Cell Membranes & Transport

• important components of cell surface membranes
• form bilayers 
• hydrophilic heads point out of bilayer, hydrophobic tails point inwards
• basis of membrane structure
• lipid solubles to freely enter and leave the cell, water solubles cannot

• arranged randomly

extrinsic: 

• surface of bilayer / partially embedded
• provide structural support
• recognition sites for identification

intrinsic: 

• extend both layers
• carriers transport water solubles 
• channels allow active transport of ions

• individual phospholipid molecues move relative to one another
• proteins embedded in bilayer vary in shape size and pattern

• cholesterol fits between phospholipid molecules increasing stability
• glycolipids found on outer layer of membrane are involved in cell to cell recognition
• glycoproteins also stick out of membranes

• lipid solubles move through the phospholipids
• water solubles pass through water filled channels
• membrane is selectively permeable to water and some solutes
• lipid solubles move through the cell easier
• uncharged molecules pass freely through the membranes 
• hydrophobic core stops ions and polar molecules
• charged and large molecules are assisted into the cell by intrinsic proteins

• passive transport
•  the movement of molecules or ions from a region of high concentration to a region of lower concentration
• net movement until equilibrium 
• affected by
  • concentration gradient- greater the difference greater the rate
  • distance- shorter the distance greater the rate
  • surface area of membrane- larger the area quicker the rate
  • thickness of exchange surface- thinner the membrane greater the rate
  • temperature- increase in temperature, increase in kinetic energy, increase in rate

diffusion = surface area X difference in concentration

                length of diffusion path

• charged particles or ions and large molecules cannot pass through cell membrane as insoluble in lipid
• facilitated diffusion is a special form of diffusion for faster movement of these molecules 
• passive and occurs down a concentration gradient
• occurs at specific points where there are protein molecules

channel proteins

• pores lined with polar groups allow charged particles and water solubles to pass
• hydrophilic
• each channel is specific to one type of ion
• open and close to fulfill need of cell

carrier proteins 

• diffusion of larger polar molecules such as sugars/ amino acids
• molecule attaches to carrier at binding site 
• carrier protein changes shape to accomidate and pushes molecule through membrane

• energy requiring process for ions and molecules to move against the concentration gradient
• ions and molecules move in opposite direction of diffusion
• energy provided by ATP, therefore anything that effects respiration effects active transport
• process uses carrier proteins
• e.g protein synthesis, muscle contraction, nerve impulse transmission
  • molecule/ion combines with specific carrier protein
  • ATP transfers phosphate group to carrier protein
  • carrier protein changes shape, carrying molecule/ion inside membrane
  • molecule/ion released, carrier protein returns to original shape
• active transport cannot take place in the presence of a respiratory inhibitor such as cyanide

• special case of diffusion moving water only
• the passage of water from a region of high water potential to a region of lower water potential through a partially permeable membrane 
• water potential is the pressure created by water molecules, units kPa
• pure water's water potential= 0kPa
• adding a solute lowers the water potential, giving it a negative value
• hgh concentration of water molecules= greater potential energy
• more concentrated solution= more negative WP

osmosis & animal cells

• animal cells have no cell walls 
• when placed in different solutions they are effected differently to plant cells:
  • hypotonic (dilute)- cell lysises/bursts
  • isotonic (same)- no net movement
  • hypertonic (concentrated)- cell shrinks

• water enters a plant causing the vacuole to expand pushing the cytoplasm against the wall
• wall can only expand so far, pressure builds up to resist entry of more water, cell is turgid
  • hypertonic=if WP of external solution is lower than solution inside cell water moves out
  • hypotonic=WP of external solution is higher than internal, water flows in
  • isotonic= WP are equal, no net movement
• if cell is placed in hypertonic solution it loses water, causing plasmolysis=vacuole shrinks, cytoplasm draws away from cell wall, cell becomes flaccid
• when the membrane is just moving away from cell wall it is at incipient plasmolysis
• if cell is placed in hypotonic solution it gains water, causing turgor= takes in water until prevented by opposing wall pressure, cell becomes turgid
• turgor is important for keeping young seedlings in shape and form

water potential= solute potential + pressure potential

• hydrostatic pressure pushes out on cell wall met by pressure potential of cell wall

• moving bigger materials or materials in bulk use different processes

endocytosis

• engulfing of material by plasma membrane to bring it inside the cell in a vesicle
  • phagocytosis
    • obtaining solid materials by having a lysosome fuse with the vesicle formed
    • digestive enzymes digest the solid material for products to be absorbed into cytoplasm
    • white blood cells do this
  • pinocytosis
    • entry of liquid via same way as a solid, producing smaller vesicles

exocytosis

• substances leaving the cell after being transported through the cytoplasm in a vesicle