Transport Across the Cell Membrane

ALL cells possess a cell membrane (mb)(~8 nm thick). Membranes function to control the passage of materials into/out of the cell (ie. membranes act as ‘gatekeepers’). Membranes are described as being selectively permeable (sometimes known as semi-permeable). To contrast this: -- a membrane that is completely permeable will allow anything and everything to pass through it. -- a membrane that is impermeable will allow nothing to pass through it. Therefore, a selectively permeable membrane will only allow certain molecules across – molecules that fit a certain set of criteria. The selection criteria are size, polarity, and/or ATP energy availability.

Other membrane functions/characteristics include: -- separating the cytoplasm from the Extracellular Fluid (ECF), in other words, ‘housing’ the organelles. -- communicating with other cells (with help from carbohydrates). -- identifying the cell (also with help from carbohydrates).

General Structure and Function of the Cell Membrane The structure of membranes primarily consists of a PHOSPHOLIPID BILAYER with proteins embedded either partially (peripheral proteins) or completely through it (integral proteins). The anchoring of integral proteins is discussed later. Peripheral proteins exist either on the outer or inner part of the membrane anchored either by covalent bonds, or by weaker, intermolecular interactions (such as hydrogen bonds, dipole-dipole forces, or London forces). Carbohydrates are evident as well, associating either with the phospholipids (forming glycolipids) or with the proteins (forming glycoproteins). These carbs only associate on the outer edge of the membrane. Cytoskeletal filaments associate with the inner portion of the membrane in order to help support the fluid nature of the phospholipid bilayer.

Fluid-Mosaic Model (fig 4.1 p.68) – A Useful Metaphor Mosaic - A picture or design made from small pieces of colored tile, glass, or other material set in mortar/grout. Phospholipids serve as the ‘grout’ or fluid portion of the membrane (membranes possess the consistency of a light olive oil; ie. in the case of cell membranes, the ‘grout’ does not harden, or else long nerve cells would crack as you moved!). Proteins/glycoproteins serve as the mosaic (ie. the tile or glass), but are able to move around within the fluid bilayer (like wading through a pool of oil). See fig. 4.2 p. 69 Cholesterol is also present in the cell membrane and serves to lend stability to the phospholipid bilayer when the temperature rises.

Conversely, cholesterol prevents a decrease in membrane fluidity when temperatures decrease. Thus, cholesterol does not allow a cell membrane to become too fluid at higher temperatures or too rigid at lower temperatures. It could be loosely described as a ‘structural buffer.’ Additionally, cholesterol acts as a physical barrier in the membrane, helping to prevent larger molecules from crossing on their own. Glycolipids and Glycoproteins serve as identifiers and communicators for the cell (ie. A ‘driver’s licence’ or ‘fingerprint’).

Phospholipid Bilayer Structure Recall that a phospholipid molecule has a polar (hydrophilic) head and two non-polar (hydrophobic) tails (fig. 2.27 p.35). Hydrophilic = “Water-loving” Hydrophobic = “Water-fearing” Phospholipid molecules are referred to as being amphipathic in that they are partially polar, and partially non-polar.

Cells are surrounded by water (ECF) and are filled with water (cytoplasm), so the polar heads face outward and inward. The non-polar tails bury (‘hide’) together to stay as much away from water as they can (ie. they are repelled by water so they face away from water) – this accounts for the need for a bilayer. The bilayer itself forms spontaneously in that it requires no ATP energy to aid in its formation.

Another simple diagram: Micelles: form when a microscopic ‘handful’ of phospholipid molecules are placed in water. Micelles are made in the lab. ECF CYTO. Very little water

Hydrogen bonds between the polar heads of the phospholipids and water on either side of the membrane help to maintain the integrity of the bilayer. London Forces (a weaker type of intermolecular bond) exist between the tails of the phospholipids, further helping to maintain the integrity of the bilayer. Phospholipids are able to move or shift in the bilayer (without ‘flipping’…ie. they ‘wade’, but don’t ‘flip’). - They are unable to flip because a head would have to pass through the non-polar core of the bilayer. This, of course, would result in the head being repelled back to where it came from!

Transporting of Substances Across Membrane Due to the highly hydrophobic core of the phospholipid bilayer, only certain molecules/substances are able to move in/out of a cell without aid of some kind. What is able to move across a membrane on its own? Water! Follows its own concentration gradient (moves from higher to lower water concentrations (ie. lower to higher solute concentrations))…this movement of water is known as osmosis. In general, the bulk flow (the strength of the flow) of water pushes it through the hydrophobic core undeterred. Small, non-polar molecules: -- relatively small hydrocarbons. -- oxygen (O2) -- carbon dioxide (CO2) One larger (but ‘skinny’), non-polar molecule-type  fatty acids. These molecules simply diffuse across the phospholipid bilayer.

What molecules/substances CANNOT move across a membrane on its own (and why not)? -- Ions/salts (Na+, K+, Ca2+, Cl-, etc.) – these are charged, polar ions requiring channel/carrier proteins (more on these later). -- Larger polar molecules (glucose, amino acids, glycerol, DNA/RNA nucleotides, etc.) – require carrier proteins as they are too large for channel proteins. -- Macromolecules (Carbohydrates, Proteins, Fats) – they are simply too large, but can enter a cell through vesicle formation (endocytosis) or exit through vesicle formation (exocytosis) (more on these later). - SEE FIG. 4.4 p. 72