Phagocytosis: How Phagocytes Work
The steps of phagocytosis, from engulfing a pathogen into a phagosome to lysosome fusion and digestion by lysozymes. Includes the 4-mark exam question, a worked model answer and the examiner trap on mixing up lysozyme and lysosome.
Chapters (8)
Exam question [4 marks]
Bacteria enter a cut in the skin. Describe how a phagocyte destroys the bacteria.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Phagocyte binds to / recognises the antigens on the bacterium and engulfs it | "receptors bind to antigens", "surrounds / takes in the bacterium", "endocytosis" | "eats the bacteria" with no reference to engulfing into the cell; "antibodies bind" |
| 2 | Bacterium is enclosed in a phagosome (vesicle) inside the cell | "vesicle", "vacuole", "bag / sac inside the cytoplasm" | "bacterium floats in the blood"; "phagosome" with the bacterium outside the cell |
| 3 | Lysosome fuses with the phagosome | "lysosome joins / merges with the vesicle" | "lysosome fuses with the cell membrane"; "lysozyme fuses with the phagosome" |
| 4 | Lysozymes (hydrolytic enzymes) are released and hydrolyse / digest the bacterium | "enzymes break down the bacterial cell wall", "digested inside the phagosome" | "lysozymes kill bacteria in the blood"; "enzymes dissolve it" with no enzyme release into the phagosome |
The mark points follow the order of the process. Mark 4 must locate the digestion inside the phagosome. Do not confuse lysosome (organelle) with lysozyme (enzyme); using them the wrong way round loses marks 3 and 4. Extra credit-worthy points that do not add marks beyond 4: phagocytes are attracted by chemicals from the bacteria; useful products are absorbed.
Model answer (4/4)
The phagocyte binds to the antigens on the surface of the bacterium and engulfs it. The bacterium is enclosed in a phagosome inside the cell. A lysosome fuses with the phagosome, releasing lysozymes, which are hydrolytic enzymes. The lysozymes hydrolyse the bacterium and break it down.
Examiner trap
- Saying lysozymes digest 'bacteria in the blood' rather than in the phagosome
- Mixing lysozyme with lysosome
- Not saying the phagosome fuses with a lysosome
Key terms
- phagocyte
- phagocytosis
- phagosome
- lysosome
- lysozyme
- hydrolytic enzymes
- pathogen
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:21Pathogen and phagocyte
A phagocyte is a white blood cell that engulfs and destroys pathogens. Neutrophils and macrophages are two examples. The process is called phagocytosis. Phagocytes patrol the blood and the tissues, and they are not fussy. They will engulf many different kinds of pathogen, so they act quickly, as one of the first lines of defence.
But how do they know what to attack? First, chemicals released by the bacteria attract the phagocyte, and it moves towards them. Then receptors on the phagocyte's surface bind to the antigens on the bacterium. Remember from the last lesson, antigens are the molecules that mark a cell out as foreign. That is what the phagocyte recognises.
1:06Engulfing the bacterium
Next, the phagocyte engulfs the bacterium. Its cell surface membrane wraps around the bacterium and pinches shut. It is the same movement as endocytosis, the cell taking something in by folding its membrane around it. Now the bacterium is trapped inside the cytoplasm, in a small vesicle called a phagosome.
Remember that word. The bacterium is inside the cell, not floating in the blood.
1:33The lysosome
The phagocyte also has lysosomes. A lysosome is a small organelle full of digestive enzymes. Those enzymes are called lysozymes, and they are hydrolytic enzymes, which means they break things down by hydrolysis. While the lysozymes stay inside the lysosome, they cannot harm the phagocyte itself. They are packaged away until they are needed.
1:56Lysosome fuses with the phagosome
The lysosome moves to the phagosome and fuses with it. That releases the lysozymes into the phagosome. The lysozymes hydrolyse the bacterial cell wall, and the bacterium is broken down. The useful products are absorbed by the phagocyte.
So the bacterium is destroyed, inside the phagosome, inside the cell, where it cannot spread.
Phagosome holds the bacterium. Lysosome is the bag of enzymes. Lysozyme is the enzyme.
2:27The five steps in order
So the order is this. Attracted to the bacteria. Binds to the antigens. Engulfs the bacterium into a phagosome. The lysosome fuses with the phagosome. Lysozymes hydrolyse the bacterium. Write those five steps in that order in the exam. Four of them carry the marks.
3:11Examiner trap and recap
The trap. Lysozymes do not digest bacteria in the blood. They work inside the phagosome, after the lysosome fuses with it.
Recap. Engulf, phagosome, fuse, hydrolyse.
But how does the phagocyte show the rest of the immune system what it found? That is next, with T lymphocytes.
Watch first
This lesson builds on:
- Antigens and Cell Recognition · AQA 7402 · 3.2.4
- Rough ER, Golgi and Lysosomes Explained · AQA 7402 · 3.2.1.1
Keep going
More from 3.2 Cells
- Antigens and Cell Recognition · AQA 7402 · 3.2.4
- T Lymphocytes: The Cellular Immune Response · AQA 7402 · 3.2.4
- B Cells, Clonal Selection and Antibodies · AQA 7402 · 3.2.4
- Primary and Secondary Immune Response · AQA 7402 · 3.2.4
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-042.