The big idea: A eukaryotic cell is divided into compartments: membranes wall off spaces inside it, each with its own job.
The points to remember
- A plasma membrane encloses the cytoplasm, which is divided into compartments by membranes.
- Each membrane-bound organelle is a compartment with its own job: the nucleus, mitochondria, ER, Golgi apparatus, vesicles and vacuoles.
- The cytoplasm has 80S ribosomes, free or attached to the rough ER.
- A cytoskeleton of microtubules and microfilaments keeps the cell's shape and moves organelles around.
Remember it as: Eukaryote: a cell with rooms, each behind its own membrane.
Real example: a liver cell: A human liver cell has hundreds of mitochondria and a large amount of rough ER, because it makes many proteins and uses much energy.
Animal, plant and fungal cells, and single-celled protists, are all eukaryotic.
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The big idea: The nucleus holds the chromosomes behind a double membrane with pores.
The points to remember
- The nucleus is surrounded by a double membrane, the nuclear envelope.
- The envelope has nuclear pores: gaps that let substances such as mRNA in and out.
- Inside are the chromosomes: long, linear DNA molecules bound to histone proteins.
- A dense nucleolus inside the nucleus is where ribosomes are made.
Chromosomes, not loose DNA: Eukaryotic DNA is linear and bound to histones. Naked, circular DNA is the prokaryote's.
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The big idea: Outside the nucleus, the cytoplasm holds many membrane-bound organelles, each doing one job.
Mitochondria
Aerobic respiration: release energy as ATP.
Rough ER
Membranes with ribosomes: make proteins for export.
Smooth ER
Membranes without ribosomes: make lipids.
Golgi apparatus
Modify proteins and pack them into vesicles.
Vesicles
Small sacs that carry substances around the cell.
Lysosomes
Vesicles of digestive enzymes that break down worn-out parts.
Vacuoles
Larger sacs that store water and other substances.
Remember it as: Make it on the rough ER, pack it in the Golgi, ship it in a vesicle.
A pancreas cell that makes digestive enzymes is packed with rough ER and Golgi apparatus, because it exports so much protein.
Membrane-bound or not: Ribosomes and the cytoskeleton have no membrane of their own. The nucleus and mitochondria have two.
How it is tested: Paper 2 asks for membrane-bound organelles found in eukaryotic cells [2]; Paper 1B and Paper 2 ask whether a cell is eukaryotic, with a reason seen in the image [1].
Cells from a pea leaf and from a human pancreas are both eukaryotic. State two membrane-bound organelles found in both of these cells.
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Eukaryotic, with a reason: Name a feature you can see: a nucleus, or membrane-bound organelles. A feature you cannot see in the image, such as 80S ribosomes, does not count.