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🧬 Biology · Cells

Cell Organelles and Their Jobs: notes and common mistakes

Study notes

Key ideas

  • Eukaryotic cells (animals, plants, fungi, protists) have a nucleus and other membrane-bound organelles. Prokaryotic cells (bacteria and archaea) have neither, but they do have DNA, ribosomes, cytoplasm, a cell membrane and usually a cell wall.
  • The nucleus stores the DNA instructions. The nucleolus makes ribosome parts. The nucleus does not think.
  • Ribosomes build proteins. Rough ER (with ribosomes) handles proteins for export, membranes and lysosomes. Smooth ER makes lipids. The Golgi apparatus modifies, sorts and packs. Vesicles carry cargo.
  • Mitochondria make most of a eukaryotic cell’s ATP by converting the energy in food molecules. They are in plant cells too. Chloroplasts (plants and algae) carry out photosynthesis.
  • Plant cells (compared with animal cells) add a cell wall, chloroplasts and a large central vacuole. Animal cells have lysosomes and no wall.
  • Cell walls are also found in fungi, most bacteria, most archaea and some protists. Cell membranes are found in every cell.

Steps

  1. Nucleus: the DNA instructions for the protein are copied into mRNA, which leaves through a nuclear pore.
  2. Ribosome on the rough ER: it reads the mRNA, builds the protein, and the protein enters the ER.
  3. Vesicle: a vesicle carries the protein from the ER to the Golgi apparatus.
  4. Golgi apparatus: the protein is finished, sorted and packed into a new vesicle.
  5. Cell membrane: the vesicle fuses with the membrane and releases the protein outside the cell.

Watch out

  • The nucleus is not a brain. It stores instructions and does not think or decide.
  • Mitochondria do not create energy. They convert the energy in food molecules into ATP, and some energy is released as heat.
  • Plant cells have mitochondria as well as chloroplasts.
  • Cell walls are not only in plants: fungi, most bacteria, most archaea and some protists have them, and animal cells do not.
  • A cell wall does not replace the cell membrane. Every cell has a membrane.
  • The protein pathway applies to proteins that are exported (and to membrane and lysosome proteins). Proteins that work in the cytoplasm are made on free ribosomes.

Why it works

  • Organelles let a cell do many jobs at once without the jobs getting in each other’s way. Membrane-bound compartments keep different chemical reactions separate.
  • Knowing which organelle does which job explains how a cell is organized and why different cells look different. A muscle cell has many mitochondria, and a cell that exports lots of protein has lots of rough ER.
  • The protein pathway connects this lesson to DNA and protein synthesis, which come later in the course.

Common mistakes

✗ MistakeThe nucleus is the brain of the cell, so it thinks and makes decisions.
✓ InsteadThe nucleus holds the DNA instructions and copies the ones the cell needs. It does not think. Think library, not brain.
✗ MistakeMitochondria are only in animal cells.
✓ InsteadPlants, fungi and most other eukaryotes have mitochondria too. A plant cell has mitochondria as well as chloroplasts.
✗ MistakeMitochondria create energy.
✓ InsteadEnergy is not created. Mitochondria convert the chemical energy in food molecules into ATP, and some energy is released as heat.
✗ MistakePlant cells have chloroplasts, so they do not need mitochondria.
✓ InsteadChloroplasts make sugar. Mitochondria release the energy in sugar as ATP. Plant cells need both, and cells without chloroplasts, such as root cells, still have mitochondria.
✗ MistakeCell walls are found only in plants.
✓ InsteadFungi, most bacteria, most archaea and some protists also have cell walls. Animal cells have none.
✗ MistakeThe cell wall and the cell membrane are the same thing.
✓ InsteadThe cell membrane is a thin boundary around every cell. A cell wall is a rigid layer outside the membrane in some kinds of cells.
✗ MistakeEvery protein goes through the rough ER and the Golgi apparatus.
✓ InsteadProteins that work inside the cytoplasm are made on free ribosomes. The ER and Golgi route is for proteins that are exported, built into membranes or sent to lysosomes.

Teach it back

Explain to a friend how a cell in the pancreas gets a protein such as insulin out of the cell, naming each organelle in order, and say what the nucleus does and does not do.

Explaining it out loud shows what you really know. Did you…

  • I put the steps in order: nucleus (copy of the instructions), ribosome on the rough ER, vesicle, Golgi apparatus, vesicle, cell membrane.
  • I said what each organelle does in one short sentence.
  • I said the nucleus stores DNA instructions and does not think, so it is a library and not a brain.
  • I said that proteins that work in the cytoplasm are made on free ribosomes and skip this route.

Your 3-week memory plan

  1. TodayRead the lesson, then try the flashcards.
  2. TomorrowRecall it from memory first, then check.
  3. In 3 daysTake the true/false and practice questions.
  4. In 1 weekMix it with older topics and teach it back.
  5. In 3 weeksA last quick review before it fades.

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