Plant and Animal Cells

Transcription

1 Plant and Animal Cells a. Explain that cells take in nutrients in order to grow, divide and to make needed materials. S7L2a b. Relate cell structures (cell membrane, nucleus, cytoplasm, chloroplasts, and mitochondria) to basic cell functions. S7L2b 1. Cells are the smallest single unit that can maintain life. Within each cell are a collection of organelles that perform specific functions. In 1855 a scientist named Rudolph Virchow consolidated the published work of other scientists and drew accurate conclusions about cells. His hypothesis, which we now call the Cell Theory, was composed of the first three statements shown: 1. All living things are made up of cells. 2. The cell is structural & functional unit of all living things. 3. All cells come from pre-existing cells. 4. Cells contain inheritable information which is passed from cell to cell during cell division. 5. Cells are basically the same in chemical composition. 6. All energy flow (metabolism & biochemistry) of life occurs within cells. Several additional facts have been added to the cell theory since then. The Modern Cell Theory also includes the last three statements as shown. A unicellular organism such as an amoeba is capable of carrying out all the necessary functions to live, reproduce, and continue as a species. A single cell can also exist as part of a multicellular organism, having many cells, carrying out varying functions according to its cell type. Cells, like any living organism, must have a way to obtain and use energy as well as a way to remove wastes. A look at the organelles and their functions will illustrate how these functions are accomplished. Cells are classified into two Domains according to their specific characteristics: A. Prokaryotic cells are very simple cells that have few organelles. These cells do not have a nucleus, but do have DNA. This DNA contains the blueprints to control the cell s growth and reproduction. There is a cell membrane surrounded by a cell wall that encloses its cytoplasm and a few other organelles. Bacteria are prokaryotic cells. Notice that some bacteria are covered with short hair-like structures known as pili as well as a long whip like flagellum that it can use to move.

2 The diagram of atypical bacteria. An electomicrophotograph of actual bacteria B. Eukaryotic cells have the organelles found in bacteria plus many more that specialize in various tasks. These different organelles function to digest food, convert food to cellular energy, break down waste products, assist with reproduction of new cells and many other activities. All cells except bacteria are eukaryotic. Image 1 B 1 illustrates key differences between prokaryotes and eukaryotes 2. Cell organelles vary in size and have a structure that best enables them to perform their tasks. The major organelles and their functions are listed below: A. Nucleus: frequently the largest and most visible organelle; a covering, called a nuclear envelope, containing many tiny holes that allow some of its contents to leave and return as needed; the cell s DNA and RNA are on chromosomes contained within the nuclear envelope; the DNA contains a complete set of the organism s genetic information and controls all aspects of the cell s life; sometimes called the control center. The image of the nucleus shows how complex an organelle it is. The nuclear pores allow its RNA to leave the nucleus as needed. DNA inside the nucleus is referred to as chromatin and is not distinctly visible until just before cell division. A dark staining area inside the nucleus is the nucleolus. The nucleolus makes ribosomes that will leave the nucleus.

3 Notice the dark region within this nucleus that is the nucleolus. B. Cell Membrane: also known as a plasma membrane due to its shifting structure; made of 2 layers with many types and shapes of holes that allow food, waste and cell products to enter or leave the cell easily; its unique structure allows certain substances to pass through it and prevents other substances from entering or leaving as needed; it is sometimes called the gatekeeper. The membrane helps keep certain substances inside the cell in the same proportions. For example, when you eat a very salty snack, the extra salt in your blood could be soaked up by your cells. This would cause the cells to have too much salt and risk damaging the cell. The membrane s job is to prevent this from happening. You will learn more about how the membrane works in another lesson. Examine the diagram of a cell membrane below. Notice that the membrane is actually two layers of lipids with embedded proteins. Some of the proteins act as tunnels while others act like pumps to assist certain substances with entering or leaving the cell. Go to the Interactive activity that shows how the cell membrane regulates transport of molecules through the membrane. Click on the molecules shown in the top right corner to see how they pass through the membrane. Cell Membrane: Just Passing Through C. Cytoplasm: a gel-like substance that fills the cell between the cell membrane and nucleus; all organelles appear to

4 be floating in the cytoplasm; tiny protein filaments and tubes within the cytoplasm help to hold the cells shape. Watch the short video clip below for more information on the nucleus, cytoplasm and cell membrane. Nucleus, Cytoplasm and Membrane D. Mitochondria: a relatively large organelle that converts food energy into a type of chemical energy called ATP the cell can use; it is sometimes referred to as the powerhouse. ATP is to living cells what gasoline is to automobiles. The drawing in diagram 1 is a simplified view of a Diagram 2 is an actual photo of a mitochondrion mitochondrion s inner structure taken with an electron microscope. E. Ribosomes: the smallest organelles with one of the biggest responsibilities; ribosomes are produced inside the nucleus and are then sent out to the cytoplasm. They are the structures responsible for assembling units of proteins that are needed to make skin, muscles, hair, and most other body tissues; sometimes known as the manufacturer. Ribosomes appear as tiny dots on the surface of rough endoplasmic reticulum or as free ribosomes scattered throughout the cytoplasm.

5 The ribosomes (#5) are the small spheres seen on the surface of the pink folds of the endoplasmic reticulum. F. Golgi Bodies: appear to resemble a small stack of flattened sacks as shown below; it assembles the protein products made by ribosomes into the usable proteins; these finalized proteins are then packed into special sacks and are sent out to their final destination; Golgies bodies are frequently described as assembly lines or shipping and receiving facilities. Diagram 1 is a simplified view of a where Diagram B is an actual photo of a Golgi body taken a Golgi body is within a cell with an electron microscope

6 G. Lysosomes: small bags of digestive enzymes; break down some of the food that enters the cell as well as waste products and some poisons; also digests and recycles worn out organelles; also known as the cell s street sweeper or garbage disposal. These are not usually found in plant cells. Lysosomes are formed when small spheres pinch off from the Golgi Body. H. Centrioles are only in animal cells; they are used to divide cells correctly when making new cells; Centrioles form a framework from one end of the cell to the other that lines up and separates the chromosomes evenly so that each new cell gets the same number and kinds of chromosomes. They look like little soda straws. Centrioles resemble tiny soda straws H. Plant cells have several organelles that are not found in animal cells. Theses include: 1. Cell Wall: outside of the cell membrane; made of somewhat more rigid substances that give plant cells support and a more geometric structure similar to little stacks of boxes; 2. Chloroplasts: these organelles contain the green substance known as chlorophyll; absorb energy from sunlight and use that energy to combine water and carbon dioxide to make sugar 3. Central Vacuole: a large balloon-like organelle that stores water in each cell; some vacuoles store other materials such as sugars made by the cell. Animal cells may have smaller vacuoles to store food and are called food vacuoles.

7 Figure is a photograph of an Elodea plant s cells. Notice the nearly rectangular shape. The diagram shown in figure shows the key differences between plant and animal cells: cell wall, chloroplasts, and the large central vacuole are in plants but not in animal cells.