Movement of substances into and out of cells

The fluid mosaic model allows an understanding of how substances enter and exit cells by a variety of different mechanisms. Investigating the effect of increasing the size of model cells allows an understanding of the constraints of obtaining resources across the cell surface and moving substances out of cells.

1. Describe and explain the processes of diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis

a) Diffusion

• Diffusion is the net movement, as a result of random motion of molecules or ions from a region of higher concentration to a region of lower concentration (down the concentration gradient)
• The random movement is caused by the natural kinetic energy of the molecules or the ions
• As a result of diffusion, molecules or ions tend to reach an equilibrium situation where they are evenly spread within a given volume of space

b) Facilitated diffusion

• The diffusion of a substance through transport proteins in a cell membrane down the concentration gradient
• Large polar molecules such as glucose and ions like Na+ cannot diffuse through the phospholipid bilayer
• They can only pass through the membrane with the help of transport proteins such as channel proteins and carrier proteins that provide a hydrophilic passageway for them 
• The transport proteins are highly specific as they only allow one type of molecule or ion to pass through it
• Channel proteins are water-filled pores that allow charged substances (usually ions) to diffuse through the membrane
• Most channel proteins are 'gated' which means that part of the protein molecule on the inside of the cell surface membrane can close or open to control the exchange of ions
• Channel proteins have a fixed shape
• However, carrier proteins can flip between two shapes
• As a result, the binding site is alternately open to one side of the membrane then the other

If the molecules are diffusing across a membrane, then the direction of movement will depend on their relative concentration on each side of the membrane. However, the rate at which this diffusion occurs depends on the number of transport proteins in the membrane and in the case of channel proteins, whether they are open or not.

c) Osmosis

• Osmosis is the net diffusion of water molecules from a region of high water potential to a region of low water potential through a partial permeable membrane
• It involves water molecules only
• Because there is a partial permeable membrane present, the solute molecules are too large to pass through the membrane
• Therefore, only water molecules can pass through from a dilute solution to a concentrated solution
• The number of solute molecules stay the same on each side of the membrane
• Over time, the water molecues will tend to spread themselves evenly over the two sides
• 

d) Active transport

• Active transport is the movement of molecules or ions through a carrier protein from a region of low concentration to a region of high concentration (against the concentration gradient) which requires energy
• Active transport requires energy because the ions or molecules move against the concentration gradient
• The energy…