3.2.3 Transport across cell membranes

Question 1

What are the functions of the phospholipid bilayer in the cell membrane?

Answer 1

allow lipid-soluble substances to enter and leave the cell
prevent water-soluble substances entering and leaving the cell
make the membrane flexible and self-sealing.

Question 2

Why are phospholipids important components

Answer 2

The hydrophilic heads of both phospholipid layers point to the outside of the cell-surface membrane attracted by water on both sides.

The hydrophobic tails of both phospholipid layers point into the centre of the cell membrane, repelled by the water on both sides.

Question 3

What causes substances to dissolve?

Answer 3

If they’re polar/charged- attracted to water molecules and vice versa
Phosphate head- neg charge FA tails- no charge

Question 4

Where are the ester bonds in a phospholipid?

Answer 4

Glycerol and fatty acid

Glycerol and phosphate (phospho ester bond)

Question 5

What do ester bonds form between?

Answer 5

Acid and Alcohol

Question 6

Who came up with the fluid mosaic model, and what are the five points?

Answer 6

Singer and Nicolson in 1972

Phospholipid bilayer

Hydrophilic heads and hydrophobic tails.

Mosaic like pattern of embedded proteins

Fluid movement

Intrinsic (inside- runs through mid of layer and forms channel) and extrinsic (outside, on surface) proteins

Question 7

What are lipids/proteins held together by and what does this cause?

Answer 7

weak hydrophilic/hydrophobic interactions. Therefore molecules are relatively “free” to move about.

Question 8

What causes the fluidity of the membrane to increase?

Answer 8

Fluidity of the membrane will increase as:

1) the temperature increases.

2) number of double bonds in fatty acids
increase.

3) decreasing amount of cholesterol (are lipid soluble, hphobic organic molecules that can stick together and go between the phospholipid layers, pulling them together)

Question 9

Function of proteins in membrane

Answer 9

Transport (e.g intrinsic proteins being hollow)
Intercellular joining
Enzymatic activity (sequential enzymes in a metabolic pathway)
Cell-cell recognition
Signal transduction (e.g. hormones)
Attachment to cytoskeleton + extracellular

Question 10

Cholesterol function and description

Answer 10

They add strength to the membranes. Cholesterol molecules are very hydrophobic and therefore play an
important role in preventing loss of water and dissolved ions from the cell. They also pull together the fatty acid tails of the phospholipid molecules, limiting their movement and that of other molecules but without making the membrane as a whole too rigid.
The functions of cholesterol in the membrane are to:
• reduce lateral movement or other molecules including
phospholipids
• make the membrane less fluid at high temperatures
• prevent leakage of water and dissolved ions from the cell.

Question 11

Permeability of the cell-surface membrane

Answer 11

most molecules do not freely diffuse
across it because many are:
• not soluble in lipids and therefore cannot pass through the
phospholipid layer
• too large to pass through the channels in the membrane
• of the same charge as the charge on the protein channels and so.
even if they are small enough to pass through, they arc repelled
• electrically charged (in other words are polar) and therefore have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer.

Question 12

What is the phospholipid bilayer permeable/ impermeable to?

Answer 12

The phospholipid bilayer is permeable to:
1) small uncharged molecules 2)Lipid soluble molecules (lipids)

Phospholipid bilayers are impermeable to:

1) Large molecules 2) All charged molecules and ions.

Question 13

Give examples of things the bilayer is permeable/impermeable to

Answer 13

Permeable: SMALL NON-POLAR MOLECULES- O2 N2
(less so than small non-polar molecs but still) SMALL UNCHARGED POLAR MOLECULES CO2 Urea Glycerol

Impermeable: LARGE UNCHARGED POLAR MOLECULES Glucose Sucrose
CHARGED MOLCULES AND IONS Na+ K+ Cl- Ca2+ Mg2+ HCO3-

Question 14

What is required to transport impermeable substances?

Answer 14

They require transport proteins.
Different molecules require different transport proteins.
Therefore, a cell can “select” which molecule to transport by putting particular transport proteins in its membrane

Question 15

What needs to be transported?

Answer 15

Nutrients e.g. oxygen, glucose, amino acids

Wastes e.g. carbon dioxide, urea

Secretion of cell products e.g. hormones, enzymes

Transport of ions to maintain concentration and electrical gradients

Question 16

Simple Diffusion

Answer 16

“The net movement of ions or molecules from a region of high concentration to a region of low concentration.”

Net= overall change

Particles move due to kinetic energy
Movement is random
Particles randomly collide with each other and their surroundings.

It is a passive process – requires no energy.

Only small non-polar molecules will diffuse through the lipid
bilayer (i.e. the substances that the membrane is permeable to).

Question 17

Facilitated Diffusion: Channel Proteins

Answer 17

• To be held in bilayer, has PHOBIC R-groups on outside (like PHOBIC tails)
• To allow water soluble ions/water to pass, has PHILIC (in tertiary structure so folded inside) channel

Question 18

Facilitated Diffusion: Carrier Proteins

Answer 18

• Shape is specific to molecules it will transport- (usually larger ones while channel is for small ions)
• When a molecule such as glucose that is specific to the protein is present, it binds with the protein. This causes it to change shape in such a way that the molecule is released to the inside of the membrane

Question 19

Summarise Channel and Carrier Proteins

Answer 19

Channel Protein
	Hydrophilic channel
	Transport of ions
	Specific to the solute
	Movement in both directions
Carrier Protein
	Complementary receptor site
	Transport of larger molecules
	Specific to the solute
	Movement in both directions

Question 20

Active Transport- what it is, if energy is required and why, what proteins it affects etc

Answer 20

“Is the transport of molecules or ions across a membrane against their concentration gradient.”

Energy is required to move the substance against its concentration gradient.  Its natural tendency is to diffuse in the opposite direction.

In cells with Specific AT carrier proteins, they use energy from ATP to transport substances across a membrane, but energy not used by a carrier protein directly

Question 21

How does ATP work?

Answer 21

P stands for phosphate -ve charge
-ve phosphate sticks to protein
changes shape

Question 22

Factors that affect rate of transport (including facilitated diffusion)

Answer 22

Concentration gradient- increase in it increases rate of diffusion- is steeper

Temperature (kinetic energy)- increase in it increases rate- more KE, more collisions FD-BUT up to a point- proteins denature at too high temp

Distance to diffuse (e.g thickness of walls of alveoli/intestines etc) - increase in it decreases rate, further away, longer it’ll take

Surface area of exchange surface- increase in it increases rate

FD- Number of carrier proteins- increase in it increases rate

Question 23

Fick’s law

Answer 23

(surface area x conc. grad)/ distance to travel

Rate increases as:
surface area increases
concentration gradient increases
distance decreases.

Question 24

Draw and interpret graphs for time (x) conc. of substance inside cell (y) and conc. of substance outside (x) rate (y)

Answer 24

On Anki and notes

Question 25

Bulk Transport Across Membrane

Answer 25

Transport of large macromolecules and solids.

Endocytosis = into the cell 1) Phagocytosis = absorbing solid material. e.g. white blood cell, amoeba.

2) Pinocytosis = absorbing dissolved material (e.g. An amoeba “eating”)

Exocytosis (vesicles formed by golgi apparatus to excrete from cell) = out of the cell e.g. releasing a neurotransmitter for cellular communication, secretion of proteins e.g enzymes

Question 26

Osmosis real def

Answer 26

the diffusion of water from a hypotonic solution to a hypertonic solution across a selectively permeable membrane.”

1) a net movement
2) equal movement in both directions between isotonic solutions.

(Think of osmosis as the diffusion of water.)

Question 27

Define: Hypotonic, hypertonic and isotonic

Answer 27

hypertonic = greater concentration of solute

hypotonic = lower concentration of solute

isotonic = the same concentration

Question 28

Why is a selectively permeable membrane needed?

Answer 28

The semipermeable membrane limits the diffusion of solutes in the water, w/o it, would just cause diff. of solutes- water wouldn’t have a conc. grad

Question 29

What is water potential? What’s the water pot. of water and solutions? What is it measured in? What water pot. does water move to?

Answer 29

Water potential is the measure of the tendency of water molecules to move. It’s given the symbol ψ (Psi)
It’s measured in MPa or KPa.

For pure water ψ = 0 KPa.

A solution will have a negative water potential.

The higher the concentration the more negative the water potential.

Water moves from high to low Ψ (high means lots of water, low means lots of solute)

Question 30

(Helps in required practicals) Water pot. equation and explain it

Answer 30

Ψ= − [isotonic concentration x P x T(ºK)] = kPa

(P = pressure constant = 8.31) 
(T = temperature measured in Kelvin = 273 + ºC)

Isotonic conc.- where graph meets x axis, (for potato experiment, conc outside is equal to conc. inside)

Question 31

REAL osmosis def.

Answer 31

Diffusion of water from a high water potential to a low water potential across a selectively permeable membrane

Question 32

What are the different effects of water potential on animal and plant cells? Give an example of what causes lysis in animal cells, and what is it called when cell memb moves away from wall

Answer 32

Ideal
Animal cells

Inside and outside same, Volume stays same

Inside Ψ more +ve, Outside -ve Volume= decreases, shrinks

Inside Ψ -ve, Outside +ve Volume: increases (phospholipid bilayer breaks), cell LYSES- e.g ecstasy, causes thirst, drink too much water

Plant Cells

Inside Ψ= Outside Ψ- ideal Volume = same, FLACCID

Inside Ψ more +ve, Outside -ve Volume= decreases, shrinks PLASMOLYSIS (cell memb moves away from wall)

Inside Ψ -ve, Outside +ve Volume: increases, TURGID

Question 33

Pressure impact on water potential, diff btw plant and animal cells

Answer 33

An increase in pressure will increase the water potential.

It increases the tendency for the molecules to move.

Animal cells burst when internal pressure is increased so it isn’t applicable- phoshpolipid bilayer breaks

But Plant cells have cell walls.

Pressure can increase inside the cell without it bursting.

Question 34

Explain, in own words, water potential in plant cells formula and state formula (simple, dont overthink)

Answer 34

Water potential = solute potential + pressure potential

ψ = ψ s + ψ p (is 0 in animal cells as they can’t handle pressure)

This can result in positive water potentials.

As water moves in, ↑ψ p causes ↓ψ s bc more water in cell, meaning more pressure, meaning higher water potential.

Question 35

Co transport and absorption of glucose in the ileum

Answer 35

1 Sodium ions are actively transported out of epithelial cells, by the sodium-potassium pump, into the blood. This takes place in one type of protein-carrier molecule found in che cell-surface membrane of the epithelial cells.

2 This maintains a much higher concentration of sodium ions in the lumen of the intestine than inside the epithelial cells.

3 Sodium ions diffuse into the epithe lial cells down thisconcentration gradient through a different type of protein carrier (co-transport protein) in the cell-surface membrane. As the sodium ions diffuse in through this second carrier protein, they carry either amino acid molecules or glucose molecules into the ceII with them.

4 The glucose/amino acids pass into the blood plasma by facilitated diffusion using another type of carrier.

1) Na+s AT out of cell by NaK pump- makes conc grad, higher outside cell
2) Na+ diff into cell through co transport protein- take aa or gl with them
3) Gl + aas conc grad, now into blood through facil