T4B3 plasma membrane

• Phospholipid molecule


  Consists of:

  A polar head which is hydrophilic (attracted to water)

  A nonpolar tail which is hydrophobic (repels water)
• The heads of the phospholipid molecule in the outer layer

  Face the extracellular fluid
• The phospholipid heads in the inner layer


  Face the cytoplasm
• The tails of the phospholipid molecule of the two layers

  Face each other
• Protein molecules embedded in the membrane


  • Partially or fully within the membrane

  Widely dispersed between the phospholipid bilayer

  Protein molecules with channels or canals are known as channel proteins

  Protein molecules that function as carriers are called carrier proteins
• Some proteins and lipids have carbohydrate chains attached to them, known as glycoprotein and glycolipid respectively
• Glycoprotein and glycolipid


  Act as receptors to hormones such as insulin

  Stabilise the membrane by forming hydrogen bonds with water

  Act as antigens for cell identification
• Cholesterol molecules


  • Found between the phospholipid molecules

  Make the phospholipid bilayer stronger, more flexible and less permeable to water-soluble substances such as ions
• The phospholipid bilayer, proteins and cholesterols are not static but form a dynamic and flexible structure
• This contributes to the 'fluid' characteristic of the plasma membrane and makes the plasma membrane more flexible
• Permeability


  A membrane is said to be permeable to a substance if the membrane allows the substance to pass through it freely. On the contrary, a membrane is impermeable if a substance is unable to move across it.
• Selectively permeable membrane


  Only allows free movement of certain molecules across it, and prevent or limit the movement of other substances.
• Plasma membrane


  • It is a selectively permeable membrane.
• The plasma membrane's selectively permeable property is due to its building structure. The phospholipid bilayer and protein determine the membrane permeability towards specific substances.</b>
• Concept of Movement of Substances Across a Plasma Membrane

  The characteristics of substances that are able to move across a plasma membrane
• Factors that determine whether a molecule can pass through a plasma membrane

  • Molecule size
  • Polar molecule
  • Ionic charge
• Nonpolar molecules


  • Fatty acid
  • Glycerol
  • Fat soluble vitamins (A, D, E, K)
  • Storoid compounds
• Polar molecules


  • Water
• Nonpolar molecules


  • Oxygen
  • Carbon dioxide
• Ions


  • K
  • Na
  • Ca²
  • Mg
• Large molecules


  • Glucose
  • Amino acid
• Simple diffusion


  The movement of molecules or ions from an area of high concentration to an area of low concentration down the concentration gradient until a dynamic equilibrium is achieved, with or without a plasma membrane
• Molecules that undergo simple diffusion

  • Oxygen, Lipid soluble molecules (fatty acids and glycerol)
  • Carbon dioxide
• Osmosis


  A passive transport process similar to diffusion, involving the net movement of water molecules from an area of high water potential (low solutes concentration) to an area of low water potential (high solutes concentration) through a selectively permeable membrane. The selectively permeable membrane is permeable to water but impermeable to some solutes such as sucrose molecules
• Facilitated diffusion


  The movement of lipid insoluble molecules such as ions, large molecules like amino acids and glucose across the membrane with the aid of transport proteins (carrier or channel proteins), down a concentration gradient without requiring energy
• Facilitated diffusion continues until a dynamic equilibrium is achieved when the concentration of molecules is the same at both sides of the membrane
• active transport

  • requires energy
  • occurs against the concentration of the gradient
  • there are accumulation and disposal of molecules or ions
• passive transport

  • occurs following the concentration of the gradient
  • occurs until a dynamic equilibrium is achieved
  • energy is not required
• passive transport & active transport


  moving a substance across a membrane & occurs through a selectively permeable membrane
• passive transport in organisms occur during:

  • gaseous exchange between an alveolus and a blood capillary through simple diffusion
  • reabsorption of water occurs by osmosis through the renal tubule in the kidney
  • absorption of water by a plant root hair cell by osmosis
  • absorption of fructose molecule in the villus by fascilitated diffusion
• active transport in organisms occurs during:

  • absorption of glucose and amino acids in the villus
  • reabsorption of glucose through the renal tubule in the kidney
  • transport of sucrose from a leaf to a phloem tissue
  • absorption of mineral ions by a plant root hair cell
• The cell cytoplasm containts solutes such as glucose and mineral salts. The fluid found in the cell is known as intracellular fluid. Each cell is surrounded by extracellular fluid.
• hypotonic solution

  • when red blood cells are placed in a hypotonic solution, water will diffuse into the cells by osmosis, causing cells to swell and finally burst
  • this is because the plasma membrane is too thin to withstand the osmotic pressure built up in the cells
  • the burst of red blood cells is known as haemolysis
• hypertonic solution

  • when red blood cells are placed in a hypertonic solution, water will move out of the cells by osmosis
  • this will cause the cells to shrink
  • the red blood cells are said to undergo crenation
• isonotic solution

  • water diffuses into and out of the cell by osmosis at the same rate
  • no net movement of water across the plasma membrane
  • the cells maintain their normal shape
• hypotonic solution (plant cells):

  • when plant cells are placed in a hypotonic solution, water will diffuse into the vacuoles by osmosis
  • the vacuoles will expand and push the cytoplasm and plasma membrane against the cell wall
  • the cells are said to be turgid
  • plant cells do not burst because the cell wall is rigid and strong
  • turgor pressure is important to plant cells because it gives support and maintains the shape of the cell
  • cell turgidity causes the guard cell to swell to allow the stomata to remain open for photosynthesis
• hypertonic solution (plant cells):

  • water will diffuse out of the vacuoles by osmosis
  • vacuoles and the cytoplasm will shrink, causing the plasma membrane to be pulled away from the cell wall
  • this process is known as plasmolysis
  • plasmolysis causes the leaves and stems to bend downwards and wilt
  • plasmolysed plant cells can regain their turgidity if the cells are returned to a hypotonic solution immediately (deplasmolysis)
• isotonic solution (plant cells):

  • when the sap of the plant cell and extracellular solution are isotonic, water potential is the same
  • the movement of water diffusion in and out of the cells is the same
  • cells become flaccid
• phenomenon of plant wilting

  Excessive use of fertilisers may cause wilting in the plants. Dissolved fertilisers will cause the soil water to be hypertonic to the sap cell of roots. Water will diffuse by osmosis from the roots' cell sap to the soil, and cells will become plasmolysed. Cells in plants will recover once they are watered. However, if the period of plasmolysis is prolonged, wilted plants will eventually die.
• rehydration drinks


  rehydration drinks such as oral rehydration salts help to recover loss of water and electrolytes in individuals with diarrhoea.