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Transcript
Cells Part 1: Cell Membranes By the end of this class you should understand: • The three major parts of all cells • The structure and function of a cell membrane • The distinction between a cell membrane and cell wall • The nature of diffusion and osmosis • The three types of membrane transport • Bulk transport and ionic transport in cells Parts of a Cell • There are two major types of cells – Prokaryotic cell – Eukaryotic cell • All cells have three parts: – Cell membrane – Genetic information – Cytoplasm Prokaryotic Cells • Prokaryotic cells are cells without a nucleus – All genetic info is bundled but not separated from cytoplasm • All known prokaryotes are single-celled bacteria – Prokaryotic cells are tiny! Eukaryotic Cells • If every cell was a company: – Prokaryotic cells are a 1man startup in a garage – Eukaryotic cells are a large corporation • Eukaryotic cells have organelles which each perform separate jobs • Genetic info stored in nucleus Eukaryotes • Some eukaryotes are single celled organisms – One eukaryotic cell performs all functions of life • Some eukaryotes are multicellular – The different cells specialize in different tasks – Each cell is still individually alive! Key Parts of Eukaryotic Cells • Nucleus – Stores genetic information • Cytoplasm – Composed of a liquid called cytosol – Filled with various organelles • Cell Membrane – Often referred to as plasma membrane Cell Membrane • The cell membrane is a flexible, selectively permeable (or semipermeable) barrier • Boundary of cell, visible under microscope only as border of cell • NOT the same as a cell wall – Cell wall is rigid Cell Membrane Structure • The cell membrane is composed of phospholipids – Hydrophilic head, hydrophobic tail • The nonpolar tails orient themselves into a hydrophobic zone surrounding the cell – Essentially a thin bubble of oil Cell Membrane Structure Cell Membrane Proteins • Proteins are macromolecules constructed from many linked amino acids • Some amino acids are hydrophobic and so mix with the inside of the cell membrane • These proteins sit inside the cell membrane Membrane Protein Functions • Cell membrane proteins serve many vital functions: – Markers – Receptors – Channels • Transporting things in and out of cells is a key function of life! Selective Permeability • Some things can move through cell membranes – Water – Lipids • Many things cannot – Ions – Proteins – Sugars – Most other things Diffusion • Diffusion is the random movement of particles • Diffusion results in particles, on the average, moving from an area of high concentration to an area of low concentration – Referred to as moving down the concentration gradient Diffusion of Water • If there is more sugar or salt inside a cell than outside, then there is less water • Water moves on average towards where there is less water • This diffusion of water is called osmosis Osmosis in Animals and Plants Relative Concentration of Solutes • Sugars and salts dissolved in water (solutes) may be more or less concentrated in cytosol than outside the cell – More concentrated = hypertonic – Less concentrated = hypotonic – Equally concentrated = isotonic Consequences of Hydration • Animal cells subjected to excessively dilute water swell up and may burst (lyse) – Plant cells are protected from lysis by their cell walls • Management of an animal’s water, sugar and salt levels is vital to life! What about Transport of Solutes? • Solutes may move in three ways through cell membranes: • Passive transport: membrane is permeable – Example: CO2 • Facilitated diffusion: membrane is impermeable but protein channels permit movement – Example: potassium ions • Active transport: energy must be expended to move solute against concentration gradient – Example: Sugars Three Types of Transport Active Transport of Ions • Actively moving charged particles to one side of the cell can create an electrical gradient – Similar to a concentration gradient – The outside of the cell may be positive or negative relative to the inside • This is how neurons send signals! Bulk Transport • Cells may also absorb or release materials using vesicles – A vesicle is a small membrane-bound organelle within the cell • Absorption: Endocytosis • Excretion: Exocytosis Vesicular Transport See you Wednesday!