NCERT Science Notes - Class 10
Chapter 7 - How do organisms reproduce?

Welcome to AJs Chalo Seekhen. This webpage is dedicated to Class 10 | Science | Chapter 7 - How do organisms reproduce?. In this chapter, students delve into the biological processes of reproduction across various organisms. The chapter covers both asexual and sexual reproduction, highlighting methods like binary fission, budding, and vegetative propagation in asexual reproduction, and processes such as fertilization, pollination, and human reproductive systems in sexual reproduction. It also explores the importance of DNA and genetic variation in the survival and evolution of species. Through this chapter, students gain a comprehensive understanding of the continuity of life and the mechanisms that ensure the transfer of genetic material from one generation to the next.

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Importance of Reproduction

• Not Essential for Individual Survival: Reproduction is not necessary for the life of an individual organism. Unlike vital processes such as nutrition, respiration, and excretion, reproduction does not contribute to the immediate survival of the individual.
• Energy Expenditure: Reproducing requires a significant investment of energy. This raises the question of why organisms would expend energy on a process that is not essential for their own survival.

Reasons for Reproduction

1. Continuation of Species: The primary reason for reproduction is to ensure the continuation of a species. Individual organisms may die, but reproduction allows for the species to persist over time.
2. Genetic Diversity: Reproduction, especially sexual reproduction, promotes genetic diversity within a population. This diversity can enhance the resilience of a species, allowing it to adapt to changing environments and resist diseases.
3. Population Growth: Reproduction leads to an increase in the population size, which can be crucial for species survival in competitive ecosystems.
4. Natural Selection: By producing offspring, organisms contribute to the process of natural selection. Offspring that are better adapted to their environment are more likely to survive and reproduce, passing on their advantageous traits.

• Similarity in Appearance: Organisms belonging to the same species typically exhibit similarities in appearance. These similarities are often used to identify and classify organisms as members of the same species.
• Reproduction and Identification: Reproduction plays a key role in maintaining these similarities across generations, as new individuals resemble their parents, contributing to the overall recognition of species.

7.1 - Do Organisms Create Exact Copies of Themselves?

Similarity in Organisms

• Body Designs: Organisms tend to look similar due to their body designs. For these designs to be similar, the genetic blueprints must also be similar.
• Role of Chromosomes: The chromosomes in the nucleus of a cell contain the information necessary for the inheritance of traits from parents to offspring in the form of DNA (Deoxyribonucleic Acid) molecules.

The Process of Reproduction

1. Copying DNA: At the core of reproduction is the creation of a DNA copy. Cells utilize chemical reactions to replicate their DNA, resulting in two copies of DNA within a reproducing cell.
2. Cell Division: The process of DNA copying is accompanied by the formation of additional cellular structures. Subsequently, the two DNA copies separate, each associated with its own cellular apparatus. This results in cell division, producing two new cells.

Are the New Cells Identical?

• Potential for Variation: While the two new cells are similar, they are not guaranteed to be absolutely identical. The accuracy of the copying reactions affects the final outcome.
• Biochemical Reliability: No biochemical reaction is entirely reliable, leading to potential variations in the DNA during replication. Consequently, the DNA copies produced will generally be similar but may exhibit some differences from the original DNA.

Consequences of Variations

• Non-viable Variations: Some variations in DNA may be significant enough that the new copy cannot function effectively with the inherited cellular apparatus, resulting in cell death.
• Survival of Variations: Other variations may not drastically impact the cell's functionality, allowing for the survival of cells that are similar yet subtly different from each other.

Implications for Evolution

• Inbuilt Tendency for Variation: The inherent tendency for variation during reproduction serves as the foundation for evolution. These subtle differences can lead to changes in traits over generations, contributing to the diversity of life.

Key Points to Remember

• Reproduction involves creating copies of genetic blueprints (DNA).
• Variations during DNA copying can result in new cells that are similar but not identical.
• The process of variation is critical for evolution, enabling species to adapt over time.

7.1.1 - The Importance of Variation

Role of Populations in Ecosystems

• Niches: Populations of organisms occupy specific roles, or niches, within ecosystems. These niches are defined by the organism's adaptations and features, allowing them to thrive in particular environments.
• DNA Consistency: Consistent DNA copying during reproduction is crucial for maintaining the body design features that enable organisms to utilize their niches effectively. This consistency supports the stability of populations.

• Niche Alterations: Niches can change due to various factors, such as:
  • Fluctuations in temperature
  • Variations in water levels
  • Environmental events like meteorite impacts
• Threat to Populations: If a population is well-adapted to a specific niche and that niche undergoes significant alteration, the population risks extinction.

• Survival of Variants: Variation within a population increases the likelihood that some individuals possess traits that enable them to adapt to new conditions. For instance:
  • In a scenario where bacteria live in temperate waters and experience a rise in temperature due to global warming, most individuals may perish. However, any variants with heat resistance may survive and reproduce.
• Long-term Species Survival: The presence of genetic variation within populations is essential for the long-term survival of species, as it provides the necessary adaptability to cope with changing environments.

• Variation as a Survival Mechanism: Genetic variation is crucial for species survival in the face of environmental changes.
• Importance for Evolution: Variation allows populations to adapt and evolve, ensuring they can persist despite challenges in their habitats.
• Adaptability and Resilience: Populations with greater genetic diversity are more likely to endure shifts in their niches, contributing to ecological balance and stability over time.

7.2 - Modes of Reproduction Used by Single Organisms

Activity 7.1: Observing Yeast and Mould Growth

Materials Required:

• 10 grams of sugar
• 100 mL of water
• 20 mL of sugar solution
• Yeast granules
• Test tube
• Cotton plug
• Microscope
• Slice of bread
• Magnifying glass

1. Yeast Observation:
   • Dissolve about 10 grams of sugar in 100 mL of water.
   • Take 20 mL of this sugar solution in a test tube.
   • Add a pinch of yeast granules to the solution.
   • Cover the mouth of the test tube with a cotton plug.
   • Place the test tube in a warm environment.
   • After 1 or 2 hours, take a small drop of the yeast culture from the test tube and place it on a slide.
   • Cover with a coverslip and observe under a microscope.
2. Mould Observation:
   • Wet a slice of bread and place it in a cool, moist, and dark location.
   • Observe the surface of the bread slice with a magnifying glass.
   • Record your observations over the course of a week.

• Yeast Growth:
  • Yeast reproduces primarily through budding, a form of asexual reproduction.
  • Under favorable conditions (sugar solution and warmth), yeast cells will divide and form new cells that bud off from the parent cell.
  • This process can be observed as the culture grows, producing gas bubbles (carbon dioxide) due to fermentation.
• Mould Growth:
  • Mould, which is a type of fungus, typically reproduces through spores.
  • When the bread is kept in a moist, dark environment, mould spores in the air settle on the bread, germinate, and grow.
  • Mould growth is characterized by the formation of a network of filaments (mycelium) that spread over the surface, leading to a fuzzy appearance.

Key Differences:

• Reproductive Methods:
  • Yeast uses budding (asexual reproduction) while mould uses spores (asexual reproduction through sporulation).
• Environmental Conditions:
  • Yeast requires a sugary solution and warmth for rapid growth.
  • Mould thrives in cool, moist, and dark conditions, utilizing organic matter (like bread) as a nutrient source.

7.2.1 - Fission

Fission is a type of asexual reproduction commonly observed in unicellular organisms. It involves the division of a single parent cell into two or more daughter cells. Different organisms exhibit various patterns of fission:

• Binary Fission: This is the most common form, where the organism splits into two equal halves. Bacteria and protozoa typically reproduce this way. For instance, in Amoeba, the division can occur in any plane, resulting in two similar-sized cells.

Activity 7.3: Observing Fission in Amoeba

Materials Required:

• Permanent slide of Amoeba
• Permanent slide of Amoeba showing binary fission
• Microscope

1. Observe the permanent slide of Amoeba under the microscope.
2. Observe the second slide that demonstrates binary fission in Amoeba.
3. Compare your observations from both slides.

1. Slide of Amoeba:
   • The Amoeba appears as an irregularly shaped single cell.
   • It exhibits pseudopodia (temporary projections) for movement and feeding.
2. Slide of Amoeba Showing Binary Fission:
   • The slide displays two identical cells, indicating that fission has occurred.
   • You may notice that the nucleus divides first, followed by the division of the cytoplasm.

Other Forms of Fission:

• Organisms with Structures: Some unicellular organisms, such as Leishmania (which causes kala-azar), have more organized body structures, including a whip-like flagellum. Binary fission in these organisms occurs in a specific orientation relative to these structures.
• Multiple Fission: In some cases, such as the malarial parasite Plasmodium, fission does not result in just two daughter cells. Instead, it can divide into many daughter cells simultaneously through a process called multiple fission.
• Budding in Yeast: As mentioned in Activity 7.1, yeast reproduces through budding, where small buds form on the parent cell, separate, and grow into new individuals.

7.2.2 - Fragmentation

7.2.3 - Regeneration

Regeneration is a remarkable ability exhibited by many fully differentiated organisms, allowing them to form new individual organisms from their body parts. This process occurs when an organism is cut or broken into pieces, enabling many of those pieces to grow into separate individuals.

Examples of Organisms That Exhibit Regeneration:

• Hydra: A small, freshwater organism known for its regenerative abilities. If a Hydra is cut into several pieces, each piece can regenerate into a complete Hydra.
• Planaria: A type of flatworm that can also regenerate. When Planaria are cut, each segment can develop into a fully functional organism.

• Specialized Cells: Regeneration is facilitated by specialized cells that proliferate (multiply) to create a large mass of cells.
• Developmental Changes: From this mass, different cells undergo changes to become various cell types and tissues. This process occurs in a coordinated sequence known as development.
• Organized Sequence: The changes that cells undergo during regeneration are not random; they follow an organized sequence that ensures the proper formation of tissues and organs.

While regeneration enables organisms to recover from damage and can produce new individuals, it is important to note that regeneration is not the same as reproduction. Here are some key points to differentiate the two:

• Dependency: Most organisms do not rely on being cut or damaged to reproduce. Regeneration is a response to injury rather than a typical reproductive strategy.
• Purpose: The primary purpose of regeneration is to restore lost body parts or recover from injury, while reproduction aims to create new individuals for the continuation of the species.

7.2.4 - Budding

Budding is a form of asexual reproduction observed in certain organisms, including Hydra. This process involves the development of a new individual as an outgrowth from the parent organism.

Process of Budding:

1. Formation of Bud: In Hydra, a bud forms as a result of repeated cell division at a specific site on the parent body. This localized growth leads to the development of an outgrowth.
2. Maturation: The bud continues to grow and develop into a tiny individual. During this time, it receives nutrients and support from the parent organism.
3. Detachment: Once the bud reaches maturity, it detaches from the parent body and becomes an independent individual. This newly formed organism is genetically identical to the parent, as it arises from the same genetic material.

• Asexual Reproduction: Budding is a form of asexual reproduction, meaning that it does not involve the fusion of gametes (sperm and egg). The offspring are clones of the parent.
• Simplicity: This method of reproduction is relatively simple and efficient, allowing organisms to increase their numbers rapidly in favorable conditions.
• Common in Aquatic Environments: Budding is commonly observed in aquatic organisms, particularly those with simple body structures, like Hydra and certain types of yeast.

Activity 7.5: Observing Vegetative Propagation in Potatoes

Activity 7.6: Observing Vegetative Propagation in Money Plants

Objective: To investigate how different cuttings from a money plant can grow and produce fresh leaves, illustrating the concept of vegetative propagation.

Materials Needed:

• Money plant (Pothos or Epipremnum aureum)
• A sharp knife or scissors
• A glass or jar with water
• A notebook for observations

1. Selecting the Plant: Choose a healthy money plant for the experiment.
2. Cutting the Plant:
   • Cut some stems that contain at least one leaf. Ensure that each cutting has a node (the point on the stem where leaves attach).
   • Cut other pieces from the stem that are located between two leaves (without nodes).
3. Preparation:
   • Dip the cut ends of all the pieces (both types) in water.
   • Place the cuttings in the glass or jar, ensuring that the cut ends are submerged in water.
4. Observation:
   • Keep the jar in a well-lit area and observe the cuttings over the next few days.
   • Note any changes, such as the appearance of roots or new leaves.

• Which cuttings grow and give rise to fresh leaves?
  • The cuttings that contain at least one leaf and have nodes will likely develop roots and produce new leaves.
  • Cuttings without nodes may not grow effectively, as they lack the necessary structure for rooting.

• From your observations, you can conclude that cuttings with nodes and leaves are more successful in growing and producing fresh leaves compared to those taken from the stem without nodes. This activity illustrates the importance of nodes in vegetative propagation and demonstrates how money plants can reproduce asexually through stem cuttings. The ability of plants to regenerate from cuttings highlights their adaptability and resilience.

Tissue Culture

• Definition: A method to grow new plants by removing tissue or separating cells from the growing tip of a plant.
• Process:
  1. Cells are placed in an artificial medium where they divide rapidly to form a small group of cells called callus.
  2. The callus is transferred to another medium containing hormones for growth and differentiation.
  3. Plantlets are placed in soil to grow into mature plants.
• Advantages:
  • Many plants can be grown from one parent.
  • Allows growth in disease-free conditions.
• Common Use: Widely used for ornamental plants.

Spore Formation

• Definition: A mode of asexual reproduction where specific structures produce spores.
• Organism Example: Bread mould (Rhizopus).
• Key Structures:
  • Hyphae: Thread-like structures that are not involved in reproduction.
  • Sporangia: Tiny blob-on-a-stick structures involved in reproduction, containing spores.
• Spores:
  • Enclosed by thick walls for protection.
  • Germinate when in contact with a moist surface, developing into new individuals.
• Reproduction Type: Asexual reproduction, allowing new generations from a single individual.

7.3 - Sexual Reproduction

Definition: A mode of reproduction involving two individuals (male and female) to produce new generations.

7.3.1 - Why the Sexual Mode of Reproduction?

1. Introduction to Sexual Reproduction:
   • Involves the creation of new cells (offspring) from two parent organisms.
   • Requires copying of DNA (genetic material) and the cellular apparatus (components of the cell).
2. DNA Copying Mechanism:
   • The process of copying DNA is not always accurate, leading to errors.
   • These errors can cause variations within populations of organisms.
   • Variations are beneficial for the survival of the species but may not protect every individual.
3. Importance of Variation:
   • Increasing variation in offspring improves adaptability to changing environments.
   • More variation means a better chance of survival for the species as a whole.
4. Limitations of Asexual Reproduction:
   • Asexual reproduction generates offspring that are genetically identical to the parent, limiting variation.
   • The process of variation through DNA copying is slow and may not keep pace with environmental changes.
5. Speeding Up Variation:
   • Each new variation builds upon accumulated variations from previous generations.
   • Different individuals have unique patterns of variations, enhancing genetic diversity.
6. Combining Variations:
   • Sexual reproduction combines DNA from two individuals, leading to new combinations of traits.
   • Each combination is unique due to the involvement of different individuals.
7. Challenge of DNA Accumulation:
   • If offspring received DNA from both parents without reduction, the amount of DNA would double each generation.
   • This could disrupt the control of cellular functions and development.
8. Meiosis as a Solution:
   • Meiosis: A type of cell division that reduces the chromosome number by half.
     • Produces germ cells (sperm and egg) that have half the DNA of regular body cells.
   • When these germ cells combine during fertilization, the normal chromosome number is restored.
9. Germ Cell Specialization:
   • In simple organisms, male and female germ cells may be similar.
   • In more complex organisms, they specialize:
     • Male Gamete: Smaller, motile, and designed for reaching the female gamete.
     • Female Gamete: Larger and contains food stores to nourish the developing embryo.
10. Differences in Reproductive Structures:
    • The need to produce different types of gametes leads to variations in the reproductive organs of males and females.
    • This can also result in differences in the physical characteristics of male and female organisms.

7.3.2 - Sexual Reproduction in Flowering Plants

7.3.3 - Reproduction in Human Beings

1. Sexual Reproduction in Humans:
   • Humans reproduce using a sexual mode of reproduction.
2. Growth and Development:
   • As individuals age, their bodies undergo various changes:
     • Continuous increase in height.
     • Development and loss of teeth (milk teeth replaced by permanent ones).
     • Changes associated with growth, which makes the body larger.
3. Changes During Adolescence:
   • Early teenage years bring about distinct changes beyond simple growth, known as sexual maturation.
   • Common Changes for Both Boys and Girls:
     • Growth of thick hair in new areas (armpits, genital area).
     • Darkening of hair color in these regions.
     • Development of thinner hair on arms and legs, and on the face.
     • Skin may become oily, leading to pimples.
     • Increased awareness of their own and others’ bodies.
   • Specific Changes in Girls:
     • Breast size increases.
     • Skin of the nipples darkens.
     • Onset of menstruation (around the onset of puberty).
   • Specific Changes in Boys:
     • Growth of thick hair on the face.
     • Voice begins to crack.
     • Penis enlargement and occasional erections during daydreams or at night.
4. Variability of Changes:
   • Changes occur gradually over months and years.
   • Not all changes happen at the same time or at an exact age; some individuals mature earlier or later.
   • Variation exists in the patterns of hair growth and the size and shape of sexual organs, similar to differences in nose and finger shapes.
5. Purpose of Sexual Maturation:
   • Sexual maturation is essential for reproduction, involving the creation of germ-cells.
   • While the body grows, resources are primarily used for growth rather than for reproductive tissue maturation.
   • As general body growth slows down, reproductive tissues mature.
   • This period of change is called puberty.
6. Linking Changes to Reproductive Processes:
   • Sexual reproduction requires the joining of germ-cells from two individuals.
   • This joining can occur through:
     • External release of germ-cells (as in flowering plants).
     • Internal transfer of germ-cells during mating (as in many animals).
   • Indicators of sexual maturity include physical changes, such as hair growth patterns, which signal readiness for reproduction.
7. Special Organs for Reproduction:
   • Internal transfer of germ-cells requires specialized organs:
     • In males, the penis becomes erect for mating.
     • In females, reproductive organs and breasts mature to support the carrying and nurturing of the baby.
     • After fertilization, the baby develops in the mother’s body for an extended period and is breast-fed later.

7.3.3 (a) - Male Reproductive System

7.3.3 (b) - Female Reproductive System

7.3.3 (c) - What Happens When the Egg is Not Fertilised?

7.3.3 (d) - Reproductive Health

Health Consequences of Sexual Activity and Contraceptive Methods

NCERT Science Notes - Class 10 | Science | Chapter 7 - How do organisms reproduce?

Class 10 | Science | Chapter 7 - How do organisms reproduce?