cell membrane transport. 1- factors affecting transport - cell membrane - chemical gradient -...
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Cell Membrane Transport
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Cell Membrane Transport1- Factors affecting transport - cell membrane - Chemical gradient - Electrical gradient - Rate of transport2- Passive transport - Diffusion - Osmosis - Facilitated diffusion3- Active transport - Pumps - phagocytosis - Endocytosis/exocytosis
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Factors affecting transport: cell membrane
• The cell needs to absorb and excrete various compounds throughout its life.
These compounds need to pass through the membrane which is made from a phospholipid bilayer
• The phospholipid bilayer is formed by phospholipid molecules bipolar molecule: the fatty acid side is hydrophobic, the phosphoric side is hydrophilic
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Figure 2.4
Stable phospholipid organizations
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• The membrane is permeable to:
- H2O- Gases (O2, CO2, N2)- Lipids- Small, neutral
molecules (such as urea)
• The membrane is impermeable to:
- Small, charged molecules
- “large molecules” such as amino acids, glucose and larger
these compounds must go through channels present in the membrane in order to enter or exit the cell
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Factors affecting transport: Chemical gradient
• Compound moves from an area of high concentration to low concentration (or concentration gradient)
• All compounds permeable to the phospholipid bilayer will move this way
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Figure 4.3
Factors affecting transport: Electrical force
• Positive ions are attracted to negative ions and vice versa
• Ions are repelled by ions of the same charge (+ against + and – against -)
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Movement across the cell membrane
• Both chemical and electrical forces (electrochemical force) drive the movement of compounds across the cell membrane
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Factors affecting the rate of transport
• The rate of transport will depend on:
- the concentration gradient
- the compound permeability to the membrane
- the type and number of charges present
on the compound
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Passive transport
• Compounds will move from area of high concentration toward area of lower concentration
• No ATP is needed for this type of transport
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Diffusion
• Compounds move toward the area of lower concentration
• Compounds permeable to the cell membrane will move through diffusion. (Compounds unable to pass through the membrane will only pass if membrane channels open)
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Osmosis
• Each compound obeys the law of diffusion
• However, some compounds are unable to cross the cell membrane (glucose, electrolytes…)
• Water can cross will enter or exit the cell depending its concentration gradient
• Note: the cell membrane is a semipermeable membrane
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Solution tonicity
• Isotonic solution: solution which has the same compound concentration as the cell
• Hypotonic solution: solution having a compound in lower concentration compared to the cell
• Hypertonic solution: solution having a compound in higher concentration compared to the cell
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Facilitated diffusion
• Some compounds are unable to diffuse through the membrane.
• They will be allow to cross if the membrane has proteins that can bind these compounds and enable to cross toward the area of lower concentration
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Figure 4.11a
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Active transport
• Compounds move from area of low concentration toward area of higher concentration
• ATP (energy) is needed pump
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ATPase pumps
• The most common: Na/K pumps reestablish membrane potential. Present in all cells.
• Two K+ ions are exchanged with 3 Na + ions
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Phagocytosis
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Figure 4.21c
Receptor-mediated endocytosis
• Cell receptors bind to a compound initiate endocytosis
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Readings:
• Chp. 4: p. 95-124.
• Clinical connections, p.121.
• Not expected: Toolbox, p. 96, p. 100, p. 105.