Cell Membranes and Transport

Cell Membranes and Transport

Cell Membranes: Components and Structure

  • Cell membranes are composed primarily of phospholipid bilayers, a fluid-like layer consisting of polar ‘heads’ and nonpolar ‘tails’.

  • Phospholipids form a barrier that selectively permits the passage of certain substances, regulating the entry and exit of molecules.

  • Proteins embedded within the membrane, known as integral proteins, perform key roles in transport, enzymatic activity, signal transduction, and cell-cell recognition.

  • The membrane also contains cholesterol molecules, which maintain the fluidity and stability of the structure.

  • Peripheral proteins are present on the inner or outer surface of the membrane providing structural support or functioning in signal transduction.

  • Glycolipids and glycoproteins contain covalently attached carbohydrates, which help in cell identification and recognition.

Passive Transport Mechanisms

  • Substances move across cell membranes via passive transport, requiring no energy.

  • Diffusion is a process where molecules spread and distribute themselves evenly.

  • Facilitated diffusion uses protein channels to move polar or charged substances across the membrane.

  • Osmosis, a specific type of diffusion, is the movement of water from an area of lower concentration to an area of higher solute concentration.

Active Transport Mechanisms

  • Active transport requires energy in the form of ATP to move substances against their concentration gradients.

  • Primary active transport directly uses ATP to fuel transport. A prime example is the sodium-potassium pump, which maintains cell potential.

  • Secondary active transport use energy stored in the form of concentration gradients established by the primary active transport.

Bulk Transport Mechanisms

  • Endocytosis allows the cell to ingest large substances or a large amount of substances by enveloping it in a portion of the cell membrane.

  • Exocytosis involves the fusion of a vesicle with the cell membrane, releasing its contents outside the cell. It is vital for the secretion of substances like hormones and enzymes.

  • Phagocytosis, a type of endocytosis, allows cells to engulf solid particles, whereas pinocytosis is the ingestion of liquids or very small particles.