CoursesPhysiologyIntroductionThe cell and its functionsTransport across the cell membrane

Transport across the cell membrane

Learning objectives

After completing this study unit, you will be able to:

Describe the concepts of diffusion, osmosis and tonicity.
Differentiate active and passive transport across the cell membrane.
Compare and contrast the types of transport across the cell membrane.

Introduction

The cell membrane carefully regulates how and when substances move into and out of the cell. Molecules and ions necessary for cellular functions are transported into the cytosol; at the same time, the cell expels waste products and releases molecules to communicate with other cells.
Transports across the cell membrane can be active or passive, depending on whether energy is expended. Substances can move across the cell membrane in different ways: through the phospholipid bilayer, via membrane proteins, or in vesicles.
Based on these two criteria, transport across the cell membrane is classified into three groups:

Passive transport includes simple diffusion (through the phospholipid bilayer) and facilitated diffusion (through channel or carrier proteins). Diffusion is the movement of substances down their concentration gradient, from areas of high concentration to areas of low concentration. The diffusion of water through the cell membrane is called osmosis.
Active protein-mediated transport includes primary and secondary transport. Primary transports like the sodium-potassium pump use ATP directly to move substances against their concentration gradient. Secondary transports use ATP indirectly to move two or more substances through symport and antiport carriers.
Active vesicular transport includes endocytosis (transport into the cell) and exocytosis (transport out of the cell). Substances are moved using vesicles made of phospholipid bilayer. Endocytic processes include phagocytosis (large molecules), pinocytosis (extracellular fluid) and receptor-mediated endocytosis (specific substances that bind to membrane receptors). Cells use exocytosis to expel waste products and to release hormones and neurotransmitters.

Explore concepts

Types of transport across the cell membrane

Substances are transported across the cell membrane in different ways.

Passive transport

Passive transport moves substances down their gradient, either through the phospholipid bilayer (simple diffusion) or through membrane proteins (facilitated diffusion).

Active protein-mediated transport

Primary active transport uses ATP directly to move substances against their concentration gradient, while secondary active transport takes advantage of concentration gradients created by primary transport.

In secondary active transport, symport and antiport carriers use concentration gradients created by primary transports to move other substances against their concentration gradient.

Active vesicular transport

Vesicular transport uses vesicles to move large molecules into (endocytosis) and out of (exocytosis) the cell.

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Summary

Key points about the transport across the cell membrane
Classification	Transports across the cell membrane are classified according to: Energy requirement (active, passive) Structure (phospholipid bilayer, membrane proteins, vesicles)
Diffusion	Movement of substances from areas of high concentration to areas of low concentration (down their concentration gradient) Can occur due to chemical gradient, electrical gradient, or osmosis
Osmosis	Movement of water from areas of low solute concentration to areas of high solute concentration
Tonicity	A solution’s ability to change the volume of water inside a cell based on the relative concentration of solutes Isotonic solution: same concentration, no net water movement Hypotonic solution: lower concentration, water moves into the cell, cell swells Hypertonic solution: higher concentration, water moves out of the cell, cell shrinks
Passive transport	Occurs by diffusion, no ATP required Simple diffusion: through the phospholipid bilayer (gases, small molecules, hydrophobic molecules) Facilitated diffusion: via channels (ions) or carriers (small molecules) Water moves via simple or facilitated diffusion (through aquaporin channels
Protein-mediated transport	Substances move using protein carriers, ATP required Primary: directly uses ATP to move substances against gradient (sodium-potassium pump) Secondary: indirectly uses ATP (gradient created by primary transport) Symporters move multiple substances in the same direction Antiporters move multiple substances in opposite directions
Vesicular transport	Substances move in vesicles, ATP required Endocytosis: engulfment of substances from the extracellular fluid Phagocytosis: endocytosis of large molecules Pinocytosis: endocytosis of random samples of extracellular fluid Receptor-mediated endocytosis: endocytosis of specific substances Exocytosis: release of waste products, hormones and neurotransmitters; integration of structures into the cell membrane Transcytosis: movement of substances across the cell