World Builders™
World Builders™
Session Five  --  Seaweeds
Session Five  --  Seaweeds

Seaweeds: Algae in the Water
Seaweeds: Algae in the Water

Home Page for Unit Five
Home Page for Unit Five

Supported  by water, algal cells began to form structures that we call sea weeds.

A Quick Review

     Life first appeared on earth about 3.8 billion years ago. The first life forms were single celled organisms, and they continued to be the only life forms here for 2.8 billion years.

      The early life forms were very small cells called prokaryotes. They survived by capturing organic molecules that were abundant in the early seas. However, after a time these molecules became less abundant, and the tiny cells needed a new way to capture energy. Some cells began to use chlorophyll to make sugar using carbon dioxide and sunlight, a process called photosynthesis. Oxygen was produced as a waste product of photosynthesis.  

     Oxygen was toxic to many of the earliest life forms.  Cyanobacteria began producing oxygen early in  prokaryotic times, but this oxygen was  absorbed by water, and most of it united with iron to form iron oxide.  This took the oxygen out of the biosphere so that it was not readily available to life forms. 

     Eventually, however, atmospheric oxygen very slowly became more abundant, and more complex cells, called eukaryotes, appeared. A eukaryotic cell has a nucleus enclosed in a membrane, and other membrane-enclosed components such as mitochondria and chloroplasts. The chloroplasts do photosynthesis inside the eukaryotic cells, creating food for them.  

       Here is a time line showing the 4.5 billion years since the formation of the earth.

     As you can see, multicellular organisms are relative newcomers, and began to appear only 700 million years ago in the history of life.  

     There are gaps in our knowledge of the story of the earth.  One gap is between non-life and life: we don't know how that gap was bridged.  Another gap is between unicellular life forms and multicellular life forms: we don't really know how that leap was made.

The Protists -- A Group of Eukaryotes

     In this lesson we study the protists, a group of eukaryotic organisms that seem to explore the the possibilities available to them as larger, more complex cells.  Many are still unicelluar organisms, in themselves complete and independent.  Others show the beginnings of communal living.  There are many protists on earth today, and scientists are finding out how unique and innovative members of this group are.  They all capture energy, grow, and reproduce, but they do so in innovative ways that amaze us.  Some members of this group are an important part of the food chain in the ocean.

Moving Toward Multicellular Organisms

     For many millions of years, cells would reproduce by dividing into two separate individual cells.   However, sometimes a cell would divide and then two or more of the new cells would remain attached together. There are strings of cells in pools and lakes today that are like very thin spaghetti.. These are not really multicellular organisms, because the cells stick together but remain self-sufficient and identical to one another in what they do..

    Truly multicellular organisms exhibit differentiation. The land plants that we are familiar with have cells adapted to forming stems, roots, leaves, flowers, and seeds. True plants evolved from algae in the water.  Have you looked closely at seaweeds, at the beach, or in an aquarium? Check the Introduction to Session Five for some thoughts about the algae that we call seaweeds.  We will be studying them in this lesson.

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  Information        Read these pages before you begin making your page.

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Do this part in class:
  • Read or print out How to Plan Your Seaweeds
  • Think about your first life forms and how they could be joined together
  • Go to Chapter 5 of the Work Sheet Master Pages to Get Ideas About Making Your Seaweeds Evolve
  • Decide which biomes your group will work with.  You need to choose as many biomes as there are members in your group.  (Example: 4 people, 4 biomes:  2 people, two biomes.)
  • Decide what the file names of your pages will be.
  • The Page Coordinator should write a draft of the Water Algae page in class tonight. Help him/her.
  • Sketch out the joined cells, and work towards forms that can survive in your environments.
  • Draw your life forms from stage to stage as they progress into aquatic seaweeds
  • Write descriptions of your aquatic plants
  • Each person will design and describe his or her own seaweed and its three or more evolved species.

Homework: Bring to class next session:

Seaweed Information Menu

Design Seaweed Page

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© 1996,1997, 1998, 1999, 2000, 2002, 2003.   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at .