Cell Membrane Activities, Osmosis and Transport
Active and Passive Transport and Osmosis are Major Membrane Events
Sep 2, 2009
Membranes help explain how cells function and survive. Originally, the outer cell membrane was considered to be a simple lipid and protein (lipoprotein) structure. Today, the outer cell membrane is recognized as a complex, integrated and dynamic structure.
Diffusion, Osmosis, Passive Transport and Active Transport
Diffusion is the random movement of molecules from one place to another. A solid can diffuse in a liquid, a liquid can diffuse in a liquid, a gas can diffuse in another gas or liquid.
Osmosis is the diffusion of water across a selectively permeable membrane. Osmosis is common and very important, and osmosis occurs during every second of a cell's existence.
Passive transport is a non-energy-requiring, protein-assisted, molecular carriage across the membrane and into the cell.
Active transport is the energy-requiring, protein-assisted, movement of materials into, or out, of a cell. ATP, or GTP, are the major energy-storage compounds used for these purposes.
Active Transport Mechanisms Include Endocytosis (Phagocytosis, Pinocytosis) and Exocytosis
When work is done within a cell, energy is required and expended. The energy that drives all these reactions is mainly ATP that is produced from glycolysis in the cytoplasm and Kreb's cycle oxidative phosphorylation reactions that occur in the mitochondria. Plants produce energy by both these means, and by photosynthetic ATP production.
In summary, ATP is needed for active transport. Active transport includes:
- sodium-potassium pump, exchanges sodium inside the cell with potassium outside the cell.
- phagocytosis, the ingestion of large cells such as bacteria, by macrophages or neutrophils.
- pinocytosis, the uptake of droplets, or small molecular complexes into the cell.
- exocytosis, the reverse of endocytosis, with vesicular export of hormones, enzymes, and other materials.
How Outer Cell Membranes are Organized and Structured
Early in the history of biology several things were discovered about outer cell membranes:
- are common to all animal and plant cells; plants also have walls surrounding their membranes.
- react with lipid, protein and sugar detection reagents; therefore, cell membranes are lipoproteins that contain some sugars.
- are selectively or differentially permeable and, therefore, regulate molecular traffic into and out of the cell.
Singer and Nicholson proposed the fluid mosaic model for membranes. This model indicated a bilayer of lipid with immersed proteins. This is membrane model has resulted in many researches and experiments that have developed and updated the original model.
A closer look at this confirmed and updated cell membrane model indicates:
- the bilayer is distinctive with lipid chains facing inward and phosphate on the outside. This accounts for hydrophilic (water-loving) properties on the inner and outer surfaces, and the hydrophobic (water-fearing) lipid chains on the inside.
- proteins are immersed and embedded in many membrane locations. Some proteins form distinct channels for transport, other proteins are passive or active transport molecules. Further analysis shows iron, copper, other metal and amino acid transporters. There are calcium channels and sodium-potassium pump channels.
- sugar and protein groups define the cell types located in different tissues and organs and different surface signatures define these cells. White blood cells (leukocytes) possess surface markers that distinguish them.
Cell membranes are like busy cities with a host of vigorous activities throughout the membrane metropolis.
Specialized Membrane Components
Chemical signals at the cell membrane move into the cell, and signals from within the cell are transported to the membrane.
Mammalian eukaryotic cells have 4 main receptors:
- G-protein coupled receptors (GPCRs), many different types.
- Tyrosine-kinase linked receptors.
- Ion channel receptors.
- Intrinsic enzyme activity receptors.
This brief trip to the cell membrane should convince anyone that the outer membrane of eukaryotic cells is, indeed, a complex, integrated and dynamic structure. Decades of research will be needed to make these mysteries of membranes less enigmatic and more understandable.
Source
Lodish, H. et al. 2000. Molecular Cell Biology. Fourth Ed., W. H. Freeman and Co., New York, N.Y.
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