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Cell structure: what each organelle is shaped for questions

The ultrastructure of eukaryotic cells: nucleus, nucleolus and nuclear envelope, rough and smooth endoplasmic reticulum, the Golgi apparatus and the route a secreted protein takes through it, mitochondria and chloroplasts, lysosomes, ribosomes at 80S and 70S, and the wall, vacuole and plastids that separate a plant cell from an animal one.

5 original questions · 16 marks · the cell structure: what each organelle is shaped for notes · Cells, microscopy and biological organisation

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• 4 marks · Compare

  Compare the structure of a mitochondrion with the structure of a chloroplast.

  Mark scheme

  1. B1 both are surrounded by two membranes, but a chloroplast has a separate internal thylakoid membrane system as well
  2. B1 the inner mitochondrial membrane is folded into cristae, whereas the chloroplast membranes form thylakoids stacked into grana
  3. B1 the fluid interior of a mitochondrion is the matrix, where the Krebs cycle runs, whereas that of a chloroplast is the stroma, holding the enzymes of the light-independent reactions
  4. B1 a chloroplast contains chlorophyll in its thylakoid membranes, whereas a mitochondrion contains no pigment

• 4 marks · Explain

  Explain why the enzymes of a lysosome must be kept behind a membrane, and explain how those enzymes reach the lysosome after they have been made.

  Mark scheme

  1. B1 a lysosome holds hydrolytic enzymes, which digest worn-out organelles and material taken in by phagocytosis
  2. B1 loose in the cytoplasm those enzymes would hydrolyse the cell that made them, so they are released only where they are wanted
  3. B1 the enzymes are proteins, made on ribosomes attached to the rough endoplasmic reticulum
  4. B1 they are packaged at the Golgi apparatus, and a lysosome is simply a vesicle budded from it and kept rather than secreted

• 3 marks · Describe

  Describe the structure of the Golgi apparatus, and describe what happens to a protein while it passes through it.

  Mark scheme

  1. B1 a stack of flattened membrane sacs, usually four to eight, with vesicles budding from one face
  2. B1 vesicles carrying newly made protein arrive at one side and the contents move through the stack
  3. B1 the protein is modified, for example by having sugar groups added to make a glycoprotein, then sorted and packaged into a vesicle addressed to its destination

• 3 marks · Suggest

  An electron micrograph of a cell from the adrenal gland shows very little rough endoplasmic reticulum but an unusually large amount of smooth endoplasmic reticulum. The adrenal gland releases steroid hormones. Suggest an explanation for these two observations.

• 2 marks · State

  State one function of smooth endoplasmic reticulum, and state the one structural feature that distinguishes it from rough endoplasmic reticulum.

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