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Depends on the discriminating barrier of the phospholipid bilayer and the nature of specific transport proteins.

The phospholipid bilayer is permeable to:

1. hydrophobic molecules. Nonpolar hydrocarbon molecules readily cross the bilayer.
2. Very small, uncharged polar molecules also readily cross the bilayer: urea, glycerol, water, as do gases such as O₂ and CO₂.

The phospholipid bilayer is impermeable to larger uncharged polar molecules such as glucose or to ions such as H⁺ or Na⁺.

For such substances, specific transport molecules are needed.

No energy required.

Diffusion of a substance from a region of higher concentration to one of lower concentration without the help of any transport protein.

Osmosis, the movement of water into cells, is an example of passive diffusion: the diffusion of water across a membrane from a region of higher concentration (dilute solution) to one of lower concentration (concentrated solution).

Facilitated Diffusion (carrier-mediated diffusion)

The uptake of molecules, typically polar molecules or ions, by specific transport proteins called carriers or channels. These proteins act by carrying or channeling (facilitating the transport of) substances from regions of higher concentration to regions of lower concentration. (Figure 5.9)

There are 2 classes of membrane transport facilitated by carrier proteins:

1. Uniport: transport of a single solute
2. Coupled transport: 2 (or more) solutes are transported.

The 2 kinds of coupled transport are

1. Symport: 2 coupled soluted are transported in the same direction. Example: Na⁺/glucose symport
2. Antiport: 2 substances are transported in opposite directions. Example: RBC HCO₃^-/Cl^- transporter protein

Facilitated diffusion systems can become saturated if [solute] is very high.

The transport of substances from a region of lower concentration to a region of higher concentration.

Active transport requires energy

2 kinds:

1. Primary Active Transport: ATP-dependent. Example: the sodium-potassium pump (Figure 5.12), an integral membrane glycoprotein found in all animal cells. Each ATP a ADP + P_i"buys" 3 Na⁺ out for 2 K⁺ in. In animal cells: [Na⁺]_in = 10 mM; [K⁺]_in = 140 mM [Na⁺]_out= 145 mM; [K⁺]_out = 5 mM ([Cl^-]_in = 5 mM; [Cl^-]_out = 110 mM). To maintain these gradients, the cell pumps sodium ions out and potassium ions in, using ATP energy. The Na⁺/ K⁺ pump uses about 1/3^rd of the animal cell’s energy output. This Na⁺/ K⁺ pump (Na⁺/ K⁺ ATPase) is also an electrogenic pump: It generates a voltage across the membrane: +_out; -_in. All cells have a membrane potential of –50 to –200 millivolts (all cells are negative inside, compared to outside).
2. Secondary Active Transport: does not use ATP directly (Figure 5.13). Instead the energy represented by higher concentration of a substance (such as the ions Na⁺ or H⁺) outside the cell is coupled to the transport of another substance (e.g., glucose) into the cell against its concentration gradient. Note that the concentration gradient of an ion across a membrane represents an electrochemical gradient that is an energized condition. Secondary active transport is a form of co-transport that takes advantage of such electrochemical gradients.