Cell Structure and Function

Transcription

1 Cell Structure and Function Can you name something that you know exists even though you can t see it with your own eyes? A drop of pond water has tiny swimming organisms and small bits of plant material, but we can t always see them with our eyes. How do we know there are tiny things in a drop of pond water? We can use a microscope to view the pond water. There are instruments people use every day to help them see things they wouldn t usually be ab le to see. Have you ever used a pair of binoculars or a magnifying glass? Have you ever had an x-ray taken of an injury? Do you need to wear glasses or contact lenses to see clearly? Vision systems are even being developed to restore vision to bind people. In this chapter, you will take a journey into a small word that was discovered when the microscope was invented the world of cells. Imagine you could shrink yourself and walk into a tiny cell. What is it like inside a cell? It s a fascinating journey! 7.1 What are Cells? Look closely at the skin on your arm. Can you see that it is made of cells? Of course not! Your skin cells are much too small to see with your eyes. Now look at one square centimeter of your arm. That square centimeter contains about 100,000 skin cells. Cells are so small that they weren t even discovered until the invention of the microscope. What are cells and how were they discovered? You Are Made Of Cells A cell is the basic unit of structure and function in a living thing. Your body is composed of trillions of cells. You have skin cells, muscle cells, nerve cells, blood cells, and many other types as well. Each type of cell has a unique structure and function, but they all share similarities. Each cell in your body shares the characteristics of all living things. Each cell can respond, grow, reproduce, and use energy. Like larger organisms, cells respond to changes in their surroundings in ways that keep them alive. Finding Out About Cells How did we learn about cells? It all started with the invention of the microscope in the late 1500s. English scientist Rober Hooke was the first to record his observations of cells. In 1663, he took a think slice of cork and placed it under a microsope that he built. Cork is made from the bark of the cork oak tree, but its cells are no longer alive. Hooke made detailed sketches of his observations. An artist s version of one of his sketches is shown in Figure 7.2. Hooke called each of the square structures a cell because they reminded him of tiny rooms. Anton van Leeuwenhoek was nota scientist. He was a Dutch craftsmn who made lenses. Yet with skill and curiosity, van Leeuwenhoek made some fo the most important discoveries in biology. He used his lenses to builde a simple microscope. With his microscope, he looked at pond water, blood, and scrapings

2 from his teeth. He was the first to observe single-celled protists, blood cells, and bacteria. As microscopes improved, scientists made more discoveries. In 1839, two German scientists, Mtthais Schleiden and Theodore Schewann, viewed plant and animal tissues under a microscope. They concluded that all plant and animals were made up of cells. Cell Image Cel ls usu ally do not glow. Scientists use fluorescent proteins to make cells glow. The cells absorb these proteins like stains. The fluorescence microscope uses filters that only let in light that matches the fluorescing material being studied. All other types of light are blocked out. The fluorescing areas shine out against a dark background, making certain cell structures glow. The mouse egg cell in Figure 7.3 have been treate to show DNA as a glowing blue. The Cell Theory Schleiden and Scchwann s theory was widely accepted by other scientists. But where did cells come from? In the 1800s, it was believed that living things came from nonliving objects. Did cells come from some tiny, noniving object/ In 1855, a German physician named Rudolf Virchow proposed that cells can only come from other cells. Rudolf Virchow The work of Hooke, van Leeuwenhoek, Schleiden, Schwann, Virchow, and others led to an improtant theory in life science. The cell theory explains that all living things are made of one or more cells, that arise from existing cells.

3 Similarities Among Cells Organisms can be an individual animal, plant, or single-celled life form. Some organisms are made of only a single cell, or unicellular. Unicellular is a single-celled organism that consists of only one cell. You are made of billions of cells, or multicellular. Multicelluar is an oganism that is made up of many cells. In multicellular organisms like you, there are many different types of specialized cells. Specialized Cell a recells that differ in structure (size, shape...) and function (the role they perform in the organism). For example, the cells that line the retina of your eye have a structure and function that is very different from your skin cells. About 200 different types of specialized cells make up the tissues and organs in your body, Even though there are many different types of cells, they all share similar characteristic. These include: 1. All cells are surrounded by a cell membrane. The cell membrane is a barrier between the inside of the cell and its environment. It also controls the movement of materials into and out of the cell. 2. All cells contain organelles. An organelle is a structure inside a cell that helps the cell perform its functions. Although all cells contain organelles, they don t all contain the same kinds. 3. All cells contian cytoplasm. The cytoplasm is a fluid mixture that contains the organelles. It also contains the compounds cells need to survive such as water, salts, enzymes, and other carbon compounds. 4. All cells contain DNA. The cell theory states that all cells come from other cells. When cells reprodue, they make copies of their DNA and pass it on to the new cells. DNA contains the instructions for making new cells and controls all cell functions.

4 Classifying Cells Based on the organization of their structures, all living cells can be be classified into two groups: prokaryotic and eukaryotic. Animals, plants, fungi, and protozoans all have eukaryotic cells. Only bacteria have prokaryotic cells.

5 Prokaryotic Cells Prokaryotes are any single cellular organism that does not have a nucleus. Prokaryotes are the smallest and simplest cells. A prokaryote is a single-celled organism that lacks a nucleus and other internal compartments. Because prokaryotes lack many specialized internal compartments, they cannot carry out many specialized functions (hence why they are simpler), and because they lack these structures, they are much smaller than eukaryotes (their size is usually about µm). Prokaryotes are the most primitive of cells (meaning they are the oldest and simplest), and they lived at least 3.5 billion years ago. For nearly 2 billion years, prokaryotes were the only organisms on Earth. The most familiar example of prokaryotes is bacteria. In prokaryotic cells, the cytoplasm is everything inside the cell membrane. Prokaryotes have a cell wall surrounding their cell membrane. The cell wall is very important because it gives prokaryotic cells their shape. The cell wall is typically made up of polysaccharides connected by short chains of amino acids. Prokaryotes also have a cytoskeleton, but it is very simple and aides in cell movement. Many (not all) prokaryotes also have a flagella, which are long, threadlike structures that protrude from the cell s surface to enable the cell to move at faster speeds. The cell wall is often times covered by a capsule, which is also made out of polysaccharides. The capsule is very sticky, and allows the prokaryote to stick to teeth, skin, food, intestines, etc. Prokaryotes may also have a pilus, which are sticky projections. The DNA shape of prokaryotes is different than eukaryotes because it consists of a single, circular molecule of DNA. The DNA is also free and loose within the cell because it is not housed in a nucleus. Eukaryotic Cells Eukaryotes are any multi cellular organism whose cells contain a nucleus and other organelles. Eukaryotic cells were the first cells to appear on earth that had specialized internal compartments. Eukaryotic cells evolved about 2.5 billion years ago, and eukaryotic cells are defined by having a nucleus. The specialized internal compartments that are found in eukaryotic cells are known as organelles meaning little organs. There are many different organelles in eukaryotic cells, and they are defined as a structure that carries out specific activities in the cell. Examples of organelles are mitochondria, endoplasmic reticulum,

6 and the Golgi body, to name a few. In Eukaryotic cells, the cytoplasm is defined as everything inside the cell membrane and outside of the nucleus. The cytosol is the fluid that is contained in the cytoplasm. The cytoskeleton of eukaryotic cells is very complex and supports the cell s shape and movement. Eukaryotic cells are very large in comparison to other types of cells (about 10 µm). Eukaryotic cells are also very complex compared to other cells because they contain many specialized organelles that each has a specific function. Though all cells have DNA, eukaryotic cells are the only cell type that has an organelle known as the nucleus (as mentioned above). The nucleus houses and protects the DNA. Prokaryotic Cells Eukaryotic Cells Bacteria No nucleus Organelles not membrane-covered All other cells Nucleus Membrane-covered organelles DNA is bunched up in the center of the cell DNA is found in the nuncleus