Living Creatures: Exploring their CharacteristicsClass 6 Science Notes

Living Creatures: Exploring their Characteristics

The world around us is filled with countless objects, some of which are living beings and others that are non-living things. It's not always easy to differentiate between them. For example, a snail's shell might appear to be a non-moving object, but it is actually an integral part of a living snail's body. This observation prompts us to ask: what fundamental characteristics define a living being?

What Sets the Living Apart from the Non-living?

To distinguish between living and non-living things, we can observe several common characteristics that all living beings share.

1. Movement: All living beings exhibit some form of movement. While animals often move from one place to another (locomotion), plants also show movements, even if they remain rooted. For example, flowers open and close, and insectivorous plants like Drosera move their hair-like projections inward to trap insects. Climbers also wind themselves around objects. This demonstrates that movement is a key characteristic of life.
2. Growth: Living beings grow and increase in size. You can't wear the clothes you wore a few years ago because your body has grown larger. Similarly, plants grow from seeds into seedlings and then into mature plants. Non-living things, like a car, do not grow.
3. Nutrition (Need for Food): All living beings require food for their growth, development, and to carry out life processes. Food provides the energy necessary for these functions.
4. Respiration: This is a vital process for all living organisms. During respiration, living beings take in air (inhale) and release air (exhale). This process involves the exchange of gases. Animals like dogs, cats, cows, and buffaloes show visible breathing movements. Plants also respire through tiny pores called stomata on the surface of their leaves, which facilitate the exchange of air.
5. Excretion: Living beings produce waste products as a result of their life processes, and these wastes must be removed from the body. This process is called excretion. For example, humans sweat, removing water and salts, and produce urine. Plants also excrete excess water and minerals, often seen as small droplets on leaves (e.g., on grasses and roses).
6. Response to Stimuli: Living beings react to changes in their surroundings. Any event or thing that prompts a living being to respond is called a stimulus (plural: stimuli). For instance, if you step on a sharp object or touch a hot cup, your body reacts instantly. Plants also respond to stimuli; the touch-me-not plant (mimosa) folds its leaves when touched, and some plants fold their leaves after sunset.
7. Reproduction: All living beings have the ability to produce new individuals of their own kind. This process, called reproduction, is essential for the continuity of life on Earth. Non-living things cannot reproduce.
8. Death: Eventually, all living beings die. When a living being can no longer exhibit its characteristics, even with all necessary resources like food, air, and water, it is considered dead.

Therefore, the essential features of living beings include movement, needing food, growth, respiration, reproduction, excretion, response to stimuli, and eventually, death. The absence of any of these features indicates that an object is non-living.

Jagadish Chandra Bose, an Indian scientist (1858-1937), conducted pioneering experiments on plant responses. He invented a machine called a crescograph to measure plant growth and their reactions to stimuli such as light, heat, electricity, and gravity, demonstrating that plants can sense and respond.

Essential Conditions for Germination of a Seed

A seed, while seemingly dormant, is a living entity. For a seed to sprout and begin to grow, a process called germination, specific conditions are required.

The primary conditions essential for seed germination are:

• Water: Seeds need water to germinate. Water is crucial because it enables the internal processes necessary for growth. It softens the seed coat (the outer covering of the seed), allowing the tiny embryo inside to develop and emerge.
• Air: Seeds require air for respiration during germination. They utilize the air present in the spaces between soil particles. Adequate air circulation is also important for root growth.
• Light and/or Dark Conditions: For most seeds, the presence or absence of light is not essential for germination itself. However, after germination, once the seed has sprouted into a seedling, sunlight becomes necessary for its continued growth (photosynthesis).

Just as humans need a balanced diet for health and growth, plants need favorable conditions and nutrients for proper growth and development.

Growth and Movement in Plants

Plants exhibit remarkable growth and movement in response to their environment.

• Response to Sunlight: Plant shoots generally grow upwards and bend towards a source of light. This movement towards light is crucial for photosynthesis.
• Response to Gravity: Roots of plants consistently grow downwards, regardless of the plant's orientation. This downward growth helps anchor the plant and allows roots to access water and nutrients deep in the soil.

Even if a plant is inverted, its roots will bend and grow downwards, and its shoots will bend and grow upwards, demonstrating their inherent responses to gravity and light.

Life Cycle of a Plant

The life cycle of a plant describes the entire process from a seed to a mature plant that produces new seeds, and then to the next generation of seeds. For example, in a bean plant:

1. A seed is sown and, under suitable conditions, begins to germinate.
2. The germinated seed develops into a young plant (seedling).
3. The young plant matures and produces flowers.
4. The flowers develop into fruits, which, in the case of a bean, are pods containing new seeds.
5. After the fruits and seeds are formed, the plant's life activities gradually come to an end, and it eventually dies.
6. The new seeds can then be sown to start the cycle again, giving rise to a new generation of plants.

This continuous process ensures the survival and propagation of the plant species.

Life Cycle of Animals

Animals also undergo a series of changes throughout their lives, from young ones to adults. These changes are part of their life cycle and are crucial for their survival and the continuation of their kind.

Life cycle of a mosquito

Mosquitoes are common insects, with female mosquitoes known for transmitting diseases like malaria, dengue, and chikungunya. Preventing mosquito breeding is important for public health. Mosquitoes lay their eggs in stagnant water.

The life cycle of a mosquito consists of four distinct stages:

1. Egg: Adult female mosquitoes lay eggs directly on or near water.
2. Larva: The eggs hatch into larvae, which are worm-like creatures that live in water. Mosquito larvae frequently come to the water surface to breathe air.
3. Pupa: Larvae develop into pupae, another aquatic stage. Pupae are also worm-like but have a different shape from larvae and also come to the water surface to respire.
4. Adult: An adult mosquito emerges from the pupa, rests briefly on the water surface, and then flies away. Adult mosquitoes typically survive for $10$ to $15$ days.

Significant changes in appearance, body shape, and structure occur between these stages, a process known as metamorphosis. For example, the egg looks very different from the larva, the larva from the pupa, and the pupa from the adult mosquito.

The silk moth also undergoes a four-stage life cycle: egg, larva, pupa, and adult. Eggs hatch into larvae, which grow in size and then secrete a thread-like material to wrap themselves, forming a pupa. These threads are the fibers used to make silk fabric.

Life cycle of a frog

Frogs, like mosquitoes, also undergo metamorphosis, with significant changes in their life cycle stages. During the rainy season, you might observe a white, jelly-like substance on the surface or edges of ponds, often attached to plants. This substance is a cluster of frog eggs, known as spawn.

The life cycle of a frog typically involves four main stages:

1. Egg Stage: Frogs lay eggs, usually in clusters (spawn), in water. These eggs contain developing embryos.
2. Tadpole Stage: The eggs hatch into tadpoles.
   • Early Tadpole: These are aquatic, fish-like creatures with a tail for swimming and no legs.
   • Late Tadpole: As they grow, tadpoles develop hind legs, and later, front legs, while still retaining their tail.
3. Froglet Stage: Tadpoles gradually transform into froglets. Froglets resemble small frogs but still have tails. They begin to spend time both in water and on land.
4. Adult Frog Stage: Froglets continue to grow, losing their tails completely. Their legs become strong for jumping and landing. They develop into fully mature adult frogs that can live both in water and on land.

Throughout these stages, frogs experience distinct changes in their body shape, structure, and even their habitat. Tadpoles are fully aquatic, while adult frogs are amphibious, living in both water and on land. These adaptations at each stage support their survival in different environments.

Both plants and animals are integral parts of the living world, undergoing various changes throughout their lives. From a tiny seed growing into a tree to a mosquito larva transforming into an adult, or a tadpole becoming a frog, these journeys are unique and essential for the survival and continuity of their species. It is our responsibility to nurture and preserve their homes, contributing to the flourishing biodiversity of our living world.