Function of lactose in newborn mammals
Lactase and its function
Lactose, which can easily be obtained from milk, is the main source of nutrients for most newborn mammals. It is a form of carbohydrates which in order for it to be absorbed by the small intestine, first requires to be broken down into its two simpler compounds: glucose and galactose. In this way, the lactose can be absorbed by cells lining the small intestine (Swagerty, Wailing & Klein 2002).
The lactase gene controls the lactase enzyme which affects the way in which lactose is absorbed in our body. Like all other enzymes, lactase, which is located on the microvilli of the small intestine, has the ability to increase the rate at which lactose break down (Huxley & Walter 2005).
Lactase has a complimentary shape to the Lactose molecule. A lactose molecule then attaches itself to the lactase enzyme where reaction rate increases. When the products, glucose and galactose, is formed, it no longer fits in the enzyme and hence leaves lactase. Lactase is free to be used in the breaking down of other lactose. It is not consumed in the reaction. (Huxley & Walter 2005). Figure one shows how the lactase enzyme functions.
People that have insufficient amounts of enzymes or genetically mutated, would therefore be unable to break down lactose. When this happens, the undigested lactase goes to the large intestine where it begins to ferment (Weise 2009). During fermentation, lactic acid, along with other organic acids, such as hydrogen and carbon dioxide cause an increase in moisture in the large intestine. Therefore causing symptoms like diarrhoea (Lactase 2009).
Most mammals generally only produce the lactase enzyme during the first two to three years of life. Beyond that, the lactase enzyme activity decreases due to the decrease intake in lactose (Bowen 2009). In the first few years of life, newborns obtain their nutrients from milk which contains lactose. Hence, it is said that adult mammals are lactose intolerant while new borns are lactose persistent.
Mutations and natural selection
A study done by Cornell University finds that descendents who has ancestors whom came from places where there is access to dairy herds have evolve to be lactose persistent (Science Daily 2005). Whereas in other places breeding of the dairy herds is unsuitable due to its biotic and abiotic factors such as its hot climate, geographical features or the presence of disease. People living in such places have the tendency not to have access to dairy products as a source of food beyond its infant years.
As lactose is not consumed, it is not necessary for the existence of large amounts of lactase in the body. Presence of lactase only takes up additional metabolic energy for such lactase to function. Over thousands of years, over many generations, the organism evolves and hence the lactase in mammals' intestine starts to disappear. Therefore, adults are unable to digest lactase and are known as lactase intolerant.
Likewise, if people have ancestors that live in places where the diary products are available to them, the lactase enzyme is necessary to be present to break down the lactose consumed. Therefore, these ancestors' descendents are more likely to have lactase enzymes at an adult age.
Most adult mammals are lactose intolerant. Hence, mutations are required in order for lactase persistant adults to exist. A possible scenario would be where mutations occur in a population. There are two types of mutations: somatic and germinal mutations. Somatic mutation occurs in all cells in the body except the reproductive cells which are the sperm and egg cell. Therefore, these mutations would not be passed on to the next generation (Schmitt 2010). However, germinal mutations, unlike somatic mutation, affects and alters the egg and sperm cell. It is evolutionary and therefore can be passed on from a generation to the next.
Tolerance to lactose is an inherited condition. Therefore the environment has little to no impact on this condition. Mammals may present either of the three genotype: homozygous dominant which are lactase persistent, homozygous recessive which are lactase non-persistent and heterozygous which carry both the lactase persistent and lactase non-persistent alleles (Laaksonen et al. 2009). However, it is known that heterozygous shows mild condition of lactose intolerance. This explains that lactose tolerance is a germinal mutation. Hence, further explaining that it may be due to alteration of genes during mutation.
The affected individuals in the population then proceed on to reproduce. Lactose tolerance is an advantageous feature as lactose tolerance individuals would have an extra source of food than compared to the rest of the population. Hence, increasing its fitness. This trait of lactose tolerance is then passed on to the next generation and so on. Therefore explaining the reason that humans are lactose tolerance. This process is called natural selection.
Natural selection is a crucial and essential process for evolution to occur. It occurs when there are a variants in a population. The variant with a higher fitness where they are able to survive better and have successive reproduction, would then pass on their trait to its offsprings.