CLASS: VIII NCERT (CBSE)
Cell-Structure and Functions
Introduction to Cells
With the help of a microscope, an English scientist,Robert Hooke, first discovered the existence of cells in 1665.Scanning electron microscopes are used to examine the external parts of various organisms.Thetransmission electron microscope is used to view the internal structure of a cell and its organelles.
Organisms that are made up of a single cell and perform all their vital activities, like reproduction, locomotion and digestion, are called unicellular organisms.Organisms that are made up of more than one cell are calledmulticellular organisms.Amoeba uses small finger-like projections called pseudopodia for locomotion and to capture prey.
Paramecium is a single cell and is built in such a way that it performs all its vital activities, like reproduction, locomotion, digestion, and so on. TheWhite Blood Corpuscle (WBC) is the only animal cell that changes its shape.The branched structure of aneuron helps it to transfer messages to all parts of the body.
Structure and Function of Cells
The black layer or the scab that you see on the wound is a result of the aggregation of dead Red Blood Cells (RBCs).The nose and the lungs form part of an organ system, the respiratory system.All organ systems work together to form a complex organism. The cytoplasm is a jelly-like fluid present between the nucleus and the cell membrane.The cell membrane allows the movement of minerals and other substances in and out of the cell.Thenucleus controls the activities of a cell.
Methylene blue is the stain placed on plant and animal cells to differentiate the nucleus under a microscope.Chromosomes are called vehicles of heredity because they carry genes and help in the inheritance or transfer of characteristics from the parents to the offspring.
Bacteria and blue-green algae that contain nucleoids are called prokaryotes.Organisms that have a well-organised nucleus are called eukaryotes.The nuclear material in unicellular organisms does not contain a nuclear membrane, and hence is called a nucleoid.
Thenucleolus is a small, spherical body in the nucleus.Chloroplast is a type of plastid involved in photosynthesis in plants.A vacuole is present in both plant and animal cells, but it looks much smaller in animal cells.