Study Material and Notes of Ch 8 Cell - Structure and Functions Class 8th Science
Introduction
• Cell
→ Discovery of cell
• Cell Theory
• Types of Organisms on the basis of cell
→ Unicellular organisms
→ Multicellular organisms
• Shape of the cells
• Size of the cells
• Cell Structure and functions
• Types of Cells
→ Prokaryotic cells
→ Eukaryotic cells
• Components of the cell
→ Cell membrane
→ Cell wall
→ Cytoplasm
→ Nucleus
→ Chromosomes
→ Vacuoles
→ Plastids
→ Endoplasmic Reticulum
→ Golgi Apparatus
→ Lysosomes
→ Mitochondria
• Differences between plany and animal cells
• Diagram of Plant Cells and Animals Cells
Cell
Discovery of the Cell
→ Cells are the basic structural units and the building blocks of all living organisms.
→ Cell was discovered by Robert Hooke in 1665 after observing a piece of cork under a magnifying device.
Introduction
→ Discovery of cell
• Cell Theory
• Types of Organisms on the basis of cell
→ Unicellular organisms
→ Multicellular organisms
• Shape of the cells
• Size of the cells
• Cell Structure and functions
• Types of Cells
→ Prokaryotic cells
→ Eukaryotic cells
• Components of the cell
→ Cell membrane
→ Cell wall
→ Cytoplasm
→ Nucleus
→ Chromosomes
→ Vacuoles
→ Plastids
→ Endoplasmic Reticulum
→ Golgi Apparatus
→ Lysosomes
→ Mitochondria
• Differences between plany and animal cells
• Diagram of Plant Cells and Animals Cells
Discovery of the Cell
→ Robert Hooke coined the term “cell”.
Cell Theory
→ According to cell theory: Cells are the basic structural and functional units of life.
→ All living organisms are made up of one or more cells.
→ New cells arise from pre-existing cells.
Types of Organisms on the basis of cell
(i) Unicellular Organisms
(ii) Multicellular Organisms
Unicellular organisms
• Number of Cells Organisms made of only a single cell are called unicellular organisms.
For example: Amoeba and Paramecium
→ All the basic functions such as digestion, respiration, excretion, etc. in these organisms performed by a single cell.
Multicellular organisms
• Organisms made up of more than one cells are called multicellular organisms.
For example: Humans, cow, rose, etc.
→ In these organisms, the cells show division of labour as particular set of cells are involved in performing a specific body function.
Shape of the Cells
→ Most of the cells have a definite shape.
→ Some cells such as that in Amoeba have no definite shape.
→ The human red blood cell (RBC) is spherical-shaped.
→ The muscle cells in humans are spindle-shaped.
→ In plants and bacteria, the cell is enclosed in a protective covering called cell wall, which gives shape and rigidity to the cells.
Size of the Cells
→ The smallest cell is 0.1 to 0.5 micrometre in bacteria.
→ The largest cell is of size 170 mm x 130 mm, which is the egg of an ostrich.
→ Size of a cell has no relation with the size of an organism.
Cell Structure and Functions
→ In multicellular organisms, each organ system is made up of several organs.
→ Organs are further made up of tissues.
→ Tissues are groups of similar cells performing a specific function.
→ Number of cells Organisms made up of only a single cell are called unicellular organisms.
For example: Amoeba and Paramecium
→ Single cell in unicellular organisms performs all the basic functions such as digestion, respiration, and excretion.
→ Organisms made up of more than one cells are called multicellular organisms.
For example: Humans, cow, etc.
→ In multicellular organisms, the cells show division of labour as a particular set of cells are involved in performing a specific body function.
Types of cell
There are two types of cells.
(i) Prokaryotic cells
(ii) Eukaryotic cells
• Prokaryotic cells
Cells which do not have a well defined nuclear membrane and the nuclear material lies freely in the cytoplasm of the cell.
For example: bacteria, blue green algae.
• Eukaryotic cells
Cells having nucleus with well defined nuclear membrane.
For example: plant and animal cells
Components of the cell
Cell membrane
→ It is the protective layer that surrounds the cell.
→ Cell membrane selectively allows the entry of only some substances and prevents the movement of other materials.
→ Hence, it checks the transport of substances in and out of the cell.
Cell wall
→ In plants, an extra protective covering of a polysaccharide, cellulose is present.
→ It is called cell wall that protects plant cells from environmental variations.
Cytoplasm
→ It is a jelly-like substance present between cell membrane and nucleus.
→ It contains various cell organelles such as mitochondria, Golgi bodies, lysosomes etc.
Nucleus
→ It is a dense spherical body located at the centre of the cell.
→ It is surrounded by porous nuclear membrane.
→ It contains spherical body called nucleolus.
→ It also contains thread-like structures called chromosomes.
Chromosomes
→ These are the structures that carry genes and play an important role in inheritance.
→ Genes are the structural and functional unit of inheritance.
→ The entire living substance in a cell is known as protoplast.
Vacuoles
→ Vacuoles are fluid-filled membrane-bound structures in the cell.
→ In plant cells, a single large vacuole is present.
→ In animal cells, numerous small vacuoles are present.
→ The membrane of the vacuole is called tonoplast.
→ This membrane encloses a fluid called cell sap.
Plastids
→ They are present only in plant cells.
→ Plastids that contain green colour pigment chlorophyll are known as chloroplasts.
→ It is the chlorophyll that gives green colour to the leaves.
→ Chloroplast traps solar energy and utilizes this energy to manufacture food for the plant.
• Plastids are of two types:
(i) Leucoplasts
(ii) Chromoplasts
→ Leucoplasts are colourless and are used to store food while chromoplasts are plastids containing pigments.
→ Chloroplasts are a type of chromoplasts.
• Chloroplasts consist of two regions:
(i) grana (stacks of sac like membrane bound structures that contain pigment chlorophyll)
(ii) stroma (ground substance containing enzymes and starch grains)
Endoplasmic Reticulum (ER)
(i) Rough Endoplasmic Reticulum (RER): It is important for the synthesis and packaging of proteins.
(ii) Smooth Endoplasmic Reticulum (SER): It acts as storage organelle. It also helps in lipid (fat) synthesis.
Golgi Apparatus
→ It is made up of parallel arranged membrane-bound vesicles called cisternae.
→ It helps in storage, modification, and packaging of products in vesicles.
→ It helps in formation of glycoproteins and glycolipids.
Lysosomes
→ It is a membrane-bound structure that holds variety of enzymes.
→ Rich in all types of hydrolytic enzymes, which are active at acidic pH.
→ It is involved in the digestion of carbohydrates, proteins, lipids, and nucleic acids.
Mitochondria
→ It is a double membrane-bound structure.
→ The inner membrane of mitochondria is deeply folded to form cristae.
→ Cristae increase the surface area in the organelle.
→ It is the site of cellular respiration and hence known as ‘power house of cell’.
→ They have their own circular DNA.
→ They divide by fission.
Differences between plant and animal cells
Plant Cells
|
Animal Cells
|
| Cell wall is present. | Cell wall is absent. |
| Nucleus is located in the periphery of the cell. | Nucleus is located in the centre of the cell. |
| Plastids are present. | Plastids are absent. |
| A large single vacuole is present in the centre of the cytoplasm. | Numerous small vacuoles are present in the cytoplasm. |
Diagram of Plant Cells and Animal Cells
