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Transport across cell membranes

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Plasma membranes

Armed with this knowledge of lipids, as well as carbohydrates and proteins, we can now explore the structure of plasma membranes, specifically in the context of the fluid-mosaic model. Phospholipids have a hydrophilic (water loving) head, and hydrophobic (water repelling) tails. This results in the formation of a phospholipid bilayer (double layer), which forms the basis for the plasma membrane.


The name of fluid-mosaic model comes from:

Fluid = the arrangement of proteins contained in the membrane is always changing
Mosaic = the proteins present are spread around in a mosaic-like fashion.



It's pretty isn't it? The proteins are crucial to cell communication as well as the selective permeability of the membrane. The glycoproteins' (sugars/carbohydrates attached to a protein) side chains act as receptorsLipid soluble stuff such as vitamins A, D and K, as well as oxygen and carbon dioxide, can pass freely though the membrane. Cholesterol can be part of the membrane to restrict the movement of other components.

It's important to understand the role of microvilli. These are elongations of plasma membrane which increase the surface area available for reaction or absorption.


Diffusion

Diffusion = the spread of particles from a region of higher concentration to a region of lower concentration, until the particles are evenly spread out.

Diffusion takes place when you use a spray in a room, for example. The particles in the spray move randomly, knocking each other, which results in them spreading throughout the room gradually, from high concentration to low concentration. Therefore, diffusion acts down (or along) a concentration gradient.

It is important to know what affects the rate of diffusion. These are:

1. Surface area - the greater the surface area, the faster diffusion will occur
2. Difference in concentration - the higher the difference (the steeper the gradient), the faster diffusion will take place
3. The thickness of the exchange surface - the thicker the exchange surface, the slower the rate of diffusion.

Of course there are other factors such as temperature (increased kinetic energy results in faster diffusion) and the diffusion pathway (distance). The latter is a side effect of (3.) The thickness of the exchange surface, in some respects.

In some cases, diffusion is aided by certain proteins. This is called facilitated diffusion. The responsible proteins speed up diffusion of substances which would otherwise take longer to pass through the plasma membrane. The key points about facilitated diffusion which differentiate it from active transport (which also uses proteins):

-it occurs down a concentration gradient
-it uses no metabolic energy

Two kinds of protein achieve facilitated diffusion: carrier proteins and ion channels. Carrier proteins transport substances from one side of the membrane to the other, usually by co-transport. For example, glucose is transported along with an Na+ ion.

Ion channels are proteins with gates that can be open or closed to allow or stop certain ions from entering, e.g. Na+ (sodium) and K+ (potassium) ions.



Osmosis

Osmosis is the diffusion of water across a semi-permeable membrane. The "concentration" of water is referred to as water potential. So osmosis is the movement of water from a higher water potential to a lower water potential across a membrane.



For osmosis to occur it is essential that there is a semi-permeable membrane separating two environments with a different solute concentration. The solute must be unable to cross the membrane (molecules too big), but the water molecules are free to pass through and lead to an equilibrium. In the above image, the right side of the beaker has a higher water potential than the left side, so water moves in from right to left.

You must also learn the term isotonic. A solution is isotonic when it has the same water potential as another solution. An example is Ringer's solution which has the same water potential as blood plasma, so can be used to keep tissues alive.


Active transport

Unlike diffusion, osmosis and facilitated diffusion, active transport requires energy in the form of ATP (adenosine triphosphate), and moves substances against a concentration gradient (from a lower concentration to a higher concentration). This process is essential in removing of all toxins from the body, as well as the movement of rare chemicals.

Active transport is achieved by specific carrier proteins in the plasma membrane, and relies on adequate oxygen supply (which results in ATP being available). Here's a quick video that shows the process:


There are certain cells which carry our active transport more than others, for example in the kidney. These cells have special adaptations, such as microvilli for increased surface area, hence more carrier proteins available, as well as many mitochondria for the production of ATP.


Co-transport

When carbohydrates from food are digested, lots of glucose molecules, along with some fructose and galactose, are produced in the small intestine (specifically the ileum). Initially the mode of transport of glucose in absorption is diffusion. It is then followed by facilitated diffusion via co-transport, where glucose molecules are absorbed along with a sodium ion (Na+) through a carrier protein. This method is much faster than diffusion by itself.


Both Na+ and glucose must be present for absorption by co-transport to take place. 

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