Homeostasis is maintained by negative feedback loops within the organism. In contrast, positive feedback loops push the organism further out of homeostasis, but may be necessary for life to occur. Homeostasis is controlled by the nervous and endocrine systems in mammals.
Answer;
The mouth
For ingested foods, the first opportunity for enzymatic digestion occurs in the mouth.
Explanation;
The digestion of food starts in the mouth.
There is both mechanical and enzymatic digestion of food in the mouth. Mechanical digestion involves the mechanical break down of food using teeth to smaller particles thus increasing the surface are for the activity by enzymes.
Enzymatic digestion in the mouth involves enzyme salivary amylase which acts on starch to form maltose.
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<u>Answer</u>: The correct statements are-
1. At any stage of mitosis a cell has twice as much as DNA in G1.
2. Metabolically active cells can exit, and sometimes re-enter, the cell cycle.
<u>Explanation</u> :
1.. The cell cycle consists of <em>4</em> distinct phases - M phase which is the mitotic phase and the G1, S and G2 phase which together form the interphase. The interphase is preparatory phase wherein the cell grows in size, replicates the DNA etc. whereas the M phase is the one where events related to cell division takes place.
2. The cell has <em>2C</em> amount of DNA during G1 but since the DNA replication takes place during S phase, the amount of DNA <em>doubles up to 4 C</em> and remains so in G2 and M phase until cytokinesis results in the formation of 2 daughter cells.
3. Sometimes, the metabolically active cells can exit the cell cycle and become <em>quiescent</em> until the conditions are favourable. When these quiescent cells recieve the right stimulus they can re enter the cell cycle.
4. Extracellular conditions such as nutrient availability, presence of certain mitogens, growth factors etc. do regulate the cell cycle progression. <em>If any of these extracellular facotrs are limiting the cell cycle is arrested.</em>
5. Once the cell cycle starts, it can be stopped without leading to cell death. When this happens it is known as <em>cell cycle arrest. </em>During this arrest several factors are assessed and if theire is need for repair of DNA then that also takes place. Once, the limitations are overcome or the repair is done, the cell cycle can then continue.
