CLASS IX
BIOLOGY
ASSIGNMENT 6
THE FUNDAMENTAL UNIT OF LIFE
GIST OF THE LESSON
• Cell is the basic structural and functional unit of life.
• Robert Hooke coined the term ‘cell’.
• Robert Brown discovered the nucleus in the cell in 1831.
• Purkinje in 1839 coined the term protoplasm for the fluid substance of the cell.
• Robert Brown discovered the nucleus in the cell in 1831.
• The Cell Theory states that:
a) All plants and animals are composed of cells.
b) Cell is the basic structural and functional unit of life. This was presented by two
scientists, Schleiden and Schwann.
c) It was further expanded by Virchow, suggesting that all cells arise from pre-existing
cells.
• A compound microscope uses two or more lens to produce a magnified image of an object,
known as a specimen, placed on a glass slide on the stage of a microscope. A plano-concave
mirror is used for reflection of light rays into the microscope.
• The electron microscope was discovered in 1940 by Knoll and Ruska. It is a microscope with
high magnification and resolution employing electron beans in presence of light and using
electron lenses.
• A temporary mount of onion peel (a typical plant cell) under compound microscope shows a
number of similar rectangular cells with an outer cell wall closely packed with each other.
Each cell has a cell membrane, cytoplasm, and a small oval nucleus at the periphery.
• Human cheek cells (a typical animal cell) under compound microscope appear as a number
of flat polygonal cells each with a distinct cell membrane, cytoplasm and a central rounded
or oval nucleus are observed. Inter-cellular spaces can be seen. Such cells are known as
squamous epithelial cells which line the inner surface of our mouth cavity.
• Cells exist in different shapes. Most cells have fixed shape, for example bone cell, nerve cell,
smooth muscle cell, ovum, sperm etc. Some cells have changing shapes like white blood cells
and Amoeba.
• The shape of cells is related to the specific function they perform. A nerve cell has a long
axon and projections to send signals over long distances. Disc-shaped RBCs carry oxygen and
travel through small blood vessels. Xylem in plants are tube-like and hollow to carry water
and dissolved minerals.
• Multicellular organisms like most fungi, higher plants and animals are composed of many
cells organised into tissues, organs and organ systems. There exists division of labour as
different cells are specialised to perform different life functions.
• The plasma membrane is the outermost covering of an animal cell that:
a) separates the inner contents of the cell from its external environment
b) helps in transporting some materials in and out of the cell.
c) It allows some materials while restricting others. So, it is selectively permeable.
• On the basis of permeability, membranes can be of the following types:
1. Impermeable membrane- It does not allow any substance to pass through eg.
Suberised cell wall.
2. Permeable membrane- Permits both solute and solvent to pass through eg.
Cellulosic cell wall.
3. Semipermeable membrane- Allows only certain molecules ( solvent) to pass
through and not the others(solute). Eg. Egg membrane.
4. Selectively permeable membrane-Allows some substances (solvent and some
solutes) to pass more readily than others. Eg. plasma membrane.
• Passive transport is the transport of molecules and ions along concentration gradient
without input of energy.
• Active transport is the transport of a substance against its concentration gradient with the
help of ATP.
• Common types of passive transport are Diffusion, Osmosis, Facilitated diffusion.
• Common types of active transport are Endocytosis, Exocytosis, Protein pump.
• Plasma membrane is made up of a phospholipid bilayer with embedded proteins (integral
and peripheral) and carbohydrate on the outer surface; glycolipids, glycoproteins and also
cholesterol in animal cells. The components have a mosaic nature and can flow and change
position easily in a membrane.
• Singer and Nicolson (1972) proposed the currently accepted fluid mosaic model of plasma
membrane.
• Diffusion is the spontaneous movement of Substances from an area of high concentration to
an area of low concentration until equilibrium is reached. Eg.: Movement of CO2 and O2
across the cell membrane.
• Osmosis is the movement of solvent molecules through a semipermeable membrane from a
less concentrated solution to a more concentrated one. (i.e with lower concentration of
solvent).
• Osmosis is a special case of diffusion through a semipermeable membrane.
• Plant roots also absorb water by osmosis.
• Facilitated diffusion, another type of passive movement is the transport of substances across
a biological membrane via transmembrane carrier proteins.
• When a cell is put in Hypotonic solution, it gains water by osmosis (endosmosis) and is likely
to swell up.
• When a cell is put in Isotonic solution, there is no net movement of water across the cell
membrane.
• When a cell is put in Hypertonic solution, the cell loses water by osmosis (exosmosis) and the
cell will shrink.
• Endocytosis is a cellular process in which substances are brought into the cell. The material
to be internalized is surrounded by an area of cell membrane, which buds off inside the cell
to form a vesicle containing the ingested material.
ASSIGNMENT 6
1. Which among the following is not a component of a compound microscope :
a) Convex mirror
b) Objective lens
c) Condenser
d) Eye piece
2. The word ‘cell’ is derived from the latin word ‘cellula’ which means “a little room”. Can
you name the scientist who coined the term cell?
a) Robert Hooke
b) Antony Van Leeuwenhoek
c) Robert Brown
d) Ernst Haeckel
3. Amoeba acquires its food through a process, termed
a) exocytosis
b) endocytosis
c) exosmosis
d) exocytosis and endocytosis both.
4. Absorption of water by plant roots occur by the process of _____________.
5. __________ coined the term ‘protoplasm’ for the fluid substance of the cell.
6. Name a pink stain used for observing plant specimen under compound microscope.
7. State the process by which gaseous exchange occurs in unicellular organisms.
8. A de-shelled egg is placed in pure water in a beaker. What would you observe after
15min?
9. State the composition of plasma membrane.
10. Define osmosis.
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DATE OF ASSIGNMENT: 13.06.2020
DUE DATE: 20.06.2020
