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Transcript
The Cell Membrane Key words: Cell Membrane Phospholipids Hydrophobic Hydrophilic Lipid Bilayer Semipermeable As you now know, all living things consist of one or more cells. Although cell structure may vary from one organism to another, the cells of all organisms are surrounded by a cell membrane. The cell membrane is a thin layer of lipid and protein that separates the cell’s contents from its environment. The cell membrane functions like a fence with gates, controlling what enters and leaves the cell. Question One: What is the function of the cell membrane? Membranes are made mostly of phospholipid molecules. A phospholipid molecule is a type of lipid made from glycerol, two fatty acids, and a phosphate group. The picture below illustrates a phospholipid. Question Two: What are the components of a phospholipid? The two ends of the phospholipid molecule have different properties in water. The phosphate head of the phospholipid is hydrophilic, meaning “water-loving.” The phosphate head dissolves easily in water. The lipid tails of the molecule are hydrophobic, meaning “water-fearing.” The lipid tails do not dissolve in water. Question Three: Label the hydrophilic and hydrophobic ends of the phospholipid. Draw a water molecule near the side that like water . When dropped in water, phospholipids will form two layers, called a lipid bilayer. The phosphate heads face the watery fluids inside the cell and outside the cell. Lipid tails are sandwiched inside the bilayer. The phospholipids in the cell membrane are constantly being formed and broken down by chemical reactions in living cells. The picture below illustrates the lipid bilayer. Protein molecules are embedded in the lipid bilayer. Most of the proteins extend all the way through the bilayer, and are similar to channels. There are many different kinds of proteins in the membrane. Some of these proteins help transport materials in and out of the cell, while others help with cell recognition. Question Four: What is the function of proteins in cell membranes? In order to live all cells must take in nutrients and eliminate wastes. In both cases these materials must pass through the cell membrane. Do all materials pass through the cell membrane the same way? The answer is no. The cell membrane is considered semipermeable, meaning that the membrane lets certain molecules pass through and prevent others molecules from crossing. Small molecules, such as water, can enter and leave the cell freely. Large molecules, such as proteins and carbohydrates, cannot. And even thought they are small, particles with strong electrical charges, such as ions, cannot pass easily through the membrane. The electrical charge prevents the ion from moving through the lipid bilayer. Question Five: What does semipermeable mean? Question Six: What two factors effect what crosses the membrane?