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Transcript
Cells and Their Environment The Cell Membrane • The Cell Membrane is composed of two layers of phospholipids (fat). Phospholipids • Each phospholipid has one end that is “water loving” containing a phosphate group. The other end is a fatty tail (lipid), and is “water fearing”. Phosphate group (water loving) Fatty tail (water fearing) • A cell membrane is formed by a double layer of phospholipid molecules in which protein molecules are embedded. Protein Molecules • Protein molecules are embedded in the cell membrane, the fatty ends of the phospholipid hold them in place. • Proteins serve as an attachment site for molecules that are entering the cell. • When an appropriate molecule comes along it attaches itself to the protein, which pulls it into the cell. Channels • Cell membranes have several channels through which calcium, sodium, and other atoms can enter or exit the cell. Cell Membrane Function • The cell membrane encloses the inside of the cell and acts as a protective barrier against molecules outside of the cell. • Cells allow some materials to enter or leave but not others. Permeable/Impermeable • Cells are permeable to some materials and impermeable to others. • Permeable means permitting passage and impermeable means to not permit passage. • The cell membrane is semipermeable (or selectively permeable); that is it allows only specific substances in and out of the cell. Movement Across the Membrane • There are various ways that substances enter and leave cells. – Passive Transport – Active Transport Passive Transport • Passive Transport involves the movement of molecules across the cell membrane without the use of energy. • Water, lipids, and lipid-soluble particles pass through the membrane in this way. • Diffusion and Osmosis are examples of passive transport. • Passive Transport: • http://www.brainpop.com/science/c ellularlifeandgenetics/passivetransp ort/preview.weml Diffusion • Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. • Molecules are constantly moving. When the molecules collide with each other they bounce off of each other. • This movement causes the molecules to gradually spread outward and become evenly distributed. This condition is called dynamic equilibrium. • Diffusion: • http://www.brainpop.com/science/m atterandchemistry/diffusion/previe w.weml Diffusion and Cells • Diffusion is one of the ways substances move into and out of the cell. • If a substance that a cell uses, such as oxygen, is in low concentration inside the cell, the molecules of the substance will diffuse across the membrane until the concentration is the same inside and outside. Concentration gradient • When a molecule moves from an area of high concentration to an area of lower concentration, it is moving down a concentration gradient. Diffusion Text Book Questions • Complete questions 1 – 5 on page 29 of the text book. Osmosis