Cell: Structure and Functions Class 8 Notes Science

Facts that Matter

• The basic structural units of an organism are the cells.
• Robert Hooke in 1665, observed a thin slice of cork under a magnifying device. He observed partitioned compartments which he termed as ‘cells’.
• The cells are microscopic but some cells like egg of hen, duck or ostrich are single celled and are big enough to be seen by the unaided eye
• Organisms show variety in cell number, shape and size:
  • The single celled organisms are called unicellular. For example, amoeba and paramecium. They do all the functions necessary for life in the single cell. Organisms made of more than one cell are called multicellular.
  • Pseudopodia: Some unicellular organisms such as amoeba have cytoplasmic projections which help them in locomotion and feeding. These projections appear and disappear as amoeba moves or feeds. Like amoeba, White Blood Cells (WBC) of our body are also unicellular and can change their shapes.
  • The smallest cell is 0.1 to 0.5 micrometre in bacteria. The largest cell measuring 170mm × 130 mm, is the egg of an ostrich.
  • The size of the cells has no relation with the size of the body of the animal or plant. The size of the cell is related to its functions. However, large animals or plants may have more cells  than smaller organisms. Nerve cell is the largest cell in humans and other animals.
• Cell Structure and Function:
  • Tissue: A tissue is a group of similar cells performing a specific function.
  • Organ: An organ is a group of tissues which together perform a specific/specialised function/functions.
  • Organ system: An organ system is a group of organs, in which each organ performs different functions. For example, digestion, assimilation and absorption in digestive system.
  • A living multicellular organism may have several organ systems to perform multiple functions.
  • A cell has cytoplasm, a jelly like substance which is bound by a cell membrane.
    The cell membrane is selective permeable that can allow certain substances but does not allow other substances to go out of the cell.
  • In plant cells, there is another thick covering called cell wall that provides rigidity, protection against variation of temperature, high wind speed, atmospheric moisture, maintenance of the plant cell structure etc.
  • The cytoplasm of a cell contains various cell organelles present between cell membrane and the nucleus, such as mitochondria, Golgi bodies, ribosomes etc.
  • Nucleus: It is generally spherical body located in the centre of the cell. Nucleus is separated from the cytoplasm by a membrane known as nuclear membrane. Nuclear membrane is porous and allows exchange of materials between cytoplasm and the nucleus. Cells may have or may not have nuclear membrane.
    •  Prokaryotes: The organisms which contain cell/cells having nuclear material without nuclear membrane (Prokaryotic cells) are called as prokaryotes. For example, bacteria and blue green algae.
    •  Eukaryotes: The organisms which have well organised nucleus with a nuclear membrane (eukaryotic cells) are eukaryotes.

Content of the Nucleus: Inside the nucleus, a spherical body is present, which can be seen under high magnification of microscope, called nucleus. Nuclear material contains thread-like structures called chromosomes. The chromosomes can be seen only when the cell divides.

Along the length of a chromosome genes are present. Genes help in inheritance or transfer of characters from the parents to the offspring. As there is inheritance of characters of both father and mother, the offspring may have some characters of the father and some characters of the mother.

Functions of Nucleus: (a) It acts as a control centre of all the activities of the cell.
(b) It helps in inheritance of character of parents (both father and mother) to the offspring.

• Protoplasm: It is known as the living substance. The entire content of the living cell including the cytoplasm and the nucleus is known as protoplasm.
• Vacuole: Vacuoles are fluid-filled structures surrounded by a membrane. Under the microscope they are blank-looking structures in the cytoplasm. Large vacuoles are common in plant cells. Vacuoles in animal cells are much smaller.
• Plastids: Plastids are found scattered in the cytoplasm of plant cells. They are of different colours.

In plants, green parts like leaves, plastids contain green pigment called chlorophyll. Chlorophyll containing plastids are called chloroplasts. They provide green colour to leaves or other parts of plants. Chlorophyll in the chloroplasts of leaves is essential for photosynthesis (that prepares food for the plant in presence of sunlight).

• Comparison of Plant and Animal Cells: Observe the Fig. 8.1. You will find some of the organelles in both the plant and the animal cells. But some organelles are present in plant cells but not in animal cells. See Table 8.1.