Plasma Membrane

• What is the fluid mosaic model?


  A concept used to describe the partially permeable cell membrane, in which various molecules, including phospholipids, proteins, glycoproteins, and glycolipids, are arranged into a phospholipid bilayer.
• What is the structure of the fluid mosaic model?


  The plasma membrane consists of phospholipids that are free to move within the layer, relative to each other, allowing for it to be flexible; with proteins imbedded in the bilayer, varying in shape, size, and positioning.
• What is the hydro structure of the plasma membrane?

  Phospholipid bilayers consists of hydrophilic phosphate heads forming the inner and outer surfaces of the membrane, with the hydrophobic fatty acid tails forming the hydrophobic core inside the membrane; allowing interaction with water for cells existing in aqueous environments.
• What is compartmentalisation?


  The formation of separate membrane bound areas in a cell, allowing for metabolic processes that involve different incompatible reactions to be carried out; allowing for different reactions to occur in varying optimum conditions, eg. pH conditions.
• What does it mean for the membrane to be partially permeable?


  This means that the membrane is selectively permeable, allowing only specific molecules to pass through via simple diffusion.
• What are some examples of molecules that can pass through the membrane?

  • Small molecules (NON POLAR) - eg. carbon dioxide, oxygen, water.
  • Lipid soluble substances (NON POLAR) - eg. some hormones.
• What are examples of molecules that cannot pass through the membrane?

  • Large molecules (POLAR) - eg. glucose.
  • Water soluble substances (POLAR) - eg. sodium ions.
• What is simple diffusion?


  The net movement of particles from a region of high to low concentration; a passive process which requires no ATP, continuing until equilibrium is reached.
• What is equilibrium?


  A state of balance or stability within a biological system.
• What is dynamic equilibrium?


  A state where the rate of the forward reaction equals the rate of the reverse reaction, resulting in no net change in concentration over time; occurring in enzymes reactions, diffusion, and osmosis.
• How do you calculated the rate of diffusion?


  rate= distance travelled/time or volume filled/time.
• What are the affecting factors of diffusion?

  Concentration gradient, temperature, diffusion distance, surface area, particle size, and permeability.
• How does the concentration gradient influence diffusion rate?


  The steeper the concentration gradient, the faster molecules will move from a high to low concentration.
• How does temperature influence diffusion rate?


  Higher temperatures cause an increase in kinetic energy to the molecules, increasing the likelihood of collisions, and increasing rate.
• How does diffusion distance influence diffusion rate?


  The shorter the distance, the shorter the amount of time it takes for molecules to travel across the membrane.
• How does surface area influence diffusion rate?


  The greater the surface area to volume ratio the more molecules can diffuse at any given time.
• How does particle size influence diffusion rate?


  Small particles will have an increased rate as they have less mass, making them easily carried by kinetic energy; they are also able to easily navigate through spaces between other molecules with less resistant, and therefore have higher mobility.
• How does permeability influence diffusion rate?


  The higher the permeability, the more easily and rapidly molecules can move across it, and less resistance there is.
• What is the role of cholesterol in the plasma membrane?


  It is present to restrict lateral movement of other molecules in the membrane, making the membrane less fluid at high temperatures, as well as preventing water and dissolved ions leaking out of the cell.
• Why does fluidity of the plasma membrane change at high temperatures?

  • High temperatures increases kinetic energy of molecules in the membrane, increasing fluidity and flexibility by weakening interactions that hold the molecules together.
  • High temperatures cause the fatty acid tails to become disordered and move apart, making the membrane less rigid.
  • Membranes with more unsaturated (double carbon bonds) fatty acids are more fluid due to the kinks in their tails, which is amplified by high temperatures.
• What would happen if the membrane was too fluid?

  The gaps between phospholipids would become too large, causing leakage, bursting or dehydration.
• What is the role of proteins in the plasma membrane?


  Proteins are imbedded across the membrane surface as either peripheral or integral proteins, with different roles dependant on which type the protein belongs to.
• What is a peripheral (extrinsic) protein?

  These proteins do not span the entirety of the membrane, loosely attached to the surface.
• What is the role of a peripheral protein?

  1. Mechanical support - linking to the cytoskeleton (network of fibres) or extracellular matrix (outside of cell), helping maintain shape, movement, endocytosis, and cell division.
  2. Cell communication - interact with carbohydrates and other surface molecules on adjacent cells, allowing communication and playing a role in immune response and tissue response.
  3. Enzymatic activity - some catalyse reactions at the membrane surface, regulating various cellular processes.
• What are the roles of a peripheral protein?

  1. Transport and anchoring - assist integral proteins in transportation by anchoring or stabilizing them, or by moving molecules from one location to another in the cell.
  2. Cell signalling - receiving signal from integral proteins and transmitting them to other parts of the cell, acting as signal transduction pathways and coordinating cellular responses to external stimuli.
• What is an integral (intrinsic) protein?

  A protein that spans through the entirety of the membranes bilayer structure.
• What is the role of integral proteins?

  1. Transport - serving as carrier proteins or channel proteins, facilitating the movement of molecules across the membrane.
  2. Receptors - acting as receptor proteins they detect signalling molecules from outside the cell.
  3. Cell adhesion - help cells stick together within tissues and organs by forming tight junctions, desmosomes, and other structures that allow cells to communicate and hold together.
  4. Anchoring - attach the membrane to cytoskeleton and extracellular membrane, maintaining shape.
• What are the roles of integral proteins?

  1. Enzymatic activity - catalysing reactions at surface, breaking down substances, synthesising molecules, and regulating metabolic processes.
  2. Signal transduction - help relay signals from outside to inside of cell, allowing cells to respond to external stimuli and changes.
  3. Cell recognition - have glycoprotein (carbohydrate chains) attached, which helps in the immune system, allowing cells to distinguish between self and non self.
• What are channel proteins?


  Proteins that create pores, forming tubes that fill with water to enable polar molecules and water soluble ions to diffuse down a concentration gradient, through the membrane. The channels are held together by interaction between hydrophobic core of the membrane and hydrophobic R-groups on the exterior of the protein.
• What are carrier proteins?


  Proteins that actively or passively transport large molecules like glucose, by binding with other carrier proteins and other molecules, changing into a conformational shape to move the substance across the membrane.
• What is a desmosome?


  A cell junction found in tissues that undergo significant mechanical stress, eg. the skin. They help provide structural integrity by anchoring cells to one another, enabling them to resist stretching an tension.
• What is conformational shape?


  The flexible three dimensional structure that a molecule can adopt, important for its biological activity.
• What are glycoproteins?


  These are imbedded proteins with attached carbohydrate chains of varying shape and length.
• What is the role of glycoproteins?

  1. Adhesion - allow cells to join to one another or the extracellular matrix, forming tight junctions in certain tissues.
  2. Receptors - when a chemical binds to a glycoprotein receptor, it elicits a response from the cell.
  3. Cell signalling - the response from the glycoprotein receptors may cause a direct response or trigger a series of events to occur within the cell.
• What is an example of when glycoproteins act as receptors?


  They act as receptors for neurotransmitters, in which the binding of a neurotransmitter triggers or prevents an impulse in the next neurone, as well as receptors for peptide hormones such as insulin.
• What are glycolipids?


  These are imbedded lipids with a carbohydrate chain of varying shape and length attached.
• What is the role of glycolipids?

  1. Cell recognition - cells recognise and interact with each other, and act as markers or antigens that are recognised by the immune system as self or non self.
  2. Cell signalling - transmitting signals from outside to inside the cell.
  3. Stability - maintain structural integrity and shape.
  4. Adhesion - facilitating adhesion of cells to each other and the extracellular matrix, helping in the formation of tissues.
  5. Protection - they form a protective layer of the cell surface that shields cells from harmful physical and chemical damage.
• What is facilitated diffusion?


  Facilitated diffusion is the passive movement of molecules across a cell membrane with the help of transport proteins. The phospholipid bilayer acts as barrier to ions and polar molecules, but these transport proteins allow them to pass via facilitated diffusion with no need for ATP.
• What is facilitated diffusion using channel proteins?

  Channel proteins allow for facilitated diffusion to take place, but the membranes are selectively permeable and some channels are specific to an ion or polar molecule, opening or closing in response to certain signals or the binding of specific molecules.
• What is facilitated diffusion using carrier proteins?

  Carrier proteins allow for facilitated diffusion to take place, done so through the carrier proteins binding to a specific molecule on one side of the membrane, in which it undergoes a conformational change, releasing the molecule on the opposite side of the membrane; this binding is specific, each carrier protein being specific to certain types of molecule.