The Cell Membrane-
The cell membrane is a semi-permeable membrane that surrounds the cytoplasm of a cell; the cell membrane is made up of phospholipids, proteins and carbohydrates. Its function is to protect the integrity of the interior of the cell allowing certain substances into the cell, while keeping other substances out. The phospholipids form a thin, flexible sheet while the proteins float in the phospholipid sheet like an ice berg, and the carbohydrates extend out from the proteins. The phospholipids are arranged in a bilayer, with their polar, hydrophilic phosphate heads facing outwards, and their non-polar, hydrophobic fatty acid tails facing each other in the middle of the bilayer. This hydrophobic layer acts as barriers to all but the smallest molecules, effectively isolating the two sides of the membranes. Different kinds of membranes can contain phospholipids with different fatty acids, affecting the strength and flexibility of the membrane, and animal cell membranes also contain cholesterol linking the fatty acid together and so stabilising and strengthening the membrane. Cell membranes have a bilayer structure through which protein materials cross from one layer to another. Some protein molecules only pass partway through the membrane while others pass all the way through. Two types of proteins are contained within the structure of a cell membrane. Integral proteins are embedded inside the membrane structure. Some integrals serve as pathways for ions and molecules to gain entry to the cell. Peripheral proteins sit on the inner and outer surfaces of the membrane, or may be attached to the ends of an integral protein material. Peripherals play a part in cell signalling processes, and help to maintain the chemical balance inside the cell. Proteins that span the membrane are usually involved in transporting substances across the membrane. Proteins on the inside surface of cell membranes are often attached to the cytoskeleton and are involved in maintaining the cell's shape, or in cell motility. They may also be enzymes, catalysing reactions in the cytoplasm. Proteins on the outside surface of cell membranes can act as receptors by having a specific binding site where hormones or other chemicals can bind. This binding then triggers other events in the cell. They may also be involved in cell signalling and cell recognition, or they may be enzymes, such as maltase in the small intestine. Carrier proteins are rooted in the cell membrane to help transport glucose and amino acids across the membrane this is because they are too large to go through ion channels. The carbohydrates are found on the outer surface of all eukaryotic cell membrane and are attached to the membrane proteins or sometimes to the phospholipids. Proteins with carbohydrates attached are called glycoproteins while phospholipids with carbohydrates attached are called glycolipids. The carbohydrates are short polysaccharides composed of a variety of different monosaccharaides, and form a cell coat or glycocalyx outside the cell membrane. The glycocalyx is involved in protection and cell recognition, and antigens such as the antigens on blood cells are usually cell-surface glycoproteins. Passive Transport-
When energy is required to carry a protein across, that’s known as active transport; when energy is not needed that’s called passive transport. Amino acids are actually transported by active transport; the transport of glucose across a cell membrane uses a transport protein but doesn’t require ATP, so it’s celled passive transport or alternatively facilitated diffusion Three different mechanisms for passive transport in bilayer membranes. Left: ion channel (through a defined trajectory); centre: ionophore/carrier (the transporter physical diffuses through with the ion); right: detergent (non-specific membrane disruption).
Passive transport activities in cell membrane structures require no energy output as molecules are able to...
References: http://antranik.org/movement-of-substances-across-cell-membranes/ (Antranik) (2014) (Accessed)
https://www.boundless.com/physiology/textbooks/boundless-anatomy-and-physiology-textbook/cellular-structure-and-function-3/transport-across-membranes-42/diffusion-330-11468/ (Boundless) (2014) (accessed)
http://www.phschool.com/science/biology_place/biocoach/biomembrane1/transport.html (Phschool) (2014) (accessed)
http://www.college-cram.com/study/biology/cell-membranes/passive-transport/ (College) (2010) (accessed)
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