BOTANY - THE CELL

PART 2

- (UPSC,TNPSC,TRB Imporatant Notes)

CELL WALL
•    Cell wall is absent in animals.
•    Plant cells, in addition to the plasma membrane, have another rigid outer covering called the cell wall. The cell wall lies outside the plasma membrane.
•    The plant cell wall is mainly composed of cellulose. Cellulose is a complex substance and provides structural strength to plants.

1.    Plasmolysis
•    When a living plant cell loses water through osmosis there is shrinkage or contraction of the contents of the cell away from the cell wall. This phenomenon is known as plasmolysis (plasma → fluid; lysis → disintegration, decomposition).
•    Only living cells, and not dead cells, are able to absorb water by osmosis. Cell walls permit the cells of plants, fungi and bacteria to withstand very dilute [hypotonic] external media without shrinkage.
•    In such media the cells tend to lose water by osmosis. The cell shrinks, building up pressure against the cell wall. The wall exerts an equal pressure against the shrunken cell.
•    Cell wall also prevents the bursting of cells when the cells are surrounded by a hypertonic medium (medium of high concentration).
•    In such media the cells tend to gain water by osmosis. The cell swells, building up pressure against the cell wall. The wall exerts an equal pressure against the swollen cell.
•    Because of their walls, plant cells can withstand much greater changes in the surrounding medium than animal cells.

Cytoplasm
•    It is the jelly-like substance present between the cell membrane and the nucleus.
•    The cytoplasm is the fluid content inside the plasma membrane.
•    It also contains many specialized cell organelles [mitochondria, golgi bodies, ribosomes, etc].
•    Each of these organelles performs a specific function for the cell.
•    Cell organelles are enclosed by membranes.
•    The significance of membranes can be illustrated with the example of viruses.
•    Viruses lack any membranes and hence do not show characteristics of life until they enter a living body and use its cell machinery to multiply.

Nucleus
•    It is an important component of the living cell.
•    It is generally spherical and located in the center of the cell.
•    It can be stained and seen easily with the help of a microscope.
•    Nucleus is separated from the cytoplasm by a double layered membrane called the nuclear membrane.
•    This membrane is also porous and allows the movement of materials between the cytoplasm and the inside of the nucleus [diffusion].
•    With a microscope of higher magnification, we can see a smaller spherical body in the nucleus. It is called the nucleolus.
•    In addition, nucleus contains thread-like structures called chromosomes. These carry genes and help in inheritance or transfer of characters from the parents to the offspring. The chromosomes can be seen only when the cell divides.
•    Gene is a unit of inheritance in living organisms. It controls the transfer of a hereditary characteristic from parents to offspring. This means that your parents pass some of their characteristics on to you.
•    Nucleus, in addition to its role in inheritance, acts as control center of the activities of the cell.
•    The entire content of a living cell is known as protoplasm [cytoplasm + nucleus]. It includes the cytoplasm and the nucleus. Protoplasm is called the living substance of the cell.
•    The nucleus of the bacterial cell is not well organized like the cells of multicellular organisms. There is no nuclear membrane.
•    Every cell has a membrane around it to keep its own contents separate from the external environment.
•    Large and complex cells, including cells from multicellular organisms, need a lot of chemical activities to support their complicated structure and function.
•    To keep these activities of different kinds separate from each other, these cells use membrane-bound little structures (or ‘organelles’) within themselves.

Chromosomes
•    The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.
•    Chromosomes contain information for inheritance of features from parents to next generation in the form of DNA (deoxyribo nucleic acid)
•    Chromosomes are composed of DNA and Protein.
•    DNA molecules contain the information necessary for constructing and organizing cells. Functional segments of dna are called genes.
•    In a cell which is not dividing, this dna is present as part of chromatin material. Chromatin material is visible as entangled mass of thread like structures. Whenever the cell is about to divide, the chromatin material gets organised into chromosomes.
•    The nucleus plays a central role in cellular reproduction, the process by which a single cell divides and forms two new cells.
•    It also plays a crucial part, along with the environment, in determining the way the cell will develop and what form it will exhibit at maturity, by directing the chemical activities of the cell.

 
Prokaryotic Cells vs. Eukaryotic Cells
•    Organisms whose cells lack a nuclear membrane, are called prokaryotes (pro = primitive or primary; karyote ≈karyon = nucleus).
•    Organisms with cells having a nuclear membrane are called eukaryotes.
•    Prokaryotic cells also lack most of the other cytoplasmic organelles present in eukaryotic cells.
•    Many of the functions of such organelles are also performed by poorly organised parts of the cytoplasm.
•    The chlorophyll in photosynthetic prokaryotic bacteria is associated with membranous vesicles (bag like structures) but not with plastids as in eukaryotic cells.

 

Prokaryotes : defined nuclear region, the membrane-bound cell organelles are absent.

Eukaryotic Cells: have nuclear membrane as well as membrane-enclosed organelles.

Description	Prokaryotes	Eukaryotes
Organisms	Monera: Eubacteria and Archebacteria	Protists, Fungi, Plants and Animals
Meaning of name	Pro = before	Eu = after
	Karyon = nucleus	Karyon = nucleus
Evolution	3.5 billion years ago (older type of cell)	1.5 billion years ago
Uni-/multicellular	Unicellular (less complex)	Multicellular (more complex)
Cell wall	almost all have cell walls (murein)	fungi and plants (cellulose and chitin): none in animals
Organelles	usually none	many different ones with specialized functions
Metabolism	anaerobic and aerobic: diverse	mostly aerobic
Genetic material	single circular double stranded DNA	complex chromosomes usually in pairs; each with a single double stranded DNA molecule and associated proteins contained in a nucleus
Location of genetic information	Nucleoid region	Nucleus
Mode of division	binary fission mostly; budding	mitosis and meiosis using a spindle: followed by cytokinesis