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CHAPTER 5: CELL FUNCTIONS 5.1 Objectives • Explain how an equilibrium is established as a result of diffusion • Distinguish between diffusion an osmosis • Explain how substances cross the cell membrane through facilitated diffusion • Explain how ion channels assist the diffusion of ions across the cell membrane Selectively permeable: Allows some molecules in and keeps other molecules out. Types of Cellular Transport PASSIVE TRANSPORT: Cell DOES NOT use energy 1. Diffusion Weee!! 2. Facilitated Diffusion 3. Osmosis high low ACTIVE TRANSPORT: Cell USES energy 1. Protein Pumps 2. Endocytosis high 3. Exocytosis This is gonna be hard work!! low TYPES OF PASSIVE TRANSPORT DIFFUSION: Movement of molecules from an area of high concentration to an area of low concentration Movement WITH the concentration gradient High http://www.youtube.com/watch?v=gXJMBgyT_hk Low Diffusion occurs until EQUILIBRIUM is reached ** When the concentration of molecules is the same throughout a space they occupy! Factors - Affect Rate of Diffusion Pressure diffusion will be faster Temperature Concentration diffusion will increase more diffusion Factors involved in moving things in & out of a cell: • Size of the particles • Whether they dissolve in water or not • Condition in and/or outside cell • Structure of the cell membrane Osmosis • The diffusion of water across a membrane. Water moves: HIGH LOW http://www.youtube.com/watch?v=GbudKs-49jo Isotonic Solutions (Iso = Same) • The concentration of water molecules is the same inside and outside the cell. • Water molecules will move across the membrane at the same rate in both directions = NO NET MOVEMENT! Hypertonic Solutions • High solute concentration/lower water concentration outside of the cell • Water will move OUT of the cell • Cell shrivels Hypotonic Solution • Lower concentration of solute on outside of cell when compared to the inside of cell • Water will move IN the cell • Cell will swell or possibly burst What type of solution are these cells in? A Hypertonic B Isotonic C Hypotonic Crossing the Cell Membrane • Why are most molecules and polar molecules not able to pass through the cell membrane? • Unable to pass through the nonpolar interior layer of the lipid bilayer. • How do these molecules get into the cell? • Transport proteins • These channels provide polar passageways Facilitated Diffusion • Movement of molecules across a membrane with an intact protein • No energy required! • Carrier proteins assist these molecules with facilitated diffusion Carrier Proteins • A transport protein that can bind to a specific substance on one side of the cell membrane, carry it across and release it on the other side. Steps of Facilitated Diffusion 1. A molecule binds to the carrier protein on either side of the cell 2. The carrier protein changes shape and transports the molecule across the cell membrane 3. The molecule is released on the other side of the cell http://www.youtube.com/watch?v=I4123hUU8xo&NR=1 Diffusion Through Ion Channels • ION CHANNEL: Membrane protein complexes that facilitate the diffusion of ions across cell membranes. • Some are always open, others are gated. • Still passive transport because ions are moving down their concentration gradient. Electrical Charge and Ion Transport • The inside of a cell is most often negatively charged • Positively charged ions are more likely to diffuse into the cell • Negatively charged ions are more likely to diffuse out of the cell 5.1 Objectives REVIEW • Explain how an equilibrium is established as a result of diffusion • Distinguish between diffusion an osmosis • Explain how substances cross the cell membrane through facilitated diffusion • Explain how ion channels assist the diffusion of ions across the cell membrane 5.2 Objectives • Distinguish between passive transport and active transport • Explain how the sodium-potassium pump operates • Compare endocytosis and exocytosis Active Transport • The transport of a substance across the cell membrane AGAINST THE CONCENTRATION GRADIENT. • Uses carrier proteins • This process is fueled by ATP • Referred to as “pumps” Sodium Potassium Pump • Transports 3 sodium ions (Na+) out of the cell and 2 potassium ions (K+) into the cell. • Sodium ions are more concentrated outside of the cell. • Potassium is more concentrated inside the cell. • Transported against their concentration gradient! http://www.youtube.com/watch?v=GTHWig1vOnY 4 Steps of Sodium-Potassium Pump 1. 3 sodium ions (Na+) and a phosphate group (P) from ATP bind to the pump. 2. Pump changes shape, transporting the 3 sodium ions across the cell membrane. 3. 2 potassium (K+) ions bind to the pump and are transported across the cell membrane 4. The Phosphate group and 2 potassium ions are released inside the cell. Why is it Important? • Prevents sodium ions from accumulating in the cell • Too much sodium causes water to enter cell and possibly burst. • HYPOTONICITY • Helps maintain the concentration gradient across the cell membrane. • Helps in transporting other substances across the cell membrane Endocytosis Taking bulky material into a cell • Uses energy • Cell membrane folds in around food particle • “Cell eating” • Forms food vacuole & digests food. **This is how white blood cells eat bacteria! Exocytosis •When substances are released from the cell through a vesicle. • Fuses with the plasma membrane to let the substance out •Used to release large molecules like proteins, waste products, and toxins. 3 ways a Cell Membrane Works 1. Cellular Identity: Proteins give cell its identity. 2. Electrical Charge: given electrical charge, proteins in membrane will open or close. • Ex: nerves 3. Chemicals: Open/close based on chemicals present. Receptor Proteins • Proteins the bind to a specific signal molecule, enabling the cell to respond to the molecule. • Ex. Exercising muscles need receptor proteins to know when to contract or relax. • Work with signal molecules • May create SECOND MESSENGERS: Acts as a signal molecule in the cytoplasm. Second Messengers • Formed by the receptor protein. • Acts as a signal molecule in the cytoplasm. • Change the function of the cell in many ways: • Activate enzymes. • Trigger a series of chemical reactions. • Open other ion channels. Receptor Protein • A protein that binds to a specific signal molecule, enabling the cell to respond to the signal molecule. Functions of Receptor Proteins • Changes permeability of the receiving cell. • Triggers formation of second messengers inside the cell. • Acts as a signal molecule in the cytoplasm. • Activate enzymes inside the cell. Changes in Permeability • Binding of signal molecules with receptor proteins can open the ion channel. • Allows specific ions to cross the cell membrane. • Very important for nervous system. Enzyme Action • Receptor proteins may act as enzymes. • This can speed up chemical reactions. • Certain drugs block receptor proteins. • This causes cells to act in different ways 5.2 Objectives REVIEW • Distinguish between passive transport and active transport. • Explain how the sodium-potassium pump operates. • Compare endocytosis and exocytosis.