Cell Transport (1.4)
Surface Area to Volume Ratio
How does SA:V Ratios related to the size of cells?
Volume = length of the side cubed. SA:V ratio = surface area / volume |
How does diffusion relate to cell size and SA:V ratio?
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Why should we care?
This is the main reason cells divide rather than continuing to grow larger. If cells become too large (and subsequently have a smaller SA:V ratio), then diffusion rates significantly lower. Lower diffusion rates mean longer times for waste to remain in the cell and longer times for nutrients to penetrate the entirety of the cell. Same reason the larger we grow the more danger higher fevers become as diffusion of the heat takes too long.
This is the main reason cells divide rather than continuing to grow larger. If cells become too large (and subsequently have a smaller SA:V ratio), then diffusion rates significantly lower. Lower diffusion rates mean longer times for waste to remain in the cell and longer times for nutrients to penetrate the entirety of the cell. Same reason the larger we grow the more danger higher fevers become as diffusion of the heat takes too long.
Overview of Cellular Transport
The main purpose of the membrane is to selectively control what goes in and out of the cell. The movement of molecules across the cell membrane is called Cellular Transport.
Two main types:
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Passive Transport
Three types of passive transport.
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Simple DiffusionSimple Diffusion works in all sorts of manners.
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Faciliated DiffusionFacilitated diffusion has many types as well. However, the one distinction from Simple Diffusion is that molecules use a transport protein to cross the membrane.
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Ball and Chain Mechanism
(a type of facilitated diffusion in nerve cells)
The ball and chain mechanism is a specific example of facilitated diffusion. It occurs primarily in our nerve cells and uses electrical charges to open and close. An example of this is the Sodium (Na+) Gated Channel in our nerves.
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See Neurons and Synapses (6.5) for more information on action potentials, voltage gated channels, and neurons
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Osmosis
Reminders: Osmosis is passive transport (down concentration gradient without energy) so its goal is equilibrium.
Osmosis: Diffusion of WATER across the membrane from an area of low solute concentration to an area of high solute concentration
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Explanation of diagram: While the definition of osmosis seems at first to indicate that water is moving from low to high concentrations, it is important to note that the water is moving based on the SOLUTE concentrations NOT on the concentration of water.
- From left to right: solute concentrations increase (0% -->10%) while water concentration decreases (100%--> 90%).
- Because osmosis is the movement of water, not the solute, water is still moving from high to low concentration, by moving left to right.
- The definition also holds true, while water moves to the right, water is moving from low solute (0%) to high solute (10%) concentrations
Three types of Osmosis
There are 3 types of osmosis:
Osmosis: Hypertonic, Hypotonic, Isotonic Solutions Hypertonic, Hypotonic, Isotonic Solutions Animation |
HypertonicHypertonic cell: there is a high solute concentration outside the cell compared to inside
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IsotonicIsotonic cell: there are equal concentrations of solute in and outside of cell
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HypotonicHypotonic cell: there is a lower solute concentration outside cell compared to inside
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Osmolality
Osmolality= total concentration of osmotically active solutes within a cell
In English: the point at which ALL solutes are balanced from inside to outside the cell.
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Active Transport
Active Transport: Uses protein pumps, requiring ATP to carry different ions across the membrane AGAINST their concentration gradient
Example: Sodium- Potassium Pump
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Endocytosis and Exocytosis
EndocytosisLarge substances taken into the cell using energy
Vesicles are pinched off a small piece of plasma membrane
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ExocytosisMovement of large substances released out of cell
Stages of exocytosis:
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