5.2 Movement In and Out of Cells

Question 1

What is the cell wall?

Answer 1

structural layer external to the plasma membrane that helps helps maintain the shape and internal composition of the cell

Question 2

What is the plasma membrane?

Answer 2

membrane that surrounds the cytoplasm of the cell, separating the inside of the cell from the outside of the cell

Question 3

What makes up the plasma membrane?

Answer 3

phospholipids with embedded proteins

Question 4

What is homeostasis?

Answer 4

active maintenance of a constant environment within the cells, is a critical attribute of cells and life

Question 5

How does the plasma membrane maintain homeostasis?

Answer 5

selective permeability:

• lets some molecules in and out freely
• lets others in and out only under certain conditions
• prevents still other molecules from passing through at all

Question 6

What is the membrane’s ability to act as a selective barrier a result of?

Answer 6

combination of lipids and embedded proteins of which it is composed

Question 7

What can move through the membrane?

Answer 7

• hydrophobic interior of the lipid bilayer prevents ions and charged polar molecules from moving across it
• macromolecules (ie. proteins and polysaccharides) are too large to cross the plasma membrane on their own
• gases (ie. oxygen and carbon dioxide) and nonpolar molecules (ie. lipids) can move across the lipid bilayer
• small uncharged polar molecules (ie. water) are able to move through the lipid bilayer to a very limited extent, but this movement is not biologically significant

Question 8

What can facilitate the movement of molecules that cannot cross on its own?

Answer 8

protein channels and transporters in the membrane

can transport molecules including ions, water, and nutrients

Question 9

What is passive transport?

Answer 9

simplest form of movement into and out of cells

Question 10

How does passive transport work?

Answer 10

by diffusion

Question 11

What is diffusion?

Answer 11

random motion of molecules, with net movement occurring from areas of higher to lower concentration of the molecules

Question 12

During diffusion, what happens if there is no longer a concentration gradient?

Answer 12

net movement stops but movement of molecules in both directions continues

Question 13

How do oxygen and carbon dioxide move into and out of a cell?

Answer 13

diffuse freely across the plasma membrane as a result of differences in concentrations between the inside and the outside of a cell

Question 14

What molecules can diffuse through the cell membrane?

Answer 14

• gases

- certain hydrophobic molecules (ie. triacylglycerols), not surprising since the lipid bilayer is also hydrophobic

Question 15

What is facilitated diffusion?

Answer 15

diffusion through a membrane protein, bypassing the lipid bilayer

some molecules that cannot move across the lipid bilayer directly can move passively toward a region of lower concentration through protein transporters

Question 16

What do diffusion and facilitated diffusion both result from?

Answer 16

random motion of molecules and net movement of the substance occurs when there are concentration differences

Question 17

What is the difference between diffusion and facilitated diffusion?

Answer 17

facilitated diffusion: the molecule moves through a membrane transporter,

simple diffusion: the molecule moves directly through the lipid bilayer

Question 18

What are the two types of membrane transporters for facilitated diffusion?

Answer 18

channel and carrier

Question 19

What are channels?

Answer 19

membrane transporters that provide an opening between the inside and outside of the cell through which certain molecules can pass, depending on their shape and charge

• some are gated, meaning that they open in response to a signal, which may be chemical or electrical

Question 20

What are carriers?

Answer 20

membrane transporters that bind to and then transports specific molecules

• exist in two conformations: one that is open to one side of the cell, and another that is open to the other side of the cell
• binding of the transported molecule induces a conformational change in the membrane protein, allowing the molecule to be transported across the lipid bilayer

Question 21

How does water move through plasma membrane?

Answer 21

by protein channels called aquaporins, which allow water to move much more readily across the plasma membrane by facilitated diffusion than is possible by simple diffusion

Question 22

What is osmosis?

Answer 22

net movement of a solvent, such as water, across a selectively permeable membrane toward the side of higher solute concentration

Question 23

What happens during osmosis?

Answer 23

net movement of water toward the side of the membrane with higher solute concentration continues until a concentration gradient no longer exists or until the movement is opposed by another force

• this force could be pressure due to gravity or the cell wall

Question 24

How can osmosis be prevented?

Answer 24

by applying force to the compartment with the higher solute concentration

Question 25

What is osmotic pressure?

Answer 25

the pressure that would need to be applied to stop water from moving into a solution by osmosis

Question 26

When does passive transport work to the cell’s advantage?

Answer 26

only if the concentration gradient is in the right direction, from higher on the outside to lower on the inside for nutrients that the cell needs to take in, and from higher on the inside and lower on the outside for wastes that the cell needs to export

Question 27

What is active transport?

Answer 27

“uphill” movement of substances against a concentration gradient requiring an input of energy

Question 28

What is most of the energy used by a cell for?

Answer 28

keeping the inside of the cell different from the outside, a function carried out by proteins in the plasma membrane

Question 29

What happens during active transport?

Answer 29

cells move substances through transport proteins embedded in the cell membrane
- some of these proteins act as pumps, using energy directly to move a substance into or out of a cell (ie. sodium-potassium pump)

Question 30

Describe the sodium-potassium pump.

Answer 30

• within cells, sodium is kept at concentrations much lower than in the exterior environment; the opposite is true of potassium
• therefore, both sodium and potassium have to be moved against a concentration gradient
• the pump actively moves sodium out of the cell and potassium into the cell

Question 31

What is primary active transport?

Answer 31

active transport that uses energy of ATP directly

movement of ions takes energy

Question 32

What is secondary active transport?

Answer 32

active transport that uses the energy of an electrochemical gradient to drive the movement of molecules

Question 33

What do many cells do because small ions cannot cross the lipid bilayer?

Answer 33

use a transport protein to build up the concentration of a small ion on one side of the membrane

the resulting concentration gradient stores potential energy that can be harnessed to drive the movement of other substances across the membrane against their concentration gradient

ie. some cells actively pump protons across the cell membrane using ATP
- as a result, in these cells the concentration of protons is higher on one side of the membrane and lower on other other side
- PUMP GENERATES A CONCENTRATION GRADIENT (CHEMICAL GRADIENT)
- concentration differences favour the movement of protons back to the other side of the membrane
- by blocking the movement of protons back to the other side, lipid bilayer creates a store of potential energy, just as a dam or battery

Question 34

Why is a concentration gradient called a chemical gradient in secondary active transport?

Answer 34

the entity of forming the gradient is a chemical

Question 35

In addition to the chemical gradient, what other force favours the movement of protons back across the membrane?

Answer 35

a difference in charge

Question 36

What is an electrical gradient?

Answer 36

because protons carry a positive charge, side of the membrane with more protons is more positive than the other side

protons move from areas of like charge to areas of unlike charge

Question 37

What is an electrochemical gradient?

Answer 37

gradient that combines the charge gradient and the chemical gradient of protons and other ions

Question 38

What happens if protons are then allowed to pass through cell membrane by a transport protein?

Answer 38

they will move down their electrochemical gradient toward the region of lower proton concentration

these transport proteins can use the movement of protons to drive the movement of other molecules against their concentration gradient

Question 39

What is always the direction of movement of protons in secondary active transport?

Answer 39

higher to lower concentration

Question 40

What is always the direction of movement of other molecules in secondary active transport?

Answer 40

lower to higher concentration

Question 41

Difference between primary and secondary transport?

Answer 41

primary: uses the chemical energy of ATP directly
secondary: uses the potential energy of an electrochemical gradient to drive the movement of molecules

Question 42

What is a common cellular strategy?

Answer 42

use of an electrochemical gradient as a temporary energy source

ie. cells use a sodium electrochemical gradient generated by the sodium-potassium pump to transport glucose and amino acids into cells
ie. cells use a proton electrochemical gradient to move other molecules and to synthesize ATP

Question 43

What do many cells use active transport for?

Answer 43

to maintain their size and composition

Question 44

What is a hypertonic solution?

Answer 44

higher solute concentration than inside the cell

water leaves the cell by osmosis and the cell shrinks

Question 45

What is a hypotonic solution?

Answer 45

lower solute concentration than inside the cell

water moves into the cell by osmosis and the cell lyses (bursts)

Question 46

How do animal cells solve the problem of water movement?

Answer 46

by keeping the intracellular fluid isotonic (the same solute concentration) with the extracellular fluid

Question 47

What do cells use the active transport of ions for?

Answer 47

to maintain equal concentrations inside and out, and the sodium-potassium pump plays an important role in keeping the inside of the cell isotonic with the extracellular fluid

Question 48

How do human red blood cells avoid shrinking or bursting?

Answer 48

by maintaining an intracellular environment isotonic with the extracellular environment (blood)

Question 49

How do single-celled organisms avoid shrinking or bursting?

Answer 49

contain contractile vacuoles

Question 50

What are contractile vacuoles?

Answer 50

type of cellular compartment that takes up excess water and waste products from inside the cell and expels them into the external environment

ie. take in water through aquaporins or take in protons through proton pumps, with water following by osmosis

Question 51

How do organisms such as plants, fungi, and bacteria maintain cell size and shape?

Answer 51

cell wall that surrounds the membrane

• plays a critical role in the maintenance of cell size and shape
• provides structure support and protection for the cell

Question 52

What is due to the cell wall being rigid and able to resist expansion?

Answer 52

it allows pressure to build up when water enter a cell

Question 53

What is turgor pressure?

Answer 53

pressure within a cell resulting from the movement of water into the cell by osmosis and the tendency of the cell wall to resist deformation

Question 54

What happens when a plant cell is placed in hypotonic solution?

Answer 54

water enters the cell by osmosis and turgor pressure created by the cell wall increases to a level to stop osmosis

Question 55

Why does turgor pressure develop?

Answer 55

because the cell wall resists being stretched and pushes back on the interior of the cell

Question 56

What does the pressure exerted by water inside the cell on the cell wall provide?

Answer 56

structural support for many organisms that is similar in function to the support provided by animals’ skeletons

Question 57

What does a vacuole do?

Answer 57

absorbs water and contributes to turgor pressure

Question 58

What does the function of vacuoles explain?

Answer 58

why plants wilt when dehydrated: loss of water from the vacuoles reduces turgor pressure, so the cells can no longer maintain their shape within the cell wall

Question 59

What is another function of plant vacuoles?

Answer 59

explain in part why plant cells are typically larger than animal cells and can store water, nutrients, ions, and wastes

Question 60

What is the cell wall made up of?

Answer 60

carbohydrates
proteins
(specifics depends on the organism)

Question 61

What is the plant cell wall composed of?

Answer 61

polysaccharides, including cellulose (most abundant biological material in nature, polymer of glucose)

Question 62

What is the algae cell wall composed of?

Answer 62

some cellulose, others made of silicon or calcium carbonate

Question 63

What is the fungi cell wall composed of?

Answer 63

chitin (polymer of sugar)

Question 64

What is the bacteria cell wall composed of?

Answer 64

primarily peptidoglycan (polymer of amino acids and sugars)