Active transport requires energy from the cell. It occurs when substances move from areas of lower to higher concentration or when very large molecules are transported. Types of active transport include ion pumps, such as the sodium-potassium pump, and vesicle transport, which includes endocytosis and exocytosis.
        
             
        
        
        
hope this helps 
I do think that they close to not allow too much of either water or air from entering. 
the hollow tube allows water to flow easily.
https://www.bbc.co.uk/bitesize/guides/z28g7p3/revision/1
 
        
             
        
        
        
Answer:
What is a good example of a response-based definition of stress?
Bruce experiences stress because his heart rate increased when he noticed Jack had a knife.
Explanation:
Stress can have many different causes. In this case, it is produced by seeing danger in Jack by having a knife. His heart rate increases because this is a common response in stress, which would enable people to scape. It is important to control stress lebels.
A stressor is distinguished as a threat. If someone anticipates that it could lead to some kind of harm, loss, or other negative consequence. However, if you see the positive side and believe that it carries the potential for gain or personal growth, it would be appraised as a Challenge.
 
        
             
        
        
        
Answer:
Methanogens are group of prokaryotes that aids digestion in cattle, deer, and other animals that obtain nutrition from cellulose.
Explanation:
Methanogens are microorganisms capable of methane production. They belong to the domain Archaea. Methanogenic archaea which reside primarily in the rumen produces methane through the reduction of methanol with hydrogen. Methane produced by the methanogens aids in keeping the hydrogen concentrations low in the rumen, this allows the growth of other species which in turns enable more efficient fermentation.
The majority of the anaerobic microbes which occupy the rumen initiate the fermentation process thereby assisting the cellulose breakdown. Methanogens uses the end products of fermentation as substrates to reduce carbon dioxide, formic acid, or methylamines to methane.
<u>Some terms explained:</u>
Prokaryotes:  are unicellular organisms and they do not have nucleus that houses their genetic material. Examples are organisms like <em>bacteria</em>, <em>methanogens.</em>
Ruminant: They include cattle, deer, goat. They have a unique digestive system which comprises of four-compartment stomach (rumen, reticulum, omasum, and abomasum). With the help of microbes in the specialized somach, they are able to ferment ingested forages to aquire nutrients.