Cell Transport Mechanisms


Cells exchange materials in order to survive. Different materials are transported in different ways.

The Fluid Mosaic Model

Specification Reference

"Know the structure of the cell surface membrane with reference to the fluid mosaic model."
All cells have a membrane. This regulates the movement of substances into and out of the cell. The membrane of all cells can be described as fluid and molecules such as proteins and cholesterol are free to move around.

These molecules combined with a phospholipid bilayer produce the cell membrane as we know it and this model is referred to as the Fluid Mosaic Model.

Passive Transport

Specification Reference

"Understand how passive transport is brought about by: diffusion, facilitated diffusion (through carrier proteins and protein channels) and osmosis."
Understand how the properties of molecules affects how they are transported, including solubility, size and charge.
The cell surface membrane is partially permeable. This allows materials to pass into and out of the cells. Increasing the temperature and the concentration of unsaturated fats increases fluidity and therefore allows substances to pass more easily.
  • Passive transport describes the movement of particles from an area of high concentration to an area of low concentration down a concentration gradient. No energy is required.
Diffusion
  • The movement of particles from an area of high concentration to an area of low concentration down a concentration gradient.
  • For example, oxygen and carbon dioxide move into and out of cells via diffusion.
  • Factors such as surface area, temperature, concentration and membrane thickness all affect the rate of diffusion.
Facilitated Diffusion
  • The movement of particles down a concentration gradient through either a carrier protein or a channel protein.
  • For example, ions such as Na+ and molecules such as glucose are charged. These particles cannot move via diffusion since the cell membrane would repel them. Instead they travel through channel or carrier proteins.
Osmosis
  • This is the movement of free water molecules from an area of high concentration to an area of low concentration.
  • This happens across a partially permeable membrane.
A solution of cells is always in one of the following states:

  1. Hypertonic - the solute concentration is high outside of the cell. This causes water to move out of the cell resulting in the cell shrinking.
  2. Isotonic - concentrations of solute are equal.
  3. Hypotonic - the concentration of solute is high inside the cell, this causes water to move into the cell causing it to swell.

Water Potential

This explains the tendency of water to move by osmosis. Pure water has the highest osmotic potential of 0. The presence of solutes lowers the water potential.

  • water potential = turgor pressure + osmotic potential
  • Turgor pressure - a measure of the inward pressure exerted by a plant cell wall (hydrostatic pressure).
  • Osmotic potential - the ability of water to move across a partially permeable membrane, influenced by the level of dissolved solutes.
Water will always move to the area with the lowest water potential (the most negative area).

Endocytosis & Exocytosis

Specification Reference

"Know that large molecules can be transported into and out of cells through the formation of vesicles, in the processes of endocytosis and exocytosis."
Vesicles are small transport vessels that can move larger molecules through a cell membrane.
  • Endocytosis - molecules are brought into the cell, cell extensions known as pilli engulf material to form a vesicle which then enters the cytoplasm.
  • Exocytosis - molecules are released from a cell when a vesicle fuses with the cell membrane. Its contents are then released.

Active Transport

Specification Reference

"Understand the process of active transport, including the role of ATP."
This is the movement of molecules from an area of low concentration to an area of high concentration against a concentration gradient.
  • Requires energy (in the form of ATP)
  • Uses carrier proteins
  • ATP binds to the carrier protein
  • The carrier protein then changes shape, carrying the molecule into or out of the cell

Phosphorylation

Specification Reference

"Know that phosphorylation of ADP requires energy and that hydrolysis of ATP provides an accessible supply of energy for biological processes."
The phosphorylation of ADP to form ATP requires energy. Therefore the hydrolysis of ATP to form ADP + Pi releases energy. This energy is now available for use in active transport.