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AQA AS (A-level) Biology > 3.2.3 Transport Across Cell Membranes > Flashcards

3.2.3 Transport Across Cell Membranes Flashcards

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1
Q

Structure of a phospholipid

A

2 fatty acids and a glycerol along with one phosphate group

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2
Q

Explain the key function of phospholipids in cell surface membranes

A
  • Form bilayers in water
  • Hydrophilic head attracts water(faces towards water) (makes contact with extra-cellular fluid or cytoplasm) - Hydrophobic tail repels water (faces away from water)
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3
Q

Which model does the cell surface membrane have?

A

Fluid mosaic model

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4
Q

Explain why cell surface membranes have a fluid structure (2)

A
  1. Phospholipids form a bilayer
  2. Molecules (that the cell surface membrane consists of) are constantly moving around (relative to one another)
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5
Q

Explain why cell surface membranes have a mosaic structure (1)

A

Protein and phospholipid molecules unevenly distributed

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6
Q

Explain why cell surface membranes are semi permeable (1)

A

Because of the type and distribution of transport proteins and phospholipid molecules present in the membrane

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7
Q

Draw and label the structure of a cell surface membrane

A

Should include:
1. Phospholipid bilayer
2. Channel protein
3. Cholesterol
4. Carrier protein
5. Receptor
6. Enzyme
7. Glycoprotein and glycolipid

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8
Q

Channel proteins

A
  • Specific tertiary structure that spans membrane and makes hydrophilic tunnels across it
  • Selective channels (only accepts one type of molecule or few closely related) for transports
  • Ions and polar molecules pass through (facilitated diffusion)
  • Aquaporins allow water to cross CSM via osmosis
  • Some open all the time, some ‘gated’ (channel opens or closes) in response to particular signal
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9
Q

Carrier proteins

A
  • Specific tertiary structure - allow transport of ions and polar molecules (facilitated diffusion)
  • Changes shape to move target molecule from one side of membrane to other
  • Selective
  • Often change shape for binding of their target molecule, moving it to opposite side of membrane
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10
Q

Enzymes in cell surface membranes

A
  • Specific, complementary active site
  • Maltese found in CSM of small intestine where it hydrolyses maltose into glucose
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11
Q

Receptors in cell surface membranes

A
  • Other protein molecules act as specific receptors for hormones with a complementary shape
  • Attach to binding site and allow cell to respond
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12
Q

Glycolipids in cell surface membranes

A

Phospholipid attached to carbohydrates - important in cell recognition

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13
Q

Glycoprotein

A

Carbohydrates and proteins - on outer surface of membrane and important in cell recognition, sometimes antigens

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14
Q

Where are glycoproteins and glycolipids produced

A

Golgi apparatus (body)

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15
Q

Cholesterol in cell surface membranes

A
  • Decreases permeability and increases stability by restricting movement of other molecules
  • Causes fatty acid chains to become compact
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16
Q

How is Fick’s law used?

A

It explains what factors influence the rate of diffusion across a cell surface membrane

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17
Q

State the equation for Fick’s law

A

Diffusion rate = (surface area x concentration gradient) ÷ diffusion distance or pathway

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18
Q

How is rate of diffusion affected when the surface area increases

A

Rate increases

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19
Q

How is rate of diffusion affected when the concentration gradient increases

A

Rate increases

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20
Q

How is rate of diffusion affected when the diffusion distance decreases

A

Rate increases

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21
Q

How is rate of diffusion affected when the temperature increases

A

Rate increases

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22
Q

How is rate of diffusion affected when the surface area deceases but concentration gradient stays the same

A

Rate stays the same

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23
Q

What is meant by a ‘passive process’?

A

Uses no ATP

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24
Q

Which type of process is simple diffusion?

A

Passive (process)

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25
Q

State the definition of simple diffusion (2)

A

The net movement of molecules from a higher concentration to a lower concentration until evenly distributed

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26
Q

What is meant by equilibrium in simple diffusion?

A

Equal number of that type of molecule on either side of the membrane - no more net movement of that molecule

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27
Q

Which type of molecules diffuse down the concentration gradient and crosses the phospholipid bilayer

A

Non polar, lipid soluble molecules

28
Q

During simple diffusion, molecules move ____________ the concentration gradient

A

Down

29
Q

How does temperature affect the rate of diffusion?

A
  • Increases kinetic energy
  • Faster movement of molecules
  • Faster rate of diffusion
30
Q

How does surface area affect the rate of diffusion?

A
  • More cell surface membrane for molecules to pass through
  • Faster rate of diffusion
31
Q

How does concentration gradient affect the rate of diffusion?

A

Increase leads to faster rate of diffusion

32
Q

How does thinner diffusion distance affect the rate of diffusion?

A

The thinner the distance, the faster the rate of diffusion

33
Q

Facilitated diffusion helps which type of molecules pass through the bilayer?

A

Polar molecules

34
Q

Which proteins does facillitated diffusion use?

A

Channel and carrier proteins

35
Q

Definition of facillitated diffusion

A

Channel and carrier proteins allow polar molecules to pass through the bilayer, DOWN the concentration gradient

36
Q

Why can’t polar molecules pass between the hydrophobic tails of the bilayer

A

Fatty acid tails are non-polar and repel polar molecules

37
Q

Facilitated diffusion is which type of process?

A

Passive

38
Q

Explain how channel and carrier proteins bind to specific molecules

A

They have specific tertiary structures and only transport molecules that have a complementary shape to the binding site

39
Q

True or false

Channel and carrier proteins are limiting factors in reactions

A

True

40
Q

Explain the effect of having more channel/carrier proteins

A

Faster rate of diffusion

41
Q

A graph shows facilitated diffusion, explain why it levels off at a certain point (1)

A

All channel or carrier proteins have filled binding sites, so are transporting at faster rate

42
Q

Explain why on a graph, simple diffusion has a higher rate of diffusion longer than facilitated diffusion (2)

A
  • Simple diffusion has the whole phospholipid bilayer or larger surface area
  • In facilitated diffusion, the channel or carrier proteins are less, so takes longer time to diffusion through
43
Q

Definition of osmosis

A

Net movement of water molecules from higher water potential to a solution with lower water potential through a partially permeable membrane

44
Q

Which channel protein does water move through

A

Aquaporin

45
Q

Explain what is meant by ‘water potential’

A

Free water molecules collide with membranes which experts pressure on them

46
Q

Water potential is measured in…

A

kPa

47
Q

How is water potential affected when there are more free water molecules?

A
  • More collisions
  • Greater the pressure
  • Increase in WP
48
Q

Pure water has a water potential of…

A

0kPa

49
Q

Why does water have a high water potential?

A

Every water molecule is free moving

50
Q

Adding solute to water makes the WP…

A

Negative

51
Q

Explain why adding solute to water makes the water potential negative, and hence, lower?

A

Water is dipole, it is attracted to charges on raise ions, so less free water molecules collide

52
Q

Hypertonic

A
  • Solution has low WP than cell
  • Water leaves cell via osmosis
  • Cell may shrivel
53
Q

Hypotonic

A
  • Solution has higher WP than cell
  • Water enters cell via osmosis
  • Cell may burst
54
Q

Isotonic

A
  • Solution has same WP as cell via osmosis
  • No net movement of water in or out of cell via osmosis
55
Q

A negative number means a ______________ water potential

A

Low

56
Q

Definition of active transport

A

Transports polar molecules across the membrane against the concentration gradient - low concentration to high concentration using carrier proteins

57
Q

Active transport is which type of process?

A

Active, requires ATP

58
Q

Describe the process of active transport (3)

A
  1. Molecule (for transport) binds to binding sites (of) specific carrier protein
  2. Hydrolysis of ATP (into ADP and Pi) provides small amount of energy (to carrier protein)
  3. (Causes) protein (to) change shape, pushing molecule through membrane/bilayer
    AND
  4. (Where) concentration of molecule is high
59
Q

A biologist isolated a sample of intestinal tissue and viewed it under an optical microscope.

He found that the sample of tissue contained lots of mitochondria.

Explain why (2)

A
  1. Mitochondria produces ATP
  2. Active transport is active process requiring ATP
60
Q

Name and describe four ways substances can move across the cell-surface membrane into a cell (4)

A
  1. (Simple) diffusion of small/non-polar molecules down a concentration gradient;

If no reference to ‘small/ non-polar’ for 1 accept this idea from ‘large/charged’ given in description of 2

  1. Facilitated diffusion down a concentration gradient via protein carrier/channel;
          *Reject if active rather than passive*
  2. Osmosis of water down a water potential gradient;
  3. Active transport against a concentration gradient via protein carrier using ATP;
61
Q

The action of the carrier protein SGLT1 is linked to a membrane-bound ATP hydrolase enzyme.

Explain the function of ATP hydrolase (2)

A
  1. (ATP to ADP + Pi ) Releases energy;
        *Reject ‘produces/makes/creates energy’*
  2. (energy) allows ions to be moved against a concentration gradient

OR

  1. (energy) allows active transport of ions;
62
Q

The movement of substances across cell membranes is affected by membrane structure.

Describe how (5)

A
  1. Phospholipid (bilayer) allows movement/diffusion of non-polar/lipid-soluble substances;
  2. and 2. Accept correct named examples
  3. and 2. Ignore water
            *Accept phospholipid (bilayer) allows movement/diffusion of O2/CO2*
        *Accept water-insoluble*
  4. Phospholipid (bilayer) prevents movement/diffusion of polar/charged/lipid-insoluble substances

OR

  1. (Membrane) proteins allow polar/charged substances to cross the membrane/bilayer;
    Accept water-soluble
  2. Carrier proteins allow active transport;
  3. Channel/carrier proteins allow facilitated diffusion/co-transport;
             *Accept aquaporins allow osmosis*
  4. Shape/charge of channel / carrier determines which substances move;
  5. Number of channels/carriers determines how much movement;
  6. Membrane surface area determines how much diffusion/movement;
  7. and 7. Accept correct reference to faster/slower/rate for ‘how much movement’
                  *Accept microvilli / Golgi (apparatus) / ER / rER*
              *Accept surface area to volume for ‘surface area’*
  8. Cholesterol affects fluidity/rigidity/permeability;
                 *Accept cholesterol affects vesicle formation/endocytosis/exocytosis/phagocytosis;*
63
Q

The cells of beetroot contain a red pigment. A student investigated the effect of temperature on the loss of red pigment from beetroot. He put discs cut from beetroot into tubes containing water. He maintained each tube at a different temperature. After 25 minutes, he measured the percentage of light passing through the water in each tube.

The student put the same volume of water in each tube.

Explain why it was important that he controlled this experimental variable (2)

A
  1. (If) too much water the concentration of pigment (in solution) will be lower / solution will appear lighter more light passes through (than expected);

OR

  1. (If) too little water the concentration of pigment (in solution) will be greater / solution will appear darker / less light passes through (than expected); So results (from different temperatures) are comparable;
          *Ignore reference to too much water so red pigment / solution too weak to measure*
64
Q

The cells of beetroot contain a red pigment. A student investigated the effect of temperature on the loss of red pigment from beetroot. He put discs cut from beetroot into tubes containing water. He maintained each tube at a different temperature. After 25 minutes, he measured the percentage of light passing through the water in each tube.

Describe a method the student could have used to monitor the temperature of the water in each tube (1)

A

(Take) readings (during the experiment) using a (digital) thermometer / temperature sensor;

65
Q

In mammals, in the early stages of pregnancy, a developing embryo exchanges substances with its mother via cells in the lining of the uterus. At this stage, there is a high concentration of glycogen in cells lining the uterus.

Suggest and explain two ways the cell-surface membranes of the cells lining the uterus may be adapted to allow rapid transport of nutrients (2)

A
  1. Membrane folded so increased / large surface area;
    OR
    Membrane has increased / large surface area for (fast) diffusion / facilitated diffusion / active transport / co-transport;
  2. Large number of protein channels / carriers (in membrane) for facilitated diffusion;
  3. Large number of protein carriers (in membrane) for active transport;
  4. Large number of protein (channels / carriers in membrane) for co-transport;
  5. Accept ‘microvilli to increase surface area’
  6. Reject reference to villi.
    Note feature and function required for each marking
    point and reference to large / many / more.
    List rule applies.
66
Q

Water and inorganic ions have important biological functions within a cell.

Compare and contrast the processes by which water and inorganic ions enter cells (3)

A
  1. Comparison: both move down concentration gradient;
  2. Comparison: both move through (protein) channels in membrane;
              *Accept aquaporins *(for water)* and ion channels*
  3. Contrast: ions can move against a concentration gradient by active transport

AQA AS (A-level) Biology (13 decks)

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