CELL: Discovery, Structure, Functions, Questions and Answers for Class 8th Science Chapter 8 NCERT/ CBSE
Introduction
Every known living entity has a cell as its fundamental structural and functional element. It is frequently referred to as the foundation of life since it is the tiniest piece of life that may be considered a living entity. There are roughly 10 trillion cells in an adult human. Only under a microscope can one see the majority of plant and animal cells, which range in size from 1 to 100 m. In 1665, Robert Hooke found the cell. All organisms are made up of one or more cells, According to the cell theory, which was first put forth in 1839 by Matthias Jakob Schleiden and Theodor Schwann. It also states that all cells are descended from preexisting cells, that vital functions of an organism take place within cells, and that all cells contain the genetic information required to control cell functions and pass information to the next generation of cells.
Cell’s discovery
Under a simple magnifier, Robert Hooke examined slices of cork in 1665. A tree’s bark includes a cork. Under a microscope, he examined small pieces of cork. In the cork slice, he observed chambers or boxes that were divided into sections.
These containers resembled a honeycomb. In addition, he observed that a wall or other obstruction separated one box from the next. For each box, Hooke created the term “cell.” Dead cells were what Hooke initially thought were “boxes” or “cells” in the cork. Only after the development of more advanced microscopes were living organisms’ cells visible. For the next 150 years following Robert Hooke’s studies, little was known about the cell. Because of enhanced, high-magnification microscopes, we now know a lot about cell structure and functions.
Types of Cells
Eukaryotic and Prokaryotic are the two different types of cells. Because it lacks a nucleus and the majority of the other organelles found in eukaryotes, the prokaryote cell is simpler and smaller than a eukaryote cell.
Number of Cells
Multicellular (multi: numerous; cellular: cell) organisms are those consisting of more than one cell. Smaller creatures have fewer cells, yet this has little bearing on how well those organisms work.
Unicellular organisms (uni: one; cellular: cell) are single-celled organisms. All the essential jobs that multicellular creatures do are done by single-celled organisms. An Amoeba is an example of a single-celled organism that can breathe, digest food, expel waste, grow, and reproduce. In multicellular organisms, clusters of specialised cells constituting various tissues perform similar duties. In turn, tissues create organs.
Parts of the Cell
All cells, Prokaryotic or Eukaryotic, contain a membrane that encloses the cell, divides it from its surroundings, controls what can enter and leave (selectively permeable), and keeps the cell’s electric potential constant. The majority of the cell’s volume is made up of a saline cytoplasm inside the membrane. DNA, the genetic material of genes, and RNA, which contains the instructions needed to create different proteins like enzymes, the cell’s main machinery, are both present in every cell. Cells contain a variety of different biomolecule types.
MEMBRANE
A cell’s plasma membrane or cell membrane encloses its cytoplasm. In prokaryotes and plants, the plasma membrane is often protected by a cell wall. The primary components of this membrane, which protects and separates a cell from its surroundings, are a double layer of lipids (hydrophobic fat-like molecules) and hydrophilic phosphorus molecules. The layer is hence known as a phospholipid bilayer. Another name for it is a fluid mosaic membrane. Several protein molecules that function as channels and pump to transport various substances into and out of the cell are embedded inside this membrane. A material (molecule or ion) can either pass through the membrane freely, partially, or not at all, which is why it is referred to as being “semipermeable.” Additionally, cell surface membranes contain receptor proteins that enable cells to recognise outside signalling chemicals, such as hormones.
Protoplasm
Protoplasm refers to everything in a cell other than the cell membrane. A cell’s protoplasm is its living substance. Alternatively, protoplasm refers to the complete inside of live cells, including their cytoplasm and nucleus.
Cell membrane
The word “cell membrane” refers to the cell’s outermost membrane. Cell membrane refers to the cell’s outermost body. Lipids and proteins make up the cell membrane. Both plants and animals have it. This membrane is alive, fragile, and thin. A very small number of molecules can pass through the cell membrane, which is a semi-permeable membrane. Since the cell membrane and plasma membrane are permeable, materials can move into and out of the cell.
Functions of cell membrane
1. The cell is supported by the cell membrane.
2. It facilitates the interchange of goods.
3. It gives the cell form.
Cell Wall
1. It is dead, external to the cell membrane, and easily permeable (All substances can pass through it)
2. It is a thick layer that is only found in plant cells.
3. Cellulose makes up the cell wall.
4. The plants also need a cell membrane in addition to the cell wall for defence against temperature changes, strong winds, atmospheric moisture, etc.
5. The leaf peel of Tradescantia, Elodea, or Rhoeo contains visible cells.
Functions of cell wall
1. It gives the cell mechanical support.
2. All substances can travel through it.
3. It keeps the form of the cell.
Cytoplasm
It is the fluid component of the cell. It has organelles in it. The jelly-like material called cytoplasm is found between the cell membrane and the nucleus. The following are additional parts or organelles of cells: Vacuoles, Plastids, and Mitochondria. There is an endoplasmic reticulum in both plants and animals.
Golgi bodies: Both plants and animals have them.
Lysosomes are found in both plants and mammals.
Plastids are only found in plants. Animals do not contain them.
The three different categories of plastids are as follows:
It is colourless, the leucoplast.
The chromoplast is a coloured structure.
The chloroplast is a green substance. It aids in plant food production, earning it the nickname “the kitchen of cells.” Plants need chlorophyll, which is found in chloroplasts, to produce food. Photosynthesis benefits from chloroplast use.
Plastid functions
1. Photosynthesis benefits from chloroplast use.
2. Leucoplast is useful for storing food, lipids, and other substances.
3. The coloured chromoplast gives flowers and fruits their colour.
Vacuole
A fluid-filled membrane is a vacuole. It can be found in plants. Large and persistent vacuoles are seen in plant cells. Animals have the potential of having them, although they are incredibly tiny and transient in size. Animals have vacuoles, which serve as storage compartments.
The role of the vacuole
1. Turgidity and stiffness are provided.
2. They are useful for keeping things in storage.
Nucleus
The cell’s nucleus is in the middle of the structure. Genes are kept in the nucleus. A cell cannot survive or perform any specific functions without a nucleus.
The nuclear membrane, nucleoplasm, and nucleolus make up the cell.
Nuclear membrane
The nuclear membrane is the membrane that separates the cytoplasm from the nucleus. This membrane permits materials to travel between the cytoplasm and the inside of the nucleus because it is porous (has pores). It also goes by the name nuclear envelope.
Nucleoplasm
It is the nucleus’ cytoplasm.
Nucleolus
Its small oval body is abundant in RNA and protein (Ribonucleic acid). The nucleolus is where ribosomes are formed. The ribosome is an organelle that is found in cells and either directly or indirectly aids in cell survival.
Nuclear material
It is composed of protein and DNA (deoxyribonucleic acid), and it has chromosomes. Genes are individual DNA strands. Genes serve as hereditary transporters. It afterwards transforms into chromosomes.
DNA and proteins make up chromosomes, which are ribbon-shaped structures. It contains pointy structures known as genes.
For instance: if both the mother and the daughter have blue eyes. This results from inherited traits. Character is passed down through genes from parents to children.
Nucleus’s Functions
It controls the cell cycle. It regulates metabolic processes. It has genes that assist parents to pass on their character traits to their children.
Questions and Answers for Class 8th Science Chapter NCERT/ CBSE
1. Indicate whether the following statements are True(T) or False(F).
(a) Unicellular organisms have a one-celled body. (T / F)
Ans: It’s true that unicellular organisms have one-celled bodies.
(b) Muscle cells are branched. (T / F)
Ans: True: Muscle cells have branches.
(c) The basic living unit of an organism is an organ. (T / F)
Ans: False: An organ is an organism’s fundamental living component.
(d) Amoeba has irregular shape. (T/F)
Ans: True, the shape of an amoeba is erratic.
Question: Sketch the human nerve cell. What function do nerve cells perform?
Ans: The functions of the nerve cells include:
1. They deliver messages to the brain after receiving them from sense organs.
2. They receive brain messages and relay them to the sensory organs.
3. The brain can manage every body part in this way.
Question: Write short notes on the following:
a) Cytoplasm b) The nucleus of a cell
Ans: Cytoplasm
Between the plasma membrane and the nucleus, the cytoplasm, a gel-like material, fills the cell. Ribosomes, Golgi bodies, mitochondria, and other cell organelles are discovered floating freely in the cytoplasm. It has been discovered that the cytoplasm facilitates material exchanges between various cell organelles.
The Nucleus of a cell
An essential part of every cell is the nucleus. It is mainly located at the cell’s centre and has a spherical shape. The nuclear membrane is the membrane that separates the cytoplasm from the nucleus. Nuclear holes in the nuclear membrane allow materials to travel back and forth between the cytoplasm and nucleus. There are two layers to the nuclear membrane. The nucleolus is a little ball-shaped structure. When a cell divides, chromosomes—tiny, thread-like structures that contain genes and aid in the transmission of traits from parents to offspring—become visible.
Question: Which part of the cell contains organelles?
Ans: The cell’s cytoplasm, which is situated between the plasma membrane and the nucleus. Organelles found in cells, such as the mitochondria, nucleus, Golgi bodies, ribosomes, and lysosomes, are found in the cytoplasm.
Question: Make sketches of animal and plant cells. State three differences between them?
Ans: Animal cell
1. The cells are compact.
2. There is no cell wall. 3. Plastids are only found in Euglena.
4. There are a few vacuoles.
Plant cell
1. The cell size is larger than that of an animal.
2. There is a cell wall.
3. There are plastids.
4. There are huge vacuoles.
Question: What distinguishes eukaryotes from prokaryotes?
Ans: Eukaryotes
(i)The majority of eukaryotes have several cells.
(ii) There is a nucleolus.
(iii) The cell’s nucleus is visible and protected by a nuclear membrane.
(iv) Organelles found in cells are present. Ribosomes, plastids, mitochondria, Golgi bodies, and other structures are among them.
(v) Animal, plant, and fungal cells are all eukaryotic.
Prokaryotes
(i) The majority of prokaryotes are single-celled.
(ii) The absence of the nucleolus.
(iii) The nucleus lacks a nuclear membrane and is ill-defined.
(iv) All cell organelles aside from ribosomes are missing.
(v) Blue-green algae and bacteria are examples of prokaryotic cells.
Question: Where are chromosomes found in a cell? State their function.
Ans: Within the nucleus are thread-like structures known as chromosomes. Character inheritance or transfer from parents to children is mostly governed by chromosomes.
Question: The fundamental building blocks of living things are cells. Explain.
Ans: The smallest unit of life, capable of performing all life processes, is known as a cell. Since cells are also the basic structural and functional unit of life, they are referred to as “the basic structural and functional unit of life.” Every cell is unique in terms of its forms, sizes, and functions. The form and size of the cell are related to its function.
Question: Explain why chloroplasts are found only in plant cells?
Ans: A specific kind of plastid called a chloroplast is used to store chlorophyll and capture sunlight for the process of photosynthesis. Chloroplasts are only found in plant cells since they are used in photosynthesis by plants.
Very Short – Answer Questions
Question: Define the following terms?
The animal cell
Ans. The fundamental structural and functional units of an animal’s body are its cells. They are eukaryotic cells, which means that the nucleus and other cell organelles are attached to a membrane.
The Nucleus
Ans. The compact oval-shaped structure called the nucleus, which is present in eukaryotic cells but less well defined in prokaryotic cells, carries genetic material. A two-layered membrane encloses it.
The Paramecium
Ans. It is an aquatic, unicellular invertebrate with a slipper-like form. Its body is coated with tiny projections that resemble hairs called cilia that aid in movement. The macronucleus and micronucleus are two nuclei.
The Nerve cell
Ans. The lengthy, branching extensions on nerve cells resemble threads. Its length exceeds one metre. It transfers brain impulses to the organ, which in turn regulates how the body moves.
Yeast
Ans. Yeast is a type of fungi that is a single-celled plant. In carbohydrate-containing foods, it leads to fermentation.
Question: Why are chromosomes called hereditary vehicles?
Ans. Genes are the fundamental hereditary units found in DNA, which is why chromosomes are referred to be hereditary vehicles. The genetic traits that parents pass on to their children through gene inheritance.
Question: Where are the chromosomes found in a cell? State their functions.
Ans. A cell’s nucleus contains chromosomes. It is made of deoxyribonucleic acid (DNA), which contains fundamental genetic building blocks known as genes. The genetic traits inherited from parents are passed on to their offspring through genes.