Membranous channel extending inward from muscle fiber membrane is called

Look at the picture below . what does this picture represent ?

Agroup of students are walking in the park, and one of them takes a picture of a pollen grain that is being blown by the wind.what caption can the student use for this picture? fahrte "littl1111111nimmt-hhhfull film# # #gene mutation in actiongene flow at workgenetic drift as it happensnatural selection in progresshii

What kinds of molecules can pass through the cell membrane without any problem?