Exploring and Understanding Concepts

Example

State the three ideas of cell theory.

1. Every living thing is built of one or more cells — there is no living thing made of none.
2. New cells arise from existing cells dividing — never from nowhere, and never assembled out of non-living material.

Example

Explain why a single cell counts as the unit of life.

1. Because a single cell is the smallest thing that carries out all of life's activities — anything smaller is working machinery, not a living whole.

Example

Name the part the arrow marks and its job.

1. The arrow marks the mitochondria: they release the energy stored in food for the cell to use. Organelles are the little structures inside a cell that each carry out one of its jobs.

Example

Name the job of the nucleus, the cell membrane and the cell wall.

1. Nucleus: directs the cell's activities and holds its instructions. Cell membrane: controls what enters and leaves the cell. Cell wall: gives a plant cell its rigid shape and support.

Example

Name the job of the chloroplasts, the vacuole and the cytoplasm.

1. Chloroplast: captures light energy to make the cell's food. Vacuole: stores water and dissolved materials. Cytoplasm: fills the cell and holds the working parts in place.

Example

Say which side is the plant cell, and name its plant-only parts.

1. Left — the plant cell: the straight-sided box is its cell wall, the green ovals are chloroplasts, and the big oval is a large central vacuole. Those three are plant-only.

Example

Say which side is the animal cell, and compare the parts both cells share.

1. Right — the animal cell: a rounded blob with no wall and no chloroplasts, and only a small vacuole.
2. Both share a nucleus, a cell membrane, cytoplasm and mitochondria.

Example

Explain why a drop of dye spreads through still water.

1. Diffusion: the particles' own motion — they spread from where they were crowded to where they were scarce until everything matched, entirely on their own.

Example

Explain why salted lettuce wilts.

1. Lettuce: osmosis — water moved out of the cells, toward the saltier side of each membrane — and cells that lose their water lose the firmness the water was providing.

Example

Explain which way water moves into a cell placed in fresh water, and what concentration means.

1. Fresh water: inward — water crosses toward the saltier side, which is the cell's inside, so the cell swells and firms up. Concentration is how much of a dissolved substance is present in a set quantity of the mixture.

Example

Compare how an amoeba and a human meet their needs.

1. Unicellular means an organism whose entire body is one cell doing every job of life. The amoeba's single cell does every job itself across its own surface, while the human divides the work among specialized cells organized into tissues, organs and systems.

Example

Explain how yeast feeds.

1. Yeast: materials pass directly across its membrane — with the whole cell touching its surroundings, dissolved food can enter and wastes can leave at the surface itself.

Example

Explain why large organisms need systems.

1. Deep cells cannot reach the surface for themselves — systems exist to deliver supplies inward and carry wastes outward, the job the amoeba's own surface does directly.

Example

Put the levels in order, and explain what a tissue is.

1. Cell, then tissue, then organ, then organ system — each level built out of the one before it.
2. A tissue: many similar cells grouped together and working on the same task.

Example

Explain what an organ is.

1. An organ: a structure built from different tissues working together on one job — the stomach, with muscle, lining and nerve tissues.

Summary

• All life is cells; cells come from cells; a cell is the unit of life.
• Each organelle has one job.
• Plant cells add a wall, chloroplasts and a large vacuole.
• Diffusion spreads; osmosis moves water toward the saltier side.
• One cell does every job; many cells divide the work into systems.
• Cell, tissue, organ, system.