5.2 Movement In and Out of Cells Flashcards
What is the cell wall?
structural layer external to the plasma membrane that helps helps maintain the shape and internal composition of the cell
What is the plasma membrane?
membrane that surrounds the cytoplasm of the cell, separating the inside of the cell from the outside of the cell
What makes up the plasma membrane?
phospholipids with embedded proteins
What is homeostasis?
active maintenance of a constant environment within the cells, is a critical attribute of cells and life
How does the plasma membrane maintain homeostasis?
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
What is the membrane’s ability to act as a selective barrier a result of?
combination of lipids and embedded proteins of which it is composed
What can move through the membrane?
- 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
What can facilitate the movement of molecules that cannot cross on its own?
protein channels and transporters in the membrane
can transport molecules including ions, water, and nutrients
What is passive transport?
simplest form of movement into and out of cells
How does passive transport work?
by diffusion
What is diffusion?
random motion of molecules, with net movement occurring from areas of higher to lower concentration of the molecules
During diffusion, what happens if there is no longer a concentration gradient?
net movement stops but movement of molecules in both directions continues
How do oxygen and carbon dioxide move into and out of a cell?
diffuse freely across the plasma membrane as a result of differences in concentrations between the inside and the outside of a cell
What molecules can diffuse through the cell membrane?
- gases
- certain hydrophobic molecules (ie. triacylglycerols), not surprising since the lipid bilayer is also hydrophobic
What is facilitated diffusion?
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
What do diffusion and facilitated diffusion both result from?
random motion of molecules and net movement of the substance occurs when there are concentration differences
What is the difference between diffusion and facilitated diffusion?
facilitated diffusion: the molecule moves through a membrane transporter,
simple diffusion: the molecule moves directly through the lipid bilayer
What are the two types of membrane transporters for facilitated diffusion?
channel and carrier
What are channels?
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
What are carriers?
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
How does water move through plasma membrane?
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
What is osmosis?
net movement of a solvent, such as water, across a selectively permeable membrane toward the side of higher solute concentration
What happens during osmosis?
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
How can osmosis be prevented?
by applying force to the compartment with the higher solute concentration
What is osmotic pressure?
the pressure that would need to be applied to stop water from moving into a solution by osmosis
When does passive transport work to the cell’s advantage?
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
What is active transport?
“uphill” movement of substances against a concentration gradient requiring an input of energy
What is most of the energy used by a cell for?
keeping the inside of the cell different from the outside, a function carried out by proteins in the plasma membrane
What happens during active transport?
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)
Describe the sodium-potassium pump.
- 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
What is primary active transport?
active transport that uses energy of ATP directly
movement of ions takes energy
What is secondary active transport?
active transport that uses the energy of an electrochemical gradient to drive the movement of molecules
What do many cells do because small ions cannot cross the lipid bilayer?
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
Why is a concentration gradient called a chemical gradient in secondary active transport?
the entity of forming the gradient is a chemical
In addition to the chemical gradient, what other force favours the movement of protons back across the membrane?
a difference in charge
What is an electrical gradient?
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
What is an electrochemical gradient?
gradient that combines the charge gradient and the chemical gradient of protons and other ions
What happens if protons are then allowed to pass through cell membrane by a transport protein?
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
What is always the direction of movement of protons in secondary active transport?
higher to lower concentration
What is always the direction of movement of other molecules in secondary active transport?
lower to higher concentration
Difference between primary and secondary transport?
primary: uses the chemical energy of ATP directly
secondary: uses the potential energy of an electrochemical gradient to drive the movement of molecules
What is a common cellular strategy?
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
What do many cells use active transport for?
to maintain their size and composition
What is a hypertonic solution?
higher solute concentration than inside the cell
water leaves the cell by osmosis and the cell shrinks
What is a hypotonic solution?
lower solute concentration than inside the cell
water moves into the cell by osmosis and the cell lyses (bursts)
How do animal cells solve the problem of water movement?
by keeping the intracellular fluid isotonic (the same solute concentration) with the extracellular fluid
What do cells use the active transport of ions for?
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
How do human red blood cells avoid shrinking or bursting?
by maintaining an intracellular environment isotonic with the extracellular environment (blood)
How do single-celled organisms avoid shrinking or bursting?
contain contractile vacuoles
What are contractile vacuoles?
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
How do organisms such as plants, fungi, and bacteria maintain cell size and shape?
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
What is due to the cell wall being rigid and able to resist expansion?
it allows pressure to build up when water enter a cell
What is turgor pressure?
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
What happens when a plant cell is placed in hypotonic solution?
water enters the cell by osmosis and turgor pressure created by the cell wall increases to a level to stop osmosis
Why does turgor pressure develop?
because the cell wall resists being stretched and pushes back on the interior of the cell
What does the pressure exerted by water inside the cell on the cell wall provide?
structural support for many organisms that is similar in function to the support provided by animals’ skeletons
What does a vacuole do?
absorbs water and contributes to turgor pressure
What does the function of vacuoles explain?
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
What is another function of plant vacuoles?
explain in part why plant cells are typically larger than animal cells and can store water, nutrients, ions, and wastes
What is the cell wall made up of?
carbohydrates
proteins
(specifics depends on the organism)
What is the plant cell wall composed of?
polysaccharides, including cellulose (most abundant biological material in nature, polymer of glucose)
What is the algae cell wall composed of?
some cellulose, others made of silicon or calcium carbonate
What is the fungi cell wall composed of?
chitin (polymer of sugar)
What is the bacteria cell wall composed of?
primarily peptidoglycan (polymer of amino acids and sugars)
