3.5 Permeability of the Plasma Membrane Flashcards
What is the function of the plasma membrane?
regulates the passage of molecules into and out of the cell
Why is the function of the plasma membrane critical?
the life of the cell depends on maintenance of its normal composition
Why can the plasma membrane regulate the passage of molecules into and out of the cell?
it is selectively permeable
What does selectively permeable mean?
certain substances can move across the membranes while others cannot
In general, what molecules can freely cross the membrane?
small, noncharged molecules such as carbon dioxide, oxygen, glycerol, and alcohol
How do molecules cross the membrane?
they are able to slip between the hydrophilic heads of the phospholipids and pass through the hydrophobic tails of the membrane
What is concentration gradient?
gradual change in chemical concentration between 2 areas of differing concentrations
What does going “down” in concentration gradient mean?
most molecules move from an area where the concentration is high to an area where there concentration is low
What does going “up” in concentration gradient mean?
some molecules are able to move from an area where their concentration is low to an area where their concentration is high
What is required when going “up” in concentration gradient?
energy
Is water polar or nonpolar?
polar
Is the membrane polar or nonpolar?
primarily nonpolar
Why would water not be expected the readily cross the primarily nonpolar membrane?
although the small size of the water molecule may allow some water to diffuse across the plasma membrane, the majority of cells have special channel proteins called AQUAPORINS that allow water to quickly cross the membrane
What molecules are unable to freely cross the membrane?
large molecules and some ions and charged molecules
How can large molecules and some ions and charged molecules cross the plasma membrane?
through channel proteins, with the assistance of carrier proteins, or in vesicles
What do channel proteins do?
form a pore through the membrane that allows molecules of a certain size and/or charge to pass
What are carrier proteins specific for?
the substances they transport across the plasma membrane (ie. sodium ions, amino acids, glucose)
What method of crossing a plasma membrane is reserved for macromolecules or even larger materials (ie. virus)?
vesicle formation is another way a molecules can exit a cell by exocytosis or enter a cell by endocytosis
Passage of Molecules Into and Out of the Cell
OSMOSIS
- direction
- requirement
- energy required?
- examples
- direction: toward lower concentration
- requirement: concentration gradient
- energy required: no
- examples: water
Passage of Molecules Into and Out of the Cell
DIFFUSION
- direction
- requirement
- energy required?
- examples
- direction: toward lower concentration
- requirement: concentration gradient
- energy required: no
- examples: lipid-soluble molecules and gas
Passage of Molecules Into and Out of the Cell
FACILITATED TRANSPORT
- direction
- requirement
- energy required?
- examples
- direction: toward lower concentration
- requirement: channels or carrier and concentration
gradient - energy required: no
- examples: some sugars and some amino acids
Passage of Molecules Into and Out of the Cell
ACTIVE TRANSPORT
- direction
- requirement
- energy required?
- examples
- direction: toward higher concentration
- requirement: carrier plus energy
- energy required: yes
- examples: sugars, amino acids, and ions
Passage of Molecules Into and Out of the Cell
EXOCYTOSIS
- direction
- requirement
- energy required?
- examples
- direction: toward outside
- requirement: vesicle fuses with plasma membranes
- energy required: yes
- examples: macromolecules
Passage of Molecules Into and Out of the Cell
ENDOCYTOSIS
- direction
- requirement
- energy required?
- examples
- direction: toward inside
- requirement: vesicle formation
- energy required: yes
- examples: macromolecules
What is diffusion?
the movement of molecules from a higher to lower concentration (down their concentration gradient) until equilibrium is achieved and they are distributed equally
What type of molecule can diffusion be observed with?
any type of molecule
Give an example of diffusion.
when a crystal of dye (solute) is placed in water (solvent), the dye and water molecules move in various directions, but their net movement (sum of motion) is toward the region of lower concentration
eventually the dye is dissolved in the water, resulting in equilibrium and a coloured solution
What is a solute?
the dissolved substance, usually a solid, in a solution
What is a solvent?
part of a solution, usually a liquid, that contains solutes
What happens once the solute and solvent are evenly distributed?
their molecules continue to move about, but there is no net movement of either one in any direction
What allows only a few types of molecules to enter and exit a cell simply by diffusion?
the chemical and physical properties of the plasma membrane
How does oxygen enter cells and carbon dioxide exit cells?
gases can diffuse through the lipid bilayer
Why does oxygen diffuse into blood?
consider the movement of oxygen from the alveoli of the lungs to the blood in the lung capillaries
after inhalation, the concentration of oxygen in the alveoli is higher than that in blood
What does diffusion also play an important role in?
maintaining the resting potential neurons using gradients of potassium and sodium ions
What factors influence the rate of diffusion?
temperature (ie. temperature increases, rate increases)
pressure
electrical currents
molecular size
What is osmosis?
the diffusion of water across a selectively permeable membrane due to concentration differences
What are percent solutions?
grams of solute per 100 mL of solvent (ie. 10% solution is 10 g of sugar with water added to make 100 mL of solution)
Give an example of osmosis.
thistle solution containing a 10% solute solution is covered at one end by a selectively permeable membrane and then placed in a beaker containing a 5% solute solution
beaker has a higher concentration of water molecules (lower % of solute) and thistle tube has a lower concentration of water molecules (higher % of solution)
diffusion always occurs from higher to lower concentration therefore a net movement of water takes place across the membrane
from the beaker to the inside of the tube
solute doesn’t diffuse out of the thistle tube because the membrane is not permeable to the solute
as water enters and the solute doesn’t exit, the level of the solution within the thistle tube rises
in the end, concentration of solute in thistle tube is less than 10% because there is now less solute per unit volume of solution
concentration of solute in the beaker is greater than 5% because there is now more solute per unit volume
water enters thistle tube due to osmotic pressure of the solution in thistle tube
What is osmotic pressure?
the pressure that develops in a system due to osmosis
THE GREATER THE OSMOTIC PRESSURE, THE MORE LIKELY IT IS THAT WATER WILL DIFFUSE IN THAT DIRECTION
How is osmotic pressure measured?
by placing a solution in an osmometer and then immersing the osmometer in pure water which creates the pressure
Where does osmosis also occur?
across the plasma membrane
In the body, what is due to osmotic pressure?
water is absorbed by the kidneys and taken up by capillaries in the tissues
What is an isotonic solution?
solution in which the solute concentration and the water concentration both inside and outside the cell are equal and therefore there is no net gain or loss of water
What does “iso-“ mean?
the same as
What does the term tonicity refer to?
the osmotic pressure or tension of the solution
Give an example of an isotonic solution?
a 0.9% solution of sodium chloride is known to be isotonic in red blood cells therefore intavenous solutions medically administered usually have this tonicity
What do terrestrial animals do to maintain the tonicity of their internal environment?
can usually take in either water or salt as needed
How do animals such as oysters, blue crabs, and some fishes able to cope with changes in the salinity (salt concentrations) of their environment?
using specialized kidneys, gills, and other structures
Animal Cells:
ISOTONIC
there is no net movement of water
Animal Cells:
HYPOTONIC
water enters the cell which may burst (cytolysis)
Animal Cells:
HYPERTONIC
water leaves the cell, which shrivels (crenation)
Plant Cells:
ISOTONIC
there is no net movement of water
Plant Cells:
HYPOTONIC
the central vacuole fills with water, turgor pressure develops, and chloroplasts are seen next to the cell wall
Plant Cells:
HYPERTONIC
the central vacuole loses water, the cytoplasm shrinks (plasmolysis), and chloroplasts are seen in the centre of the cell
What is a hypotonic solution?
solution that causes cells to swell, or even burst, due to an intake of water
What does “hypo” mean and refer to?
“less than”
refers to a solution with a lower concentration of solute (higher concentration of water) than inside the cell
What happens when an animal cell is placed in a hypotonic solution?
water enters the cell and the net movement of water s from the outside to the inside of the cell
What solution is hypotonic to red blood cells?
any concentration of a salt solution lower than 0.9%
What is the difference between cytolysis and hemolysis?
cytolysis: refers to disrupted cells
hemolysis: refers to disrupted red blood cells
What happens when a plant cell is placed in a hypotonic solution?
the cytoplasm expands because the large central vacuole gains water and the plasma membrane pushes against the rigid cell wall
THE PLANT CELL DOES NOT BURST BECAUSE THE CELL WALL DOES NOT GIVE WAY
What creates turgor pressure?
the swelling of the plant cell in a hypotonic solution
Why is turgor pressure extremely important?
important to the maintenance of the plant’s erect position, plants wilt due to turgor pressure
How do organisms that live in fresh water prevent their internal environment form becoming hypotonic?
- many protozoans (ie, paramecia) have contractile vacuoles that rid the body of excess water
- fishes have well-developed kidneys that excrete a large volume of dilute urine, they have to take in salt at their gills
What is a hypertonic solution?
solution that causes cells to shrink or shrivel due to loss of water
What does “hyper” mean and refer to?
“more than”
refers to a solution with a higher percentage of solute (lower concentration of water) than the cell
What happens when an animal cell is placed in a hypertonic solution?
water leaves the cell and the net movement of water is from the inside to the outside of the cell
What solution is hypertonic to red blood cells?
any concentration of a salt solution higher than 0.9%
What is crenation?
the shriveling of a cell in a hypertonic solution
Why are meats sometimes preserved by salting them?
the salt kills any bacteria present because it makes the meat a hypertonic environment
What happens when a plant cell is placed in a hypertonic solution?
the plasma membrane oulls away from the cell wall as the large central vacuole loses water
What is plasmolysis?
shrinking of the cytoplasm due to osmosis
Why might you see dead plants along a salted roadside?
they were exposed to a hypertonic solution during the winter
How do marine animals cope with their hypertonic environment to prevent losing water?
- sharks increase or decrease the urea in their blood until their blood is isotonic with the environment
- marine fishes and other types of animals excrete salts across their gills
What are carrier proteins required for?
facilitated and active transport
What is facilitated transport?
the act of a carrier assisting the passage of a molecule such as glucose and amino acids, across the plasma membrane with no expenditure of energy
What happens during facilitated transport?
a carrier protein speeds the rate at which the solute crosses the plasma membrane toward a lower concentration
What happens to the carrier protein during facilitated transport?
it undergoes a change in shape as it moves a solute across the membrane
Why doesn’t facilitated transport require ATP?
the molecules are moving down their concentration gradient in the same direction they tend to move anyways
What is active transport?
the movement of molecules or ions through the plasma membrane against their concentration gradient and requires ATP
What happens during active transport?
molecules or ions move through the plasma membrane, accumulating either inside or outside the cell
molecules have moved to the region of higher concentration, exactly opposite of the process of diffusion
ie.
- iodine collects in the cells of the thyroid gland
- glucose is completely absorbed from the gut by the cells lining the digestive tract
- sodium can be almost completely withdrawn from urine by cells lining the kidney tubules
What do cells involved primarily in active transport have?
a large number of mitochondria near membranes where active transport is occurring because ATP is required for the carrier to combine with the substance to be transported
What are proteins involved in active transport often called?
pumps because just as a water pump uses energy to move water against the force of gravity, proteins use energy to move a substance against its concentration gradient
Give an example of a pump that is active in animal cells.
SODIUM-POTASSIUM PUMP
especially active in nerve and muscle cells
moves sodium ions to the outside of the cell and potassium ions to the inside of the cell
What allows the carrier to combine alternatively with sodium ions and potassium ions?
a change in carrier shape after the attachment of a phosphate group and again after its detachment
When is the phosphate group donated by ATP?
when it is broken down enzymatically by the carrier
What does the sodium-potassium pump result in?
both a solute concentration gradient and an electrical gradient for these ions across the plasma membrane
Why is the inside of the cell negatively charge compared to the outside?
3 sodium ions are carried outward for every 2 potassium ions carried inward
How do macromolecules enter and exit a cell?
because they are too large to be transported by carrier proteins, macromolecules are transported into and out of the cell by vesicle formation
What is vesicle formation called?
membrane-assisted transport because membrane is needed to form the vesicle
Does vesicle formation require energy?
yes, requires an expenditure of cellular energy
What is a benefit of vesicle formation?
vesicle membrane keeps the contained macromolecules from mixing with molecules within the cytoplasm
What is exocytosis?
a way substances can exit a cell
How does exocytosis work?
a vesicle fuses with the plasma membrane as secretion occurs
hormones, neurotransmitters, and digestive enzymes are secreted from cells in this manner
What produces the vesicles that carry these cell products to the membrane?
Golgi apparatus
Why can exocytosis be a normal part of cell growth?
during exocytosis, the membrane of the vesicle becomes a part of the membrane which is thereby enlarged
What happens to the proteins released from the vesicle during exocytosis?
- adhere to the cell surface
- become incorporated in an extracellular matrix
What do pancreatic cells do?
produce digestive enzymes or insulin
What do pituitary cells do?
produce growth hormone, among other hormones
What do vesicles do in pancreatic and pituitary cells?
secretory vesicles accumulate near the plasma membrane, and the vesicles release their contents only when the cell is stimulated by a signal received at the plasma membrane
What is regulated secretion?
ie. a rise in blood sugar signals pancreatic cells to release the hormone insulin
vesicles fuse with the plasma membrane only when it is appropriate to the needs of the body
What is endocytosis?
a way substances can enter a cell
How does endocytosis work?
cells take in substances by vesicle formation, a portion of the plasma membrane folds in on itself to envelop the substance and then the membrane pinches off to form an intracellular vesicle
What are the 3 ways in which endocytosis can occur?
- phagocytosis
- pinocytosis
- receptor-mediated cytosis
What is phagocytosis?
process in which the material taken in by endocytosis is large, such as viruses, food particles, or another cell
Where is phagocytosis common?
unicellular organisms such as amoebas
Where does phagocytosis occur in humans?
certain types of human white blood cells are amoeboid (mobile like amoeba) and are able to engulf debris such as worn-out red blood cells or viruses
When does digestion occur?
when an endocytic vesicle fuses with a lysosome
What is pinocytosis?
occurs when vesicles form around a liquid or around very small particles
Give examples of what use pinocytosis.
- blood cells
- cells that line the kidney tubules or the intestinal wall
- plant root cells
all use pinocytosis to ingest substances
What microscope can phagocytosis be seen with?
light microscope
What microscope can pinocytic vesicles be seen with?
electron microscope (they are no larger than 0.1 - 0.2 micrometers)
Why does pinocytosis involve a significant amount of the plasma membrane?
it occurs continuously
What is the loss of plasma membrane due to pinocytosis balanced by?
the occurrence of exocytosis
What is receptor-mediated endocytosis?
a form of pinocytosis that is quite specific because it uses a receptor protein shaped so that a specific molecule (ie. vitamin, peptide hormone, lipoprotein) can bind to it
Where are the receptors found?
at one location in the plasma membrane called a COATED PIT because there is a layer of protein on the cytoplasmic side of the pit
Once formed, what happens to the vesicle? (receptor-mediated endocytosis)
vesicle becomes uncoated and may fuse with a lysosome
When the coated pit is empty, what happens to the vesicles and the receptors?
used vesicles: fuse with the plasma membrane
receptors: return to their former location
Why is receptor-mediated endocytosis selective and much more efficient than ordinary pinocytosis?
it is involved in uptake and also in the transfer and exchange of substances between cells
ie. such exchanges take place when substances move from maternal blood into fetal blood at the placenta
The importance of receptor-mediated endocytosis is demonstrated by a genetic disorder called familial hypercholesterolemia. Describe it.
cholesterol is transported in the blood by a complex of lipids and proteins called low-density lipoprotein (LDL)
usually, body cells take up LDL when LDL receptor gather in a coated pit but in some individuals, the LDL receptor is unable to properly bind to the coated pit and the cells are unable to take up cholesterol
instead, cholesterol accumulates in the walls of arterial blood vessels, leading to high blood pressure, blocked arteries, and heart attacks
When does phagocytosis occur?
when the substance to be transported into the cell is large
amoebas ingest by phagocytosis
digestion occurs when the resulting vacuole fuses with a lysosome
When does Pinocytosis occur?
when a macromolecule such as a polypeptide is transported into the cell
the result is a vesicle (small vacuole)
What happens during receptor-mediated endocytosis?
(a form of pinocytosis) molecules first bind to specific receptor proteins, which migrate to or are already in a coated pit
the vesicle that forms contains the molecules and their receptors
Describe exocytosis.
vesicle fuses with plasma membrane as secretion occurs
(Golgi apparatus produces vesicles that carry cell products to membrane)
membrane of vesicle becomes part of plasma membrane which is thereby enlarged
proteins released from vesicle adheres to cell surface or becomes incorporated in extracellular matrix
