Exocytosis

Where it happens

Exocytosis happens at the plasma membrane, where a vesicle formed inside the cell, often by the Golgi apparatus, releases its contents. It is used whenever a substance made inside the cell is too large to cross the membrane through carrier or channel proteins.

The cells that rely on it most are secretory cells: the cells of the pancreas that release digestive enzymes, goblet cells of the intestine that release mucus, endocrine cells that release hormones such as insulin, and the axon terminals of neurones that release neurotransmitter. In every case the product is first packaged inside the cell and only leaves in bulk when the vesicle reaches the membrane.

Inputs and outputs

• Input: a secretory vesicle containing the product, budded off from the Golgi apparatus after the product was made on the rough endoplasmic reticulum.
• Input: ATP from aerobic respiration in the mitochondria, used to move the vesicle along the cytoskeleton and to drive membrane fusion.
• Input: a signal that triggers release, for example a nerve impulse arriving at a synapse or a rise in blood glucose at a pancreatic cell.
• Output: the product, such as an enzyme, hormone, mucus or neurotransmitter, released outside the cell in one burst.
• Output: the vesicle membrane, which becomes part of the plasma membrane and increases its surface area.
• Output: ADP and phosphate, returned to the mitochondria to be rebuilt into ATP.

The steps

1. The product is made on the rough endoplasmic reticulum and moved to the Golgi apparatus, where it is modified and packaged.
2. A vesicle containing the substance to be released buds off from the Golgi apparatus.
3. The vesicle moves through the cytoplasm towards the plasma membrane, using energy from ATP.
4. The vesicle membrane makes contact with the inner face of the plasma membrane.
5. The two membranes fuse together, opening the vesicle to the outside of the cell.
6. The contents of the vesicle are released outside the cell.
7. The vesicle membrane is absorbed into the plasma membrane, so the membrane lost during endocytosis is replaced.

Why it matters and how it is controlled

Exocytosis lets a cell secrete substances it has made, such as digestive enzymes from gland cells, hormones, or neurotransmitters released at a synapse. Because the membrane must actively move and fuse, exocytosis needs energy from respiration, unlike diffusion and osmosis.

The process is controlled by signals rather than running continuously. A pancreatic beta cell stores insulin in vesicles and releases it only when blood glucose rises.

A presynaptic knob stores neurotransmitter and releases it only when a nerve impulse arrives, which lets calcium ions enter and trigger fusion. This control means the cell can respond within seconds, because the product is already made and only needs to be released.

Exocytosis also balances endocytosis. Every vesicle that enters by endocytosis removes a patch of plasma membrane; every vesicle that fuses during exocytosis adds one back.

Without this exchange, a phagocytic cell would shrink and a secretory cell would swell.

How it is examined

Questions may ask you to describe the stages of exocytosis, to explain why cells that secrete a lot, such as gland cells, have abundant mitochondria, or to link exocytosis to the release of a neurotransmitter at a synapse.

A common structured item shows a diagram of a secretory cell and asks you to name the organelles in sequence: rough endoplasmic reticulum, Golgi apparatus, vesicle, plasma membrane. Another asks you to predict what happens to secretion when a respiratory inhibitor is added, which tests whether you know the process is active.

In the essay paper, exocytosis often appears as one paragraph inside a wider answer comparing passive and active movement across the membrane.

Common misconceptions

Worked exam-style question

Question. A student examined electron micrographs of two cells. Cell P, from the pancreas, contained a large rough endoplasmic reticulum, a prominent Golgi apparatus, abundant vesicles near the plasma membrane and abundant mitochondria.

Cell Q, a mature red blood cell, contained none of these organelles. (a) Name the process by which cell P releases digestive enzymes.

(b) Explain how the organelles listed in cell P work together to carry out this process. (c) Explain why cell Q cannot carry out the same process.

Model answer. (a) Exocytosis. (b) Enzymes are proteins, so they are made by ribosomes on the rough endoplasmic reticulum.

They are transported to the Golgi apparatus, which modifies and packages them into secretory vesicles. The vesicles move to the plasma membrane and fuse with it, releasing the enzymes outside the cell.

The mitochondria supply ATP, because movement of the vesicle and fusion of the membranes are active and need energy. (c) Cell Q has no ribosomes or rough endoplasmic reticulum to make protein, no Golgi apparatus to package it into vesicles, and no mitochondria to supply ATP, so it cannot form or release vesicles by exocytosis.

Source:SRC-DSKP-EN

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