NCERT Solutions for Class 11 Biology Chapter 8 The Unit of Life

Ans : (a) Robert Brown did not discover the cell. The cell was discovered by Robert Hooke. 

Ans : (c) According to the biogenic theory, new cells can only arise from pre-existing cells. Only complete cells, in favourable conditions, can give rise to new cells. 

Ans : (c) Membrane-bound organelles are organelles surrounded by a double membrane. Nucleus, mitochondria, chloroplasts, etc., are examples of such organelles. These cell organelles are absent from prokaryotes. 
(a) Only eukaryotic cells have nuclei. They are absent from prokaryotes. 
(b) Cell walls are only present in plant cells. They are absent from all animal cells. 
(d) All cells arise from pre-existing cells. 

Ans : Mesosome is a convoluted membranous structure formed in a prokaryotic cell by the invagination of the plasma membrane. Its functions are as follows: 
(1) These extensions help in the synthesis of the cell wall, replication of DNA. They also help in the equal distribution of chromosomes into the daughter cells. 
(2) It also increases the surface area of the plasma membrane to carry out various enzymatic activities. 
(3) It helps in secretion processes as well as in bacterial respiration. 

Ans : Plasma membrane is the outermost covering of the cell that separates it from the environment. It regulates the movement of substances into the cell and out from it. It allows the entry of only some substances and prevents the movement of other materials. Hence, the membrane is selectively-permeable. 
Movement of neutral solutes across the cell membrane — Neutral molecules move across the plasma membrane by simple passive diffusion. Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration. Movement of polar molecules across the cell membrane — The cell membrane is made up of a phospholipid bilayer and proteins. The movement of polar molecules across the non-polar lipid bilayer requires carrier-proteins. Carrier-proteins are integral protein particles having certain affinity for specific solutes. As a result, they facilitate the transport of molecules across the membrane.