Organization of Organisms
‘You might recall that matter makes up everything around you. The chair you are sitting on, the clothes you are wearing, and the air you are breathing are all made of matter! Matter is made of atoms that combine to form molecules. These molecules combine to form cells. A cell is the smallest unit of life. Organisms can be unicellular—made of one cell—or multicellular—made of two or more cells. Unicellular organisms, such as amoebas and algae, still carry out all the life processes needed for survival, such as absorbing nutrients and getting rid of wastes. On the other hand, the way in which multicellular organisms carry out life processes is more complex.

You may recall that scientists organize ecosystems into different levels in a system called hierarchical organization. Hierarchical organization is a system of organization that begins with the simplest level and each level becomes more complex. This system enables scientists to study ecosystems at different scales, whether it be an entire ecosystem or an individual organism. The composition of multicellular organisms can also be organized into hierarchical levels. Just as each level in the hierarchical organization of an ecosystem has a role, so does each level in organism organization. Let's explore these levels to learn how the parts of the system work together to form an organism.

You may recall that scientists organize ecosystems into different levels in a system called hierarchical organization. Hierarchical organization is a system of organization that begins with the simplest level and each level becomes more complex. This system enables scientists to study ecosystems at different scales, whether it be an entire ecosystem or an individual organism. The composition of multicellular organisms can also be organized into hierarchical levels. Just as each level in the hierarchical organization of an ecosystem has a role, so does each level in organism organization. Let's explore these levels to learn how the parts of the system work together to form an organism.

Cells
Cells are at the lowest level of the hierarchical organization of organisms. Every living thing on Earth is made of cells. Larger organisms, such as a panda, will have more cells than smaller organisms, such as an ant. Scientists estimate there are 37.2 trillion cells in the human body! Regardless of the type of multicellular organism, all cells are organized in a way that enables the body to function. Cells work together like the employees at a restaurant work together. Each member of the team, from servers to cooks, has a job that keeps the restaurant running. Similarly, multicellular organisms have different types of cells that each perform a specific job for the survival of the organism.

As multicellular organisms grow, cells divide to produce new cells. The first cells made can become any type of cell through the process of cell differentiation (dihf uh ren shee AY shun). Cell differentiation is the process by which cells become different types of cells. Animal cells can become different types of cells, including muscle cells, nerve cells, blood cells, bone cells, or skin cells.

Differentiation of Plant Cells

Even though plants and animals have different needs and life processes, cell differentiation also takes place in plants. However, plants have different types of cells with specialized structures and functions for making food, storing food, protecting the plant, or transporting materials. These cells might become parts of stems, leaves, flowers, or roots. 

Tissues
Similar types of cells are organized into groups. A tissue is a group of similar types of cells that work together to carry out specific tasks. Tissues are the next level in the hierarchical organization of organisms. Most animals, including humans, have four main types of tissue—muscle, connective, nervous, and epithelial (eh puh THEE lee ul). Each type of tissue has a specific task. Muscle tissue causes movement. Connective tissue provides structure and support and often connects other types of tissue together. Nervous tissue carries messages to and from the brain. Epithelial tissue forms the protective outer layer of the skin and the lining of major organs and internal body Cavities.

Plant Tissues
Like animal cells, similar plant cells are also organized into tissue. However, because animals and plants have different life processes, they also have different types of tissues. The three main types of plant tissue are dermal, vascular (VAS kyuh lur), and ground tissue. Dermal tissue provides protection and helps reduce water loss. Vascular tissue transports water and nutrients
from one part of a plant to another. Ground tissue provides storage and support and is where photosynthesis takes place.

Organs

The life processes that are carried out by organisms are complex jobs that require more than one type of tissue. An organ is a group of different tissues working together to perform a particular job. For example, your heart is an organ specialized for moving blood throughout your body. It is made of all four types of tissue: muscle, epithelial, nervous, and connective. Each type of tissue performs a specific function necessary for the heart to work properly. Muscle tissue pumps blood out of the heart to the rest of the body. Epithelial tissue lines blood vessels to allow blood to flow smoothly through the organ. Nervous tissue controls the beating of the heart, and connective tissue is found in the walls of blood vessels.

Plant Organs
Plants also have organs, including leaves, stems, and roots. Just like in animals, plant organs are groups of tissue working together to carry out a specific function. Leaves are organs specialized for photosynthesis. Each leaf is made of dermal, ground, and vascular tissues. Dermal tissue covers the outer surface of a leaf. The leaf is a vital organ because it contains ground tissue that produces food for the rest of the plant. Ground tissue is where photosynthesis takes place. The ground tissue is tightly packed on the top half of the leaf. The vascular tissue moves both the food produced by photosynthesis and water throughout the leaf and the rest of the plant.

Organ Systems
Usually, organs do not function alone. Instead, organs work in groups to perform certain functions. An organ system is a group of organs that work together and perform a specific task. For example, the digestive system is made of many organs, including the stomach, the small intestine, the liver, and the large intestine. These organs and others all work together to break down food and take it into the body. Complex multicellular organisms are made up of many organ systems, including the circulatory system, respiratory system, nervous system, and muscular System,

Not all multicellular organisms have the same organ systems. For example, earthworms do not have a respiratory system and sea sponges do not have a nervous system. However, all complex multicellular organisms follow this hierarchical organization,  including mammals, reptiles, birds, fish, amphibians, and insects.

Plant Organ Systems
Plants have two major organ systems—the shoot system and the root system. The shoot system includes leaves, stems, and flowers. Food, minerals, and water are transported throughout the plant by the shoot system. Even though the roots of most plants are never seen, they are vital to a plant's survival. Root systems are the only part of the plant that absorb water and soil nutrients, which are essential for the plant's survival. The roots anchor the plant, either in soil or onto another plant or object.

Organisms
An organism is the highest level in the hierarchical organization of plants and animals. An organism is something that has or once had all the characteristics of life. Multicellular organisms usually have many organ systems. These systems work together to carry out all the jobs needed for the survival of the organisms. Each organ system depends on the others and cannot work alone to keep the organism functioning.