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Transcript
Cell Boundaries The roles of the cell membrane and the cell wall….it’s all about transport (active and passive) Structures that enclose the cell • Cell membrane is the outer boundary of every cell • Role is to separate and protect the cell from its surroundings • Cell membrane is selectively permeable • Phospholipids and the lipid bilayer • Role of proteins and carbohydrates In plants: the cell wall • Cell wall located outside the cell membrane and serves to support and protect the cell • Found in PLANTS ONLY • Cell walls are very porous; allow water, gases to pass through easily • Primarily cellulose (carbohydrate) and some protein It’s about Transport • Cell membranes help the cell regulate its internal chemistry • Cells maintain a constant internal environment by compensating for environmental changes. • This is called homeostasis • Use a combination of Passive Transport and Active transport Passive Transport • Substances cross the cell membrane without the cell expending energy • Process is diffusion: this is how substances spread through a liquid or a gas • Substances move from regions of high concentration to regions of lower concentration Passive transport (diffusion) • Small molecules like alcohol, water and small lipids diffuse directly across the cell membrane. • Cell membranes have hundreds of different kinds of protein channels to allow diffusion of specific molecules (like glucose) • This use of channels is called facilitated diffusion Special case: diffusion of water • Diffusion of water across the cell membrane is called OSMOSIS • Like all other substances water diffuses form regions of high concentration to regions of low concentration • Pure water has a higher concentration of water than a solution does • This has important consequences for a cell The consequence: Osmotic pressure • Movement of water causes powerful pressure; high enough to burst the cell • Cells deal with this: – By having cell walls (physical strength) – Pumping out water (special structures) – By having blood to bathe the cells (blood and cells have same concentrations of dissolved substances) Active Transport • Movement of substance against a concentration gradient • ALWAYS requires energy; can be compared to a pump • Used to transport large molecules into the cell, pareticles of food and sometimes whole other cells Active Transport • Endocytosis: the cells engulfs large particles and takes the particle into the cell • Endocytosis is also called phagocytosis – Protozoans – White blood cells • Exocytosis: cells use energy to expel particles from the cell