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Transport Across the Cell Membrane
• maintains homeostasis of cell
• membrane is selectively permeable – some things can pass through but others can’t
• 3 types: active transport, passive transport, and bulk transport
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1. Passive Transport
• passive = no energy req’d
• move through membrane due to differences in concentration gradient
• 3 different types
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1. Diffusion
• Movement of molecules from an area of high concentration to an area of low concentration across a concentration gradient
• Used transport small molecules like CO2 H2O, and O2
• Animation: How Diffusion Works
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2. Osmosis
• Diffusion of waterwater from a region of high concentration to a region of low concentration
• Water can diffuse into or out of a cell, it depends on the concentration on either side of the cell membrane
• Animation: How Osmosis Works
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Water potentials
• Water moves from a place with a LESS NEGATIVE (higher) water potential to a place with a MORE NEGATIVE (lower) water potential
• The water potential of pure water is 0 (zero).• Solutions have negative water potentials – the
more concentrated the solution, the more negative the water potential.
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3 Types of Osmosis
1. Hypotonic • contain a low concentration of
solute relative to another solution (e.g. the cell's cytoplasm).
• cell is placed in a hypotonic solution, the water diffuses into the cell, causing the cell to swell and possibly explode.
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2. Hypertonic
• contain a high concentration of solute relative to another solution (e.g. the cell's cytoplasm).
• when cell is placed in a hypertonic solution, water diffuses out of the cell, causing the cell to shrivel.
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3. Isotonic• contain the same
concentration of solute as an another solution (e.g. the cell's cytoplasm).
• when cell placed in an isotonic solution, the water diffuses into and out of the cell at the same rate.
• fluid that surrounds the body
cells is isotonic.
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http://www.zerobio.com/flashmx/transport.swf
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3. Facilitated diffusion
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Key features
• how glucose/charged ions moves into cells• passive – does not require energy• uses carrier proteins• solute molecules “combine” with carrier
proteins in the membrane. • carrier molecules speed (or facilitate) the
passage of the solute molecules across the membrane.
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Spot the difference!
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2 kinds of proteins involved:
1.CARRIER PROTEINS
• bind to a specific type of diffusing molecule.
• have a highly specific hydrophilic region to which the solute molecule binds.
• binding cause the protein to undergo a change in shape that moves the solute across the bilayer and release it on the other side
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Carrier proteins
Animation: How Facilitated Diffusion Works
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Carrier proteins
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2. ION CHANNELS• formed by proteins with a
central pore that is lined with charged groups.
• help the diffusion of charged particles such as Ca2+, Na+, K+, HCO3- and Cl ions.
• Some channels are gated and allow cells to regulate the flow of ions from one cell to another.
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Ion channel
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Factors affecting Rate of Diffusion
1. Concentration Difference• happens ONLY when a concentration gradient is
present and solute travels ALONG (down) a concentration gradient
2. Saturation• there are only a limited number of carrier
molecules per unit area of membrane.• rate of movement reaches a max. when all carrier
molecules are fully loaded with solute molecules
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2. Active Transport
• the transport of molecules or ions across a membrane by carrier proteins against a concentration gradient.
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• requires energy
• involves carrier proteins in the membrane.
• hydrolysis of ATP releases the energy required for active transport.
• Cells involved in active transport have a large number of mitochondria to provide the ATP required
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Spot the difference
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Sodium-Potassium Pump
• ex. of active transport
• Exists in most cell membranes.
• Actively removes sodium ions from the cell while actively accumulating potassium ions into them from their surroundings
• Animation: How the Sodium Potassium Pump Works
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3. Bulk Transport
• used for materials to large to enter via passive or active transport
• vesicles created by folding of cell membrane onto itself to either engulf or expel materials
• 2 types: endocytosis & exocytosis
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Endocytosis• the transport of large particles into the cell
in vesicles formed by folding in of the cell surface membrane
• 3 types:
1. Pinocytosis (cell drinking)• intake of small droplet of
extracellular fluid along with solute particles
• occurs in all cells often
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2. Phagocytosis (cell eating)
• intake of large droplet of extracellular fluid including particulate matter (bacteria or organic matter)
• occurs only in specialised cells like amoeba or macrophages (bacteria fighting immune cells)
• Animation: Phagocytosis
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3. Receptor-assisted endocytosis
• involves intake of specific molecules that attach to special protiens in cell membrane that serve as receptors
• have a unique shape that fit only to one specific molecule
• ex. animal cells use this to bring cholesterol into cell
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Exocytosis• the reverse process and is used to secrete
proteins, e.g digestive enzymes, out of the cells.
• vesicle forms inside cell moves to membrane and empties contents outside of cell
• ex. pancreas secretes insulin