STEMscopedia: CELL STRUCTURES AND HOMEOSTASIS B1A

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1 Reflect B1A Have you wondered why you shiver in the cold? Or maybe why your stomach gets upset after eating something bad? These and many other reactions are ways your body is responding to a change in its internal environment. Different responses and processes will occur in your body to help it reach a state known as homeostasis. When an external change occurs, such as a decrease in the outside temperature, our body has to compensate for what effect this decreased temperature will have on us. The internal body will pick up on signals and tell it how to respond to ensure the body stays at its optimal internal temperature. For this particular example, shivering increases body temperature to compensate for the cooler temperatures on the outside. How COOL is that? But HOW does the body adjust to these changes? The body counts on cells, and the parts that make up a cell, to carry out their normal functions, allowing the body to change and adapt. Cells are the smallest unit of life, meaning that within each cell there are parts that allow these microscopic living beings to grow, change, protect the cell, reproduce, obtain nutrients, and discard waste. These parts are called organelles, or small organs. Each organelle has a function, or role, to play to maintain the cells balance. Look Out Think for a moment about all the living things on Earth. There is great diversity among organisms, from microscopic bacteria to massive whales, the largest animals on the planet. Despite the tremendous variety of life, all organisms have something in common they are all made of cells. Some organisms are unicellular, composed of just a single cell. Other organisms are multicellular, composed of more than one cell. In fact, the human body is made of about 100 trillion cells! The two categories of cells are prokaryotic cells and eukaryotic cells. A prokaryotic cell is a simple cell that does not contain a nucleus or other membrane-bound organelles. In contrast to prokaryotic cells, eukaryotic cells are more complex. They contain a nucleus and other membrane-bound organelles that perform specific functions that contribute to the overall metabolism and growth of the cell. Eukaryotic cells are found in multicellular organisms including plants, animals, fungi, and protists. They can also be unicellular protists. 1

2 capsule cell wall Prokaryotic Chart The capsule is the thin, outermost layer of the cell that provides protection. The cell wall surrounds the cell and maintains the cell s shape. plasma membrane cytoplasm DNA nucleoid plasmids ribosomes pili flagella Individual membranes do not surround internal structures. However, a single plasma membrane surrounds the entire cell. The membrane helps move materials into and out of the cell. Prokaryotic cells contain a gel-like fluid called cytoplasm. Cytoplasm takes up most of the space inside the cell. DNA within a prokaryotic cell is a single, circular molecule that is not enclosed in a membrane-bound compartment. DNA carries the instructions and genetic code for the cell. Although DNA is not enclosed in a nucleus, it is generally confined to a central region called the nucleoid. Circular structures called plasmids are found inside prokaryotic cells. Plasmids are a genetic element of the cell but are not part of the main DNA strand. They are involved in cell activities, such as growth and metabolism. Prokaryotic cells contain ribosomes that play roles in manufacturing proteins. Hollow, hairlike structures called pili surround prokaryotic cells. Pili enable prokaryotic cells to attach to other cells. Long, whip-like structures called flagella (singular: flagellum) help prokaryotic cells move. A cell may have one flagellum, or it may have several flagella. In addition to the structures shown, prokaryotic cells contain a central area around the DNA called the nucleoid. 2

3 Eukaryotic Chart cell wall plasma membrane cytoplasm nucleus DNA mitochondria endoplasmic reticulum (ER) Golgi body ribosomes lysosomes chloroplast central vacuole The cell wall surrounds the cell and maintains the cell s shape. Individual membranes do not surround internal structures. However, a single plasma membrane surrounds the entire cell. The membrane helps move materials into and out of the cell. Prokaryotic cells contain a gel-like fluid called cytoplasm. Cytoplasm takes up most of the space inside the cell. The nucleus is the central organelle that holds DNA. DNA within a prokaryotic cell is a single, circular molecule that is not enclosed in a membrane-bound compartment. DNA carries the instructions and genetic code for the cell. The mitochondria play major roles in transforming the energy in food into a usable form of energy called ATP. The cell then uses ATP to carry out activities such as reproduction and growth. The endoplasmic reticulum, or ER, transports proteins and helps produce lipids. The Golgi body helps package and distribute proteins and lipids within the cell. Like prokaryotic cells, eukaryotic cells contain ribosomes that play roles in manufacturing proteins. However, the ribosomes in eukaryotic cells are larger and more complex. Lysosomes contain enzymes that help break down food or break down the cell when it dies. Plant cells and some protists contain chloroplasts. These structures contain the green pigment chlorophyll, which captures the energy of sunlight for use in photosynthesis. Many plant cells contain a large central vacuole, which stores water, food, and waste. Animal cells contain vacuoles, but they are much smaller than the central vacuole found in plant cells. In addition to the structures shown in this animal cell, plant cells contain a cell wall, a central vacuole, and chloroplasts. 3

4 What Do You Think? Take a look at the following images of cells. Based on organelle complexity, can you determine which one is eukaryotic and which one is prokaryotic? Discover Science: The Cell Theory Before the 1600s, people did not know that cells existed at all. This may be hard to believe, considering how much we now know about cells. But until scientists were able to fully observe cells and their functions, people believed life arose spontaneously. Thanks to the work of Robert Hooke, Antonie van Leeuwenhoek, Matthias Jakob Schleiden, Theodor Schwann, Rudolf Virchow, and other scientists, the cell theory was developed. British scientist Robert Hooke was the first person to observe matter that made up what he called cells. In the 1660s, Hooke used a microscope to look at cork from the bark of an oak tree. He noted the cork looked like it was made of small compartments that reminded him of the rooms, or cells, in which monks lived. For this reason, Hooke named the structures he observed cells. Hooke was unknowingly observing nonliving cell walls. The next major development came in the later part of the 1600s when Antonie van Leeuwenhoek observed living cells under a microscope. He examined what he called animalcules, what we now call microorganisms. Based on his notes, scientists today think that van Leeuwenhoek was observing algae and bacteria. Matthias Jakob Schleiden, Theodor Schwann, and Rudolf Virchow are the three scientists who are typically given credit for the development of the cell theory. Schleiden studied plants and discovered they were made of cells. At the same time, Schwann discovered animals were made of cells. In 1838, Schleiden and Schwann proposed the first two parts of the current cell theory. The theory states that all living things are made of cells and that cells are the basic units of structure and function in living things. Virchow developed the third part of the cell theory, which states that all cells arise from pre-existing cells. This magnified image of cork tissue is from Robert Hooke s book Micrographia. 4

5 Try Now Each organelle plays a role in how a cell maintains balance, or homeostasis, within the body. Take a look at the following images and predict what organelles are active in helping to maintain balance for each individual. Connecting With Your Child To help your child learn more about how organelles help maintain homeostasis, have them draw or create a three-dimensional model of a eukaryotic cell. Ask them to choose either a plant cell or an animal cell. If drawing their models, have them use colored pencils to sketch the cells and their structures. They should include labels and list the functions of each structure. If creating threedimensional models, help them brainstorm ideas of materials they can use such as pipe cleaners, wax craft sticks, pompoms, and string. Three-dimensional models should also include labels. Next, have your child use toothpicks, tape, and small pieces of paper to create numbered labels. Then he or she can create a written numbered key on a sheet of paper. For example, a toothpick taped with the number 1 can be placed on the nucleus. The written key would indicate that number 1 is a nucleus and is the centrally located organelle that contains DNA. Here are some questions to discuss with your child: 1. What type of cell did you create? 2. Do you think you would be able to see all of the structures under a microscope? Explain. 3. Can you give some examples of how these organelles are maintaining balance within the cell? 5

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