4.2 Cell Transport Mechanisms

Question 1

why can gases diffuse directly into cells of small organisms?

Answer 1

they have short diffusion distance from outside and inside organs
large surface area to volume ratio
low metabolic demands

Question 2

name the three chemical components of a cell

Answer 2

lipids- phospholipids and cholesterol
proteins
carbohydrates

Question 3

how and why do bilayers form?

Answer 3

phospholipid bilayers form due to the hydrophobic tail of the phospholipid and hydrophilic head. this simple lipid bilayer allows fat, soluble organic molecules to pass through

Question 4

why is it called the fluid mosaic model?

Answer 4

fluid lipid bilayer with a mosaic of proteins, glycoproteins and glycolipids floating on it

Question 5

give the function of integral proteins

Answer 5

main transport system of membrane that can form permanent pores or other transport mechanisms (e.g carrier proteins)

Question 6

give the function of peripheral proteins

Answer 6

can be enzymes that regulate transport

Question 7

give the function of glycoproteins

Answer 7

often act as antigens- are important for cell recognition or act as receptors for hormones

Question 8

name two things that proteins in cell membrane can do

Answer 8

form channels and carriers for passage of substances

can act as specific receptor molecules

Question 9

give 4 key pieces of evidence for the fluid mosaic model

Answer 9

• monolayer of phospholipids is found to be 2x larger than cell surface area
• microscope images show proteins sticking out of cell surfaces
• some water-soluble substances pass in + out of cells- evidence of channels
• ionic + polar molecules do not pass through membranes but lipid-soluble molecules do

Question 10

name the three types of passive transport

Answer 10

diffusion
facilitated diffusion
osmosis

Question 11

describe active transport and give two examples

Answer 11

movement of substances across membrane of cells using ATP
e.g exocytosis- movement of large molecules through vesicle formation out cells
endocytosis - movement of large molecules through vesicle formation into cells

Question 12

when would facilitated diffusion be used?

Answer 12

when specific substances like large molecules with strong charges need to be moved into the membrane down a concentration gradient

Question 13

describe the process of facilitated diffusion

Answer 13

1. amino acids are carried across the membrane in protein carriers of specific shape
2. the protein carriers change shape + the molecule is passed into the cell
3. the protein carrier returns to its original shape which allows more molecules to enter

Question 14

what is a gated channel?

Answer 14

a channel that only opens when a specific molecule is present

Question 15

does facilitated diffusion occur up or down a concentration gradient?

Answer 15

it can only occur down the concentration gradient

Question 16

define osmosis

Answer 16

net movement of water molecules from an area of high concentration to an area of low concentration through a semi-permeable membrane

Question 17

what is an isotonic solution?

Answer 17

solution with the same osmotic concentration as the cell

Question 18

what is a hypotonic solution?

Answer 18

solution with a lower osmotic concentration than the cells so water moves in

Question 19

what is a hypertonic solution?

Answer 19

solution with a higher osmotic concentration than the cells so water moves out

Question 20

what happens if too much water enters a cell?

Answer 20

cell bursts - osmotic lysis

Question 21

what happens if too much water leaves a cell?

Answer 21

cell shrinks - plasmolysis (in hypertonic solutions)

Question 22

describe how pressure potential increases in a plant cell

Answer 22

the rigid nature of cellulose cell wall stops plant cells from bursting
if the surrounding fluid is hypotonic to the cytoplasm, water enters the cells via osmosis
as the cytoplasm swells, it generates hydrostatic pressure
the inward pressure of the cell wall on cytoplasm increases to oppose the entry if water - this is pressure potential

Question 23

define turgor

Answer 23

state of a plant cell when solute potential that causes water to move into the cell by osmosis is balanced by the force of the cell wall pressing on the protoplasm

Question 24

what happens when water leaves the cell?

Answer 24

turgor is lost and incipient plasmolysis occurs

Question 25

define water potential

Answer 25

a measure of the potential for water to move out of a solution by osmosis

Question 26

why does pure water have the highest water potential?

Answer 26

the water molecules will always move from pure water to any solution on the other side of the membrane

Question 27

what is the maximum water potential (wP of pure water)?

Answer 27

0 - all other solutions will have a negative value for water potential because they have a lower concentration of water molecules

Question 28

define osmosis in terms of water potential

Answer 28

net movement of water molecules from an area of high water potential to an area of low water potential through a partially permeable membrane

Question 29

define turgor pressure

Answer 29

the hydrostatic pressure caused by cell contents pushing against the cell wall

Question 30

how does turgor pressure increase?

Answer 30

• water moves into cell via osmosis and protoplasm swells
• as water continues to move into the cell, hydrostatic pressure is exerted by cell wall against cell contents
• turgor pressure increases until osmotic force pulling water into the cell is balanced by turgor pressure opposing entry of water

Question 31

does turgor pressure have a positive or negative value? how can it be measured?

Answer 31

usually has a positive value

can be measured directly using a pressure probe

Question 32

define osmotic potential

Answer 32

a measure of the potential of a solution to cause water to move into the cell across a partially permeable membrane

Question 33

does osmotic potential have a positive or negative value? explain why.

Answer 33

pure water has the highest osmotic potential of 0 so all other solutions must have a lower osmotic value which is negative

Question 34

define osmosis in terms of osmotic potential

Answer 34

net movement of water molecules from an area of high osmotic potential to low osmotic potential through a partially permeable membrane

Question 35

when turgor pressure + osmotic potential are equal…

Answer 35

• cell is at full turgor

- water potential of cell is 0

Question 36

give the equation to determine water potential of a cell

Answer 36

water potential = turgor pressure + osmotic potential

Question 37

describe the role of ATPase in active transport

Answer 37

it catalyses the hydrolysis of ATP, breaking one bond and forming two more to provide energy needed by carrier in membrane or to release transported substances

Question 38

name two things needed for active transport to occur

Answer 38

a carrier protein that spans the whole membrane

energy from ATP

Question 39

define active transport

Answer 39

movement of substances into or out of the cell using ATP produced during respiration

Question 40

describe the process of active transport

Answer 40

1. proteins act as carriers that are specific to molecule being transported in one direction (against concentration gradient)
2. release of energy from ATP hydrolysis causes change in carrier protein shape that moves molecules across membrane
3. protein carrier returns to original shape to allow more molecules to enter

Question 41

give 4 pieces of evidence for active transport

Answer 41

• only occurs in living cells
• rate of active transport depends on temperature and CO2 concentration
• cells that carry out a lot of active transport have lots of mitochondria
• poisons that stop respiration/prevent ATPase from working also stop active transport

Question 42

give two similarities of endocytosis and exocytosis

Answer 42

• both are possible because of fluid nature of the membrane

- both are active processes, requiring energy from ATP

Question 43

explain endocytosis

Answer 43

substance needed to be transported is englufed and move in by phagocytosis

Question 44

explain exocytosis

Answer 44

intracellular vesicle fuses with cell membrane and ejects contents that need transporting