Cell and Cell Organelles Class 8 Science Notes Maharashtra State Board
Structure of the Cell
According to the functions, cells of different sizes and structures are found in different organs.
Components of a Cell
1. Cell Wall:
It is found around the cells of algae, fungi, and plants. Animal cells lack cell walls. The cell wall is an elastic and strong coat around the cell membrane. It is mainly composed of carbohydrates like cellulose and pectin. Afterward, polymers like lignin, suberin, and cutin are added to the cell wall as needed. The function of the cell wall is to support and protect the cell by preventing the entry of excess water into the cell.
2. Plasma Membrane/Cell Membrane:
It is a thin, fragile, and elastic covering that separates the cell components from the outer environment. Structure of plasma membrane Protein molecules are embedded in two layers of phospholipids.
The plasma membrane is said to be a selectively permeable membrane as it allows some substances to enter the cell while preventing other substances. Due to this property, useful molecules of water, salt, and oxygen enter the cell and CO2 exits the cell. If any changes occur outside the cell, the cellular environment does not change due to the plasma membrane. This condition is called homeostasis.
Activities that Consume Cellular Energy
- Endocytosis – To gulp/engulf food and other substances from the outer environment.
- Exocytosis – To excrete waste materials out of the cell.
Processes those that don’t Consume Cellular Energy
- Diffusion – Entry or exit of small molecules like O2, CO2
- Osmosis – The traveling 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.
- Isotonic Solution – Medium outside and inside the cell has the same proportion of water, water doesn’t go in or out.
- Hypotonic Solution – The cell has less water than the outside medium, so water enters the cell. This is called endomosis.
Eg. If raisins are kept in water after some time they swell and become turgid. - Hypertonic Solution – The cell has more water than the outside medium so water comes out of the cell.
Eg. If fruit pieces are kept in a thick saturated sugar solution, the water from fruit pieces enters the sugar solution. So the fruit pieces shrink after a while. If a plant cell or animal cell is kept in a hypertonic solution, water comes out of the cell by the process of exosmosis and there is a contraction of the cytoplasm. This process is called plasmolysis.
3. Cytoplasm
Cytoplasm is the fluid between the plasma membrane and the nucleus. It is a moving, sticky substance. Many cell organelles are suspended in the cytoplasm. Cytoplasm is the medium for cellular chemical reactions. The part of the cytoplasm other than organelles is the cytosol. Cytosol stores vital substances like amino acids, glucose, vitamins, etc. In animal cells, cytoplasm is more granular and dense while in plant cells, it is thin and mostly pushed to the periphery due to a larger central vacuole.
Cell Organelles:
An organelle is a specialized subunit having a specific function within the cell. They are ‘organs of the cell.’ Each organelle has its lipoprotein membrane. Except nucleus and chloroplast, all other organelles can be seen only with an electron microscope.
Nucleus
Activity – Take a drop of water on a clean glass slide. Using an ice cream spoon, gently scrape the inner surface of your cheek. With a needle, transfer a little material from the spoon to the water drop on the slide and spread it evenly. Put a drop of methylene blue stain on the smear. Put a cover slip and observe under the microscope. Did you observe the cells with blue nuclei? The dark round spot seen under the microscope while observing the onion peeling stained with iodine is the nucleus of the cell.
When observed under an electron microscope, we can see a nucleus covered by a double-layered membrane with small pores. These pores allow the passage of material in and out of the nucleus. The nucleus has one round nucleolus and a network of chromatin fibers. Chromatin fibres are thin thread-like structures, that condense to form short thick chromosomes at the time of cell division. Functional segments on chromosomes are called genes.
Functions of Nucleus
- It controls all metabolic activities of the cell and also the cell division.
- It is involved in the transmission of hereditary characters from parents to offspring.
Due to the loss of the nucleus, a large quantity of hemoglobin can be accommodated in the RBC, and thereby a large amount of oxygen can be transported. Due to the loss of the nucleus of the sieve tubes of the plant phloem, they become hollow, and thereby transportation of food becomes easy.
Endoplasmic Reticulum (ER)
The organelle that conducts various substances inside the cell is called the endoplasmic reticulum. E.R. has a net-like structure consisting of interconnected miniature tubes and sheets filled with fluid. E.R. is connected to the nucleus from the inner side to the plasma membrane from the outer side. Rough ER has ribosome granules on its outer surface.
Functions of ER
- It is the framework that supports cells.
- Conduction of proteins.
- Toxins that have entered the body through food, air, and water are made water soluble by ER and then flushed out of the body.
Golgi Complex:
It is made up of 5-8 hollow and flat sacs placed parallel to each other. These sacs are called ‘cisternae’ and are filled with different enzymes. The proteins coming from the ER are enclosed in vesicles, which come towards the Golgi complex via cytoplasm. They fuse with the formation face of the Golgi membranes and empty their contents in the cisternae.
As they pass through the cisternae, they are chemically modified with the help of enzymes. They are again packed in the vesicles. These vesicles exit from the maturation face. Thus, cisternae work like a packing department that packs and distributes substances.
Functions
- The Golgi complex is the secretory organ of the cell.
- It modifies, sorts, and packs materials synthesized in the cell (enzymes, mucus, proteins, pigments, etc.) and dispatches them to various targets like plasma membrane, lysosome, etc.
- It produces vacuoles and secretory vesicles.
- It helps in the formation of cell walls, plasma membranes, and lysosomes.
Camilio Golgi described the Golgi complex for the first time. He developed the staining technique called ‘Black reaction’ and with the help of it, he studied the nervous system. He won the Nobel Prize of 1906 along with Santiago Cajal for the study of the structure of the nervous system.
Lysosomes
Due to various metabolic activities in the cell, organic waste is generated. Lysosomes digest the waste. Lysosomes are simple, single membrane-bound sacs, filled with digestive enzymes.
Functions
- Immune system – It destroys viruses and bacteria that attack the cell.
- Demolition squads – It destroys worn-out cellular organelles and organic debris. (Autolysis)
- Suicide Bags – When a cell becomes old or is damaged, lysosomes burst and enzymes digest their cells.
- During starvation, lysosomes digest stored proteins and fats.
Mitochondria
Each cell requires energy and this energy is produced in the mitochondria. Under the electron microscope, a mitochondrion is seen as a double membrane structure.
The outer membrane is porous and the inner membrane is deeply folded. These folds are called ‘cristae’. The inner cavity is filled with a proteinaceous gel-like matrix containing ribosomes, phosphate granules, and DNA. Therefore it can produce its proteins. With the help of enzymes, mitochondria oxidize carbohydrates and fats in the cell. The energy released in this process is stored in the mitochondria in the form of ATP (Adenosine Tri Phosphate). Plant cells have fewer mitochondria than animal cells.
Functions
- To produce energy-rich compound- ATP.
- Synthesis of proteins, carbohydrates, lipids, etc. by using the energy in ATP.
Mitochondria are absent in RBCs. Due to this, the oxygen which is carried by them is not used for themselves.
Vacuoles
Vacuoles are the storage sacs for solid or liquid contents. They don’t have any typical shape or size. The structure of the vacuole changes according to the needs of the cell. A vacuole is bound by a single membrane.
Functions
- To maintain the osmotic pressure of the cell.
- To store metabolic byproducts and end products. (Glycogen, proteins, water, etc).
- Animal cells, store waste products and food, while amoeba store food before digestion.
- In plant cells, vacuoles are full of cell sap and provide turgidity, and rigidity to them.
Plastids:
Why are plant leaves green and flowers red, yellow, orange, or blue? The organelle that gives such colors is present only in the plant cells – it is called a plastid. Plastids have double membranes and are of two types:
- Leucoplasts – White or colorless plastids
- Chromoplasts – Coloured plastids
Chloroplast can be converted into other types of chromoplasts.
Eg. Raw green tomatoes turn red due to the breakdown of chlorophyll and the synthesis of lycopene.
Chloroplast
Activity – Take out a thin peel of Rheo or Croton leaf and observe the chromoplasts under the compound microscope.
Chloroplasts are important for the photosynthesis process that takes place in the leaves. Chlorophyll in chloroplast traps solar energy and converts it to chemical energy. Stroma in the chloroplast contains enzymes, DNA, ribosomes, and carbohydrates that are necessary for photosynthesis.
Functions
- Chloroplasts convert solar energy to chemical energy (food).
- Chromoplasts give different colors to flowers and fruits.
- Leucoplasts are involved in the synthesis and storage of food like starch, oils, and proteins.
Mitochondria and plastids can produce proteins as well as replicate themselves, as they have their DNA and ribosomes. After studying the cell and cell organelles, you must have come to know that all the functions in plant and animal cells are taking place smoothly due to the cell organelles. Such developed cells are called eukaryotic cells. Last year, you have studied prokaryotic cells of bacteria. Let’s now compare them.
| Eukaryotic Cell | Prokaryotic Cell |
| Size: 5-100 micrometers | 1-10 micrometer. |
| Number of chromosomes: More than one | Only one. |
| Nucleus – with nuclear membrane, nucleolus, and nucleoplasm | Nucleoid resembling nucleus |
| Mitochondria and plastids – present | They don’t have membrane-bound cell organelles |
| Examples – Present in highly evolved unicellular and multicellular plants and animals. | Bacteria |
Good Maharashtra State Board Class 8 Science Notes Cell and Cell Organelles can simplify complex concepts and make studying more efficient.