Chapter 7 Cell Structure and Function The cell basic unit of life, all living things are made of a cell (unicellular) or more than one cell (multicellular). LIFE IS CELLULAR The invention of the microscope in the 1600 s made possible for cells to be discovered. Robert Hooke examined cork saw many boxes. Anton van Leeuwenhoek - discovered that pond water contained many tiny living organisms. Cell Theory 1. All living things are composed of one or more cells. 2. In organisms, cells are the basic units of structure and function. 3. New cells are only produced from other existing cells. Cells Are Small A meter is not useful for measuring cells. Scientists uses a unit called a micrometer (micron) to measure cells. 1 micrometer=1/1000 of a millimeter. There are 1000 microns in a mm. All cells have a cell membrane, genetic material, and cytoplasm The cell membrane (aka plasma membrane) is a flexible, thin barrier that: Separates the cell from its surroundings. Regulates what moves in and out of the cell. Genetic material provides instructions that control the cell s activities. The cytoplasm is the material inside the cell that supports the organelles. What is a prokaryote? Prokaryotic Cell Single celled organisms that lack a nucleus and other internal membrane-bound organelles. Genetic material (DNA or RNA) is found free floating in a cell, not in a compartment or nucleus. All bacteria (ex. E. Coli, Streptococcus) are prokaryotic. Prokaryotes are unicellular. 1
Eukaryotic Cell What is a eukaryote? (YOU are a eukaryote!) Have membrane bound organelles (specialized organ-like structures). Genetic material is found in a nucleus. The Kingdoms Animal, Plant, Fungi, and Protista are Eukaryotes. Eukaryotes can be unicellular or multicellular. What is the nucleus? Nucleus Often found in the center of the cell. Largest and most easily seen organelle. Surface is bound by a double lipid membrane called the nuclear envelope. Is a double membrane system. Contains nuclear pores - allow for protein and other molecules to pass through. Genetic material, or DNA, is found here (in the form of chromosomes) in eukaryotic organisms only! The nucleolus is Nucleolus Structure found inside the nucleus. Responsible for making ribosomes which will function in protein synthesis. Remember that proteins regulate cellular processes and make you you! Cellular boundaries What is the cell wall? Found in many organisms like plants, fungi, algae, and nearly all prokaryotes. Not found in animal cells. Function is to support, provide strength, and protect the cell. Composed of the polysaccharide cellulose. Are porous and allow water, oxygen, CO2, and other substances to pass through. Cellular boundaries All cells contain a cell membrane Composed of a phospholipid bilayer Regulates what enters and leaves the cell Protects and supports the cell 2
Organelles that build proteins What is a ribosome? Protein manufacturer. Makes proteins to be used within the cell or be exported out of the cell to be used in other places. Are composed of RNA and protein. Produced by the nucleolus within the nucleus. Organelles that build proteins The endoplasmic reticulum is Found in eukaryotic cells. Structure is an extensive system of internal membranes. Location where lipid components of the cell membrane are assembled, proteins are exported. Organelles that build proteins Rough ER Ribosomes found on the surface. Functions to make proteins and transport proteins that will be released/secreted from the cell. Also functions to make proteins destined for lysosomes or other locations in the cell. Smooth ER Does NOT have ribosomes. Contains enzymes for making lipids, detoxification of drugs. Organelles that build proteins What is the Gogi apparatus: Receives vesicles (transport vacuoles) from the RER that contains proteins. Structure in eukaryotic cells that is a stack of flattened membranes. Modifies molecules that it receives, packages, and distributes molecules into vesicles. Marks the molecules inside the vesicle to determine what their final destination will be. Destination is often storage in a vacuole or release to the outside of the cell. Organelles that store, cleanup, and support What is a lysosome? Organelle filled with digestive enzymes. Digests and recycles the used components of a cell for reuse. Destroy bacterial cells. Found in animal cells and specialized plant cells. Organelles that store, clean-up, and support What is the role of the vacuole? Store waste products, large amounts of water and nutrients. Help plant cells hold their shape. Plant cells have large vacuoles; animal cells have very small vacuoles. Vesicles are transport vacuoles found in eukaryotic cells. Move materials between organelles and to and from the cell surface. 3
Organelles that store, clean-up, and support What is the cytoskeleton? Mesh-like network that helps the cell keep its shape. Supports the organelles of the cell. Microtubules hollow protein tubes that form the tracks for the organelles to move on. Centrioles - special microtubules found in animal cells involved in cell division. Cilia - short thread-like structures important for movement of unicellular organisms. Flagella - whip-like microtubules that help unicellular organisms move. Microfilaments support the cell and function in helping cytoplasm move through the cell. Organelles that capture & release energy (plastids) What are chloroplasts? Capture sunlight to make sugars photosynthesis. Ultimately are the energy source for all living things. Are surrounded by two membranes. Contain the green pigment chlorophyll. Are not found in animal cells. Contains its own DNA. Organelles that capture & release energy (plastids) What are mitochondria? Convert the chemical energy stored in food into useable compounds. ATP is the energy molecule created by the cell. Made of a double membrane (inner is folded). Also contains its own DNA (from Mom). Present in nearly all eukaryotic cells. Movement Through the Membrane All cells exist in a liquid environment. The cell membrane functions as a barrier but must let certain materials in and out of the cell. Structure of the Plasma/Cell Membrane Plasma Membranes consist of 2 types of molecules phospholipids and proteins. A phospholipid has 2 regions head and tail. Polar head region with a phosphorus group. Remember, water is polar. Non-polar 2 tail region composed of long carbon chains (fatty acids). 4
Structure of a phospholipid Diagram of a Phospholipid Polar head attracts water molecules (hydrophilic), nonpolar tails repel water molecules (hyrdrophobic). Phospholipid bilayer Due to the hydrophobic and hydrophilic nature of the phospholipid, a double layer is formed where the heads protect the tails from the water. This is called the phospholipid bilayer. The bilayer is fluid, not a hard shell. Structure of phospholipid bilayer The phospholipid bilayer is called semipermeable or selectively permeable it allows some materials through and blocks others. Polar (head) portion is the true boundary between a cell and its surroundings. Proteins are Embedded Within the Bilayer Proteins act as passageways for nonpolar (hydrophobic) molecules to pass through. Without proteins embedded in the bilayer, essential molecules needed for communication, energy, etc., would not reach the interior of the cell. Carbohydrates are attached to the proteins. Allow for cell recognition and communication. Proteins are Embedded Within the Bilayer (continued) The model of the lipid bilayer with embedded proteins is called the Fluid Mosaic Model. 5
TWO Types of Transport Through a Cell Membrane 1. Passive Transport the movement of molecules through a cell membrane without the expenditure of energy. 2. Active Transport the movement of molecules through a cell membrane using energy. 3 Types of Passive Transport 1. Diffusion - Movement of molecules from areas of high concentration to low concentration. 2. Osmosis - Diffusion of water molecules from higher concentration of water to lower concentration of water. 3. Facilitated Diffusion - Diffusion through pores or channel proteins. Diffusion Molecules tend to move from areas where there is more of them (high concentration) to where there is less (low concentration). This is called moving WITH the concentration gradient. Diffusion continues until equilibrium is reached, or where molecules are equally distributed. Once equilibrium is reached, particles will move at the same rate across the membrane in both directions. If a membrane separates two regions of liquid the solutes within the liquid will move from high to low. Some solutes will be able to move through when others will not. Diffusion (continued) http://programs.northlandcollege.edu/biology/biology1111/animations/transport1.html Facilitated Diffusion Molecules like small lipids and alcohol can diffuse directly through a membrane (diffusion). Facilitated diffusion occurs because the cell membrane is semi-permeable and some molecules cannot diffuse through on their own, they will need assistance (need facilitated). Facilitated Diffusion Membranes have protein channels that are designed to fit specific solutes. When a difference in concentration occurs, solute molecules will diffuse through these protein channels. Hundreds of proteins exist to help particles cross membranes. Solute must have a specific size, shape, or polarity in order to fit through the protein channel. Is fast and specific! http://programs.northlandcollege.edu/biology/biology1111/animations/transport1.html 6
Osmosis: an example of facilitated diffusion OSMOSIS Water passes through cell membranes rapidly. Osmosis is a specialized form of facilitated diffusion that moves water molecules through membranes via aquaporins. Aquaporins are special water channel proteins in membranes. Types of Solutions A hypertonic solution is one in which the solute concentration is HIGHER in the solution than in the cell. Water will move out of the cell. A hypotonic solution is one in which the solute concentration is LOWER in the solution than in the cell. Water will move in to the cell. An isotonic solution is one in which there are EQUAL solute concentrations in the solution and in the cell. Water will move in and out of the cell at equal rates. Osmosis: an example of facilitated diffusion Osmotic pressure the pressure that occurs when more water molecules accumulate inside a cell. Caused by water moving into or out of a cell. http://programs.northlandcollege.edu/biology/biology1111/animations/transport1.html Active processes expend energy Active transport is the use of energy to move molecules from LOW concentration to HIGH concentration (against or opposite the concentration gradient). Some molecules exist in low amounts on one side of a membrane and need to be moved to areas of high amounts. Cell membranes have mechanisms to move these molecules against their concentration gradients. Types of Active Transport Molecules can be carried through the membrane by membrane-associated protein pumps. Na, K, Ca ions are transported in this manner. Cells have an ability to engulf large particles by forming a vesicle around a particle in a process called endocytosis. If the particles have been partially broken down into a liquid of tiny dissolved molecules, it is called pinocytosis (cell drinking). If the particles are cell fragments or organic matter, it is called phagocytosis (cell eating). http://programs.northlandcollege.edu/biology/biology1111/animations/transport1.html 7
Types of Active Transport Exocytosis is the process by which wastes or secretions (hormones) are brought to the cell membrane, packaged into vesicles and sent out of the cell. It is the reverse of endocytosis. Unicellular vs. Multicellular Unicellular organisms single celled. Include both prokaryotes and eukaryotes. Maintain homeostasis by growing, responding to the environment, transforming energy, and reproducing. Multicellular organisms many cells working together to create an organism. Cells are specialized different cell types play different roles. Ex. Move, respond, communicate, substance production. Cells communicate with one another to maintain homeostasis for the organism. Levels of organization Multicellular organisms have increasing levels of organization: Organelles Cells Tissue Organs Organ systems Multi-cellular organisms The organization allows them to divide labor in order to maintain homeostasis. Cellular communication Cells in large organisms must communicate with one another in order to work together to maintain homeostasis! Occurs via chemical signals that influence the behavior of the cells receiving the signals. Receptors are often found on the outside of cells where they bind chemical signaling molecules and influence cellular activity. 8