NCERT Class 9 Science Solutions: The Fundamental Unit of Life

Question:

Cell membrane structure (fluid mosaic model), permeability (semi-permeable), and transport mechanisms (diffusion, osmosis, active transport).


Substances like CO2 and water move across the cell membrane through various transport mechanisms based on the principles of diffusion and osmosis, which are passive processes.

Carbon dioxide (CO2) is a small, nonpolar molecule. It moves into and out of the cell primarily by diffusion. Diffusion is the movement of a substance from a region of higher concentration to a region of lower concentration. Inside the cell, cellular respiration produces CO2, leading to a higher concentration of CO2 within the cell compared to the extracellular environment. Therefore, CO2 diffuses out of the cell. Conversely, if the extracellular environment has a higher concentration of CO2, it will diffuse into the cell. The cell membrane is freely permeable to CO2, allowing for this rapid movement.

Water, a polar molecule, moves across the cell membrane through osmosis. Osmosis is a special type of diffusion where water moves across a semi-permeable membrane from a region of higher water concentration (lower solute concentration) to a region of lower water concentration (higher solute concentration). The cell membrane acts as a semi-permeable barrier, allowing water to pass through but restricting the passage of many solutes. Therefore, if the solute concentration outside the cell is lower than inside, water will move into the cell. If the solute concentration outside is higher, water will move out of the cell. Proteins called aquaporins embedded in the cell membrane also facilitate the rapid movement of water by osmosis. While water can also move by simple diffusion, aquaporins significantly increase the rate of water transport.

Question:

The question asks about the fundamental differences between prokaryotic and eukaryotic cells. To answer this, one needs to recall the defining characteristics of these two cell types, focusing on structural and organizational features. Key concepts include the presence or absence of a nucleus, membrane-bound organelles, genetic material organization, cell size, and cell wall composition.


Prokaryotic cells are simpler and smaller than eukaryotic cells. Their defining feature is the absence of a true nucleus and membrane-bound organelles like mitochondria, endoplasmic reticulum, and Golgi apparatus. The genetic material (DNA) in prokaryotes is usually a single circular chromosome located in a region called the nucleoid, not enclosed by a membrane. Ribosomes are present in both cell types but differ in size. Prokaryotes often have a cell wall made of peptidoglycan (in bacteria).

Eukaryotic cells are more complex and generally larger. They possess a true nucleus, which encloses their linear chromosomes. Eukaryotic cells also contain various membrane-bound organelles, each with specialized functions. Examples include mitochondria for energy production, endoplasmic reticulum for protein and lipid synthesis, and Golgi apparatus for modification and packaging of proteins. Ribosomes in eukaryotes are larger. The presence or absence of a cell wall varies; plant cells have a cell wall made of cellulose, while animal cells do not. Fungal cells have cell walls made of chitin.

In summary, the primary distinctions lie in the presence of a nucleus and membrane-bound organelles, the organization of genetic material, and differences in size and cell wall composition.

Question:

The question asks for the justification of why a cell is considered the fundamental unit of life. This requires understanding the basic roles and characteristics of cells in living organisms. Key concepts include:
– Definition of a cell
– Structural organization of living beings
– Functional processes occurring within living beings


The cell is called the structural unit of life because all living organisms, from the simplest to the most complex, are made up of cells. In unicellular organisms, a single cell performs all life processes. In multicellular organisms, different cells are specialized to perform specific functions, and these cells organize to form tissues, organs, and organ systems, which together constitute the entire organism. Therefore, the cell provides the basic framework and building blocks for all living structures.

The cell is also called the functional unit of life because all the essential life processes necessary for survival and reproduction occur within the cell. These processes include metabolism (like respiration and digestion), growth, reproduction, response to stimuli, and excretion. Even in multicellular organisms, these fundamental life functions are carried out by individual cells or coordinated groups of cells. Without cellular activity, life as we know it would not be possible. Thus, the cell is the site where all vital activities take place, making it the functional basis of life.

Question:

Cell organelles, eukaryotic cells, genetic material, DNA, independent replication


The question asks to identify two organelles within a cell that possess their own genetic material. This property is a key characteristic of certain organelles that were once believed to be independent prokaryotic organisms engulfed by early eukaryotic cells (endosymbiotic theory). These organelles have their own DNA, which is separate from the nuclear DNA of the cell, and they can replicate independently.

The two organelles that fit this description are:
1. Mitochondria: These are often referred to as the “powerhouses” of the cell as they are responsible for cellular respiration and ATP production. Mitochondria contain a circular DNA molecule that is distinct from the DNA found in the cell’s nucleus. They also have their own ribosomes and can synthesize some of their own proteins.

2. Chloroplasts: Found in plant cells and some algae, chloroplasts are the sites of photosynthesis. Similar to mitochondria, chloroplasts also contain their own circular DNA and ribosomes. This allows them to carry out some of their own protein synthesis and to replicate independently of the cell.

Therefore, the two organelles that contain their own genetic material are mitochondria and chloroplasts.

Question:

The question asks about two fundamental types of cell division: one responsible for increasing the number of cells in an organism for growth and to fix damaged tissues, and another that produces reproductive cells. To answer this, you need to recall the distinct purposes and mechanisms of mitosis and meiosis. Mitosis is known for producing genetically identical daughter cells, essential for somatic cell proliferation. Meiosis, on the other hand, involves two rounds of division to produce haploid gametes, crucial for sexual reproduction.


The type of cell division required for growth and repair of the body is MITOSIS. Mitosis is a process of asexual reproduction in which a parent cell divides into two identical daughter cells. This process increases the number of somatic cells, leading to the growth of an organism and enabling the replacement or repair of damaged tissues.

The type of cell division involved in the formation of gametes is MEIOSIS. Meiosis is a specialized type of cell division that reduces the chromosome number by half, creating four haploid cells (gametes). These gametes, such as sperm and egg cells, are essential for sexual reproduction. When gametes fuse during fertilization, they restore the diploid number of chromosomes in the offspring.

Question:

Cellular Organization and Function: Cells are the basic units of life, and their internal organization is crucial for carrying out essential life processes. This organization involves specific structures (organelles) performing specialized functions.
Cellular Integrity: Maintaining the structural integrity of the cell and its organelles is vital for survival. Physical or chemical damage can disrupt this integrity.
Consequences of Disruption: Destruction of cellular organization leads to the failure of vital functions, ultimately leading to cell death.


If the organization of a cell is destroyed due to some physical or chemical influence, its ability to perform essential life functions will be severely compromised. The cell’s internal environment will become unstable, and its organelles, which are responsible for specific tasks like energy production, protein synthesis, and waste removal, will cease to function correctly. This disruption can lead to a breakdown of metabolic processes, impaired communication with the environment, and an inability to maintain homeostasis. Ultimately, if the damage is extensive enough, the cell will be unable to survive and will undergo programmed cell death (apoptosis) or necrosis.

Question:

The question requires understanding the fundamental structural differences between prokaryotic and eukaryotic cells, focusing on characteristics like nuclear region, chromosomes, and the presence of membrane-bound organelles.


The table highlights key distinctions between prokaryotic and eukaryotic cells. Let’s fill in the gaps:

For point 2 under Prokaryotic cell:
The nuclear region in prokaryotic cells is not enclosed by a membrane. It is described as ‘undefined’ or ‘poorly defined’ and is a region containing the genetic material. This region is known as the ‘nucleoid’.

So, the gap for point 2 under Prokaryotic cell should be:
undefined or poorly defined
lacks a nuclear membrane
nucleoid

For point 4 under Eukaryotic cell:
The statement for prokaryotic cells is “Membrane-bound cell organelles absent”. The corresponding point for eukaryotic cells should state the opposite, which is the presence of these organelles.

So, the gap for point 4 under Eukaryotic cell should be:
Membrane-bound cell organelles
present
Examples include mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, etc. (any two or three of these would suffice).

Question:

Cell Membrane Structure and Synthesis: The cell membrane is primarily composed of lipids (phospholipids) and proteins. Understanding the cellular organelles responsible for the synthesis of these biomolecules is crucial.


The lipids that constitute the cell membrane, primarily phospholipids, are synthesized in the endoplasmic reticulum (ER). Specifically, the smooth endoplasmic reticulum (SER) is the main site for phospholipid synthesis.

The proteins that are embedded within or associated with the cell membrane are synthesized in ribosomes. Ribosomes attached to the rough endoplasmic reticulum (RER) synthesize membrane proteins that are destined for secretion, insertion into membranes, or delivery to other organelles. Free ribosomes in the cytoplasm synthesize proteins that function within the cytoplasm or are targeted to other organelles like mitochondria or the nucleus. However, for proteins that become part of the cell membrane, they are generally synthesized on ribosomes associated with the RER, after which they are processed and transported through the Golgi apparatus before reaching their final destination in the cell membrane.

Question:

The plasma membrane is a biological membrane that surrounds the cytoplasm of a cell. It is composed primarily of a phospholipid bilayer with embedded proteins. Its function is to regulate the passage of substances into and out of the cell.


The plasma membrane is called selectively permeable because it allows certain molecules or ions to pass through it by means of active or passive transport. This selective permeability is due to the structure of the phospholipid bilayer and the presence of transport proteins. The hydrophobic tails of the phospholipids form an interior barrier that prevents the free passage of polar molecules and ions. Small, nonpolar molecules like oxygen and carbon dioxide can pass through easily. Larger molecules and charged ions, however, require the assistance of specific transport proteins embedded within the membrane to cross. These proteins act like channels or carriers, binding to specific substances and facilitating their movement across the membrane. Therefore, the membrane “selects” which substances can enter or leave the cell and at what rate, hence being selectively permeable.

Question:

The question asks about the cellular location of protein synthesis. This involves understanding the roles of different organelles within a cell, specifically those involved in the production of macromolecules like proteins. Key organelles to consider are the nucleus, ribosomes, endoplasmic reticulum, and Golgi apparatus. The central dogma of molecular biology, which describes the flow of genetic information from DNA to RNA to protein, is also relevant.


Proteins are synthesized inside the cell by structures called ribosomes. Ribosomes are often described as the “protein factories” of the cell. They can be found freely floating in the cytoplasm or attached to the surface of the endoplasmic reticulum. The process of protein synthesis, known as translation, involves reading the genetic code carried by messenger RNA (mRNA) and assembling amino acids into a polypeptide chain. This mRNA molecule carries the instructions transcribed from DNA in the nucleus to the ribosomes. Therefore, ribosomes are the primary sites of protein synthesis within the cell.

Question:

The key concept is the movement of water molecules across a selectively permeable membrane. Understanding what a selectively permeable membrane is and the direction of water movement based on concentration differences is crucial.


Osmosis is the net movement of solvent molecules (typically water) through a selectively permeable membrane from a region of higher solvent concentration to a region of lower solvent concentration. This movement occurs to equalize the solute concentration on both sides of the membrane. A selectively permeable membrane allows certain molecules or ions to pass through by diffusion, and occasionally specialized facilitated diffusion. In simpler terms, it’s the movement of water from an area where there’s more water (and less solute) to an area where there’s less water (and more solute), until the concentrations are balanced.

Question:

Cell: The basic structural and functional unit of all known living organisms.
Microscope: An optical instrument used to observe very small objects, such as specimens or samples, that cannot be seen with the naked eye.
Observation: The process of noticing and describing events or processes in a careful, orderly way.


Robert Hooke discovered cells. In 1665, he observed thin slices of cork under a microscope. He noticed that the cork was made up of many small, hollow compartments, which he called “cells” because they reminded him of the small rooms in a monastery called cells. He drew these observations in his book, Micrographia. While Hooke was the first to observe and name cells, it was later that scientists like Antonie van Leeuwenhoek, using more advanced microscopes, observed living cells and made significant contributions to cell biology.

Question:

Cellular organelle, lysosomes, digestive enzymes, cell membrane rupture, autolysis


Lysosomes are membrane-bound organelles within animal cells that contain powerful digestive enzymes. These enzymes are capable of breaking down waste materials and cellular debris. In normal conditions, the lysosomal membrane prevents these enzymes from damaging the rest of the cell. However, if the cell is damaged, or under certain stressful conditions like starvation or exposure to toxins, the lysosomal membrane can rupture. Once the membrane breaks, the digestive enzymes are released into the cytoplasm and begin to break down the cell’s own components, leading to its self-destruction. This process is known as autolysis, and because lysosomes are responsible for initiating this self-digestion, they are referred to as “suicide bags.”

Question:

Amoeba is a unicellular organism belonging to the kingdom Protista. It feeds by a process called phagocytosis, which involves engulfing food particles using its pseudopodia.


An Amoeba obtains its food through a process called phagocytosis. When an Amoeba encounters a food particle, it extends temporary finger-like projections called pseudopodia from its cell membrane. These pseudopodia surround the food particle and fuse together, enclosing it within a food vacuole inside the cytoplasm. Digestive enzymes are then secreted into the food vacuole, breaking down the complex food material into simpler substances that can be absorbed by the cell. Undigested food material is then expelled from the cell.

Question:

Cellular respiration, ATP, energy production, organelles


The mitochondrion is known as the powerhouse of the cell. This is because mitochondria are the primary sites of cellular respiration, a process that generates adenosine triphosphate (ATP). ATP is the main energy currency of the cell, providing the energy required for most cellular activities, such as muscle contraction, nerve impulse propagation, and synthesis of molecules. Therefore, the organelle responsible for producing the vast majority of the cell’s usable energy is called the powerhouse.