Another process by which substances are transported across plasma membranes is osmosis. Not all substances can pass through the cell surface membrane; water molecules do but larger solute molecules do not. This means the membrane is partially permeable. Osmosis is defined as the passage of water from a region where it is highly concentrated to a region where its concentration is lower through a partially permeable membrane. The greater the concentration of the solution, the greater the osmotic potential (the potential of solution to pull water into it). Osmosis depends on factors other than the differences in the number of water molecules. Solute molecules make weak chemical bonds with water molecules. Solutions with many solute molecules bind most of the water molecules. However, a solution with few solute molecules binds only a few water molecules. Substances, which are small enough to pass across membranes, do so at different rates.
When dynamic equilibrium is established it is called isotonic. Isotonic solutions have the same concentrations of water molecules, so the rate at which water molecules diffuse into and out of the cell is the same.
Hypertonic solutions have a lower concentration of water molecules compared to the inside of a cell and so there is a net movement of water molecules out of the cell.
Hypotonic solutions have a higher concentration of water molecules compared to the inside of a cell so there is a net movement of water molecules into the cell. This causes the cell to swell; a cell that is full of water but has not burst is said to be turgid.
An example of osmosis taking place is in the vacuole of a plant cell.
Facilitated diffusion is a passive process, as it does not need energy. (There is no requirement for ATP as there is no energy consumption.) It uses a carrier protein to transfer a molecule across a membrane along its electrochemical gradient. There are different transport proteins for different molecules or ions. Transport proteins have a highly specific tertiary structure, which gives the molecule a distinctive shape. Facilitated diffusion makes diffusion easier because it is a more rapid exchange due to the channels produced by the carrier proteins. The carrier proteins bind with the molecules causing it to change shape allowing the molecules to be released on the other side of the membrane.
This is considerably faster then normal diffusion and is used to transport molecules such as glucose, fructose, non fat-soluble vitamins, urea and many ions across the membrane.
Substances can pass in and out of cells along their concentration gradient in several different ways, none of which requires the input of energy. When the cells need to move substances against their concentration gradient, the cells need to expend some of their energy. Some intrinsic protein (proteins which span the entire width of the membrane) molecules act as molecular pumps. They allow the cell to perform active transport to accumulate glucose or ions against their concentration gradient. Animal and plant cells that specialise in absorption usually have abundant mitochondria to provide the ATP needed to power active transport.
As I said before, the membrane is made up of a phospholipid bilayer. Therefore, every process by which substances are transported, the substance has to pass through the phospholipid bilayer. The phospholipid bilayer is not very permeable to monosaccharides. Water is small enough to pass between phospholipid molecules. Gases and other small-uncharged molecules diffuse across the phospholipid bilayer. Small lipid-soluble molecules such as steroid hormones diffuse through the phospholipid bilayer. Molecules that are too large to diffuse through the phospholipid bilayer may diffuse through protein channels
There are also the processes of endocytosis and exocytosis. Endocytosis is the process of taking in materials. Exocytosis is the process of removing materials/substances away from the cell. The nutrient particles/bacterium enter the phagocytic vesicle formed by pinching of the cell membrane. This then breaks away and forms a vacuole with the nutrient/bacterium inside. A lysosome with lytic enzymes fuses with the phagocytic vessel, and then the lysosome releases enzymes which break down nutrient particles/bacterium. The soluble products are absorbed into the cytoplasm and the insoluble material is removed as the vesicle regains.
Macromolecules and larger particles, such as bacteria, are taken into cells by endocytosis. Large molecules, such as proteins, are transported out of cells by exocytosis.
Endocytosis includes phagocytosis and pinocytosis. Phagocytosis is when cells obtain particles that are too large to be absorbed by diffusion or active transport. Pinocytosis is similar to phagocytosis, but pinocytosis takes in small droplets of the external solution, forming vesicles.
There are different processes for transporting any substance between cells, some which require energy and some which don’t. Cells are complex factories and they need constantly to import raw materials and get rid of waste. Molecules and ions with different sizes and electrical charges enter and leave all the time. Some of the exchange of materials occurs as a result of passive processes such as diffusion and osmosis, and some occur as a result of processes which require energy such as facilitated diffusion and active transport.
