LONG ESSAY QUESTIONS Plasma membranes are semipermeable membranes that allow sub
ID: 74453 • Letter: L
Question
LONG ESSAY QUESTIONS
Plasma membranes are semipermeable membranes that allow substances to enter and exit based on their chemical make up in relation to the membrane. Explain the composition of a plasma membrane and how it uses active transport, passive transport and bulk transport to move substances in and out of the cell. In your explanation be sure to include the following:
1) What makes up a plasma membrane (Include proteins!)
2) The polarity of the monomer in the membrane
Discuss head vs. tails
3) What is passive transport? What is active transport?
Discuss concentration gradient and energy use
4) Explain how simple diffusion, facilitated diffusion and osmosis work (discuss concentration gradient and energy use)
5) Explain how active transport works (discuss concentration gradient and energy use)
6) Explain the terms endocytosis, exocytosis, phagocytosis and pinocytosis
7) Give an example of what material is moved during each of these: simple diffusion, facilitated diffusion, osmosis, active transport, phagocytosis and pinocytosis
Explanation / Answer
Plasma Membrane
The plasma membrane is the boundary between the cell and its environment. It regulates what enters and exits the cell. Cells must maintain an appropriate amount of molecules to function inside them. They must also have a way to keep things out or to allow things to enter. This is the job of the plasma membrane. The plasma membrane is like the guard at a gated community.
Plasma Membrane Structure
The plasma membrane is composed of a phospholipid bilayer. This is two layers of phospholipids back-to-back. Phospholipids are lipids with a phosphate group attached to them. The phospholipids have one head and two tails. The head is polar and hydrophilic, or water-loving. The tails are nonpolar and hydrophobic, or water-fearing.
It basically looks like a phospholipid sandwich with the heads forming the bread and the tails forming the meat in the middle.
Located all throughout the surface of the plasma membrane are cholesterol molecules. These molecules help stabilize the phospholipids and keep them in position. The plasma membrane is also dotted with proteins. Some of the proteins are peripheral proteins that only go halfway through the membrane, while others are integral proteins that go entirely through the membrane. These proteins serve as channels to allow the molecules to enter and leave the cell.
· The membrane primarily consists of a bilayer of phospholipid molecules. These molecules can move about by diffusion in their own layer.
· Width is about 7nm on average
· Some of the phospholipids are saturated and some are unsaturated. This affects the fluidity of the membrane, a heavily unsaturated membrane means a more fluid membrane. This is due to the kink in saturated tails causing the molecules to not sit closely together.
· Phospholipid tails point inwards, facing each other, meaning that inside the membrane it is non-polar hydrophobic.
· The protein molecules within the structure can move around although some are fixed to structures inside the cell and do not move. Also, some of them span the width of the membrane, some are only on the inner layer and some on the outer layer.
· Many proteins and lipids have short carbohydrate chains attached to them, forming glycoproteins and glycolipids.
· Contains cholesterol.
How Cell Substances Transport through the Plasma Membrane
The plasma membrane surrounding animal cells is where the exchange of substances inside and outside of cells takes place. Some substances need to move from the extracellular fluid outside cells to the inside of the cell, and some substances need to move from the inside of the cell to the extracellular fluid.
Active transport
When the molecules are too big to easily flow across the plasma membranes or dissolve in the water, they can be filtered through the membrane. In these cases, the cells must put out a little energy to help get molecules in or out of the cell.
Embedded in the plasma membrane are protein molecules, some of which form channels through which other molecules can pass. Some proteins act ascarriers — that is, they are “paid” in energy to let a molecule attach to itself and then transport that molecule inside the cell.
Passive transport of molecules
A membrane can allow molecules to be passively transported through it in three ways: diffusion, osmosis, and filtration.
· Diffusion: Sometimes organisms need to move molecules from an area where they are highly concentrated to an area where the molecules are less concentrated. This transport is much more easily done than moving molecules from a low concentration to a high concentration. To go from a high concentration to a low concentration, in essence the molecules need to only “spread” themselves, or diffuse, across the membrane separating the areas of concentration.
· Osmosis: The diffusion of water (osmosis) works as described in the preceding bullet. However, with osmosis, the concentration of substances in the water is taken into consideration. If a solution is isotonic, that means the concentrations of the substances(solutes) and water (solvent) on both sides of the membrane are equal. If one solution is hypotonic, there is a lower concentration of substances (and more water) in it when compared to another solution. If a solution ishypertonic, there is a higher concentration of substances in it (and less water) when compared to another solution.
e.g., the blood in your body contains a certain amount of salt. The normal concentration is isotonic. If suddenly there is too high a concentration of salt, the blood becomes hypertonic (too many salt molecules). This excess of salt forces water out of the blood cells in an attempt to even things out. But the effect this action has is actually that of shrinking the blood cells.
This shrinking of cells is called crenation . If too much fluid is in the bloodstream, the blood cells have too few molecules of salt in comparison, making them hypotonic. Then, the blood cells take in water in an attempt to normalize the blood and make it isotonic. However, if the blood cells need to take in too much water to bring everything back into balance, they can swell until they burst.
Filtration: The last form of passive transport is used most often in the capillaries. Capillaries are so thin (their membranes are only one cell thick) that diffusion easily takes place through them. But remember that animals have a blood pressure. The pressure at which the blood flows through the capillaries is enough force to push water and small solutes that have dissolved in the water right through the capillary membrane.
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