Nucleus, Mitochondria and Chloroplasts
The structure and function of the nucleus, mitochondria, chloroplasts and ribosomes in eukaryotic cells, with cell theory as the unifying idea. Includes the 4-mark exam question, a worked model answer and the examiner trap on saying mitochondria make ATP, not energy.
Chapters (8)
Exam question [4 marks]
Describe how the structure of the nucleus and the structure of the mitochondrion are related to their functions.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Nuclear envelope has pores, so messenger RNA can leave the nucleus | "nuclear pores let mRNA / molecules out" | "pores let everything in and out"; "the nucleus has holes" with no function |
| 2 | Chromosomes of protein-bound, linear DNA carry the genetic code / instructions for making proteins | "DNA wound round histones / proteins", "codes for proteins", "genes" | "circular DNA"; "DNA controls the cell" alone |
| 3 | Nucleolus makes ribosomal RNA and assembles ribosomes | "makes rRNA", "makes ribosomes" | "nucleolus contains DNA"; "nucleolus makes proteins" |
| 4 | Inner membrane folded into cristae gives a large surface area for enzymes of aerobic respiration, making ATP | "folded inner membrane", "more room for enzymes / ATP synthase / ATP synthesis"; "enzymes in the matrix for the Krebs cycle release energy to make ATP" (earns mark 4 instead, not an extra mark) | "mitochondria make energy"; "cristae" with no function; "large surface area" with no reference to enzymes or ATP |
Each mark links a structure to a function; a function with no structure scores nothing. "Makes energy" is never credited; ATP is the product. The matrix and its enzymes are creditable only if linked to respiration, and cannot add marks beyond mark 4 (the one mitochondrion mark) here. Chloroplasts and ribosomes are not asked in this question.
Model answer (4/4)
The nucleus has an envelope with nuclear pores, so messenger RNA can leave to the cytoplasm. It contains chromosomes made of protein-bound, linear DNA, which carries the genetic code for making proteins. The nucleolus makes ribosomal RNA and assembles ribosomes. The mitochondrion has an inner membrane folded into cristae, giving a large surface area for the enzymes of aerobic respiration, so more ATP can be made.
Examiner trap
- Saying the nucleolus contains DNA only (it makes ribosomal RNA and ribosomes)
- Saying mitochondria 'make' energy
- Not naming the structure and function together
Key terms
- cell theory
- nucleus
- nuclear envelope
- nuclear pore
- chromosome
- nucleolus
- mitochondrion
- cristae
- matrix
- chloroplast
- granum
- stroma
- ribosome
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:25Cell theory
First, a big idea that runs through all of this: cell theory. Cells are the basic unit of life. All living things are made of cells. And new cells only come from existing cells. Now let's look inside a eukaryotic cell, starting with the nucleus.
0:43The nucleus
The nucleus is surrounded by a nuclear envelope, which is two membranes. The envelope has nuclear pores, and these let molecules such as messenger RNA leave to the cytoplasm. Inside are the chromosomes. Each one is a linear molecule of DNA, wound round proteins. The DNA carries the genetic code, the instructions for making proteins.
And there are one or more nucleoli. A nucleolus makes ribosomal RNA, and uses it to assemble ribosomes. Remember that one. The nucleolus makes ribosomal RNA and assembles ribosomes.
1:23Mitochondria
Next, the mitochondrion. This is where the later stages of aerobic respiration happen, and where most of the ATP is made. It has an outer membrane, and an inner membrane that is folded into cristae. The folds give a large surface area, so there is room for lots of the enzymes that make ATP. Inside is the matrix, which contains enzymes for the middle stages, the link reaction and the Krebs cycle.
So energy released in respiration is used to make ATP. The mitochondrion does not make energy.
1:58Chloroplasts
Plants and algae also have chloroplasts, the site of photosynthesis. Inside are stacks of flattened discs called grana, which hold the chlorophyll that absorbs light. The fluid around them is the stroma, where sugars are made. Animal cells have no chloroplasts, but they do have mitochondria. Plant cells have both.
2:20Ribosomes
Finally, ribosomes. They are tiny, with a large and a small subunit, and they are where proteins are made. Some float free in the cytoplasm, and some sit on the rough endoplasmic reticulum, which is the next lesson.
3:16Examiner trap and recap
The trap. Mitochondria do not make energy. They make ATP, using energy released from respiration. And always name the structure with its job.
Recap. Nucleus: pores, chromosomes, nucleolus. Mitochondria: cristae and ATP. Chloroplasts: grana and photosynthesis. Ribosomes: proteins.
Next lesson, the rough endoplasmic reticulum, the Golgi and lysosomes, and how a protein travels through them.
Keep going
More from 3.2 Cells
- Rough ER, Golgi and Lysosomes Explained · AQA 7402 · 3.2.1.1
- Cell Wall, Vacuole and Specialised Cells · AQA 7402 · 3.2.1.1
- Prokaryotic vs Eukaryotic Cells · AQA 7402 · 3.2.1.2
- Viruses: Structure and Non-Living Status · AQA 7402 · 3.2.1.2
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-022.