Transport across cell membranes Flashcards

1
Q

structure of cell-surface membrane

A

-phospholipid bilayer where molecules have hydrophobic phosphate head = attracted to water and hydrophobic fatty acid tail = repelled from water
-many components - phospholipids, proteins, cholesterol, glycolipids. glycoproteins

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2
Q

functions of phospholipids in cell-surface membranes?

A

-Lipid soluble substances to pass through
-Flexibility and self-sealing
-Stop water soluble substance entering and leaving cells

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3
Q

Describe the bilayer

A

-formed from two layers of phospholipids.
-hydrophilic heads which face out of the bilayer
-hydrophobic tails which face into the bilayer, so they don’t come in to contact with water .

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4
Q

2 ways in which proteins are emebedded in phospholipid bilayer

A

-surface of bilayer and never extend completely across it > mechanical support to membrane or in conjunction with glycolipids, as cell receptors for molecules like hormones

-channel proteins or carrier proteins

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5
Q

What are 2 types of proteins found in cell membranes?

A

-integral; span the whole of the membrane (e.g transport proteins)
-extrinsic; only one side of the bilayer
(give support or act with glycolipids as cell receptors)

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6
Q

Function of channel proteins

A

-form water-filled tubes > allow water soluble-ions to diffuse across the membrane

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7
Q

function of carrier proteins

A

-bind to ions/ molecules eg glucose and a.a then change shape in order to move these molecules across the membrane

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8
Q

6 functions of proteins in cell-surface membranes?

A
  • structural support
  • act as protein channels > transport water soluble substances
  • allow active transport across membrane through carrier proteins
  • form cell-surface receptors for identifying cells
  • help cells adhere together
  • act as receptors
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9
Q

Describe cholesterol

A

-occur within phospholipid bilayer of cell-surface membrane
-add strength to membranes
-very hydrophobic > preventing water loss and dissolved ions from cell.
-pull together fatty acid tails of phospholipid molecules > limiting their movement and other molecules without making whole membrane too rigid

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10
Q

Function of cholesterol in membrane

A

-reduce lateral movement of other molecules including phospholipids
-make membrane less fluid at high temperature
-prevent leakage of water and dissolved ions from cell

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11
Q

Describe glycolipids

A

-made of carbohydrate covalently bonded with a lipid
-carbohydrate portion extends from phospholipid bilayer into watery environment outside cell where it acts as a cell-surface receptor for specific chemicals eg human ABO blood system

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12
Q

function of glycolipids

A

-act as recognition sites
-help maintain the stability of the membrane
-help cells to attach to one other and so form tissues

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13
Q

Describe glycoproteins

A

-carbohydrate chains attached to many extrinsic proteins on outer surface on cell membrane
-also act as cell-surface receptors for hormones and neurotransmitters

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14
Q

Function of glycoproteins

A

-act as recognition site
-help cells to attach to one another and so form tissues
-allows cells to recognise one another eg lymphocytes recgonise own cells

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15
Q

permeability of cell surface membranes

A

cell surface membrane controls movement of substances in and out cell. Most molecules don’t freely diffuse across because many are:

  • not soluble in lipids > can’t pass through phospholipid layer
    -too large to pass through channels in membrane
    -of same charge as charge on protein channels > repel
    -polar > have difficulty passing through non-polar hydrophobic tails in phospholipid bilayer
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16
Q

Explain fluid mosaic structure

A

fluid- individual phospholipid molecules can move relative to one another > gives membrane a flexible structure and is constantly changing shape
mosaic- proteins that are embedded in bilayer vary in shape, size and pattern

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17
Q

How does temperature impact cell surface membranes?

A

an increase in temperature increases the fluidity of the membrane, as a result increasing it’s permeability

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18
Q

How does pH impact cell surface membranes?

A

it impacts the protein structure of the proteins in the cell membrane

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19
Q

How does solute concentration impact cell surface membranes?

A

Phospholipid bilayer is dissolved more easily, increasing its permeability

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20
Q

How does colour indicate the permeability of membranes when investigating using beetroot?

A

The darker the solution of beetroot, the more pigment has been released because the membrane is more permeable. A darker solution will have a higher reading for absorbance

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21
Q

Why is fluidity an important characteristic of cell membranes?

A

It allows flexibility for the cell, increased motion needed in transport

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22
Q

Define simple diffusion

A

net movement of molecules/ions from a region where there more highly concentrated to one where their concentration is lower, until they’re evenly distributed

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23
Q

Explain how simple diffusion occurs

A
  • Particles have kinetic energy and move randomly
    -They will distribute themselves evenly . - Net movement from a higher concentration to a lower
    concentration
24
Q

Describe molecules that travel by simple diffusion and explain why they can

A

-Small, non polar molecules such as oxygen and carbon dioxide. They are small enough
-not charged, so can pass through the phospholipid bilayer.
-Lipid soluble substances can also diffuse through the phospholipid bilayer,

-net movement of lipid-soluble and non polar or very small substances eg co2, o2 , steroid hormones
-from area of high conc to area of a lower conc down a conc gradient
-across phospholipid bilayer

25
Define facilitated diffusion
Diffusion involving the presence of protein carrier and channel molecules to allow the passive movement of charged and larger molecules, charged ions and polar molecules across plasma membranes
26
Why does facilitated diffusion occur
charged ions and polar molecules don't diffuse easily due to hydrophobic nature of fatty acid tails of phospholipids in membrane
27
Explain why facilitated diffusion is a passive process?
There is no ATP from respiration used in the process, the only energy used is the built-in (kinetic) energy of the molecules themselves
28
Explain how facilitated diffusion occurs including the roles of carrier and channel proteins
-Facilitated diffusion is a passive process that relies on the kinetic energy of the diffusing molecules. -Diffusion down a concentration gradient can only occur where there are specific carrier of channel proteins.
29
Describe the difference between carrier proteins and channel proteins
- Channel proteins - water filled hydrophilic channels across the membrane which only open to allow specific ions across. - Carrier proteins - when a specific molecule eg glucose binds and conformational change occurs which releases the molecule to the other side of the membrane
30
Describe protein channel (water-soluble -polar- substances)
-water-filled hydrophobic channels across membrane -allow specific water-soluble ions to pass through -channels =selective, so open in presence of specific ion and remain closed if not present (so controls in and exit of ions) -ions bind with protein causing it to change shape in a way that closes it to one side of the membrane and opens it to the other side
31
Describe carrier proteins (slightly larger substances)
-molecule like glucose that's specific to protein is present, it bind with protein -causes change in shape so molecule is releases to inside of membrane -no external energy needed -molecules move from high conc to lower conc using kinetic energy of the molecules
32
Describe the molecules that travel by facilitated diffusion
-Ions and polar molecules cannot pass through the phospholipid bilayer because of the hydrophobic fatty acid tails. -Larger molecules cannot pass through the phospholipid bilayer because of their size. -Transmembrane protein channels and carriers are required to facilitate (help) diffusion.
33
Define osmosis
the passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively permeable membrane
34
What is meant by the term ‘water potential’?
a measure of the pressure caused by water molecules, and the likelihood of them moving
35
way of finding of water potential of cells have taken place
-place in series of solutions of different water potentials -where there's no net gain/loss of water from cells/tissues -water potential inside cells/tissues must be same as that of external solution
36
Explain how osmosis occurs in terms of water potential
-Water potential is the tendency for water molecules to move, it represents the pressure created by the water molecules. -An area of high water potential has a lower solute concentration, the solute cannot move through the partially permeable membrane unlike water. -The water moves from a higher water potential (less negative number) to a lower water potential (more negative number) through the partially permeable membrane until dynamic equilibrium is reached.
37
lower conc in left and higher conc on right. explain osmosis
-solution on left has lower conc of solute molecules while solution on right has higher conc of solute molecules -both solute and water molecules are in random motion due to their kinetic energy -selectively permeable plasma membrane only allows water molecules across it and not solute molecules -water molecules diffuse from left (higher water potential) to right (lower water potential) down a water potential gradient -at point where water potentials on either side of plasma membrane are equal, dynamic eqm is established and there's no net movement of water
38
highest value of water potential (pure water) is what
0 -the more negative the value the lower the water potential
39
Describe and explain the effect of solutions of different water potentials on cells
Animal cells: -external solution higher water potential - cells bursts as net movement of water in to cell -external solution equal water potential - dynamic equilibrium, no net movement of water -external solution lower water potential - cells shrink as net movement of water out of cell Plant cells: -external solution higher water potential - TURGID net movement of water in to cell - cell does not burst due to cell wall - protoplast pushed against cell wall -external solution equal water potential - INCIPIENT PLASMOLYSIS dynamic equilibrium, no net movement of water -external solution lower water potential - PLASMOLYSIS net movement of water out of cell - protoplast pulled away from cell wall
40
RBC and osmosis
-absorb water by osmosis due to having a lower water potential -cell surface membranes are thin and can't stretch to great extent -cell surface membrane will break -bursting cell and releasing content prevent: -animals cells normally live in a liquid which has the same water potential as the cells -if rbc placed in a solution with water potential lower than its own , water leaves via osmosis and cell shrinks and becomes shrivelled
41
What is a selectively permeable membrane?
42
How might a cell be adapted to increase the rate of osmosis?
Larger surface area would increase the rate of osmo
43
Explain how K+ ions are involved in the opening and closing of stomata
-K+ ion concentration in guard cells increases -lowers water potential inside guard cell -water moves from higher water potential into the lower water potential outside of the cell, by osmosis -cells become turgid and change shape, opening the stomata
44
Define active transport
-The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins -occurs AGAINST a conc gradient -requires metabolic energy by ATP
45
uses of active transport
-directly move molecules -individually move molecules using conc gradient which has already been set up y active transport (co-transport)
46
Explain how active transport occurs inc' importance of carrier proteins and hydrolysis of atp (direct active transport of a single molecule /ion)
-Specific carrier protein for the molecule/ion spans the membrane Molecule/ion binds to receptor on the carrier protein -On the inside of the membrane ATP binds to the carrier protein and splits into ADP and Pi -Energy from ATP causes carrier protein to change shape and open on opposite side of membrane -Molecule/ion released to other side of membrane Phosphate released from carrier protein causing it to revert to original shape -ATP reformed from ADP and P during respiration
47
What is the role of ATP in active transport?
ATP provides the energy needed, it undergoes ATP hydrolysis, forming ADP and Pi
48
How does an increase in the number of carrier proteins impact the rate of active transport?
more carrier proteins will increase the rate of active transport, as more molecules can be moved at the same time
49
Give three differences between active transport and facilitated diffusion
-Active transport requires energy, facilitated diffusion does not -active transport moves from low concentration to high, facilitated diffusion moves from a high concentration to low -Active transport only uses carrier proteins, facilitated diffusion used channel and carrier proteins
50
why is sodium potassium pump essensial
essential to number of processes in organism , inc' nerve impulse
51
Define co-transport
Transport of one substance coupled with the transport of another across the plasma membrane in the same direction through the same carrier protein
52
Explain how co-transport occurs using absorption of sodium ions and glucose by cells lining the mammalian ileum as an example
-Sodium ions actively transported out of epithelial cells by sodium potassium pump (carrier protein) into the blood -This maintains a higher concentration of sodium ions in the lumen of the ileum compared with the inside of the epithelial cells -Sodium ions diffuse into the epithelial cell through a co-transport protein (carrier protein) -The movement of sodium ions carries glucose/amino acids through the same protein into the epithelial cell -Glucose/amino acids move into the blood by facilitated diffusion (through another carrier protein)
53
Describe how cells are adapted for rapid exchange across membranes
-Microvilli on epithelial lining > finger-like projections on the cell surface membrane provide a larger surface area for carrier proteins involved in facilitation diffusion, active transport and co transport -High density of carrier proteins and protein channels on the membrane
54
Role of diffusion in absorption
-carbohydrates and proteins being digested continuously, and there's normally a greater conc of glucose and a.a within ileum than in blood -so there's conc gradient down which glucose moves by facilitated diffusion from inside ileum into blood -blood constantly being circulated by heart , glucose absorbed into it is constantly being removed by cells as they use it up during respiration -maintain conc gradient between inside of ileum and blood -so rate of movement by facilitated diffusion across epithelial cell-surface membrane is increased
55
Role of active transport in absorption
-diffusion only results in conc of either side of intestinal epithelium becoming equal -means not all available glucose and a.a can be absorbed in this way + some may pass out the body > not happening as glucose and a.a being absorbed by active transport > glucose and a.a should be absorbed into blood
56
Why would the rate of active transport eventually level off, even if the extracellular concentration of a solute increased?
As all the carrier proteins would be fully saturated
56
What carrier proteins are involved in the transport of glucose
Na-K pump, transports Na+ ions into the blood (lowers conc. inside the cell) Na-glucose, moves glucose against conc. gradient, as Na+ ions are moved down conc. gradient