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Biology > 2.1.5: Biological Membranes > Flashcards

2.1.5: Biological Membranes Flashcards

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1
Q

List the roles of membranes in cells

A

1.partially permeable barriers between the cell and its environment, between organelles and the cytoplasm and within organelles
2. sites of chemical reactions
3. sites of cell communication (cell signalling).
4. Regulates transport of materials into and out of cells
5. Contain enzymes involved in specific metabolic pathways
6. Has antigens so the organism’s immune system can recognise the cell as ‘self’ and not attack it

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2
Q

What is the theory for the model of the membrane structure?

A

The fluid mosaic model

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3
Q

What are the 4 main componenets the fluid mosaic model consists of?

A
  • Phospholipids
  • Cholesterol
  • Glycoproteins and glycolipids
  • Transport proteins
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4
Q

Describe the fluid mosaic model

A

Fluid
* The phospholipid bilayer in which individual phospholipids can move gives membrane their flexible shape

Mosaic
* Intrinsic and extrinsic proteins of different shapes and sizes are embedded.

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5
Q

Explain the role of phospholipids in the membrane

A
  • Form phospholipid bilayer
  • Tails = hydrophobic core, heads= inner and outer surface of membrane
  • Bilayer acts as a barrier to most water soluble substances as the non-polar fatty acid tails prevent polar molecules or ions from passing across the membrane
  • This stops water-soluble molecules (eg sugars, amino acids and proteins) from leaking out of cell + unwanted water soluble molecules cannot get in
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6
Q

Explain the role of cholestrol in the membrane

A
  • Cholestrol = a lipid with a hydrophilic end and a hydrophobic end
  • Positioned between phospholipids in bilayer with the hydrophillic end interacting with the heads and the hydrophobic end acting with the tails
  • This causes the phospholipids to pack more closely together
  • This stabilises the cell membrane at higher temperatures by stopping the membrane from becoming too fluid
  • Increases the fluidity of the membrane, stopping it from becoming too rigid at low temperatures
  • This allows cells to survive at lower temperatures
  • Increases the mechanical strength and stability of membranes
  • Without this, the membranes would break down the cells burst
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7
Q

Explain the role of membrane proteins in the membrane

A

Membrane proteins have 2 important roles in various functions.
There are 2 types of proteins:
- extrinsic
- intrinsic

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8
Q

Explain what intrinsic/integral proteins in the membrane are

A
  • These are transmembrane proteins that are embedded through both layers of a membrane
  • They have amino acids with hydrophobic R-groups on their external surface
  • These interact with the hydrophobic core of the membrane and keeps them in place

Channel and carrier proteins are intrinsic proteins. Glycoproteins are also intrinsic proteins

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9
Q

Explain the role of channel proteins in the membrane

A
  • Provide a hydrophilic channel
  • This allows passive movement of polar molecules and ions down a concentration gradient through membranes
  • They are held in position by interactions between the hydrophobic core of the membrane and the hydrophobic R-groups on the outside of the proteins
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10
Q

Explain the role of carrier proteins in the membrane

A
  • Role in passive transport (down a concentration gradient)
  • Role in active transport (against concentration gradient)
  • Active transport involves the shape of the protein changing.
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11
Q

Explain the role of glycoproteins in the membrane

A
  • Intrinsic proteins
  • Embedded in cell-surface membrane with attatched carbohydrate (sugar) chains of varying lengths and shapes
  • Role in cell adhesion (when cells join together to form tight junctions in certain tissues)
  • Role as receptors for chemical signals
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12
Q

Explain cell communication/signalling

A
  • It is when the chemical binds to the receptor which elicits a response in the cell
  • This may be a direct response or set off a cascade of events inside the cell.

Some examples:
- Receptor= insulin receptor on liver cell, chemical signal= insulin
- Dopamine receptor –> dopamine

Some drugs act by binding to cell receptors. Eg beta blockers are used to reduce response of heart to stress

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13
Q

Explain the role of glycolipids in the membrane

A
  • Similar to glycoproteins
  • Lipids with attatched carbohydrate (sugar) chains which are called antigens or cell markers
  • The antigens can be recognised by the cells immune system as self or non self
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14
Q

Explain the role of extrinsic proteins/peripheral proteins in the membrane

A
  • Present in one side of the bilayer
  • Have hydropholic R-groups on outer surfaces
  • These interact with the polar heads of the phospholipids or with intrinsic proteins
  • Present in either layer and some move between layers
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15
Q

Factors Affecting Membrane Structure & Permeability- temperature

A

Proteins + lipids are both effected by temperature:
As temperature increases:
* lipids become more fluid
* This increased fluidity reduces the effectiveness of the cell membrane as a barrier to polar molecules
* This means polar molecules can pass through

Diffusion also occurs at a high speed due to higher kinetic energy

As temperature reduces:
* the lipids will return to their normal levels of fluidity

After optimum tempersture:
* Proteins denature
* This disrupts the membrane structure, meaning it no longer forms an effective barrier
* substances can pass freely through the disrupted membrane
* This is irreversible

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16
Q

Factors Affecting Membrane Structure & Permeability- solvents

A
  • Organic substances can increase cell membrane permeability
  • as they dissolve the lipids in the membrane
  • causing the membrane to lose its structure
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17
Q

Investigating cell membrane permeability – the effect of temperature on beetroot cell membranes:

A
  1. Use a scalpel to cut five equal sizes of beetroot.
  2. Rinse the pieces to remove any pigment released during cutting.
  3. Add the 5 pieces to 5 different test tubes (labelled with the temperature of water they will be going in), each containing 5 cm3 of water.
  4. Place each test tube of in a water bath at different temperatures for the same length of time using a stopwatch.
  5. Remove the pieces of beetroot, leaving just the liquid.
  6. Carry out colorimetry (use the blue filter): pipette the solution into the cuvette and place in colorimete with the transparent sides pointing correct directions.
  7. The higher the permeability, the more pigment released, the higher the absorbance reading.
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18
Q

Define diffusion

A

The net movement of particles from a region of high concentration of that particle to a region of low concentration of that particle.
* This is down a concentration gradient
* No metabolic energy is required so its a pssive process

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19
Q

Why does diffusion occur?

A

As a result of the random motion of its molecules or ions. The random movement is caused by the natural kinetic energy of the molecules or ions.
A dynamic equillibrium will eventually be reached.

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20
Q

Is diffusion fast + why?

A
  • Particles move at slow speeds + are constantly colliding
  • This slows down their overall movement
  • So, over short distances diffusion is fast
  • But, as diffusion distance increases, diffusion speed decreases
  • As, more collisons have taken place
21
Q

Factors effecting rate of diffusion- temperature

A
  • The higher the temperature,
  • The higher the rate of diffusion
  • This is because the particles have more kinetic energy
  • And move at higher speeds
22
Q

Factors effecting rate of diffusion- concentration gradient

A
  • The greater the difference in concentration between the two regions,
  • the faster the rate of diffusion.
  • Because, the overall movement from the higher concentration to the lower concentration will be larger.
23
Q

Factores that effect the rate of diffucion PAG 8- concentration

A
  1. Make some agar jelly with phenolphthalein and dilute sodium hydroxide. This will
    make the jelly pink.
  2. Prepare 5 test tubes containing HCl in increasing concentrations e.g. 0.2M, 0.4M,
    0.6M, 0.8M and 1.0M (synoptic link – serial dilutions).
  3. Using a scalpel, cut out 5 equal size cubes from the agar jelly.
  4. Put one of the cubes into the first test tube and use a stopwatch to time how long it
    takes for the cube to turn colourless.
  5. Then repeat for the other concentrations, using a new cube each time.
  6. The highest concentration of HCl will have the fastest colour change as it has the
    steepest concentration gradient.
24
Q

Factors that effect the rate of diffusion PAG 8- surface area

A
  1. Make some agar jelly with phenolphthalein. This will make the jelly pink in the presence of an alkali.
  2. Prepare 5 test tubes containing the same concentration and volume of sodium hydroxide
  3. Using a scalpel, cut out 5 cubes from the agar jelly. Increase the surface area of these each time. Eg 1 x 1 x1, 2 x 2 x2, 3 x 3 x 3, 4 x 4 x 4, 5 x 5 x 5 cm
  4. Put each cube into a test tube for 10 miniutes
  5. Remove cubes and measure with a ruler how far the sodium hydroxide had diffused (where the sodium hydroxide had diffused would be pink)
  6. The smallest cube with the greatest surface area to volume ratio would have the highest level of sodium hydroxide
25
Q

Diffusion across membranes:
* What is involved
* What can pass through
* why are membranes partially permeables

A
  • Involves passing through the phospholipid bilayer
  • Can only occur if the membrane is permeable to the particles
  • Non-polar molecules eg, oxygen diffuse freely down a conc gradient
  • The hydrophobic interior of the membrane repel substances with a positive or negative charge, (so repel ions)
  • So these cannot easily pass through
  • Polar molecules can pass through at a slow rate (have both positive + negative sections) eg water
  • Small polar molecules diffuse more easily than larger ones
  • So membraned are partially permeable
26
Q

The rate at which molecules diffuse or pass through membranes is effected by:

A
  1. surface area- the large the area if the exchange surface, the higher the rate of diffusion
  2. thickness of the membrane- the thinner the exchange surface, the higher the rate of diffusion
27
Q

Define facilitated diffusion

A

Movement of molecules from an area of high concentration of that molecule to an area of low concentration of that molecule, through a partoally permeable membrane view protein channels of carrier proteins. It doesn’t involve metabolic energy

28
Q

Membranes with protein channels are called ———— as ——————————

A

Membranes with protein channels are called selectively permeable as most protein channels are specific to one molecule or ion

29
Q

Facilitated diffusion can involve 2 types of proteins:

A
  1. Channel proteins- where polar molecules and ions can pass through
  2. Carrier proteins- These change shape when a specific molecule binds
30
Q

How does the number of protein channels effect facilitated diffusion

A

The more protein channels, the higher the rate of diffusion overall

31
Q

Explain what is meant by active transport

A
  • The movement of molecules or ions into or out of a cell, from a region of lower concentration to a region of higher concentration.
  • It requires energy so uses ATP made in aerobic respiration (so will stop if theres no oxygen available or enzymes are disrupted)
  • Involves a transmembrane protein pump/carrier protein
  • Each protein pump is highly specific so there is a different pump for each molecule to be transported
32
Q

Describe the process of active transport

A
  1. The molecule or ion to be transported binds to receptors in the channel of the carrier protein on thr outside of the cell
  2. On the inside of the cell ATP binds to the carrier protein and is hydrolysed into ADP and phosphate
  3. Binding of the phosphate molecule to the carrier protein causes the protein to change shape- opening up to the inside of the cell
  4. The molecule or ion is released to the inside of the cell
  5. The phosphate molecule is released from the carrier protein and recombines with ADP to form ATP
  6. The carrier protein returns to its original shape

The process is selective- specific substances are transported by specific carrier proteins

33
Q

What is bulk transport and what is involved

A
  • Another form of active transport
  • Large molecules eg, enzymes, hormones and whole cells like bacteria are too large to move through carrier proteins and channel proteins
  • So they are moved by bulk transport
  • Involves vesicles which move through cytoplasm and fuse onto membranes
  • ATP requires to form + move vesicles
34
Q

Describe and explain endocytosis

A
  • Bulk transport of materials into cells
  • 2 types: phagocytosis (for solids) and pinocytosis (for liquids).
  • Example: white blood cell engulfing bacteria which have invaded the body

Process:
* The cell surface membrane first invaginates (bends inwards) when is comes into contact with the material to be transported
* The membrane enfolds the naterial until eventually the membrane fuses- forming a vesicle
* The vesicle pinches off and moves into the cytoplasm to transfer the material for further processing

35
Q

Descibe and explain exocytosis

A
  • Reverse of endoxytosis
  • The removal of waste substances
  • Secretion: The removal of useful substances to the outside of the cell
  • Examples: Digestive enzymes by pancreatic cells into the pancreatic duct, the hormone insulin by pancreatic cells into the bloodstream

Process:
* Vesicles (usually formed by the Golgi apparrats) move towards and fuse with the cell surface membrane
* The contents of the vesicle are then released outside the cell

36
Q

How do water molecules move through cell membranes

A
  • Water molecules are polar
  • Polar molcules do not pass through the phospholipid bilayer
  • BUT water molecules are so small and unusual that they CAN move through the gaps between phospholipids
  • Water also moves through protein channels
  • They move passively via diffusion
37
Q

Define osmosis

A

Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane, down a concentration gradient.

38
Q

What is water potential

A
  • Measures the pressure exerted by water molecules as they collide with the membrane or a container
  • Measures in Pascals
  • It is the relative tendance of water to move from one area to another/ the measure of the ability of water molecules to move freely in a solution
  • Water potention of pure water = 0kPa
  • Presence of solute lowere the water potential to a negative value.
  • The higher the concentration, the lower the water potential
39
Q

Osmosis continues until a …… is reached

A

dynamic equilibrium

40
Q

What is hydrostatic pressure

A

AN increase in pressure due to the diffusion of water into a solution (in a closed system eg a cell)

At a cellular level, the pressure is relatively large and potentially damaging.

41
Q

What depth is the cell surface membrane

A

7 nm

42
Q

When cells are placed in solutions they can be described in 3 ways.
* What is the net movement of water in each solutionn
* Which has a greate water potential: cell or solution

A

Isotonic
1. No net movement of water. At equilibrium
2. Cutoplasm and solution both have same water potential

HypOtonic (O because cell swells)
1. Net movement of water into cell, from solution
2. Water potential of solution is higher than cells

Hypertonic
1. Net movement from cell into solution
2. Water potention of solution is lower than cells

43
Q

Describe + explain what happens to Plant Cells when placed in hypertonic solutions

A

**Water moves out of cells as water potential of solution is lower **
* Water leaves cell through partially permeable sell surface membrane by osmis
* Volume of cell decreases as water keaves the vacuole
* Protoplast gradually shrinks + stops exerting pressure on cell wall
* Protoplast pulls away from cell wall
* Cell is plasmolysed
* Cell becomes flaccid

44
Q

Describe + explain what happens to Plant Cells when placed in hypotonic solutions

A

Net movement of water into cells as there is a high water potential outside of cell
* Water enters vacuole so volume of cell increase
* Protoplast expands and psushes against cell wall
* Inelastic cell wall prevents cell from bursting ans stops too much water entering
* When fully inflated with water it becomes fully turgid
* Turgidity provides support + strength

Without enough water plant wilts

45
Q

What is incipient plasmolysis

A

The point at whicj tje plasma membrane is just about to pull away from cell wall

46
Q

Describe + explain what happens to Plant Cells when placed in isotonic solutions

A

Water constantly leaves and enters cell at equal rates
* Dynamic equillibrium
* Protoplast begins to pull away from cell wall
* No change to cell

47
Q

Describe + explain what happens to Animal Cells when placed in hypertonic solutions

A

Net movement of water outside of cell as water potential is lower outside cell
* Water leaves cell through partially permeable cell surface membrane via osmosis
* Vell shrinks and shrivels up
* This is crenation which is fatal for the cell
* In red blood cells: haemoglobin becomes more concentrated giving a darker appearance

48
Q

Describe + explain what happens to Animal Cells when placed in hypotonic solutions

A

Net movement of water into cell as cell has a lower water potential
* Water enters through partially permeable cell surface membrane via osmosis
* Gains water until cell membrane is stretched too far and cell burts- this is cytolysis
* Cytolysis occurs as there is no cell wall to withstand pressure increases
* This is fatal for cell
* Haemolysis (the destruction of red blood cells) can occur

49
Q

Describe + explain what happens to Animal Cells when placed in isotonic solutions

A
  • No net movement of water as the water is moving in and out of cells at equal rates
  • No change in state of cell

Biology (7 decks)

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