Answer:
The myelin sheath are the glial cells which present in the neurons. It acts as an insulator for the impulse. Inside the myelin sheath the impulse propagates in high speed. Because there is no depolarization occured in it. Therefore, the nerve impulse jumps from one sheath to other and it is called saltatory movement.
The gap between the two myelin sheath is called nodes of ranvier. It contains sodium ions and have voltage gated channels. When the action potential passes through the myelin sheath, the depolarisation happens due to entry of excess sodium ions into the non - myelinated sheath / nodes of ranvier.
When there is excess sodium ions present in the axon, again repolarization occurs and action potential occurs in the myelin sheath. Myelin sheath contains lipids which enhance the action potential.
<u>Phenylketonuria</u>
Explanation:
Phenylketonuria (PKU) is a type of a rare inherited metabolic disorder.
Etiology: PKU results from excessive accumulation of the amino acid phenylalanine in the body. Normally, phenylalanine will be converted to tyrosine (forms neurotransmitters) by the action of the enzyme phenylalanine hydroxylase.
Among people with PKU, a defect in the PAH gene which codes to synthesize this enzyme occurs which leads to the absence of this enzyme. So the phenylalanine which is normally present in dietary protein is not metabolized and gets accumulated. This gene is inherited by the child only if both the parents are with the defective PAH gene.
Symptoms: Stunted growth, behaviroral disturbances, learning difficulties, seizures, tremors etc.
Treatment: Basic treatment is to follow a diet which limits dietary sources of phenylalanine. High protein foods like meat, egg should be avoided. Children can be given PKU formula to supplement their nutritional needs.
a. it limits water loss through transpiration
The stomata are small openings in epidermis (found on the leaf) that allow carbon dioxide, water, and oxygen to enter and exit the leaf. Transpiration is water movement, including evaporation, in a plant. So just think about it. If it's a hot, sunny day, wouldn't it make sense for the plant to keep its stomata closed so that it doesn't lose water through evaporation? The cacti would close it during the day and leave it open at night since the sun isn't out, and therefore wouldn't lose as much water. Hope this helped :)