A2.2.4

• The plasma membrane is the outer boundary of the cell and encloses all of its contents. It controls the entry and exit of substances. It can pump substances in or prevent them from entering.
• The plasma membrane allows the cell to maintain a concentration of substances even if it is very different to those outside.
• The permeability of the plasma membrane relies on the phospholipid bilayer.
• Plasma membranes burst occasionally. This is called lysis and can be caused by excess pressure or by a virus.
• Lysis always leads to the death of the cell.
• All cells must have an outer border to maintain an internal chemistry that is different to the exterior (homeostasis).
• Water is the main component of cytoplasm and there are many substances dissolved in this water.
• Enzymes in the cytoplasm catalyse a lot of different chemical reactions which are the metabolism of the cell.
• Metabolism provides a cell with energy and produces all the proteins and other substances that make up the structure of a cell.
• Genes (made of DNA) contain the information needed for a cell to carry out all its functions (instruction manual).
• DNA can be copied a passed on to daughter cells (mitosis, meiosis).
• Plant and animal cells have a nucleus which contains their DNA. In bacteria the DNA is in the cytoplasm because they do not have a nucleus.