Describe the fluid-mosaic model of a plasma membrane. Discuss the role of the membrane in the movement of materials through it by each of the following processes:
a. Active transport
b. Passive transport
The plasma membrane is a semi permeable barrier that separates the inside of the cell from the outside environment. The plasma membrane is made up of carbohydrates, cholesterol, proteins, and a lipid bilayer, or double layer of lipids. The plasma membrane may be known as a fluid mosaic model where the membrane is a fluid structure with various proteins embedded in or attached to the bilayer of phospholipids. The plasma membrane possesses hydrophilic tails and hydrophobic tails, which may be referred to as amphiphilic. There are various ways that materials may pass through the membrane. Movement across the membrane may be classified into two different categories, passive transport and active transport.
Passive transport does not require energy to occur. During passive transport the molecules will move from a place of high concentration to a place of low concentration. In other words, the molecules are moving down their concentration gradient. A concentration gradient is the increase or decrease in the density of a chemical substance in an area. The three types of passive transport are diffusion, osmosis, and facilitated diffusion. Diffusion is the tendency for molecules of any substance to spread out into the available space. The plasma membrane is semi permeable so diffusion across the plasma membrane may only occur with a few substances such as oxygen, carbon dioxide, and alcohol. Osmosis is the diffusion of water across a selectively permeable membrane. When osmosis occurs in the plasma membrane the molecules will move from hypotonic to hypertonic. Facilitated Diffusion is polar molecules and ions impeded by the lipid bilayer of the plasma membrane diffusing passively with the help of transport proteins that span the membrane. The transport protein,...
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