Being Multicellular

What are the advantages of having lots of cells?

If you want to be bigger than an Amoeba you need either giant cells or lots of cells. As the SA:Vol ratio limits cell size the only option is to be an organism made from lots of cells. The human body contains trillions of cells. This activity is about the characteristics of living things, cell differentiation and the complexity that emerges from simple cells joining together to make organs, emergent properties. A series of video clips and questions lead students to understand the main concepts and make a few notes.

Lesson Description

Guiding Questions

What are the characteristics which all living things have, always?

How can you make big animals, or plants when cells are limited in size by the SA to Vol ratio?

Can new characteristics or new properties appear when many cells live together?

Activity 1 The characteristics of living things

One of the most famous single-celled organisms is the amoeba. Watch the two videos below then answer the questions on the Being Multicellular student worksheet

  A single celled amoeba engulfs two single celled animals

  The Characteristics of living things - using the acronym "Mrs H. Gren"

This is a great 60 second review of the characteristics of living things.

Questions

  1. How do you think an amoeba can be "sensitive" to its environment?

    Receptors on the plasma membrane of the cell might detect chemicals in the environment around the amoeba.

  2. What substances in the amoeba may be kept in a constant state by homeostasis?

    Oxygen, Carbon dioxide, water, are three easy examples.

  3. What you have just seen in the amoeba is a good example of nutrition, how does excretion happen?

    The amoeba has engulfed its food by 'endocytosis', excretion can happen by a reversal of this process 'exocytosis'. Excretion can also happen by diffusion too.

  4. How many of the characteristics of living things do humans and amoebas share?

    Both humans and amoeba share all seven of these characteristics of living things.

Students can use the  Being Multicellular student worksheet

Activity 2 What happens when one cell divides to make a multicellular organism?

If you want to be bigger than an Amoeba you really have to be multicellular.
What happens as groups of cells grow? Do any new properties emerge?

Watch the videos below then describe the process of differentiation and the emergence of new properties when many cells work together. These properties could not exist in a single celled organism like an amoeba.

  From embryo to fetus - becoming multicellular

Start watching from 1min30s

  Human embryo development - focus on cells

This video shows how the cells differentiate during early embryo development.

  Making a Human - Simple Science (Nature magazine)

This is an excellent interview with an MIT professor. It could make an activity in itself.

Questions

  1. Name four different types of specialist cells which a single celled human zygote can differentiate into.
    What makes them different from each other?

    Bone cells, blood cells, muscle cells, skin cells, nerve cells.
    They have different shape, they do different things, eg. red blood cells are small and round and nerve cells are long and thin. If you tried to get nerve cells to flow in the blood you would quickly see the difference.

  2. Suggest a property of one of these groups of specialist cells which could not exist if there were not a large umber of cells living together?

    some types of skin cells (epithelial cells) can form tubes which carry blood around the body, or food through the digestive system. These structures are emergent, you wouldn't expect them if you only studied individual cells, and they couldn't exist if there was just one epithelial cell.

  3. It is said that thoughts we have in our brains is an emergent property. Explain why this is 'emergent'.

    The test of an emergent property is that it would not be expected by studying cells on their own. Nerve impulses in the brain, for example, are not emergent because they exist in individual nerve cells. Thoughts however are made from many junctions of nerve cells and many impulses between them. A biologist who never studied a whole brain would not know of their existence.

Teachers notes

This topic is important as a link between the study of cells and the study of larger organisms like humans. The appearance of emergent properties has inspired the relatively new field of systems biology. We could not learn all there is to know about the human body by studying cells on their own.

There are two short activities where students watch films and answer questions.

The activity should take about 50 minutes.

There is plenty of opportunity for extension work,

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