Confidence votes 9.4K. Animal cells do not have cell walls, they only have cell membranes. The cell membrane mainly consists of a phospholipid bilayer, and also contains embedded proteins and cholesterol. Plant cell walls are mainly made of cellulose, bacterial cell walls are mainly made of peptidoglycan.
In the Northern Hemisphere, the official summer season begins with the June solstice, which takes place on June 20th, 21st or 22nd each year, and ends with the September equinox, which takes place on September 22nd or 23rd each year; the exact duration of summer depends on when the solstice and equinox will take place and can range from 92 to 95 ...
The year is divided into the four seasons based on the two equinoxes and the two solstices. The summer and winter solstices, which typically land around June 21 and December 22, mark the shortest and the longest days of the year. The autumnal and vernal equinoxes, which fall around September 23 and March 21, mark the points in the year when the day and the night are of equal lengths.