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Endocytosis

• Large particles cannot cross the cell membrane → they enter the cell by endocytosis.

• Types of endocytosis:

  • Pinocytosis

  • Phagocytosis

Pinocytosis

• Means drinking by the cell.

• for intake of macromolecules.

• Occur continuously in most cells.

Cell expresses receptors on cell membrane for molecules that it wants to take in

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These receptors are located on coated pits

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The molecule binds the receptors

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Proteins underneath the pit pull the membrane inside the cell & ...

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break the mouth of the pit

↓ Formation of vesicle inside the cell containing the molecule and extracellular fluid

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Vesicle formed by pinocytosis is called pinocytotic vesicle. These vesicles are very small.

• This process required energy from ATPs.

Phagocytosis

• Means eating by cell.

• for intake of larger particles like bacteria, whole cell etc.

• Only some cells have capacity for phagocytosis e.g. tissue macrophage.

• Mechanism of phagocytosis is similar to pinocytosis.

Som ligand on the particle to be ingested binds with the cell surface receptor

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Membrane around this attachment evaginates to cover the particle

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the membrane merges on the other side

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forms vesicle inside the cell

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Vesicle formed by phagocytosis is called phagocytic vesicle. It's large.

Digestion of Ingested Material

The vesicle formed by pinocytosis/phagocytosis merges with lysosome

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Lysosome empties its hydrolase enzymes in the vesicle

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Now the vesicle is called digestive vesicle

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Hydrolase enzymes break down the ingested particles

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1. Smaller molecules like amino acid, glucose, etc → cross the membrane of vesicle → enter the cytoplasm.

2. Indigestible material is left inside the vesicle → excreted out of the cell by exocytosis.

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