CELL – STRUCTURE AND FUNCTIONS class 9

 

 

BIOLOGY (class-IX)

 

Chapter-1:          CELL – STRUCTURE AND FUNCTIONS

 

Introduction

The term cytology is concerned with the study of the structure of the cell and its organelles, whereas cell biology is concerned with the physiological and biochemical aspects of the cell and its components.

Cell is the structural and functional unit of the living organism. It is also called as the building block of life. Each cell is comprised of living substance called protoplasm (the living content of the cell), which is composed of various constituents like water, ions, salts, organic molecules (proteins, carbohydrates, fats) and nucleic acids (DNA and RNA) etc.

The protoplasm contains the living constituents within the plasma membrane, including nucleus and cytoplasm while cytoplasm is the viscous semi-fluid like material enclosed within the plasma membrane or plasmalemma, embedded with in it are the various organelles.

THE CELL THEORY

Mathias J. Schleiden (1838) and Theodore Schwann (1839), proposed cell theory as per their observation on plant and animal cells respectively:

• All living things are composed of cells and their products.
• All cells arise from pre-existing cells.
• All cells are basically alike in chemical composition and metabolic activities.
• The function of an organism as a whole is the outcome of the activities and interaction of the constituent of cells.

Rudolf Virchow (1821–1902) gave his famous expression Omnis cellula e cellula, i.e., all cells arise from pre-existing cells. The four generalizations of the now famous cell doctrine are as follows:

• All living things are composed of cells and their products.
• All cells arise from pre-existing cells, either by cell division or cell fusion.
• All cells are basically alike in chemical composition and metabolic activities.
• The function of an organism as a whole is the outcome of the activities and interaction of the constituent of cells.

Prokaryotic cell	Eukaryotic cell
·           Size of the cell is generally small (1-10 mm)	·           Size of cell is generally large (5–100 mm).
·           Nucleus is absent (Nuclear region ‘nucleoid’ is not surrounded by a nuclear membrane).	·           Nucleus is present (Nuclear material is surrounded by a nuclear membrane).
·           It contains single chromosome.	·           It contains more than one chromosome.
·           Nucleolus is absent.	·           Nucleolus is present.
·           Membrane bound cell organelles are absent.	·           Cell organelles such as mitochondria, plastids, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, etc., are present.
·           Cell division takes place by fission or budding (no mitosis).	·         Cell division occurs by mitotic or meiotic cell division.
·           Ribosome is 70 S.	·           Ribosome is 80 S & 70 S type in chloroplast & mitochondria.
·           Cell wall surrounds the plasma membrane in most cases. It is composed of peptidoglycans comprising polysaccharides linked to amino acids. Strengthening material is murein.	·           Cell wall surrounds the plasma membrane in some protists, most fungi and all plants. It is composed of polysaccharides. Main strengthening material is chitin in most fungi and cellulose in others. Animal cells lack cell wall.
·           Cell membrane bears respiratory enzymes.	·          Cell membrane lacks respiratory enzymes.
·           Cell membrane may infold to form mesosomes or photosynthetic lamellae (thylakoids). The latter occur free in the cytoplasm.	·         Cell membrane does not form mesosomes or photosynthetic lamellae. Thylakoids, if present, occur within the chloroplasts.
·           They include one kingdom only such as monera. e.g. Bacteria.	·       They include four kingdoms such as Protista, fungi, Plantae, and Animalia.
·           Cytoplasm lacks organelles (endoplasmic reticulum, mitochondria, Golgi apparatus, centrosome, microfilaments, microtubules, intermediate fibres, microbodies), except ribosomes.	·          Cytoplasm contains organelles, viz. endoplasmic reticulum, mitochondria, Golgi apparatus, lysosomes, centrosome, microfilaments, intermediate fibres, microtubules and microbodies, besides ribosomes.

Unicellular to multicellular organisms

Advantages of Unicellularity. Unicellular organisms have the following advantages:

• They are capable of performing all the life functions within the limits of a single cell.
• They do not lose any organelle or any function during their life.
• They have a well-marked power of regeneration.
• The same cell is sensitive to all types of stimuli.
• All of them can reproduce by asexual means.
• They are immortal. An adult organism, after division, starts living as two or more young organisms.

Disadvantages of Unicellularity. Unicellular organisms have disadvantages too. These are: ·           Since a single cells carries on all the vital activities these cannot attain a high level of efficiency. One activity may interfere with another. ·           Injury can lead to the death of a unicellular organism. ·           Life-span (The period from birth to natural death) is short, perhaps due to heavy load of work.

Prokaryotic Cell

• Cell body cannot attain a large size; limit being imposed by the surface area to volume ratio.
• Cell body has to face the environment on all sides. This costs a lot of energy.
• Sexual reproduction, if present, occurs only once during the lifetime.

Plant and Animal Cells

This table summarizes the distinctions between plant and animal cells:

Structure	Typical Plant Eukaryotic Cell	Typical Animal Eukaryotic Cell
Cell Wall	Yes	No
Centrioles	No	Yes
Chromosomes	Many	Many
Cilia or Flagella	Present in some	Present in some
Vacuole	Yes	No, except unicellular organism like amoeba
Endoplasmic Reticulum	Yes (some exceptions)	Yes (some exceptions)
Golgi Complex	Yes	Yes
Lysosomes	No	Common
Mitochondria	Yes	Yes
Nucleus	Yes	Yes
Peroxisomes	Common	Common
Plasma Membrane	Yes	Yes
Ribosomes	Yes	Yes

Eukaryotic Cell and its organisation

• Eukaryotic cells, by definition, have a nucleus, which contains nuclear material enclosed by a double-layered membrane.
• All plants and animal cells fall under this category. In general, the eukaryotic cells have components like cell wall (absent in animal cells and in some protists), plasma membrane, cytoplasm and cellular organelles.
• The motile cells have additional appendages for locomotion such as cilia, flagella and pseudopodia.
• The animal cells are comparatively irregular in shape due to the absence of rigid cell wall whereas plant cells are much rigid as they possess cell wall.

PLASMA MEMBRANE Þ       Fluid mosaic model by S..Jonathan Singer and Nicholson (1972).

Structure of Plasma Membrane

As per this model membrane this is two dimensional solutions of oriented globular proteins and lipids. The lipid bilayer is a fluid in which the proteins are dispersed to give a mosaic appearance. The lipid bilayer is, in the membrane. They are amphipathic i.e. they are structurally asymmetric with polar hydrophilic and non-polar hydrophobic groups. On the outer side some of the lipids have sugar chains attached on their polar heads and are hence known as glycolipids. Proteins are of two kinds :-

• Peripheral or extrinsic proteins : They are completely external and are loosely attached to the polar heads of the lipids on both the surfaces.
• Integral or the intrinsic proteins : The small integral proteins partially project from either surface of the membrane while the larger ones span the entire thickness of the membrane and project from one or both the surface.

Functions:

• Biomembranes compartmentalize the cells. This separates (not isolates) the cell interior from the external environment. The membranes also allow the adjacent intra-cellular compartments (e.g., mitochondria, chloroplasts) to maintain their distinct identity and physiology.
• Plasma membrane binds the semi-fluid protoplasmic contents of the cell.
• Plasma membrane controls the substances to be passed inwardly and outwardly from the cell (selective permeability and retentivity).
• Plasma membrane controls the entry and exit of molecules thus acting as a selectively permeable membrane.

Almost all the cells maintain a difference in ionic concentration on the inside and outside of the cell membrane. The usual ions are Na⁺ and K⁺, with a higher concentration of Na⁺ outside the cell, than inside. The ionic concentration of Na⁺ and K⁺ is altered when impulse arrives at a cell. The impulse is then conducted along the length of the cell such as nerve or muscle cell.

Plasma membrane is capable of ingesting liquids (pinocytosis) and solid materials (phagocytosis) by the formation of an invagination and later absorbing then into the cell.

Secretory materials and waste products are thrown out of the cell by plasma membrane by exocytosis.

CELL WALL

l     It is outer rigid protective supportive and semi-transparent covering of plant cells, fungi and some protists.

• Cell wall was first seen in cork cells by Hooke in 1665. Its thickness varies in different types of cells from 0.1 m
• Cell wall is a non-living extracellular secretion or matrix of the cell which is closely attached to it. Cell wall is metabolically active and is capable of growth.
• Cell wall is present only in plant cells. It is the non-living component and it is secreted by the cell itself.
• It is laid down during development of the cell and starts as a thin organic material called pectin, beneath which, cellulose secreted by the outer part of cytoplasm is laid down (primary wall). Further layers of cellulose constitute the secondary wall.

Functions

• Protects the protoplasm against mechanical injury
• Protects the cell from attack of pathogens.
• Counteracts osmotic pressure.
• Walls of sieve tubes, tracheid and vessels are specialized for long distance transport.
• Cutin and suberin of the cell wall reduce the loss of water through transpiration.
• Cell wall gives definite shape to the plant cells.
• It provides mechanical strength and protection to the cell.
• Cell wall prevents the cell from desiccation.

NUCLEUS

         The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide. Chromosomes contains information for inheritance of features from parents to next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules. 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 entangled mass of thread like structures. Whenever the cell is about to divide, the chromatin material gets organized into chromosomes.

Functions

• It controls all the cellular activities of the cell.
• The nucleus contains chromosomes having a characters which are passes from parents to their offsprings.

CYTOPLASM

• The cytoplasm is the fluid content inside the plasma membrane. It also contains many specialized cell organelles. Each of these organelles performs a specific function for the cell.
• In a broad sense, a cell is differentiated into cytoplasm and nucleus. Cytoplasm is surrounded by cell or plasma membrane and the nucleus by nuclear membrane. A typical animal cell or plant cell consists of plasma membrane, cell wall (only in plant cells) cytoplasm and nucleus. Cytoplasm is filled with both minute and large dispersed particles and organelles. The portion of cytoplasm immediately below the cell membrane is gel like and is called The cytoplasm between ectoplasm and nuclear membrane is liquefied and is called endoplasm. The typical cell structure of plant cell and animal cell is represented in Figures.
• Embedded in the cytoplasm are the cell organelles, the endoplasmic reticulum, ribosomes, Golgi bodies, lysosomes, vacuoles, centrioles and nucleus. Each one has been described in detail in the following paragraphs. To study the details of cell organelles an Electron Microscope is used. A part from the cell organelles, several inorganic and organic substances are present in the cytoplasm. The different substances that make up the cell are collectively called protoplasm. The varied substances that constitute the cytoplasm are water (70 – 85%), electrolytes (K⁺, Mg⁺⁺, , , , NaCl and Ca), proteins, lipids and carbohydrates.

A Eukaryotic Cell

 

 

Cell Organelles

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EXERCISE

 

Very Short Answer Questions

1. What is plasmalemma?
2. What are the two subunits of 80 S ribosomes?
3. Which is the principal site for the development of ribosomal RNAs?
4. What are plasmodesmata?
5. Name the different types of endoplasmic reticulum?

 

Short Answer Questions

6. What are the main differences between animal and plant cells?
7. What organic macromolecules are found in cytoplasm?
8. Arrange the following from smallest to largest – cell, organ, organism, tissue, system, organelle.
9. If prokaryotic cells such as bacteria do not have organelles, how do they function?
10. How does technology advances the cell knowledge?

 

Long Answer Questions

 

11. Describe the electron microscope structure of biomembrane and the two models proposed to explain it?
12. What is meant by active transport across a cell membrane?
13. List the functions of rough and smooth endoplasmic reticulum and Golgi bodies.
14. Write an account of lysosomes and their role in cellular metabolism.
15. Distinguish between prokaryotic and eukaryotic cells.

 

MISCELLANEOUS Questions

16. Why were the scientists not able to observe most of the cell organelles before 1940?
17. What is the function of the cell wall?
18. Why are lysosomes called digestive bags or suicide bags?
19. Why are peroxisomes mostly found in kidney and liver cells?
20. Why are mitochondria also called ‘The Power House of cell’?
21. There would be no plant life if chloroplasts did not exist. Justify.
22. Why is the Golgi apparatus called the secretory organelle of the cell?
23. Why is variation not possible in mitotic division of cells?
24. The daughter cell of meiotic division is different from the parent cells. Explain.
25. Differentiate between smooth and rough endoplasmic reticulum.
26. Give the importance of cristae in mitochondria.
27. Give one similarity and one dissimilarity between plastids and mitochondria.
28. In many plant cells the nucleus and other cell organelles are pushed near the boundary walls.
    Explain.
29. Differentiate between the cell of an elephant and a plant cell (2 differences).
30. During cell division, the number of chromosomes in a cell becomes half of what it was
    initially. What type of cell is it? Name the type of cell division.
31. Why does not a child exactly resemble his father or mother?
32. Identify the diagrams given below. To which stage of mitosis do they belong? Give reasons.
33. In which phase of cell division do the following occur:
    a) Disappearance of nuclear envelope
    b) Development of spindle
    c) Reappearance of nuclear membrane
    d) Reorganization of nucleus at the poles of the dividing cells
34. Give two characteristics unique to meiotic division.
35. Name two components that the cell membrane encloses.
36. What are the two types of endoplasmic reticulum (ER)?
37. Define the endoplasmic reticulum.
38. What is the cell theory? Who proposed it? When?
39. What are lysosomes?
40. What are peroxisomes?
41. Name the important function of peroxisomes.
42. What is the difference between vacuoles of plant and animal cells?
43. When and how is the cell plate formed?
44. In meiosis, why is a reduction in number of chromosomes necessary in the germ cell?
45. Give two differences between Meiosis and Mitosis.

 

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