OCR GCSE Biology A B1.1 Cell Structures Exam Questions

About OCR GCSE Biology A B1.1 Cell Structures

B1.1 opens the OCR Gateway Biology A course by looking at cells as the basic unit of life and the microscopes used to study them. You need to compare plant, animal and bacterial cells, link each sub-cellular structure to its function, and explain how electron microscopy changed our understanding of cells. It is assessed in Paper 1 (Foundation) or Paper 3 (Higher), and it underpins almost every later topic, from respiration in mitochondria to photosynthesis in chloroplasts. Examiners regularly test magnification calculations with unit conversions, the difference between magnification and resolution, and the light microscopy practical (PAG B1).

What you need to know

• Light microscopes Light passes through a thin specimen and is focused by an objective and eyepiece lens; total magnification is eyepiece magnification multiplied by objective magnification. Stains such as iodine (starch) or methylene blue (nuclei) increase contrast so structures can be seen.
• Electron microscopes Electron microscopes use a beam of electrons, which has a much shorter wavelength than light, so they have far higher resolution and magnification. This revealed ribosomes, the internal structure of mitochondria and chloroplasts, and the detail of membranes.
• Magnification and resolution Magnification = size of image / size of real object. Resolution is the smallest distance between two points that can still be seen as separate, so a higher resolution shows more detail, not just a bigger image.
• Eukaryotic cells Animal and plant cells are eukaryotic: they have a nucleus containing DNA, a cell membrane, cytoplasm, mitochondria and ribosomes. Plant cells also have a cellulose cell wall, chloroplasts and usually a permanent vacuole.
• Prokaryotic cells Bacteria are prokaryotic: they have no nucleus, and their DNA is a single loop in the cytoplasm, often with extra small rings called plasmids. They have a cell wall, cell membrane, cytoplasm and ribosomes, and are much smaller than eukaryotic cells.
• Units and standard form You must convert between mm, micrometres (um) and nanometres (nm): 1 mm = 1000 um and 1 um = 1000 nm. Very small sizes are often written in standard form, for example 2 um = 2 x 10^-6 m.

Common mistakes in B1.1 Cell Structures

• Saying an electron microscope is better simply because it magnifies more. The key point is its higher resolution, because electrons have a shorter wavelength than light.
• Forgetting to convert units before using magnification = image size / real size. Both sizes must be in the same unit, usually micrometres.
• Stating that plant cells have chloroplasts instead of mitochondria. Plant cells have both, because they respire as well as photosynthesise.
• Writing that bacteria have a nucleus or that their DNA is in chromosomes in a nucleus. Bacterial DNA is a single circular chromosome free in the cytoplasm, plus plasmids.
• Describing the cell membrane as fully permeable. It is partially permeable and controls what enters and leaves the cell.

Try three B1.1 Cell Structures exam-style questions