Science Class 9 Chapter 2 The fundamental Unit Of Life: Cell Full Chapter With Book Image
What is Cell
the smallest unit that can live on its own and that makes up all living organisms and the tissues of the body. A cell has three main parts: the cell membrane, the nucleus, and the cytoplasm. ... Parts of a cell. A cell is surrounded by a membrane, which has receptors on the surface.
Difference Between Unicellular And Multicellular Organisms
As stated initially, one major difference between unicellular and multicellular organisms is the cellularity or the number of cells. Read on the explore more differences between the two:
| Unicellular Organisms | Multicellular Organisms |
|---|---|
| Unicellular organisms are composed of a single cell | Multicellular organisms are composed of more than one cell |
| Simple body organization | Complex body organization |
| A single cell carries out all necessary life processes | Multiple cells perform different functions |
| The total cell body is exposed to the environment | Only the outer cells are exposed to the environment |
| Division of labour is at the organelle level | Division of labour is at cellular, tissue, organs and organ system level |
| Includes both eukaryotes and prokaryotes | Includes only eukaryotes |
| A lifespan of a unicellular organism is usually short | Multicellular organisms have a comparatively longer lifespan |
| Injury to the cell leads to the death of the organism | Injury to a cell does not cause the death of the multicellular organism |
| Asexual reproduction is predominant, however, sexual reproduction is also seen (conjugation) | Reproduction happens sexually as well as asexually |
| Cell differentiation is absent | Cell differentiation is present |
| They can be autotrophs or heterotrophs | They include both autotrophs and heterotrophs |
| They are microscopic in nature | Most are macroscopic in nature |
| Bacteria, amoeba, paramecium and yeast are examples of unicellular organisms | Humans, animals, plants, birds and insects, are examples of multicellular organisms |
Difference Between Light Microscope And Electron Microscope
Prokaryote definition
Prokaryotes are unicellular organisms that lack membrane-bound structures, the most noteworthy of which is the nucleus. Prokaryotic cells tend to be small, simple cells, measuring around 0.1-5 μm in diameter. While prokaryotic cells do not have membrane-bound structures, they do have distinct cellular regions. In prokaryotic cells, DNA bundles together in a region called the nucleoid.
Prokaryotic cell features
Here is a breakdown of what you might find in a prokaryotic bacterial cell.
- Nucleoid: A central region of the cell that contains its DNA.
- Ribosome: Ribosomes are responsible for protein synthesis.
- Cell wall: The cell wall provides structure and protection from the outside environment. Most bacteria have a rigid cell wall made from carbohydrates and proteins called peptidoglycans.
- Cell membrane: Every prokaryote has a cell membrane, also known as the plasma membrane, that separates the cell from the outside environment.
- Capsule: Some bacteria have a layer of carbohydrates that surrounds the cell wall called the capsule. The capsule helps the bacterium attach to surfaces.
- Fimbriae: Fimbriae are thin, hair-like structures that help with cellular attachment.
- Pili: Pili are rod-shaped structures involved in multiple roles, including attachment and DNA transfer.
- Flagella: Flagella are thin, tail-like structures that assist in movement.
Examples of prokaryotes
Bacteria and archaea are the two types of prokaryotes.
Do prokaryotes have mitochondria?
No, prokaryotes do not have mitochondria. Mitochondria are only found in eukaryotic cells. This is also true of other membrane-bound structures like the nucleus and the Golgi apparatus (more on these later).
One theory for eukaryotic evolution hypothesizes that mitochondria were first prokaryotic cells that lived inside other cells. Over time, evolution led to these separate organisms functioning as a single organism in the form of a eukaryote.
Eukaryote definition
Eukaryotes are organisms whose cells have a nucleus and other organelles enclosed by a plasma membrane. Organelles are internal structures responsible for a variety of functions, such as energy production and protein synthesis.Eukaryotic cells are large (around 10-100 μm) and complex. While most eukaryotes are multicellular organisms, there are some single-cell eukaryotes.
Eukaryotic cell features
Within a eukaryotic cell, each membrane-bound structure carries out specific cellular functions. Here is an overview of many of the primary components of eukaryotic cells.
- Nucleus: The nucleus stores the genetic information in chromatin form.
- Nucleolus: Found inside of the nucleus, the nucleolus is the part of eukaryotic cells where ribosomal RNA is produced.
- Plasma membrane: The plasma membrane is a phospholipid bilayer that surrounds the entire cell and encompasses the organelles within.
- Cytoskeleton or cell wall: The cytoskeleton or cell wall provides structure, allows for cell movement, and plays a role in cell division.
- Ribosomes: Ribosomes are responsible for protein synthesis.
- Mitochondria: Mitochondria, also known as the powerhouses of the cell, are responsible for energy production.
- Cytoplasm: The cytoplasm is the region of the cell between the nuclear envelope and plasma membrane.
- Cytosol: Cytosol is a gel-like substance within the cell that contains the organelles.
- Endoplasmic reticulum: The endoplasmic reticulum is an organelle dedicated to protein maturation and transportation.
- Vesicles and vacuoles: Vesicles and vacuoles are membrane-bound sacs involved in transportation and storage.
Other common organelles found in many, but not all, eukaryotes include the Golgi apparatus, chloroplasts and lysosomes.
Examples of eukaryotes
Animals, plants, fungi, algae and protozoans are all eukaryotes.
Difference between Prokaryotic and Eukaryotic Cells
Though these two classes of cells are quite different, they do possess some common characteristics. For instance, both possess cell membrane and ribosomes, but the similarities end there. The complete list of differences between prokaryotic and eukaryotic cells are summarized as follows:
| Prokaryotes | Eukaryotes | |
| Type of Cell | Always unicellular | Unicellular and multi-cellular |
| Cell size | Ranges in size from 0.2 μm – 2.0 μm in diameter | Size ranges from 10 μm – 100 μm in diameter |
| Cell wall | Usually present; chemically complex in nature | When present, chemically simple in nature |
| Nucleus | Absent. Instead, they have a nucleoid region in the cell | Present |
| Ribosomes | Present. Smaller in size and spherical in shape | Present. Comparatively larger in size and linear in shape |
| DNA arrangement | Circular | Linear |
| Mitochondria | Absent | Present |
| Cytoplasm | Present, but cell organelles absent | Present, cell organelles present |
| Endoplasmic reticulum | Absent | Present |
| Plasmids | Present | Very rarely found in eukaryotes |
| Ribosome | Small ribosomes | Large ribosomes |
| Lysosome | Lysosomes and centrosomes are absent | Lysosomes and centrosomes are present |
| Cell division | Through binary fission | Through mitosis |
| Flagella | The flagella are smaller in size | The flagella are larger in size |
| Reproduction | Asexual | Both asexual and sexual |
| Example | Bacteria and Archaea | Plant and Animal cell |
1.#Plasma membrane#
The plasma membrane, or the cell membrane, provides protection for a cell. It also provides a fixed environment inside the cell. And that membrane has several different functions. One is to transport nutrients into the cell and also to transport toxic substances out of the cell. Another is that the membrane of the cell, which would be the plasma membrane, will have proteins on it which interact with other cells. Those proteins can be glycoprotein, meaning there's a sugar and a protein moiety, or they could be lipid proteins, meaning there's a fat and a protein. And those proteins which stick outside of the plasma membrane will allow for one cell to interact with another cell. The cell membrane also provides some structural support for a cell. And there are different types of plasma membranes in different types of cells, and the plasma membrane has in it in general a lot of cholesterol as its lipid component. That's different from certain other membranes within the cell. Now, there are different plants and different microbes, such as bacteria and algae, which have different protective mechanisms. In fact, they have a cell wall outside of them, and that cell wall is much tougher and is structurally more sound than a plasma membrane is.
What is Diffusion ?
Diffusion
Diffusion is the net movement of anything from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in concentration.
What is Osmosis ?
Osmosis
Osmosis is the spontaneous net movement or diffusion of solvent molecules through a selectively permeable membrane from a region of high water potential to a region of low water potential, in the direction that tends to equalize the solute concentrations on the two sides.
Difference Between Osmosis and Diffusion
Definition of Osmosis
Osmosis refers to a type of diffusion in which a substance crosses a semipermeable membrane so as to balance the concentrations of another substance. Furthermore, in the field of biology, it happens when a solvent such as water flows into a cell or out of it on the basis of a solute’s concentration like salt. Also, the occurrence of osmosis takes place spontaneously without any use of cell energy.
Definition of Diffusion
Diffusion can be defined as a physical process that refers to the net movement of molecules from a region of high concentration to a lower concentration region. This definition certainly makes the difference between osmosis and diffusion more clear. Furthermore, the material that diffuses could be of any state of matter-solid, liquid, or gas.
2.#Cell Wall #
cell wall, specialized form of extracellular matrix that surrounds every cell of a plant. The cell wall is responsible for many of the characteristics that distinguish plant cells from animal cells. Although often perceived as an inactive product serving mainly mechanical and structural purposes, the cell wall actually has a multitude of functions upon which plant life depends. Such functions include: (1) providing the living cell with mechanical protection and a chemically buffered environment, (2) providing a porous medium for the circulation and distribution of water, minerals, and other small nutrient molecules, (3) providing rigid building blocks from which stable structures of higher order, such as leaves and stems, can be produced, and providing a storage site of regulatory molecules that sense the presence of pathogenic microbes and control the development of tissues.
Certain prokaryotes, algae, slime molds, water molds, and fungi also have cell walls. Bacterial cell walls are characterized by the presence of peptidoglycan, whereas those of Archaea characteristically lack this chemical. Algal cell walls are similar to those of plants, and many contain specific polysaccharides that are useful for taxonomy. Unlike those of plants and algae, fungal cell walls lack cellulose entirely and contain chitin. The scope of this article is limited to plant cell walls.
Cell Wall Structure
The cell wall is the outer covering of a cell, present adjacent to the cell membrane, which is also called the plasma membrane. As mentioned earlier, the cell wall is present in all plant cells, fungi, bacteria, algae, and some archaea. An animal cell is irregular in their shape and this is mainly due to the lack of cell wall in their cells. The compositions of the cell wall usually vary along with organisms.
The plant cell wall is generally arranged in 3 layers and composed of carbohydrates, like pectin, cellulose, hemicellulose and other smaller amounts of minerals, which form a network along with structural proteins to form the cell wall. The three major layers are:
Primary Cell Wall
The Middle Lamella
The Secondary Cell Wall
2.#Nucleus #
What is a Nucleus?
The most integral component of the cell is the nucleus (plural: nuclei). It is derived from a Latin word which means “kernel of a nut”.
Nucleus Definition:
A nucleus is defined as a double-membraned eukaryotic cell organelle that contains the genetic material.
The nucleus is one of the most obvious parts of the cell when you look
at a picture of the cell. It's in the middle of the cell, and the
nucleus contains all of the cell's chromosomes, which encode the genetic
material. So this is really an important part of the cell to protect.
The nucleus has a membrane around it that keeps all the chromosomes
inside and makes the distinction between the chromosomes being inside
the
nucleus and the other organelles and components of the cell staying
outside. Sometimes things like RNA need to traffic between the nucleus
and the cytoplasm, and so there are pores in this nuclear membrane that
allow molecules to go in and out of the nucleus. It used to be thought
that the nuclear membrane only allowed molecules to go out, but now it's
realized that there is an active process also for bringing molecules
into the nucleus.
As stated above, the nucleus is found only in eukaryotes and is the defining characteristic feature of eukaryotic cells. However, some cells, such as RBCs do not possess a nucleus, though they originate from a eukaryotic organisms.
Structure Of Nucleus
- Typically, it is the most evident organelle in the cell.
- The nucleus is completely bound by membranes.
- It is engirdled by a structure referred to as the nuclear envelope.
- The membrane distinguishes the cytoplasm from the contents of the nucleus
- The cell’s chromosomes are also confined within it.
- DNA is present in the Chromosomes, and they provide the genetic information required for the creation of different cell components in addition to the reproduction of life.
Nucleus Function
Following are the important nucleus function:
- It contains the cell’s hereditary information and controls the cell’s growth and reproduction.
- The nucleus has been clearly explained as a membrane-bound structure that comprises the genetic material of a cell.
- It is not just a storage compartment for DNA, but also happens to be the home of some important cellular processes.
- First and foremost, it is possible to duplicate one’s DNA in the nucleus. This process has been named DNA Replication and produces an identical copy of the DNA.
- Producing two identical copies of the body or host is the first step in cell division, where every new cell will get its own set of instructions.
- Secondly, the nucleus is the site of transcription. Transcription creates different types of RNA from DNA. Transcription would be a lot like creating copies of individual pages of the human body’s instructions which may be moved out and read by the rest of the cell.
- The central rule of biology states that DNA is copied into RNA, and then proteins.
What are Cell Organelles?
The cellular components are called cell organelles. These cell organelles include both membrane and non-membrane bound organelles, present within the cells and are distinct in their structures and functions. They coordinate and function efficiently for the normal functioning of the cell. A few of them function by providing shape and support, whereas some are involved in the locomotion and reproduction of a cell. There are various organelles present within the cell and are classified into three categories based on the presence or absence of membrane.
Organelles without membrane: The Cell wall, Ribosomes, and Cytoskeleton are non-membrane-bound cell organelles. They are present both in prokaryotic cell and the eukaryotic cell.
Single membrane-bound organelles: Vacuole, Lysosome, Golgi Apparatus, Endoplasmic Reticulum are single membrane-bound organelles present only in a eukaryotic cell.
Double membrane-bound organelles: Nucleus, mitochondria and chloroplast are double membrane-bound organelles present only in a eukaryotic cell.
Let us learn more in detail about the different cell organelles in brief.
Endoplasmic Reticulum
The Endoplasmic Reticulum is a network of membranous canals filled
with fluid. They are the transport system of the cell, involved in
transporting materials throughout the cell.
There are two different types of Endoplasmic Reticulum:
- Rough Endoplasmic Reticulum – They are composed of cisternae, tubules, and vesicles, which are found throughout the cell and are involved with protein manufacture.
- Smooth Endoplasmic Reticulum – They are the storage organelle, associated with the production of lipids, steroids, and also responsible for detoxifying the cell.
Cytoplasm
The cytoplasm is present both in plant and animal cells. They are jelly-like substances, found between the cell membrane and nucleus. They are mainly composed of water, organic and inorganic compounds. The cytoplasm is one of the essential components of the cell, where all the cell organelles are embedded. These cell organelles contain enzymes, mainly responsible for controlling all metabolic activity taking place within the cell and are the site for most of the chemical reactions within a cell.
Mitochondria
Mitochondria are called the powerhouses of the cell as they produce energy-rich molecules for the cell. The mitochondrial genome is inherited maternally in several organisms. It is a double membrane-bound, sausage-shaped organelle, found in almost all eukaryotic cells.
The double membranes divide its lumen into two distinct aqueous compartments. The inner compartment is called ‘matrix’ which is folded into cristae whereas the outer membrane forms a continuous boundary with the cytoplasm. They usually vary in their size and are found either round or oval in shape. Mitochondria are the sites of aerobic respiration in the cell, produces energy in the form of ATP and helps in the transformation of the molecules.
For instance, glucose is converted into adenosine triphosphate – ATP. Mitochondria have their own circular DNA, RNA molecules, ribosomes (the 70s), and a few other molecules that help in protein synthesis.
Plastids
Plastids are large, membrane-bound organelles which contain pigments. Based on the type of pigments, plastids are of three types:
- Chloroplasts – Chloroplasts are double membrane-bound organelles, which usually vary in their shape – from a disc shape to spherical, discoid, oval and ribbon. They are present in mesophyll cells of leaves, which store chloroplasts and other carotenoid pigments. These pigments are responsible for trapping light energy for photosynthesis. The inner membrane encloses a space called the stroma. Flattened disc-like chlorophyll-containing structures known as thylakoids are arranged in a stacked manner like a pile of coins. Each pile is called as granum (plural: grana) and the thylakoids of different grana are connected by flat membranous tubules known as stromal lamella. Just like the mitochondrial matrix, the stroma of chloroplast also contains a double-stranded circular DNA, 70S ribosomes, and enzymes which required for the synthesis of carbohydrates and proteins.
- Chromoplasts – The chromoplasts include fat-soluble, carotenoid pigments like xanthophylls, carotene, etc. which provide the plants with their characteristic color – yellow, orange, red, etc.
- Leucoplasts – Leucoplasts are colorless plastids which store nutrients. Amyloplasts store carbohydrates (like starch in potatoes), aleuroplasts store proteins, and elaioplasts store oils and fats.
Ribosomes
Golgi Apparatus
Golgi Apparatus also termed as Golgi Complex. It is a membrane-bound organelle, which is mainly composed of a series of flattened, stacked pouches called cisternae. This cell organelle is primarily responsible for transporting, modifying, and packaging proteins and lipid to targeted destinations. Golgi Apparatus is found within the cytoplasm of a cell and are present in both plant and animal cells.
Cell Organelles
| Cell Organelles | Structure |
Functions |
| Cell membrane | A double membrane composed of lipids and proteins. Present both in plant and animal cell. | Provides shape, protects the inner organelle of the cell and acts as a selectively permeable membrane. |
| Centrosomes | Composed of Centrioles and found only in the animal cells. | It plays a major role in organizing the microtubule and Cell division. |
| Chloroplasts | Present only in plant cells and contains a green-coloured pigment known as chlorophyll. | Sites of photosynthesis. |
| Cytoplasm | A jelly-like substance, which consists of water, dissolved nutrients and waste products of the cell. | Responsible for the cell’s metabolic activities. |
| Endoplasmic Reticulum | A network of membranous tubules, present within the cytoplasm of a cell. | Forms the skeletal framework of the cell, involved in the Detoxification, production of Lipids and proteins. |
| Golgi apparatus | Membrane-bound, sac-like organelles, present within the cytoplasm of the eukaryotic cells. | It is mainly involved in secretion and intracellular transport. |
| Lysosomes | A tiny, circular-shaped, single membrane-bound organelles, filled with digestive enzymes. | Helps in the digestion and removes wastes and digests dead and damaged cells. Therefore, it is also called as the “suicidal bags”. |
| Mitochondria | An oval-shaped, membrane-bound organelle, also called as the “Power House of The Cell”. | The main sites of cellular respiration and also involved in storing energy in the form of ATP molecules. |
| Nucleus | A largest, double membrane-bound organelles, which contains all the cell’s genetic information. | Controls the activity of the cell, helps in cell division and controls the hereditary characters. |
| Peroxisome | A membrane-bound cellular organelle present in the cytoplasm, which contains the reducing enzyme. | Involved in the metabolism of lipids and catabolism of long-chain fatty acids. |
| Plastids | Double membrane-bound organelles. There are 3 types of plastids:
|
Helps in the process of photosynthesis and pollination, Imparts colour for leaves, flowers and fruits and stores starch, proteins and fats. |
| Ribosomes | Non-membrane organelles, found floating freely in the cell’s cytoplasm or embedded within the endoplasmic reticulum. | Involved in the Synthesis of Proteins. |
| Vacuoles | A membrane-bound, fluid-filled organelle found within the cytoplasm. | Provide shape and rigidity to the plant cell and helps in digestion, excretion, and storage of substances. |
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