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Direction of Movement - Coggle Diagram
Direction of Movement
Chemical factors
Chemical properties of a substance indicate how it will behave in the extracellular environment and affect its transport across cell membranes.
Uncharged molecules
easily penetrate and cross membranes in either direction allowing them to dissolve in the phospholipid bilayer.
Charged ions
are hydrophilic, meaning they cannot cross the hydrophobic interior of the membrane. They use transport proteins- ion channels- which are specific for the substance they carry.
Physical factors
Physical properties of size and shape affect whether or not a substance moves through the plasma membrane and how it is transported.
Small molecules
pass easily and quickly slip between the phospholipids and diffuse passively from high concentrations to low (water, oxygen, and carbon dioxide)
Large molecules
are physically too big to move this way so they are transported by specific membrane carrier proteins that span the plasma membrane.
The specificity of transport protein depends on the physical shape of the molecule fitting in into the carrier protein. Through the process of facilitated diffusion, the direction of movement is determined by the concentration gradient of the substance, with the substance moving from a high concentration to low.
Concentration gradient
The directional movement of a substance across the cell membranes is generally determined by its relative concentration on either side of the membrane.
The substance will move from where it is in a higher concentration down its concentration gradient to where it is in a lower concentration and the steeper the difference in the concentration, the faster the substance will diffuse. The closer the gradient comes to its equilibrium the slower the rate of diffusion will be.
However the exception to this is when a substance is moved with an expenditure of energy, by active transport.
In this case the substance is moved against its concentration gradient (from a lower to higher concentration). The substance may move either in vesicles, by endocytosis, or via a membrane transport protein pump using energy from ATP.
Electrochemical gradient
This type of active transport where your using the energy that was stored up through another form of active transport is called secondary active transport
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