Monday, April 4, 2011
Cellular Transport
Shubin's book covers a lot of important topics that we learned this year. However, one topic that is barely covered is cell transport. Without cellular transport, all of the systems in our bodies would be rendered useless. Explain the different types of transport and the importance of osmosis.
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The simplest type of cell transport is passive transport because a cell does not have to use any energy to make it happen. Passive transport, in scientific terms, is described as “the diffusion of a substance across a biological membrane” (Campbell 132). Diffusion is “the movement of molecules of any substance so that they spread out evenly into the available space” (Campbell 132). In other words, molecules will naturally spread out into areas of smaller concentration. For example, water will naturally diffuse out of a cell if the inside of a cell contains a higher concentration of water than its surroundings.
ReplyDeleteAnother type of cell transport is facilitated diffusion, “the spontaneous passage of molecules or ions across a biological membrane with the assistance of specific transmembrane transport proteins” (Campbell G-14). There are two types of transport proteins: channel proteins and carrier proteins. These transmembrane proteins exhibit the relationship between structure and function. Channel proteins literally form small tunnels that allow specific molecules or ions to cross the membrane (Campbell 134). The shape of these proteins is similar to tubes that connect a cell’s inside to its outer-environment, a perfect shape for a protein that simply allows molecules to move across a membrane while using virtually no energy. A specific example of a channel protein is an aquaporin, a channel protein for water. There are also ion channels, “which open or close in response to a stimulus… The stimulus may be electrical or chemical” (Campbell 135). Ion channels are usually closed, meaning no ions are allowed to pass through the membrane, but when stimulated by a specific stimulus, the channel literally opens and allows the passage of certain ions. Carrier proteins are proteins that “seem to undergo a subtle change in shape that somehow translocates the solute-binding site across the membrane” (Campbell 135). For example, when the specific carrier protein for glucose binds with a molecule of glucose at its solute-binding site, the protein changes its shape in such a way that the solute-binding site is moved from facing the cell’s environment to facing the inside of the cell. The glucose molecule is then detached from the binding site and moves into the cell. The structure of these proteins is like a “lock and key” relationship between an enzyme and its substrate. The binding site for the glucose carrier protein is shaped in such a way that only glucose can bind to it, and when the binding site attaches a molecule of glucose to the protein the molecule is pushed into the cell.
Osmosis is “the diffusion of water across a selectively permeable membrane” (Campbell 133). This movement of water is crucial to the balance of water inside the cells of living things, such as animals. Osmosis is specifically involved in tonicity, “the ability of a solution to cause a cell to gain or lose water” (Campbell 133). Animal cells live best in an isotonic environment, when there is no net movement of water in or out of the plasma membrane. However, when an animal cell is put into a hypertonic environment, one that has a higher concentration of solute, it shrivels because the net movement of water moves outside the cell. The opposite of hypertonic is hypotonic, an environment that has a lower concentration of solutes than the cell. In this case, water moves into the cell and the cell is lysed, which causes the cell to burst because its plasma membrane can’t deal with the pressure posed by the net movement of the water going into the cell. All in all, osmosis is a major factor in the health of a human body cell.
Mikey Ling (mikeyling@ymail.com)
the following response has been posted as two seperate comments
ReplyDeleteI believe that the most important point that Mikey covered is that passive transport occurs without the use of energy. The ability to transport important molecules like oxygen, water, and carbon dioxide without using energy is what allows organisms to thrive. For example, as we all know, oxygen is needed in order for humans to survive. Oxygen is used in cellular respiration and this is the basis for all life. Cellular respiration results in the formation of ATP which then serves as the energy source for all functions in the body. You can say that the “benefit” of this process is ATP. In order for the oxygen to reach all the cells of the body, it has to travel through the mouth, into the lungs, into the heart, then finally to the cells of the body. Before it can reach the cells, it has to have a means to get there. If Oxygen required energy to get to the cells, the “cost” or amount of ATP used might exceed the amount that the oxygen would produce. Then, the body wouldn’t be able to generate ATP for other uses such as the digestion of polysaccharides or signaling cells to attack pathogens in the body (1). That is why it is important that oxygen can diffuse by concentration gradient from the alveoli of the lungs into the capillaries of the body without the use of energy (Campbell 920). In other words, the “cost” of making ATP can’t outweigh the “benefits” of its production.
This cost vs. benefit relationship is an important. Governing under Darwin’s theory of natural selection, mathematical formulas like this one are being used to determine how every action and behavior contributes to Darwinian fitness (2). Darwinian fitness is the relative success of one organism in surviving and reproducing to pass on its genes to its offspring (3). Furthermore, natural selection dictates that an adaptation in one member of a species leads to a selective advantage which allows those that possess it to have a better chance at obtaining important resources and thus passing down these traits to their offspring until eventually all members of the species have the selective advantage (Campbell 15). The cost vs. benefit relationship is better known throughout the biology world as the optimization theory. Recently, this theory has fallen under scrutiny. Changes in the mathematical formulas used have now pointed to the idea that the evolutionary idea of adaptation of one member in a society is untestable rendering it unscientific and useless. This would mean that the idea of optimization is completely and utterly meaningless. However, it can be proven that individual components of this theory are reliable therefore proving the optimization theory actually does make sense. By looking at how sex ratios affect a species, mammals move in their community, and foraging strategies, it is liable that the optimization theory does place. Couple that with the possibilities of adaptations that create a selective disadvantage for certain members of a species and it is easy to see that optimization although itself isn’t provable, clearly can be used to determine how well members of a certain species are able to gain access to resources (2).
This ability to gain access to resources better than other members of the same species is the backbone of the evolutionary process. Selective advantages or disadvantages all become meaningless if they don’t increase the ability to access resources for an animal. In the simplest way, how the resources are attained, and if they can be attained using the least amount of energy, will allow the animal the best chance to survive and reproduce. This relates back to the optimization theory and is known as the optimal foraging model (Campbell 1133). Although this isn’t the same theory that relates to diffusion, they follow the same principles. In our society today, this type of behavior might be referred to as laziness. However, science proves that although it is most certainly bad to sit on the couch all day, it might benefit a person better if someone were to bring them all their food and feed them directly. Although, by the same token, that person may never be able to fend for themselves and that idea itself completely encompasses the cost vs. benefit relationship.
ReplyDelete1. http://www.brighthub.com/science/genetics/articles/25894.aspx
2. http://courses.cit.cornell.edu/jdv55/teaching/167win10/maynard%20smith%2078%20-%20optimization%20theory.pdf
3. http://www.biology-online.org/dictionary/Darwinian_fitness
-Robbie Thomashow
(diehardcubsfan93@comcast.net)
Although Mikey is correct in saying passive transport is the simplest and easiest form of cell transport, he is forgetting about the two other very important forms of cells transport. Active transport and bulk transport are very important for cells, too. Active transport is needed to carry molecules across the concentration gradient with the help of carrier proteins and the use of ATP (Campbell 135). The ability to use ATP and move molecules across the membrane is a selective advantage for animals and cells. This ties in the theme of evolution since the ability to actively transport larger molecules is benefit to those that have the ability to spend more energy. Although the process of active transport is by creating a gradient across the membrane to create a high concentration of ions on the outside, the benefit is that the cell is able to keep out unwanted molecules which could benefit the organism (http://staff.jccc.net/pdecell/cells/activet.html).
ReplyDeleteAs a matter of fact, humans are very efficient in this process and evolution has given us the selective advantage to use this transport. Along with other animals, the ability to do active transport allows the cells to maintain an isotonic concentration within the cell compared to the environment preventing the cells to lyse. This is a huge benefit to animals as less cell death means less energy spent to create more cells and a smaller chance of dying (http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/D/Diffusion.html#active). One famous pump to create the concentration gradient is the sodium potassium pump. This pumps works by pumping out sodium ions from inside the cell to outside the cell with the help of ATP. Then the energy from ATP changes the integral protein to fit the shape of the potassium ion outside the cell. The potassium ion then binds to the integral protein to be placed inside the cell to create the electrochemical gradient, and the protein returns to its original shape (Campbell 136).
Co-transport is a form of active transport, and symport is one method of co-transport that uses integral proteins to simultaneously transport the ion and the molecule into the cell (http://academic.brooklyn.cuny.edu/biology/bio4fv/page/sympo.htm). One example of symport in plants is the expulsion of H+ protons into the environment then passively transporting the H+ ion and sucrose into the cell (Campbell 137). Anti-port is the opposite because it transports two different molecules across the membrane in separate directions. While one molecule is moving into the cell, the other molecule is moving out of the cell (http://course1.winona.edu/sberg/ANIMTNS/symport.htm). These types of co-transport are an evolutionary benefit for larger animals nowadays because they have the ability to keep certain unwanted molecules out of the cells and certain wanted molecules in the cell. With this ability, cells are able to maintain a constant environment within the cell. Although, this takes up energy, ultimately, this process helps animals and plants survive.
Benny Jeong
bennyjeong218@gmail.com
This comment has been removed by the author.
ReplyDeleteMikey and Benny did a nice job of summarizing many of the different types of transport. An important type that they missed is bulk transport. Bulk transport is the direct expulsion or intake of fluids or particles. Intake occurs by invagination and expulsion occurs by evagination. This type of transport includes the processes of phagocytosis, pinocytsosis, endocytosis, and exocytosis (1).
ReplyDeleteEndocytosis includes the processes of receptor mediated endocytosis, phagocytosis, and pinocytosis. The plasma membrane of a cell captures particles in a vesicle by fusing together parts of its cell membrane. The process begins with invagination of the cell. By folding inwards, particles surrounding the cell are able to fill the now empty space. The ends of the invagination eventually begin to pinch when the space created by the fold becomes big enough. (Campbell 138).Phagocytosis is the ingestion of necessary particle by the cell and is known as “cell eating.” The new vesicle formed by this process enters the cell and fuses with a lysosome. The lysosome contains hydrolytic enzymes that will then digest the particle inside of the vesicle (Campbell 139). Pinocytosis is a type of endocytosis. The cell membrane invaginates around a substance. The invagination closes off and becomes a vesicle. Mainly, this process brings in substances that will dissolve in the cell and is therefore often referred to as cellular drinking (2). Receptor mediated endocytosis is a more specialized version of endocytosis. In the membranes of each cell are specific receptor proteins. Ligands bind to the receptor sites causing invagination and formation of vesicle around the ligand and the materials inside the ligand. An example of this process is the intake of cholesterol by cells. Cholesterol travels in LDLs that act as ligands. The cell has receptor sites in its membrane for this ligand. This is a more efficient process that ensures that the cell will receive certain particles it needs (Campbell 139).
Exocytosis is the secretion of particles from a cell by fusing vesicle containing the particles with the cell’s plasma membrane. The vesicle membranes and plasma membranes fuse when they come into contact by rearranging the lipid bilayers of each membrane. Insulin is an example of a product of exocystosis. Formed in the pancreas, insulin is secreted into the extracellular by exocytosis. Insulin then stops the use of fat for energy by glucagon. Without exocytosis, insulin wouldn’t be able to leave the cells it is formed in because it is too big and the owner of the cells will suffer from diabetes. (Campbell 138).
-Robbie Thomashow
(diehardcubsfan93@comcast.net)