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Lecture 2

BISC 101 - Lecture 2 - Cell Membrane.docx

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Biological Sciences
BISC 101
Christopher Kennedy

Biological Science 101 – General Biology Lecture Two: Cell Membrane Textbook: Chapter 7, pg. 125-139 Fig. 7.2-7.22 Composition of cell membranes Plasma membrane: The membrane that surrounds the cell. - Other membranes around: nucleus, mitochondria, Golgi apparatus. - Membranes are selectively permeable – “some things can go in and some things can go out”. Lipids and proteins are in the membrane (they also contain carbohydrates). - Lipids and proteins are bond together in a way that makes up a “fluid mosaic” model; because it is a fluid model it is flexible. -9 - It is very thin 8 x 10 (8,000 membranes = thickness of one paper page). Lipids = Phospholipids that have a hydrophilic end and a hydrophobic end. Proteins Present in Membrane Amphipathic = Hydrophilic + Hydrophobic Embedded Proteins (integral) and External (peripheral). Carbohydrates in membrane -> short-chain (oligosaccharides). Functions of Carbohydrates: 1) Cell-to-cell recognition. E.g. Foreign cells. 2) Determine cell features. E.g. Blood types (A, B, O, AB) Functions of Proteins: 1) Cell-to-cell recognition: - Some glycoproteins serve as identification tags that are specifically recognized by membrane proteins of other cells. This type of cell-to-cell binding is usually short-lived compared to that shown in intercellular joining. 2) Intercellular joining: - Membrane proteins of adjacent cells may hook together in various kinds of junctions (see textbook figure 6.32). - This type of binding is more long-lasting that that shown in cell-to-cell recognition. 3) Attachment to the cytoskeleton and extracellular matrix (ECM): - Microfilaments of other elements of the cytoskeleton may be noncovalently bound to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that can bind to ECM molecules can coordinate extracellular and intracellular changes (see textbook figure 6.30). Movement of Molecules: Hydrophilic End --- Hydrophobic End - Sugars and amino acids are difficult to transport. - Water is very easy. Passive Transport = No energy required. 1) Diffusion: Molecules spread into a space, from high concentration to low concentration. 2) Osmosis: The movement of water, from a low solute (hypotonic) to a high solute (hypertonic). - It equalizes the distribution of solute, which is important for cell viability. 3) Facilitated Diffusion: Diffusion with the help of transport proteins. - Specific for molecules to transport. - Water uses “aquaporins” (water channels). Active Transport = Moving molecules against a concentration gradient. - Gradient from low concentration to high concentration. - Against concentration gradient. - Cells uses energy in the form of ATP: 1) Sodium – Potassium Pump: Sodium out, Potassium in. - Cells need high K inside. - Potassium ions have to enter and Sodium ions have to leave. 2) Proton Pump (Electrogenic Pump): - Hydrogen ions go out of the cell. - Negative charge inside. - Electrical potential created (membrane potential) - 50 mV to -200 mV - Creates a pathway for ions to enter cell. 3) Co-transport: - Solute transport is accompanied by the proton pump. E.g. Plants – transport sucrose into the cell. Bulk Transport: Moving large molecules E.g. Carbohydrates and Proteins. 1) Exo
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