Lesson B6: Unicellular and Multicellular Organisms

Key Ideas     Unit Checklist 

  Video Lesson

What are the differences between unicellular and multicellular organisms? Watch this video to learn more.

 
 

  Lesson B6: Unicellular and Multicellular Organisms

Figure B.2.6.1 – The human digestive system depends on helpful bacteria.
Figure B.2.6.2 – Body odour is caused by bacteria on your skin.


Figure B.2.6.3 – We brush our teeth to remove bacteria and the food particles they digest.
Reading and Materials for This Lesson

Science in Action 8
Reading: Pages 110–114

Materials:
2% milk (1 litre), ½ cup powdered milk, live culture plain yogurt (the container will say “active bacteria” on the side), thermometer, 3 empty and clean glass jars with lids, cooking pot, large cooking spoon, stove, oven mitts, large heatproof bowl, small insulated cooler box, electric kettle, tablespoon measure, funnel, ladle, bath towels, 2 empty clean pop bottles with lids, 2 balloons, baking yeast, warm (but not hot) tap water, sugar, measuring cup, measuring spoons.

Bacteria and You
Both the inside and outside of your body are covered with unicellular bacteria. We rely on bacteria to stay alive, and bacteria rely on us to feed and reproduce.

Human skin is covered with helpful bacteria. These bacteria occupy space on your skin and prevent harmful bacteria from moving in. Skin bacteria reproduce very well in sweaty, warm areas that are not exposed to air. For example, bacteria thrive on your feet and armpits. When bacteria eat sweat and dead skin in these areas, they release stinky waste materials, which are also known as body odour. It’s a good idea to shower often to prevent bacterial overgrowth and body odour on these areas of your body.

Your inner digestive system contains a large variety of bacteria. These bacteria help you digest different types of food. A type of bacteria in your digestive system called Lactobacillus helps you digest milk. A type of bacteria called Escherichia coli, or E. coli is found in your intestines. Good varieties of E. coli make vitamin K to help blood clotting, and they also stop harmful bacteria from reproducing in your digestive system. Some other types of E. coli are harmful and cause food poisoning.

 Watch More

Body Bacteria

There are 100 trillion microorganisms on your body! Watch this video to find out more about where they live and what they do.

 
 
 
 
The different types of microorganisms in your digestive system have different jobs. Watch this video to learn more about all the bacteria in your digestive system.

 
 
 
 
Why do bacteria cause your feet to smell? Watch this video to find out more.

 
 
 
 
Bacteria that live in your mouth are responsible for bad breath, tooth decay, and other harmful effects to your health.

 
 
Figure B.2.6.4 – Yogurt is a popular fermented food.
Figure B.2.6.5 – Kimchi is a traditional Korean food. Kimchi is spicy fermented cabbage and vegetables.

Fermentation

Figure B.2.6.6 – Miso is a popular flavouring in Japanese soup. Miso is made from fermented soybeans.


Fermentation is a process that changes food. Fermentation happens when bacteria or other unicellular organisms are exposed to a carbohydrate, or sugar, food source. Fermentation needs to happen in an anaerobic environment, which means that there is no oxygen. When bacteria consume sugars without oxygen, they produce alcohol or acid, as well as carbon dioxide gas.

Many tasty foods are created by the process of fermentation. Bacteria can ferment vegetables like cabbage to make sour-tasting sauerkraut and kimchi. A certain type of bacteria ferments warm milk to produce yogurt. The bacteria in yogurt are very helpful and healthy for your digestive system. Many types of cheese are made with the help of bacteria and mold. Bacteria ferment wine into acidic vinegar.

Figure B.2.6.7 –Sourdough bread is made from fermented dough.
Figure B.2.6.8 – Sauerkraut is fermented cabbage that is often eaten on hot dogs.

 Watch More

Fermentation and Food

Microorganisms create many delicious foods through the process of fermentation. Watch these videos to learn more.

 
 
 
 
 
 
 
 
This video explains how yogurt is made.

 
 

  Try It! 

Homemade Yogurt

Try this experiment to ferment milk and make your own homemade yogurt. 


Materials: 

  • 2% milk (1 litre)
  • ½ cup powdered milk
  • Live culture plain yogurt (the container will say “active bacteria” on the side)
  • Thermometer
  • 3 empty and clean glass jars with lids
  • Cooking pot
  • Large cooking spoon
  • Stove
  • Oven mitts
  • Large heatproof bowl
  • Small insulated cooler box
  • Electric kettle
  • Tablespoon measure
  • Funnel
  • Ladle
  • Bath towels

This activity involves hot fluids. It must be completed with the supervision of an adult. DO NOT attempt this activity by yourself.

Hot fluids and metal can burn you or others if you are not careful.
Wear oven mitts when handling the hot cooking pot.
Never leave a hot stove unattended.
Take care when pouring hot fluids.
Instructions:  (You may want to watch the video at the end of the instructions before you start.)
  1. Put the glass jars into the insulated cooler box.

  2. Boil water in the electric kettle. Fill the cooler with hot water, surrounding the bottom of the glass jars. The water should only cover the bottom of the jars. Close the cooler.

  3. Pour the milk into a cooking pot.

  4. While stirring with the spoon, heat the milk on the stove over medium heat to 85°C. Check the temperature of the milk frequently. Constantly stir the milk so it doesn’t burn.

  5. Once the milk reaches 85°C, turn off the stove and remove the milk from the heat.

  6. While wearing oven mitts, pour the milk into the large heatproof bowl. Let the milk cool slowly to 45°C. It should take about 1 hour. Stir the milk every 15 minutes.

  7. Once the milk reaches 45°C, add 2 tablespoons of live culture yogurt to the milk and stir well.

  8. Take the glass jars out of the insulated cooler box.

  9. Place a funnel in top of one jar. Ladle milk through the funnel into the jar until it is three-quarters full. Repeat for the rest of the jars.

  10. Close the glass jars and place them back into the cooler box.

  11. Add additional water to the cooler box, to the height of the yogurt in the jars. Mix cold and hot water so the cooler box water is 43°C (110°F).

  12. Close the cooler box and surround it with several layers of bath towels.

  13. Every 2 hours, check the temperature of the cooler water. If the water cools below 43°C (110°F), remove some cool water and add more hot water to bring the temperature up.

  14. After 6 hours, open one yogurt jar to see if the yogurt has thickened. If the yogurt is still thin, allow it to remain in the cooler at 43°C for 4 more hours.

  15. Watch this video to see this experiment and its results:

 
 

Questions: 

Think about the following questions very carefully. Then, type or write your answers. After you have your answers, click the questions for feedback.


The milk was initially heated to a high temperature to kill any harmful bacteria in the milk.
Keeping the yogurt at 43°C provided the warm conditions necessary for the yogurt bacteria to reproduce. As more bacteria formed, they ate the sugars in the milk and converted it to yogurt.
Figure B.2.6.9 – Bread yeast is sold as tiny pellets.
Figure B.2.6.10 – Yeast makes bread dough rise.

Yeast

Figure B.2.6.11 – Kombucha is a popular tea drink made using yeast and bacteria.


Yeast is a type of unicellular fungus. Humans use yeasts for fermentation. When yeasts eat carbohydrates, or sugars, they ferment the sugars into alcohol and carbon dioxide gas. You might be most familiar with yeast from breadmaking. When yeast, flour, and water are combined, the yeast eats the flour carbohydrates to make a little bit of alcohol and a lot of carbon dioxide gas. The gas forms bubbles in the bread dough, causing the bread to rise and form a light fluffy structure.

Yeast also ferments tea into kombucha, grape juice into alcoholic wine, and mashed-up grains into alcoholic beer. Kombucha and beer are bubbly, which is a result of the carbon dioxide gas of fermentation. Carbon dioxide bubbles are usually removed from wine, but some wines like champagne still contain their carbon dioxide bubbles.

 Watch More

Yummy Yeast Treats

Watch this video to learn more about how yeast helps bread rise.

 
 
 
 
This video shows how yeast and bacteria help make sourdough bread.

 
 

  Try It! 

Figure B.2.6.12 – As yeast digests sugars, it produces carbon dioxide gas as waste.
Hungry Yeast

Try this experiment to observe what happens when yeast and sugar are combined.

Materials: 
  • 2 empty clean pop bottles with lids
  • 2 balloons
  • Baking yeast (2 packages or a jar)
  • Warm tap water
  • Sugar
  • Measuring cup
  • Measuring spoons
  • Funnel

Instructions:

  1. Measure 1 cup of warm (but not hot) tap water into the measuring cup.

  2. Place the funnel in the top of one empty pop bottle.

  3. Pour the warm tap water through the funnel into the pop bottle.

  4. Add 1 package or 1 tablespoon of yeast through the funnel into the pop bottle.

  5. Repeat steps 1 to 4 for the second pop bottle.

  6. Add 1 tablespoon of sugar through the funnel into the second pop bottle.

  7. Close both pop bottles and shake them well.

  8. Remove the lids from both bottles and stretch a balloon over the opening of each pop bottle.

  9. Let the pop bottles and balloon sit for several hours. What changes do you observe?

  10. Watch this video to see this experiment and its results:

 
 

Questions: 

Think about the following questions very carefully. Then, type or write your answers. After you have your answers, click the questions for feedback.

When yeast consumes sugar, it produces carbon dioxide gas. When yeast and sugar were combined in warm, moist conditions, the yeast consumed sugar to produce carbon dioxide gas, which inflated the balloon.
The yeast in water had no sugar to consume, so it could not produce carbon dioxide gas to inflate the balloon.
Figure B.2.6.13 – E coli is a type of bacteria that can cause food poisoning.
Figure B.2.6.14 – Salmonella is a bacteria that causes food poisoning.

Harmful Unicellular Organisms

Figure B.2.6.15 – Raw chicken can be contaminated with Salmonella.


A large number of unicellular organisms are harmful to the human body. Many infectious diseases are caused by unicellular bacteria. A type of unicellular amoeba called Naegleria fowleri can enter people’s noses and eat their brains!

Food poisoning occurs when bacteria, viruses, or other toxins contaminate human food. The first symptoms of food poisoning are usually vomiting and diarrhea, and sometimes food poisoning can cause death. One common type of food poisoning is caused by Salmonella bacteria. Salmonella is a type of bacteria found in some animal feces, including that of reptiles and chickens. Salmonella can contaminate raw chicken meat and raw eggs. Another type of food poisoning bacteria that sometimes contaminates raw beef is a harmful strain of E. coli bacteria.

To prevent food poisoning, meat and eggs should be cooked thoroughly and never left out at room temperature. Knives and cutting boards that are used to cut meat should not touch vegetables or other food before they are washed in soap and hot water. Always wash your hands after handling raw meat.

 Watch More

Food Poisoning and Brain Eating Unicellular Organisms

Watch this video to learn more about how food becomes contaminated with Salmonella bacteria.

 
 
 
 
This video explains more about E. coli food poisoning.

 
 
 
 
Watch this video to learn more about the brain-eating amoeba.

 
 


Multicellular Organisms are Made Up of Many Cells

When you think of multicellular organisms, you might think of very large organisms. However, some multicellular organisms are so tiny that they are very hard to see with the naked eye.

For example, head lice are tiny insects that are usually only 2 millimeters long. They live in human hair and bite the scalp to feed on human blood. Lice lay tiny eggs called nits on hair strands. If a person gets lice, they need to use a medicated shampoo on their hair to kill the lice, and get rid of nits with a fine-toothed comb.

Figure B.2.6.16 – Head lice are tiny multicellular organisms that live in hair.
Figure B.2.6.17 – Lice are 2 mm in length.

 Watch More

Tiny Multicellular Organisms

Watch this video for a microscopic look at head lice.

 
 
 
 
Dust mites are tiny invisible multicellular organisms that are found all around us. Watch this video to learn more about dust mites.

 
 



  Make sure you have understood everything in this lesson. Use the Self-Check below, and the Self-Check & Lesson Review Tips to guide your learning.

Unit B Lesson 6 Self-Check

Instructions


Complete the following 6 steps. Don't skip steps – if you do them in order, you will confirm your understanding of this lesson and create a study bank for the future.

  1. DOWNLOAD the self-check quiz by clicking here.

  2. ANSWER all the questions on the downloaded quiz in the spaces provided. Think carefully before typing your answers. Review this lesson if you need to. Save your quiz when you are done.

  3. COMPARE your answers with the suggested "Self-Check Quiz Answers" below. WAIT! You didn't skip step 2, did you? It's very important to carefully write out your own answers before checking the suggested answers.

  4. REVISE your quiz answers if you need to. If you answered all the questions correctly, you can skip this step. Revise means to change, fix, and add extra notes if you need to. This quiz is NOT FOR MARKS, so it is perfectly OK to correct any mistakes you made. This will make your self-check quiz an excellent study tool you can use later.

  5. SAVE your quiz to a folder on your computer, or to your Private Files. That way you will know where it is for later studying.

  6. CHECK with your teacher if you need to. If after completing all these steps you are still not sure about the questions or your answers, you should ask for more feedback from your teacher. To do this, post in the Course Questions Forum, or send your teacher an email. In either case, attach your completed quiz and ask; "Can you look at this quiz and give me some feedback please?" They will be happy to help you!

Be a Self-Check

Superhero!



Self-Check Quiz Answers

Click each of the suggested answers below, and carefully compare your answers to the suggested answers.

If you have not done the quiz yet – STOP – and go back to step 1 above. Do not look at the answers without first trying the questions.

Multicellular organisms have a huge number of cells and complex organ systems, compared to simple unicellular organisms. Large organ systems with many cells require much more time to grow compared to tiny unicellular organisms that split in half to reproduce.
Both yeast and humans eat sugars to produce energy. The process of using sugars to produce energy makes carbon dioxide. Yeast release carbon dioxide, and humans breathe out carbon dioxide.
Antibiotics kill both helpful and harmful bacteria in the body. Yogurt contains bacteria that are helpful in the digestive system. Eating yogurt replaces helpful bacteria that may have been killed by antibiotics.
Unicellular organisms are much smaller than most multicellular organisms. Since unicellular organisms are tiny, great numbers of them can fit into smaller spaces. Unicellular organisms are simple organisms, which means they can reproduce quickly into large populations.
Helpful bacteria occupy space on human skin to prevent the growth of harmful bacteria. Helpful bacteria are found in the human digestive system and help digest food. Helpful bacteria ferment some foods, which creates healthy and delicious foods for humans.