Cells and stem cells, at a glance

Living organisms consist of cells; a cell is the smallest unit of life; new cells arise from pre-existing cells.

• All organisms are made of cells.
• New cells arise from pre-existing cells.

Keep in mind: Energy use alone does not define a cell. Organelles carry out particular processes within the organised living cell.

Cell structures create useful spaces and surfaces. Identify their visible features, then connect those features to a specific function.

• Nuclear envelope surrounds the nucleus; the nucleolus lies inside.
• Cristae and thylakoid stacks are different membrane arrangements.
• A cell wall lies outside the cell surface membrane.

Keep in mind: The nucleolus is a region within the nucleus and is not membrane-bound. It is associated with ribosomal RNA production and subunit assembly.

Ribosomes make polypeptides; rough ER and the Golgi process and route many proteins; vesicles carry material between compartments.

• Rough ER bears ribosomes; smooth ER does not.
• Lysosomes digest material within a membrane-bound compartment.

Keep in mind: Ribosomes assemble the chain. The Golgi modifies, sorts and packages products that reach it.

Use several visible features and the image scale. A tidy textbook colour is not an identification test.

• Describe the visible clue before naming the structure.
• Stain colour does not prove organelle identity.

Keep in mind: A diagram is a model. Section angle, specimen preparation and resolution alter the appearance in a real image.

A typical bacterium is small and unicellular, with circular DNA, 70S ribosomes and a peptidoglycan wall, but no membrane-bound nucleus or other membrane-bound organelles.

• Bacterial wall: peptidoglycan; plant wall: cellulose.
• Bacterial DNA is not enclosed by a nuclear envelope.

Keep in mind: They lack membrane-bound organelles but possess non-membrane-bound 70S ribosomes.

Stem cells can self-renew and generate differentiated cells. Potency describes the range of cell types they can produce.

• Zygotic: totipotent; embryonic: pluripotent; blood stem cell: multipotent.
• Self-renewal and potency answer different questions.

Keep in mind: Totipotency includes the extraembryonic tissues needed for a complete organism; pluripotency is the capacity to form the body's cell types.

Embryonic stem cells supply diverse cell lineages during development. Blood stem cells continually replace short-lived specialised blood cells.

• Embryonic stem cells support development.
• Blood stem cells support lifelong replacement of several cell lineages.

Keep in mind: Differentiation primarily changes gene expression. Most specialised nucleated cells retain essentially the same genome, although particular cell types such as mature mammalian red cells later lose their nucleus.