Membranes and Transport
General Biology 1
Cell Membrane
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Semi-permeable
o Controls what goes in and out of the cell
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Peripheral proteins
Peripheral area of the
membrane
Involved with enzymes,
receptors
Integral proteins
Go through the membrane
Involved in transport
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Fluid Mosaic Model
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Model for the structure of the plasma membrane and other
cell membranes
Fluid implies movement (the phospholipids float around;
flexibility of the membrane)
Mosaic refers to the arrangement of many pieces
Types of Cell Transport
Transport
Passive
Phospholipid Bilayer
Simple Diffusion
Active
Facilitated Diffusion
Osmosis
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Passive
High to low concentration
Molecules flow across the
membrane through diffusion
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Polar head
Hydrophilic (water-loving)
Phosphate-containing
o Has an empty shell—can
form bonds
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Non-polar head
Hydrophobic (water-fearing)
Fatty acid
Active
Low to high concentration
Molecules are pumped
across the membrane
Requires energy
Passive Transport
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Involves water
Moves from high water potential to low water potential
Moves from low concentration gradient to high concentration
gradient
Terms:
o Concentration gradient refers to the other molecules
(i.e. solutes)
Plant
Cell in a:
Hypotonic
Solution
Isotonic
Solution
Hypertonic
Solution
Stable
Low WP, High CG
High WP, Low CG
Cell will shrink
Diagram
Simple Diffusion
• Net (overall) movement of a substance traveling down its
concentration gradient
• Moving from high concentration to low concentration
• Small molecules such as water and oxygen
High WP, Low CG
Low WP, High CG
Cell will expand
Facilitated Diffusion
• Requires a protein channel
o Because the hydrophobic tails repel hydrophilic
substances (e.g. charged ions and polar substances)
Osmosis
High Water Potential
Low Concentration Gradient
Solution
Cell
Effect
à
Low Water Potential
High Concentration Gradient
Active Transport
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Requires energy (i.e. ATP)
o Adenosine triphosphate has three phosphates; when
one phosphate breaks a bond, it releases energy
Moving from low concentration to high concentration
Sodium-Potassium Pump
• Phosphate released by the ATP attaches to the membrane
protein
o The membrane protein then changes it shape