AS Biology Edexcel - how do substances pass through cell membranes?

Diffusion (high --> low)

The net movement of molecules or ions from a region of high concentration to a region of low concentration. 

Diffusion will continue until equilibrium (when the substance is evenly spread throughout the whole volume)

Small uncharged particles diffuse across a cell membrane passing between the phospholipid bilayer as they move down the concentration gradient:

• Oxygen
• Carbon dioxide (although it is polar, its small size allows rapid diffusion)

Diffusion, whether facilitated or not, is often referred to as 'passive transport' because no metabolic energy is needed for the transport

Hydrophilic (polar - charged) molecules that are larger than carbon dioxide cannot simply diffuse through the phospholipid bilayer. They are insoluble in lipids (due to the hydrophobic tails) so instead they move across the membrane with the aid of proteins in a process called facilitated diffusion...

• They may diffuse through water-filled pores within channel proteins that span the membrane. There are different channel proteins for transporting different molecules, and each type of channel protein has a specific shape that permits the passage of only one particular ion or molecule. Some channels can be opened or closed by the presence or absence of a hormone or a change in potential difference (voltage) across the membrane - these channels are called gated channels
• Carrier Proteins - the ion or molecule binds onto a specific site on the protein, the protein changes shape and as a result the ion or molecule crosses the membrane. The movement can occur in either direction. 

Osmosis is the net movement of water molecules from a solution with a lower concentration of solute to a solution with a higher concentration of solute through a partially permeable membrane.

Osmosis is due to the random movement of water molecules across the membrane and is a particular type of diffusion.

If solute molecules are present, water molecules form hydrogen bonds with them and this reduces the movement of these water molecules. If a solute is present there are fewer water molecules free to collide with and move across the membrane

Energy is required if substances need to be moved against the concentration gradient (from low --> high concentration). Specific carrier proteins are also needed.

The energy comes from respiration and is supplied by the energy transfer molecule ATP 

• The substance to be transported binds to the carrier protein
• Energy from ATP changes the shape of the carrier protein
• The substance is then released at the other side of the membrane

Active transport occurs in every cell (e.g. transport of ions across epithelial cells, plant cell roots, muscle cells and nerve cells). It also occurs between the mitochondria and cytoplasm.

Exocytosis

Used when VERY LARGE molecules need to be transported across cell surface membranes

Exocytosis - the release of subtances, usually proteins or polysaccharides, from the cell as vesicles (small membrane-bound sacs) fuse with the cell membrane E.g. Insulin is released into the blood by exocytosis

Endocytosis

Substances are taken into the cell by the creation of a vesicle (membrane-bound sac). Part of the cell membrane engulfs the solid or liquid material to be transported.

(In some cases, the substance to be absorbed attaches to a receptor in the membrane and is then absorbed by endocytosis.

This is how cholesterol is taken up into cells. White blood cells ingest bacteria by endocytosis.