`star` Facilitated diffusion
`star` Active Transport


● 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."`


● `color{brown"Active transport"` uses `color{violet}"energy"` to pump molecules `color{violet}"against a concentration gradient"`.

● Active transport is carried out by `color{violet}"membrane-proteins"`.

● Hence `color{violet}"different proteins"` in the membrane play a major role in both `color{violet}"active as well as passive"` transport.

● `color{violet}"Pumps are proteins"` that use energy to `color{violet}"carry substances"` across the cell membrane.

● These `color{violet}"pumps can transport"` substances from a `color{violet}"low concentration"` to a `color{violet}"high concentration"` (`color{violet}"uphill transport"`).

● Transport rate reaches a `color{violet}"maximum"` when all the `color{violet}"protein transporters"` are being used or are `color{Brown}"saturated."`

● Like `color{violet}"enzymes"` the carrier protein is `color{violet}"very specific"` in what it carries across the membrane.

● These proteins are `color{violet}"sensitive to inhibitors"` that react with `color{violet}"protein side chains"`.