Glycocalyx : A sticky cell coating consisting of oligosaccharides and glycoprotiens. Hydrophillic: Having an affinity for water; tending to mix readily with water. Hydrophilic compounds are typically polar compounds, with charged or electronegative atoms. Hydrophobic: Lacking affinity for water; tending to repel and not absorb water. Hydrophobic compounds are typically non-polar compounds and when in water often cluster together.
Integral Membrane Protein: A general term describing a protein that completely spans the hydrophobic interior of the membrane. Lipid-Anchored Proteins: Proteins that are covalently bonded to a lipid in the membrane. Examples include G proteins and certain kinases. Lipid Bilayer: The foundational structure of plasma membranes; it is composed of two layers of phospholipids positioned such that their polar hydrophilic heads face outward and their nonpolar hydrophobic tails are directed inward, blocking entry of water and water-soluble material into the cell.
Peripheral Membrane Protein: Proteins that adhere only loosely to the biological membrane with which they are associated. These molecules do not span the lipid bilayer core of the membrane, but attach indirectly, typically by binding to integral membrane proteins, or by interactions with the polar end of the lipid bilayer. Phospholipids: Main lipid component of cell membranes.
Phospholipids are a heterogeneous type of molecule composed of glycerol, phosphate, two fatty acid residues, and 'headgroups' with different chemical properties. Plasma Membrane: The external, limiting lipid bilayer membrane of cells, which is a thin membrane around the cytoplasm of a cell; controlling passage of substances in and out of the cell.
Polypeptide: A molecule made up of a string of amino acids. A protein is an example of a polypeptide. Protein: The fundamental components of all living cells and include many substances, such as enzymes, hormones, and antibodies, that are necessary for the proper functioning of an organism.
Proteins are molecules composed of one or more chains of amino acids in a specific order. This order is determined by the base sequence of nucleotides in the gene coding for the protein. Transmembrane Protein: An integral membrane protein that spans from the internal to both sides of the exterior surface of the lipid bilayer in which it is embedded.
Some proteins associated with the cell membrane simply connect with one surface or other of the lipid bilayer. They may be attached by way of carbohydrate links, or be complexed with other proteins already embedded in the hydrophobic container. However, many other proteins extend their structures completely though the bilayer, crossing from one side to another. These transmembrane proteins have regions that easily associate with water i. Other proteins have regions that are hydrophilic, which have no problems in the aqueous environment outside the cell, but also are linked to chains of carbohydrate oligosaccharides and then to a separate phospholipid, which has no difficulty fitting into the membrane.
These proteins, called peripheral membrane proteins , are only associated with one side of the membrane or the other - never both. Only the transmembrane proteins can operate on both sides of the membrane at once, and they often serve to "signal" events taking place outside the cell, to vital functions inside the cell. They also serve as exits and entrances, transporting vital materials from one side of the membrane to the other.
Those regions of the protein that must interact with the strongly hydrophobic center of the lipid bilayer have sequences of polypeptide that are made up of amino acids with hydrophobic R -groups, such as alanine, leucine, glycine, serine and tyrosine.
Normal eukaryotic cells contain lots and lots of different proteins, making a study of just those proteins associated with the membranes quite difficult.
This problem can be solved, however, if the plasma membranes of human red blood cells are used as a starting material. Such cells are available in very large numbers, they only have one membrane as part of their structure the plasma membrane , and it is easy to prepare "ghosts" of these cells by bursting them open in very dilute salt solution and setting free their only contents - the protein hemoglobin.
The remaining, almost pure plasma membrane, can then be studied directly without the problems of contaminating cytoplasmic proteins. Very often the sheets of pure membrane are resealed into tiny globules; tiny "cells" which can be studied for their membrane properties. Spectrin is found on the inner, cytoplasmic, side of the cell membrane. Small nonpolar molecules can readily diffuse to their concentration gradient across the membrane whereas polar molecules will be prohibited from doing so.
The hydrophobic lipid bilayer forms a barrier between the interior and the exterior of the cell. Thus, the transport of polar molecules needs to be modulated. Polar molecules, such as water and certain proteins, and ions need a transporter in the plasma membrane to cross. This is the function of membrane proteins. Since membrane proteins are also amphipathic molecules, they can interact with the hydrophobic lipid bilayer and thereby insert themselves in the membrane.
At the same time, they provide a transport mechanism by which polar and charged molecules to pass through. Thus, while the lipid bilayer prevents them to enter or leave the cell, the membrane proteins are their vehicles for entry and exit. This is important for the cell to ensure that the cytosolic components are kept at optimal levels, thereby maintaining homeostasis. The selective permeability of the plasma membrane is a fundamental feature of biological membranes. Thus, the membraned organelles such as nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, chloroplasts, and vesicles are able to similarly regulate the passage of molecules.
Cholesterol molecules are another essential amphiphile. They are present in the plasma membrane of animal cells and are responsible for membrane fluidity and structural integrity of animal cells.
Because of them, the animal cells do not need a cell wall. Their presence in the animal cell membrane ensures cellular integrity. While they keep the membrane stable, they also allow an animal cell to alter its shape and move. They are also involved in intracellular transport, selective permeability, cell signaling, and nerve conduction.
Glycolipids are another plasma membrane component. They provide stability to the cell. They also permit cell to cell interactions.
They enable tissue formation by cell adhesion. Also, they facilitate cellular recognition, which is essential in immunologic functions.
A micelle is an aggregate of surfactant molecules wherein the hydrophilic head regions face the aqueous solution and the hydrophobic tail regions are oriented towards the center.
Thus, it is often spherical in shape. Due to the amphipathic nature of bile acids, they are able to form micelles. Bile acid-containing micelles aid in lipid digestion. They bring the lipids near the intestinal brush border membrane to prompt fat absorption. The body is comprised of different elements with hydrogen, oxygen, carbon, and nitrogen as the major four.
This tutorial will help you understand the chemical composition of the body. This will come in handy when considering the various interactions between cells and structures. Read More. The gastrointestinal system breaks down particles of ingested food into molecular forms by enzymes through digestion and then transferred to the internal environment by absorption.
Find out more about these processes carried out by the gastrointestinal system through this tutorial A typical eukaryotic cell is comprised of cytoplasm with different organelles, such as nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, and so on.
The cellular contents are surrounded by a double layer, cell membrane. These cellular structures and cell junctions are elaborated in this tutorial Skip to content Main Navigation Search. Dictionary Articles Tutorials Biology Forum.
Table of Contents. Amphipathic molecules are chemical compounds containing both polar and nonpolar apolar portions in their structure.
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