The Invisible Living World: Beyond Our Naked Eye · Lesson 3 of 9
Cell Shapes and Levels of Organisation
“Connect a cell’s shape to its job and trace how cells build a whole organism.”
• Use examples to explain how cell shape suits function. • Distinguish cell, tissue, organ, organ system and organism. • Explain how specialised cells cooperate in digestion. • Describe how repeated division of an egg can lead to a multicellular organism.
Why cells take different shapes
The onion and cheek observations showed that cells need not look alike. Shape often helps a cell perform its job. Thin, flat cheek cells make a protective lining. A nerve cell has long, branched extensions that help pass messages between parts of the body. A spindle-shaped muscle cell can shorten and relax; some plant cells form long tubes that help move water.
Do not try to identify a cell’s job from shape alone. Instead ask how its features could help it perform an observed task. A long tube offers a path for transport, while a flat sheet can cover and protect a surface.
Problem
A plant must move water through its body. Which described cell shape seems helpful?
- 1.Water needs a connected route over a distance.
- 2.Long tube-like transport cells can form that route more readily than isolated flat lining cells.
Problem
Why is a nerve cell long and branched rather than simply round?
- 1.Messages must travel between different regions.
- 2.Its extensions help reach and communicate with other cells.
Specialised cells cooperate
Think about food moving from the mouth to the stomach. Groups of muscle cells in the food pipe contract and relax in a wave-like sequence, pushing food onward. Muscle cells in the stomach wall help churn it. Other stomach-lining cells produce digestive juices and acid that help break food down. Digestion depends on these different jobs working together, not on one identical cell doing everything.
The way different cells develop structures suited to particular tasks in a multicellular organism.
Food movement and digestion involve groups of cells with different jobs. A single muscle cell is not an entire digestive system.
From cells to an organism
When similar cells work together they form a tissue. Different tissues build an organ, such as a stomach. Organs cooperate in an organ system, such as the digestive system. The systems together form an organism. The brick-and-wall comparison helps with the idea of building from smaller units, but living cells also carry out processes and interact with one another.
A group of similar cells working together to perform a particular function.
A body structure made from different tissues working together.
A group of organs working together in a major body function.
Problem
Identify the levels involved when stomach muscles churn food.
- 1.A muscle cell is one working unit; similar muscle cells together form muscle tissue.
- 2.Muscle tissue contributes to the stomach organ, which belongs to the digestive system of the organism.
Many cells from one beginning
A complex plant or animal can develop from a single egg cell that divides repeatedly. The resulting cells become numerous and take on different roles. An ostrich egg’s yolk is an unusually large single cell, about 130–170 mm across in the source. The protective shell and white material are extra material around it; their presence does not make all of it one giant cell. Size alone therefore does not tell us how many cells an organism has.
Check your understanding
Work in both directions: trace how one specialised cell contributes to a whole organism, and trace an organ’s function back to cooperating cells.
Quiz
What feature helps a nerve cell pass messages between distant regions?
What immediately follows tissue in the organisation sequence?
Which cells help push food down the food pipe?
Why are thin, flat cheek cells suitable for their role?
Which statement about an ostrich egg is supported by the chapter?
Practice Problems
- Draw the cell-to-organism sequence and give a digestive-system example at each level where possible.
- Explain how shape helps each of a muscle cell and a nerve cell perform its task.
- Describe two different cellular contributions to stomach digestion.
- A friend claims that a large body must begin with many egg cells. Explain the role of repeated cell division.
- Why does the brick-wall analogy help, and where is it incomplete?
Key Takeaways
• Cell structures and shapes often suit particular functions. • Specialised cells cooperate within tissues, organs and organ systems. • The organisation sequence is cell → tissue → organ → organ system → organism. • Many-celled organisms can develop through repeated division of one egg cell.