Endocytosis
Endocytosis is the bulk movement of material into a cell, formed when the plasma membrane folds around the material and pinches off as a vesicle inside the cell. It needs energy because it involves active movement of the membrane.
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Where it happens
Endocytosis happens at the plasma membrane of a cell, whenever the material to be taken in is too large to cross through carrier or channel proteins. It is used for solid particles, such as bacteria, and for liquid droplets containing dissolved substances.
The most familiar example in the syllabus is a phagocyte, a type of white blood cell, engulfing a bacterium during the body's defence response. Amoeba feeds the same way, wrapping pseudopodia around a food particle to form a food vacuole.
Pinocytosis, the uptake of droplets, happens in cells lining the small intestine that take in dissolved nutrients, in kidney tubule cells that recover small proteins from the filtrate, and in cells of a developing embryo. The membrane's phospholipid bilayer can fold and fuse because it is fluid, and the cytoskeleton beneath it, powered by ATP, provides the pulling force.
Inputs and outputs
- Input: a solid particle such as a bacterium, a dead cell fragment or a food particle (phagocytosis), or a droplet of extracellular fluid carrying dissolved solutes (pinocytosis).
- Input: ATP from aerobic respiration in the mitochondria, used to reshape the membrane and move the vesicle inward.
- Input: a section of the plasma membrane, which is temporarily used up to form the vesicle wall.
- Output: a membrane-bound vesicle inside the cytoplasm, called a phagosome or food vacuole when it contains a solid particle.
- Output after digestion: soluble products such as amino acids and glucose released into the cytoplasm when the vesicle fuses with a lysosome.
- Output: undigested residue, which is later expelled by exocytosis, and membrane that is recycled back to the cell surface.
The steps
- Receptors or the sticky surface of the plasma membrane make contact with the particle or droplet outside the cell.
- Using energy from ATP, the membrane folds inward (invaginates) or pushes outward as pseudopodia that flow around the material.
- The extending edges of the membrane meet on the far side of the material.
- The two membrane edges fuse, sealing the material inside a pocket lined by membrane.
- The pocket pinches off from the plasma membrane and becomes a free vesicle in the cytoplasm, a phagosome for solids, a pinocytic vesicle for liquids.
- The vesicle is moved deeper into the cell along the cytoskeleton.
- A lysosome fuses with the vesicle and releases hydrolytic enzymes, which digest the contents.
- Soluble products diffuse into the cytoplasm, and any residue is removed from the cell by exocytosis.
Why it matters and how it is controlled
Endocytosis allows a cell to take in material that is far too large for diffusion or active transport, such as whole bacteria or large droplets of fluid. Phagocytosis is endocytosis of solid particles, and pinocytosis is endocytosis of liquid droplets; both require energy from respiration to reshape the membrane.
Endocytosis is controlled rather than continuous. A phagocyte engulfs only material it recognises as foreign, so the process begins when receptors on the membrane bind to antigens on a pathogen or to antibodies already attached to it.
Cells also regulate how much membrane they lose: because each vesicle removes a patch of the surface, the cell balances endocytosis with exocytosis, which returns membrane to the surface. Anything that reduces ATP supply, such as a lack of oxygen, a respiratory poison or low temperature, slows or stops endocytosis, and this is the standard evidence used to show that it is an active process.
In terms of the syllabus, endocytosis completes the set of membrane transport mechanisms: simple diffusion for small, non-polar molecules; facilitated diffusion and osmosis for polar molecules and water moving down a gradient; active transport for ions and molecules moving against a gradient; and endocytosis and exocytosis for material too large for any protein channel. Being able to place a substance in the right category is the skill most examination questions on this chapter test.
Endocytosis compared with other membrane transport
| Feature | Simple diffusion | Active transport | Endocytosis | Exocytosis |
|---|---|---|---|---|
| Size of material | Small molecules or ions | Small molecules or ions | Large particles or droplets | Large molecules such as enzymes, hormones |
| Direction | Down the gradient | Against the gradient | Into the cell | Out of the cell |
| Route across the membrane | Through the phospholipid bilayer | Through a carrier protein | Enclosed in a vesicle formed from the membrane | Vesicle fuses with the membrane |
| Energy from ATP | Not needed | Needed | Needed | Needed |
| Change to the membrane | None | None | Membrane surface area decreases | Membrane surface area increases |
| Example | Oxygen entering a red blood cell | Glucose uptake in the ileum | Phagocyte engulfing a bacterium | Secretion of digestive enzymes |
How it is examined
You may be asked to describe the stages of endocytosis using a diagram, to explain why it needs energy while diffusion does not, or to link endocytosis to a white blood cell taking in a pathogen.
The stages are usually presented as a sequence of four or five diagrams to be arranged in order or described in words, and the mark scheme looks for the vocabulary: invagination, vesicle, fuse, pinch off, lysosome. A common comparison question gives two processes and asks for similarities and differences; a strong answer states that both endocytosis and active transport need ATP, then contrasts the route taken, vesicle versus carrier protein, and the size of material moved.
Questions linking to the immune system ask how a phagocyte destroys a bacterium, and expect the sequence engulf, phagosome, lysosome fusion, enzyme digestion.
Common misconceptions
Worked exam-style question
Question. A researcher observed white blood cells taking in bacteria in two culture dishes. Dish 1 was kept at 37 °C.
Dish 2 was kept at 37 °C but a chemical that blocks the production of ATP was added. After 30 minutes, cells in dish 1 contained bacteria inside vesicles, while cells in dish 2 had bacteria attached to the surface only.
(a) Name the process observed in dish 1 and state the type of endocytosis it represents. (b) Describe how the vesicle containing a bacterium is formed.
(c) Explain the result in dish 2. (d) Describe what happens to the bacterium after the vesicle has formed.
Model answer. (a) Endocytosis; specifically phagocytosis, because a solid particle is taken in. (b) The plasma membrane binds to the bacterium and folds inward / extends pseudopodia around it; the membrane edges meet and fuse, and the pocket pinches off to form a vesicle (phagosome) in the cytoplasm.
(c) Endocytosis is an active process that needs energy from ATP to reshape the membrane; the chemical stops ATP production, so the membrane cannot fold and fuse, and the bacteria stay attached at the surface. Temperature is the same in both dishes, so ATP supply is the only difference.
(d) A lysosome fuses with the vesicle and releases hydrolytic enzymes, which digest the bacterium; useful products are absorbed into the cytoplasm and residue is removed by exocytosis.
Source:SRC-DSKP-EN
Frequently asked questions
Does endocytosis need energy?
What is the difference between phagocytosis and pinocytosis?
How is endocytosis different from active transport if both use energy?
What happens to the membrane used to make the vesicle?
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