Chapter 3, Cells PDF

CHAPTER 3* Cells *with optional homework Introduction to cells Why are cells small? The cell membrane Passive transport (diffusion) Active transport Internal structures of the cell Cellular respiration (forms ATP)  The cell theory of biology All living things composed of c...

CHAPTER 3* Cells *with optional homework Introduction to cells Why are cells small? The cell membrane Passive transport (diffusion) Active transport Internal structures of the cell Cellular respiration (forms ATP)  The cell theory of biology All living things composed of cells and cell products (fingernails = cell products) Single cells: smallest unit with the all characteristics of life All cells come from pre-existing cells (sperm and egg cells for humans)  Interesting cells in the human body Skeletal muscle cells: numerous mitochondria for making ATP, stored glycogen for ATP production. Multiple nucleii Specialized protein fibers for contraction (actin / myosin proteins) Slide 3.2  Interesting cells in the human body Neurons: generate electrical signals. Often long and thin to carry electrical signals over long distances (toes to spinal cord, for example).  Interesting cells in the human body Sperm cells in men: the only human cells with flagella: contain ½ of man’s DNA, swim to fertilize woman’s egg / oocyte.  Interesting cells in the human body Rod and cone cells in retina with photopigments that are sensitive to light. Slide 3.2  Cells are small, why? Cells must maintain their metabolic activities. Metabolic activity is proportional to cell volume. Cells must exchange materials for metabolic activity (glucose, waste, oxygen etc.) with surroundings. Exchange is proportional to cell surface area. Increased cell size à volume increases faster than surface area. At some point, exchange (surface area) is too slow to support metabolism (volume). (Cubes represent cells): if cells are too large then Supply (surface area) / Demand (volume) is too small 1 cm 2 cm Vol. = 1cm3 Vol. = 8cm3 S.A. = 6cm2 S.A. = 24cm2 S.A./Vol. = 6/1cm S.A./Vol. = 3/1cm Small intestine cells absorb nutrients. They have microvilli to increase surface area dramatically relative to cell volume. Purposes of cell membrane: 1) Separate interior of cell from external environment. 2) Selectively allow some substances in and out (control what comes in and out of the cell). 3) Transmits hormonal and nervous signals 4) Has glycoproteins that identify cell, so white blood cells do not destroy it. The cell membrane is made of: 1) Phospholipid molecules (lipid bilayer) 2) Cholesterol molecules 3) Various proteins with different functions Phospholipids form a lipid bilayer. Polar heads of molecules exposed to water, hydrophobic tails in interior. Non-polar and non-ionic substances can pass through. (Carbon dioxide, oxygen, steroid hormones). What cannot pass through? Cholesterol molecules: make cell membrane more rigid. Proteins: Stuck (embedded) in cell membrane. Different proteins with different functions. Channel proteins: always open, but selective. Allow a specific polar molecule or ion in and out (which cannot go through the lipid bilayer). Examples: water, Na+, K+ channel proteins. Gated channel proteins: not always open. Receive certain signals: changes shape to “open” position, then act like channel proteins. These are important in neurons: Na+, K+ gated channel proteins. Transport proteins: change shape to allow certain molecules or atoms across. Passive or active transport (no energy vs. energy required). Examples: glucose (passive transport), sodium/potassium pump (active) Receptor proteins: bind non-steroid hormones. Receptor changes shape, causes internal chemical changes. Non-steroid hormone does not enter cell, but “message” gets into the cell.  Receptor Proteins 1) Non-steroid hormone binds to receptor protein 2) Receptor protein changes shape inside the cell. In this new shape, the receptor protein causes a change inside the cell. Note: non-steroid hormone 2 1 does not get into cell, but its message gets through the cell membrane. Glycoproteins: carbohydrates attached to protein. These are like “flags” that identify the cell as “self”. Foreign cells are attacked (by white bloods cells of the immune system). Glycoproteins: the immune system recognizes our cells and does not attack our own cells How do molecules or ions cross the cell membrane? Passive transport (diffusion): With the chemical gradient (high to low concentration) Does not require energy H L Active transport: Against the chemical gradient (low to high concentration) Requires energy (ATP) L H Endocytosis and exocytosis Bulk transport of large amounts of proteins or large particles Requires energy (ATP)  Endocytosis and Exocytosis Move Materials in Bulk and require ATP Endocytosis moves material into cell (like bacteria into white blood cells) Exocytosis moves material out of cell (releasing hormones, mucus, neurotransmitters, as examples) Exocytosis  Diffusion: Passive Transport Passive transport: no ATP energy required Diffusion: movement from high concentration to low concentration by random atomic / molecular movement and collisions. The concentration later becomes uniform throughout the volume. Many things move in our bodies through simple diffusion: oxygen into our lungs, for example. Diffusion demonstration of scent molecules Three Forms of Passive Transport Passive transport follows the concentration gradient (High concentration to low concentration). In the cell it occurs as: Diffusion through the lipid bilayer (oxygen, carbon dioxide) Diffusion through channel proteins (water, various ions) Facilitated transport: transport proteins move molecules across the membrane, following the concentration gradient (glucose). Three Forms of Passive Transport (diffusion)  Osmosis: Diffusion of Water Osmosis: diffusion of water Here osmosis is occurring across a semi-permeable membrane, (like the cell membrane) Copyright © 2001 Benjamin Cummings, an imprint of Addison Wesley Longman, Inc. Extracellular fluids must remain isotonic with cells Isotonic (same) concentrations maintained outside and inside cells A red blood cell in H pure water will swell L H with water and burst L A red blood cell in very salty water will lose water and shrink  Active Transport Active transport: energy in ATP used to move substances from low concentration to high concentration. H L  Active Transport: The Sodium/Potassium Pump Sodium/potassium pump expels 3 sodium ions, imports 2 potassium ions, maintains cell volume The pump keeps the extracellular fluid isotonic with the cell’s interior, so water does not flow into cells, bursting them, and cells do not shrink either. ATP (active transport) is used to expel three sodium ions for every two potassium ions brought into the cell 3 2 Goal: maintain constant cell volume, by pumping ions across the cell membrane, which controls how much water is going into and out of the cell. H2O L H H2O 3 2 Cell is swelling in volume: increase the sodium/potassium pump = more ions out of cell = lower concentration of water outside of cell, so water diffuses out of the cell and the cell gets smaller. H2O H L H2O 3 2 Cell is shrinking in volume: decrease the sodium/potassium pump. Sodium ions diffuse into the cell slowly, through sodium ion channels = lower concentration of water inside the cell, so water diffuses into the cell = cell gets bigger. Na diffuses in slowly Internal Structures of an Animal Cell The nucleus Structure and Function of the Nucleus Functions: Contains the genetic information of the cell (DNA) Controls the cell (DNA has information for proteins) Features: Double-layered nuclear membrane Nuclear pores (DNA cannot get out, mRNA and small proteins can move in and out) The Nucleus The Nucleus Nuclear membrane and pores  Ribosomes: read mRNA to make amino acid sequence Ribosomes: “translate” mRNA to make amino acid sequence (making protein) Ribosomes are free or membrane bound. Free ribosomes: proteins are for use inside the cell. Membrane bound ribosomes: proteins go into the endoplasmic reticulum. Proteins in ER are often for export or for cell membrane. Slide 3.16A A membrane-bound ribosome, with the protein entering the endoplasmic reticulum The Endoplasmic Reticulum (ER) Endoplasmic Reticulum (ER) and Ribosomes  Endoplasmic Reticulum (ER) Smooth ER: no ribosomes. Lipid synthesis including phospholipids (for cell membrane) and steroid hormones Rough ER: has ribosomes. Modifies proteins, including adding carbohydrates / metals to them. The ER then packages proteins in vesicles to take them to the Golgi apparatus. Smooth Rough The Golgi Apparatus The Golgi Apparatus  The Golgi Apparatus Receives proteins in vesicles from the ER, modifies them chemically, and packages them in vesicles for secretion via exocytosis or for the cell membrane. Also constructs special vesicles: lysosomes and peroxisomes. Transport vesicles Figure 3.16 Slide 3.17 Special vesicles Special vesicles Peroxisomes: vesicles with enzymes that break down toxins in the cell (like alcohol). Other functions too. Lysosomes: vesicles with enzymes that break down bacteria, old mitochondria and other large intracellular particles. Summary of protein production DNA of a gene copied (“transcribed”) into mRNA. mRNA leaves nucleus and becomes associated with a ribosome. Ribosome reads (“translates”) mRNA message and forms the amino acid sequence. Membrane-bound ribosomes insert the protein inside the endoplasmic reticulum. Protein (amino acid sequence) in endoplasmic reticulum is chemically modified. Protein (amino acid sequence) is further modified in the Golgi. Final working protein may be exported via exocytosis, may become part of the cell membrane, or may remain inside the cell in a special vesicle.  Cell Structures: Cilia Cilia: Small “hairs” of cell for movement Cilia line the inside of oviducts, transporting eggs down to uterus Cilia line the trachea, transporting mucus with trapped dust and bacteria upwards towards the mouth. Long-term smoking destroys these! The Mitochondria Mitochondria: Provide Energy to the Cell Energy from chemical bonds in food molecules used to create ATP, thus they are the “cell’s powerhouse”. Take in: oxygen, food molecules (glucose, lipids, amino acids), ADP + P Produce: ATP and waste (CO2, water) Figure 3.18A Slide 3.19 Outside of mitochondria (cytoplasm): Glucose (6 carbon) split into 2 pyruvate (3 carbon each) This does not require oxygen, and produces 2 ATP Anaerobic cellular respiration In mitochondria: Pyruvate and oxygen interact through a series of complex chemical processes to form 34 ATP, generating 6 CO2 and 6 water molecules as waste. Aerobic cellular respiration Overall, potential energy in glucose à potential energy in ATPà kinetic energy for cell (work)  Muscle proteins 36 ADP + P 36 ATP 34 ADP + P 2ADP + P 6 CO2 Mitochondria 6 H20 2Pyruvate Glucose 6 O2 Anaerobic Pathways: ATP Without Oxygen Anaerobic metabolism: when cells are short of oxygen (when does this occur?). Lactic acid builds up, and you have an “oxygen debt”: to break it down you keep breathing hard after you stop exercising. Lactic acid in muscles produces burning sensations and sometimes muscle cramps. Fats and Proteins: Additional Energy Sources Figure 3.25 Slide 3.27 Proteins and fats: broken down, enter mitochondria as 2 or 3 carbon molecules, potential energy in chemical bonds converted to ATP. Fats produce the most ATP / gram Proteins produce ammonia as a waste product, which is toxic to cells Toxic ammonia à less toxic urea (conversion at liver) Both ammonia and urea excreted in urine (and small amounts in sweat)  Chiras DD Human Biology Health, Homeostasis, and the Environment Jones and Bartlett Publishers, Sudbury, Mass, 2002 http://www.northrup.org/photos/Animals/nl-33.htm Campbell, Reese, Mitchell, Biology,Benjamin/Cummings,Menlo Park, CA,1999 http://www.haverford.edu/biology/Courses/bio300/BioGallery00/presentation.llem/EM/EM%20Figure%208