Transport across cell membrane

Question 1

-What are molecules that pass through the plasma membrane:

Answer 1

Lipid soluble substances + very small molecules ( e.g , CO2, O2, H2O )

Question 2

-what are molecules that cannot pass through the plasma membrane

Answer 2

-water soluble (polar) substances
(Sodium ions) and large molecules e.g—> glucose

Question 3

why is the plasma cell membrane known as fluid mosaic model

Answer 3

-due to mixture and movement of phospholipids, proteins, glycoproteins and glycolipids that make it

Question 4

list the key components of cell membrane

Answer 4

-phospholipid bilayer
-cholesterol
-proteins
-glycoproteins
-glycolipids

Question 5

decribe the phospholipid

Answer 5

Each phospholipid molecule is made up of a hydrophilic ‘head’ and a hydrophobic ‘tails’.

This causes phospholipids to arrange themselves into a bilayer so that the hydrophilic heads are facing out (towards water) and the hydrophobic tails are facing in (away from water).

This arrangement creates a hydrophobic centre in the bilayer so that water-soluble substances cannot pass through.

Question 6

function of phospholipid in the cell 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

describe cholesterol
+ function

Answer 7

-cholesterol is hydrophobic and binds to the hydrophobic fatty acid tails

-helps make the membrane more rigid and less fluid at higher temp

-reduces lateral movement of other molecules (e.g phodpholipids)

prevents dissolved ions and water leaking

Question 8

describe extrinsic/ peripheral proteins

Answer 8

• do not extend completely across the membrane

-provide mechanical strength

• some are connected to proteins or lipids to form glycoproteins or glycolipids to become receptors for cell recognition

Question 9

describe intrinsic/ integral proteins

Answer 9

-are protein carriers or channel proteins

-involved in the transport of molecules across the membrane

Question 10

describe function of glycoproteins

Answer 10

-Acts as recognition sites
-allows cells to recognise one another
-helps cells attach to one another and so form tissues

Question 11

describe function of glycolipids

Answer 11

-acts as recognition sites
-helps maintain the stability of the membrane
-helps cells attach to one another and so form tissues

Question 12

-what do the protein channels do

Answer 12

form tubes filled with water which enables water-soluble ions to pass through the membrane

Question 13

-what is needed for molecules to diffuse across the membrane

Answer 13

lipid soluble and small

Question 14

what do protein carriers do

Answer 14

-protein will bind to other ones and larger molecules such as glucose and amino acids, change shape to transport them to the other side of the membrane

Question 15

what is the role of the cell surface membrane

Answer 15

-these surround cells to act as a barrier between the cell and its environment, controlling which substances enter and leave the cell.

Question 16

-define simple diffusion

Answer 16

net movement of particles from an area of high concentration to an area of low down a concentration gradient

-passive process so no ATP required

Question 17

-why does diffusion only take place in liquids and gases

Answer 17

-because they have Kinetic Energy that enables them to constantly move /flow

Question 18

-define facilitated diffusion

Answer 18

movement of polar molecules and ions in and out of the cell with the use of carrier and channel proteins

Question 19

-how do carrier proteins enable a molecule such as glucose to diffuse across the membrane.

Answer 19

-carrier proteins binds to the glucose molecule
-causes a change in shape of the protein
-enables glucose molecule to be released to the other side

Question 20

factors affecting rate of diffusion:

Answer 20

1) the higher concentration/steeper it is, the faster the rate of diffusion

2) diffusion distance: shorter diffusion distance, faster rate of diffusion

3) surface area : larger the surface area faster rate of diffusion

Question 21

define active transport

Answer 21

movement of molecules and ions through a cell membrane from a region of low concentration to a region of high concentration using ATP and carrier proteins

Question 22

factors affecting the rate of active transport

Answer 22

1) the speed of individual carrier proteins - the faster they work , the faster the rate of active transport.

2) the number of carrier proteins present - the more proteins there are , faster the rate of active transport

3) the rate of respiration in the cell and the availability of ATP. if respiration is inhibited, active transport cant take place

Question 23

define osmosis

Answer 23

movement of water from an area of higher water potentials to an area of lower water potential across a partially membrane

Question 24

define water potential

Answer 24

pressure created by water molecules
-measured in kPa and represented with the symbol Ψ

Question 25

what is the water potential of pure water

Answer 25

0

Question 26

when does the Ψ become negative

Answer 26

when solutes are dissolved in water

-the more negative the water potential the more solute dissolved

Question 27

define isotonic solution

Answer 27

when the water potential is the same in the solution and the cell within the solution

Question 28

define hypotonic

Answer 28

when the water potential of a solution is more positive ( closer to 0) than the cell

Question 29

define hypertonic

Answer 29

when the water potential of a solution is more negative than the cell

Question 30

how does direct active transport occur?

Answer 30

1) carrier proteins bind to the molecule
2) the molecule/ion binds to the specific receptor site on the carrier proteins
3) On the inside of the cell, ATP will be hydrolysed into ADP+Pi
4) This causes the shape of the protein to change and open to the opposite side of the membrane
5) the molecule is then released to the other side of the membrane
6) the phosphate ion is then released and the protein returns to its original shape

Question 31

the movement of substances across the membrane is affected by membrane structure. Describe how ( 5)

Answer 31

1) the phospholipid bilayer allows the diffusion of non polar substances
2) the phsopholipid bilayer prevents the diffusion of polar substances
3) The channel /carrier proteins allow facilitated diffusion/co transport
5) shape of the carrier protein/channel protein determines which substances are transported
-number of carrier/channel proteins determine how much movement
6) the surface area of the membrane determines how much diffusion occurs
7) cholesterol affects fluidity

Question 32

name and describe 5 ways substances can move across the cell surface membrane

Answer 32

1) simple diffusion of small molecules down a concentration gradient
2) facilitated diffusion down a concentration gradient with the use of protein channels and carrier proteins
3) active transport against a concentration gradient with the use of protein carriers using ATP
4) Osmosis of water down a water potential gradient
5) co transport of 2 different substances using a carrier protein

Question 33

The action of the carrier protein X in Figure 1 is linked to a membrane-bound ATP hydrolase enzyme.
Explain the function of this ATP hydrolase.

Answer 33

ATP–> ADP+Pi which releases energy
energy therefore allows active transport of the ions

Question 34

Describe and explain two features you would expect to find in a cell
specialised for absorption.

Answer 34

1) Microvilli: large surface area for absorption
2) large number of carrier proteins for active transport

Question 35

Give two similarities in the movement of substances by diffusion and by
osmosis.

Answer 35

1) both occurs from an area of high concentration to an area of low concentration

2) both passive processes

Question 36

Water and inorganic ions have important biological functions within cells.

Compare and contrast the processes by which water and inorganic ions
enter cells.

Answer 36

Both move down a concentration gradient
Both move through channel proteins in the membrane

Contrast: ions can move against a concentration gradient by active transport

Question 37

describe how glucose is absorbed via facilitated diffusion in the lumen of ileum

Answer 37

1) after digestion, there is a high glucose concentration in the lumen of the ileum
2) glucose molecules move down the concentration gradient by facilitated diffusion with the use of carrier proteins
3) bloodstream then carriers glucose molecules away

-however concentration gradient fall so facilitated diffusion cannot absorb all of the glucose molecules
so glucose is absorbed by active transport

Question 38

describe the sodium potassium pump

Answer 38

-sodium ion is actively transported out of the epithelial cells and into the bloodstream using ATP from the mitochondria in the epithelial cell

-now there a low concentration of sodium ions in the epithelial cells + lumen of the ileum contains high concentration of Na+ ions
-results in a concentration gradient for Na+ ion between the lumen of the ileum and the inside of the epithelial cell

Na+ ions can diffuse through the sodium glucose transporter down their concentration gradient into the epithelial cell

at the same timer sodium-glucose co transporter also transport glucose molecules into the cell against their concentration gradient at a fast rate

the energy for this glucose transport comes indirectly from the concentration gradient of the sodium ions

Question 39

purpose of the membrane of the epithelial cell being folded into a large number of microvilli

Answer 39

-increase the SA of the membrane –> providing more space for the membrane proteins needed for facilitated diffusion and active transport

Question 40

why is the glucose molecules being absorbed rapidly carried away by the bloodstream

Answer 40

-means there’s a steep concentration gradient between the epithelial cell and the blood

so facilitated diffusion of glucose into the bloodstream takes place rapidly

Question 41

Contrast the processes of facilitated diffusion and active transport.

Answer 41

• facilitated diffusion is passive process ( doesn’t use ATP
  whereas active transport uses ATP

-facilitated diffusion uses carrier proteins or channel proteins whereas active transport only uses carrier proteins

-facilitated diffusion occurs down a concentration gradient whereas active transport occurs against a concentration gradient