3.2.3 - transport across cell membranes

Question 1

what is the basis of all cell membranes (cell-surface membranes and the membranes around cell organelles of eukaryotes)?

Answer 1

a phospholipid bilayer (their basic structure is the same)

Question 2

what are the functions of membranes in cells?

Answer 2

controlling the movement of substances in and out of the cell/organelle, acting as barriers, separating one part of an organelle from another, location for chemical reactions, contain receptors for molecules like hormones, allow adjacent cells to stick together

Question 3

which molecules are able to pass through the cell-surface membrane?

Answer 3

lipid soluble molecules eg. steroid hormones - they can pass through the hydrophobic centre

Question 4

which molecules can’t pass through the cell-surface membrane?

Answer 4

water soluble molecules - they are hydrophilic and polar, so they can’t pass through the non-polar region of the membrane (hydrophobic centre)

Question 5

what its the current model of the cell membrane known as and why?

Answer 5

the fluid mosaic model
fluid - phospholipid molecules move around in each layer so the membrane is flexible
mosaic - membrane is studded with protein molecules and their arrangement varies

Question 6

what are the main components of the cell membrane?

Answer 6

phospholipids, cholesterol, glycoproteins, glycolipids, proteins

Question 7

what is the function of the cholesterol in the cell membrane?

Answer 7

increases the strength of the membrane so it is less likely to get damaged
reduces sideways movement of phospholipids which controls fluidity of membrane
packs the space between phospholipids to reduce movement of water-soluble chemicals across the cell-surface membrane

Question 8

why is cholesterol able to provide these functions?

Answer 8

cholesterol molecules have a polar hydrophilic group at one end which attracts the polar heads on phospholipid molecules, however the rest is non-polar and hydrophobic and attracts the non-polar fatty acids

Question 9

what are the two categories of proteins found in the cell membrane?

Answer 9

intrinsic/integral and extrinsic/peripheral

Question 10

what do intrinsic proteins do?

Answer 10

they are fully embedded in the membrane from one side to the other and have hydrophobic amino acids on their outside surface - these interact with the hydrophobic fatty acids in the phospholipid bilayer

Question 11

what are the two types of intrinsic protein?

Answer 11

channel proteins and carrier proteins

Question 12

what is the function of channel proteins?

Answer 12

they have a channel lined with hydrophilic amino acids and filled with water molecules, which allows water soluble molecules to diffuse through

Question 13

what is the function of carrier proteins?

Answer 13

to transfer molecules from one side of the membrane to the other by changing their shape or position

Question 14

how are extrinsic proteins different to intrinsic proteins?

Answer 14

they are only found on one side of the membrane or the other, but may be attached to intrinsic proteins

Question 15

what are the functions of extrinsic proteins?

Answer 15

they may play a structural role in the membrane, act as enzymes, or act as receptors for molecules like hormones

Question 16

what are the functions of glycoproteins?

Answer 16

allowing cells to attach to each other to form tissues, working in the immune system, acting as receptors to hormones

Question 17

what are the functions of glycolipids?

Answer 17

cell communication - glycolipids on one cell can be recognised by another cell to determine whether the cells come into contact
may act as antigens to determine blood group

Question 18

what is simple diffusion?

Answer 18

the net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration

Question 19

what are key features of simple diffusion?

Answer 19

molecules or ions move down the concentration gradient
random movement is caused by the natural kinetic energy of the molecules/ions
diffusion is passive so doesn’t require metabolic energy

Question 20

what happens as a result of diffusion?

Answer 20

molecules or ions reach an equilibrium situation so they are evenly spread within a given volume of space

Question 21

what factors affect the rate of a substance’s diffusion across a membrane?

Answer 21

concentration gradient, temperature, surface area, size of molecule, charge on molecule

Question 22

factors affecting diffusion - concentration gradient?

Answer 22

a greater difference in concentration on two sides of a membrane means a greater difference in the number of molecules passing in the two directions, so a faster rate of diffusion

Question 23

factors affecting diffusion - temperature?

Answer 23

at higher temperatures, molecules and ions have more kinetic energy and diffuse faster

Question 24

factors affecting diffusion - surface area?

Answer 24

the greater the surface area across which diffusion occurs, the greater the number of molecules/ions that can cross it at a time, so diffusion occurs faster
as SA:V ratio decreases, diffusion slows down because the distance is too great (so cell membranes are thin)

Question 25

factors affecting diffusion - size of molecule?

Answer 25

large molecules diffuse more slowly because they require more energy to move

Question 26

factors affecting diffusion - charge on molecule?

Answer 26

uncharged and non-polar molecules diffuse directly across the non-polar phospholipid bilayer and more quickly than polar ones because they are soluble in it

Question 27

which substances can’t diffuse through the phospholipid bilayer?

Answer 27

large polar molecules eg. glucose, and ions eg. sodium ions (hydrophilic)

Question 28

what is facilitated diffusion?

Answer 28

when highly specific intrinsic proteins (channel proteins and carrier proteins) enable hydrophilic or charged substances to cross the phospholipid bilayer

Question 29

how do channel proteins enable facilitated diffusion?

Answer 29

they are pores filled with water and lined with hydrophilic amino acids which allow charged substances to diffuse through the cell membrane
they are usually ‘gated’ to control the exchange of ions - part of the protein on the inside membrane can move to open or close the pore

Question 30

how do carrier proteins enable facilitated diffusion?

Answer 30

they switch between two shapes - this means the binding site of the carrier protein is open to one side of the membrane first, and then open to the other side when it switches shape

Question 31

how do you calculate rate of diffusion?

Answer 31

number of molecules or ions which diffuse/time (usually in seconds)
unit= s^-1

Question 32

what is active transport?

Answer 32

when specific carrier proteins in the cell membrane transport a chemical from a region of lower concentration to a region of higher concentration (against the concentration gradient)

Question 33

what is the key feature of active transport?

Answer 33

it requires ATP (Adenosine Tri-Phosphate), which is produced in the mitochondria during aerobic respiration

Question 34

how does ATP enable active transport?

Answer 34

ATP is hydrolysed to ADP (Adenosine Di-Phosphate) which releases energy (from the bond which breaks when the third phosphate is released) and causes a change in the shape of the membrane protein to allow the molecule to pass through

Question 35

what is the process of active transport occurring in a cell?

Answer 35

1. the molecule/ion attaches to a receptor site on a carrier protein
2. an ATP molecule binds to the carrier protein
3. the ATP molecule undergoes hydrolysis to produce phosphate and an ADP molecule
4. the phosphate attaches to the carrier protein so it changes shape, and the molecule/ion is transported to the other side of the membrane and released
5. the phosphate leaves the carrier protein so it returns to its original shape
6. the ADP and phosphate reform to ATP in respiration

Question 36

where does co-transport occur?

Answer 36

in the mammalian ileum, part of the small intestine which absorbs molecules produced by digestion

Question 37

how does glucose initially move from ileum into the bloodstream?

Answer 37

there is a high concentration in the lumen of the ileum after digestion , so molecules move via facilitated diffusion into the epithelial cells which line the ileum and then into the bloodstream

Question 38

what is the problem with glucose moving into the bloodstream via facilitated diffusion?

Answer 38

the concentration gradient falls as glucose moves into the epithelial cells, so the rate of diffusion does too - this means not all available glucose can move in this way

Question 39

how is remaining glucose absorbed into the bloodstream?

Answer 39

1. a carrier protein called the sodium-potassium pump (antiport) uses ATP to transport sodium ions from the epithelial cells into the bloodstream via active transport (and potassium ions into the epithelial cells)
2. the low concentration of sodium ions in the epithelial cells but high concentration in the lumen creates a concentration gradient
3. sodium ions diffuse (facilitated diffusion) into the epithelial cell via a protein in the membrane called the sodium-glucose transporter (symport)
4. simultaneously, glucose molecules are transported into the epithelial cell through the protein via active transport (against their conc gradient - energy comes indirectly from conc gradient of sodium ions)

Question 40

how are epithelial cells adapted for their role in active transport?

Answer 40

contain lots of mitochondria to produce ATP for sodium potassium pump
cell membrane is folded into microvilli to increase the surface area so there is more space for membrane proteins

Question 41

what is a uniport membrane protein?

Answer 41

transports one type of molecule/ion in one direction

Question 42

what is an antiport membrane protein?

Answer 42

transports two types of molecule/ion - when one enters, the other leaves

Question 43

what is a symport membrane protein?

Answer 43

transports two types of molecule/ion - both move in same direction

Question 44

what is endocytosis?

Answer 44

when particles or molecules are transported into the cell including large molecules like proteins and cels like bacteria

Question 45

what is exocytosis?

Answer 45

when material is moved out of the cell including secreted proteins like enzymes and hormones

Question 46

what is osmosis?

Answer 46

the net movement of water molecules across a partially permeable membrane from an area of higher water potential to an area of lower water potential

Question 47

what is water potential measured in and what does it mean?

Answer 47

measured in kPa - it relates to the pressure free water molecules exert on the walls of their container

Question 48

what is the water potential of pure/distilled water?

Answer 48

0 kPa (highest)

Question 49

what happens when a solute is added to pure water?

Answer 49

1. some water molecules associate with the solute, so it dissolves and there are less free water molecules
2. as there are fewer free water molecules in the container
   , the pressure they collectively exert on the walls is lower
3. the more solute added, the lower the water potential becomes

Question 50

what happens to cells in a hypertonic solution?

Answer 50

there is a lower water potential outside the cell so water moves out of the cell causing crenation

Question 51

what happens to cells in a hypotonic solution?

Answer 51

there is a lower water potential inside the cell so water moves into the cell causing lysis

Question 52

what happens to cells in an isotonic solution?

Answer 52

the water potential is the same inside and outside the cell so there is no net movement of water