Basic Ideas About Cells

Question 1

Plasma membranes and lipid bilayers
- four things plasma membrane must do

Answer 1

Plasma membrane must do four things:
- prevent contents of cell getting out but let out waste products as necessary
- let in nutrients as needed
- act as sensor/communications system
- change shape without tearing so cell can grow and move

Question 2

Lipid bilayers

Answer 2

• lipid molecules have hydrophilic head and hydrophilic tail
• a bilayers with hydrophobic ends pointing towards each other resolves this
• structure means lipids will self-assemble into bilayers and micelles in the presence of water even when lipid molecules are initially at random

Question 3

Lipid bilayers properties

Answer 3

• layer needs to be reasonably fluid to allow proteins to move around / allow membranes to fuse and divide
• allows water and small non-polar molecules through but not ions - other molecules must use membrane transport proteins
• membranes supported by network of proteins, membrane itself is weak

Question 4

Pressurising cells

Answer 4

• plant cells and bacteria often have high internal (osmotic) pressures
• if a membrane separates two solutions with different concentrations of ions, and ions cannot get through the membrane but the water can, water will pass from the more dilute solution to the concentrated one
• this builds pressure in cells
• eventually pressure stops water transfer

Question 5

Tonocity and osmotic pressure

Answer 5

Hypertonic: solutions have higher concentration of solute than the cell - water leaves cell
Isotonic: solutions have same concentration - cell and solution are in equilibrium
Hypotonic: solutions have a lower concentration of solute than the cell - it will swell and often burst

Question 6

Holding cells together

Answer 6

• Plant cells have rigid wall that supports and stops it bursting - resulting turf or pressure helps keep stems rigid and the leaves extended
• animal cells have a protein mesh (part of the cytoskeleton) but has limited ability to resist pressure thus active control of the osmotic pressure is necessary by pumping ions out of the cell

Question 7

Membrane transport

Answer 7

• the bilayer allows small hydrophobic and uncharged polar molecules - otherwise membrane transfer proteins are necessary
• small water soluble molecules transferred either by carrier proteins or channel proteins (most are ion channels)
• macromolecules transported using vesicles - small sacs - small portions of the plasma membrane that enclose them

Question 8

The cytoskeleton

Answer 8

• network of proteins filaments that extends through the interior of the cell
• acts as skeleton and muscles
• also acts are pathway for transporting material using vesicles
• built on” intermediate filaments, spectrum and actin filaments, microtubules

Question 9

Intermediate filaments

Answer 9

• important for cells subjects to mechanical stress
• common in axons, muscle cells and epithelial cells
• distribute effect of local forces
• made of keratin twisted into ropes, have great tensile strength

Question 10

Spectrum and actin filaments

Answer 10

• spectrin is a flexible polymer about 100nm long connected to membrane through attachment proteins
• actin found as globular or filamentous - essential for cell motion , together with spectrin, involves in the mesh that supports the plasma membrane through attachment

Question 11

Microtubules

Answer 11

• the thickest individual filaments are microtubules of the protein tubulin
• polar
• rapidly assemble and dis-assemble
• create tracks in the cell from the centre some towards the edge - unstable and rapidly remodel
• provide tracks for vesicles, organelles and other components to be moved around the cell using moto proteins