Plasma Membrane: Structure, Functions & Signaling

Posted on Mar 8, 2025 in Biotechnology

Plasma Membrane

Every cell (eukaryotic or prokaryotic) has a plasma membrane that allows:

  • To maintain the composition of the cell
  • To regulate the passage of material into and out of the cell

In eukaryotes, there are other membranes in several organelles. All of them have a common structure: a lipid bilayer with proteins and carbohydrates. Thickness: 7 to 10 nm.

Membrane Composition

Chemically, the cell membrane is composed of three components, namely:

  • Lipids (3 types:
    1. Phospholipids
    2. Glycolipids
    3. Cholesterol
  • Proteins
  • Carbohydrates

1. Phospholipids

Amphipathic (or amphiphilic) molecules:

  • Polar head (hydrophilic)
  • 2 nonpolar tails (hydrophobic)

In the membrane, there are 4 PL with different polar heads (phosphatidylethanolamine, phosphatidylserine, phosphatidylcholine, sphingomyelin).

2. Glycolipids

Amphipathic, contain sugar residues.

Proteins

Integral proteins:

  • Incorporated within the lipid bilayer
  • More hydrophobic regions
  • More interactions with lipids
  • Harder to extract

Peripheral proteins:

  • Less hydrophobic regions
  • More hydrophilic regions exposed to water on either side of the membrane
  • Less interaction with lipids
  • More easily extractable

Most of these proteins are not bound rigidly in place and are able to move laterally.

Mosaic: The different components are like the pieces of a mosaic.

Fluidity: The membrane is a bidimensional fluid. Its fluidity depends on the movement of the components, particularly, the lipids.

Lipid Rafts

Specialized membrane patches with higher concentrations of cholesterol and saturated fatty acids which reduce lipid fluidity. In these domains, proteins are usually less mobile (for instance, proteins involved in the transduction of signals from outside the cell).

Functions of the Plasma Membrane

The plasma membrane is the site where materials are exchanged between the cell and the environment.

  • Lipid bilayer
    • Constitutes the outer boundary of the cell
    • Allows to regulate the composition of the cell
  • Proteins and carbohydrates
    • Allow specific functions (transport, enzymatic, signaling, etc.)

Specific functions of membrane proteins:

  1. Physical barrier
  2. Selective permeability
  3. Generation of electrochemical gradients
  4. Communication
  5. Cell surface marker
  6. Cell adhesion
  7. Attachment to the cytoskeleton

2. Selective Permeability

Regulates entry and exit of nutrients, ions, and waste molecules through the membrane.

Types:

  • Microtransport
    • Transport of ions, water, and small molecules
    • Without folding of the plasma membrane
  • Macrotransport
    • Transport of large molecules into and out of the cell
    • Pinocytosis and phagocytosis

Microtransport

  • Simple diffusion
    • Small, nonpolar molecules cross directly through the lipid bilayer (without proteins)
    • Lipophilic (fat-soluble) diffuse readily and small uncharged molecules (as water) diffuse very slowly
  • Channel-mediated transport
    • Ions and small molecules pass selectively through channels (multipass proteins forming transmembrane pores)
    • Cells open and close channels for Na+, K+, Ca++, etc.
    • Water usually crosses through channel proteins called aquaporins
  • Carrier-mediated transport
    • Carriers are transmembrane proteins that bind small molecules and translocate them via conformational changes

3. Generation of Electrochemical Gradients

Allow reception (excitability) and conduction (conductivity) of electrical signals in electrically excitable cells:

  • Neurons
  • Skeletal muscle cells

Excitability

The plasma membrane reacts to stimuli with a reversal of the ionic gradient (membrane depolarization) that spreads and is propagated as an action potential or nerve impulse.

4. Communication

Cell-cell communication in a multicellular organism is necessary to:

  • Regulate tissue and organ development
  • Control growth and proliferation
  • Coordinate functions

Forms of Intercellular Signaling Mechanisms

  • Secreted molecules
    1. Endocrine signaling – hormones
    2. Local mediators: autocrine and paracrine signaling (Example: growth factors)
    3. Synaptic signaling – neurotransmitters

    Most of these signals are hydrophilic molecules that bind transmembrane protein receptors to initiate changes in the target cell.

  • Yuxtacrine signaling – Contact-dependent. The signaling molecules are cell-membrane-bound proteins.
  • Gap junctions