Science Facts: What is life? Classification Cells, tissues, and organs

 

There are millions of different kinds of living things, from germs that are too small to see to elephants, whales, and towering trees. Living things are also known as organisms.

 

Characteristics of life

Most of the living things we see around us are animals and plants. Although animals and plants look very different, they share certain characteristics in common with all organisms. These are the characteristics of life.

 

1. Getting food

All organisms need food, which gives them both energy and the raw materials they need to grow. Animals get food by eating other organisms. Plants get food by making it, using sunlight, air, and water.

 

2. Getting energy

All living things use energy. They get it from food by a chemical process known as respiration, which takes place inside cells. Most organisms need a continual supply of oxygen from the air for respiration, which is why they need to breathe.

 

3. Sensing

All organisms can sense things in their surroundings. Animals can sense light with their eyes, sound with their ears, smells with their nose, touch and heat with their skin, and the taste of food with their tongue.

 

4. Removing waste

Lots of processes happening inside an organism produce waste products that must be removed from the body in a process called excretion. This is because the waste products may harm the body if they are allowed to build up.

 

Count the species

See how many different types of organisms you can identify in a backyard in only one minute. A good place to find small animals is under rocks or plant pots, where small creatures like to hide and keep out of the sun.

 

 

5. Moving

All living things move, though some move so slowly that we hardly notice. Animals move quickly by using their muscles. Plants move by growing—their shoots grow upward to the light and their roots grow down into the soil.

 

6. Reproducing

All organisms strive to create new organisms by a process called reproduction. Plants, for example, create seeds that grow into new plants. Animals lay eggs or give birth to babies.

 

7. Growing

Young organisms grow into mature ones, getting larger as they age. Some organisms simply get bigger as they age, but others also change. An acorn, for instance, grows into an oak tree and a caterpillar grows into a butterfly.

 

Classification

There are nearly two million known species (types of organisms) on Earth. These species are classified into groups based on the common ancestors they share, just like a family tree.

 

Divisions of life

Every organism on Earth belongs to one of several major divisions of life, such as the animal kingdom and the plant kingdom.

 

1. Animal kingdom

Animals are multicellular organisms that eat other organisms. They have sense organs to detect changes in their surroundings, and nervous systems and muscles so they can respond quickly.

 

2. Plant kingdom

Plants are multicellular organisms that produce food by capturing sunlight. Most plants have leaves to absorb sunlight and roots to anchor them in place and absorb water from the ground.

 

3. Fungus kingdom

Fungi absorb food from dead or living organic matter, such as soil, rotting wood, or dead animals. Members of this kingdom include mushrooms, toadstools, and molds.

 

 

4. Microorganisms

Microorganisms are so tiny they can only be seen with a microscope. Many types consist of just a single cell. Microorganisms can be divided into three kingdoms.

 

Classifying animals

Earth’s animals are divided into two major groups: animals with backbones (vertebrates) and animals without backbones (invertebrates). These are then divided into even more groups.

 

 

Cells

All living things are made up of microscopic units called cells. The smallest living things have only one cell each, but animals and plants are made up of millions of cells working together.

 

Animal cells

Animal cells and plant cells have many features in common, but animal cells lack a sturdy wall and so are often irregular in shape. All cells work like miniature factories, performing hundreds of different tasks every second of the day. Many of these tasks are carried out by tiny bodies called organelles inside the cell.

 

Cell size

Most cells are just a fraction of a millimeter long. This is too small for the human eye to see, so scientists use microscopes to study cells. On average, plant cells are slightly larger than animal cells.

 

Plant cells

Plant cells have many of the same organelles as animal cells, but they also have a fluid store called a vacuole and bright green organelles called chloroplasts, which capture and store energy from sunlight. Plant cells also have tough outer walls that make them more rigid than animal cells.

 

• A vacuole in the center of the cell stores water. When you water a plant, its vacuoles swell with water, making the plant’s stem and leaves sturdy and firm.

 

• Chloroplasts use the energy in sunlight to create energy[1]rich sugar molecules from air and water. This process is called photosynthesis.

 

• A cell wall surrounds and supports a plant cell. It is made of a tough, fibrous material called cellulose the main ingredient in paper, cotton, and wood.

 

Microscopes

Microscopes are viewing devices that make it possible to see tiny objects such as cells. Using a series of curved glass lenses that work like magnifying glasses, they can make objects look hundreds of times bigger. The sample of cells is placed on a thin piece of glass, and a light is shined through this to help make the cells visible.

 

Cells, tissues, and organs

 

The cells in the human body are joined in groups that work together, known as tissues. Different tissues are joined to form organs, and organs work together in groups called systems.

 

 

Types of cells

There are many different shapes and types of cells, each one specialized to do a specific role. Every cell has the same basic structure: an outer coating called a membrane; a jellylike cytoplasm containing many structures called organelles, which bring the cell to life; and a nucleus—the cell’s control center.

 

Tissues

Most cells are joined together in layers to form tissues. Epithelial cells, for instance, are tightly packed together to form a protective wall of tissue that lines the inside of the mouth, stomach, and intestines.

 

Organs

Different types of tissue combine to form organs. The stomach is an organ that stores food and digests it. It is lined with epithelial tissue, but its wall also contains muscle tissue and glandular tissue that secretes digestive juices.

 

Systems

The stomach is just one organ in the digestive system—the collection of organs that break down food so the body can absorb it. Groups of organs that work together in this way are called organ systems. The digestive system includes the esophagus, stomach, small and large intestines, liver, and pancreas. Other systems include the muscular system, nervous system, and respiratory (breathing) system.