CELLULAR NEUROPHYSIOLOGY LEARNING OBJECTIVES
Membranes and Movement of Molecules and Ions Across Them
2. Explain how hydrophobic and hydrophilic forces favour spontaneous assembly of the lipid bilayer. What is on either side of the lipid bilayer and how does it help stabilize the bilayer?
What kinds of physical interactions are involved?
This lipid bilayer formation is spontaneous since the hydrophobic interactions are energetically favorable to the structure. The lipid bilayer is a noncovalent assembly. The proteins and lipid molecules are held together by noncovalent interactions such as Van der Waals forces (which holds the hydrophobic tails together) and hydrogen bonding (which binds the hydrophilic heads with water), which help to stabilize the lipid bilayer structure.
3.In what ways do membrane proteins associate with the lipid bilayer in the fluid mosaic model?
There are two types of proteins: integral membrane proteins and peripheral proteins Cell membranes consist of a phospholipid bilayer and various integral proteins. The proteins act as carriers that move needed molecules across the membrane channel. There are four different types of integral proteins: Channel, transport, adhesion, and receptor proteins. Channel proteins allow hydrophilic substances to enter the cell through a passageway. Transport proteins allow for the transfer of substances across the membrane due to using up energy (ATP), which is also known as active transport. The function of Adhesion proteins is to attach cells to neighboring cells. The Receptor proteins provide binding sites for enzymes such as ATP. The fluid-mosaic model also comes into play, as it is the make-up of the cell membrane. The hydrophilic head of the phospholipids faces outward to allow for absorption of water, nucleic acids, and other miscellaneous substances needed for the cells survival. The hydrophobic tails make up the inside of the bilayer. Proteins extend out through the plasma membrane of animal cells, and non-polar regions of the proteins fastening them to the membrane's non-polar interior. Carbohydrate chains are often bound to the extra cellular portions of these proteins, as well as to the membrane phospholipids. These chains serve as a way to identify the cell as one of its own or a foreign cell.
4. Give two examples of substances that can readily diffuse through a lipid bilayer. What are the physical properties of these substances that make this possible?
Nonpolar and lipid-soluble substances diffuse directly through the lipid bilayer. Such substances include oxygen, carbon dioxide, and fat-soluble vitamins.
6. If two solutions of differing concentrations are separated by a selectively permeable membrane, what are the factors that determine the rate of net movement of the solute from one side to the other? if it is permeable to that substance, the molecules will pass from one side of the membrane to the other. This is how oxygen passes from the air we inhale into the bloodstream. Dialysis treatment for kidney patients is based on diffusion of solutes through artificial dialysis membranes.
Diffusion rates are very important to cell survival because they determine how quickly a cell can acquire nutrients or rid itself of wastes. Some factors that affect the rate of diffusion through a membrane are as follows:
Temperature. Diffusion is driven by the kinetic energy of the particles, and temperature is a measure of that kinetic energy. The warmer a substance is, the more rapidly its particles diffuse. This is why sugar diffuses more quickly through hot tea than through iced tea.
Molecular weight. Heavy molecules such as proteins move more sluggishly and diffuse more slowly than light particles such as electrolytes and gases. Small molecules also pass through membrane pores more easily than large ones.
“Steepness” of the concentration gradient. The steepness of a gradient refers to the concentration difference between two points. Particles
