REVISION NOTES CELL STRUCTURE AND FUNCTIONS

SCIENCE CELL – STRUCTURE AND FUNCTIONS

Cell – Structure and Functions
Cell as the Basic Unit of Life
 The cell is the basic structural and functional unit of all living organisms.
 It is the smallest part of the body of an organism, is capable of independent existence and is able to perform the essential functions of life.

Discovery of the Cell
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Robert Hooke observed cork cells under a simple microscope designed by him.
He observed compartments resembling honeycombs and each compartment was separated by a wall.
He termed each compartment as a ‘cell’.
It was the first indication that living organisms are made of cells.

Cell Theory
 In 1838, Matthias Schleiden and Theodor Schwann proposed the basic cell theory. In 1858, another scientist Virchow made an addition to the existing cell theory.
 The postulates of the modern cell theory are
1. The cell is the smallest unit of structure of all living things.
2. The cell is the unit of function of all living things.
3. All cells arise from pre-existing cells.

Variety in Cells
On the basis of the number of cells
Single-celled
 Organisms made of a single cell. They are called unicellular organisms.  Examples: Bacteria,
Yeast,
Chlamydomonas,
Amoeba,
Paramoecium www.topperlearning.com Few-celled
 Organisms made of a few hundred to few thousand cells.  Examples:
Spirogyra, Volvox

Multi-celled
 Organisms made of millions to billions of cells.
They are called multicellular organisms.
 Examples: Man, cow, mango tree, crow 2

SCIENCE CELL – STRUCTURE AND FUNCTIONS
On the basis of size of cells
 Smallest cell: Examples: Bacteria (0.3–5.0 µm), red blood cells (7 µm)
 Longest cell: Example: Nerve cell in the neck of a giraffe (˃3 m long)
 Largest cell: Example: Ostrich egg (170 mm × 130 mm)
Smallest cell

Longest cell

Largest cell

On the basis of shape of cells
 Columnar: Epithelial cells
 Spherical: Human ovum
 Oval: Fat cells
 Spherical, biconcave: Red blood cells
 Rectangular: Spirogyra
 Spiral: Sperm cell
 Rod-shaped: Bacteria
 C-shaped: Cartilage cells
 Cylindrical: Striated muscle fibre cells
 Branched: Nerve cells
 Spindle-shaped: Smooth muscle cells
 Bean-shaped: Guard cell from a plant leaf
 Irregular: Amoeba

Amoeba is irregular in shape. It changes its shape continuously due to the presence of pseudopodia.
The change in shape helps Amoeba in movement and in capturing food.

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SCIENCE CELL – STRUCTURE AND FUNCTIONS

Structural Organisation of a Cell

CELL ORGANELLES
NATURE AND
OCCURRENCE

MAIN CHARACTERISTICS

MAIN FUNCTIONS

1. Plasma membrane/Cell membrane
 Forms the outermost covering in animal cells
 Lies next to the cell wall in plant cells
 Separates cellular material from its surroundings
 Acts as an effective barrier and regulates the entry of substances in and out of the cell

2. Cell wall (in plant cells only)
 Found in plant cells
 Situated just outside the plasma membrane
 Mainly composed of cellulose
 Provides protection
 Gives rigidity and shape to plant cells

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SCIENCE CELL – STRUCTURE AND FUNCTIONS

3. Cytoplasm
 Transparent jelly-like material
 Centre of all metabolic activities
 Different organelles contained in it perform different functions

4. Nucleus
 Largest cell organelle
 Mostly spherical and dense
 Nuclear membrane with pores, which allow substances to enter and leave the nucleus
 Regulates cell functions
 Contains chromosomes, made of genes, which control hereditary characteristics
5. Nucleolus
 Embedded within the nucleus of the cell.
 One or more in number
 Produces ribosomes
 Participates in protein synthesis by forming and storing RNA
6. Chromatin fibres
 Network of thread-like structures made of DNA
 Chromosomes carry hereditary information or
Genes

7. Vacuoles
 Fluid-filled membrane bound spaces
 Larger and permanent in plant cells
 Small and temporary in animal cells
 Storage of water and other substances, food, pigments and waste products
 Provides turgidity to the cells

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SCIENCE CELL – STRUCTURE AND FUNCTIONS
8. Plastids
 Three kinds of plastids.
 Chromoplasts: Impart colour to flowers and fruits
 Chloroplasts: Trap solar energy for photosynthesis
 Leucoplasts: Store starch
 Chloroplasts are chromoplasts which are disc-shaped and are filled with green colour chlorophyll.

The gene is a unit of inheritance in every living organism. It is responsible for the transfer of hereditary characteristics from parents to offspring.
However, the offspring may receive different characteristics due to a different combination of genes from parents.

Study of Plant and Animal Cells

Similarities between Plant and Animal Cells

Presence of cell membrane Presence of cytoplasm

Presence of nuclear membrane www.topperlearning.com

Presence of nucleus

Presence of mitochondria 6

SCIENCE CELL – STRUCTURE AND FUNCTIONS
Differences between Plant and Animal Cells

PLANT CELL
Presence of a definite cell wall made of cellulose
Present internal to the cell wall
Presence of one or more prominent vacuoles
Presence of plastids
Usually larger with distinct outlines Not so dense
Only a thin lining of cytoplasm, which is mostly pushed to the periphery FEATURE

ANIMAL CELL

Structural differences
Cell wall
Absence of cell wall
Cell membrane
Vacuoles

Forms the boundary of the cell
Presence of small and temporary vacuoles Absence of plastids

Plastids
Functional differences
Size
Usually smaller with less distinct boundaries Cytoplasm
Denser and more granular
Arrangement of
Cytoplasm fills up almost the entire cytoplasm cell

Rectangular

Other differences
Shape
Spherical

Starch

Storage material

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Glycogen

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SCIENCE CELL – STRUCTURE AND FUNCTIONS

Prokaryotic and Eukaryotic Cells
PROKARYOTIC CELL

FEATURE

Absence of well-defined nucleus

Nucleus

Absent
Presence of a single length of only
DNA

Nucleolus
Genetic
material

Presence of smaller ribosomes
Absence of other cell organelles Ribosomes
Cell
organelles

Cell division occurs by fission or budding but not by mitosis

Cell division

Bacteria, blue green algae

Examples

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EUKARYOTIC CELL

Presence of well-defined nucleus with a nuclear membrane
Present
Presence of several lengths of
DNA wound around certain proteins Presence of larger ribosomes
Presence of several other cell organelles such as mitochondria, ER, chloroplasts etc.
Cell division occurs by mitosis or meiosis
Euglena, Amoeba, plants, animals 8