Cell Structure and Organisation: Question 10
Syllabus 2.1
A food scientist studies a species of bacterium used to make yoghurt. A diagram of a single cell shows a cell wall, a cell membrane, cytoplasm containing ribosomes, one large loop of circular DNA, and several small plasmids.
Describe the function of each of the following structures in this bacterial cell.
(a) cell wall [1]
(b) circular DNA (bacterial chromosome) [2]
(c) plasmid [1]
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Worked solution
Part (a): Function of the cell wall
The cell wall is a rigid layer surrounding the cell membrane. Its function is to support the cell and give it a fixed shape. (Note that a bacterial cell wall is made of a different material from a plant cell wall, so its exact structure differs, but its supporting role is similar.)
Part (b): Function of the circular DNA
A bacterial cell has no nucleus, so its main loop of circular DNA (sometimes called the bacterial chromosome) lies free in the cytoplasm rather than being enclosed by a nuclear membrane. Its function is to carry the cell’s genetic material. The genes that provide the instructions needed to control the cell’s activities, such as which proteins are made.
Part (c): Function of a plasmid
A plasmid is a small, circular piece of DNA that is separate from the main loop of circular DNA. Plasmids often carry extra genes that are not essential for everyday survival but can be useful in particular conditions, for example a gene that gives the bacterium resistance to a particular antibiotic. Because plasmids are small and separate, they can be copied and passed between bacterial cells.
Final answers
- (a) Cell wall: supports the cell and maintains its shape.
- (b) Circular DNA: carries the genetic material that controls the cell’s activities (not enclosed by a nucleus).
- (c) Plasmid: a small, separate loop of DNA that often carries additional genes, such as antibiotic resistance, and can be passed between bacteria.