Class 9 Science NCERT Notes – Chapter 5: The Fundamental Unit of Life (PDF, MindMap, Q&A, Quizzes)

Chapter 5 (Biology): The Fundamental Unit of Life – Class 9 NCERT Science Detailed Study Notes.

1. The Discovery and History of the Cell

• Robert Hooke (1665): First discovered cells by observing a thin slice of cork with a self-designed, primitive microscope. He saw honeycomb-like compartments, which he named “cells” (from the Latin word for ‘a little room’). This was the first observation that living things consist of separate units.
• Leeuwenhoek (1674): Using an improved microscope, he was the first to discover free-living cells, which he observed in pond water.
• Robert Brown (1831): Discovered the nucleus within the cell.
• Purkinje (1839): Coined the term “protoplasm” to describe the fluid substance of the cell.
• The Cell Theory:
  • Initially proposed by two biologists, Schleiden (1838) and Schwann (1839).
  • It stated that all plants and animals are composed of cells and that the cell is the basic unit of life.
  • The theory was later expanded by Virchow (1855), who suggested that all cells arise from pre-existing cells.
• Electron Microscope (1940): The invention of the electron microscope allowed for detailed observation and understanding of the complex structure of the cell and its organelles.

2. Types of Organisms

• Unicellular Organisms: A single cell constitutes the whole organism. Examples include Amoeba, Chlamydomonas, Paramoecium, and bacteria.
• Multicellular Organisms: Many cells group together to perform different functions and form various body parts. Examples include fungi, plants, and animals. Every multicellular organism originates from a single cell.

3. The Structural Organisation of a Cell

All living cells possess certain basic functions and are able to live due to specific components called cell organelles. This internal “division of labour” allows the cell to perform functions like making new material, clearing waste, and respiration. Three features are found in almost every cell: the plasma membrane, the nucleus, and the cytoplasm.

3.1 Plasma Membrane (or Cell Membrane)

• Structure: The outermost covering of the cell, composed of organic molecules called lipids and proteins. Its structure is flexible and can only be observed with an electron microscope.
• Function:
  • Separates the cell’s contents from the external environment.
  • It is a selectively permeable membrane, meaning it allows or permits the entry and exit of some materials while preventing the movement of others.
• Transport Mechanisms:
  • Diffusion: The spontaneous movement of a substance from a region of high concentration to a region of low concentration. This is crucial for gaseous exchange (e.g., CO₂ moving out and O₂ moving into the cell).
  • Osmosis: A special case of diffusion involving the movement of water molecules across a selectively permeable membrane from a region of higher water concentration to one of lower water concentration (or toward a higher solute concentration).
  • Endocytosis: The flexibility of the membrane allows the cell to engulf food and other material from its external environment. This is how Amoeba acquires its food.

3.2 Effects of External Solutions on a Cell (Osmosis)

• Hypotonic Solution: The surrounding medium has a higher water concentration (is more dilute) than the cell. The cell will gain water through osmosis and swell up.
• Isotonic Solution: The surrounding medium has the exact same water concentration as the cell. There is no net movement of water, and the cell will stay the same size.
• Hypertonic Solution: The surrounding medium has a lower water concentration (is more concentrated) than the cell. The cell will lose water through osmosis and shrink.

3.3 Cell Wall

• Presence: Found in plant cells, fungi, and bacteria, located outside the plasma membrane.
• Composition: In plants, it is mainly composed of cellulose, a complex substance that provides structural strength.
• Function:
  • Provides rigidity and structural support to the cell.
  • Allows cells to withstand very dilute (hypotonic) external media without bursting. When the cell takes up water, it swells and builds pressure against the cell wall, which exerts an equal pressure back.
• Plasmolysis: In a hypertonic solution, a living plant cell loses water, causing the contents of the cell to shrink and pull away from the cell wall. This phenomenon is known as plasmolysis.

3.4 Nucleus

• Structure:
  • A large, often spherical or oval, organelle.
  • Covered by a double-layered nuclear membrane which contains pores to allow material transfer to and from the cytoplasm.
  • Contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.
• Content:
  • Chromosomes are composed of DNA (Deoxyribonucleic Acid) and protein.
  • DNA contains the information for constructing and organizing cells and for the inheritance of characters (genes). Genes are functional segments of DNA.
  • In a non-dividing cell, DNA is present as chromatin material, an entangled mass of thread-like structures.
• Function:
  • Plays a central role in cellular reproduction (cell division).
  • Directs the chemical activities of the cell, determining how the cell will develop and what form it will exhibit at maturity.

3.5 Cytoplasm

• Definition: The fluid content inside the plasma membrane, enclosing the nucleus and various cell organelles. It takes up very little stain when observed under a microscope.
• Function: It is the site of many chemical reactions and contains the specialized cell organelles.

4. Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cell	Eukaryotic Cell
Size	Generally small (1-10 µm)	Generally large (5-100 µm)
Nuclear Region	Poorly defined, not surrounded by a nuclear membrane. Called a nucleoid.	Well-defined, surrounded by a double nuclear membrane.
Chromosome	Single	More than one chromosome
Membrane-bound Organelles	Absent	Present (e.g., ER, Golgi, mitochondria)
Examples	Bacteria	Fungi, Plants, Animals

5. Cell Organelles (Eukaryotic)

• Endoplasmic Reticulum (ER):
  • A large network of membrane-bound tubes and sheets.
  • Rough ER (RER): Has ribosomes attached to its surface. Ribosomes are the sites of protein manufacture.
  • Smooth ER (SER): Helps in the manufacture of fat molecules (lipids). In liver cells of vertebrates, SER detoxifies poisons and drugs.
  • Functions: Serves as a channel for transport of materials (especially proteins), and contributes to building the cell membrane (membrane biogenesis).
• Golgi Apparatus:
  • A system of membrane-bound, flattened sacs (vesicles) arranged in stacks called cisterns.
  • Functions: Storage, modification, and packaging of products (synthesized near the ER) in vesicles. It is also involved in the formation of lysosomes and can make complex sugars from simple sugars.
• Lysosomes:
  • Membrane-bound sacs filled with powerful digestive enzymes (made by RER).
  • Functions: Act as the cell’s waste disposal system, digesting foreign material (like bacteria) and worn-out organelles.
  • Known as “suicide bags” because if the cell is damaged, lysosomes may burst and their enzymes can digest their own cell.
• Mitochondria:
  • Known as the “powerhouses of the cell.”
  • Structure: Have two membranes; the outer is porous, and the inner is deeply folded to increase surface area.
  • Function: Release energy for chemical activities in the form of ATP (Adenosine triphosphate) molecules. ATP is the energy currency of the cell.
  • Special Feature: Have their own DNA and ribosomes, allowing them to make some of their own proteins.
• Plastids:
  • Present only in plant cells.
  • Chromoplasts: Coloured plastids. Those containing chlorophyll are called chloroplasts and are the site of photosynthesis.
  • Leucoplasts: White or colourless plastids that store materials like starch, oils, and protein granules.
  • Special Feature: Like mitochondria, plastids have their own DNA and ribosomes.
• Vacuoles:
  • Storage sacs for solid or liquid contents.
  • Small-sized in animal cells but very large in plant cells (can occupy 50-90% of cell volume).
  • Function in Plants: Filled with cell sap, they provide turgidity and rigidity to the cell and store substances like amino acids, sugars, and organic acids.

6. Cell Division

The process by which new cells are made. It is essential for growth, replacement of old/injured cells, and reproduction.

• Mitosis:
  • The process by which most cells divide for growth and repair.
  • One mother cell divides to form two identical daughter cells.
  • The daughter cells have the same number of chromosomes as the mother cell.
• Meiosis:
  • Occurs in specific reproductive cells to form gametes (sperm and egg).
  • Involves two consecutive divisions.
  • One mother cell produces four new cells.
  • The new cells have half the number of chromosomes as the mother cell.

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Q&A Section

Short-Answer Questions

Answer each question in 2-3 complete sentences.

1. Who first discovered cells, and what was he observing?
2. What are the three main points of the cell theory, including Virchow’s expansion?
3. Explain the difference between unicellular and multicellular organisms, providing an example of each.
4. Why is the plasma membrane described as a “selectively permeable membrane”?
5. Describe what happens to an animal cell when placed in a hypotonic solution and explain the process involved.
6. What is plasmolysis, and under what conditions does it occur in a plant cell?
7. What is the main function of the cell wall in plants, and what is it primarily composed of?
8. Describe the structure of the nucleus, including its membrane and its contents in a non-dividing state.
9. What are genes, and where are they located within a eukaryotic cell?
10. List three key differences between prokaryotic and eukaryotic cells.
11. What are the two types of endoplasmic reticulum, and what is the primary function of each?
12. Explain the role of the Golgi apparatus in the cell.
13. Why are lysosomes known as the “suicide bags” of the cell?
14. Why are mitochondria referred to as the “powerhouses of the cell”?
15. What is ATP, and what is its function within the cell?
16. Name the two types of plastids found in plant cells and state their primary functions.
17. What two organelles contain their own DNA and ribosomes, separate from the nucleus?
18. How do vacuoles in plant cells differ from those in animal cells in terms of size and function?
19. Define endocytosis and name an organism that uses this process to obtain food.
20. What is the purpose of cell division in organisms?
21. What is the process of diffusion, and how is it important for a cell’s gas exchange?
22. Briefly describe the structure and role of ribosomes.
23. What is the primary purpose of mitosis?
24. How does the outcome of meiosis differ from mitosis in terms of the number of cells produced and their chromosome count?
25. Explain the term “membrane biogenesis” and which organelles are involved.

Multiple-Choice Questions (MCQs)

Choose the single best answer for each question.

1. The term “cell” was first coined in 1665 by: a) Antonie van Leeuwenhoek b) Robert Brown c) Robert Hooke d) Rudolf Virchow
2. The organelle responsible for manufacturing proteins is the: a) Lysosome b) Ribosome c) Smooth ER d) Golgi apparatus
3. Which of the following statements is NOT part of the cell theory? a) All living things are composed of cells. b) All cells have a nucleus and a cell wall. c) The cell is the basic unit of life. d) All cells arise from pre-existing cells.
4. A cell placed in a highly concentrated salt solution will: a) Swell up due to osmosis. b) Shrink due to osmosis. c) Stay the same size. d) Burst.
5. The flexible outer boundary of an animal cell is the: a) Cell wall b) Nuclear membrane c) Plasma membrane d) Cytoplasm
6. Which structure is found in plant cells but NOT in animal cells? a) Mitochondria b) Nucleus c) Plastids d) Ribosomes
7. The “energy currency” of the cell is: a) DNA b) Glucose c) Protein d) ATP
8. In a eukaryotic cell, DNA is primarily found in the form of chromatin within the: a) Cytoplasm b) Nucleus c) Ribosome d) Mitochondrion
9. Which process is used by organisms like Amoeba to engulf food? a) Osmosis b) Diffusion c) Endocytosis d) Plasmolysis
10. The detoxification of poisons and drugs in liver cells is a function of the: a) Rough Endoplasmic Reticulum (RER) b) Smooth Endoplasmic Reticulum (SER) c) Lysosomes d) Golgi apparatus
11. A prokaryotic cell lacks: a) A plasma membrane b) A cell wall c) Ribosomes d) A nuclear membrane
12. The scientist who discovered the nucleus in 1831 was: a) Schleiden b) Schwann c) Purkinje d) Robert Brown
13. The main component of the plant cell wall is: a) Lipids b) Proteins c) Cellulose d) Starch
14. The organelle that functions in the storage, modification, and packaging of cellular products is the: a) Endoplasmic Reticulum b) Golgi apparatus c) Vacuole d) Mitochondrion
15. The process of cell division that results in two identical daughter cells for growth and repair is: a) Meiosis b) Mitosis c) Endocytosis d) Photosynthesis
16. Lysosomes contain powerful: a) Photosynthetic pigments b) Genetic material c) Digestive enzymes d) Stored energy
17. The undefined nuclear region in a prokaryotic cell is called a: a) Nucleolus b) Chromosome c) Nucleoid d) Karyon
18. The process of cell division that produces gametes with half the number of chromosomes is: a) Mitosis b) Meiosis c) Fertilization d) Diffusion
19. In plant cells, vacuoles are full of cell sap and provide: a) Energy b) Turgidity and rigidity c) Genetic information d) Sites for protein synthesis
20. Organisms like bacteria and Paramoecium are classified as: a) Multicellular b) Eukaryotic c) Unicellular d) Plants

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Answer Keys

Short-Answer Questions – Answer Key

1. Robert Hooke discovered cells in 1665. He was observing a thin slice of cork through a self-designed microscope and saw structures that resembled a honeycomb.
2. The cell theory states that all plants and animals are composed of cells, the cell is the basic unit of life, and all cells arise from pre-existing cells. The first two points were from Schleiden and Schwann, while the third was added by Virchow.
3. Unicellular organisms are made of a single cell that performs all life functions, such as Amoeba. Multicellular organisms consist of many cells that are grouped to perform specialized functions, such as plants and animals.
4. The plasma membrane is called selectively permeable because it regulates the passage of substances into and out of the cell. It permits the movement of some materials while preventing the movement of others.
5. When placed in a hypotonic solution, an animal cell will gain water through osmosis because the water concentration is higher outside the cell. This net influx of water will cause the cell to swell up.
6. Plasmolysis is the shrinkage or contraction of the contents of a living plant cell away from the cell wall. It occurs when the cell is placed in a hypertonic solution, causing it to lose water via osmosis.
7. The main function of the cell wall is to provide structural strength and rigidity to plant cells. It is mainly composed of a complex substance called cellulose.
8. The nucleus is covered by a double-layered nuclear membrane that has pores for material transfer. Inside, in a non-dividing cell, the genetic material exists as an entangled mass of thread-like structures called chromatin material.
9. Genes are functional segments of DNA molecules that contain information for the inheritance of characters. In eukaryotic cells, they are located on chromosomes, which are found inside the nucleus.
10. Three key differences are: (1) Prokaryotic cells lack a nuclear membrane while eukaryotic cells have one; (2) Prokaryotes have a single chromosome, while eukaryotes have more than one; (3) Prokaryotes lack membrane-bound organelles, which are present in eukaryotes.
11. The two types are Rough ER (RER) and Smooth ER (SER). RER has ribosomes and is the site of protein manufacture, while SER helps in the manufacture of lipids and detoxifies poisons.
12. The Golgi apparatus functions to store, modify, and package materials synthesized in the cell into vesicles. It then dispatches these materials to various targets inside and outside the cell and is also involved in forming lysosomes.
13. Lysosomes are called “suicide bags” because they contain powerful digestive enzymes. If the cell’s metabolism is disturbed or the cell gets damaged, the lysosomes may burst, releasing these enzymes which can then digest the cell itself.
14. Mitochondria are called “powerhouses” because they perform the chemical reactions that release energy for the cell’s various activities. This energy is released in the form of ATP molecules.
15. ATP stands for Adenosine triphosphate and it is known as the energy currency of the cell. The body uses the energy stored in ATP for mechanical work and for making new chemical compounds.
16. The two types of plastids are chromoplasts and leucoplasts. Chromoplasts (like chloroplasts) are colored and involved in photosynthesis, while leucoplasts are colorless and primarily function in storing materials like starch and oils.
17. Mitochondria and plastids both contain their own DNA and ribosomes. This allows them to synthesize some of their own proteins independently of the nucleus.
18. In plant cells, vacuoles are very large, often occupying 50-90% of the cell volume, and provide turgidity and rigidity. In animal cells, vacuoles are small and are used for temporary storage.
19. Endocytosis is the process by which the flexibility of the cell membrane enables a cell to engulf food and other material from its external environment. Amoeba acquires its food through this process.
20. The purpose of cell division is for organisms to grow, to replace old, dead, and injured cells, and to form gametes required for reproduction.
21. Diffusion is the spontaneous movement of a substance from an area of high concentration to an area of low concentration. It is vital for gas exchange, allowing waste CO₂ to move out of the cell and essential O₂ to move in.
22. Ribosomes are small particles found in all active cells, often attached to the RER or free in the cytoplasm. They are the sites where protein synthesis (manufacture) occurs.
23. The primary purpose of mitosis is to produce new cells for the growth and repair of tissues in an organism. The resulting cells are genetically identical to the parent cell.
24. Meiosis produces four new cells, whereas mitosis produces two. The cells produced by meiosis have only half the number of chromosomes as the mother cell, while cells from mitosis have the same number.
25. Membrane biogenesis is the process of building the cell membrane. The proteins and lipids required for this process are synthesized by the endoplasmic reticulum (RER for proteins, SER for lipids).

Multiple-Choice Questions – Answer Key

1. c) Robert Hooke
2. b) Ribosome
3. b) All cells have a nucleus and a cell wall.
4. b) Shrink due to osmosis.
5. c) Plasma membrane
6. c) Plastids
7. d) ATP
8. b) Nucleus
9. c) Endocytosis
10. b) Smooth Endoplasmic Reticulum (SER)
11. d) A nuclear membrane
12. d) Robert Brown
13. c) Cellulose
14. b) Golgi apparatus
15. b) Mitosis
16. c) Digestive enzymes
17. c) Nucleoid
18. b) Meiosis
19. b) Turgidity and rigidity
20. c) Unicellular

Essay Questions and Answers

1. Describe the structure of the plasma membrane and explain in detail the processes of diffusion and osmosis, providing examples of how they are vital to a cell’s life.

The plasma membrane is the outermost covering of a cell, separating its contents from the external environment. It is a flexible structure made of organic molecules called lipids and proteins. This membrane is selectively permeable, meaning it controls which substances can enter and leave the cell.

Two vital processes for transport across this membrane are diffusion and osmosis. Diffusion is the spontaneous movement of a substance from a region of high concentration to one of low concentration. This is crucial for gaseous exchange; for example, carbon dioxide (CO₂), a cellular waste product, accumulates to a high concentration inside the cell and diffuses out into the environment where its concentration is lower. Conversely, oxygen (O₂) enters the cell by diffusion when its concentration inside decreases.

Osmosis is a special case of diffusion specifically involving the movement of water across a selectively permeable membrane. Water moves from an area of higher water concentration to one of lower water concentration. This is critical for maintaining the cell’s water balance. For example, unicellular freshwater organisms and plant roots absorb water from their environment through osmosis.

2. Compare and contrast prokaryotic and eukaryotic cells, highlighting at least four major differences in their structural organization.

Prokaryotic and eukaryotic cells represent two fundamental types of cellular organization. While both are the basic units of life, they differ significantly in their structure.

• Nuclear Region: The most defining difference is the nucleus. Eukaryotic cells have a true nucleus, where the genetic material is enclosed within a double-layered nuclear membrane. In contrast, prokaryotic cells lack a nuclear membrane; their nuclear region is poorly defined and is called a nucleoid.
• Chromosomes: Eukaryotic cells typically have more than one chromosome, which are composed of DNA and protein. Prokaryotic cells have a single chromosome, which consists only of nucleic acid.
• Membrane-Bound Organelles: Eukaryotic cells exhibit a complex internal organization with numerous membrane-bound organelles, such as the endoplasmic reticulum, Golgi apparatus, mitochondria, and lysosomes. Prokaryotic cells lack these membrane-bound organelles.
• Size: There is a significant size difference. Prokaryotic cells are generally small, typically ranging from 1 to 10 micrometers, whereas eukaryotic cells are generally much larger, ranging from 5 to 100 micrometers.

3. Explain the concept of “division of labour” within a single eukaryotic cell by describing the specific functions of at least four different membrane-bound organelles.

The concept of “division of labour” within a eukaryotic cell means that different functions are carried out by specific components known as cell organelles, much like different organs perform different functions in a multicellular organism. This organization allows the cell to operate efficiently. Four key organelles exemplify this:

1. Mitochondria: Known as the “powerhouses,” their specific function is to generate energy. They carry out chemical reactions that release energy in the form of ATP, which powers all other cellular activities.
2. Endoplasmic Reticulum (ER): This organelle serves as a manufacturing and transport system. The Rough ER, studded with ribosomes, synthesizes proteins, while the Smooth ER synthesizes lipids and detoxifies poisons. The ER then transports these materials throughout the cell.
3. Golgi Apparatus: This organelle acts as the cell’s packaging and shipping center. It receives proteins and lipids from the ER, then modifies, sorts, and packages them into vesicles for delivery to other destinations inside or outside the cell.
4. Lysosomes: These are the waste disposal system. Filled with digestive enzymes, their specialized role is to break down and recycle foreign materials, such as bacteria, and old, worn-out cell organelles, thus keeping the cell clean.

4. Discuss the structure and function of the plant cell wall and explain the phenomenon of plasmolysis. Why are plant cells better able to withstand hypotonic environments than animal cells?

The plant cell wall is a rigid outer covering that lies outside the plasma membrane. It is primarily composed of cellulose, a complex carbohydrate that provides structural strength to plants. Its main functions are to provide a definite shape to the cell, protect it from mechanical stress, and prevent excessive water uptake.

Plasmolysis is a phenomenon that occurs when a living plant cell is placed in a hypertonic solution (a solution with a lower water concentration than the cell). Due to osmosis, water moves out of the cell and into the surrounding solution. This loss of water causes the cell’s contents, enclosed by the plasma membrane, to shrink and pull away from the rigid cell wall.

Plant cells are better able to withstand hypotonic environments than animal cells because of the cell wall. In a hypotonic medium, both cell types will take in water via osmosis. An animal cell, which only has a flexible plasma membrane, will continue to swell and eventually burst. However, a plant cell swells only to a certain point, as the incoming water creates pressure (turgor pressure) against the inelastic cell wall. The wall exerts an equal counter-pressure, preventing the cell from taking in too much water and bursting.

5. Detail the processes of mitosis and meiosis. What is the purpose of each type of cell division, and how do their outcomes differ?

Mitosis and meiosis are the two main types of cell division. Mitosis is the process by which most body cells divide for the purpose of growth and the repair or replacement of tissues. In this process, a single “mother cell” divides once to form two genetically identical “daughter cells.” A crucial outcome of mitosis is that the daughter cells have the exact same number of chromosomes as the mother cell, ensuring genetic consistency throughout the organism’s tissues.

Meiosis, on the other hand, is a specialized type of cell division that occurs only in the reproductive organs or tissues to produce gametes (e.g., sperm and egg cells). This process involves two consecutive divisions (Meiosis I and Meiosis II). A single mother cell undergoes these two divisions to produce four new cells. The most significant difference in outcome is that the four new cells produced by meiosis are not identical to the mother cell; they contain only half the number of chromosomes. This reduction in chromosome number is essential for sexual reproduction, as it ensures that when two gametes fuse during fertilization, the resulting offspring will have the correct, full number of chromosomes.

6. Describe the path and processing of a protein from its synthesis to its dispatch from the cell, mentioning the key organelles involved.

The synthesis and export of a protein from a eukaryotic cell is a coordinated process involving several organelles. The journey begins at the ribosomes located on the surface of the Rough Endoplasmic Reticulum (RER). Here, the genetic information is translated to manufacture the protein.

Once synthesized, the protein is sent into the channels of the RER, which acts as a transport network. The RER processes and folds the protein, then transports it, often in small membrane-bound sacs, to the Golgi apparatus.

The Golgi apparatus, a system of flattened sacs called cisterns, functions as the cell’s post office. It receives the protein, further modifies it, sorts it, and packages it into vesicles. These vesicles are then dispatched to their final targets. If the protein is destined for export, the vesicle moves to the plasma membrane, fuses with it, and releases the protein outside the cell.

7. Why is a virus considered to be on the border of living and non-living? Relate its structure to this characteristic.

Viruses are considered to be on the border of living and non-living because they lack the characteristics of life on their own but can replicate once inside a living host cell. A key reason for this is their simple structure; viruses lack any membranes and do not have the cellular machinery, such as organelles, required for metabolism or reproduction. They are essentially genetic material (DNA or RNA) enclosed in a protein coat.

Outside of a living cell, a virus is inert and shows no signs of life. However, once a virus enters a living body and infects a host cell, it hijacks the host’s cellular machinery—its ribosomes, enzymes, and energy—to multiply. By using the host’s resources, it forces the cell to produce new virus particles, thereby demonstrating the living characteristic of reproduction. This complete dependence on a living cell for replication places viruses in a unique category between the living and the non-living.

8. Explain the roles of both Smooth ER (SER) and Rough ER (RER) in the process of membrane biogenesis.

Membrane biogenesis is the process of building the cell membrane, which is composed of lipids and proteins. Both the Smooth Endoplasmic Reticulum (SER) and the Rough Endoplasmic Reticulum (RER) play crucial, distinct roles in this process.

The RER is studded with ribosomes, which are the sites of protein synthesis. Therefore, the RER is responsible for manufacturing the proteins that will be embedded within or attached to the cell membrane.

The SER, which lacks ribosomes, is responsible for the manufacture of fat molecules, or lipids. These lipids are the fundamental building blocks that form the bilayer structure of the cell membrane. Thus, the RER produces the protein components and the SER produces the lipid components. These materials are then transported and assembled to form new sections of the cell membrane, allowing the cell to grow and repair its boundary.

9. What are the key structural features of mitochondria that relate to their function as the “powerhouse of the cell”?

The structure of mitochondria is uniquely adapted to their function of energy production. Mitochondria have two membrane coverings: a smooth, porous outer membrane and a deeply folded inner membrane. These folds are a critical feature, as they dramatically increase the surface area available for the chemical reactions that generate ATP (Adenosine triphosphate).

ATP is the “energy currency” of the cell, and its production is the core function of mitochondria. By having a large surface area on the inner membrane, the cell can accommodate more of the enzymes and proteins required for these ATP-generating reactions, making the process highly efficient. Therefore, the distinct double-membrane structure with extensive inner folds is directly responsible for the mitochondrion’s ability to act as the cell’s powerhouse, releasing the energy required for all life activities.

10. Describe the function and importance of vacuoles in a mature plant cell.

In a mature plant cell, the vacuole is a large, membrane-bound organelle that can occupy as much as 50-90% of the total cell volume. Its primary function is as a storage sac for both solid and liquid contents. The vacuole contains cell sap, which is a solution of important substances such as amino acids, sugars, various organic acids, and some proteins, as well as waste products.

The vacuole is critically important for maintaining the structural integrity of the plant cell. By absorbing water, the vacuole swells and pushes the cytoplasm against the cell wall, creating pressure. This state of being full of water provides turgidity and rigidity to the cell. This internal pressure helps support the plant’s leaves and stems, preventing wilting. Thus, the vacuole is essential not only for storage but also for the physical support of the plant cell and, by extension, the entire plant.

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Glossary of Key Terms

• ATP (Adenosine triphosphate): The “energy currency” of the cell; a molecule that stores and releases energy for chemical activities.
• Cell: The fundamental structural and functional unit of all living organisms.
• Cell Division: The process by which new cells are made, including mitosis and meiosis.
• Cell Membrane (Plasma Membrane): The selectively permeable outer boundary of the cell, composed of lipids and proteins.
• Cell Theory: The scientific theory stating that all organisms are made of cells, cells are the basic unit of life, and all cells come from pre-existing cells.
• Cell Wall: A rigid outer layer found in plants, fungi, and bacteria that provides structural support. In plants, it is made of cellulose.
• Cellulose: A complex substance that is the main component of plant cell walls.
• Chloroplast: A type of plastid containing chlorophyll, where photosynthesis occurs in plant cells.
• Chromatin: The material of which chromosomes are composed, consisting of DNA and proteins, visible as an entangled mass in a non-dividing cell.
• Chromosome: Rod-shaped structures found in the nucleus that contain genetic information in the form of DNA.
• Cisterns: Stacks of flattened, membrane-bound sacs that make up the Golgi apparatus.
• Cytoplasm: The fluid content inside the plasma membrane, in which the organelles are suspended.
• Diffusion: The spontaneous movement of a substance from a region of higher concentration to a region of lower concentration.
• DNA (Deoxyribonucleic Acid): The molecule that contains the genetic information for constructing and organizing cells.
• Endocytosis: A process where the cell membrane engulfs food or other material from the external environment.
• Endoplasmic Reticulum (ER): A network of membrane-bound tubes and sheets involved in protein and lipid synthesis and transport.
• Eukaryote: An organism whose cells contain a nucleus and other membrane-bound organelles.
• Gametes: Reproductive cells (e.g., sperm and egg) formed through meiosis.
• Genes: Functional segments of DNA that carry information for the inheritance of characters.
• Golgi Apparatus: An organelle that modifies, sorts, and packages proteins and lipids for storage or transport.
• Hypertonic Solution: A solution that has a lower water concentration (higher solute concentration) than the cell.
• Hypotonic Solution: A solution that has a higher water concentration (lower solute concentration) than the cell.
• Isotonic Solution: A solution that has the same water concentration as the cell.
• Lysosome: An organelle containing digestive enzymes; the cell’s waste disposal system. Known as “suicide bags.”
• Meiosis: A type of cell division that produces four new cells (gametes) with half the number of chromosomes as the parent cell.
• Membrane Biogenesis: The process of forming the cell membrane using lipids and proteins synthesized by the ER.
• Mitochondria: The “powerhouses” of the cell; organelles that generate most of the cell’s supply of ATP.
• Mitosis: A type of cell division that results in two identical daughter cells, used for growth and repair.
• Nuclear Membrane: The double-layered membrane enclosing the nucleus in eukaryotic cells.
• Nucleoid: The undefined region in a prokaryotic cell where the genetic material is located.
• Nucleus: A large organelle in eukaryotic cells that contains the genetic material and directs cell activities.
• Organelle: A specialized structure within a cell that performs a specific function.
• Osmosis: The movement of water molecules across a selectively permeable membrane.
• Plasmids: Organelles found only in plant cells, including chromoplasts (for color/photosynthesis) and leucoplasts (for storage).
• Plasmolysis: The process in which the contents of a plant cell shrink away from the cell wall when the cell loses water.
• Prokaryote: A single-celled organism that lacks a nucleus and other membrane-bound organelles.
• Protoplasm: The fluid substance of the cell, a term coined by Purkinje.
• Ribosomes: Small particles that are the sites of protein synthesis.
• Selectively Permeable Membrane: A membrane that allows certain molecules or ions to pass through it by means of active or passive transport.
• Unicellular: An organism composed of a single cell.
• Vacuole: A membrane-bound sac used for storage; very large in plant cells, small in animal cells.