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Biology - AS Level > Transport Across Cell Membranes > Flashcards

Transport Across Cell Membranes Flashcards

1
Q

what is the importance of the cell membrane

A

-prevent organelles from escaping
-controls what can enter and leave
-cell recognition and communication
-compartmentalise
-site of biochemical reactions

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2
Q

what is the model of the cell surface membrane called

A

fluid mosaic model

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3
Q

what does the membrane consist of

A

-phospholipid bilayer being free to move about by lateral diffusion
-tails point inwards to the interior of the membrane (hydrophobic) and heads point outwards (hydrophilic)
-the tails can be saturated or unsaturated
-cholesterol- plays an important role in the mechanical stability of the membrane and prevents uncontrolled diffusion
-protein molecules that are free too move about
-intrinsic membrane protein - completely span the bilayer
-extrinsic ‘’’- dont go fully through

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4
Q

what is the difference between sat and unsat

A

unsat makes the membrane more fluid as they fit together more loosely- higher percentage

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5
Q

what other type of lipid does the bilayer contain

A

cholesterol

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6
Q

how does cholesterol help the membrane

A

provides mechanical stability/support
the higher the cholesterol the more fluid the membrane is and vice-versa
Cholesterol molecules sit in between the phospholipids, preventing them from packing too closely together when temperatures are low; this prevents membranes from freezing and fracturing.
Interaction between cholesterol and phospholipid tails also stabilises the cell membrane at higher temperatures by stopping the membrane from becoming too fluid
Cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them and causing phospholipids to pack more closely together

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7
Q

what other substance does the bilayer contain

A

proteins
-intrinsic - completely span the bilayer
-extrinsic - only on one side of the bilayer

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8
Q

what are the roles of intrinsic proteins

A

-transportation
-channel proteins
-gated channel proteins
-receptor proteins

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9
Q

what are the roles of extrinsic proteins

A

-informational
-enzymes
-receptors
-cell surface antigens
-components in energy transfer

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10
Q

what do some lipids and proteins have in the bilayer

A

short, branchy carbs
gylcoproteins and gylcolipids

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11
Q

why is the membrane described as fluid and mosaic

A

The fluid mosaic model describes cell membranes as ‘fluid’ because:
The phospholipids and proteins can move around via diffusion
The phospholipids mainly move sideways, within their own layers
The many different types of proteins interspersed throughout the bilayer move about within it (a bit like icebergs in the sea) although some may be fixed in position
The fluid mosaic model describes cell membranes as ‘mosaics’ because:
The scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above

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12
Q

what is the membrane also described as

A

partially/selectively permeable barrier

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13
Q

what is diffusion

A

The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration.

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14
Q

what factors affect simple diffusion

A

-size and nature of the particles - small-lipid soluble and uncharged can pass through whereas large polar water molecules struggle
-conc gradient- greater concentration gradient/difference the faster the overall net movement
-length of diffusion path-shorter = quicker eg capillary walls and villi and alveoli walls
-surface area bwt two regions -larger SA = faster rate
eg villi and microvilli in small intestine, alveoli , fish gills and tracheal system in insects
-temp - higher temp = more kinetic energy and diffuse quicker

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15
Q

what is fricks law

A

calcs rate of diffusion
surface area x conc difference/ thickness of membrane

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16
Q

what is facilitated diffusion

A

refers to the transport of large polar molecules and ions into and out of a cell with aid of channel/carrier protein

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17
Q

what helps these ions / polar molecules diffuse

A

These substances can only cross the phospholipid bilayer with the help of certain protein

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18
Q

what is a carrier protein

A

carrier proteins can switch between two shapes
This causes the binding site of the carrier protein to be open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape
The direction of movement of molecules diffusing across the membrane depends on their relative concentration on each side of the membrane
Net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient (from an area containing many of that specific molecule to an area containing less of that molecule)
substance deposited on the other side of the membrane

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19
Q

what is a channel protein

A

Channel proteins are water-filled pores
They allow charged substances (eg. ions) to diffuse through the cell membrane- pass through protein avoiding membrane
The diffusion of these ions does not occur freely, most channel proteins are ‘gated’, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore
this is due to an electrochemical conc gradient
This allows the channel protein to control the exchange of ions
these proteins have a specific shape so will only let a specific molecule or ion through- due to tertiary structure

20
Q

what does diffusion depend on

A

size
solubility
charge

21
Q

what is osmosis

A

net movement of water from a region of higher water potential to a region of their lower water potential through a partially permeable membrane down conc gradient

22
Q

what do the solute molecules in the solution do (osmosis)

A

interact with water and form weak bonds meaning the water cannot move as freely

23
Q

what is water potential

A

Pressure created by water molecules
Measure of the extent at which a solution gives out water

24
Q

in which way does water move

A

high to low

25
Q

what is the water potential of pure water

A

0

26
Q

why do solutions with a solute in them have a -ve water potential

A

presence of solute reduces potential of water molecules to move

27
Q

the more concentrated the solution

A

the more negative it is

28
Q

in which way does the water move

A

to an area where the water potential is more negative

29
Q

what is an isotonic solution

A

same conc of solute and water molecules no change in cell structure

30
Q

what is a hypertonic solution

A

lower conc of water and high conc of solute - lower water potential so water moves out of cell by osmosis and cells shrink

31
Q

what is hypotonic solution

A

high conc of water less solute so higher external water potential and water moves in by osmosis causing them to swell

32
Q

what is turgor pressure

A

when water enters a plant cell vacuole causes them to swell due to high pressure potential which keeps the cell turgid- high water conc outside

33
Q

why does the vacuole swell in osmosis

A

cell sap has high conc of ions and very negative solute potential and water potential

34
Q

plant cell in hypertonic

A

pressure/ turgor pressure drops and cell becomes flaccid and cytoplasm shrinks away from cell wall - cell is plasmolysed

35
Q

what does water potential depend on

A

solute potential -attractive forces of solute and water molecules reduce water molecules movement
pressure potential- membrane exerts force onto cell which makes conc of water molecules increase so causes the cell to squeeze water out

36
Q

water potential equation

A

water potential= solute + pressure

37
Q

what is active transport

A

the movement of substances against a concentration gradient which requires energy in the form of ATP

38
Q

what is the role of transporter proteins in active transport

A

transport one substance at a time - changes shape to fit molecule
transport one things in and one thing out eg Na= and K+ ion
transport two things on the same direction- eg Na+ glucose

39
Q

what are some examples of active transport

A

-absorption of amino acids in gut
-sodium-potassium pump
-excretion of urea and hydrogen ions in the kidney
-absorption of mineral ions by plant roots

40
Q

what is the sodium-potassium pump

A

inside cell- binds 3 molecules of Na+ and a molecule of ATP
in protein- ATP splits and changes shape of the channel and sodium ions driven through
outside cell- sodium ions released and new shape allows 2 K+ ions to bind these are then released inside the cell

41
Q

describe the process of the uptake of glucose by epithelial cells

A

stage 1 - sodium ions pumped out of the cell into the blood plasma by active transport and potassium ions are pumped into the cell- this sets up a concentration gradient
stage 2 - there is now a low conc of sodium ions in the cell as there is more sodium ions outside in the lumen than in enabling a conc gradient
stage 3 - sodium ions diffuse through the carrier protein by binding to it - this enables a glucose molecule to also bind to the protein enabling the glucose to enter the epithelial cell
stage 4 - the glucose then enters the blood via facilitated diffusion as it is a large molecule so needs a carrier protien

42
Q

what factors affect rate of movement of molecule in AT

A

-conc gradient
-Surface area
-density of carrier proteins

43
Q

reading AT from a graph

A

-graph levels off bc all channel proteins are saturated

44
Q

how does oxygen affect rate of uptake of mg ions

A

in solutions of high oxygen- enough required for production of ATP via respiration

45
Q

Required practice beetroot

A
  • cut evenly sliced sections of plant tissue using a scalpel and weigh
  • make up solutions of 20 cm3 using different conc of sucrose eg 0,0.2,0.4,0.6,0.8,1
    -make up the rest with distilled water
    -add to water bath and add thermometer make sure water bath reaches 30
    -place chips in tt and time for 20 min
    -remove ,blot and re weigh to see change in mass and work our % change
    -plot graph of % change against sucrose
    -Draw a line when graph reaches x
    -then use another source to find water potential

Biology - AS Level (13 decks)

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