Tuesday, March 22, 2011
sense of smell
In chapter 8, Shubin discusses how being able to smell odors is a result of external factors sending impulses to the brain. Odor molecules are sucked into our nostrils, where nerve cells will transmit the odor to the brain, and tell us about the world around us. Many animals determine what is safe or hazardous to eat for survival by noting the smell of its food. Yet, the earliest single celled organisms, or plants even, don't have a sense of smell. What environmental factors have led to the selective advantages of being able to detect odor molecules? Why was it not necessary for plants or earlier organisms to be able to smell? How come smell is beneficial when there are five other senses to depend on for survival?
Subscribe to:
Post Comments (Atom)
This comment has been removed by the author.
ReplyDeleteThis comment has been removed by the author.
ReplyDeleteLike any sense, the sense of smell helps active animals locate things (food, mates, danger, etc.). As with many features involving our senses, the ability to smell is shared with our evolutionary relatives, with slight variations. Our noses have millions of olfactory cells, which are a special type of neuron located in the olfactory epithelium, a patch of neurons right behind the nostrils. These neurons have protein receptors on the outside of the cells that bind to specific olfactory molecules (scents). When such molecules are present, they attach to specific neuron receptors. This interaction is a lock-and-key mechanism, a pattern also seen between enzymes and their substrates and hormones and their receptors. The biological theme of structure and function is represented here, with each olfactory neuron receptor matching up to a single olfactory molecule. These neurons, once activated by their specific olfactory molecule, send a message up the olfactory nerve to the olfactory bulb in the brain, where all the activated neurons' signals are interpreted as a single smell. In human noses, we have about 5 million of these cells. Rabbits have 100 million, and dogs have about 220 million.
ReplyDeletePlants and bacteria don't smell because their evolutionary path was simply different. Bacteria are too small to have the need to actively seek out their food, and plants are immobile so being able to sense smell is not even very helpful. Our evolutionary ancestors have developed different numbers of cells because it was an advantage to those who had more. The dog with more cells found more food, and the rabbit evaded being eaten. It was not exactly "necessary" for any organism to be able to smell, it was just helpful.
Your Inner Fish
http://www.sirc.org/publik/smell_human.html
http://www.ncbi.nlm.nih.gov/pubmed/21086527
http://www.wisegeek.com/how-does-the-sense-of-smell-work.htm
Jeremy Solomon
imabum14@gmail.com
I agree with Jeremy that smell is used to locate different things based on stimuli in the environment. However, I think the reason that animals evolved a sense of smell was based solely on their environment and the stresses that pushed for selective advantages. Among mammals, it is well developed in the carnivores and ungulates, or hooved mammals, which must always be aware of each other, and in those that smell for their food, like moles. Having a strong sense of smell is referred to as macrosmatic (Gilad 5). Some animals that are macrosmatic have developed that way because their other senses are very poor. For example, the sense of smell is less developed in the catarrhine primates (Catarrhini), and nonexistent in cetaceans, which compensate with a well-developed sense of taste (Hussein 3). This is because these primates are well protected in trees from most predators, so they do not need to smell them. They can distinguish food from the taste, and even if there’s a small amount of a harmful substance, the catarrhine will not eat it.
ReplyDeleteFish too have a well-developed sense of smell, even though they inhabit an aquatic environment. Salmon utilize their sense of smell to identify and return to their home stream waters. Catfish use their sense of smell to identify other individual catfish and to maintain a social hierarchy. Many fishes use the sense of smell to identify mating partners or to alert to the presence of food. Sharks especially have an incredibly powerful sense of smell. They can detect one drop of blood in a million drops of water (25 gallons or 100 liters) and can smell blood 0.25 mile (0.4 km) away. Its paired nostrils are on the underside of its snout. Water continually flows through the nostrils, giving the shark olfactory information. Unlike humans, shark nostrils have nothing to do with breathing - they are not even connected to the mouth. Some sharks, like the nurse shark, have sensory projections near the nostrils and mouth called nasal barbels. These barbels are whisker-like feelers used to taste and feel (Voganov 2).
benitorosenberg12@comcast.net
http://www.ncbi.nlm.nih.gov/pubmed/19237578
http://www.ncbi.nlm.nih.gov/pubmed/12612342
http://www.enchantedlearning.com/subjects/sharks/anatomy/Senses.shtml