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transport across cell membrane

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Describe the five processes which allow the transport across the cell membrane. Structure of a plasma membrane: The cell membrane is semi-permeable membrane that is made up of proteins, carbohydrates and phospholipid molecules. There are different types of protein molecules randomly embedded in the bi-layer which carry out various cell functions like cell-cell contact, surface recognition, cytoskeleton contact, signalling, enzyme activity, or transporting substances across the membrane. The types of proteins in the membrane are Transport channel proteins, Globular proteins, Glycoprotein's, Peripheral, Integral, and Surface proteins and Alpha-Helix proteins. These proteins make up around 50% of the membrane where as only 5% is made up from carbohydrates. The cell membrane has phospholipids, glycolipids, and steroids present in it. Other lipids present are the cholesterol molecule which helps maintain the fluid condition in the bi-layer. The phospholipid molecules form the outer and inner layers (bi-layer) and are constantly moving giving a fluid like structure. The outer layers consist of the heads of the phospholipids and the inner layer consists of the tails of the phospholipids. The phospholipid polar heads are hydrophilic (they like water) and the non polar tails are hydrophobic (they dislike water). The bi-layer allows lipid soluble molecules to pass through it but not ions and polar molecules. ...read more.


Water potential is the tendency of water to move from an area of high concentration to one of a lower concentration. A presence of a solute decreases the water potential of a substance. Therefore, there is more water present in a glass of fresh water than in a glass of sea water. Hypertonic or hyperosmotic solutions have more solute in them and have a lower water potential. Isotonic or Isosmotic solutions have an equal amount of water and solute and therefore have an equal amount of water potential. Hypotonic or Hypoosmotic solutions have less solute and a higher water potential. On of the main functions of blood in animals is to maintain an isotonic (equal) internal environment. The cells vacuole does this by pumping water out of the cell. If this didn't happen the cell would swell up and eventually burst. Ridding water out of the cell requires energy in the form of ATP as the water is moving against the concentration gradient. Endosmosis is the movement of a liquid through a membrane from the outside to the inside, whereas exosmosis is the movement of a liquid from the inside to the outside. ...read more.


Passive Transport Passive transport doesn't use any chemical energy. Examples of passive transport are diffusion, facilitated diffusion and osmosis. Bulk Transport Endocytosis is a process where objects are taken from the outside of a cell and absorbed into the cell. The cell does this by engulfing the object in its membrane. All cells use this process as most substances it needs are large polar molecules (dissolved in water) and which cannot pass through the phospholipid bi layer due to the lipids hydrophilic nature. There are two types of endocytosis, these are phagocytosis and pinocytosis: Phagocytosis (meaning cell-eating), is the process by which cells ingest large objects, for example cells which have undergone apoptosis (a form of programmed cell death), bacteria or viruses. The membrane folds around the object and the object is sealed off into a large vacuole known as a phagosome (a food vacuole). Pinocytosis (meaning cell-drinking), is an alternative name for endocytosis. This process is concerned with the uptake of solutes and single molecules such as proteins. Exocytosis is the opposite of Endocytosis. It is a strong process by which a cell directs secretory vesicles to the cell membrane. These membrane-bound vesicles contain soluble proteins to be secreted to the extracellular environment, as well as membrane proteins and lipids that are sent to become components of the cell membrane. ...read more.

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