A-LevelBiologyCell structure9700/22 · Oct/Nov 2021 · Q19 Marks

There are two types of cell, prokaryotic and eukaryotic. Bacterial cells are prokaryotic and plant cells are eukaryotic.

(a)There are differences in the structural features that are common to bacterial cells and plant cells. For example, the cell surface membrane in a plant cell contains cholesterol, but in a bacterial cell the membrane contains molecules known as hopanoids. Cholesterol and hopanoids have the same function. Some of the main structural features common to both types of cell are shown in Table 1.1. Complete Table 1.1 by giving one difference between a bacterial cell and a plant cell for each structural feature listed. The difference between the cell surface membranes of the two types of cell has been completed for you. Table 1.1 | feature common to bacterial and plant cells | bacterial cell | plant cell | |---------------------------------------------|-------------------------|-----------------------| | cell surface membrane | contains hopanoids | contains cholesterol | | ribosome | | | | DNA | | | | cell wall | | |[3]
(b)One role of the cell surface membrane of bacterial cells and plant cells is the transport of substances into and out of cells. Explain how membrane carrier proteins and membrane channel proteins are involved in the transport of substances into and out of cells.[3]
(c)Fig. 1.1 is a photomicrograph showing chloroplasts in plant leaf cells. [Figure 1.1] Explain why the chloroplasts are seen only around the periphery (edge) of each plant cell.[1]
(d)Fig. 1.2 shows plant cells in a root tip where cell division by mitosis is taking place. [Figure 1.2] Identify two cells in Fig. 1.2 that are in different stages of mitosis. Draw a label line to each cell and add the name of the stage of mitosis that is shown by the cell.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2021). The mark or mark type is shown in brackets.

[3] one mark max for each feature
ribosome | 70S / smaller / 18-20nm I no 80S ribosomes | 80S / larger / 22-30nm ; A and 70S
DNA | circular / closed loop I ref. to plasmids | linear ; AW
naked A no histones | (complexed) with, histones / (basic) proteins ;
free in, cytoplasm / cytosol A not enclosed / AW, by nuclear envelope A found in nucleoid region | enclosed by / AW, nuclear envelope ; A in a nucleus
cell wall | (mainly composed of) peptidoglycan / murein | (mainly composed of) cellulose ;
[3] any three from:
A particles for substances I ref. to size
1 for transport of ions and, hydrophilic / polar, molecules / substances AW or for transport of substances that cannot pass (directly) across the (phospholipid) bilayer / hydrophobic core ; AW R if implied that all substances cannot cross the bilayer I substances, bypass / do not need, to cross the bilayer
2 carrier proteins for active transport and facilitated diffusion and channel proteins for facilitated diffusion;
3 further detail carrier protein ; I water-filled / hydrophilic interior e.g. have specific binding sites are specific in, molecule / ion / named example, transported AW carry out conformational change (following binding) / AW use, ATP / energy active transport) moves substances against the concentration gradient / AW (active transport) moves substances down the concentration gradient / AW (facilitated diffusion) accept once in mp3 or 4 I moves substances by diffusion
4 further detail channel protein ; e.g. hydrophilic / water-filled, pore / channel A hydrophilic lining some, ion channels / proteins channels for ions, are selective (for size and charge) (most) channel proteins are not selective aquaporins for passage of (larger quantities of) water some, are gated/ voltage-gated moves substances down the concentration gradient / AW
[1] I ref. to light absorption
any one from:
chloroplasts at periphery because pushed by vacuole ;
vacuole is, turgid / filled with cell sap ;
(large permanent) vacuole is in the centre (of the cell) ;
[2] metaphase label to metaphase cell;
prophase label to one of the prophase cells ;