Saturday, May 25, 2013

9th THE FUNDAMENTAL UNIT OF LIFE: Solved [Study Guide By JSUNIL ]

Discovery of Cell Robert Hooke at first examining a thin slice of dead cork [the bark of a tree] through a self-designed microscope in 1665. Robert Hooke predicts that the cork resembled the structure of a honeycomb consisting of many little boxes. He called these boxes cells. He described about this in his book “Micrographic”.

 Cell is a Latin word which means ‘a little room’. The word cell was derived from a Greek word “Cellulae” which means small room. First living cell was discovered by A.V. Leeuwenhoek with the improved microscope in pond water.
Q. What are Living Organisms made up of?

Answer: All living organisms that we observe around us are made up of cells.

Q. Who gave the cell theory? What does it state? Which organism is an exception of cell theory?

Answer: Two biologists, “Schleiden and Schwann” gave the “Cell theory” which was later on expanded by “Rudolf Virchow”. Cell theory states that

(a) All plants and animals are composed of cells. (b) Cell is the basic unit of life. (c) All cells arise from pre-existing cells.

=>Viruses are the exceptions of cell theory.

Q. Why do we put peel off the skin of onion (called epidermis) immediately in a watch-glass containing water? Answer: This will help to keep peel wet.

Q. All cells come from pre-existing cells .Justify?

Answer: All organisms around are made up of cells. There are single cells organism called unicellular like amoeba where as some single body are made up of many cells called multi-cellular organism. Cells divide to produce cells of their own kind. Hence, all cells come from pre-existing cells.

Q. What are the types of cell on the basis of type of organization?

Answer: (i) Prokaryotic cells: Cells having less developed nucleus without nuclear membrane and nucleolus. E.g. Bacteria. These are primitive and incomplete cells.

(ii) Eukaryotic cells: Cells having well developed nucleus with unclear membrane and nucleolus. e.g. Plants and animals.

Q. How do all cells look alike in terms of shape and size?

Answer: Cells are of variable shapes and sizes. Their shape is according to the function. Generally cells are spherical but they may be long and branched(nerve cell), Kidney shaped (guard cell in plant’s leaves), discoidal (RBC), spindle shaped (muscle cell) etc.

Size of cell varies from 0.2 mm to18 cm in diameter. Some are microscopic while some are visible with naked eyes.

For example:

=>  Size of a typical cell in a Multicellular organism ranges from 20-30 mn.

=> The largest cell is ostrich egg (15 cm. in diameter with shell & 8 cm. in diameter without shell)

=> The longest cell is nerve cell. (up to 1m. or more) and Red Blood cells are the smallest cell in our body.

=> Smallest cells so far known are PPLOs e.g. mycoplasma (0.1mm in diameter.)

=> Human egg is 0.1 mm. in diameter.

Þ Unicellular alga, Acetabularia is about 10cm long.

Q. How does a living cell perform basic functions?

Answer: a living cell perform basic functions by division of labour among specific components within it known as cell organelles.

In Text:

1. Who discovered cells, and how?

Answer: Robert Hooke at first examining a thin slice of dead cork [the bark of a tree] through a self-designed microscope in 1665 and discovered cell.

2. Why is cell called the structural and functional unit of life?

Answer: All living things are made up of small structures called cells. Cells also perform all the basic functions in all living things. This is why cell is called the structural and functional unit of life.

Q. What is a cell made up of?

Answer: A cell is made up of plasma membrane, nucleus and cytoplasm

Q. What is the function of plasma membrane?

Answer: plasma membrane is the outermost covering of the cell present in both plants [below the cell wall] and animal cell.

Singer and Nicholson gave the fluid mosaic model of plasma membrane according to him it consists of a protein layer between two layers of lipids.

(a) It separates the contents of the cell from its external environment.

(b) It allows the entry and exit of some materials in and out of the cell.

Q. How does the movement of substances take place into the cell?

Answer: Some substances like carbon dioxide or oxygen can move across the cell membrane by a process of diffusion whereas water can move across the cell membrane by a process of osmosis.

Q. What are diffusion and osmosis?

Answer: Diffusion is a spontaneous movement of a substance from a region of high concentration to a region to a region of concentration is low through a selectively permeable membrane.

The movement of water molecules through a selectively permeable membrane from a region of high water concentration to a region of low water concentration is called osmosis. Therefore, osmosis is a special case of diffusion through a selectively permeable membrane.

Q. How do substances move out of the cell?

Answer: As there is a difference of concentration of CO2 inside and outside a cell, CO2 moves out of the cell, from a region of high concentration, to a region of low concentration outside the cell by the process of diffusion.

Q. What will happen if we put an animal cell or a plant cell into a solution of sugar or salt in water?

Answer: The cell swell up as the cell will gain water by osmosis. This happens because the concentration of the solution outside the cell is lesser than that of cytoplasm of cell.

Q. What are isotonic solution, hypertonic solution and hypotonic solutions?

Answer: Types of solution on the basis of concentration:

(A)Isotonic solution: When the concentration of the solution outside the cell is equal to the concentration of cytoplasm of the cell it is called as isotonic solution.

(B) Hypertonic solution: When the concentration of the solution outside the cell is less than the concentration of the solution inside the cell it is called as hypertonic solution.. Due to this cell looses water and shrink and becomes plasmolysed.

(C) Hypotonic solution: When the concentration of the solution outside the cell is more than that of cytoplasm of cell. Due to this cell swells up and bursts.

Q. Remove the shell of an egg by dissolving it into dilute hydrochloric acid. Put the egg in pure water and left it for 5 minutes. What do we observe?

Answer: The egg swells because the cell will gain water by osmosis. This happens because the medium surrounding the cell has a higher water concentration than the cell

Q. When we put dried raisins or apricots in plain water and leave them for some time they swell. Why?

Answer: Each dried raisins or apricots gains water by osmosis and swells when placed in water.

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