What is your body made of? You might say that you are made of atoms or cells. You might even say you are made of organs, like skin and a heart. These answers are all correct. Each focuses on a different level of organization of the human body. Atoms are a nonliving unit of the body. But cells and organs are living units. How do these different living units relate to each other? Where do body systems, like the digestive system, fit into these levels of organization? Organization in Animals In all living things, the cell is the smallest unit of life. Some organisms are unicellular. They are made of a single cell functioning on its own. Bacteria and yeasts are two examples of single celled organisms. Animals are multicellular, meaning they are composed of more than one cell. In fact, the human body is made up of about 100 trillion cells! Cells have a variety of different shapes and structures because they each have a different function. For example, muscle cells tend to be long to allow for contraction. Nerve cells tend to have many branches to help with communication. One main function of red blood cells is to transport oxygen from the lungs to other cells throughout the body. An important function of certain bone cells is to release hormones that help form bones. Nerve cells have many branches that help them send signals throughout the body. Cells that are similar in structure and function form tissues. The cells that make up tissues work together to perform a specific activity. Animals have four main types of tissue: Muscular tissue is involved in movement. For example, skeletal muscles help move the body. The stomach has smooth muscle tissue, which helps to churn foods and break the foods down into smaller pieces. The smooth muscle of the stomach can also expand to hold large amounts of food and then relax when empty. 1
Epithelial tissue is found on the surface of organs and lines the inner spaces of the organs. This tissue type also covers the entire body because it forms skin. The main functions of epithelial tissue are protection, secretion (producing and releasing materials) and absorption (taking in materials). Epithelial tissue that lines the stomach secretes chemicals that help with digestion. Nervous tissue functions to sense stimuli from the environment and send signals throughout the body. For example, nervous tissue in the stomach sends signals to the brain to let a person know when he or she is hungry. Nervous stimuli: things thatan tissue also works with muscle tissue to help the body move. For organism can sense example, when you touch a hot object, the nervous tissue in your hand sends signals to the brain to contract your muscles in order to move your hand away. Connective tissue has a variety of functions. This type of tissue connects and holds together structures in the body, providing support and structure. Connective tissue includes bones and fat tissue. Connective tissue is also found under epithelial tissue and helps to support the cells of the epithelial layer. In the stomach, connective tissue can be found under its epithelial tissue also. The next level of organization in animals is the organ. Each organ is made of tissues with similar structure and function. Examples of organs include the heart, skin (the largest human organ), lungs, and stomach. The organ systems are the next level of organization. An organ system consists of two or more organs working together to perform a specific function for the organism. The human body is organized into several main organ systems: circulatory, nervous, skeletal, muscular, integumentary, endocrine, digestive, immune, reproductive, excretory, and respiratory systems. Each system performs specific functions. For example, the integumentary system is made of the skin, hair, nails, and glands. This system receives stimuli from the external environment and protects the body s deeper tissues and organs. The digestive system is made of a number of organs. The stomach stores food and helps with digestion. The intestines digest food and absorb nutrients. The liver works as part of the digestive system by secreting bile, which is a substance that helps with the break down of fats, or lipids. A single organ can work with multiple organ systems. For example, the liver also works with the circulatory system to filter wastes in the blood. glands: organs that make and release chemicals called hormones; hormones control and regulate body processes 2
The final and most complex level of organization in animals is the organism. Organisms are entire living things that perform basic life processes. Organisms take in materials, release energy from food, release wastes, grow, respond to the environment, and reproduce. Simple The living units of organization from smallest to largest are cells, tissues, organs, organ systems, and organisms. The images here show, from left to right, are an epithelial cell, epithelial tissue, a stomach, the digestive system, and a human body. Note that the images are not to scale. Complex Suppose one organ in an organ system failed to work properly. How would the organ system be affected? Would it still be able to function? Explain your reasoning. Not all animals have the same levels of organization. For example, sponges are simple aquatic animals. They are multicellular; however, their cells are not organized into welldefined tissues. They do not have organs or organ systems. A sponge s essential life functions are carried out on the cellular level. Getting Technical: Magnetic Resonance Imaging Magnetic Resonance Imaging, or MRI, is a procedure used by doctors to help diagnose organ or organ system diseases. The procedure uses magnets and radio signals to generate images of the inside of the body. The magnets and the radio signals in an Sponges commonly live and grow on coral reefs. MRI machine interact with the water in a patient s body. The machine is then able to measure how much water is present in any given area. This creates an image with various shades of grey from 3
light (where there is a lot of water) to dark (where there is little water). Because the magnetic coils are able create a magnetic force encircling the entire body, the MRI machine is able to image the body in three dimensions, or in 3-D. This is displayed as a series of pictures showing one slice of the body at a time. Doctors examine the images to look for structural abnormalities that may be present in the organs. An MRI machine generates a magnetic force that encircles the entire body. An MRI machine generated this image of the brain and other structures. Organization in Plants Like animals, plants are made of specialized cells that are organized into tissues. For example, xylem is a tissue that moves water, while the phloem is a tissue that moves sugars. Plant tissues are organized into tissue systems. Plants have three tissue systems: The dermal tissue system covers the outside of the plant and provides protection. The dermal tissue system includes epidermal tissue and tiny openings, called stomata, that open and close to allow for gas exchange. The ground tissue system is a general tissue system with a variety of functions including photosynthesis and food storage. The ground tissue contains the photosynthetic cells (chloroplasts) of the plant. These plant cells group together to form tissue. 4
The vascular tissue system is the system responsible for the transport of materials, including water and nutrients, throughout the plant. Xylem and phloem are parts of the vascular tissue system. Which tissue system in plants is most similar to the integumentary system in animals? Explain your reasoning. Plant tissue systems are organized into organs. Plants have four main organs: Leaves are the main organs used to capture sunlight used in photosynthesis. Roots are the main organs used to absorb water and nutrients from the soil. Stems are the main organs used to transport materials between leaves and roots. Reproductive organs (like flowers) produce seeds that grow into new plants. Flowers usually consist of colorful petals that attract insects and other pollinators. Plant organs group together to form the two main organ systems of plants: the root system and the shoot system. The root system is typically underground and includes the root and associated fibers that branch off the main root. This system functions to anchor the plant and absorb water and nutrients from the soil. The shoot system is typically above the ground and includes the stem, the leaves, and the reproductive organs, such as flowers. This system has many functions including photosynthesis and reproduction. As with animals, the organ systems of plants work together to make up the structure and function of the entire organism. The general organization in plants, from smallest to largest units, is shown below. cells tissues tissue system organ organ system organism 5
A specific example of this organization begins with long, narrow, specialized plant cells that work together to form the xylem, which is a tissue. The xylem works with the phloem (another tissue) to form the vascular tissue system. The vascular tissue system works with the dermal tissue system and the ground tissue system to form leaves, which are organs. Finally, leaves work with other organs (roots, stems, and reproductive structures) to make up a plant. Not all plants have all the tissue systems and organs. Mosses do not have vascular tissue. Without vascular tissue, mosses cannot transport water long distances. This is why they can only grow in wet areas and why they cannot grow very tall. Mosses also do not reproduce using flowers. They use a simplified reproductive cycle involving tiny structures called spores. What do you know? The chart on the next page includes images of different structures found in animals or plants. For each structure, describe the smaller living unit that makes up the structure. Also, describe the next largest living unit. Write your answers directly in the chart next to each image. Watch out! Some structures may not have a smaller or larger living unit. The first one is done for you. 6
Plant Tissue systems make up leaves. Leaves group together to form part of the shoot system. Leaf Nerve Cells Circulatory System Ground Tissue System Brain 7
Identifying Plant Structures To help students learn more about units of organization in plants, dissect plant organs and identify the tissue systems found within the organs. You will need a veined leaf (such as an oak or maple leaf), a stalk of celery, a carrot, a knife, and a hand lens (if available). Begin by identifying the veins of the leaf. The veins represent the vascular tissue system of the leaf, which is an organ. The dermal tissue system can be peeled off carefully to show the ground tissue system beneath. To peel off the dermal tissue system, bend the leaf in half until it snaps, and then peel back the clear dermal tissue system. Be sure to do this in an area of the leaf that is furthest from the thick veins. Next, explain to your child that the part of celery that we eat, the stalk, is actually a stem. Make a cross section of the stem; the dermal tissue system is on the outside. The vascular tissue system will look like clusters of circles. This is easiest to see if a hand lens is available. The rest of the cells in the stem are in the ground tissue system. Now explain to your child that the part of a carrot that we eat is a root. Make a crosssection of the carrot. The root will look much like the stem, but the vascular tissue system is generally found in a single large bundle in the center of the root. Here are some questions to discuss with students: What was the smallest living unit of a plant that you were able to observe? What was the largest living unit you were able to observe? What might happen if a plant organ, such as a leaf or stem, became damaged? How would the organism be affected? 8