Notes-Class 8-Science-Chapter-10-Cell and Cell Organelles-Maharashtra Board

Topics to be learn :

Structure of the cell
Components of the cell
Processes in the cell
Cell organelles

Cell : Cell is the structural and functional unit of living organisms.

Cells are of different sizes and structures based on their functions. They form every organ in the body.

Types of Cells : Cells are of two main types, viz. Prokaryotic cells and Eukaryotic cells. Eukaryotes has two subtypes. Viz. Plant cells and animal cells.

To observe the cells microscope is required.
Only compound microscope can be used to observe the details of cells.

Components of a cell :

Components of a cell :

1) Cell wall :

Found around cells of algae, fungi and plants.
Animal cells lack cell wall.
Structure : Cell wall is an elastic and strong coat around cell membrane.

Characteristics : It is mainly composed of carbohydrates like cellulose and pectin. Afterwards, polymers like lignin, suberin, cutin are added in cell wall as per need.
Functions : To support and to protect the cell by preventing entry of excess water in the cell.

2) Plasma membrane/cell membrane : Present in all types of cell.

Structure : It is a thin, fragile and elastic covering that separates the cell components from outer environment.

Characteristics - Protein molecules are embedded in two layers of phospholipids.
Functions : Selectively permeable membrane. It allows some substances to enter the cell, while prevents other substances. Due to this property, useful molecules of water, salt and oxygen enter the cell and CO₂ exits the cell. Maintain homeostasis.

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Q. How do substances travel in the cell?

Answer :

Ans. Substances travel inside the cell by different processes such as diffusion and osmosis.

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Activities that consume cellular energy :

Endocytosis : The entry of some substances into the cell from outer environment is endocytosis.
Exocytosis : The removal or exit of some substances from inside the cell to the outer environment is called exocytosis.

Processes that do not consume cellular energy :

Diffusion :

The movement of molecules from region of high concentration to the region of low concentration is called diffusion. Liquid molecules or gaseous molecules like O₂, CO₂ show diffusion.

Osmosis :

The movement of solute or solvent molecules that take place through the semipermeable membrane is called osmosis. OR The travelling of water from a part with more water to a part with less water, through a selectively permeable membrane is osmosis.
It is a physical process with 3 possibilities.
The solute molecules always move from region of low concentration to region of high concentration. The solvent molecules move from region of their high concentration to region of their low concentration.
It is a physical process and passive in nature as it does not require energy.

Explanation of Osmosis :

Explanation of Osmosis: The solution is made up of a solute and a solvent. For convenience let’s use the word ‘water’ for solvent.

The cell’s plasma membrane acts as a semipermeable membrane during osmosis.
When three similar cells are placed in three types of solutions, i.e. isotonic, hypotonic and hypertonic, the process of osmosis will take place differently.
Water will always move from region of its high concentration to region of its low concentration.
Solute will always move from region of its low concentration to the region of high concentration.
Therefore, in isotonic solutions, the cell will remain (in equilibrium) the same as there will be no movement of water molecules.
In hypotonic solution, the water from outside the cell will come inside the cell. The cell will thus swell. This process is endosmosis.
In hypertonic solution, the water from cell will come out in the cell. The cell will thus shrink. This process is exosmosis.

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Process with 3 possibilities :

Isotonic solution : Medium outside and inside the cell has same proportion of water, water doesn’t go in or out, i.e. no movements as concentration is same inside the cell and in outside medium.

Hypotonic solution : A hypotonic solution is one in which the concentration of solutes is greater inside the cell than outside of it. In other words, the concentration of water is lesser inside the cell than outside of it, so water enters the cell. This is called endomosis. E.g. If raisins are kept in water, after sometime they swell and become turgid.

Hypertonic solution : Hypertonic solution is one where the concentration of solutes is greater outside the cell than inside it. In other words, the concentration of water is greater inside the cell than outside of it.

The cell has more water than the outside medium so water comes out of the cell.

Example : If fruit pieces are kept in thick saturated sugar solution, the water from fruit pieces enter the sugar solution. So the fruit pieces shrink after a while.

If plant cell or animal cell is kept in hypertonic solution, water comes out of the cell by the process of exosmosis and there is contraction of cytoplasm. This process is called plasmolysis.

Cytoplasm :

Cytoplasm :

Protoplasm = Cytoplasm + Nucleus.
Cytoplasm—cell organelles: cytosol
Cytoplasm is the material bounded by plasma membrane.
Many cell organelles and nucleus are present in the cytoplasm.
Cellular chemical reactions take place in cytoplasm.
Vital substances like amino acids, glucose, vitamins, etc. are present in the cytoplasm.
Animal cell cytoplasm : More granular and dense.
Plant cell cytoplasm : Thin and pushed to the periphery with a larger central vacuole.

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Cell organelles : They are organs of the cell. Specialized sub unit with speciﬁc Cellular function.

Each organelle has its own lipoprotein membrane.
Except nucleus and chloroplast, all other organelles can be seen only with electron microscope.

1) Nucleus :

Nucleus :

Structure :

Covered by a double layered porous membrane.
Pores allow the passage of material in and out of the nucleus.
One round nucleolus present.
Network of chromatin ﬁbres present.
Short, thick chromosomes formed from chromatin ﬁbres by condensation at the time of cell division.
Functional segments on chromosomes are called genes.

Functions : -

Control of all metabolic activities of the cell and cell division.
Transmission of hereditary characters from parents to offspring.
Nucleus is lost in RBCS of mammals to accommodate more haemoglobin.
Nucleus is also lost in sieve tubes of phloem for making them hollow for transportation of food.

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2) Endoplasmic Reticulum (ER) :

Endoplasmic Reticulum (ER) :

Structure :

Net like structure having interconnected miniature tubes and sheets ﬁlled with ﬂuid.
Conducts various substances inside the cell.
E.R. is connected to nucleus from inner side to plasma membrane from outer side.
Rough ER has ribosome granules on its outer surface
Smooth ER are without ribosomes.

Functions :

Framework for supporting cell.
Conduction of proteins.
Detoxication process. Toxins that have entered are made water soluble by ER for their rapid ﬂushing out of the body.

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3) Golgi Complex :

Golgi Complex :

Structure :

5-8 cisternae which are hollow and ﬂat sacs placed parallel to each other,
Cisternae have different enzymes_
The proteins coming from ER are enclosed in vesicles.
Vesicles come to Golgi complex via cytoplasm.
Vesicles fuse with the formation face of the Golgi membranes.
They empty their contents in the cisternae.
The secretory contents are chemically modiﬁed with the help of enzymes while passing through the cisternae.
They are again packed in the vesicles which exit from the maturation face.

Functions :

Secretory organ of the cell.
It modiﬁes, sorts, packs and dispatches materials synthesized in the cell (enzymes, mucus, proteins, pigments etc.) to plasma membrane, lysosome etc.
Production of vacuoles and secretory vesicles.
Helps in the formation of cell wall, plasma membrane and lysosomes.

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4) Lysosomes :

Lysosomes :

Structure :

Simple, Single membrane bound sacs containing digestive enzymes.
Digesting the organic wastes produced through metabolic activities in the cell.

Functions :

Immune system - Destruction of viruses and bacteria that attack the cell
Demolition squads - It destroys worn out cellular organelles and organic debris. (Autolysis)
Lysosomes are called suicidal bags of the cell as the old and damaged cells are digested by bursting and releasing the digestive enzymes. Thus, digesting their own cells.
Lysosomss digest stored proteins, fats during starvation.

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5) Mitochondria :

Mitochondria :

Structure :

Double membrane bound structure.
Porous outer membrane and deeply folded inner membrane.
Folds are called ‘cristae'.
The inner cavity has matrix containing ribosomes. phosphate granules and DNA. It is like a proteinaceous gel.

Functions :

Oxidation of carbohydrates and fats in the cell with the help of enzymes.
Release of energy. Energy is stored in the form of ATP in mitochondria.
To produce energy-rich compound ATP.
Synthesis of proteins, carbohydrates, lipids etc. by using the energy in ATP.

Important :

Mitochondria produces its own proteins.
Plant cells have less mitochondria than animal cells.
Mitochondria are absent in RBCs. The oxygen transported by RBCS don‘t use this oxygen due to lack of mitochondria.

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6) Vacuoles :

Vacuoles :

Structure :

Storage sacs for solid or liquid contents.
No typical shape or size.
Structure changes according to the need of the cell.
Bound by single membrane.

Functions :

Maintain the osmotic pressure of the cell.
Storage of metabolic by-products and end products. Such as glycogen, proteins, water, etc.
In animal cell - storage of waste products and food, In amoeba storage food before digestion.
Plant cell vacuoles—full of cell sap and provide turgidity, rigidity to them.

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7) Plastids :

Plastids : It is the organelle which gives colours and present only in the plant cells. Important for the photosynthesis process. Mitochondria and plastids are able to replicate on their own and they also produce their own proteins, as in them, ribosomes and DNA is present.

Structure :

Plastids have double membrane -outer and inner.
Thylakoids present which as grana and stroma in which photosynthetic pigment is present.
Stroma contains enzymes, DNA, ribosomes and carbohydrates that are necessary for photosynthesis.

Functions of three types of plastids :

1) Chloroplasts-Green Plastids : Chlorophyll converts solar energy to chemical energy during photosynthesis.

2) Chromoplasts-Coloured Plastids : Give different colours to ﬂowers and fruits.

3) Leucoplasts -White or colourless Plastids : Synthesis and storage of food like starch, oils and proteins

Pigments-Colour:

Chlorophyll – Green, in leaves
Carotene – Orange, in carrots
Xanthophyll – Yellow, in ﬂowers
Anthocyanin - Purple, blue
Betalains - Dark pink, in Beet root.

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What is the beneﬁt of foldings of inner membrane in mitochondria?

Answer

Ans. The structure of the inner mitochondrial membrane is extensively folded and compartmentalized. The numerous invaginations of the membrane are called cristae. This folded inner membrane increases the area which is about 5 times more than that the outer membrane due to cristae. Cristae membranes have small round protein complexes known as F1 particles. In these particles the process of energy production goes on.

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Main differences between the Prokaryotic and eukaryotic cells :

Prokaryotic and eukaryotic cells :

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Work of Institutes :

National Centre for Cell Science (NCCS) is an independent institute working under
Biotechnology Department of Government of India.
Situated at Pune in Savitribai Phule Pune University campus. It is involved in research in cytology.
National Animal cell repository is also associated with this institute. It’s main work is research about cancer treatment.