7.2 Cell Structure

Cellular Boundaries *What is the function of the cell membrane?

1) A working factory needs walls and a roof to protect it from the environment outside, and also to serve as a barrier the keeps its products safe and secure until they are ready to be shipped out. All cells are surrounded by a barrier known as the cell membrane. Many cells, including most prokaryotes, also produce a strong supporting layer around the membrane known as a cell wall. Cell Walls 2) Many organisms have cell walls in addition to cell membranes. The main function of the cell wall is to support, shape, and protect the cell. - Most prokaryotes and many eukaryotes have cell walls. - Animal cells DO NOT have cell walls. Cell walls lie outside the cell membrane. Most cells walls are porous enough to allow water, oxygen, carbon dioxide, and certain other substances to pass through easily. 3) Cell walls provide much of the strength needed for plants to stand against the force of gravity. In trees and other large plants, nearly all of the tissue we call wood is made up of cell walls. The cellulose fiber used for paper as well as the lumber used for the building comes from these walls. Cell membranes 4) All cells contain cell membranes, which almost always are made up of a double-layered sheet called a lipid bilayer. 5) The lipid bilayer gives cell membranes a flexible structure that forms a strong barrier between the cell and its surroundings. 6) *The cell membrane regulates what enters and leaves the cell and also protects and supports the cell. >The Properties of Lipids 7) The layered structure of cell membranes reflects the chemical properties of the lipids that make them up. 8) You may recall the many lipids have fatty acid chains attached to chemical groups that interact strongly with water. In the language of a chemist, the fatty acid portions of this kind of lipid are hydrophobic, or "water-hating" while the opposite end of the molecule is hydrophilic, or "water-loving." 9) When these lipids, including the phospholipids that are common in animal cell membranes, are mixed with water, their hydrophobic fatty acid "tails" cluster together while their hydrophilic "heads" are attracted to water. A lipid bilayer is the result. 10) The head groups of lipids in a bilayer are exposed to the outside of the cell, while the fatty acid tails form an oily layer inside the membrane that keeps water out. >The Fluids Mosaic Model 11) Embedded in the lipid bilayer of most cell membranes are protein molecules. Carbohydrate molecules are attached to many of these proteins. Because the proteins embedded in the lipids bilayer can move around and "float" among the lipids, and because so many different kinds of molecules make up the cell membrane, scientists describe the cell membrane as a "fluid mosaic". 12) A mosaic is a kind of art that involves bits and pieces of different colors or materials. What are all these different molecules doing? As you will see, some of the proteins form channels and pumps that help to move material across the cell membrane. Many of the carbohydrate molecules act like chemical identification cards, allowing individual cells to identify one another. Some proteins attach directly to the cytoskeleton, enabling cells to respond to their environment by using their membranes to help move or change shape. 13) Some things are allowed to enter and leave a factory, and some are not. The same is true for living cells. Although many substances can cross biological membranes, some are too large or too strongly charged to cross the lipid bilayer. If a substance is able to cross a membrane, the membrane is said to be permeable to it. 14) Most biological membranes are selectively permeable, meaning that some substances can pass across them and others cannot. Selectively permeable membranes are also called semipermeable membranes.

Organelles that Capture and Release Energy *What are the functions of chloroplasts and mitochondria?

1) All living things require a source of energy. Factories are hooked up to the local power company, but how do cells get energy? Most cells are powered by food molecules that are built using energy from the sun. Chloroplasts 2) Plants and some other organism contain chloroplasts. 3) *Chloroplasts capture the energy from sunlight and convert it into food that contains chemical energy in a process called photosynthesis. 4) Two membranes surround chloroplasts. Inside the organelle are large stacks of other membranes, which contain the green pigment chlorophyll. Mitochondria 5) Nearly all eukaryotic cells, including plants, contain mitochondria (singular: mitochondrion). 6) Mitochondria are the power plants of the cell. 7) *Mitochondria convert the chemical energy store in food into compounds that are more convenient for the cell to use. 8) Like chloroplasts, two membranes-- an outer membrane and an inner membrane-- enclose mitochondria. The inner membrane is folded up inside the organelle. 9) One of the most interesting aspects of mitochondria is the way in which they are inherited. In humans, all or nearly all of our mitochondria come from the cytoplasm of the ovum, or egg cell. This means that when your relatives are discussing which side of the family you should take credit for your best characteristics, you can tell them you got your mitochondria from Mom! 10) Another interesting point: Chloroplasts and mitochondria contain their own genetic information in the form of small DNA molecules. This observation has led to the idea that they may be descended from independent microorganisms. This idea is called the endosymbiotic theory.

Cell Organization *What is the role of the cell nucleus?

1) It's easy to divide a eukaryotic cell into two major parts: the nucleus and the cytoplasm. - the cytoplasm is the portion of the cell outside the nucleus. - the nucleus and cytoplasm work together in the business of life. - Prokaryotic cells have cytoplasm too, even though they do not have a nucleus 2) In our discussion of cell structure, we consider each major component of plant and animal eukaryotic cells-- some of which are also found in prokaryotic cells-- one by one. Because many of these structures act like specialized organs, they are known as organelles (literally "little organs") - Understanding what each organelle does helps us understand the cell as a whole. Comparing the Cell to a Factory 3) In some respects, the eukaryotic cell is much like a living version of a factory. The different organelles of the cell can be compared to the specialized machines and assembly lines of a factory. Cells, like factories, follow instructions and produce products. The Nucleus 4) In the same way the main office controls a large factory, the nucleus is the control center of the cell. 5) *The nucleus contains nearly all the cell's DNA and, with it, the coded instructions for making proteins and other important molecules. 6) Prokaryotic cells lack a nucleus, but they do have DNA that contains the same kind of instructions. 7) The nucleus is surrounded by a nuclear envelope composed of two membranes. 8) The nuclear envelope is dotted with thousands of nuclear pores, which allow material to move into and out of the nucleus. 9) Like messages, instructions, and blueprints moving in and out of a factory's main office, a steady stream of proteins, RNA, and other molecules move through the nuclear pores to and from the rest of the cell. 10) Chromosomes, which carry the cell's genetic information, are also found in the nucleus. Most of the time, the threadlike chromosomes are spread throughout the nucleus in the form of chromatin-- a complex of DNA bound to proteins. - When a cell divides, its chromosomes condense and can be scene under a microscope. 11) Most nuclei also contain a small dense region known as the nucleolus. The nucleolus is where the assembly of ribosomes begins.

Organelles That Build Protein *What organelles help make and transport proteins

1) Life is a dynamic process, and living things are always working, building new molecules all the time, especially proteins, which catalyze chemical reactions and make up important structures in the cell. Because proteins carry out so many of the essential functions of living things, a big part of the cell is devoted to their production and distribution. Proteins are synthesized on ribosomes, sometimes in association with the rough endoplasmic reticulum in eukaryotes. Ribosomes 2) One of the most important jobs carried out in the cellular "factory" is making proteins. 3) *Proteins are assembled on ribosomes. 4) Ribosomes are small particles of RNA and protein found throughout the cytoplasm in all cells. 5) Ribosomes produce proteins by following coded instructions that come from DNA. 6) Each ribosome, in its own way is like a small machine in a factory, turning out proteins on orders that come from its DNA "boss". Cells that are especially active in protein synthesis often contain large numbers of ribosomes. Endoplasmic Reticulum 7) Eukaryotic cells contain an internal membrane system known as the endoplasmic reticulum, or ER. 8) The ER is where lipid components of the cell membrane are assembled, along with proteins and other materials that are exported from the cell. 9) The portion of the ER involved in the synthesis of proteins is called rough endoplasmic reticulum, or rough ER. It is given this name because of the ribosomes found on its surface. Newly made proteins leave these ribosomes and are inserted into the rough ER, where they may be chemically modified. 10) *Proteins made on the rough ER include those that will be released, or secreted, from the cell as well as many membrane proteins and proteins destined for lysosomes and other specialized locations within the cell. 11) Rough ER is abundant in cells that produce large amounts of protein for export. Other cellular proteins are made on "free" ribosomes, which are not attached to membranes. 12) The other portion of the ER is known as smooth endoplasmic reticulum (smooth ER) because ribosomes are not found on its surface. In many cells, the smooth ER contains collections of enzymes that perform specialized tasks, including the synthesis of membrane lipids and the detoxification of drugs. Liver cells, which play a key role in detoxifying drugs, often contain large amounts of smooth ER. Golgi Apparatus 13) In eukaryotic cells, proteins produced in the rough ER move next into an organelle called the Golgi apparatus, which appears as a stack of flattened membranes. 14) As proteins leave the rough ER, molecular "address tags" get them to the right destinations. As these tags are "read" by the cell, the proteins are bundled into tiny vesicles that bud from the ER and carry them to the Golgi apparatus. 15) *The Golgi apparatus modifies, sorts, and packages proteins and other materials from the endoplasmic reticulum for storage in the cell or release outside the cell. 16) The Golgi apparatus modifies is somewhat like a customization shop, where the finishing touches are put on proteins before they are ready to leave the "factory." From the Golgi apparatus, proteins are "shipped" to their final destination inside or outside the cell.

Organelles That Store, Clean Up, and Support *What are the functions of vacuoles, lysosomes, and the cytoskeleton?

1) Many of the organelles outside the nucleus of a eukaryotic cell have specific functions, or roles. Among them are structures called vacuoles, lysosomes, and cytoskeleton. These organelles represent the cellular factory's storage space, cleanup crew, and support structures. Vacuoles and Vesicles 2) Every factory needs a place to store things, and so every cell does. Many cells contain large, saclike, membrane-enclosed structures called vacuoles. 3) *Vacuoles store materials like water, salts, proteins, and carbohydrates. 4) In many plant cells, there is a single, large central vacuole filled with liquid. The pressure of the central vacuole in these cells increases their rigidity, making it possible for plants to support heavy structures, such as leaves and flowers. 5) Vacuoles are also found in some unicellular organisms and in some animals. The paramecium contains an organelle called a contractile vacuole. By contracting rhythmically, this specialized vacuole pumps excess water out of the cell. 6) In addition, nearly all eukaryotic cells contain smaller membrane-enclosed structures called vesicles. Vesicles store and move materials between cell organelles, as well as to and from the cell surface. Lysosomes 7) Even the neatest, cleanest factory needs a clean up crew. 8) Lysosomes are small organelles filled with enzymes. 9)*Lysosomes break down lipids, carbohydrates, and proteins into small molecules that can be used by the rest of the cell. They are also involved in breaking down organelles that have outlived their usefulness. 10) Lysosomes perform the vital function of removing "junk" that might otherwise accumulate and clutter up the cell. A number of serious human diseases can be traced to lysosomes that fail to function properly. 11) Biologists once thought that lysosomes were only found in animal cells, but it is now clear that lysosomes are also found in a few specialized types of plant cells as well. The Cytoskeleton 12) Factory building is supported by steel or cement beams and by columns that hold up its walls and roof. 13) Eukaryotic cells are given their shapes and internal organization by a network of protein filaments known as the cytoskeleton. 14) Certain parts of the cytoskeleton also help transport materials between different parts of the cell, much like the conveyor belts that carry materials form one part of a factory to another. 15) Cytoskeletal components may also be involved in moving the entire cells in cell flagella and cilia. 16) *The cytoskeleton helps the cell maintain its shape and is also involved in movement. 17) Fluorescence imaging clearly shows the complexity of a cell's cytoskeletal network. Microfilaments (pale purple) and microtubules (yellow) are two of the principal protein filaments that make up the cytoskeleton. >Microfilaments 18) Microfilaments are threadlike structures made up of a protein called actin. They form extensive networks in some cells and produce a tough, flexible framework that supports the cell. 19) Microfilaments also help cells move. Microfilament assembly and disassembly are responsible for the cytoplasmic movements that allow amoebas and other cells to crawl along surfaces. >Microtubules 20) Microtubules are hollow structures made up of proteins known as tubulins. In many cells, they play critical roles in maintaining cell shape. 21) Microtubules are also important in cell division, where they form a structure known as the mitotic spindle, which helps to separate chromosomes. 22) In animal cells, organelles call centrioles are also formed from tubules. - Centrioles are located near the nucleus and help organize cell division. Centrioles are NOT found in plants cells. 23) Microtubules also help build projections from the cell surface -- known as the cilia (singular: cilium) and flagella (singular: flagellum)-- that enable cells to swim rapidly through liquid. The microtubules in cilia and flagella are arranged in a "9 + 2" pattern. 24) Small cross-bridges between the microtubules in these organelles use chemical energy to pull on, or slide along, the microtubules, producing controlled movements.