The Fundamental Unit Of Life, Science, Class 9 - Test - Class 9 MCQ

The inner membrane of mitochondria is folded because:
- Increased Surface Area: The folding of the inner membrane of mitochondria greatly increases its surface area. This is important because the inner membrane is the site of many crucial metabolic reactions, including the production of ATP through oxidative phosphorylation. The increased surface area allows for more space for these reactions to occur, increasing the efficiency of energy production.
- Enhanced Transport: The folding of the inner membrane also facilitates the transport of molecules across the membrane. The folds, known as cristae, create numerous small compartments within the mitochondria, allowing for more efficient movement of molecules such as metabolites, ions, and proteins. This enhances the overall functioning of the mitochondria.
- Optimal ATP Production: The folding of the inner membrane is essential for the production of ATP, the main energy currency of the cell. The inner membrane houses the electron transport chain and ATP synthase, which work together to generate ATP. The folding increases the number of these complexes, maximizing ATP production.
- Space Optimization: The folding of the inner membrane helps to optimize space within the mitochondria. By creating numerous compartments, the overall volume of the mitochondria can be increased without significantly increasing its size. This allows for a higher density of metabolic enzymes and other components, further enhancing the efficiency of energy production.
In conclusion, the folding of the inner membrane of mitochondria is crucial for increasing surface area, enhancing transport, optimizing ATP production, and maximizing space utilization. These adaptations contribute to the overall functioning and efficiency of mitochondria in cellular energy metabolism.

Proteins are formed in:
Ribosomes:
- Proteins are primarily formed in ribosomes, which are small organelles found in the cytoplasm of cells.
- Ribosomes are composed of ribosomal RNA (rRNA) and proteins, and they are responsible for protein synthesis.
- Ribosomes read the genetic information encoded in messenger RNA (mRNA) and use it to assemble amino acids into a polypeptide chain, which then folds into a functional protein.
Golgi Bodies:
- Golgi bodies, also known as Golgi apparatus or Golgi complex, are involved in modifying, sorting, and packaging proteins.
- While Golgi bodies play a crucial role in protein processing and distribution within the cell, they do not directly create proteins.
Nucleus:
- The nucleus is the control center of the cell and contains the genetic material in the form of DNA.
- DNA provides the instructions for protein synthesis, but the actual synthesis of proteins occurs outside the nucleus in the cytoplasm, specifically in the ribosomes.
Plastids:
- Plastids are a group of organelles found in plant cells, including chloroplasts, chromoplasts, and leucoplasts.
- While plastids are involved in various metabolic processes, such as photosynthesis in chloroplasts, they are not directly responsible for protein synthesis.
Therefore, the correct answer is D: Ribosomes. Ribosomes are the primary site of protein synthesis in cells.

Answer:
The organelle that helps in membrane biogenesis is the endoplasmic reticulum (ER).
Explanation:
The endoplasmic reticulum is a network of membrane-bound tubules and sacs that are involved in various cellular processes, including the synthesis and modification of proteins and lipids. Here is how the endoplasmic reticulum helps in membrane biogenesis:
1. Protein synthesis: The rough endoplasmic reticulum (RER) is studded with ribosomes and is responsible for the synthesis of membrane proteins. These proteins are synthesized by ribosomes attached to the ER and then inserted into the ER membrane.
2. Lipid synthesis: The smooth endoplasmic reticulum (SER) is involved in the synthesis of lipids, including phospholipids, which are the major components of cell membranes. Lipids are synthesized in the SER and then transported to the ER membrane.
3. Protein modification: The endoplasmic reticulum also plays a role in the modification of proteins. It adds carbohydrate groups to proteins, a process known as glycosylation. These modified proteins are then transported to other organelles or the cell membrane.
4. Vesicle formation: The endoplasmic reticulum is involved in the formation of vesicles, which are small membrane-bound sacs that transport materials within the cell. Vesicles can transport lipids and proteins from the ER to other organelles or the cell membrane.
In summary, the endoplasmic reticulum is crucial for membrane biogenesis as it synthesizes membrane proteins and lipids, modifies proteins, and participates in vesicle formation for the transport of these components. Therefore, option C, the endoplasmic reticulum, is the correct answer.

Explanation:
The process of osmosis involves the movement of water across a semi-permeable membrane from an area of lower solute concentration to an area of higher solute concentration. When a cell is placed in a solution, the direction of water movement depends on the relative concentration of solutes inside and outside the cell.
In the given question, we are asked to identify the solution in which a cell will gain water by osmosis. This means that the solute concentration inside the cell is higher than the solute concentration outside the cell. Let's examine the options:
A: Isotonic solution:
- An isotonic solution has the same concentration of solutes as the cell.
- In an isotonic solution, there is no net movement of water across the cell membrane.
- Therefore, a cell will not gain water by osmosis in an isotonic solution.
B: Hypertonic solution:
- A hypertonic solution has a higher concentration of solutes than the cell.
- In a hypertonic solution, water will move out of the cell by osmosis, causing the cell to shrink.
- Therefore, a cell will not gain water by osmosis in a hypertonic solution.
C: Hypotonic solution:
- A hypotonic solution has a lower concentration of solutes than the cell.
- In a hypotonic solution, water will move into the cell by osmosis, causing the cell to swell.
- Therefore, a cell will gain water by osmosis in a hypotonic solution.
D: Both (a) and (b):
- This option is incorrect because both isotonic and hypertonic solutions do not result in a cell gaining water by osmosis.
Therefore, the correct answer is C: Hypotonic solution where a cell will gain water by osmosis.

The root hair absorbs water by the process called osmosis.
Explanation:
Osmosis is the process by which water molecules move across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. Root hairs are specialized structures found in the epidermis of plant roots, which increase the surface area for water absorption.
Here is a detailed explanation of how root hairs absorb water through osmosis:
1. Presence of a concentration gradient: The soil surrounding the root hairs has a higher solute concentration compared to the cytoplasm of the root hair cells. This creates a concentration gradient that drives the movement of water.
2. Semipermeable membrane: The plasma membrane of the root hair cells acts as a semipermeable membrane. It allows water molecules to pass through but restricts the movement of solutes.
3. Water uptake: The root hairs actively transport mineral ions from the soil into the cytoplasm of the root hair cells using energy. This creates a higher solute concentration inside the cells.
4. Osmotic pressure: The higher solute concentration inside the root hair cells creates an osmotic pressure. Water molecules, being smaller and able to pass through the semipermeable membrane, move from the area of lower solute concentration (soil) to the area of higher solute concentration (root hair cells) by osmosis.
5. Water movement: As water enters the root hair cells through osmosis, it moves through the interconnected cell walls and cytoplasm of the root cells, eventually reaching the xylem vessels. From the xylem vessels, water is transported upwards to other parts of the plant.
In conclusion, osmosis is the process by which root hairs absorb water from the soil, utilizing the concentration gradient and the semipermeable membrane of the root hair cells.

The animal cell which does not possess nucleus is a red blood cell (RBC).
Explanation:
Red blood cells, also known as erythrocytes, are the most common type of blood cells in the human body. They are responsible for transporting oxygen from the lungs to the tissues and removing carbon dioxide from the body.
Here are some key points to explain why red blood cells do not possess a nucleus:
- RBCs undergo a process called enucleation during their development in the bone marrow. This means that the nucleus is expelled from the cell before it is released into the bloodstream.
- The absence of a nucleus in RBCs allows for more space to be available for hemoglobin, a protein that binds to and carries oxygen. This increases the efficiency of oxygen transport.
- Without a nucleus, RBCs have a biconcave shape, which enables them to squeeze through narrow blood vessels and increases their surface area for gas exchange.
- The lack of a nucleus also means that RBCs cannot divide or undergo protein synthesis. They have a limited lifespan of approximately 120 days before being removed by the spleen and liver.
Other options:
- Egg of hen: The egg of a hen is a complete cell with a nucleus.
- White blood cell: White blood cells, also known as leukocytes, do possess a nucleus. They play a crucial role in the immune system by defending the body against infections and foreign substances.
- Nerve cell: Nerve cells, also called neurons, are specialized cells that transmit electrical signals in the nervous system. They have a distinct nucleus and are involved in various functions such as sensory perception, motor control, and information processing.

Discovery of the Cell Nucleus
The discovery of the cell nucleus is an important milestone in the field of biology. Let's explore the contributions of various scientists in this discovery:
1. Robert Hooke:
- Robert Hooke was an English scientist who, in 1665, observed thin slices of cork under a microscope.
- He coined the term "cell" to describe the box-like structures he observed, which resembled the cells of a monastery.
- However, he did not discover the nucleus itself.
2. Antonie van Leeuwenhoek:
- Antonie van Leeuwenhoek was a Dutch scientist who made significant contributions to the development of microscopy.
- He improved the design of the microscope and made detailed observations of various biological specimens, including cells.
- However, he did not specifically identify the nucleus within the cell.
3. Robert Brown:
- Robert Brown, a Scottish botanist, made the discovery of the cell nucleus in 1831.
- While studying plant cells, he observed a dense, centrally located structure within the cells, which he named the "nucleus."
- Brown's discovery of the nucleus was a groundbreaking finding in cell biology.
4. Jan Evangelista Purkinje:
- Jan Evangelista Purkinje, a Czech anatomist, also made significant contributions to the field of biology.
- He was the first to use the term "protoplasm" to describe the substance within living cells.
- However, he did not specifically discover the cell nucleus.
Conclusion:
The correct answer is option C: Robert Brown. He was the scientist who discovered the nucleus of the cell in 1831. Although Robert Hooke and Antonie van Leeuwenhoek made significant contributions to the field of microscopy and cell observation, it was Robert Brown who specifically identified the nucleus within the cell. Jan Evangelista Purkinje, on the other hand, made important contributions to the understanding of protoplasm but did not discover the cell nucleus.

The plant cells are more rigid than the animal cell due to presence of cell-wall.

Opening and Closing of Stomata
The opening and closing of stomata, which are small pores found on the surface of plant leaves, is a crucial process for regulating the exchange of gases and water vapor between the plant and its environment. This process is mainly regulated by the movement of water through osmosis.
Explanation:
Stomata open during the day to allow the entry of carbon dioxide (CO2) for photosynthesis and the exit of oxygen (O2) and water vapor (H2O) through transpiration. At night, stomata close to conserve water and prevent excessive loss through transpiration.
The opening and closing of stomata is primarily controlled by the movement of water in and out of guard cells, which surround the stomatal pore. Here is a detailed explanation of how this process occurs:
1. Light Intensity: Stomata open in response to light intensity. When light intensity is high, photosynthetic activity increases, and the plant requires more carbon dioxide for photosynthesis. This stimulates the opening of stomata.
2. Osmosis: The opening and closing of stomata are mainly regulated by the movement of water through osmosis. When stomata open, the concentration of solutes (such as potassium ions) increases inside the guard cells, creating a lower water potential. As a result, water flows into the guard cells by osmosis, causing them to become turgid and bend outwards, thus opening the stomatal pore.
3. Guard Cell Shape: The shape of the guard cells also plays a role in stomatal opening and closing. When the guard cells become turgid and swell with water, they curve outward, creating a larger pore size for gas exchange. Conversely, when the guard cells lose water, they become flaccid and close the stomatal pore.
4. Environmental Factors: Environmental factors such as temperature, humidity, and water availability can also influence stomatal opening and closing. For example, high temperatures and low humidity can lead to increased transpiration rates, causing stomata to close to conserve water.
In conclusion, the opening and closing of stomata is primarily regulated by the movement of water through osmosis, which is influenced by light intensity, guard cell shape, and environmental factors. This process allows plants to regulate gas exchange and water loss, ensuring their survival and optimal functioning.

Prokaryotic cells are primitive cells. They do not possess many membrane bound organelles as mitochondria, lysosomes etc. They also do not haven well-defined membrane-bound nucleus. Nuclear material of a prokaryotic cell consists of a single chromosome which is in direct contact with cytoplasm. Here, the undefined nuclear region in the cytoplasm is called as nucleoid, i.e., there is no nuclear membrane.