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IB Biology SL > Cell membrane transport 1.4 > Flashcards

Cell membrane transport 1.4 Flashcards

1
Q

What 2 key qualities do cell membranes possess?

A

They are:
- Semi-permeable - only certain materials may cross freely (large and charged substances are typically blocked)
- Selective - membrane proteins may regulate the passage of material that cannot freely cross

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2
Q

In what two ways can movement of material across membranes occur?

A
  • Actively
    or
  • Passively
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3
Q

How does material move across the membrane in passive transport?

A

It moves along a concentration gradient
- High concentration –> low concentration

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4
Q

Does passive transport require ATP?

A

No, because the materials are moving down a concentration gradient so does not require energy

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5
Q

What are the three main types of passive transport?

A
  • Simple diffusion - movement of small or lipophilic molecules (e.g. O2, CO2, etc.)
  • Osmosis - movement of water molecules (dependent on solute concentrations)
  • Facilitated diffusion - movement of large/ charged molecules via membrane proteins (e.g. ions, sucrose, etc.)
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6
Q

How is material moved across the membrane in active transport?

A

The material moves against a concentration gradient (low concentration –> high concentration)

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7
Q

Does active transport require ATP/energy?

A

Yes, because materials are moving against the gradient

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8
Q

What are the two main types of active transport?

A
  • Primary (direct) active transport - involves the direct use of metabolic energy (ATP hydrolysis) to mediate transport
  • Secondary (indirect) active transport - involves coupling the molecule with another moving along an electrochemical gradient.
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9
Q

What is diffusion?

A

The net movement of molecules from a region of high concentration to a region of low concentration

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10
Q

Is diffusion passive or active?

A

Passive

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11
Q

What is it called when molecules become evenly dispersed?

A

Equilibrium

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12
Q

What molecules freely diffuse across cell membranes in simple diffusion?

A

Small + non-polar (lipophilic) molecules
- e.g. CO2, O2, Glycerol

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13
Q

What factors can affect the rate of diffusion?

A
  • Temperature - affects kinetic energy of particles in a solution
  • Molecular size - larger particles are subjected to greater resistance within a fluid medium
  • Steepness of gradient - rate of diffusion will be greater with a higher concentration gradient
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14
Q

What is osmosis?

A

The net movement of water molecules across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration (until equilibrium is reached)

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15
Q

Why is water considered the universal solvent?

A

Because it will associate with, and dissolve, polar or charged molecules (solutes)

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16
Q

Why does water move to equalise two solutions?

A

Because solutes cannot cross a cell membrane unaided

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17
Q

How many free water molecules can be found in a higher concentration of solutes?

A

There are less free water molecules in solution as water is associated with the solutes

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18
Q

What is osmosis essentially?

A

The diffusion of free water molecules and hence occurs from regions of low solute concentration

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19
Q

What is osmolarity?

A

A measure of solute concentration, as defined by the number of osmoles of a solute per litre of solution (osmol/L)

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20
Q

What osmolarity is categorised as hypertonic?

A

Solutions with a relatively high osmolarity (high solute concentration => gains water)

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21
Q

What osmolarity is categorised as hypotonic?

A

Solutions with a relatively low osmolarity (low solute concentration => loses water)

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22
Q

What osmolarity is categorised as isotonic?

A

Solutions that have the same osmolarity (same solute concentration => no net water flow)

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23
Q

How do you estimate the osmolarity of a tissue?

A

The osmolarity of a tissue may be interpolated by bathing the sample in solutions with known osmolarities

  • The tissue will lose water when placed in hypertonic solutions and gain water when placed in hypotonic solutions
  • Water loss or gain may be determined by weighing the sample before and after bathing the solution
  • Tissue osmolarity may be inferred by identifying the concentration of solution at which there is no weight change (i.e. isotonic)
24
Q

What negative effects in regards to cell viability will uncontrolled osmosis in hypertonic solutions have?

A

Water will leave the cell causing it to shrivel (crenation)

25
Q

What negative effects in regards to cell viability will uncontrolled osmosis in hypotonic solutions have?

A

Water will leave the cell causing it to burst (lysis)

26
Q

In plant tissues, why is the effect of uncontrolled osmosis moderated?

A

Because of the presence of an inflexible cell wall

27
Q

What will the effect of uncontrolled osmosis in hypertonic solutions have on plant tissues?

A

The cytoplasm will shrink (plasmolysis) but the cell wall will maintain a structured shape

28
Q

What will the effect of uncontrolled osmosis in hypotonic solutions have on plant tissues?

A

The cytoplasm will expand but be unable to rupture within the constraints of the cell wall (turgor)

29
Q

What is facilitated diffusion?

A

The passive movement of molecules across the cell membrane via the aid of a membrane protein

30
Q

What molecules utilise facilitated diffusion?

A

By molecules that are unable to freely cross the phospholipid bilayer
- e.g. large, polar molecules + ions

31
Q

Which two transport proteins mediate the process of facilitated diffusion?

A
  • Channel proteins
    and/or
  • Carrier proteins
32
Q

What are carrier proteins and how do they work in facilitated diffusion?

A

Integral glycoproteins which bind a solute + undergo a conformational change to translocate the solute across the membrane
- Carrier proteins will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction
- Carrier proteins can move molecules against concentration gradients in the presence of ATP (i.e. are used in active transport)
- Carrier proteins have a much slower rate of transport than channel proteins

33
Q

What are channel proteins and how do they work in terms of membrane transport?

A

Integral lipoproteins which contain a pore via which ions may cross from one side of the membrane to the other
- Channel proteins are ion-selective and may be gated to regulate the passage of ions in response to certain stimuli
- Channel proteins only move molecules along a concentration gradient (i.e. are not used in active transport)
- Channel proteins have a much faster rate of transport than carrier proteins

34
Q

How do the axons of nerves transmit electrical impulses?

A

By translocating ions to create a voltage difference across the membrane

  • At rest, the sodium-potassium pump expels sodium ions from the nerve cell while potassium ions are accumulated within
  • When the neuron fires, these ions swap locations via facilitated diffusion via sodium and potassium channels
35
Q

How do Potassium channels work?

A
  • Integral proteins with a hydrophilic inner pore via which potassium ions may be transported
  • The channel is comprised of four trans-membrane sub-units, while the inner pore contains a selectivity filter at its narrowest region that restricts passage of alternative ions
  • Potassium channels are typically voltage-gated and cycle between an opened and closed conformation depending on the transmembrane voltage
36
Q

What is active transport?

A

Active transport uses energy to move molecules against a concentration gradient

37
Q

How can the energy used in active transport be generated?

A
  • The direct hydrolysis of ATP (primary active transport)
  • Indirectly coupling transport with another molecule that is moving along its gradient (secondary active transport)
38
Q

What proteins does active transport require the use of?

A

Carrier proteins called protein pumps (due to their use of energy)

39
Q

How does active transport work?

A
  • A specific solute will bind to the protein pump on one side of the membrane
  • The hydrolysis of ATP (to ADP + Pi) causes a conformational change in the protein pump
  • The solute molecule is consequently translocated across the membrane (against the gradient) + released
40
Q

What is a sodium potassium pump?

A

An integral protein that exchanges 3 sodium ions (moves out of cell) with two potassium ions (moves into cell)

41
Q

What key steps are involved in the process of energy-dependent ion exchange against the gradient?

A
  1. Three sodium ions bind to intracellular sites on the sodium-potassium pump
  2. A phosphate group is transferred to the pump via the hydrolysis of ATP
  3. The pump undergoes a conformational change, translocating sodium across the membrane
  4. The conformational change exposes two potassium binding sites on the extracellular surface of the pump
  5. The phosphate group is released which causes the pump to return to its original formation
  6. This translocates the potassium across the membrane, completing the ion exchange
42
Q

What are vesicles?

A

Membranous containers that transport materials destined for secretion around the cell

43
Q

What is the endoplasmic reticulum (ER)?

A

A membranous network responsible for synthesising secretory materials

44
Q

What is the difference between the rough ER and the smooth ER?

A
  • Rough ER - embedded with ribosomes and synthesises proteins destined for extracellular use
  • Smooth ER - involved in lipid synthesis + also plays a role in carbohydrate metabolism
45
Q

When are materials transported from the ER?

A

When the membrane bulges + then buds to create a vesicle surrounding the material

46
Q

Where is a vesicle transported to after it leaves the ER?

A

The Golgi Apparatus
- and it fuses to the internal (cis) face of the complex

47
Q

What happens to the materials after they reach the golgi apparatus?

A
  • Materials move via vesicles from the internal (cis) face of the Golgi to the externally oriented (trans) face
  • While within the Golgi apparatus, materials may be structurally modified (e.g. truncated, glycosylated, etc.)
48
Q

What happens to materials after they are sorted within the Golgi apparatus?

A

Either:
- Secreted externally
or
- May be transported to the lysosome

or
- Released immediately into the extracellular fluid (constitutive secretion)
- Stored within an intracellular vesicle for a delayed release in response to a cellular signal (regulatory secretion)

49
Q

What happens to vesicles containing materials destined for extracellular use after they are transported to the cell membrane?

A

The vesicle will fuse with the cell membrane and its materials will be expelled into the extracellular fluid

50
Q

What do the weak hydrophobic associations between the fatty tails of phospholipids in the membrane allow for?

A

Membrane fluidity and flexibility, as the phospholipids can move around to some extent
- this allows for the spontaneous breaking + reforming of the bilayer, allowing large materials to enter or leave the cell without having to cross the membrane

51
Q

Is the spontaneous breaking + reforming of the membrane bilayer a passive or active process?

A

This is an active process and requires ATP hydrolysis

52
Q

What is endocytosis?

A

The process by which large substances (or bulk amounts of smaller substances) enter the cell without crossing the membrane

53
Q

How does endocytosis work?

A
  • An invagination of the membrane forms a flask-like depression which envelopes the extracellular material
  • The invagination is then sealed off to form an intracellular vesicle containing the material
54
Q

What are the two main types of endocytosis?

A
  • Phagocytosis - the process by which solid substances are ingested (usually to be transported to the lysosome)
  • Pinocytosis - the process by which liquids / dissolves substances are ingested (allows faster entry than via protein channels)
55
Q

What is exocytosis?

A

The process by which large substances (or bulk amounts of small substances) exit the cell without crossing the cell membrane

56
Q

How does exocytosis work?

A
  • Vesicles (typically derived from the Golgi) fuse with the plasma membrane, expelling their contents into the extracellular environment
  • The process of exocytosis adds vesicular phospholipids to the cell membrane, replacing those lost when vesicles are formed via endocytosis

IB Biology SL (15 decks)

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