transport across cell membrane

Question 1

what do all membranes around cells ( including those around and within cell organelles) have in common

Answer 1

they all have the same basic structure and are known as plasma membranes

Question 2

what is the cell surface membrane

Answer 2

the cell surface membrane is the name specifically given to the plasma membrane that surrounds cells and forms the boundary between the cell cytoplasm and environment

Question 3

what are the different molecules that make up the cell surface membrane structure

Answer 3

• phospholipids
• proteins
• cholesterol
• glycolipids
• glycoprotiens

Question 4

what is the role of phospholipids in the cell surface membrane and why are they important

Answer 4

phospholipids form a bilayer

They are important components of cell - surface membrane for the following reasons:

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

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

Question 5

what material can pass through the phospholipid portion

Answer 5

lipid soluble materials move through the membrane via the phospholipid portion

Question 6

what is the function of phospholipids in the membrane

Answer 6

• 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 7

what are the two ways that proteins are embedded in the phospholipid bilayer

Answer 7

proteins are interspersed throughout the cell surface membrane

they are embedded in the phospholipid bilayer in two main ways:

• some proteins occur in the surface of the bilayer.
  They never extend completely across it

They act either to give mechanical support to support the membrane or, with glycolipids, cell receptors for molecules such as hormones ( cell receptors are certain sites that allows the cell to bind with cells)

• other proteins completely span the phospholipid bilayer from one side to another

some of these proteins are protein channels

others are carrier proteins

Question 8

what are protein channels

Answer 8

some of the proteins in the cell surface membrane are protein channels which form water-filled tubes to allow water-soluble ions to diffuse across the membrane

Question 9

what are carrier proteins

Answer 9

carrier proteins bind to ions or molecules like glucose and amino acids, then they change shape in order to move these molecules across the membrane

Question 10

what are the functions of proteins in the cell surface membrane

Answer 10

• provide structural support
• acts as channels transporting water - soluble substance across the membrane
• allow active transport across the membrane through carrier proteins
• form cell surface receptors for identifying cells
• helps cells adhere together
• acts as receptors e.g. for hormones

Question 11

where is the cholesterol in the cell surface membrane

Answer 11

the cholesterol occurs in the phospholipid bilayer of the cell surface membrane

they add strength to the membranes

Question 12

how do cholesterol prevent the loss of water and dissolved ions from the cell

Answer 12

cholesterol molecules are very hydrophobic and therefore play an important role in preventing loss of water and dissolved ions form the cell

Question 13

how does cholesterol reduce the lateral movement of the other molecules and phospholipids

Answer 13

they pull together the fatty acid tails of the phospholipids molecules but without making the membrane too rigid

Question 14

what is the functions of the cholesterol in the cell surface membrane

Answer 14

reduces lateral movement of molecules including phsopholipids

make membrane less fluid at high temperatures

prevent leakage of water and dissolved ions from the cell

Question 15

what are glycolipids

Answer 15

glycolipids are made up of carbohydrates covalenty bonded with a lipid

Question 16

where are the glycolipids in the cell surface membrane

Answer 16

the carbohydrate portion of the glycolipid extends from the phospholipid bilayer into the watery environment outside the cell where it acts as a cell - surface receptor for specific chemicals

e.g. AOB blood system operates as a result of glycolipids on the cell surface membrane

Question 17

what is the function of the glycolipids

Answer 17

acts as recognition sites

help maintain stability of the membrane

help cells to attach to one another and so form tissues

Question 18

what are glycoproteins

Answer 18

carbohydrate chains that are attached to extrinsic proteins on the outer surface of the cell membrane

Question 19

what are the functions of the glycolipids

Answer 19

• act as cell surface receptors, more specifically for hormones and neurotransmitters
• act as recognition sites ]]helps cells to attach to one another forming tissues
• allows cells to recognise one another, e.g. lymphocytes can recognise organisms own cells

Question 20

what are the substances that can move in and out of the cell

Answer 20

the cell surface membrane controls the movement of substances into and out of the cell

In general, cells do not diffuse freely across it because many are:

• not soluble in lipids and therefore does not pass through the lipid-soluble layer
• too large to pass through channels and into the membrane
• of the same charge of the proteins channels and so they are repelled ( even if the molecule is small)
• if are polar and therefore have difficulty passing through the non - polar hydrophobic tails in the phospholipid bilayer

Question 21

what is the fluid mosaic model of the cell surfaced membrane

Answer 21

the way in which all the various molecules are combined into the structure of the cell surface membrane

this arrangement is known as the fluid mosaic model

Question 22

why is the fluid mosaic model “fluid”

Answer 22

fluid

because the individual phospholipids molecules can move relative to one another.
This gives the membrane a flexible structure that is constantly changing in shape

Question 23

why is the fluid mosaic model “mosaic”

Answer 23

mosaic because the protiens that are embedded in the phospholipid bilayer vary in shape, size and pattern in the same way as the stones/ tiles of a mosaic

Question 24

what are the two types of transport

Answer 24

the exchange of substances between cells and environments occurs in ways that require metabolic energy (active transport) and in ways that do not (passive transport)

a form of passive transport is diffusion

Question 25

what is passive transport

Answer 25

all movements involves energy therefore it is confusing that we describe diffusion as passive transport

In this sense, “passive” refers to the energy that comes from the natural inbuilt motion of particles (kinetic energy) rather than some external source such as ATP

Question 26

what is diffusion

Answer 26

diffusion is the net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distributes

Question 27

what molecules can easily diffuse across plasma membranes

Answer 27

plasma membranes are not readily permeable to all molecules.
They are only permeable to small, non - polar molecules like oxygen which can diffuse across them easily

Question 28

what is facilitated diffusion

Answer 28

charged ions and polar molecules do not diffuse easily because the hydrophobic nature of the fatty acid tails of the phospholipid membranes

The movement of these molecules is made easier (facilitated) diffusion. This is a passive process.

It relies only on the inbuilt - motion (kinetic energy) of the diffusing molecules. There is no external input of the ATP from respiration.
It occurs down a concentration gradient but differs in that it occurs at specific points on the plasma membrane where there are special protein molecules:

• protein channels
• carrier proteins

Question 29

what are protein channels

Answer 29

these proteins form water - filled hydrophilic channels across the membrane

They allow specific water soluble ions to pass through

The channels are selective, each opening in the presence of a specific ion. If the particular ion is not present, the channel remains closed.
In this way, there is control over the entry and exit of ions. The ions bind with the proteins causing it to change in a way that causes it to close on one side of the membrane and opens it to the other side

Question 30

what are carrier proteins

Answer 30

an alternative form of facilitated diffusion involves carrier proteins that span the plasma membrane

when a molecule, such as glucose, 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

no external energy is needed for this. The molecules move from a region where they are highly concentrated to one of lower concentration, using only the kinetic energy of the molecules themselves

Question 31

what is osmosis

Answer 31

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

Question 32

what are selectively permeable membranes

Answer 32

cell surface membranes and other plasma membranes, such as those around organelles, are selectively permeable.

This means they are permeable to water molecules and a few other small molecules, but not to larger molecules

Question 33

what is a solute

Answer 33

a solute is any substances that is dissolved in a solvent

Question 34

what makes a solution

Answer 34

solute + solvent = a solution

Question 35

what is water potential

Answer 35

water potential is represented by Greek letter psi and is measured in units of pressure, usually kiloPascals kPa

water potential is the pressure created by water molecules. Under standard conditions of temperature and pressure (25 C and 100kPa), pure water is said to have a water potential of zero

Question 36

what happens when we add more solute to pure water

Answer 36

it will lower its water potential

therefore, the more solute in a solution, the lower its water potential

Question 37

how does water potential relate to osmosis

Answer 37

water will move by osmosis from a region of higher water potential ( less negative) to one of lower water potential ( more negative)

Question 38

give an example of osmosis in terms of water potential

Answer 38

a selectively permeable plasma membrane separates two solutions

• the solution on the left has a lower concentration of solute molecules while the solution on the right has a higher concentration of solute molecules
• both the solute and water molecules are random motion due to their kinetic energy
• the selectively permeable plasma membrane, however, only allow water molecules across it and not solute molecules
• at the point where the water potentials on either side of the plasma membrane are equal, a dynamic equilibrium is established and there is no net movement of water

Question 39

what is the highest value of water potential

Answer 39

the highest value of water potential ( pure water) is zero

all the other values are negative
the more negative the value, the lower the water potentials

Question 40

what do all animal cells contain

Answer 40

all animal cells contain a variety of solutes dissolved in their watery cytoplasm

Question 41

what happens when an animal cell is placed in pure water

Answer 41

it will absorb water by osmosis because it has a lower water potential

Question 42

what happens when an animal cell absorbs too much water

Answer 42

cell surface membranes are very thin (7nm) and, although they are flexible, they cannot stretch to any great extent

the cell - surfaced membrane will therefore break, bursting the cell and releasing its contents ( in red blood cells this is called haemolysis)

Question 43

how do we prevent a cell form bursting due to osmosis

Answer 43

animal cells normally live in a liquid which has the same water potential as the cells

e.g. for red blood cells, this liquid would be the blood plasma

Question 44

what happens when animal cells loose to much water

Answer 44

if an animal cell is placed in a solution with a water potential lower than its own, water leaves by osmosis and the cell shrinks and becomes shriveled

Question 45

what is active transport

Answer 45

active transport is 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

This process uses metabolic energy

Question 46

how is ATP used in active transport

Answer 46

ATP directly move

substances against a concentration gradient, that is from a lower concentration to a higher one

Question 47

how does active transport work

Answer 47

• carrier proteins span the plasma membrane and bind to the molecule or ion to be transported on one side of it
• the molecule or ions binds to receptor sites on the carrier protein
• on the inside of the cell/ organelle, ATP binds to the protein, causing it to split into ADP and a phosphate molecule. As a result, the protein molecule changes shape and opens to the opposite side of the membrane
• The molecule or ion is then released to the other side of thee membrane
• the phosphate molecules is released from the protein which causes the protein to revert to its original shape, ready for the process to be repeated. The phosphate molecule then recombines with the ADP to form ATP during respiration

Question 48

how is active transport different to facilitated diffusion

Answer 48

active transport occurs against a concentration gradient but uses carrier proteins like facilitated diffusion

however, facilitated diffusion occurs down a concentration gradient. This means that facilitated does not require metabolic energy, while active transport does.

Question 49

what is a sodium - potassium pump

Answer 49

sometimes more than one molecule or ion may be moved in the same direction at the same time by active transport

ion/ molecule moves into cells/ organelles at the same time one is being removed from it e.g. sodium potassium pump

in the sodium potassium pump, sodium ions are actively removed from the cell/ organelle while potassium ions are actively taken in from the surrounding. This process is essential to a number of important processes in the organisms, including the creation of a new impulse

Question 50

what are microvilli

Answer 50

epithelial cells lining the ileum possess microvilli

these are finger like projections of the cell cell surface membrane about 0.6 micrometres in length

they are collectively termed a “brush border” because, when viewing them under a light microscope, they look like the bristles on a brush

Question 51

what is the function of the microvilli

Answer 51

microviilli provide more surface area for the insertion of carrier proteins through which diffusion, facilitated diffusion and active transport can take place

Question 52

how else can we increase transport across membranes

Answer 52

increase the number of protein channels and carrier proteins in any given area of the membrane

Question 53

what is the role of diffusion in absorption

Answer 53

diffusion is the net movement of molecules or ions from a region where they are highly concentrated to a region where their concentration is lower

as carbohydrates and proteins are being digested continuously, there is normally a greater concentration of glucose and amino acids within the ileum than in the blood

therefore, there is a concentration gradient down which glucose moves by facilitated diffusion from inside the ileum into the blood

given that the blood is constantly being circulated by the heart, the glucose absorbed into it is continuously being removed by the cells as they use it during respiration. This means the rate of movement by facilitated diffusion across epithelial cell - surface membranes is increased

Question 54

what is the role of active transport in absorption

Answer 54

at best, diffusion only results in the concentrations either side of the intestinal epithelium becoming equal. This means that not all the available glucose and amino acids can be absorbed in this way and may pass out of the body.

However, this does not happen because glucose and amino acids are being absorbed by active transport. This means that all then glucose and amino acids should be absorbed into the blood

The mechanism by which they are absorbed from the small intestine is an example of co - transport

Question 55

how can we find out the water potential of a plant tissue

Answer 55

1. Make a serial dilution
2. measure the change in mass
3. produce a calibration curve

Question 56

how do we make a serial dilution

Answer 56

1. line up five test tubes in a rack
2. Add 10cm3 of the initial 2M sucrose solution to the first test tube and 5cm3 of distilled water to the other four test tubes
3. Then, using a pipette, draw 5cm3 of the solution from the first test tube, add it to the distilled water in the second test tube and mix the solution thoroughly.
4. Repeat this process three more times to create solutions of 0.5M, 0.25M, and 0.125M

Question 57

how would you make a serial dilution (maths skills)

Answer 57

if you want to make 15cm3 of 0.4 sucrose solution

get a known solution e.g 1M

1) 1 / 0.4 = 2.5
This means that the solution is 2.5 times weaker than the one you have

To make the solution weaker we use 2.5 times less of it
15 / 2.5 = 6 cm3

Transfer this amount to a clean test tube and top it up to the volume you need

Question 58

how do you measure the change of mass in a plant tissue

Answer 58

once you have made serial dilutions, you can use them to find the water potential of potato cells
- first you need to measure how much mass the potato cells gain or lose in each solution

1. use a cork borer to cut potatoes into identically sized chips about 1cm in diameter
2. place one group into each of your sucrose solutions and leave the chips in the solution for 20 minutes at least
3. remove the chips and pat dry gently with a paper towel. Weigh each group again and record your results.
   Calculate the percentage change in the mass for each group.

Question 59

when does the potato chip gain mass

Answer 59

the potato chips will gain water (and therefore mass) in solutions with a higher water potential than the chips and lose water in a solution with a lower water potential

Question 60

how do we produce a calibration curve

Answer 60

after measuring the change in mass, you can produce a calibration curve by plotting percentage change in mass against the concentration of sucrose solution

the calibration curve can be used to determine the water potential of the potato cells

The point at which your calibration curve crosses the x - axis (where the percentage change in mass is 0) is the point at which the water potential of the sucrose solution is the same as the water potential of the potato cells

We can then find the concentration at this point, then look up the water potential for that concentration of sucrose solution e.g. textbook

Question 61

what are some factors affecting the rate of osmosis

Answer 61

1. the water potential gradient - the higher water potential gradient, the faster the rate of osmosis.
   As osmosis takes place, the difference in water potential on either side of the membrane decreases, so the rate of osmosis levels off overtime
2. the thickness of the exchange surface - the thinner the exchange surface, the faster the rate of osmosis
3. the surface area of the exchange surface - the larger the surface area the faster the rate of osmosis

Question 62

what are co transporters

Answer 62

co - transporters are a type of carrier protein

They bind two molecules at a time
The concentration gradient of one of the molecules is used to move other molecules against its own gradient

Question 63

what is an example of a co - transport

Answer 63

co - transport of sodium ions and glucose

sodium ions move across the membrane down their concentration gradient
This moves glucose across the membrane too, against its concentration gradient

Question 64

what is the role of co transport in the absorption of glucose

Answer 64

Glucose is absorbed into the bloodstream in the small intestine.
In the ileum, the concentration of glucose is too low for glucose to diffuse into the blood so the glucose is absorbed from the lumen (middle) of the ileum by co - transport

STEP 1:
- sodium ions are actively transported out of the epithelial cells in the ileum, into the blood by the sodium potassium pump
This creates a concentration gradient - there’s now a higher concentration of sodium ions in the lumen of the ileum than inside the cell

STEP 2:
This causes sodium ions to diffuse from the lumen of the ileum into the epithelial cell, down their concentration gradient
They do this via a sodium - glucose co - transporter proteins .
The co transporter carries glucose into the cell with the sodium. As a result the concentration of glucose inside the cell increases

STEP 3:
glucose diffuses out of the cell into the blood, down its concentration gradient through a protein channel, by facilitated diffusion