cell membranes and transport

• the phospholipid bilayer is the main component of the plasma membrane
• Fluid mosaic model: individual phospholipid and protein molecules move around within their monolayer. The word ‘mosaic’ describes the pattern produced by scattered protein molecules when the surface of the membrane is viewed from above.
• Phospholipid bilayer: This provides the basic structure of membranes; it is selectively permeable and acts as a barrier to most water-soluble substances.
• The more unsaturated the tails, the more fluid the membrane
• Cholesterol increases rigidity of the membrane
• Proteins are embedded into the lipid bilayer or attached to its outer surfaces
• why is the membrane more fluid when the tails are more unsaturated?

  unsaturated fatty acid tails are bent and therefore fit together more loosely.
• the longer the tail?

  the less fluid the membrane
• what are micelles?


  phospholipid molecules that arrange themselves in a spherical form in aqueous solutions
• what is cholesterol?


  it regulates the fluidity of a membrane because its hydrophobic region prevents polar molecules from passing through the membrane. ex: myelin sheath
• what happens to cholesterol at low temperatures?

  it's prevented from becoming to rigid and increases the fluidity of the membrane.
• what happens to cholesterol at higher temperatures?

  it helps stabilize cells when the membrane could otherwise become too fluid and helps with mechanical support
• what are glycolipids and glycoproteins?

  :Carbohydrate chains that are attached to membrane protein (glycoprotein) and phospholipids (glycolipid) project out into the watery fluids surrounding the cell where they form hydrogen bonds to stabilize the membrane structure
• what do carbohydrate chains mainly act as?

  receptors
• Signalling receptors: The receptors recognise messenger molecules like hormones and neurotransmitters. When the messenger molecule binds to the receptor, a series of chemical reactions is triggered inside the cell.
• Endocytosis: These group of receptors bind to molecules that are to be engulfed by the cell surface membrane.
• Cell adhesion: binding cells to other cells in tissues and organs. Some glycolipids and glycoproteins act as antigens, allowing cell–cell recognition
• what are proteins?

  Transport proteins provide hydrophilic channels for ions and polar molecules.
• what helps maintain the shape of the cell?

  the cytoskeleton made of protein filaments
• Intrinsic/integral proteins: Proteins that are found embedded within the membrane. They may be found in the inner layer, the outer layer or, most commonly, spanning the whole membrane, known as transmembrane proteins.
• Extrinsic/peripheral proteins: found on the inner or outer surface of the membrane. Many are bound to intrinsic proteins or to phospholipids.
• Channel proteins: water-filled pores that allow charged substances, usually ions, to diffuse through the membrane. They have a fixed shape and can be gated to control ion exchange. This does not use ATP and is in facilitated diffusion.
• Carrier proteins: can flip between two shapes, and is mainly in active transport where it uses ATP to change shape and carry ions/molecules up the concentration gradient. It is also involved in passive transport (facilitated diffusion) down the concentration gradient without the use of energy.
• Cell surface receptors: These are present in membranes and bind with particular substances, e.g.: hormones which are chemical messengers which circulate in the blood but only bind to specific target cells.
• Cell surface antigen: These act as cell identity markers. Each type of cell has its own antigen. This enables cells to recognise other cells and behave in an organised way.
• Cell signalling: Cells communicate by sending and receiving signals.
• Cell signalling

  1. Signal arrives at a specific protein receptor in a cell surface membrane

  2. Signal brings about a conformational change in the shape of the receptor, spanning the membrane

  3. Message is passed to the inside of the cell (signal transduction)

  4. Changing the shape of the receptor allows it to interact with the G protein

  5. G protein brings about the release of a 'second messenger'

  6. Second messenger activates a cascade of enzyme catalysed reactions

  7. Required change is brought about
• Cell signalling is an active process involving ATP use
• what is diffusion?

  the net movement of molecules or ions from a region of high concentration to a region of low concentration. It is a passive process (molecules have natural kinetic energy). As a result of diffusion, molecules reach equilibrium.
• what increases the rate of diffusion?

  Steeper concentration gradient, higher temperature and increased surface areas all increase rate of diffusion.
• what can diffuse through the membrane?

  non-polar molecules ex: steroid hormones, gases and water because it is small despite it being polar
• Facilitated diffusion: Movement of molecules from a region of high concentration to a region of low concentration down a concentration gradient. The movement is passive; however, molecules go through transport proteins instead of passing through phospholipids. This allows for the passage of large polar ions and molecules e.g. glucose, amino acids, Na+, Cl
• Osmosis: the diffusion of water molecules from a region of higher water potential (ψ) (less negative) to a region of lower ψ (more negative) through a selectively permeable membrane.
• what is water potential?

  ψ is the tendency of water to move out of a solution; pressure potential (ψp) on liquid increases ψ
• what happens to red blood cells in a dilute solution?

  the cell bursts because water moves into the cell through osmosis
• what happens to a red blood cell in a solution with the same solution as the red blood cell?

  the cell remains normal, no net movement of water
• what happens to a red blood cell in a concentrated solution?

  the cell shrinks because water moves out of the cell through osmosis
• what is a protoplast?

  the living part of the cell inside the cell wall of a plant cell
• what happens to a plant cell in pure water?

  water enters the cell by osmosis, and the cell wall pushes back against the expanding protoplast, building up pressure rapidly, becoming turgid. This is the ψp, and it increases the ψ of the cell until equilibrium is reached
• in a concentrated solution?

  water will leave the cell by osmosis. The protoplast gradually shrinks until it is exerting no pressure on the cell wall. The protoplast continues to shrink and pulls away from the cell wall, so the cell is plasmolysed