HL 1 Biology

2.4a Cells: Membranes

<<Membrane Structure>> Membranes are composed of phospholipids and proteins.

1) Phospholipids: major component of a membrane.  In each structure, they consist of:

  • 2 Hydrophobic tails: hydrocarbon tails; are not attracted to water
  • Hydrophilic phosphate head: negatively charged; attracted to water

Phospholipids arrange themselves into double layers (heads outwards, tails inwards) called phospholipid bilayer, when mixed with water. This supports the membrane and provides protection with the multiple strong bonds between the tails are hard to tear apart.

2) Proteins:

  • Integral Proteinsproteins that are embedded in the phospholipid
  • Peripheral Proteinsproteins loosely attached to the surface of the membrane

Functions of membrane proteins:

    • Hormone Binding Sites: one specific hormone binds to the outer site, sends signal inside cell (e.g. insulin receptor)
    • Enzymes: either outside or inside (depend on the which has the active site)
    • Electron Carriers: two beside each other, move electrons from one to the other
    • Channels for Passive Transport: a passage way for specific substance to pass/ hydrophilic particles by facilitated diffusion
    • Pumps for Active Transport: use the energy released from ATP to pass specific substances across membrane

<<Diffusion>> “Passive movement of particles from a region of high concentration to a lower concentration.” e.g. Oxygen and carbon dioxide moving in and out of cells. Transport systems (pumping fluid consisting particles): the blood system.

  • Simple Diffusion: particles passing through the permeable phospholipid bilayer, only when the concentration of the particle is higher on one side of the membrane than the other (concentration gradient).
  • Facilitated Diffusion: particles passing through the membrane from a higher concentration to a lower concentration with channels.

<<Osmosis>> “the passive movement of water molecules, across a partially permeable membrane; from a region of lower solute concentration to a higher solute concentration.”

  • Solvent: liquid in which particles dissolve
  • Solute: dissolved particles

1) Animal cells

  • in same solute concentration as cytoplasm: good shape
  • in a higher concentration: shrink, shrivel up
  • in a lower concentration: burst,

2) Plant Cells

  • Plasmolysis: when the cytoplasm and plasma membrane are pulled away from the cell wall, as water leaves the cell by osmosis.
    • Can be observed using a microscope.
    • Entry/exit of water can be observed by changes in the mass of plant tissue.

Independent Variable: can be chosen before the experiment.

Dependent Variable: measured during the experiment (may/may not be affected by iv)


Diffusion of Proteins in Membranes (pg. 27)

1.  (a) The movement of the membrane proteins are both random and yet do not differ between species. It also has the ability to move inside the bilayer.

(b) ATP energy is unnecessary for the movement of membrane proteins (passive movement), as ATP energy is only essential during active transport.

(c) The markers were not mixed by fusion after the breaking of the pieces since regrouping is not possible.

(a) For temperatures between 15 and 35˚C, it is apparent that the percentage of cells with markers fully mixed is increasing drastically. As the temperature increases, there’s a increase in the markers actions. This may be due to the molecules moving faster with the increase in temperature. After about 30˚C we can see that the graph is gradually becoming linear once again.

(b) For the temperatures below 15˚C, there is no apparent movement with the markers and thus are barely mixed together. It seems as though the membrane proteins do not cause much movement when the temperature is low.

3  If the experiment was repeated using cells from Arctic fish rather than from mice or humans, the markers are predicted to mix together faster at the same temperature because fish are used to living in a colder environment than mice/humans.

Patch Clamp Analysis (pg. 31)

1 (a)  0pA (current)

(b) 1 amp =  1,000,000,000,000 picoamps  or 10^12 picoamps

2 (a) Scale bar: 200ms (actual)/ 11mm (image) ≈ 18ms per mm

Maximum length channel remains open: 3mm (Image) * 18ms = 54 ms

(b) About -4pA

3  The natural source of ACh is in living muscle fibre may be the neuron synapse.

4 (a) The ion channels are opened as the ACh concentrations attach to receptors. The higher ACh concentrations increased the frequency of opening of channels because the ACh stays within the receptor region, thus the channel opens repetitively.

(b) Higher ACh concentrations did not increase the average time that the channels remained open because there is a set level of concentration necessary to open the channel. Thus the system is set so the channel either opens fully or does not open at all, but there is a different specific function which controls the time period in which the channel stays open.

5 (a) Both graphs show that the current stays 0pA when the ion channels are closed. Although when the channels are open in a membrane from a mouse, the current flowing is greater as it could go up to -12pA. Also, the channels in the membrane stay open for longer than those in a muscle fibre.

(b) The difference between the two traces are the structure of each channel, the level in which the channels react to each respective glycine/ACh, and the membrane receptors (one is activated by glycine ).

Albumin in the Blood (pg. 32)

1 (a) Control has the highest level of albumin out of the three groups of children, with about 37g dm-3. Marasmus (lack of food) children have about 28g dm-3, which is between the amount of albumin the control and kwashiorkor contain. Kwashiorkor has 15g dm-3, the lowest level of albumin in the three groups. 

(b) The less protein in the blood of the children, the lower level of albumin they have. This explains how children with marasmus and kwashiorkor, which is caused by lack of proteins/food, have less amounts of albumin in their bloodstream.

2 The albumin dissolved in blood plasma causes reabsorption of water into the blood as the water moves from a high water concentration to a low concentration. This movement occurs from the bonds between albumin and water molecules, creating a region with low water concentration.

The group of kwashiorkor children were most at risk of edema since they reabsorb the least amount of blood and have the lowest level of albumin. Their tissues will most likely become swollen because of the fluid retention.

Osmosis in Plant Tissues (pg. 33) 

1 (a) Water moved into the tissues, since mass increased. 

(b) Water moved out of the tissues for the cactus, sweet potato, and butternut squash as their mass decreased. Yet the water moved into the tissues for pine kernel because it’s mass increased.

The cactus had the lowest solute concentration in its cytoplasm. The tissue showed a vast decrease in mass when it was placed in a 0.1mol dm-3 concentration. This shows how the cactus should have a lower solute concentration than 0.1mol since water moved out of the tissue, causing the mass to decrease.

Each tissues have different solute concentrations because they have adapted to different types of environments. It is likely that the cactus has a large amount of water stored inside its tissues, to survive the dry environment it is mostly found in, thus resulting to a low solute concentration. On the other hand, the pine kernels have high solute concentrations because they adapt in moist environments where they could absorb much water rapidly.

Percentage mass change is used rather than the actual mass change in this type of experiment because the initial mass may vary as well as, not all the tissues have the same density. Using the percentage mass change may be the best method to compare the different tissues more efficiently.

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