What is your body made of? You might say that you are made of organs like skin and a heart. You might say that you are made of tissue, cells, or even atoms. All these answers are correct. Multicellular organisms, like yourself, other animals and plants, have various levels of organization within them. The levels of organization are a hierarchy from simple to complex: cells tissues organs organ systems organisms Organization in Animals The Cell 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 shapes and structures because each has a different function. For example, muscle cells tend to be long to allow for contraction and nerve cells tend to have many branches to help with communication. Nerve cells have many branches that help them send signals throughout the body. One 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. Tissues Cells that are similar in structure and function form tissues. The cells that make up tissues work together to perform a specific activity and are specialized (e.g., evidence from data about the kinds of cells found in different tissues, such as nervous, muscular, and epithelial, and their functions). Specialized tissues comprise each organ, enabling the specific organ functions to be carried out (e.g., the heart contains muscle, connective, and epithelial tissues that allow the heart to receive and pump blood). 1
Muscular tissue is involved in movement. For example, skeletal muscles help move the body. The stomach has smooth muscle tissue, which helps to churn food and break it down into smaller pieces. The smooth muscle of the stomach can also expand to hold large amounts of food and then relax when empty. Epithelial tissue is found on the surface of organs and lines the inner spaces of organs. This type of tissue also covers the entire body as 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 (plural of stimulus) from the environment and send signals throughout the body. stimuli: things that an organism can sense For example, nervous tissue in the stomach sends signals to the brain to let a person know when the body is hungry. Nervous tissue also works with muscle tissue to help the body move. 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 support the cells of the epithelial layer. In the stomach, connective tissue can also be found under its epithelial tissue. 2
Organs The next level of organization in animals is the organ. Specialized tissues comprise each organ, enabling the specific organ functions to be carried out. For example the heart is a major organ that contains muscle, connective, and epithelial tissues that allow the heart to receive and pump blood throughout the body. Different organs can work together as subsystems to form organ systems that carry out complex functions (e.g., the heart and blood vessels work together as the circulatory system to transport blood and materials throughout the body). Each organ is made of tissues with similar structure and function. Other examples of organs include the skin (the largest human organ), lungs, brain, kidneys, and stomach. Organ Systems The organ systems are the next level of organization. The body contains organs and organ systems that interact with each other to carry out all necessary functions for survival and growth of the organism (e.g., the digestive, respiratory, and circulatory systems are involved in the breakdown and transportation of food and the transportation of oxygen throughout the body to cells, where the molecules can be used for energy, growth, and repair). 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 organ systems such as: circulatory, nervous, skeletal, muscular, digestive, excretory, and respiratory systems. 3
Circulatory System The major organs of the circulatory system are the heart, arteries, veins, and capillaries. The heart pumps blood through its chambers to all parts of the body by cycles of contracting and relaxing. Arteries and veins carry blood to and from the heart. Arteries carry pumped blood under high pressure away from your heart through smaller branched tubes called capillaries. Veins carry deoxygenated blood from tissues back into the heart with less force. The circulatory systems works with the respiratory system to deliver blood to the body. With every breath, oxygen mixes with blood in the lungs, while the heart pumps blood to all cells in the body through the arteries. Oxygenated blood then returns back to the heart, and the process repeats. The nervous system coordinates all these interactions. Skeletal System Respiratory System When we breathe, we inhale oxygen and exhale carbon dioxide. Inside the lungs are thousands of bronchial branches ending with millions of alveoli sacs where oxygen and carbon dioxide exchange. Around the alveoli are microscopic capillaries (part of the circulatory system) that carry carbon dioxide rich blood from the heart to the lungs to be exhaled and bring oxygen rich blood back to the heart. Lastly, the muscular system helps the lungs expand and contract. The nervous system coordinates all these interactions. The skeletal system protects and holds organs in place, provides support for the body and coordinates movement with the muscular system attached to the bones, stores minerals, and contains resources to generate new blood cells. The major organs of this system include bones and joints. Bones are hard and white. A mineral compound called calcium phosphate is what makes up the composition of bones. The bone arrangement of the skeleton is important to provide a wide range of functions. These functions include supporting delicate and soft organs, anchoring muscles, and protecting the brain, lungs, and heart. The skeletal system also interacts with the circulatory system: Inside of the bones is bone marrow, the jelly-like substance where red and white blood cells form. The function of various bones includes the following: The backbone provides structure, which enables you to stand up straight. The skull acts as a hard, safety helmet protecting the brain. The vertebrae surround your spinal cord. The rib cage protects the heart and lungs. 4
The Muscular System The muscular system attaches to the skeletal system to allow the body to move and also permits movement of internal organs, such as the heart and intestines. It also provides strength, posture, balance, and heat for body warmth. Organs of this system include: muscles (three types: skeletal, smooth, and cardiac), ligaments, and tendons. Smooth muscles make up the walls of hollow organs, specifically utilized in the digestive, circulatory, respiratory, and reproductive systems. Cardiac muscle is the heart s muscle tissue. Ligaments and tendons are strong, fiber-like connectors assisting in efficient physical movement and stability. Ligaments connect bones to other bones, while tendons connect muscles to bones. The nervous system coordinates these interactions. The Digestive System The digestive system processes food into small enough nutrients to travel through the blood to all cells and passes through material that cannot be digested as waste. In the mouth, saliva and teeth break down food into smaller components. Esophagus muscles push the smaller food particles into the stomach where hydrochloric acid chemically breaks down food and destroys most bacteria. After digestion in the stomach, a thick liquid called chyme is formed which passes through the small intestine where the small intestine absorbs most of the nutrients. Finally, the solid waste passes through the large intestine and is excreted through the rectum and anus. The nervous system coordinates these interactions. The Excretory System The excretory system filters excess fluids, chemicals, vitamins, minerals, salts, and works with the circulatory system to eliminate wastes from the bloodstream into the kidneys. The major organs of the excretory system are the kidneys, bladder, and urethra. The kidneys filter excess fluids for the right consistency, salt concentrations, and wastes. Waste collects in the kidneys, where it is processed into urine and passes into a stretchy pouch called the bladder. When the bladder becomes full, it sends a signal through the nervous system to release the urine through the urethral opening. 5
Organisms 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 shown above, 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 Sponges commonly live and grow on coral reefs. 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 MRI machine interact with the water in a patient s body. 6
The machine then measures how much water is present in any given area. This creates an image with various shades of grey from light (where there is a lot of water) to dark (where there is little water). Because the magnetic coils create a magnetic force encircling the entire body, the MRI machine can 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 and other materials from the roots to the stems and leaves, while the phloem is a tissue that moves sugars throughout the plant. Plant tissues are organized into tissue systems. Plants have three types of tissue: The Dermal Tissue covers the outside of the plant and provides protection. The dermal tissue includes epidermal tissue and tiny openings, called stomata, that open and close to allow for gas exchange. The Ground Tissue is a general tissue with a variety of functions, including photosynthesis and food storage. The ground tissue contains the photosynthetic cells (chloroplasts) of the plant. 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. These plant cells group together to form tissue. 7
Which tissue type in plants is most similar to the integumentary system in animals? Explain your reasoning. Plant tissues 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 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. 8
Not all plants have all of the tissue types and organs. For example, mosses do not have vascular tissue. Without vascular tissue, mosses cannot transport water over 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. 9
For each structure, describe the smaller living unit that makes up the structure. Also, describe the next larger living unit. Write your answers 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. Plant Leaf Tissues make up leaves. Leaves group together to form part of the shoot system. Nerve Cell Circulatory System Ground Tissue System Brain 10
Identifying Plant Structures To help your child learn more about units of organization in plants, dissect plant organs, and identify the tissues 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 of the leaf, which is an organ. The dermal tissue can be peeled off carefully to show the ground tissue beneath. To peel off the dermal tissue, bend the leaf in half until it snaps and then peel back the clear dermal tissue. 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 is on the outside. The vascular tissue 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 the ground tissue. Now, explain to your child that the part of a carrot that we eat is a root. Make a cross-section of the carrot. The root will look much like the stem, but the vascular tissue is generally found in a single large bundle in the center of the root. Here are some questions to discuss with your child: What was the smallest living unit of a plant 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? 11