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Cell Transport
The movement of molecules into and out of a cell
How is a phospholipid constructed?
Interactions of phospholipid molecules with waterThe basis of phospholipid bi-layers
The phospholipid bilayer is the fundamental structure of the membrane
The cell membrane contains both hydrophilic and hydrophobic regions
The Cell Membrane
• The cell membrane is composed of 2 layers of phospholipids
• Also called the phospholipid bilayer or plasma membrane
• The Polar heads face the outside of the membrane where water exists
• The Nonpolar tails face each other in the inside of the bilayer creating a hydrophobic region
Membranes are primarily composed of phospholipids and proteins
Properties of Membrane
Membranes have the important function of regulating the movement of substances into or out of cells or organelles
The Fluid-Mosaic Model of a membrane
Two types of proteins in membranes – peripheral proteins and integral proteins
peripheral protein integral protein
The Passage of Molecules across the Plasma Membrane
Hydrophobic molecules and non-polar molecules (such as O2) cross the membrane with ease.
Hydrophilic molecules cannot, however, slip through the extensive hydrophobic core of the membrane.
The transport of hydrophilic compounds requires the help of the integral proteins of the membrane.
Diffusion -- a spontaneous process in which particles move from an area of high concentration to an area of low concentration across a concentration gradient
When two different solutes are diffusing at the same time, they will each follow their own concentration gradients from high to low.
The net diffusion occurs from a region of high concentration toward a region of low concentration.
High concentration Low concentration
What happens if we remove the barrier?
Diffusion – fills the space – it is spontaneous, no work is done
High concentration Low concentration
Diffusion Summarized
Some molecules can simply pass through
Example: small molecules
No energy required to pass: passive transport
Move from high low concentration: diffusion
Diffusion will occur until equilibrium is reached
OsmosisOsmosis – the diffusion of water across a selectively-permeable membrane
higher solute concentration
Solute is defined as polar or charged molecule or ion
Water is attracted to solutes!
OsmosisOsmosis – the diffusion of water across a selectively-permeable membrane
water level rises
Osmosis Summarized
Water can simply pass through Special type of diffusion: osmosis No energy required: passive transport
Tonicity – the ability of a solution to cause a cell to loose or gain water
Cellwith higher
soluteconcentration
Water with low solute concentration
hypotonic
H2O
Hypotonic solution – contains fewer solutes than cell (Cell will enlarge when placed in this solution!)
Tonicity – the ability of a solution to cause a cell to loose or gain water
Cell(with solutes)
Water with high solute concentration
hypertonic
H2O
Hypertonic solution – contains more solutes than cell (Cell will shrink when placed in this solution!)
Tonicity – the ability of a solution to cause a cell to loose or gain water
Cell(with solutes)
Water with equal solute concentration
isotonic
H2O
Isotonic solution – contains equal solutes as cell (Cell will retain size when placed in this solution!)
Tonicity in animal cells
Tonicity in plant cells
Turgid(normal)
Flaccid(slight water stress)
Plasmolyzed(severe water stress)
Facilitated Diffusion
No ATP energy required
Molecules follow their concentration gradients
They are just “helped” across the hydrophobic core of the membrane by a transport protein
Facilitated Diffusion
Net rate of diffusion from high concentration toward low concentration
channel
gate
Facilitated Diffusion Summarized
Some molecules are too large to pass normally Example: monosaccharides Carrier proteins assist: facilitated diffusion No energy required: passive transport
Facilitated Diffusion Summarized
Ions have a charge: cannot normally pass Examples: Na+, Ca+, Cl-
Can pass through ion channel No energy required: passive transport
Active Transport
Molecules go against their concentration gradients
They are “helped” across the hydrophobic core of the membrane by a transport protein, but only when energy is expended.
Work is done by the cell! Requires ATP (Energy)!!
Active Transport Summarized
Molecules often move from low high concentration Such movement requires energy: active transport Carrier protein used as cell membrane “pump” Example: Na+ and K+ pump
Review the Differences
Bulk Transport
• EndocytosisEndocytosis – movement of large molecules into the cell– Phagocytosis – movement of foods (cell
eating)– Pinocytosis – movement of fluids (cell
drinking)
• ExocytosisExocytosis – movement of large molecules out of the cell