In simple words, an abstract is a summary of the contents of a book, presentation, article or a speech. You would see abstracts in the beginning of any scientific research paper. Usually in universities and research institutions, if someone is going to do a presentation or a speech, he will send out an abstract to his audience so they will have an idea of what he is going to present beforehand. Also in some books (usually research based publications), you will see an abstract in the beginning of each chapter.
The idea is to summarize a lengthy article, discussion or a speech and to give condensed yet complete overview of the full work. Usually publishers or organizers of scientific conferences indicate word limits for the abstract (both lower and upper limits). Since you (or your teacher) have not indicated such limits, I am going to keep my abstract to 100-200 words.
When you write an abstract, its important to write an outline of the subject being discussed. That way you will not miss the key elements.
1. What is osmosis?
2. Where does it happen?
3. What is the significance?
Osmosis is the net movement of solvent through semi-permeable membrane into a region of higher solute concentration (lower solvent concentration). This is a spontaneous reaction and it acts as a mean of equalizing solute concentrations. Osmotic pressure is defined as the minimum pressure which is required to be maintained so that there is no net movement of solvent. Osmosis is a key mechanism in biological processes especially in cells. Osmosis is responsible for drawing up water by plants from soil and also play a key role in animal cells when maintaining optimum solute concentrations. Reverse osmosis is variation of osmosis where an external pressure is applied to reverse the movement of solvent, i.e.; to move the solvent from the region with higher solute concentration to lower solute concentration and it is widely used in the industry as a way of concentrating chemical solutions.