The homeostasis of the human body works like a machine set at a particular standard. If the factors deviate from the standard values, the homeostatic mechanism come into action.
For example, if a cell requires certain protein, the respective genes get a signal and get activated. The cell start synthesizing the protein and when sufficient amount is made, there is a feedback loop, which allows the same protein to stop the synthesis as well. A lot of organs and cellular systems are involved in regulating the synthesis of this protein.
This is similar to the cooling system. If we set the temperature of the cooling system to certain degree, it will start cooling the room till the required temperature is reached. As soon as the temperature is acquired, the system cut-off automatically and the required temperature is maintained.
Replication of the DNA and cell growth occur during the S-Phase of the cell cycle.
Lets look at your first question. A food chain is a linear (line) relationship between organisms. It shows who eats what and who gets eaten. The energy is passed up the chain from one organisms to the next. A food web is a series of interconnected food chains. It can show one organism being preyed upon by several predators. The answer choice A is really referring to an energy pyramid, but it is true in any ecosystem that only 10% of available energy is passed to the next trophic level. B and C are both true statements as well. D is completely false because 90% of energy is lost, not retained. So with that said, there is more than one correct answer here. I have included a diagram so you can see the difference between the two.
For Question 8, the key to understanding this is that sunlight is the main source of energy for any ecosystem. Plants (autotrophs) use the sunlight to convert it to sugars (glucose) which is a compound that heterotrophs can utilize. This process is called photosynthesis. :-)
Question 9: So in a food chain, you typically start with the producers (plants), then to the primary consumer, then secondary consumer, then tertiary consumer, and in some cases you may have a quaternary consumer. That is 4 links. However, we are forgetting the role of decomposers at all levels of this food chain, and that would add an extra link. In summary, you wouldn't typically see more than 5 links in a food chain.
I hope all of my explanations are helpful! Good luck! :-)
The main responsible for the delay among the atrial and
ventricular contractions is the action potential conduction velocity of the AV
node cells. If the atria and ventricles contract simultaneously, atria cannot
give any help in filling the ventricles, which is needed, especially in those
with stiff ventricles as in the elderly. Hence atria have to contract first and
then the ventricles.