transport across cell membranes - semantic scholar · 2017. 10. 24. · cell membrane structure a...
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Transport Across Cell Membranes
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CELL MEMBRANE STRUCTURE
� A phospholipid bilayer makes up the main part of the cell membrane
� Each phospholipid molecule contains a charged polar head (H2O-loving) and non-polar, fat-soluble tails (H O fearing)tails (H2O fearing)
� This gives the cell membrane it’s fluid-like nature
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CELL MEMBRANE STRUCTURE
� Proteins form a ‘mosaic’ pattern (scattered) throughout the cell membrane
� Proteins are embedded in the cell membrane and on its surface
Some are carrier proteins that bring H O, amino � Some are carrier proteins that bring H2O, amino acids, ions, glucose… in/out of the cell (require ATP) eg. Sodium/potassium pump in nerve transmission
� Some proteins have channels/pores to allow passage of H2O, dissolved ions & small molecules
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CELL MEMBRANE STRUCTURE
� Some proteins are receptors that a specific molecule can bind to (eg hormones)
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FLUID-MOSAIC MODEL
� The fluid nature of the phospholipids coupled with the random scattering of proteins led to the development of the fluid-mosaic model of cell membrane structure.
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GLYCOLIPIDS & GLYCOPROTEINS
� Glycolipids & Glycoproteins – are found on the outside of the cell membrane and serve as recognition sites allowing organisms to recognize foreign cells/molecules
� Glycolipids & Glycoproteins are not found on � Glycolipids & Glycoproteins are not found on intracellular membranes (ie mitochondria, ER, golgi)
� Glycolipids are carbohydrate chains attached to a phospholipid head, glycoproteins are carbohydrate chains attached to a protein
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PROTEIN FUNCTION
� Allow/select certain molecules in/out of cell through channels (eg H2O, O2, CO2, or ions)
� Carry molecules selectively in/out of the cell by carriers (eg Na+, K+, C6H12O6, HCO3
-, Ca+)
Catalyzes reactions on the surface of the cell � Catalyzes reactions on the surface of the cell (enzymatic proteins ie enzymes for ATP metabolism)
� Provide receptor sites (receptor proteins, eg for hormone binding)
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PHOSPHOLIPIDS
� Allow for diffusion of lipid soluble molecules (O2, CO2, and alcohol)
� Allows for flexibility of the cell membrane
� Excludes water & ions
� Act as a boundary keeping organelles within the cell
� Create hydrophobic (water-fearing) and hydrophilic (water-loving) portions of the cell membrane
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HYDROPHOBIC/HYDROPHILIC
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SELECTIVELY PERMEABLE MEMBRANE
� Selectively permeable has replaced the term semi-permeable when referring to cell membranes.
� Selectively permeable refers to the selective way that membranes and their proteins allow molecules in/out of the cell.in/out of the cell.
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PASSIVE TRANSPORT: DIFFUSION/OSMOSIS
� Diffusion is the movement of particles from an area of high concentration to an area of low concentration
� Osmosis is the movement of water from an area of high concentration to an area of low concentration
Any time a substance is moving from high � Any time a substance is moving from high concentration to low concentration we say that it is moving down the concentration gradient
� No energy (ATP) is required for these processes
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PASSIVE TRANSPORT: DIFFUSION/OSMOSIS
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PASSIVE/FACILITATED TRANSPORT
� Facilitated transport is still considered passive as no energy (ATP) is required for the is required for the process.
� Particles move from high concentration to low concentration.
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FACTORS THAT AFFECT DIFFUSION RATES
� CONCENTRATION:
� Solutions consist of a solvent (often water) and a solute (particles dissolved in the solvent)
� A difference in solute concentrations on either side of a cell membrane creates a concentration gradient of a cell membrane creates a concentration gradient and diffusion will occur
� The larger the difference in solute concentrations the greater the rate of diffusion
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FACTORS THAT AFFECT DIFFUSION RATES
� TEMPERATURE:
� An increase in temperature increases the kinetic energy of the particles in a solution (they move more quickly) and therefore increases the rate of diffusion
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FACTORS THAT AFFECT DIFFUSION RATES
� IONIC/MOLECULAR SIZE:
� Smaller molecules will collide less frequently and therefore diffuse more quickly than larger molecules
� Large molecules (starches, proteins...) simply do not diffuse across cell membranesdiffuse across cell membranes
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FACTORS THAT AFFECT DIFFUSION RATES
� SHAPE OF ION/MOLECULE
� Similar to size, the shape of a molecule may prevent it from diffusing rapidly
� Other molecules have a configuration (shape) that facilitates their diffusionfacilitates their diffusion
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FACTORS THAT AFFECT DIFFUSION RATES
� VISCOSITY:
� Solutions with lower viscosity (fluid density) allow molecules to diffuse more quickly through them
� For example, particles will diffuse more quickly through water than they will through syrupthrough water than they will through syrup
� MOVEMENT OF THE MEDIUM:
� Currents (in water, air, or cytoplasmic streaming) will aid in the process of diffusion
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ISOTONIC
� Isotonic Solutions: contain the same concentration of solute as an other solution (e.g. the cell's cytoplasm). When a cell is placed in When a cell is placed in an isotonic solution, the water diffuses into and out of the cell at the same rate. The fluid that surrounds the body cells is isotonic.
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HYPOTONIC
� Hypotonic Solutions: contain a low concentration of soluterelative to another solution (e.g. the cell's cytoplasm). When a cell cytoplasm). When a cell is placed in a hypotonic solution, the water diffuses into the cell, causing the cell to swell and possibly explode (lysis).
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HYPERTONIC
� Hypertonic Solutions: contain a high concentration of solute relative to another solution (e.g. the cell's cytoplasm). When a cell cytoplasm). When a cell is placed in a hypertonic solution, the water diffuses out of the cell, causing the cell to shrivel (crenation).
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ACTIVE TRANSPORT
� Movement from low concentration to high concentration (against the concentration gradient)
Requires energy (ATP)� Requires energy (ATP)
� Example: sodium-potassium pump in neurons
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ENDOCYTOSIS
� 2 types – pinocytosis & phagocytosis
�Move objects too large to fit through the membrane
� Pinocytosis = cell drinking (small objects)� Pinocytosis = cell drinking (small objects)
� Phagocytosis = cell eating (larger objects)
� Both processes require ATP
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PINOCYTOSISPINOCYTOSIS PHAGOCYTOSISPHAGOCYTOSIS
ENDOCYTOSIS
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EXOCYTOSIS
� This is the opposite process to endocytosis
� Vesicles produced by the golgi body move toward the cell membrane, fuse with the membrane and with the membrane and release their contents to the outside of the cell
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LIMITS TO CELL SIZE
� Cell size is limited by their surface area to volume ratio
� As cells grow bigger their surface area to volume ratio gets smaller
Once cells reach a certain size they must divide or � Once cells reach a certain size they must divide or they will die