Plasma Membrane

• The plasma membrane consists of two layers of phospholipids with embedded proteins.
• The plasma membrane is the outermost layer that surrounds all cells.
• the plasma membrane also has other components such as intergral proteins, peripheral proteins, cytoskeletal filaments, glycolipids, glycoproteins and cholesterol
• Integral proteins are transmembrane proteins which span across both layers of the lipid bilayer
• Cholesterol molecules are also present in the membrane, helping to maintain its fluidity at different temperatures.
• Cytoskeleton filaments are made up of actin microfilaments and intermediate filaments
• Peripheral proteins are attached to integral proteins or the head groups of the phospholipids on either side of the membrane
• The head of a phospholipid is polar and hydrophilic, it helps attract liquid to the cell and within the cell
• the tails of phospholipids are hydrophobic and non-polar. They face the membranes interior, they ensure only very small or hydrophobic molecules can diffuse through the membrane.
• glycoproteins are proteins that have carbohydrate chains attached to them, they help with cell-cell recognition and regulation in antibodies, hormones and viruses.
• Glycolipids are lipids with carbohydrates attached to their heads, they are found mainly in animal cells and are used for cell-cell recognition.
• The plasma membrane is a fluid structure meaning that individual phospholipid molecules and some proteins, are free to move around within the layer.
• The fluidity of the phospholipid bilayer depends on the percentage of unsaturated fatty acids in the phospholipids, the greater the percentage of unsaturated fatty acids, the more fluid the phospholipid bilayer
• Plasma membrane permeability is controlled by the selective permeability of the membrane to specific substances
• nutrients include O2, H2O, Monosaccharides, amino acids, lipids, and various ions
• Waste product include H2O, Various ions, Nitrogenous waste (e.g ammonia or urea)
• Urea: a colourless crystalline compound which is the main nitrogenous breakdown product of protein metabolism in mammals and is excreted in urine.
• small uncharged molecules like oxygen or carbon dioxide are permeable to the membrane
• lipid soluble, non-polar molecules like alcohol, chloroform, steroids are permeable to the membrane.
• small polar molecules such as water and urea can be semipermeable or permeable to the membrane
• small ions like like Potassium ions (P+), sodium Ions (Na+), and chloride ions (Cl-) are non-permeable molecules and can only pass through protein channels
• Large, polar and water soluble molecules like amino acids and glucose are non-permeable and pass through protein channels
• Most water soluble molecules, ions, and polar molecules must pass through designated protein channels.
• Diffusion: The movement of solute particles along the concentration gradient. It is a passive process.
• simple diffusion is the movement of molecules from an area of high concentration to an area of low concentration
• If the concentration on both sides of the membrane are the same, there will always be approx. the same amount moving across in each direction, therefore the net movement is zero
• if the concentration is higher on one side, it will move to the area with the lower substance concentration. (down the concentration gradient)
• Semipermeable: (of a material or membrane) allowing certain substances to pass through it but not others, especially allowing the passage of a solvent but not of certain solutes
• The higher the higher the temperature, the faster or higher the rate of diffusions. This is because increasing the temperature will increase the speed of particle/molecule movement
• The smaller the particles, the faster the rate of diffusion
• the greater the difference in concentration of the reactants, the greater the rate of reaction
• facilitated diffusion is the movement of molecules across a membrane from a region of high concentration to a region of low concentration through proteins (channel/carrier) in the membrane. large hydrophilic molecules (e.g glucose) and charged atoms (ions) that are unable to diffuse through the lipid membrane. Hydrophilic molecules or ions need help from transmembrane proteins.
• Active transport requires an active process that uses energy from ATP hydrolysis to pump substances against their concentration gradients, will use carrier proteins which may be called an ion pump if transporting ions ( sodium and potasium)
• Endocytosis: the way via macromolecules enter the cell, the cell membrane forms a vesicle to encloses the molecules, the vesicle is transported to the appropriate site within the cell, this process require ATP.

  
  

  Phagocytosis - solid food particles enter

  pinocytosis - liquid enters
• Exocytosis: The way macromolecules exit the cell, molecules are enclosed in a vesicle that fuses with the cell membrane. The molecules pass out of the cell without directly passing through the lipid bi-layer. This process requires ATP.

  
  

  Wastes molecules are excreted, as well as cell products are secreted such as proteins and such enzymes.
• Phospholipids are amphiphatic meaning they have both polar and non-polar regions
• Osmosis & simple diffusion: Both allow molecule movement across the phospholipid bi-layer without use of ATP. Osmosis is diffusion of H2O, while diffusion is movement of small non-polar molecules (eg O2 & CO2)
• membrane benefits

  • protect cells from the environment
  • control substances entering and exiting
  • divide cell into compartments so that chemical reactions can occur more efficiently
  • allows a larger surface area
  • isolate potentially harmful enzymes
• phospholipid bi-layer


  the phospholipid bi-layer has hydrophilic heads and hydrophobic tails that allow that only allows hydrophobic and non-polar or tiny molecules to pass through (e.g water)
• transmembrane proteins


  proteins embedded in the lipid bilayer and some span the entire membrane, these act as channels for specific molecules as well as receptors for hormones