5 - Plasma membranes

Question 1

compartmentalisation
- why is it important

Answer 1

formation of separate membrane bound areas in a cell
- metabolism includes many different and often incompatible reactions. each with their own specific conditions

Question 2

general membrane structure

Answer 2

plasma membrane - the cell surface membrane that separates the cell from its external environment

they are formed from phospholipid bilayer - the hydrophilic phosphate heads form the inner and outer surface of a membrane and the tails form the hydrophobic core inside the membrane

Question 3

fluid mosaic model

Answer 3

• proteins occupy various positions in the membrane
• phospholipids are free to move within the layer, this gives the membrane flexibility

Question 4

intrinsic proteins

Answer 4

or integral proteins
are transmembrane proteins that are embedded through both layers of the membrane
- this means that they have AA with hydrophobic R-groups in their external surfaces which interacts with the hydrophobic tails of the inner core

Channel proteins
Carrier proteins
glycoproteins

Question 5

channel proteins

Answer 5

intrinsic proteins

• provide a hydrophobic channel that allows the passive movement of polar molecules and ions down a conc gradient through membranes
• they are held in place by the interactions of the hydrophobic R-groups of the AA of the channel protein and the core of the membrane

Question 6

carrier proteins

Answer 6

role in passive and active transport into a cell
- it involves the shape of the protein changing

Question 7

glycoproteins

Answer 7

• intrinsic proteins
• embedded in the cell-surface membrane with attached carbohydrate chains
• play a role in cell adhesions and as receptors for chemical signals (cell signalling)
  -when a chemical binds to the receptors it causes a response from the cell (like a direct response or a cascade)
• e.g neurotransmitters or peptide hormones

Question 8

are glycolipids intrinsic proteins

Answer 8

NOPE
glycolipids are lipids with an attached carbohydrate chain- these are not proteins

• these molecules are cell markers/antigens and can be recognised by the immune system as self or non-self

Question 9

what are extrinsic proteins

Answer 9

they are proteins present on one side of the bilayer

• usually have hydrophilic r-groups that interact with the polar heads of the phospholipids

Question 10

Cholesterol as a component of the cell-surface

Answer 10

• it is a lipid with a hydrophobic end and hydrophilic end
• it regulates the fluidity of the cell membrane#
• positions between phospholipids in a membrane
• its hydrophobic end interacts with the tails and the hydrophilic end interacts with the heads pulling them together
• this increases stability of the plasma membrane
• it also prevents them from becoming too solid by preventing phospholipids from becoming too close together and crystallising

Question 11

how does temperature affect membrane structure

Answer 11

• when temp is increased the kinetic energy of the PM increases and they will move more
• this makes the PM more fluid and it begins to lose its structure
• if temperature increases too far the cell will eventually break down its membrane
• this lose of structure increases the permeability of PM
• intrinsic proteins will also be denatured at these high temperatures - affecting permeability of the PM

Question 12

water as a solvent for PM

Answer 12

.water is essential for formation of the bilayer
- non-polar tails are orientated away from the water forming a bilayer with the a hydrophobic core
- charged heads interact withy the water

Question 13

organic solvents and how they effect PM structure
more conc vs less conc alcoholic solvents

Answer 13

• organic solvents are usually less polar than water like alcohols
• organic solvents will dissolve membranes disrupting cells
• in strong alcoholic solutions they are very toxic to cells and destroy them
• in less conc alcohol solutions they can enter the cell membrane and disrupt the molecule
  -> the membrane becomes more fluid and more permeable

Question 14

what is passive movement

Answer 14

utilises the energy from the natural motion of the particles rather from an energy source like ATP

Question 15

diffusion

Answer 15

the net or overall movement of particles from a region of higher conc to a region of lower con

• it is passive and will continue until a concentration equilibrium is met

(only occurs between the different conc of the same substance)

• movement of liquid or gas is random. the equilibrium doesn’t mean no movement, but that the movements are equal in both directions

Question 16

is diffusion faster over short or large distances

Answer 16

• particles move at high speeds and collide, which slows them down
• over short distances the diffusion is fast, but as diffusion distance increases the rate of diffusion decreases as more collisions have taken place (reason why its better for cells to be small)

Question 17

how does temp affect diffusion

Answer 17

it increases diffusion rate as molecules have more kinetic energy and move at higher speeds

Question 18

how does conc difference affect rate of diffusion

Answer 18

the greater the difference in the conc between two regions the faster the rate of diffusion. because the overall movement from the higher conc to the lower conc is larger

Question 19

simple diffusion

Answer 19

diffusion in the absence of a barrier of membrane

Question 20

what molecules can diffuse across a membrane

Answer 20

• diffusion across a membrane involves the phospholipid bilayer
• it can only happen if the membrane is permeable to the particles
• non polar molecules like O2 can diffuse freely across
• the hydrophobic interior repel substances with a charge like ions, so they cant easily pass through
• polar molecules like H2O can diffuse through but only at a slow rate
• small membrane pass easier than large ones

therefore it is described as partially permeable

Question 21

what can change the rate at which molecules can pass through a membrane

Answer 21

surface area- the larger the surface area the higher rate of diffusion

thickness of the membrane - the thinner the exchange surface the higher the rate of diffusion

Question 22

facilitated diffusion

Answer 22

it is diffusion across a membrane through channel proteins
- polar ions and molecules can pass through the protein channels
- membranes with protein channels are selectively permeable
- can involve a carrier protein
- rate of facilitated diffusion can be affected by temperature, conc grad, surface area and thickness. as well as number of channel proteins present

Question 23

active transport

Answer 23

• the movement of molecules or ions across a membrane from a lower conc to a higher conc
• This process requires the input of energy, usually in the form of ATP (adenosine triphosphate), to drive the movement of substances against their electrochemical gradient.
• can involve carrier proteins

Question 24

explain the process of active transport through a carrier protein

Answer 24

1) molecule binds to the receptor in the channel of the carrier protein on the outside of the cell
2) on the inside of the cell ATP binds to the carrier protein and is hydrolysed to ADP and Pi
3) binding of Pi causes carrier protein to change shape and open up to the inside of the cell
4) molecule is released into the cell
]5) Pi is released form carrier and combines with ADP to form AT{
6) carrier protein returns to normal

Question 25

what is bulk transport

Answer 25

the movement of large substances such as enzymes hormones and whole cells like bacteria
- these r too large to go through channel or carrier proteins so are moved by bulk transport

Question 26

what is endocytosis

Answer 26

• bulk transport of materials into cells
  two types:
• phagocytosis is for solids
• pinocytosis is for liquids
  -> the cell surface membrane invaginates when it comes into contact with the material
  -> membrane enfolds the material until the membrane fuses forming a vesicle
  -> the vesicle pinches of and moves into the cytoplasm to transfer the material

Question 27

what is exocytosis

Answer 27

• bulk transport of materials out of the cell
• vesicles usually formed by the golgi move towards and fuse with the cell membrane and release the contents outside of the cells

Question 28

osmosis

Answer 28

the net movement of water molecules across a selectively permeable membrane from an area of higher water potential (lower solute concentration) to an area of lower water potential (higher solute concentration), down the concentration gradient. Osmosis occurs until equilibrium is reached, where there is an equal concentration of water molecules on both sides of the membrane.

Question 29

water potential

Answer 29

• the pressure exerted by water molecules when they collide with a membrane of container
• measures in Pa or kPa

Question 30

water potential of pure water
- how is it relative to other water potentials

Answer 30

0kPa (at RTP)
- it is the highest value for Ψ as the presence of a solute lowers the Ψ below )
- the more negative the Ψ the more solutes it has (the more concentrated)

Question 31

hydrostatic pressure

Answer 31

the pressure exerted by a fluid at equilibrium at a given point within the fluid

Question 32

animal cell is placed in a solution with higher Ψ than in the cytoplasm

Answer 32

• net movement of water into the cell
• increasing hydrostatic pressure
• swells and bursts - cytolysis

Question 33

animal cell is placed in a solution with lower Ψ than in the cytoplasm

Answer 33

• net movement of water out of the cell
• decreasing hydrostatic pressure
• crenation

Question 34

plant cells response to fluid control vs plants

Answer 34

• plant cells cant control the Ψ of the fluid around them
• they have strong cellulose walls

Question 35

plant cell is placed in a solution with higher Ψ than in the cytoplasm

Answer 35

• net movement of water into the cell
• increasing hydrostatic pressure
• swells and becomes turgid

Question 36

plant cell is placed in a solution with lower Ψ than in the cytoplasm

Answer 36

• net movement of water out of the cell
• decreasing hydrostatic pressure
• plasmolysis, contents shrink