Bacteria can be classified according to the type of cell wall they have. Gram-negative bacteria have a cell wall with an outer layer known as the outer membrane. Gram-positive bacteria do not have an outer membrane, but have a much thicker peptidoglycan layer than Gram-negative bacteria. Fig. 4.1 is a diagram of a section through the cell wall of a Gram-negative bacterium. [Figure 4.1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2025). The mark or mark type is shown in brackets.

1. [3] any three from: / similarities, both have / phospholipids ; / channel/transport, protein; A intrinsic / integral / transmembrane, protein / R carrier / pump, protein / a bilayer / two layers ; / max 2 for similarities, max 2 for differences / differences / do not need to have 'outer membrane' 'bacterial' if context is clear / (bacterial outer membrane) has lipopolysaccharides ; ora / (cell surface membrane / eukaryote) has, phospholipid bilayer / both layers with phospholipids ; ora / if ora also given (bacterial) only has outer layer of (outer) membrane with phospholipids, can also award mp1 / AVP;; e.g. cell surface membrane has, glycocalyx / glycolipids / glycoproteins / cell surface membrane has cholesterol / outer membrane does not have extrinsic proteins
2. [1] any one from: / formation of O-IMV / involves a more complex process; e.g. greater variety of, contents / cell structures / R organelles / ref. to needing to make a vesicle with two 'membranes' / AW A ‘layers' / peptidoglycan, chains / layer / bonds, need to be broken ; A peptidoglycan wall needs to be broken / causes, (temporary) weakness in cell wall / risk of lysis; / idea that more resources required to form vesicle or too great a loss of important cell, components / structures / ref. to take more time to produce ; / uses (more) energy to, break wall / repair wall / make the vesicle ;I ref. to organelles / AVP; e.g. suggestion that periplasmic proteins need to be released in greater numbers so more OMVs formed
3. [1] any one suggestion that: / must be in context of a fully formed O-IMV / function of O-IMVs require, energy / ATP ; / A suggested function e.g. DNA transcription / metabolic reactions / for active transport I cell processes / O-IMV (formation) has cytoplasm, which contains ATP ; / OMVs formed only from periplasm, which has no ATP ; A no ATP in outer membrane space / AVP; e.g. no / few, ATP transporters / transport proteins, for exit of ATP into periplasm (in area where OMV forms)
4. [2] ref. to DNA in O-IMVs being passed on to other bacteria ; e.g. O-IMVs may, fuse with / be taken up by / AW, / plasmids / DNA, in (degraded) O-IMVs may enter other bacteria / qualified ref. to O-IMV DNA and antibiotic resistance ; e.g. / O-IMVs have, alleles / genes / plasmid, that, give / confer / AW, antibiotic resistance / DNA may mutate to give (a, gene / allele, for) antibiotic resistance / DNA, codes for / expresses / AW, a protein involved in antibiotic resistance / example of gene conferring resistance e.g. enzyme to breakdown antibiotic / change to ribosome structure (to prevent antibiotic attachment) / efflux pump for / membrane protein to pump out, antibiotic