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Transport Across the Cell Membrane

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• ALL cells possess a cell membrane (mb)(~8 nm thick).• Membranes function to control the passage of

materials into/out of the cell (act like “gatekeepers”).• Membranes are described as being selectively

permeable.-- a mb that is completely permeable will allow

anything and everything to pass through it.-- a mb that is impermeable will allow nothing to

pass through it.• Therefore, a selectively permeable membrane will only

allow certain molecules across.• The ‘selection’ criteria may be based on size, polarity,

ATP (energy) availability or a combination of any of these factors.

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IMPERMEABLE:

OIL SLICKERS

COMPLETELY PERMEABLE

MAPLE LEAFS’ GOALTENDING

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• Other mb functions include:

-- separating the cytoplasm from the Extracellular Fluid (ECF), in other words, ‘housing’ the organelles.

-- communication with other cells.

-- identifying the cell.

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General Structure and Function of the Cell Membrane

• The primary structure of all membranes is a PHOSPHOLIPID BILAYER with proteins embedded either partially (peripheral proteins – more for support purposes) or completely through it (integral proteins).

• The anchoring of integral proteins is discussed later; peripheral proteins occur either on the outside or inside of the mb anchored either by covalent bonds, or by weaker, intermolecular interactions.

• Carbohydrates are evident as well, associating with either the phospholipids (forming glycolipids) or with the proteins (forming glycoproteins). These carbs only associate on the outer edge of the mb. *more later…

• Cytoskeletal filaments associate with the inner portion of the mb in order to anchor it.

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Fluid-Mosaic Model of a Cell Membrane (fig 4.1 p.68)

• Phospholipids serve as the ‘grout’ or fluid portion of the membrane (membranes possess the consistency of a light olive oil; ie. The ‘grout’ does not ‘harden’, or else long nerve cells would crack as you moved!).

• Proteins/glycoproteins serve as the mosaic, but can (and do) 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 mb and serves to lend stability to the phospholipid bilayer when the temperature rises.

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• Conversely, cholesterol prevents a decrease in mb fluidity when temperatures decrease.

• Ie. Cholesterol does not let the cell mb become too fluid at higher temperatures and also does not let the mb become too rigid at lower temperatures.

• A structural buffer???• As well, cholesterol acts as a physical barrier in

the mb, disallowing 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’).

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Fluid-Mosaic Model Picture

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Phospholipid Bilayer Structure

• Each phospholipid molecule in the mb has a POLAR (charged or hydrophilic) head and two NON-POLAR (uncharged or hydrophobic) tails (see fig. 2.27 p.35).

• Hydro-philic = Water-loving• Hydro-phobic = Water-hating• Phospholipid molecules are referred to as

being amphipathic in that they are part polar, and part non-polar.

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• Cells are surrounded by water (ECF) and are filled with water (cytoplasm), so the heads face outward and inward.

• The non-polar tails bury (hide) together to stay as much away from water as they can (hence, the double layer).

• Very little water (if any) in between the layers.• This bilayer arrangement is spontaneous (requires no

energy)

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• Another simple diagram:

• Micelles: form when a ‘handful’ of phospholipid molecules are placed in water.

• Micelles are made artificially.

Very little water

ECF

CYTO.

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• Hydrogen bonds between the polar heads of the phospholipids and water on either side of the mb help to keep the bilayer intact.

• So do the hydrophobic interactions (London Forces) that exist between the tails of the phospholipids.

• Phospholipids are able to move or shift in the bilayer since the individual molecules are not bound to each other. However, they can only shift in one layer, they very rarely ‘flip’ over to the opposite layer. Why not?

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Transporting of Substances Across Mb

• Due to the highly hydrophobic (non-polar) inner core of the phospholipid bilayer, only certain molecules/substances are able to move in/out of a cell without aid of some kind.

• What, therefore, is able to move across a mb on its own?– Water! Follows a concentration gradient (moves from higher to lower

water concentrations (ie. lower to higher solute concentrations))…the diffusion of water is known as osmosis. In general, the strength of the ‘flow’ (bulk flow) of water pushes it through the hydrophobic core undeterred.

– Small, non-polar molecules:

• -- relatively small hydrocarbons.

• -- oxygen (O2)

• -- carbon dioxide (CO2)– One larger, non-polar molecule-type fatty acids.

– These molecules simply diffuse across the phospholipid bilayer.

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• What molecules/substances CANNOT move across a mb on its own (and why not)?

-- Ions/salts (Na+, K+, Ca2+, Cl-, etc.) – these are charged, and therefore polar ions requiring channel/carrier proteins (more on these later). As well, they tend to be surrounded by water molecules, making them seem bigger than they really are, allowing the hydrophobic core of the mb to repel them more easily.

-- Larger polar molecules (glucose, amino acids, glycerol etc) – require carrier proteins as they are too large for channel proteins.

-- Macromolecules (Carbohydrates, Proteins, Fats) – they are simply too big, but can enter through vesicle formation (endocytosis) or exit through vesicle formation (exocytosis) (more on these later too).

- SEE FIG. 4.4 p. 72

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