potassium permanganate solution for 5 minutes then slice as per the Student Prac book. It works really well.