Page 17 - 84 human physiolofy part-1

Cell Physiology

Active Transport Across the Cell Membrane

Active transport is the movement of solutes against a gradient and requires the expenditure of
energy, usually in the form of ATP. Active transport is achieved through one of these two mechanisms:

Protein Pumps

+
+
• Transport proteins in the plasma membrane transfer solutes such as small ions (Na , K ,
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Cl , H ), amino acids, and monosaccharides.
• The proteins involved with active transport are also known as ion pumps.
• The protein binds to a molecule of the substance to be transported on one side of the
membrane, then it uses the released energy (ATP) to change its shape, and releases it on the
other side.
• The protein pumps are specific, there is a different pump for each molecule to be
transported.
• Protein pumps are catalysts in the splitting of ATP → ADP + phosphate, so they are called
ATPase enzymes.

Cystic fibrosis is a genetic disorder that results in a misshapen chloride ion pump. By not
regulating chloride levels properly the cells produce thick mucus.

Vesicular Transport

• Vesicles or other bodies in the cytoplasm move macromolecules or large particles across
the plasma membrane. Types of vesicular transport include:

1. Exocytosis, which describes the process of vesicles fusing with the plasma membrane and
releasing their contents to the outside of the cell. This process is common when a cell produces
substances for export.
2. Endocytosis, which describes the capture of a substance outside the cell when the plasma
membrane merges to engulf it. The substance subsequently enters the cytoplasm enclosed in a
vesicle.

There are three kinds of endocytosis:
• Phagocytosis or cellular eating, occurs when the dissolved materials enter the cell. The
plasma membrane engulfs the solid material, forming a phagocytic vesicle.
• Pinocytosis or cellular drinking occurs when the plasma membrane folds inward to form
a channel allowing dissolved substances to enter the cell. When the channel is closed, the
liquid is encircled within a pinocytic vesicle.
• Receptor-mediated endocytosis occurs when specific molecules in the fluid surrounding
the cell bind to specialized receptors in the plasma membrane. As in pinocytosis, the
plasma membrane folds inward and the formation of a vesicle follows.
Note: Certain hormones are able to target specific cells by receptor-mediated endocytosis.

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