Cell Membranes

Cell Membranes
The boundary of the cell, sometimes called the plasma membrane, separates internal metabolic events from the external environment and controls the movement of materials into and out of the cell. This membrane is very selective about what it allows to pass through; this characteristic is referred to as “selective permeability.” For example, it allows oxygen and nutrients to enter the cell while keeping toxins and waste products out. The plasma membrane is a double phospholipid membrane, or a lipid bilayer, with the nonpolar hydrophobic tails pointing toward the inside of the membrane and the polar hydrophilic heads forming the inner and outer surfaces of the
membrane.
Membrane transport
*Passive Transport Across the Cell Membrane
Passive transport describes the movement of substances down a concentration gradient and does not require energy use the following .
• Bulk flow is the collective movement of substances in the same direction in response to a force, such as pressure. Blood moving through a vessel is an example of bulk flow.
• Simple diffusion is the net movement of substances from an area of higher concentration to an area of lower concentration.
• Facilitated diffusion is the diffusion of solutes through channel proteins in the plasma membrane. .
• Osmosis is the diffusion of water molecules across a selectively permeable membrane.
• Dialysis is the diffusion of solutes across a selectively permeable membrane.



*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 :
Protein Pumps
• Transport proteins in the plasma membrane transfer solutes such as small ions (Na ,K), amino acids, and monosaccharides.
• The proteins involved with active transport are also known as ion pumps.
* Protein pumps are catalyses in the splitting of ATP to ADP + phosphate, so they are called ATPase enzyme.
* The sodium-potassium pump actively moves sodium out of the cell and potassium into the cell. and are essential in transmission of nerve impulses and in muscular contractions.
*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.

Eubacteria Lacking Cell Walls
These are microorganisms that lack cell walls (commonly called mycoplasmas and comprising the class Mollicutes) and do not synthesize the precursors of peptidoglycan. They are enclosed by a unit membrane, the plasma membrane They resemble the L forms that can be generated from many species of bacteria (notably gram-positive eubacteria); unlike L forms, however, mycoplasmas never revert to the walled state, and there are no antigenic relationships between mycoplasmas and eubacterial L forms.