real science for today's homeschooler

Yeast – Examining Living Cells

Yeast - Examining Living Cells

Yeast . . . it turns grapes into wine . . . it makes bread rise . . . but did you know it’s actually a living one-celled fungus? Yeast provides a safe way for children to observe a few of the life processes of living cells.

1. Living Cells Need Water – Add dry yeast to very warm water to activate them. Explain to children that the yeast must have water in order to carry out life processes. They are able to survive in a dormant state without water, but they won’t become active and grow until they have water.

2. Living Cells Need Food – Put some of the hydrated yeast culture in two small containers (preferably clear). Add sugar to one of the containers, but not to the other. Let children observe the differences they see over time. Does “feeding” the yeast cells make them more active?

3. Living Cells Produce Waste – The yeast culture with sugar will give off noticeable amounts of carbon dioxide gas in the form of bubbles. Explain to children that the cells are getting rid of waste just like they do . . . by “breathing” out carbon dioxide gas.

4. Living Cells Reproduce – If a microscope is available make a slide from a drop of the yeast culture with sugar. Look carefully and you may find a cell that is undergoing “budding.” Budding is the way yeast cells reproduce. First they double all the material inside the cell that’s needed to keep it alive. Then they separate out one set of the material and pinch it off in a little pocket on the side of the cell. The new pocket will eventually pop off and form a new yeast cell! Children enjoy seeing the “baby” yeast cells. 🙂

Extension – Make bread or another pastry that requires yeast to make it rise. Ask children to use what they have learned about the yeast cells to explain what is happening inside the dough. (The yeast is eating the sugar and using the water in the dough to grow. As it grows it produces waste in the form of carbon dioxide gas. The gas bubbles are what makes the bread rise. As the yeast reproduces, more and more yeast cells can produce more and more gas bubbles and the dough gets bigger and bigger!)

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Growing Bacteria at Home

Growing Bacteria at Home

First, a warning . . . if you grow bacteria at home there is always a possibility you could grow large amounts of harmful bacteria that could make someone in your household sick. Always use proper safety precautions when growing bacteria! Some safety hints are included below, but always, always use common sense when handling any bacteria culture.

When bacteria cultures are grown in the lab the bacteria is grown in shallow containers (Petri dishes) on a layer of nutrient agar. These supplies can be expensive, but you can simulate this setup at home with substitute ingredients.

First, the containers . . . any shallow, disposable container will work. The smaller the container, the less base material you’ll need. Very important safety tip . . . the container should NEVER be closed up air tight! Some of the most harmful types of bacteria are anaerobic, meaning they grow when there is no oxygen present. Be sure your bacteria culture is always exposed to oxygen! Leave the lids of your containers loose.

Next, the nutrient agar. Agar is just a plant-based gelatin. You can buy it from science supply stores, or you can use a substitute. I’ve heard of people using plain gelatin powder from the grocery store, but animal-based gelatin melts at around room temperature. Also, some bacteria are able to produce a chemical that breaks down animal-based gelatin. So, I don’t recommend using plain gelatin powder like Knox, as you may come out with a watery mess!

Instead, look for “agar agar” which is a flake type gelatin made from seaweed. It’s used as a thickener in many Asian foods and it can usually be found in Asian grocery stores, or any large grocery that has an Asian foods section. You just dissolve the flakes in boiling water and then cool to room temperature to solidify.

Gelatin alone won’t serve as a food source to encourage bacteria growth, so you need to add some type of nutrient media to the agar agar when heating. I would suggest adding a beef bouillon cube. It adds a food source to your gelatin, and its high salt content will often suppress the growth of bacteria that is at home in the human body (and may cause illness). Again, this doesn’t mean you can’t grow a harmful bacteria, just that it helps lessen the possibility! Caution should still be used in handling the bacteria cultures!

A final note about safety . . . never, never allow someone to culture their throat or nose, or cough into a dish. A relatively “safe” source of microorganisms to culture for small children is soil bacteria and fungi. Dig up some dirt, add enough water to thoroughly soak the soil, and allow the mixture to soak for 10 minutes. Then pour a little of the water onto the growth medium and spread it around. Soil is full of harmless bacteria and fungi that will grow a very impressive culture in the container!

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Edible Cell Models

Edible Cell Models

When teaching about the parts of the cell, increase your child’s interest by having them build an edible model of a cell. The type of cell and the type of model depend on the age of the child and your snack preference for them.

If your child is older they can research “parts of a cell” online to find many different diagrams of different types of cells. Help your child find a diagram at an appropriate level for their age and understanding. If you have a younger child, you may want to select the diagram for them. Children should then decide which cell parts they want to include in their model. Encourage them to find out more about each part . . . what does it look like and what does it do for the cell?

Next, decide on what type of edible model you want your child to make. Two popular choices are cookies and pizzas, but any snack that provides a flat surface to “decorate” will work. Whichever snack you choose, prepare the “base” (cookie, pizza crust, etc.), and collect a variety of toppings your child can use to represent the cell parts. A wide variety of shapes and colors will allow for more creativity and interest.

Children should select a topping that best represents each part of the cell they have decided to include in their model. Help them decorate the snack base to accurately model a real cell. Relative size, shape, and number of cell parts should be considered. Do stress to your child that the diagram they are using is just another person’s model of a cell and they do not have to look exactly alike!

Challenge your child to tell you the name and function of each cell part in their edible model before they eat it! You may also want to take pictures of the model before it disappears.

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