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BIOL 1406: Ch. 6 & 7 (Cell Membrane (Cellular membranes are fluid…
BIOL 1406: Ch. 6 & 7
Microscopes
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Cell walls were first seen by Robert Hooke in 1665 as he looked through a microscope at dead cells from the bark of an oak tree.
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Energy Converters
Chloroplasts, found in plants and algae, are the sites of photosynthesis. Solar energy is converted to chemical energy by absorbing sunlight and using it to drive the synthesis of organic compounds such as sugars from carbon dioxide and water.
Mitochondria are the sites of cellular respiration, the metabolic process that uses oxygen to drive the generation of ATP by extracting energy from sugars, fats, and other fuels.
In eukaryotic cells, mitochondria and chloroplasts are the organelles that convert energy to forms that cells can use for work.
The endosymbiont theory states that an early ancestor of eukaryotic cells engulfed an oxygen-using non-photosynthetic prokaryotic cell, becoming endosymbiont (a cell living within another cell). Evolution caused the two cells to merge into a eukaryotic cell with a mitochondrion.
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Peroxisome is a specialized metabolic compartment bounded by a single membrane that cooperates with mitochondria and chloroplasts in certain metabolic functions.
Cell Membrane
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The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others.
A phospholipid is an amphipathic molecule, meaning it has both hydrophilic region and a hydrophobic region.
The fluid mosaic model is the currently accepted model of ell structure, which envisions the membrane as a mosaic of protein molecules drifting laterally in a fluid bilayer of phospholipids.
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Peripheral proteins are not embedded in the lipid bilayer at all, they are loosely bound to the surface of the membrane, often to exposed parts of integral proteins.
Functions of membrane proteins include transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix.
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Transport Systems
Bulk Transport
In phagocytosis, a cell engulfs a particle by extending pseudopodia around it and packaging it within a membranous sac called a food vacuole.
In pinocytosis, a cell continually "gulps" droplets of extracellular fluid into tiny vesicles, formed by infoldings of the plasma membrane.
In endocytosis, the cell takes in molecules and particulate matter by forming new vesicles from the plasma membrane.
Exocytosis - the cell secretes certain molecules by the fusion of vesicles with the plasma membrane.
Receptor-mediated endocytosis is a specialized type of pinocytosis that enables the cell to acquire bulk quantities of specific substances.
Active Transport
Active transport is the use of energy to pump a solute across a membrane against its gradient requiring work.
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Membrane potential is the voltage across a membrane, acting like a battery, and energy source that affects the traffic of all charged substances across the membrane.
A chemical force and electrical force drive diffusion of ions across a membrane acting on an ion together referring to the electrochemical gradient.
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The main electrogenic pump of plants, fungi, and bacteria is a proton pump.
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Passive Transport
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Diffusion is the movement of particles of any substance so that they spread out into the available space.
Any substance will diffuse down its concentration gradient, the region along which the density of a chemical substance increases or decreases.
The diffusion of free water across a selectively permeable membrane, whether artificial or cellular, is called osmosis.
Tonicity
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Hypertonic meaning the cell will lose water, shrivel, and probably die.
Hypotonic meaning water will enter the cell faster than it leaves, and the cell will swell and lyse (burst).
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Endomembrane System
Includes the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, various kinds of vesicles and vacuoles, and the plasma membrane.
The membranes of this system are related either through direct physical continuity or by the transfer of membrane segments as tiny vesicles (sacs made of membrane).
Many of the different membrane-bounded organelles of the eukaryotic cell are part of the endomembrane system.
The endoplasmic reticulum us such an extensive network of membranes that it accounts for more than half the total membrane in many eukaryotic cells.
Smooth ER is named because of its outer membrane lacking ribosomes. Functions include synthesis of lipids, metabolism of carbohydrates, detoxification of drugs and poisons, and storage of calcium ions.
Round ER is studded with ribosomes on the other surface of the membrane and thus appears rough through the electron microscope. Functions as a membrane factory for the cells and the making of secretory proteins.
The Golgi apparatus is the warehouse for receiving, sorting, and shipping, and even some manufacturing in the cell.
A lysosome is a membranous sac of hydrolytic enzymes that many eukaryotic cells use to digest (hydrolyze) macromolecules.
Vacuoles are large vesicles derived from the endoplasmic reticulum and Golgi apparatus. Perform a variety of functions in different kinds of cells and includes food vacuoles, contractile vacuoles, and a central vacuole.
Cellular Activity
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Between primary walls of adjacent cells is the middle lamella, a thin layer rich in sticky polysaccharides called pectins.
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Animals cells have an extracellular matrix that are composed of glycoproteins and other carbohydrate-containing molecules secreted by the cells.
The most abundant glycoprotein in the ECM of most animal cells is collagen, which forms strong fibers outside the cells.
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Fibronectin and other ECM proteins bind to cell-surface receptor proteins called integrins that are built into the plasma membrane.
Cell Junctions
Cells in an animal or plant are organized into tissues, organs, and organ systems.
Cell walls of plants are perforated with plasmodesmata, channels that connect cells.
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Cytoskeleton
Intermediate filaments are named for their diameter, which is larger than the diameter of microfilaments but smaller than that of microtubules.
Microfilaments are thin sold rods also called actin filaments because they are built from molecules of actin, a globular protein.
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In eukaryotes, a specialized arrangement of microtubules is responsible for the beating of flagella and cilia, microtubule-containing extensions that project from some cells and motile appendages.
In animal cells, microtubules grow out from a centrosome, a region that is often located near the nucleus. Within the centrosome is a pair of centrioles, each composed of nine sets of triplets microtubules arranged in a ring.
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The eukaryotic cytoskeleton, which plays a major role in organizing the structures and activities of the cell, is composed of three types of molecular structures: microtubules, microfilaments, and intermediate filaments.
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