Diffusion
The definition of diffusion is, the movement of particle form an area of high concentration to an area of low concentration. Diffusion is a passive transport, as the molecules are going in the direction of the gradient and does not use any energy. This is shown when you but a drop of blueink into a beaker full of water. After a short time the molecules would have spaced evenly throughout the beaker, so all the water would have turned blue. This is what happens with gases, inside our body. They are spreading evenly through our cells via the phospholipid bilayer. There are a few aspects that change the rate of diffusion, they are the concentration of the gases being diffused, the exchange surface, as in our lungs the alveoli’s are moist to have a optimum diffusion rate. Final the thickness of the surface that the gases have to diffuse across. These three factors that affect the rate of diffusion are linked together by Fick’s law and this equation,
The rate of diffusion through the exchange surface is proportional to:
Surface area difference in concentration
Thickness of surface
Facilitated diffusion
Large and charged molecules are unable to pass freely though the phospholipid bilayer. So they have to go though proteins, which are on both the layer of bilayer. There are two different proteins that work on the bilayer and they are carrier proteins and ion channels. This is also a passive transport, as the molecules are going in the direction of the gradient and does not use any energy
Carrier proteins
Carrier proteins allow large molecules like glucose, amino acids thorough, which are too large to pass through the phospholipid bilayer. The carrier protein catches the large molecules, and then changes shape, and opens the bottom part of the protein and release the molecules inside the cell.
Ion channel
This process allows charged particles to enter the cells. The proteins that is used for the ion channels are lined with charged groups. This allows charged molecules like, Ca +, Na+, K+, HCO – and Cl – ions, through which are attracted by the ion channel. Some of the channels are able to control the amount to ions passing into the cells by having gates that are able to open and close, on the top of the ion channels.
Osmosis
The definition of osmosis is- osmosis is the movement of water molecules across a partially permeable membrane from a region of high water concentration to an area of low water concentration. This process is also passive transport, as the water molecules move in the direction on the gradient.
In osmosis only small molecules are able to get though, just as the phospholipid bilayer did. That is why it is stated definition, that it is a partially permeable membrane.
Inside the cells, their proteins that are unable to pass through the partially permeable membrane, as they are too large, so there will be a build up of proteins along the plasma membrane. To counter this build up, water molecules diffuse into the cell and decrease the concentration of proteins inside the cell, so it is at equilibrium with the outside of the cell.
Sometimes too many water molecules enter the cell, and cause it to expand to a dangerous size and will eventually burst, which is called heamolysis. This can not happen in plant cells as they have a cellulose wall, which supports the cell, and prevents it form bursting. On the other extreme, too many water molecules may leave the cell, and cause the cell to become crenation.
Active transport
All of the other process for Transport across Plasma Membranes that I have mentioned, are all passive transports, so they do not use any energy, to perform the tasks, as the molecules are moving with their gradient. Some tasks need to move molecules against their gradient and need energy to complete this task, and this is called active transport.
These protein carriers are found in the phospholipid bilayer. This process it very similar to facilitated diffusion but the proteins work against the gradient. The way that the carrier protein works is, it binds with the molecules, and changes shape by opening the bottom part of the protein and drops the molecules into the cell. The energy that is used it, ATP, which is converted to ADP.
Exocytosis
This is when the vesicles bud off from the golgi apparatus or from other tubular structures in the cytoplasm. The contents in the vesicles are provided by the ribosomes. The vesicle then travels to the plasma membrane and fuses into the membranes, which secretes their contents out of the cell. Many substances are secreted this way, for example insulin and glucagon.
Endocytosis
This is when part of the outer membrane bud-off and seals its self and flows back into the cell through the cytoplasm. The plasma membrane, which budded off, contains some substances from outside the cell, which is used inside the cell. During endocytosis, when solid materials are brought into the cell, it is called phagocytosis and when liquids are brought in, they are called pinocytosis. Another example of endocytosis, is when white blood cell engulfs bacteria.
