Diffusion is affected by temperature. Heat speeds up the process and cold slows down the process.
Diffusion across (in and out) of the cell membrane involves the movement of substances that pass through the membrane until a state of equilibrium is reached. Water, carbon dioxide and oxygen are a few of the simple molecules that can cross the membrane by diffusion or osmosis.
Diffusion is passive processes not requiring any energy.
An example of diffusion takes place in the lungs. Blood will continue to take in oxygen from the alveolar air spaces as long as there is more oxygen in the air spaces than in the blood. The oxygen diffuses across the alveolar walls in to the blood.
Diffusion of water across a semi permeable membrane is termed osmosis.
The diffusion of oxygen and carbon dioxide are passive transport and do not require energy.
Osmosis
Osmosis is a special type of diffusion involving water molecules passing through a semi permeable membrane, i.e. the cell membrane, separating solutions of different concentrations. The water passes through from a less concentrated solution to more concentrated solution until equilibrium is reached. If you apply external pressure to the more concentrated solution it will cease osmosis. Water potential is the tendency of water to move from an area of high concentration to one of a lower concentration. A presence of a solute decreases the water potential of a substance. Therefore, there is more water present in a glass of fresh water than in a glass of sea water.
Hypertonic or hyperosmotic solutions have more solute in them and have a lower water potential.
Isotonic or Isosmotic solutions have an equal amount of water and solute and therefore have an equal amount of water potential.
Hypotonic or Hypoosmotic solutions have less solute and a higher water potential.
On of the main functions of blood in animals is to maintain an isotonic (equal) internal environment. The cells vacuole does this by pumping water out of the cell. If this didn’t happen the cell would swell up and eventually burst. Ridding water out of the cell requires energy in the form of ATP as the water is moving against the concentration gradient.
Endosmosis is the movement of a liquid through a membrane from the outside to the inside, whereas exosmosis is the movement of a liquid from the inside to the outside.
Osmosis of water is a passive process and does not require any energy.
An example of osmosis takes place in the stomach and in the roots of plants.
Facilitated Diffusion
Facilitated diffusion is the same principle as diffusion with regards to substances diffusing down the concentrations gradient, from a high concentration to a lower concentration, but the process is aided by channel and carrier proteins in the cell membrane. The proteins completely span the membrane and have binding points for specific molecules or ions that are to be transported through. Small uncharged non polar molecules, such as oxygen, can easily diffuse across the plasma membrane because the have not been dissolved in water. Non polar molecules are poorly soluble in water. However, charged ions and polar molecules that have dissolved in water cannot diffuse through the bi layer due to the hydrophilic (water disliking) nature of the lipids. The proteins completely span the membrane and have binding points for specific molecules or ions that are to be transported through.
Channel proteins have a fixed shape and aid the transportation of charged ions and polar molecules (have dissolved in water), are unable to pass through. They regulate the flow of ions and molecules through them by opening and closing their gates.
Carrier proteins, however, change shape whilst aiding the transport of larger molecules, such as glucose and amino acids, through them. Carrier proteins are also used in active transport but require energy.
Facilitated diffusion is a passive process not requiring any energy as it moves along the concentration gradient, from high to low.
Active Transport
Active transport is the movement of substances across the cell membrane using carrier proteins, just like facilitated transport, but this time against the concentration gradient, from low concentration to a high concentration. This process requires stored chemical energy in the form of adenosine triphosphate (ATP). ATP is the major energy source within the cell to drive a number of biological processes.
Passive Transport
Passive transport doesn’t use any chemical energy. Examples of passive transport are diffusion, facilitated diffusion and osmosis.
Bulk Transport
Endocytosis is a process where objects are taken from the outside of a cell and absorbed into the cell. The cell does this by engulfing the object in its membrane. All cells use this process as most substances it needs are large polar molecules (dissolved in water) and which cannot pass through the phospholipid bi layer due to the lipids hydrophilic nature.
There are two types of endocytosis, these are phagocytosis and pinocytosis:
Phagocytosis (meaning cell-eating), is the process by which cells ingest large objects, for example cells which have undergone apoptosis (a form of programmed cell death), bacteria or viruses. The membrane folds around the object and the object is sealed off into a large vacuole known as a phagosome (a food vacuole).
Pinocytosis (meaning cell-drinking), is an alternative name for endocytosis. This process is concerned with the uptake of solutes and single molecules such as proteins.
Exocytosis is the opposite of Endocytosis. It is a strong process by which a cell directs secretory vesicles to the cell membrane. These membrane-bound vesicles contain soluble proteins to be secreted to the extracellular environment, as well as membrane proteins and lipids that are sent to become components of the cell membrane.
Bibliography:
Cell Membrane Picture:
http://en.wikipedia.org/wiki/Plasma_membrane
Diffusion Picture:
Osmosis Pictures:
http://faculty.southwest.tn.edu/rburkett/GB1-osmosis.htm
Facilitated Diffusion Picture:
Active Transport Picture:
Bulk Transport Pictures:
http://pirates.afsc.k12.ar.us/DHS/King/ch5notes.htm
