A level-readyAll subjects · Biology · WBI12 (Unit 2)
Cell theory, tissues and bacterial structures
Every organism is made of cells. In multicellular organisms cells are organised into tissues, organs and systems, while a bacterium does everything inside one small cell.
Cell theory and common features
Cell theory states that all living organisms are made of one or more cells, the cell is the basic unit of life, and new cells come only from existing cells.
All cells share some features: a cell surface membrane, cytoplasm, ribosomes for protein synthesis and DNA as genetic material.
Levels of organisation
- Tissue: a group of similar cells working together for a particular function (for example squamous epithelium, xylem).
- Organ: several tissues working together (for example the heart, a leaf).
- Organ system: organs working together (for example the circulatory system).
This division of labour lets each cell type become specialised.
Prokaryotic structures and their functions
Learn the function of each structure.
| Structure | Function |
|---|---|
| Cell wall (peptidoglycan) | Support; stops the cell bursting when water enters |
| Capsule | Protection from drying out and from phagocytes; helps cells stick to surfaces |
| Circular DNA | Main genetic material, free in the cytoplasm |
| Plasmids | Small DNA loops with extra genes (for example antibiotic resistance); can be passed to other bacteria |
| Flagellum | Rotates to move the cell |
| Pili | Attach to surfaces or other cells; some transfer plasmids between bacteria |
| 70S ribosomes | Protein synthesis |
Worked example
A bacterium survives better inside a host because white blood cells struggle to engulf it. Which structure is most likely responsible?
- Phagocytes engulf bacteria.
- A capsule makes the bacterium harder to engulf.
Answer: The capsule.
Check yourself
- Which is the best definition of a tissue?
- Several organs working together
- A single specialised cell
- A group of similar cells working together to carry out a function
- Any group of organelles
Show answer
A group of similar cells working together to carry out a function. Tissues are made of similar cells with a shared function.
- Which of these is an organ?
- A leaf
- Xylem
- A red blood cell
- A mitochondrion
Show answer
A leaf. A leaf contains several tissues, such as epidermis, mesophyll and xylem.
- What is the function of pili in bacteria?
- Respiration
- Storing starch
- Attachment to surfaces or other cells
- Making ATP by photosynthesis
Show answer
Attachment to surfaces or other cells. Pili help bacteria stick to cells and can transfer plasmids.
- What is the role of a bacterial flagellum?
- Protecting against drying out
- Moving the cell
- Protein synthesis
- Storing extra genes
Show answer
Moving the cell. The flagellum rotates to propel the bacterium.
- Why are plasmids important in the spread of antibiotic resistance?
- They make the cell wall thicker
- They contain the main chromosome
- They destroy antibiotics directly
- They can carry resistance genes and be passed between bacteria
Show answer
They can carry resistance genes and be passed between bacteria. Plasmids can be transferred, spreading resistance genes.
- Which structure is found in all cells, prokaryotic and eukaryotic?
- Nucleus
- Ribosomes
- Mitochondria
- Chloroplasts
Show answer
Ribosomes. All cells make proteins on ribosomes.
Sources
Last updated 7 November 2026. Spotted a mistake? Tell us.