OxfordAQA International A Level Biology
OxfordAQA International A Level Biology 3.1.2 Cells and Cell Structure Exam Questions
Free OxfordAQA International A Level Biology revision for 3.1.2 Cells and Cell Structure: what you need to know, common mistakes and exam-style questions with answers. Covers eukaryotic organelles, prokaryotic cells, optical and electron microscopes, magnification and resolution, cell fractionation and ultracentrifugation.
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About OxfordAQA International A Level Biology 3.1.2 Cells and Cell Structure
Cells and cell structure in Unit 1 asks you to link the structure of each organelle in a eukaryotic cell to its function and to compare eukaryotic and prokaryotic cells. You also need to understand how optical microscopes, transmission electron microscopes and scanning electron microscopes work, and why resolution matters more than magnification. Cell fractionation and ultracentrifugation are tested as a method with reasons for each step. The topic links to membranes, protein synthesis, binary fission and disease. Expect questions that show an electron micrograph to identify, a magnification calculation, and explanations of why the homogenate is kept cold, isotonic and buffered.
What you need to know
- Eukaryotic organelles Know the structure and function of the plasma membrane, nucleus, mitochondria, chloroplasts, Golgi apparatus, lysosomes, ribosomes, rough and smooth endoplasmic reticulum, cell wall and cell vacuole. For example, rough ER is covered in ribosomes and processes proteins, and the Golgi apparatus modifies and packages proteins and forms lysosomes.
- Prokaryotic cells Prokaryotic cells are much smaller, have no nucleus and no membrane-bound organelles, and have smaller ribosomes. Their DNA is circular, free in the cytoplasm and not associated with proteins. The cell wall contains murein, a glycoprotein, and many also have plasmids, a capsule and one or more flagella.
- Microscopes Optical microscopes use light and have limited resolution because of its long wavelength. Transmission electron microscopes give the highest resolution but need thin, dead specimens in a vacuum; scanning electron microscopes give 3D surface images at lower resolution. Artefacts can appear in prepared specimens.
- Magnification and resolution Magnification is how many times larger the image is than the object: magnification = size of image / size of object. Resolution is the minimum distance apart two points can be and still be seen as separate. Convert units carefully: 1 mm = 1000 micrometres.
- Cell fractionation Tissue is homogenised in a cold, isotonic, buffered solution. The homogenate is filtered and then ultracentrifuged at increasing speeds, so the heaviest organelles (nuclei) form the first pellet, followed by mitochondria and then lighter organelles.
- Levels of organisation In complex multicellular organisms, specialised cells are organised into tissues, tissues into organs and organs into systems.
Common mistakes in 3.1.2 Cells and Cell Structure
- Confusing magnification with resolution. A microscope can magnify without increasing resolution, so the image just gets bigger and blurrier.
- Saying the solution is kept cold to stop the cells dying. It is kept cold to reduce enzyme activity that would digest the organelles.
- Saying prokaryotes have no ribosomes or no DNA. They have smaller ribosomes and circular DNA that is not associated with histone proteins.
- Forgetting to convert units in magnification questions. Put the image and the object into the same unit before dividing.
Try three 3.1.2 Cells and Cell Structure exam-style questions
1. A mitochondrion is 4.5 micrometres long. In a micrograph its image is 27 mm long. Calculate the magnification. (2 marks)
Show the mark scheme answer
Convert 27 mm to 27 000 micrometres; magnification = 27 000 / 4.5 = x 6000.
2. Explain why the tissue is placed in an ice-cold, isotonic, buffered solution before cell fractionation. (3 marks)
Show the mark scheme answer
Cold reduces enzyme activity so organelles are not digested; isotonic so there is no osmotic movement of water and organelles do not burst or shrink; buffered so pH does not change and proteins / enzymes are not denatured.
3. Give three structural differences between a prokaryotic cell and a eukaryotic cell. (3 marks)
Show the mark scheme answer
Prokaryotic cell has no nucleus / DNA free in cytoplasm; no membrane-bound organelles; smaller ribosomes; circular DNA not associated with proteins; cell wall of murein; may have plasmids, capsule or flagella. (Any 3)
3.1.2 Cells and Cell Structure FAQs
What does OxfordAQA International A Level Biology 3.1.2 Cells and Cell Structure cover?
It covers eukaryotic organelles, prokaryotic cells, optical and electron microscopes, magnification and resolution, cell fractionation and ultracentrifugation.
Which unit is 3.1.2 Cells and Cell Structure examined in?
It is part of Unit 1, The diversity of living organisms, which is examined at International AS. The Unit 5 synoptic paper at A2 can also draw on this content.
What is the order of pellets in ultracentrifugation?
At the lowest speed nuclei form the first pellet, then mitochondria (and chloroplasts in plant cells), then smaller organelles such as ribosomes at the highest speeds. Heavier organelles always sediment first.
