Chapter 7: Cell Structure and Function The Discovery of the Cell - microscopes invented in 1600 s - Robert Hooke observed cork in 1665 and described them as little boxes he called cells - Hooke did not know cells were basic units of living things - Anton van Leeuwenhoek looked at a drop of pond water and observed tiny creatures (single celled protests) Cell Theory - 1838 a German botanist named Matthias Schleiden concluded that all plants were made of cells - 1839 German biologist Theodor Schwann stated that all animals were made of cells - 1855, the German physician Rudolf Virchow concluded that new cells could be produced only from the division of existing cells. - Observations of these scientist lead to the cell theory The cell theory states: 1. All living things are composed of one or more cells. 2. Cells are the basic units of structure and function in living things. 3. New cells are produced from existing cells. Common Characteristics of Cells 1. Cell Membrane a. Semi-permeable membrane that controls the flow of materials in and out of the cell (food, water, waste) b. Separates cell from external environment 1
c. Composed of lipids and proteins d. Helps maintain homeostasis e. Protects and supports 2. Nucleus a. Control center of cell b. Contains DNA c. Regulates production and structure of proteins within cells d. Surrounded by nuclear membrane that allows materials to pass in and out of nucleus 3. Cytoplasm a. Everything in cell except the genetic material b. Fluid portion of the cytoplasm is called cytosol Prokaryotes and Eukaryotes 1. Prokaryotes - pro means before - these are small (1-2um) and simple cells - do not have internal compartments so they can not carry out many specialized functions - Some contain internal membranes, but are generally less complicated than eukaryotes - genetic material is single, circular not enclosed in a membrane-bound compartment - for about 2 billion yrs they were the only organisms that existed on earth 2. Eukaryotes - eu means true; karyote means kernel - evolved about 1.5 billion yrs ago probably from prokaryotes - contain a nucleus in which their genetic material is separated from the rest of the cell. - contain dozens of membrane bound internal compartments called organelles to carry out specific functions - display great variety. single-celled organisms to multicellular organisms (Plants, animals, fungi, and protists are eukaryotes) Eukaryotic Cell Structure Nucleus Nucleus - The nucleus contains nearly all the cell's DNA and with it the coded instructions for making proteins and other important molecules nuclear envelope - composed of two membranes that surrounds the nucleus. The nuclear envelope has thousands of nuclear pores, which allow material to move into and out of the nucleus chromatin - consists of DNA bound to protein. Most of the time, chromatin is spread throughout the nucleus. When a cell divides,chromatin condenses to form chromosomes chromosomes distinct, threadlike structures contain the genetic information nucleolus - dense region of nuclei where the assembly of ribosomes begins Ribosomes: - small particles of RNA and protein found throughout the cytoplasm - Protein Factories - Proteins are assembled on ribosomes - Ribosomes are the structures in which proteins are made. - Ribosomes are among the smallest of organelles. 2
Endoplasmic Reticulum (ER ) - The endoplasmic reticulum (abbreviated ER) is the site where lipid components of the cell membrane are assembled, along with proteins and other materials that are exported from the cell it is an extensive system of internal membranes that transports materials through the inside of the cell. - The ER creates a series of channels between the membranes that isolates some spaces as membraneenclosed sacs called vesicles - There are two types of ER: smooth ER and rough ER. - The rough ER has ribosomes on its surface which are involved in protein synthesis. - Proteins released or exported from a cell are synthesized on the rough ER, as are many membrane proteins. - Rough ER is abundant in cells that produce large amounts of protein for export such as digestive enzymes or hormones are exported by the cell - Smooth ER does not have ribosomes on its surface - Smooth ER specializes in making lipids for brain and intestinal cells, synthesis of membrane lipids and the detoxification of drugs - The manufacture of some proteins or lipids on the ER are used to replace damaged or worn parts of the plasma membrane. Golgi apparatus - The function of the Golgi apparatus is to modify, collects, sort, and distributes package proteins and other materials from the endoplasmic reticulum for storage in the cell or secretion outside the cell. - Protiens and lipids destined for export from the cell are enclosed in vesicles and the golgi apparatus contains enzymes that serve as molecular addresses labels that determine where they will go in the cell or to exported out of the cell Lysosomes - Lysosomes are small membrane-bound structures, found in animal cells, that contain many different enzymes necessary for digesting certain materials in the cell - Lysosomes perform the vital function of digesting and recycling worn out material such as proteins, nucleic acids, lipids and carbohydrates from the cell. - Lysosomes are also involved in breaking down organelles that have outlived their usefulness. - Lysosomes perform the vital function of removing junk that might otherwise accumulate and clutter up the cell. - A number of serious human diseases, including Tay-Sachs disease, can be traced to lysosomes that fail to function properly. - The enzymes must be kept in vesicles - Lysosomes are formed by the Golgi apparatus. Vacuoles - vacuoles are saclike structures that store materials such as water, salts, proteins, and carbohydrates - In plant cells the central vacuole stores waste products in a large internal space - large amounts of water and nutrients are stored in vacuoles 3
- the water helps keep the plant rigid and stand upright; when the vacuoles are empty of water the plant becomes limp and will wilt - the central vacuole in plant cells makes it possible for plants to support heavy structures such as leaves and flowers. - Vacuoles are also found in some single-celled organisms and in some animals - paramecium contains a vacuole called a contractile vacuole specialized for pumping excess water out of the cell. Mitochondria and Chloroplasts - All living things require a reliable source of energy from the sun or food sources. - Mitochondria and chloroplasts are key organelles that change energy from one form to another. - Mitochondria change the chemical energy stored in food into compounds that are more convenient for the cell to use - Mitochondria are enclosed by two membranes an outer membrane and an inner membrane. - The inner membrane is folded up inside the organelle - In humans, all or nearly all of our mitochondria come from the cytoplasm of the ovum, or egg cell - Chloroplasts are organelles that capture the energy from sunlight and convert it into chemical energy in a process called photosynthesis. - Chloroplasts are found only in plant cells and algae - chloroplasts are surrounded by two membranes - Inside the chloroplast are large stacks of other membranes, which contain the green pigment chlorophyll Organelle DNA chloroplasts and mitochondria contain their own genetic information mitochondria and chloroplasts may be the descendants of ancient prokaryotes. endosymbiotic theory - prokaryotic ancestors of these organelles evolved a symbiotic relationship with early eukaryotes, taking up residence within the eukaryotic cell. 1. prokaryotes with ability to use oxygen to generate ATP evolved into mitochondria 2. prokaryotes that carried out photosynthesis evolved into chloroplasts. Cytoskeleton: Framework - The cytoskeleton is composed of a network several kinds of protein filaments and fibers that support cell structure and drive cell movement. - microtubules and microfilaments are two of the principal protein filaments that make up the cytoskeleton. 1. Microtubules - hollow tubules made of proteins which provide support for cell shape, and help move chromosomes during cell division - play a special role in cell division by forming centrioles (centrioles are found in animal cells, but not in plant cells) - help to build projections from the cell surface, which are known as cilia and flagella that enable cells to swim rapidly through liquids. - also help substances move along the cell's surface 2. Microfilaments 4
- threadlike structures made of a protein called actin - form extensive networks in some cells and produce a tough, flexible framework that supports the cell. - They permit cytoplasm movement, called cytoplasmic streaming. Cell Boundaries cell membrane - All cells are surrounded by a thin, flexible barrier - regulates what enters and leaves the cell and also provides protection and support - lipid bilayer - a double-layered of lipids (most contain protein molecules that are embedded in it with carbohydrate molecules are attached to many of these proteins) cell wall - The main function of the cell wall is to provide support and protection for the cell. - present in plants, algae, fungi, and many prokaryotes. - Cell walls lie outside the cell membrane. - Plant cell walls are composed mostly of cellulose, a tough carbohydrate fiber - Most cell walls are porous enough to allow water, oxygen, carbon dioxide, and certain other substances to pass through easily 5
Diffusion Through Cell Boundaries - One of the most important functions of the cell membrane is to regulate the movement of dissolved molecules from the liquid on one side of the membrane to the liquid on the other side - Diffusion - particles tend to move from an area where they are more concentrated to an area where they are less concentrated, - equilibrium - When the concentration of the particles is the same throughout a system - concentration - the mass of solute in a given volume of solution, or mass/volume - Passive Transport - Because diffusion depends upon random particle movements, substances diffuse across membranes without requiring the cell to use energy - some molecules are too large or too strongly charged to cross the lipid bilayer and are impermeable - If a substance is able to diffuse across a membrane, the membrane is said to be permeable - Most biological membranes are selectively permeable, meaning that some substances can pass across them and others cannot. Osmosis - is the diffusion of water through a selectively permeable membrane isotonic - same strength or concentration of solutes in cells is equal to concentration outside the cell Hypotonic solution when solution has a lower concentration of solutes than in the cell Hypotonic Solution H20 ------------> Hypertonic Cell Into cell (cell expands) Hypertonic Solution when a higher concentration of solutes in the solvent relative to cell H20 Hypertonic Solution <----------- Hypotonic Cell Out of cell (cell srinks) Osmotic Pressure - For organisms to survive, they must have a way to balance the intake and loss of water. - cells in large organisms are bathed in fluids, such as blood, that are isotonic and are not in danger of expanding - for plant cells and bacteria, which do come into contact with fresh water, the cell walls prevent the cells from expanding, even under tremendous osmotic pressure. Facilitated Diffusion - molecules, such as glucose, that cannot diffuse across the cell membrane's lipid bilayer on their own move through protein channels instead down the concentration gradient - ability to pass through a protein channel depends on the size, polarity and shape of the molecule - cell membrane protein is said to facilitate, or help, the diffusion of glucose across the membrane - Hundreds of different protein channels have been found that allow particular substances to cross different membranes - it is a passive process and the cell expends no energy for the movement 6
Active Processes Active Transport using energy to transport particles through a membrane against a concentration gradient - Many amino acids and sugars are in low concentration outside the cell Proton Pumps - move or pump H+ ions through the plasma membrane of mitochondria and chloroplast Inside a cell, proton pump channels actively transport protons through the internal membranes of chloroplasts and mitochondria. In a chloroplast, protons that build up on one side of the membrane diffuse back to the other side through. protein channels by facilitated diffusion. These channels use the force of the proton pushing through to power the manufacture of ATP. Sodium Potassium Pumps use ATP to transport Na+ ions out through a cell s membrane - the Na-K pump is the most important energy-using process - more than 1/3 of all energy by human cells is used to transport Na+ - nerve cells use the difference in Na-K concentrations to send signals throughout the body - Na-K pumps also help transport food particles into cells 7
For every 3 Na+ ions pumped out of the cell, the energy from one molecule of ATP is used and two K+ ions are pumped in. As a result, most of your cells have a high concentration of Na+ ions outside their membrane. As the Na+ ions diffuse back into the cell it brings along a with it a sugar molecule. Endocytosis - some food molecules are too large to pass through protein channels - therefore the cell engulfs the particles by a process called endocytosis by means of infoldings, or pockets, of the cell membrane. - The pocket that results breaks loose from the outer portion of the cell membrane and forms a vacuole within the cytoplasm. - Large molecules, clumps of food, and even whole cells can be taken up in this way. - if the engulfed material is liquid with dissolved molecules, this type of endocytosis is called pinocytosis Phagocytosis is when another cell or fragment of organic matter is engulfed. An example is when white blood cells protect you from infection by phagocytosis. Also, unicellular eukaryotes commonly devour their prey by phagocytosis. 8
Exocytosis - reverse of endocytosis where excretions or waste products are dumped outside of the cell - this happens when waste vacuoles fuse with the cell membrane and dump the contents outside the cell - exocytosis also happens when certain cells release certain hormones into the blood stream The Diversity of Cellular Life Unicellular Organisms - A single-celled organism - Unicellular organisms do everything that you would expect a living thing to do. - 1. They grow - 2. respond to the environment - 3. transform energy - 4. reproduce. Multicellular Organisms - Organisms that are made up of many cells - cell specialization - Cells develop in different ways to perform different tasks. Levels of Organization - The levels of organization in a multicellular organism are individual cells, tissues, organs, and organ systems - In multicellular organisms, cells are the first level of organization. - tissue is a group of similar cells that perform a particular function. - Most animals have four main types of tissue: muscle, epithelial, nervous, and connective tissue - Organs - many groups of tissues working together For example, each muscle in your body is an individual organ. Within a muscle, there are nerve tissues and connective tissues working together - Organ Systems - A group of organs that work together to perform a specific function 9