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Wombats and wallabies, fish and frogs, crows and crabs are all complex organisms that perform a variety of functions. They breathe, reproduce, digest food and get rid of wastes. To perform these functions, the cells of multicellular organisms are organized into tissues, which in turn are organized into organs and then into systems.

Animal tissues and organs

Many different things need to happen in an organism if it is going to stay alive and function well. There is a need for some form of organization. In complex organisms (like those in Figure 2.4.1) there are several different levels of organization. These levels are shown in Figure 2.4.2.

• Tissues: Cells that perform the same function are not scattered throughout the body; they are grouped together to form tissues.
• Organs: The next level of organization is into organs. A n organ is a structure that contains at least two different types of tissue working together to complete a task.
• Organ systems:Organs are arranged into organ systems. Organ systems have two or more different organs that work together.

Tissues

In the human body there are four different types of tissues:

• epithelium
• connective tissue
• muscle tissue
• nerve tissue.

Epithelium is another name for skin. The cells of an epithelium form a continuous layer that is more than just the skin covering the outside of your body. It continues into the body. It lines the inside of your mouth right down to your stomach and through the rest of your insides. It also covers all the organs of the body.You can see epithelium cells in Figure 2.4.3.

Connective tissue adds support and structure to the body. Examples of connective tissue are fat, bone, cartilage, ligament and tendon such as that shown in Figure 2.4.4. Some types of connective tissue are strong and fibrous. Ligaments hold bones together at the joints. If you sprain your ankle or wrist badly, then it is likely that you have torn some of the fibers of the ligaments holding the bones together.Tendons join muscles to bones.

Muscle tissue is a specialized tissue that can contract, becoming shorter and fatter. For example, the muscle on the front of your upper arm (the bicep) bulges when you pull up your forearm. You can see this in Figure 2.4.5. Each individual cell of the muscle has contracted. However, one cell on its own, or even a few cells contracting, would not be strong enough to pull up your arm. It takes thousands of cells working together to move the bones of your arm.

Nerve tissue such as that in Figure 2.4.6 can send electrical signals around the body. These electrical signals are managed by nerve tissue in the brain and passed down the spinal cord (a large bundle of nerve tissue) to the body. One nerve cell would not be able to pass information around your body effectively. Thousands of nerve cells are grouped together to form your nerves

Organs

Tissues are grouped together into organs. All the different tissues contribute to the job that the organ has to do. There are many different organs in your body, such as the liver, kidneys and heart. Even your skin is an organ-the largest organ of your body. As you can see in Figure 2.4.7, the skin has many different parts.

Heart

In the figure of the heart below, the tissues are grouped together for the specific purpose of ensuring that the blood flows around your body, carrying to the cells all the things they need to function. Your blood also carries away wastes that could harm your cells.

Organ systems

The organs do not work independently. Organs are organized so that they can work together to complete a task. These organs form an organ system. For example, a series of organs work together to change the food you eat into a form that your body can use. This system digests the food. For this reason the system is called the digestive system. It is shown in Figure 2.4.9.

There are other systems in your body such as the:

• respiratory system, which gets oxygen from the air into your body and gets rid of waste carbon dioxide
• skeletal system, which is your skeleton and muscles
• excretory system, which gets rid of wastes from the body
• nervous system, which sends messages from your sense organs to the brain and from the brain to other parts of the body, including the muscles
• reproductive system.

Plant tissues and organs

Although flowering plants are simpler than animals, they have tissues and organs too. These tissues and organs enable the plant to carry out essential functions such as photosynthesis, transport and reproduction. For example, many plants use flowers to reproduce and so flowers are their reproductive organs.

Transport tissue

Materials are transported around plants in two types of cells:

• Xylem carries water from the roots. Xylem cells are long thin tubes with strong walls that also form an important part of the skeleton of the plant. You can see them clearly in Figure 2.4.10.
• Phloem carries glucose from the leaves to all other plant parts.

The main organs of a plant are its roots, stems and leaves. These organs work together to bring together the raw materials required for photosynthesis. Photosynthesis is the process by which plants make glucose. Plants use energy from glucose for their own life processes; without it they could not survive. For photosynthesis to take place, water, sunlight, chlorophyll and carbon dioxide are needed. Oxygen is produced as a waste product.

The root is the plant organ that takes water into the plant from the soil. The structure of the root is shown in Figure 2.4.12. Water moves through cells to the vascular tissue. It moves up the xylem to the stem.

The stem is the organ that holds the leaves of the plant up to the light. Stems also carry water from the root to the leaves and transport glucose to all parts of the plant.

The leaves are the organs in plants that carry out photosynthesis. Leaves have:

• a large surface area to capture as much light as possible
• cells near the surface that have large numbers of chloroplasts to trap energy from the Sun
• stomata (holes) in the leaf surface to let carbon dioxide into the leaf and let water vapor and oxygen out
• air spaces within the leaf that allow gases to move quickly through the leaf to the cells where they will be used or away from the cells that produced them as waste
• large amounts of vascular tissue to carry water to the cells and to carry glucose away from the cells to other parts of the plant.

Photosynthesis

Photosynthesis uses carbon dioxide from the air and water from the soil. The carbon dioxide and water react together to produce glucose, a sugar plants use as their source of energy. Sunlight is needed for photosynthesis to take place. Light energy from the Sun is converted into chemical energy and stored in the glucose that is produced. Chlorophyll is also needed for the reaction. Chlorophyll is a chemical that converts the Sun’s energy into chemical energy the plant can use.

2.Cells Chapter review