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Biology 1113

by: Jeftar Kusi

Biology 1113 1113

Jeftar Kusi
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Chapter 6: Lipids, Membranes, and the First Cells
Biological Sciences: Energy Transfer and Development
Dr. Goodell
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This 6 page Class Notes was uploaded by Jeftar Kusi on Wednesday September 14, 2016. The Class Notes belongs to 1113 at Ohio State University taught by Dr. Goodell in Fall 2016. Since its upload, it has received 22 views. For similar materials see Biological Sciences: Energy Transfer and Development in Biology at Ohio State University.


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Date Created: 09/14/16
Biology: Chapter 6  RNA is a nucleic acid  Lipids have low solubility in water  Saturated Fat: Fatty acid tail lacks double bond Steroids  Cholesterol: An important component of animal cell membranes.  Estradiol: The primary female sex hormone; produced in ovaries.  Testosterone: The most abundant male sex hormone; produced in testes.  Vitamin D: Aids in calcium and phosphate metabolism.  All steroids have four linked rings.  Phospholipids are composed of a phosphate group, a glycerol, and fatty acids.  Phospholipids can be recognized by the presence of a head and two tails.  Olive oil is plant oil, and most plant oils are rich in unsaturated fats.  Water rejects non-polar molecules such as fats, so fats travel inside particles that are coated with polar parts of phospholipids and proteins.  Lipids include the family of molecules called steroids. Some members of the family are vital components of membranes. Others steroids help to control growth, development, and metabolism.   f LDL exceeds HDL, your body is probably storing excess cholesterol, which may clog the arteries and cause heart disease.  The plasma membrane is selectively permeable.  The transport protein is allowing solute molecules to enter the cell.  Ions, such as hydrogen ions, and hydrophilic molecules, such as water and glucose, cannot rapidly pass directly through the phospholipids of a plasma membrane. To move rapidly through the membrane, they must pass through membrane transport proteins.  Diffusion: The tendency for particles of any kind to spread from high concentration to low concentration; concentration gradient.  Diffusion is occurring via a transport protein.  Diffusion across the biological membrane is called passive transport.  Osmosis is the diffusion of water across a selectively permeable membrane.  Phospholipids contain both a polar head and a non-polar hydrocarbon tail, both of which are necessary for their ability to form membrane bi- layers.  Lipids are non-polar molecules, whereas sugar is polar.  What property of dishwashing liquid (detergent) makes it useful to wash grease from pans? Answer: Amphipathic nature. Detergents form micelles around the grease, which are then washed away because the polar head groups facing outward on the micelle are water-soluble.  Small non-polar molecules such as oxygen can diffuse across cell membranes.  The water-soluble portion of a phospholipid is the polar head, which generally consists of a glycerol molecule linked to a phosphate group. The hydrophilic, or water-loving, portion of a phospholipid is the polar head, whereas the hydrophobic portion is the non-polar tail.  If a red blood cell is placed in a salt solution and bursts, the tonicity of the solution relative to the interior of the cell is hypotonic. The salt concentration in the solution is lower than it is in the cell, so water enters the cell, causing it to burst.  A cell will gain water when placed in a hypotonic solution.  In a hypotonic solution plant cells experience a net gain of water and become turgid.  A cell will lose water when placed in a hypertonic solution.  Sometimes a cell needs to move a solute against its concentration gradient. This process is called active transport, and it requires input of energy from ATP. For instance, most animal cells need to expel sodium ions (Na ) and take in potassium ions (K ), both against their concentration gradients.  Here is how the sodium-potassium pump works: Sodium ions bind to a transport protein. ATP transfers a phosphate group to the protein, providing the energy that causes the protein to change shape and push the sodium ions across the membrane, where they are released outside the cell.  Potassium ions now bind to the transport protein and the phosphate group is released. This causes the protein to return to its original shape, releasing the potassium ions inside the cell. The transport protein is now ready to repeat the process.  The transport protein facilitates the movement of solute across the plasma membrane.  Each dye molecule and the water molecules that surround it are in constant motion due to their thermal energy. Any individual molecule’s motion is random because of the frequent collisions among all of the molecules.  If a concentration gradient exists for a population of molecules, the motion of the individual molecules in that population will result in a net (directional) movement from higher to lower concentration. For example, in the initial condition, there is a concentration gradient for the orange dye. As a result, the orange dye molecules diffuse down the concentration gradient, with net movement from side A to side B. Once diffusion has eliminated the concentration gradient and equilibrium is reached, net movement stops, but the random motion of each molecule continues (as indicated by the red arrows in the image below).  Some solutes pass readily through the lipid bi-layer of a cell membrane, whereas others pass through much more slowly, or not at all.  Small non-polar (hydrophobic) molecules, such as dissolved gases (O , 2 CO ,2N 2 and small lipids, can pass directly through the membrane. They do so by interacting directly with the hydrophobic interior of the lipid bi-layer.  Very small polar molecules such as water and glycerol can pass directly through the membrane, but much more slowly than small non-polar molecules. The mechanism that permits small polar molecules to cross the hydrophobic interior of the lipid bi-layer is not completely understood, but it must involve the molecules squeezing between the hydrophobic tails of the lipids that make up the bi-layer.  Polar molecules such as glucose and sucrose have very limited permeability.  Large molecules such as proteins cannot pass through the lipid bi-layer.  Ions and charged molecules of any size are essentially impermeable to the lipid bi-layer because they are much more soluble in water than in the interior of the membrane.  Carrier proteins and channels are both transport proteins involved in facilitated diffusion, the passive transport of solutes across a membrane down their concentration or electrochemical gradient. As integral membrane proteins, both carriers and channels protect polar or charged solutes from coming into contact with the hydrophobic interior of the lipid bi-layer. Furthermore, all transport proteins are specific for the solutes they transport, owing to the specificity of the interactions between the solute and the transport protein.  Channels are protein-lined pores across the membrane. A channel may be open at all times (non-gated), or may be gated such that the channel opens and closes under specific conditions. Channels transport inorganic ions or water.   In contrast, carrier proteins do not have a pore. Binding of the transported solute to the carrier protein on one side of the membrane induces a conformational change in the protein that exposes the solute binding site to the opposite side of the membrane, where the solute is released. Carriers transport small polar solutes such as sugars and amino acids.  


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