Introduction

There are many ways substances move in and out of cells across the membrane. First we'll start with simple and facilitated diffusion.

The process of diffusion

Diffusion is the net movement of particles (molecules or ions) from an area of high concentration to an area of lower concentration. Molecules will move both ways, but the net movement will be to the area of lower concentration. This continues until particles are evenly distributed throughout the liquid or gas. The concentration gradient is the path from an area of higher concentration to an area of lower concentration. Particles diffuse down a concentration gradient.

Diffusion is a passive process - no energy is needed for it to take place. Particles can diffuse across cell membranes as long as they can move freely through the membrane. When molecules diffuse directly through the cell membrane, it is known as simple diffusion. For example, oxygen and carbon dioxide can diffuse easily through cell membranes because they are very small, so they can pass through spaces between the phospholipid bilayer.

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Factors affecting the rate of diffusion

The rate of diffusion across both external and internal cell membranes can vary. Some specialised cells are adapted for rapid transport across their membrane. The rate of diffusion depends on:

The concentration gradient - the higher it is the faster the rate of diffusion. As diffusion takes place, the difference in concentration between the two sides of the membrane decreases until it reaches an equilibrium (the concentration is equal on both sides). This means diffusion slows down over time.
The thickness of the exchange surface - the thinner the exchange surface (i.e. the shorter the distance the particles have to travel), the faster the rate of diffusion.
The surface area - the larger the surface area (e.g. of the cell surface membrane), the faster the rate of diffusion.

As an example, some specialised cells (e.g. epithelial cells in the small intestine) have microvilli - projections formed by the cell surface membrane folding up on itself. Microvilli give the cell a huge surface area - in human cells microvilli can increase the surface area by 600 times.

Facilitated diffusion

Some larger molecules (e.g. amino acids, glucose) would diffuse extremely slowly through the phospholipid bilayer because they are so big. Charged particles, e.g. ions and polar molecules, would also diffuse slowly - that is because they are water soluble, and the centre of the bilayer is hydrophobic. So to speed things up, large or charged particles diffuse through carrier proteins or channel proteins in the cell membrane instead - this is called facilitated diffusion.

Like diffusion, facilitated diffusion moves particles down a concentration gradient, from a higher to a lower concentration. It is a passive process - it does not use energy. There are two types of proteins involved - carrier and channel proteins.

Carrier proteins

Carrier proteins move large molecules across the membrane, down their concentration gradient. Different carrier proteins facilitate the diffusion of different molecules. Here is how they work:

First, a large molecule attaches to a carrier protein in the membrane.
Then the protein changes shape.
This releases the molecule on the other side.

Channel proteins

Channel proteins form pores in the membrane for charged particles to diffuse through (down their concentration gradient). Different channel proteins facilitate the diffusion of different charged particles.

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Factor affecting the rate of facilitated diffusion

The rate of facilitated diffusion depends on:

The concentration gradient - the higher the concentration gradient, the faster the rate of facilitated diffusion (up to a point). As equilibrium is reached, the rate of facilitated diffusion will level off.
The number of channel or carrier proteins - once all the proteins in a membrane are in use, facilitated diffusion cannot happen any faster, even if the concentration gradient is increased.

A good example of a carrier protein are used is the GLUT 1 carrier protein that is used to facilitate the diffusion of glucose into red blood cells.

A good example of a channel protein are the aquaporins that assist in the facilitation of water through cell membranes. Some kidney cells are adapted to have many of these.

Osmosis

Osmosis is a special case of diffusion for water molecules.

What is osmosis?

Osmosis is the diffusion of water molecules across a partially permeable membrane, from an area of high water potential (i.e. high concentration of water molecules) to an area of low water potential (i.e. lower concentration of water molecules). Water potential is the potential (likelihood) of water molecules to diffuse out of or into a solution.

Water molecules are small and can diffuse easily through the cell membrane, but large solute molecules cannot.

Pure water has the water potential of zero. Adding solutes to pure water lowers its water potential - so the water potential of any solution is always negative. The more negative the water potential, the stronger the concentration of the solutes in the solution.

If two solutions have the same water potential they are said to be isotonic. Cells in an isotonic solution will not lose or gain any water - there is no net movement because they are there is no difference in water potential.

If a cell is placed in a solution that has a higher water potential it will swell as water moves into it by osmosis. Solutions with a higher water potential compared with the inside of the cell are called hypotonic. If a cell is placed in a solution that has a lower water potential it may shrink as water moves out of it by osmosis. Solutions with a lower water potential than the cell are called hypertonic.

Factor affecting the rate of osmosis

The factors affecting the rate of osmosis are similar to those affecting the rate of diffusion:

The water potential gradient - the higher the water potential gradient, the faster the rate of osmosis. As osmosis takes place, the difference in water potential on either side of the membrane decreases, so the rate of osmosis levels off over time.
The thickness of the exchange surface - the thinner the exchange surface, the faster the rate of osmosis.
The surface area of the exchange surface - the larger the surface area, the faster the rate of osmosis,

Active transport

Another method of transport across a cell membrane, this time using energy (hence - active transport).

Active transport - the basics

Active transport uses energy to move molecules and ions across a plasma membrane, usually against a concentration gradient. Carrier proteins and co-transporters are involved in active transport.

Carrier proteins

The process is pretty similar to facilitated diffusion - a molecule attaches to the carrier protein, the protein changes shape and this moves the molecule across the membrane, releasing it on the other side. There are two main differences between active transport and facilitated diffusion:

1. Active transport usually moves solutes from a low to a high concentration - in facilitated diffusion they always move from a high to a low concentration.

2. Active transport requires energy - facilitated does not.

ATP ( a molecule produced by respiration) is a common source of energy in the cell, so it is important for active transport. ATP undergoes a hydrolysis reaction, splitting into ADP and P_i(inorganic phosphate). This releases energy so the solutes can be transported.

Co-transporters

Co-transporters are a type of carrier protein. They bind two molecules at a time. The concentration gradient of one of the molecules is used to move the other molecule against its concentration gradient.

For example, in the co-transport of sodium ions and glucose, sodium ions move across the membrane down their concentration gradient. This moves glucose across the membrane too, against its concentration gradient.

Factor affecting the rate of active transport

When active transport moves molecules and ions against their concentration gradient, a decreasing concentration gradient does not affect the rate of active transport. The rate of active transport is affected by:

The speed of individual carrier proteins - the faster they work, the faster the rate of active transport.
The number of carrier proteins present - the more proteins there are, the faster the rate of active transport.
The rate of respiration in the cell and the availability of APT. If respiration is inhibited, active transport cannot take place.

Now watch this Amoeba Sisters video on active transport.