biology 1.5

• Cell membrane


  Separates the internal environment of the cell from its surroundings
• Cell membrane


  • Mostly consists of two kinds of molecules - phospholipids and other lipids such as cholesterol and membrane proteins
  • According to the fluid-mosaic model, the structure includes a bilayer or two layer framework of phospholipids with membrane proteins embedded, phospholipid heads positioned on the outer and inner edges, phospholipid tails positioned in the centre of the bilayer, glycoproteins which are membrane proteins with carbohydrate chains attached, and glycolipids which are phospholipids with carbohydrate chains attached
• The cell membrane is vital in controlling the exchange of materials between the internal and external environment of the cell
• Selectively permeable


  The cell membrane permits it to select what passes across it - some materials may pass across it, but not others
• The proportions of chemicals in the intracellular environment of cells is different from those in the extracellular environment of cells
• Cells need the correct pH, glucose concentrations and the correct water and solute balance to ensure that cell reactions can occur
• The levels of waste products like carbon dioxide and urea need to be kept low inside the cell
• Passive transport


  The movement of atoms and molecules from a region of high concentration to a region of lower concentration, without the expenditure of energy
• Passive transport


  1. Diffusion

  2. Osmosis
• Diffusion


  The passive transport of a substance from a region of high concentration to a region of lower concentration until an overall even concentration is reached
• Osmosis


  The passive transport of water from a region of low solute concentration to a region of higher solute concentration across a selectively permeable membrane
• Facilitated diffusion


  The passive transport of a substance across a membrane through a membrane transport protein
• Active transport


  The movement of substances from a region of low concentration to a region of high concentration, involving the expenditure of energy
• Endocytosis


  The process where larger particles or molecules are taken into the cell, enclosed in a vesicle formed by the cell membrane
• Exocytosis


  The release of a substance from a cell, where the substance is packaged into vesicles by organelles and then fused with the cell membrane to be released
• Surface area to volume ratio (SAV)

  The numerical value that represents the relationship between the amount of external surface area of the cell membrane and volume of the cytoplasm
• Cells that are smaller which have a larger ratio of surface area to volume are able to obtain their nutrients and dispose of their wastes much more efficiently than larger cells