components of plasma membrane

• composition of the plasma membrane

  -phospholipid bilayer- hydrophilic heads on the outside and hydrophobic tails on the inside

  -phospholipids make up around 75% of the plasma membrane- other components include: protein, glycoproteins, cholesterol, polysaccharides
• phospholipids

  -phosphate group head is hydrophilic

  -fatty acid tails are hydrophobic

  -tails will not mix with water- formation of bilayers
• phospholipids in the membrane


  -phosphate head- hydrophilic

  -fatty acid tails- hydrophobic

  -molecules with evenly distributed charges don't mix well with water- repel- hydrophobic

  -molecules with an uneven distribution of charges do mix well with water- hydrophilic

  -leads to the formation of the bilayer

  -phospholipids completely surrounded by water forms the bilayer
• proteins are also present in membranes. fluid -mosaic model- proteins can move freely in lipid bilayer
• phospholipids


  -basic membrane structure- bilayer

  -only fat- soluble (non-polar) molecules can diffuse directly through

  -hydrophobic centre prevents diffusion of polar molecules

  -unsaturated fatty acids pack in less tightly due to a kinked molecular shape- more fluid membrane
• cholesterol


  -regulates membrane fluidity

  -fits between fatty acid tails and bind them together to make a more complete barrier
• proteins


  -enzymes catalyse reactions

  -channel proteins- allow passive diffusion of polar molecules (facilitated diffusion). channel is hydrophilic

  -carrier proteins- active transport

  -receptors in cell signalling
• glycoproteins


  -protein with a polysaccharide attached on outside of the membrane

  -acts as a receptor for cell recognition

  -antigens

  -stabilise plasma membrane by forming hydrogen bonds with surrounding water molecules
• glycolipids


  -phospholipids with polysaccharides attached on outside

  -cell recognition and communication

  -glycolipids and glycoproteins together make up glycocalyx layer (used for cell recognition so cells form tissues)

  -stabilise membrane by forming hydrogen bonds with surrounding water molecules
• fluid mosaic model


  -proteins can move freely through the bilayer

  -the ease at which they move depends on number of phospholipids with unsaturated or saturated fatty acid tails
• in bilayers, the hydrophilic heads form h-bonds with the surrounding water
• describe fluid mosaic model of membranes


  FLUID- phospholipid bilayer- individual phospholipids can move giving the membrane its flexible shape

  MOSAIC- extrinsic and intrinsic proteins of different sizes are embedded
• explain the role of cholesterol in membranes


  -steroid hormone in some plasma membranes- connects phospholipids and reduces fluidity to make bilayer more stable
• explain the role of glycolipids in a plasma membrane


  -cell signalling and recognition
• functions of extrinsic proteins in a membrane


  -binding sites/ receptors for hormones etc

  -antigens (glycoproteins)

  -bind cells together

  -involved in cell signalling
• function of intrinsic proteins in membranes 


  -electron carriers- photosynthesis or respiration

  -channel proteins

  -carrier proteins
• function of membranes in cells


  -isolate organelles from cytoplasm for specific metabolic reactions

  -allows entrance and exit of molecules- selectively/ partially permeable

  -provides a reaction surface
• functions of cell surface membrane


  -isolate cytoplasm from extracellular environment

  -partially permeable to regulate transport of substances

  -cell signalling and recognition