FACILITATED DIFFUSION
● As pointed out earlier, a `color{violet}"gradient"` must already be present for `color{brown}"diffusion"` to occur.
● The `color{violet}"diffusion rate"` depends on the `color{violet}"size of the substances"`; obviously `color{violet}"smaller substances"` diffuse faster.
● The diffusion of any substance across a membrane also depends on its `color{violet}"solubility in lipids"`, the major constituent of the `color{violet}"membran"`e.
● Substances `color{violet}"soluble in lipids"` diffuse through the membrane`color{violet}"faster"`.
● Substances that have a `color{violet}"hydrophilic moiety"`, find it difficult to pass through the membrane; their movement has to be `color{violet}"facilitated"`.
● `color{Brown}"Membrane proteins"` provide sites at which such molecules `color{violet}"cross the membrane"`.
● They do not set up a `color{violet}"concentration gradient"`: a concentration gradient must already be present for molecules to diffuse even if facilitated by the proteins.
● This process is called `color{Brown}"facilitated diffusion"`.'
● In facilitated diffusion `color{violet}"special proteins"` help move substances across membranes `color{violet}"without expenditure"` of ATP energy.
● Facilitated diffusion cannot cause `color{violet}"net transport"` of molecules from a `color{violet}"low to a high concentration"` – this would require `color{violet}"input of energy"`.
● `color{violet}"Transport rate"` reaches a maximum when all of the `color{brown}"protein transporters"` are being used (`color{brown}"saturation"`).
● Facilitated diffusion is `color{violet}"very specific"`: it allows cell to `color{violet}"select substances"` for uptake.
● It is `color{Brown}"sensitive to inhibitors"` which react with `color{violet}"protein side chains"`.
● The proteins form `color{brown}"channels"` in the membrane for `color{violet}"molecules"` to pass through.
● Some channels are `color{violet}"always open"`; others can be `color{violet}"controlled"`.
● Some are `color{violet}"large"`, allowing a `color{violet}"variety of molecules"` to cross.
● The `color{Brown}"porins"` are proteins that form `color{violet}"huge pores"` in the outer membranes of the `color{violet}"plastids"`, `color{violet}"mitochondria"` and some `color{violet}"bacteria"` allowing molecules up to the size of small proteins to pass through.
● Figure shows an `color{violet}"extracellular molecule"` bound to the `color{violet}"transport protein"`; the transport protein then rotates and releases the molecule inside the cell, e.g., `color{Brown}"water channels"` – made up of eight different types of`color{violet}" aquaporins"`.
`star` `color{green}"Passive symports"` `color{green}"and antiports"`
● Some `color{violet}"carrier or transport proteins"` allow diffusion only if `color{violet}"two types of molecules"` move together.
● In a `color{brown}"symport"`, both molecules cross the membrane in the `color{violet}"same direction"`; in an `color{brown}"antiport"`, they move in `color{violet}"opposite directions"`.
● When a molecule moves across a membrane `color{violet}"independent of other"` molecules, the process is called `color{brown}"uniport."`
● The `color{violet}"diffusion rate"` depends on the `color{violet}"size of the substances"`; obviously `color{violet}"smaller substances"` diffuse faster.
● The diffusion of any substance across a membrane also depends on its `color{violet}"solubility in lipids"`, the major constituent of the `color{violet}"membran"`e.
● Substances `color{violet}"soluble in lipids"` diffuse through the membrane`color{violet}"faster"`.
● Substances that have a `color{violet}"hydrophilic moiety"`, find it difficult to pass through the membrane; their movement has to be `color{violet}"facilitated"`.
● `color{Brown}"Membrane proteins"` provide sites at which such molecules `color{violet}"cross the membrane"`.
● They do not set up a `color{violet}"concentration gradient"`: a concentration gradient must already be present for molecules to diffuse even if facilitated by the proteins.
● This process is called `color{Brown}"facilitated diffusion"`.'
● In facilitated diffusion `color{violet}"special proteins"` help move substances across membranes `color{violet}"without expenditure"` of ATP energy.
● Facilitated diffusion cannot cause `color{violet}"net transport"` of molecules from a `color{violet}"low to a high concentration"` – this would require `color{violet}"input of energy"`.
● `color{violet}"Transport rate"` reaches a maximum when all of the `color{brown}"protein transporters"` are being used (`color{brown}"saturation"`).
● Facilitated diffusion is `color{violet}"very specific"`: it allows cell to `color{violet}"select substances"` for uptake.
● It is `color{Brown}"sensitive to inhibitors"` which react with `color{violet}"protein side chains"`.
● The proteins form `color{brown}"channels"` in the membrane for `color{violet}"molecules"` to pass through.
● Some channels are `color{violet}"always open"`; others can be `color{violet}"controlled"`.
● Some are `color{violet}"large"`, allowing a `color{violet}"variety of molecules"` to cross.
● The `color{Brown}"porins"` are proteins that form `color{violet}"huge pores"` in the outer membranes of the `color{violet}"plastids"`, `color{violet}"mitochondria"` and some `color{violet}"bacteria"` allowing molecules up to the size of small proteins to pass through.
● Figure shows an `color{violet}"extracellular molecule"` bound to the `color{violet}"transport protein"`; the transport protein then rotates and releases the molecule inside the cell, e.g., `color{Brown}"water channels"` – made up of eight different types of`color{violet}" aquaporins"`.
`star` `color{green}"Passive symports"` `color{green}"and antiports"`
● Some `color{violet}"carrier or transport proteins"` allow diffusion only if `color{violet}"two types of molecules"` move together.
● In a `color{brown}"symport"`, both molecules cross the membrane in the `color{violet}"same direction"`; in an `color{brown}"antiport"`, they move in `color{violet}"opposite directions"`.
● When a molecule moves across a membrane `color{violet}"independent of other"` molecules, the process is called `color{brown}"uniport."`
