CCEA A Level Biology 1.4 Cells Exam Questions

About CCEA A Level Biology 1.4 Cells

Section 1.4 is the core cell biology of Unit AS 1. You need to compare prokaryotic and eukaryotic cells, describe the fluid mosaic model and the functions of membrane components, and know the structure and role of each organelle as seen in electron micrographs. CCEA also asks you to compare plant, fungal and animal cells, explain light, transmission and scanning electron microscopy in terms of magnification and resolution, and describe homogenisation and centrifugation. Expect to identify organelles on micrographs, calculate actual size and magnification, and use a graticule and stage micrometer. This feeds into cell physiology (1.5) and tissues (1.7).

What you need to know

• Prokaryotic cells No nucleus, mitochondria or ER; naked circular DNA, small (70S) ribosomes, often plasmids, and a cell wall.
• Eukaryotic organelles Nucleus with nucleolus (makes rRNA) and double membrane envelope with pores; mitochondria with cristae and matrix; chloroplasts with thylakoids, grana and stroma; rough ER with ribosomes; smooth ER for lipids; Golgi for modifying and packaging; lysosomes with hydrolytic enzymes; microtubules forming centrioles, spindle and cytoskeleton.
• Fluid mosaic model A phospholipid bilayer with intrinsic and extrinsic proteins, glycoproteins and glycolipids forming the glycocalyx, and cholesterol in animal cells. Proteins act as carriers, channels, enzymes, receptors and antigens; cholesterol stabilises the membrane.
• Plant, fungal and animal cells Plant cells have a cellulose wall, chloroplasts, a large permanent vacuole with tonoplast and a middle lamella of calcium pectate. Fungal cells have chitin walls and are often multinucleate. Animal cells have no wall or chloroplasts but have centrioles.
• Microscopy Magnification is how many times bigger the image is; resolution is the ability to distinguish two points as separate. TEM gives the highest resolution of internal detail; SEM gives 3D surface images; light microscopes can view living cells.
• Cell fractionation Tissue is homogenised in cold, isotonic, buffered solution, then centrifuged at increasing speeds so nuclei, then mitochondria, then smaller organelles form pellets.
• Calculations Magnification = image size / actual size. Calibrate an eyepiece graticule against a stage micrometer, and convert units carefully (1 mm = 1000 micrometres).

Common mistakes in 1.4 Cells

• Confusing magnification and resolution. Increasing magnification does not show more detail unless resolution also increases.
• Saying prokaryotes have no DNA or no ribosomes. They have circular DNA free in the cytoplasm and smaller ribosomes.
• Forgetting to convert units in magnification calculations. Put image size and actual size in the same unit before dividing.
• Describing the cell wall as part of the cell membrane. The wall is outside the plasma membrane and is fully permeable.
• Explaining a cold, isotonic buffer only as keeping cells alive. Cold reduces enzyme activity, isotonic prevents organelles bursting or shrinking by osmosis, and buffer keeps pH constant.

Try three 1.4 Cells exam-style questions