Cell Processes - Lecture 1 Flashcards

1
Q

How thick is a membrane structure

A

8nm (8x10^-9 m)

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2
Q

general features of a membrane (structure / barrier)

A
  • thin
  • flexible
  • sturdy
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3
Q

how can we describe a membrane structure

A

fluid mosaic model

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4
Q

what is a membrane made up of

A

50% lipid
50% protein
held together by Hydrogen bonds

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5
Q

What is a lipid

A

barrier to entry or exit of POLAR substances

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6
Q

what are proteins?

A

“gatekeepers” - regulate traffic

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7
Q

can molecules cross the lipid bilayer?

A

some can, some cannot

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8
Q

What is the role of cholesterol in the lipid bilayer

A

it is scattered among a double row of phospholipid molecules
- is a lipid which can insert in phospholipid backbone
- can change composition or properties of lipid bilayer

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9
Q

what is the role of glycolipids in the lipid bilayer

A

it is scattered among a double row of phospholipid molecules
- is a sugar group
- attached to hydrophobic lipid molecule which can embed in lipid bilayer

NOTE: it is a sugar group in and out of the membrane

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10
Q

Give the properties of phospholipids

A
  • comprises 75% of lipids
  • phospholipid bilayer = 2 parallel layers of molecules
  • each layer in the bilayer known as a leaflet
  • each molecule is amphipathic (has both polar & non-polar regions)
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11
Q

features of membrane fluidity

A
  • membranes are fluid structures
  • cholesterol changes fluidity
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12
Q

features of lipids according to membrane fluidity

A
  • lipids can move around within the plane of the membrane leaflet
  • lipid rarely flip flop between the leaflets
  • due to reduced flip flop, lipid composition of leaflets can be asymmetric
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13
Q

what is membrane fluidity determined by

A
  • lipid tail length –> longer tail = less fluid of the membrane
  • number of double bonds –> more double bonds = increased fluidity
  • amount of cholesterol –> more cholesterol = decreased fluidity
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14
Q

Two different types of membrane proteins and where they can be found

A
  1. Integral proteins
    extend into or completely across cell membrane
    (also known as a transmembrane protein)
  2. Peripheral proteins
    attached to inner or outer surface of cell membrane
    can be easily removed from it
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15
Q

features of integral membrane proteins

A
  • amphipathic
  • have hydrophobic regions which span hydrophobic core of lipid bilayer
  • hydrophobic region consists of non-polar amino acids coiled into helices
  • hydrophilic ends / regions interact with aqueous solution
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16
Q

what are the different functions of membrane proteins

A

Can act as:
- receptor proteins
- cell identity markers
- linkers
- enzymes (can form enzymes)
- ion channels
- transporter proteins

Ion channels & transporter proteins move things across cell membrane

17
Q

what does permeability mean

A

ability to cross membrane

18
Q

what causes selective permeability

A

the molecular organisation
^^ the membrane allows some substances to cross but excludes others

19
Q

How can be apply selective permeability to a lipid bilayer

A
  • permeable to non-polar, uncharged molecules (O2, N2, benzene)
  • permeable to lipid soluble molecules (steroids, fatty acids, some vitamins)
  • permeable to small uncharged polar molecules (water, urea, glycerol, CO2)
  • impermeable to large uncharged polar molecules (glucose, amino acids)
  • impermeable to ions (Na+, K+, Cl-, Ca2+, H+)
20
Q

How do large uncharged polar molecules AND ions move across the membrane?

A

via transport protein
^^ it moves them across the membrane

21
Q

How do substances which cannot permeate the hydrophobic core of the lipid bilayer get across? and give examples

A

membrane proteins mediate the transport of the substances across the membrane.
this is:
ion channels
transport protiens

22
Q

when does diffusion occur

A

if we have a difference in concentrations & molecules will move across the membrane until they reach equilibrium

23
Q

How does net diffusion work

A

net diffusion from a higher dye concentration to the region of lower dye

24
Q

How do we know equilibrium has been reached?

A

when the dye (or colour) is spread out evenly / evenly distributed

25
What are the principles of diffusion
- random mixing of particles in a solution due to a particles kinetic energy - more molecules move away from an area of high conc to an area of low conc - greater difference in concentration between 2 sides of the membrane = faster the rate of diffusion - higher temp = faster rate of diffusion - larger size of diffusing substance = slower rate of diffusion - increase in surface area = higher rate of diffusion - increasing diffusion distance = slower rate of diffusion
26
what are the physical consequences of diffusion
- rate of diffusion sets a limit on the size of cells of about 20nm - to increase diffusion a cell can increase membrane area available for exchange (diffusion) of a substance - thicker membrane = slower rate of diffusion - small distances = fast diffusion
27
What are the two gradients across the cell membrane
- concentration gradient - electrical gradient
28
Concentration gradient
- non charged molecules will diffuse down their concentration gradients NOTE: driving force for dilution will solely be conc gradient because they are non-charged so no electrical gradient acting the selective permeability of the membrane enables: - a difference in concentration OR - concentration gradient across the membrane to be established
29
electrical gradient
- ions will be influenced by membrane potential in addition to their conc gradient NOTE: electrochemical gradient drives cells can maintain a difference in charged ions between the inside & outside of membrane establishing an electrical gradient or membrane potential
30
what is the movement of ions influenced by
electrochemical gradient
31
What do membranes mimic
capacitors: can seperate AND store charge
32
ion gradients across the membrane
- cells use ~30% of resting energy to maintain concentration and electrical gradients - gradients represent stored energy
33
what is osmosis
net movement of water through a selectively permeable membrane from an area of high water concentration to an area of low water concentration NOTE: we want to have hemeostatis
34
features of osmosis
- only occurs if membrane is permeable to H2O but not to certain solutes - this is the situation in biological membranes - if an osmotic gradient exists water will move to eliminate it
35
osmosis equation and meaning of the variables
Pw = Pd + Pf Pw = membrane permeability to water Pd = through lipid bilayer - small - mercury insensitive - temp dependent (lipid fluidity) Pf = through water channel - large - mercury sensitive - temp independent NOTE: - Pf is mediated by the aquaporins (9 isoforms) - cells have different Pw because they express different aquaporin isoforms
36
osmotic pressure
is the pressure applied to a solution to prevent inward flow of water across semi-permeable membrane