Biology - membranes

• fluid mosaic model


  Structural model of the plasma membrane where molecules are free to move sideways within a lipid bilayer.
• structure of membranes

  phospholipid bilayer, cholesterol, receptor molecules, channel and carrier proteins
• Intrinsic proteins


  Extend from one side of the membrane to the other
• Extrinsic proteins


  Embedded in the bilayer on one side but don't extend through
• Size of membranes


  7-10nm
• Functions of membranes

  Acts as a barrier, keeps cell contents inside, can be a site of chemical reactions
• cell signaling


  The process of cell-to-cell communication mediated by signaling molecules and membrane receptors
• Can polar molecules cross the membrane?

  No because they have a charge - except water due to being a small particle
• Can non-polar molecules cross the membrane?

  Yes because they are uncharged particles
• Volume of a sphere


  4/3 pi r3
• Surface area of a sphere


  4πr²
• surface area to volume ratio

  a variable that decreases as cells grow
• What organelles have cell membranes

  Nuclear envelope, thylakoid, inner mitochondria, Golgi, ER, tonoplast, vesicles and lysosomes
• passive diffusion


  movement of substances across a partially permeable membrane with the concentration gradient; this process does not require metabolic energy
• Examples of passive diffusion


  Facilitated, osmosis
• Factors that affect the passive rate of diffusion

  Surface area, temperature, concentration, diffusion distance and the size/charge of molecules
• Endocytosis


  process by which a cell takes material into the cell by infolding of the cell membrane and encircling the particle making a vesicle
• Exocytosis


  Process by which a cell releases large amounts of material - Golgi releases vesicles and then they fuse with the cell membrane to secret molecules
• Extrinsic proteins - roles


  Structural role, act as enzymes or receptors for other molecules
• channel proteins


  provide corridors which are linked with hydrophilic animo acids that allow a specific soluble molecule or ion to diffuse across the membrane
• carrier proteins


  They can change shape to transfer molecules or ions from one side to another
• Glycoproteins


  Carbohydrates attached to proteins
• role of glycoproteins

  Allows cells to attach to each other forming tissues, play a role in the immune system or act as receptors for hormones
• role of glycolipids


  Cell signalling and cell recognition or act as antigens e.g determining your blood group
• Glycolipids


  carbohydrates attached to phospholipids
• active transport


  the movement of materials through a cell membrane using metabolic energy against a concentration gradient
• What diffusion do carrier and channel proteins use?

  Facilitated diffusion
• What uses active transport?

  sodium potassium pump (NaK) and the uptake of glucose in the digestive system
• What do we find in cells that carry out a lot of active transport?

  Mitochondria
• Are carrier proteins always open?

  Yes however they only respond to a certain trigger - chemical binding to the protein channel or a change in voltage
• How to channel proteins change shape?

  They have binding sites for a specific chemical - when it binds it causes the tertiary structure of the protein to change
• Osmosis


  Diffusion of water through a selectively permeable membrane
• Diffusion


  Movement of molecules from an area of higher concentration to an area of lower concentration.
• water potential

  the potential energy of a volume of water, expressed as a pressure
• What is the water potential of pure water?

  0 kPa
• What will the water potential of a solution containing water be?

  A negative water potential - bigger negative number = lower water potential
• hydrostatic pressure


  The pressure of water against the walls of its container.
• What happens if water leaves an animal cell?

  Shrinks
• What happens if too much water enters an animal cell?

  Bursts
• What happens when too much water enters a plant cell

  Protoplast pushes outwards against cell wall increasing hydrostatic pressure