Description Example
Simple Diffusion The movement of molecules along the concentration gradient (no energy required) across a partially permeable membrane 1)      Alveoli

2)      Gut

3)      Leaves (movement of CO2)

Facilitated diffusion Transport of substances across a membrane by trans-membrane proteins 1)      Absorbing of glucose

2)      Movement of Ions

Active Transport Movement of molecules against the concentration gradient requiring a protein pump (Required energy) 1)      Root hair cells (absorbing minerals from the soil)
Osmosis The movement of water across a selective membrane towards an area of high water potential (positive area) to an area of low water potential (negative area) 1)      Roots (absorbing water from the ground)





  • Water is a polar molecule and therefore many molecules can dissolve in it. These dissolved substances are called solutes
  • Water diffuses by Osmosis from a region of high Water Potential to a region of low Water Potential through the Water Potential Gradient.
  • Hypertonic – More Concentrated solute
  • Isotonic – Equal Concentration
  • Hypotonic – Less concentration solute
  • Osmosis is the diffusion of water across a cell membrane
  • Cell contains solutions of different solutes
  • Water molecules can diffuses freely across the phospholipid bilayer, but always down the concentration gradient
  • Water therefore diffuses from a dilute to a concentrated solution


Active Transport


  • Active transport is the pumping of substances across a membrane by a trans-membrane protein pump molecule
  • The protein bonds to a molecule of the substance to be transported on one side of the membrane, change shape and released onto the opposite side.
  • The protein pumps are specific so there is a different protein pump for each molecule to be transported


Transport through the membrane

The cell membrane is primarily made up of phospholipids bilayers with various channels to allow molecules into the cells through integral proteins.

Simple Diffusion

Simple Diffusion is the random movement of particles from an area of high concentration to an area of low concentration.

The movement of gas is random as it moves away from its source using its own kinetic energy they travel across an area until an equal amount is found throughout the area.

Facilitated diffusion

Facilitated diffusion is the transport of substances across a membrane by a transmembrane protein molecule.

Transport proteins tend to be specific to one molecule that can cross a membrane if it contains the appropriate protein, this is a passive diffusion process as it uses the kinetic energy of the molecules to change the shape of the transmembrane protein.

The molecules can only move along their concentration gradient

Protein Channels

They form water filled hydrophilic channels across the membrane. They allow specific water soluble ions to pass through. The channels are selective, only opening to specific ions . They are intrinsic proteins, so span across the whole membrane

The channel that the channel proteins make, are full of water. This means only water soluble substances can pass through.

Facilitated diffusion happens here. Some channels are also gated and/or selective. Gated means it opens only when appropriately stimulated.

Carrier Proteins

Spanning across the plasma membrane, when a molecules, such as glucose, is specific to the protein that is present it binds to the protein.  This Changes the shape which allows the molecule to be released to the inside of the membrane.

No extra energy is required as it follows the concentration gradient, using only the kinetic energy of the molecules.

Na+ K+ Pump


  • This transported protein is present in the cell membrane of all animal cells and is the most abundant and important of all the membrane pumps.
  • They are found in neurons as they require a positive (+) charge, so there it pumps Sodium (Na) out of the cell and K+
  • 3 Naions are removed, to 2K+ which gives the internal cell a +1 charge and the outside of the cell a relative -1 charge




  • Protein pumps are also ATPase as they split ATP into ADP + phosphate
  • The energy released in them is then used to charge the shape of the pump molecule
  • Therefore pumping is an active process and is only transported mechanism that can transport substances against (up) the concentration gradient


Water Potential


  • Osmosis can be quantified using water potential (Ψ) so therefore you can calculate which way water molecules will move and how quickly
  • Water potential is a measure of the water molecules potential for movement in a solution
  • Water potential is measured in Pascal
  • Water always moves by osmosis from less negative to more negative water potential
  • 100% pure water has a water potential (Ψ) of 0. This is the highest water potential so all solutions have Ψ less than 0. No water potential can be greater than 0.


Osmosis in Cells


  • Water molecules diffuses into a cell because there if a less/lower water potential of water molecules within the cell
  • Different biological molecules have different solutes in them, whether they be salts or sugars, therefore they have lower water potentials than that is within the cell
  • In animal cells, as they do not contain a cell wall if the water potential is higher on the outside than inside the cell, they will expand until they burst open; becoming Haemolysed. If the water potential is greater on the inside then the water will diffuse out, thus the cell will look wrinkled and become crenated.
  • In plant cells, as they do contain a cell wall if the water potential is higher on the outside than inside of the cell, it will expand and become turgid. If the water potential is greater on the inside then the water will diffuse out, thus the cytoplasm will pull away from the cell wall. The cell will become Plasmolysed.


Factors that affect the rate of diffusion:


  • Surface Area -temperature
  • Length of Diffusion pathway
  • Concentration of molecules


Simple vs Facilitated Diffusion

The key differences between Simple and Facilitated Diffusion is diffusion in a general term describes the movement of any particle from an area of high concentration to low concentration. Facilitated diffusion is more specific to the movement of materials in and out through the cell membrane. The use of protein channels and carrier proteins allow for only specific molecules into the cell. Facilitated diffusion is a passive process as no additional energy is required, rather is uses the kinetic energy of the molecule to travel into the cell

Movement of Glucose

Glucose is transported into cells by pores in the proteins that span across a cell phospholipid bilayer. This is due to them not being a soluble lipid.