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The Fundamental Unit of life Notes

All living organisms are made up of cells. In unicellular organisms (like bacteria), a single cell performs all life functions. In multicellular organisms (like humans), groups of cells form tissues, which form organs and organ systems — forming the whole organism. Therefore, cell is called the fundamental unit of life. Our cells carry out all essential life processes like respiration, digestion, excretion, reproduction, growth etc. These functions keep the organism alive. Different cells have varied sizes. They range in size from 0.0001 mm to nearly 150 mm in some cases. The study of cells from its basic structure to the functions of every cell organelle is called Cell Biology or cytology. Each cell can grow, reproduce, respond to stimuli, and perform metabolic activities necessary for survival. A cell can replicate itself independently. Hence, they are known as the building blocks of life.  Each cell contains a fluid called the cytoplasm, which is enclosed by a membrane. Also present in the cytoplasm are several biomolecules like proteins, nucleic acids and lipids. Moreover, cellular structures called cell organelles are suspended in the cytoplasm.

Cell Definition

“A cell is the smallest, basic structural and functional unit of life that is capable of carrying out all the life processes. Cells are the building blocks of all living organisms”.

Discovery of cell

Robert Hooke was the first Biologist who discovered cells. All organisms are made up of cells. They may be made up of a single cell (unicellular), or many cells (multicellular). Mycoplasmas are the smallest known cells. Cells are the building blocks of all living beings. They provide structure to the body and convert the nutrients taken from the food into energy. Cells are complex and their components perform various functions in an organism. They are of different shapes and sizes, pretty much like bricks of the buildings. Our body is made up of cells of different shapes and sizes. Cells are the lowest level of organisation in every life form. From organism to organism, the count of cells may vary. Humans have more number of cells compared to that of bacteria.

Cells comprise several cell organelles that perform specialised functions to carry out life processes. Every organelle has a specific structure. The hereditary material of the organisms is also present in the cells.

Discovery of Cells

The discovery of the cell is one of the most important milestones in biological science. Robert Hooke discovered the cell in 1665.He observed a thin slice of cork under his self-designed compound microscope. Hooke saw tiny, box-like compartments which reminded him of monks’ rooms in a monastery. Consequently, he named these “rooms” as cells. However, his compound microscope had limited magnification, and hence, he could not see any details in the structure. Owing to this limitation, Hooke concluded that these were non-living entities.

Later Anton Van Leeuwenhoek observed cells under another compound microscope with higher magnification. This time, he had noted that the cells exhibited some form of movement (motility). As a result, Leeuwenhoek concluded that these microscopic entities were “alive.” Eventually, after a host of other observations, these entities were named as animalcules.

In 1883, Robert Brown, a Scottish botanist, provided the very first insights into the cell structure. He was able to describe the nucleus present in the cells of orchids.

Characteristics of Cells

Following are the various essential characteristics of cells:

  1. Cells are the structural and Functional Units as they provide structure and shape to all living organisms. They perform essential life functions (e.g., respiration, digestion)
  2. All cells have a plasma membrane that encloses the cytoplasm and controls the movement of substances in and out.
  3. The jelly-like substance inside the cell where most of the cell's chemical reactions occur is called Organelles are suspended in the cytoplasm.
  4. Cells contains genetic Materials -DNA (or RNA) which holds genetic information for growth, reproduction, and functioning
  5. Cells have specialized structures called organelles (e.g., nucleus, mitochondria, ER, Golgi) to perform different tasks
  6. Cells produce energy through cellular respiration (in mitochondria) or photosynthesis (in chloroplasts, in plant cells)
  7. Cells can divide to form new cells by mitosis (for growth and repair) or meiosis (for reproduction).
  8. Cells grow in size and may divide to form tissues and organs in multicellular organisms.

Types of Cells

Cells are similar to factories with different labourers and departments that work towards a common objective. Various types of cells perform different functions. Based on cellular structure, there are two types of cells called prokaryotes and eukaryotes

Prokaryotic Cells

  1. No true nucleus – Genetic material is in a region called the nucleoid (not enclosed by a membrane).
  2. Unicellular organisms – e.g., Bacteria, Archaea, Cyanobacteria.
  3. Small size – Typically 1 to 0.5 µm in diameter.
  4. Genetic material – Can be DNA or RNA, usually circular.
  5. Reproduction – Mainly by binary fission (asexual). Some use conjugation for genetic exchange (not true sexual reproduction).
  6. No membrane-bound organelles (e.g., mitochondria, ER).
  7. Simple cell structure.

 Eukaryotic Cells

  1. True nucleus – DNA is enclosed within a nuclear membrane.
  2. Includes – Plants, Animals, Fungi, Protozoa.
  3. Larger size – Typically 10 to 100 µm in diameter.
  4. Cell membrane – Controls transport and enables cell-to-cell communication.
  5. Reproduction – Both sexual (meiosis) and asexual (mitosis).
  6. Membrane-bound organelles present – e.g., mitochondria, ER, Golgi apparatus.
  7. Eukaryotic Plant cells have chloroplasts, large vacuoles, cell walls, while animal cells have centrioles, smaller vacuoles, no cell wall or chloroplasts.

Differences between prokaryotic and eukaryotic cells

Prokaryotes

Eukaryotes

Cells of organism lacks nuclear membrane.

Cells of organism have nuclear membrane.

Nucleolus is absent.

Nucleolus is present.

Single chromosomes.

Single or multi chromosomes

Reproduction is always asexual.

Reproduction is both sexual and asexual.

Always unicellular.

Often multicellular.

Membrane bound cell organelles are absent.

Membrane bound organelles are present like mitochondria.

Centriole is absent.

Centriole is present only in animals cell.

Cell division is by binary fission.

Cell division is by mitosis or meiosis.

Example: Bacteria, Blue green algae, etc.

Example: Fungi, Plant cell, Animal cell etc.

Plant cells and animal cells

Plant cells are the basic structural and functional units of plants. They are eukaryotic cells, meaning they have a true nucleus and membrane-bound organelles.

 Key Features of Plant Cells:

  1. Cell Wall is the rigid outer covering made of cellulose. It provides shape, strength, and protection.
  2. Cell Membrane is located just inside the cell wall. It controls the entry and exit of substances.
  3. Cytoplasm is the jelly-like substance where cell organelles are suspended. It provides space for many chemical reactions.
  4. Nucleus of a plant cell contains DNA which controls cell activities. It is enclosed by the nuclear membrane.
  5. Chloroplasts found in plant cells contain chlorophyll, the green pigment. It assists in photosynthesis by converting sunlight into food.
  6. Plant cells have large vacuoles, they are usually central, and fluid-filled to maintain cell pressure, stores nutrients and wastes.
  7. Plastids are the organelles involved in storage and synthesis. There are 3 types of plastids
  1. Mitochondria is the powerhouse of the cell; site of cellular respiration.
  2. Golgi Bodies are involved in modifying, packaging, and transporting
  3. Endoplasmic Reticulum (ER) is if 2 types; Rough ER helps in Protein synthesis (has ribosomes) and smooth ER: Lipid synthesis and detoxification.
  4. Ribosomes are the sites of protein synthesis.

Animal cells

Animal cells are eukaryotic cells, meaning they have a true nucleus and membrane-bound organelles. Unlike plant cells, they lack a cell wall and chloroplasts.

 Key Features of Animal Cells:

  1. Cell Membrane (Plasma Membrane)
    • Flexible outer layer that surrounds the cell.
    • Controls the movement of substances in and out (selectively permeable).
  2. Cytoplasm
    • Jelly-like fluid that fills the cell.
    • Organelles are suspended here, and many chemical reactions occur in it.
  3. Nucleus
    • Contains DNA (genetic material).
    • Controls growth, metabolism, and reproduction.
    • Surrounded by the nuclear membrane.
  4. Mitochondria
    • Known as the "powerhouse of the cell".
    • Site of cellular respiration, producing ATP (energy).
  5. Endoplasmic Reticulum (ER)
    • Rough ER: Has ribosomes; helps in protein synthesis.
    • Smooth ER: No ribosomes; helps in lipid synthesis and detoxification.
  6. Ribosomes
    • Tiny structures either floating in cytoplasm or attached to rough ER.
    • Site of protein synthesis.
  7. Golgi Apparatus (Golgi Bodies)
    • Modifies, packages, and transports proteins and lipids.
  8. Lysosomes
    • Contain digestive enzymes to break down waste and worn-out cell parts.
    • Known as the "clean-up crew" of the cell.
  9. Centrioles
    • Involved in cell division (mitosis).
    • Present only in animal cells.
  10. Vacuoles
    • Small and temporary in animal cells.
    • Used for storage and transport of substances.

Differences between Plant Cells and Animal Cells

Animal Cell Plant Cell
Cell wall is absent. Cell wall is present.
Plastids are absent. Plastids are present.
Centrioles are present. Centrioles are absent.
Golgi bodies are present. Golgi bodies are present and called dictyosome.
Vacuoles are absent. If present, they are small. Vacuoles are present and large in size.
Centrosome is present with one or two centrioles. Centrosome is absent.

Structure of Cell

The cell structure comprises individual components with specific functions essential to carry out life’s processes. These components include- cell wall, cell membrane, cytoplasm, nucleus, and cell organelles. Read on to explore more insights on cell structure and function.

Cell Membrane or plasma membrane

Diffusion

The movement of molecules from a region of high concentration to a region of lower concentration is known as diffusion. E.g. carbon dioxide and oxygen move across the cell membrane by diffusion.

Osmosis in Selectively Permeable Membrane

Osmosis is the movement of water across a semi-permeable membrane. Osmosis is a selective process since the membrane does not allow all molecules to pass through it. Water is usually the only free-flowing molecule across this membrane.

Isotonic, Hypotonic, and Hypertonic Solutions

Cell Wall

Cytoplasm

Nucleus

  1. Nuclear membrane/ Nuclear envelope/Karyotheca
  2. Nuclear sap/Nucleoplasm/Karyolymph.
  3. Chromatin threads.

Cell Organelles

Cells are composed of various cell organelles that perform certain specific functions to carry out life’s processes. The different cell organelles, along with its principal functions, are as follows:

Cell Organelles and their Functions

Nucleus

Nucleolus

The nucleolus is the site of ribosome synthesis. In addition, it is involved in controlling cellular activities and cellular reproduction.

Nuclear membrane

The nuclear membrane protects the nucleus by forming a boundary between the nucleus and other cell organelles.

Chromosomes

Chromosomes are the genetic material present in the nucleus. It contains DNA with associated proteins. They are present as threads called chromatin. During the growth phase of the cell, the chromatin condenses into a much thicker structure called a chromosome. Human cells contain 23 pairs of chromosomes (46).

Chromatin

Chromatin is a thread-like structure which serves as the genetic material present inside the nucleus of the cell. It is made up of DNA and protein molecules. DNA contains the hereditary information needed for the structure and function of the organism.

Endoplasmic reticulum

Endoplasmic reticulum is a complex network of membrane bound channels or sheets, tubules and vesicles. It does not work alone. The ER works closely with the Golgi apparatus and ribosomes. It creates a network of membranes found through the whole cell. The ER may also look different from cell to cell, depending on the cell’s function.

Two types of Endoplasmic Reticulum ;

1. Smooth and Rough Smooth ER/Agranular ER : It has smooth membranes which do not bear ribosomes. It acts as a storage organelle. It is important in the synthesis and storage of steroids. It is mainly made up of vesicles and tubules.

2. Rough ER/Granular ER : It has a rough membrane because a number of ribosomes are attached to its outer surface. RER is engaged in protein synthesis. It is mainly made up of cisternae. Tubules are very few.

Golgi Bodies:

Discovered by Camillo Golgi (1898) in nerve cells of owls. Golgi carried out a revolutionary method of staining individual nerve and cell structures. It is located near the nucleus. Golgi bodies are pleomorphic structures, because components of the golgi body are different in structure & shape in different cells.

Functions :

1. It is involved in cell secretion and acts as storage, modification and condensation or packaging membrane. It forms the acrosome of sperm.

2. It forms the lysosomes and secretory vesicles.

3. It is the site of formation of glycolipids and glycoproteins. 

4. Synthesis of cell wall material (polysaccharide synthesis).

5. Cell plate formation during plant cell division.

Ribosomes

Ribosomes are essential cellular structures responsible for protein synthesis, acting as the cell's protein factories. They are composed of RNA and protein and exist as two subunits, a large and a small subunit, that assemble to read the mRNA sequence and link amino acids together.

Mitochondria:

Mitochondria are double membrane bound organelles of eukaryotic cells. Mitochondria are semi autonomous organelles bound by an envelope of two unit membranes and filled with a fluid matrix. The outer membrane is smooth and has porous proteins which form channels for the passage of molecules through it. The inner membrane is semipermeable.

It usually produces numerous infolds called cristae. The cristae greatly increases the inner surface area of the mitochondria to hold a variety of enzymes. Cristae bears minute, regularly spaced tennis racket shaped particles known as F1 particles or oxysomes.

Oxysomes are concerned with ATP synthesis. Matrix contain various respiratory enzymes.

Plastids:

Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell. Plastids often contain pigments used in photosynthesis and the types of pigments present can change or determine the cell's color. Plastids are responsible for photosynthesis, storage of products like starch. Chloroplasts are the sites of photosynthesis. They contain enzymes and coenzymes necessary for the process of photosynthesis.

Lysosomes:

Lysosomes are spherical bag-like structures covered by a single membrane. Lysosomes hold enzymes that were created by the cell. The purpose of the lysosome is to keep the cell clean by digesting any foreign materials as well as worn-out cell organelles. They might be used to digest food or break down the cell when it die

Chloroplast:

Chloroplasts are the primary organelles for photosynthesis. It contains the pigment called chlorophyll.

Plastids are of three types:

Chromoplasts - For pigment synthesis and storage. Chromoplasts are red, yellow and orange in color and are found in petals of flowers and in fruits. Their color is due to two pigments, carotene and xanthophyll.

Leucoplasts - Leucoplasts are colorless or white plastid. They occur in plant cells not exposed to light, such as roots and seeds. Leucoplasts are the center of starch grain formation and they are also involved in the synthesis of oil and proteins.

Chloroplasts - Chloroplasts are probably the most important among the plastids since they are directly involved in photosynthesis. They are usually situated near the surface of the cell and occur in those parts that receive sufficient light. e.g. the palisade cells of leaves. The green color of chloroplasts is caused by the green pigment chlorophyll.

Vacuoles

Vacuoles are mostly defined as storage houses of irregular shapes which are found in cells. They are fluid-filled organelles enclosed by a membrane. The vacuole stores the food or a variety of nutrients that a cell might need to survive. In addition to this, it also stores waste products. The waste products are eventually thrown out by vacuoles. Thus, the rest of the cell is protected from contamination. The animal and plant cells have different size and number of vacuoles. Compared to the animals, plant cells have larger vacuoles.

Cell Theory

The German scientists, Theodor Schwann, Matthias Schleiden, and Rudolf Virchow proposed cell Theory. The cell theory states that:

A modern version of the cell theory was eventually formulated, and it contains the following postulates:

Functions of Cell

A cell performs major functions essential for the growth and development of an organism. Important functions of cell are as follows:

1. Provides Support and Structure

All the organisms are made up of cells. They form the structural basis of all the organisms. The cell wall and the cell membrane are the main components that function to provide support and structure to the organism. For eg., the skin is made up of a large number of cells. Xylem present in the vascular plants is made of cells that provide structural support to the plants.

2. Facilitate Growth Mitosis

In the process of mitosis, the parent cell divides into the daughter cells. Thus, the cells multiply and facilitate the growth in an organism.

3. Allows Transport of Substances

Various nutrients are imported by the cells to carry out various chemical processes going on inside the cells. The waste produced by the chemical processes is eliminated from the cells by active and passive transport. Small molecules such as oxygen, carbon dioxide, and ethanol diffuse across the cell membrane along the concentration gradient. This is known as passive transport. The larger molecules diffuse across the cell membrane through active transport where the cells require a lot of energy to transport the substances.

4. Energy Production

Cells require energy to carry out various chemical processes. This energy is produced by the cells through a process called photosynthesis in plants and respiration in animals.

5. Aids in Reproduction

A cell aids in reproduction through the processes called mitosis and meiosis. Mitosis is termed as the asexual reproduction where the parent cell divides to form daughter cells. Meiosis causes the daughter cells to be genetically different from the parent cells.

Thus, we can understand why cells are known as the structural and functional unit of life. This is because they are responsible for providing structure to the organisms and perform several functions necessary for carrying out life’s processes.

Chapter question answers

  1. Who discovered cells, and how?

Answer:

In 1665, Robert Hooke discovered cells while examining a thin slice of cork through a self-designed microscope. He observed that the cork resembled the structure of a honeycomb consisting of numerous tiny compartments. The minuscule boxes are referred to as cells.

  1. Why is the cell called the structural and functional unit of life?

Answer:

Cells form the structure of an entity. A group of cells form a tissue, further an organ and ultimately an organ system. They perform fundamental functions and life processes such as respiration, digestion, excretion etc., in both unicellular and Multicellular entities. They perform all the activities independently. Hence, cells are referred to as structural and fundamental units of life.

Exercise-5.2.1

  1. How do substances like CO2and water move in and out of the cell? Discuss.

Answer:

CO2 moves by diffusion. The cellular waste accumulates in high concentrations in the cell, whereas the concentration of CO2 in the external surroundings is comparatively lower. This difference in the concentration level inside and outside of the cell causes the CO2 to diffuse from a region of higher (within the cell) to a lower concentration.

H2O diffuses by osmosis through the cell membrane. It moves from a region of higher concentration to a lower concentrated region through a selectively permeable membrane until equilibrium is reached.

  1. Why is the plasma membrane called a selectively permeable membrane?

Answer:

The plasma membrane is called as a selectively permeable membrane as it permits the movement of only certain molecules in and out of the cells. Not all molecules are free to diffuse.

Exercise-5.2.2-5.2.4

  1. Fill in the gaps in the following table, illustrating the differences between prokaryotic and eukaryotic cells.

Prokaryotic Cell

Eukaryotic Cell

1. Size: Generally small (1-10 μm)

1 μm = 10-6m

2. Nuclear region:

______________________________

______________________________

and known as ___________________

3. Chromosome: single

4. Membrane-bound cell organelles absent.

1. Size: Generally large (5-100 μm)

2. Nuclear region: well-defined and surrounded by a nuclear membrane.

3. More than one chromosome.

4. ______________________________

______________________________

______________________________

Answer:

Prokaryotic Cell

Eukaryotic Cell

1. Size: Generally small (1-10 μm)

1 μm = 10-6m

2. The nuclear region is poorly defined due to the absence of a nuclear membrane and is known as the nucleoid.

3. There is a single chromosome.

4. Membrane-bound cell organelles absent.

1. Size: Generally large (5-100 μm)

2. Nuclear region: well-defined and surrounded by a nuclear membrane.

3. There is more than one chromosome.

4. Membrane-bound cell organelles present.

Exercise-5.2.5

  1. Can you name the two organelles we have studied that contain their own genetic material?

Answer: The two organelles which have their own genetic material are

  1. Mitochondria
  2. Plastids
  1. If the organisation of a cell is destroyed due to some physical or chemical influence, what will happen?

Answer: In the event of any damage to cells and when the revival of cells is not possible, Lysosomes burst, and enzymes digest such cells. This is why lysosomes are often referred to as ‘suicide bags’.

  1. Why are lysosomes known as suicide bags?

Answer: When there is damage to the cell and when revival is not possible, lysosomes may burst, and the enzymes digest their own cell. Consequently, lysosomes are known as suicide bags.

  1. Where are proteins synthesised inside the cell?

Answer: Protein synthesis in cells takes place in ribosomes. Hence, ribosomes are also referred to as protein factories. Ribosomes are particles that are found attached to the rough endoplasmic reticulum.

Exercise

  1. Make a comparison and write down ways in which plant cells are different from animal cells.

Answer:

The following table depicts the differences between plant cells and animal cells.

Characteristic

Plant Cell

Animal Cell

Cell Wall

Present

Absent

Shape of Cell

With distinct edges, the shape is either rectangular or square-shaped.

Round and irregular shape

Nucleus

Present. It lies on one side of the cell

Present. It lies in the centre of the cell

Lysosomes

Rarely present

Always present

Plastids

Present

Absent

Structure of Vacuoles

Single or a few large vacuoles that are centrally located

Presence of numerous and small vacuoles
  1. How is a prokaryotic cell different from a eukaryotic cell?

Answer:

The following are the differences between prokaryotic and eukaryotic cells.

Prokaryotic Cell

Eukaryotic Cell

1. Size: Generally small (1-10 μm)

1 μm = 10-6m

2. The nuclear region is not well defined as the nuclear membrane is absent and is referred to as the nucleoid.

3. There is a single chromosome.

4. Membrane-bound cell organelles absent.

1. Size: Generally large (5-100 μm)

2. Nuclear region: well-defined and girdled by a nuclear membrane.

3. There is more than one chromosome.

4. Membrane-bound cell organelles present.
  1. What would happen if the plasma membrane ruptures or breaks down?

Answer:

If the plasma membrane ruptures or breaks down, then molecules of some substances will freely move in and out of the cells. As the plasma membrane acts as a mechanical barrier, the exchange of material from its surroundings through osmosis or diffusion in a cell won’t take place. Consequently, the cell would die due to the disappearance of the protoplasmic material.

  1. What would happen to the life of a cell if there was no Golgi apparatus?

Answer:

The Golgi apparatus consists of stacks of membrane-bound vesicles whose functions are as follows:

Without the Golgi apparatus, the cells will be disabled from packing and dispatching materials that were produced by the cells. The Golgi apparatus is also involved in the formation of cells. Hence, in the absence of the Golgi apparatus, cells will not be produced.

  1. Which organelle is known as the powerhouse of the cell? Why?

Answer:

Mitochondria are known as the powerhouse of the cell. It is because it releases the energy required for different activities of life. Mitochondria releases energy in the form of ATP (Adenosine triphosphate) molecules, essential for numerous chemical activities of life. Hence, ATP is often referred to as the ‘energy currency of the cell’.

  1. Where do the lipids and proteins constituting the cell membrane get synthesized?

Answer:

Lipids and proteins are synthesized in the ER (Endoplasmic Reticulum).

  1. How does an Amoeba obtain its food?

Answer:

Through the process of endocytosis, an Amoeba obtains its food. As its cell membrane is flexible enough, food particles are engulfed, forming a food vacuole girdling it, which is assisted by the pseudopodia. Amoeba secretes digestive enzymes to bring about digestion of the engulfed particle once the food is trapped.

  1. What is osmosis?

Answer:

The process of movement of a water molecule from a region of higher concentration to a region of lower concentration through a semi permeable membrane is known as osmosis.

  1. Carry out the following osmosis experiment:

Take four peeled potato halves and scoop each one out to make potato cups. One of these potato cups should be made from a boiled potato. Put each potato cup in a trough containing water. Now,

(a) Keep cup A empty

(b) Put one teaspoon sugar in cup B

(c) Put one teaspoon salt in cup C

(d) Put one teaspoon sugar in the boiled potato cup D.

Keep these for two hours. Then observe the four potato cups and answer the following:

(i) Explain why water gathers in the hollowed portion of B and C.

(ii) Why is potato A necessary for this experiment?

(iii) Explain why water does not gather in the hollowed-out portions of A and D.

Answer:

(i) Water accumulates in the hollowed portions of B and C as a difference in the water concentration

is observed. Thereby, endosmosis occurs as the cells act as a semi permeable membrane.

(ii) Potato A is essential in this experiment as it is significant to compare different scenarios seen in

potato cups B, C and D. Potato A in this experiment clearly shows that the potato cavity on its

own cannot bring about water movement.

(iii) Cup in A does not show any change in the water flow concentration for osmosis to occur, which

requires concentration to be higher than the other. Cells in cup D are dead; thus, there is no

existence of a semipermeable membrane for water flow. Consequently, osmosis does not occur.

  1. Which type of cell division is required for the growth and repair of the body, and which type is involved in the formation of gametes?

Answer:

There are two ways in which a cell divides:

Mitosis is the type of cell division that is involved in the growth and repair of the body, whereas meiosis is a type of cell division which results in the formation of gametes.

Multiple Choice Questions

  1. The inner membrane of mitochondria is folded because
    (a) it has no space inside (b) it helps in transportation of material
    (c) it increases the surface area (d) it stores more food
    2. Proteins are formed in
    (a) Golgi bodies (b) nucleus (c) plastids (d) ribosomes
    3. The organelle that helps in the membrane biogenesis is
    (a) lysosome (b) Golgi bodies (c) endoplasmic reticulum (d) ribosome
    4. The solution in which a cell will gain water by osmosis is termed as
    (a) isotonic solution (b) hypertonic (c) hypotonic solution (d) both (a) and (b)
    5. The root hair absorbs water by the process called
    (a) diffusion (b) osmosis (c) endocytosis (d) plasmolysis
    6. The animal cell which does not possess nucleus is
    (a) egg of hen ‘ (b) white blood cell (c) red blood cell (d) nerve cell
    7. The nucleus of the cell was discovered by
    (a) Robert Hooke (b) Leeuwenhoek (c) Robert Brown (d) Purkinje
    8. The plant cells are more rigid than the animal cell due to
    (a) cell wall (b) vacuoles (c) plastids (d) both (a) and (b)
    9. The opening and closing of stomata is due to
    (a) sunlight (b) osmosis (c plasmolysis (d) endocytosis
    10. The cells with jio membrane bound organelles, and the chromosomes are composed of only nucleic acids are
    (a) plant cells (b) animal cells (c) prokaryotic cells (d) eukaryotic cells
    Answer. 1—(c), 2—(d), 3—(4. 4—(c), 5—(b), 6—(c), 7—(4, 8—(d), 9—(b), 10—(c).

Very Short Answer Type Questions

Question 1. What are plastids? Name the different types of plastids found in a plant cell.
Answer: Plastids are organelles found only in plants. They are:
(a) Chloroplast-Containing chlorophyll
(b) Chromoplast-Containing carotenoids and xanthophyll (coloured plastids)
(c) Leucoplast-Wllite or colourless plastids

Question 2. What is plasma membrane made up of?
Answer: Plasma membrane is made up of proteins and lipids.

Question 3. What did Robert Hooke observed first in cork cell?
Answer: Robert Hooke observed that cork consists of box like compartments which formed a honeycomb structure.

Question 4. Name the autonomous organelles in the cell.
Answer. Chloroplasts and mitochondria are the autonomous organelles in the cells.

Question.5. What does protoplasm refer to?
Answer: Protoplasm refer to cytoplasm and nucleus.

Question 6. Name two cells which keep changing their shape.
Answer: Amoeba and white blood cells.

Question 7. Name the smallest cell and the longest cell in human body.
Answer: The smallest cell is the red blood cell or sperm cell in male. Longest cell is the nerve cell.

Question 8. Name 3 features seen/present in almost every cell.
Answer: Plasma membrane, nucleus and cytoplasm.

Question 9.What is diffusion?
Answer: When gases like C02, 02, move across the cell membrane, this process is called diffusion.

Question 10.What is osmosis? This takes place from high water concentration to low water concentration.
Answer: The movement of water molecules through a selectively permeable membrane is called osmosis. This takes place from high water concentration to low water concentration.

Question 11. What is the full form of DNA?
Answer: DNA —> Deoxyribo Nucleic Acid.

Question 12. What is the function of chromosome?
Answer: Chromosomes contain information for the inheritance of features from parents to next generation in the form of DNA molecules.

Question 13. Name the organelles present in liver of animals for detoxifying many poisons and drugs. 
Answer: In the liver of animal cells smooth endoplasmic reticulum helps in detoxifying many poisons and drugs.

Question 14. What is the energy currency of the cell?
Answer: ATP—Adenosine Triphosphate.

Question 15. What is the function of ribosome?
Answer: Ribosomes help in protein synthesis.

Question 16. Where are genes located in the cell?
Answer: Genes are located in the chromosomes in the nucleus of the cell.

Question 17. Name the cell organelles that helps in packaging?
Answer: Golgi apparatus.

Question 18. Name the cell organelle which helps in the transportation of material.
Answer: Endoplasmic reticulum.

Question 19. Name the cell organelle due to which leaves, flowers and fruits get their colour.
Answer: Chromoplast.

Question 20. Name the cell organelle which helps in the formation of lysosome.
Answer: Golgi apparatus.

Short Answer Type Questions

Question 1. State two conditions required for osmosis.
Answer: (i) The difference in the concentration of water, one should have higher concentration than the other.
(ii) Semi-permeable membrane is also required through which water will flow.

Question 2. What is plasmolysis?
Answer: When a living plant cell loses water through osmosis there is shrinkage or contraction of the contents of the cell away from the cell wall. This phenomenon is known as plasmolysis.

Question 3. How does fungi and bacteria can withstand much greater changes in the surrounding medium than animal cells?
Answer: The cell wall present in fungi and bacteria permits these cells to withstand very dilute external medium without bursting.
The cells take up water by osmosis, swells, and builds the pressure against the cell wall. The wall exerts an equal pressure against the swollen cell. It is because of the cell wall, such cells can withstand much greater changes in the surrounding medium than animal cells.

Question 4. Give the function of nuclear membrane.
Answer: The nuclear membrane present as outer covering in the nucleus allows the transfer of material inside and out of the nucleus to cytoplasm.

Question 5. Name the cell-organelles that have their own DNA and ribosomes.
Answer: The cell organelles with their own DNA and ribosomes are mitochondria and plastids.

Question 6. State the difference between smooth endoplasmic reticulum and rough endoplasmic reticulum.
Answer:

Question 7. What is endocytosis?
Answer: The cell membranes flexibility allows the cell engulf in food and other material from its external environment. This process is known as endocytosis. E.g., Amoeba acquires its food through such processes.

Question 8. What is the function of vacuoles?
Answer: Vacuoles are storage sacs for solid or liquid content. In plant cells it provides turgidity and rigidity to the cell. In single-celled organisms vacuoles store food, e.g., Amoeba.

Question 9. When we put raisins in water, why do they swell?
Answer: Raisins are dry with less water inside, when they are kept in water, osmosis takes place, water flows through the cell wall, cell membrane of the raisins and therefore it swells.

Question 10. Why are lysosomes called suicidal bags?
Answer: Lysosomes contain digestive enzymes in it and helps in the cleaning of cell by digesting any foreign materials entering the cell, such as bacteria, food and old cell organelles.
When the lysosomes burst, the digestive enzyme digest its own cell. Hence it is called as suicidal bag.

Question 11. What is nucleoid?
Answer: The nuclear region in some cells are poorly defined due to the absence of a nuclear membrane, it contains only nucleic acid. This undefined nuclear region with nucleic acid in it is called nucleoid.

Question 12. What is the role of cell organelles in the cell?
Answer: Each kind of cell organelles performs a specific function such as making new material, clearing of the waste, transporting material, etc.

Question 13. Label the figure and answer the questions:
(i) A – It is the packaging organelle
(ii) B – Provides energy
(iii) C – helps in the transport of material
(iv) D – Carries the information.

Answer. (i) A – Golgi body (ii) B – Mitochondria
(iii) C – Endoplasmic reticulum (iv) D – Nucleus

Question 14. What is the function of nucleus in a cell?
Answer: The nucleus plays a very important role in the reproduction of cells. It also helps the single cell to divide and form two new daughter cells.
It plays an important role in determining how the cell will develop and what form it will exhibit at maturity, by directing the chemical activities of the cell.

Question 15. What is the function of plastids?
Answer: Plastids are present only in plant cells. There are two types of plastids chromoplasts (coloured plastids) and leucoplasts (white or colourless)
Chromoplast—Consists of coloured pigments and gives different colours to flowers, fruits and leaves. The green colour pigment present in leaf is called chlorophyll which helps in the photosynthesis and a plastid with chlorophyll is called chloroplast.
Leucoplast—It stores starch, oil and protein granules in it.

Question 16. Do vacuoles store some material? If yes, name them.
Answer: Yes, vacuoles also store some important substances required in life of the plant cell. These are amino acids, sugars, various organic acids and some proteins. In some unicellular organisms, e.g. Amoeba, vacuoles also store food.

Question 17. Explain the structure and function of Golgi bodies.
Answer: Structures: Golgi bodies consist of a system of membrane-bound vesicles arranged in stacks parallel to each other called cisterns. These membranes have connections with the membrane of endoplasmic reticulum (ER).
Functions:
(1) The material synthesised near the ER is packaged and dispatched to various target inside and outside the cell through Golgi apparatus.
(2) It also stores, modifies and helps in the packaging of products in vesicles.
(3) In some cases, complex sugars may be made from simple sugars in it.
(4) It also helps in the formation of lysosomes.

Question 18. What are ribosomes? Where are they located in the cell? What is their function?
Answer: Ribosomes are spherical organelles present in the cell which are either freely distributed in the cytoplasm or may be attached to the endoplasmic reticulum.
It consists of ribosomal RNA (Ribonucleic acid) and proteins.
Functions of Ribosomes: It helps in the synthesis of proteins.

Question 19. What is the difference in chromatin, chromosomes and gene?
Answer: (1) Chromatin: It is a fine network of thread-like structure made up of DNA or RNA. It gets condense to form chromosomes.
(2) Chromosome: The chromosomes are made from chromatin material and are located iri the cell.
(3) Genes are found in chromosomes.

Question 20. Why do plant cells have more in number and big-sized vacuoles as compared to the animal cells?
Answer: Plant cells attain turgidity and rigidity due to the more number of vacuoles as well as large-sized vacuoles help the plant cells to withstand the wear and tear, external environmental conditions.
They also help in the storage of essential material required by plants for their growth like amino acids, sugar and various organic substances.