cell physiology Flashcards
what 4 ways do materials enter and leave cells
- Diffusion (simple)
- Facilitated diffusion
- Active transport
- Endocytosis / exocytosis
describe process simple diffusion
(Simple) Diffusion
•Non polar substances (hydrophobic, lipid soluble substances)
• e.g. gases like oxygen and carbon dioxide can pass through unaided between spaces between phospholipid molecules in the phospholipid bilayer. (i.e protein molecules are not involved)
•Very small molecules such as water can also diffuse between spaces between the phospholipid molecules because its molecules are small.
•Large polar water soluble molecules and charged molecules generally are unable to pass through by simple diffusion due to the hydrophobic nature of the centre of the phospholipid bilayer
define diffusion
NET MOVEMENT of molecules (or ions) from a higher concentration to a region of lower concentration until equilibrium is achieved
Diffusion across membranes is affected by a number of factors, what are they?
- CONCENTRATION GRADIENT-
- SIZE of the molecule
- TEMPERATURE
- THICKNESS of the exchange surface
- SURFACE AREA of the membrane
affect of concentration gradient
The greater the concentration gradient across the membrane the faster the diffusion
E.g The concentration gradient is maintained by the constant blood flow to and from alveoli and also ventilation which replaces the air in the lungs with every breath
affect of size of molecule
Small molecules diffuse faster than larger molecules
affect of temperature
Diffusion normally takes place quicker at higher temperatures, as the higher temperatures give the diffusing molecules more kinetic energy.
affect of thickness of exchange surface
Biological membranes are generally very thin therefore ideal for rapid diffusion.
affect of surface area
The greater the surface area,
the faster the diffusion.
•E.g In many cells where diffusion is important, the cell surface membrane is extended to increase the area across which diffusion can take place, e.g. by the presence of microvilli.
What factors increase the rate of diffusion across membranes?
- Increased CONCENTRATION GRADIENT-
- Smaller SIZE of the molecule
- Increased TEMPERATURE
- Smaller THICKNESS of the exchange surface
- Larger SURFACE AREA of the membrane
what is the process of facilitated diffusion
Faciliated Diffusion
•In facilitated diffusion, substances i.e Ions and large polar, water soluble molecules e.g. glucose, amino acids that would otherwise not pass through the membrane are allowed to do so by a specific protein molecule present in the membrane.
what are 2 main types of proteins in facilitated diffusion?
•There are two main types of protein involved: •Carrier proteins
•Channel proteins
CARRIER PROTEINS
CARRIER PROTEINS-
•They take in the diffusing molecule, such as glucose (large polar), change shape and release the molecule on the other side of the membrane
•These protein carriers have binding sites that match specific molecules and they assist the movement of these molecules across the membrane
•The carrier protein is therefore selective to what it can carry.
CHANNEL PROTEIN
•These channels are formed by proteins with a central hydrophilic pore that enables charged particles (ions) to pass through.
• **They allow charged ions such as Na+, K+, Ca2+ and Cl- to pass through because they are lined with polar groups.
• Charged ions cannot diffuse easily across the non-polar centre of the phospholipid bilayer.
•**Note channel proteins are also specific what they transport across the membrane. E.g sodium channels only transport sodium ions across the membrane
channel proteins can be…
•Some of these ion channels are permanently open but others are gated.
•Gated channels can open or close allowing control of ion movement
does diffusion and facilitated diffusion require energy ?
NO
Both diffusion and facilitated diffusion only transport molecules down the concentration gradient and neither involved the expenditure of metabolic
energy-i.e. the processes are PASSIVE.
ACTIVE TRANSPORT
•Sometimes molecules and ions need to be moved across the membrane AGAINST THE CONCENTRATION GRADIENT (i.e. from LOW to HIGH concentration).
•Consequently, ENERGY is required.
•Active transport involves PROTEIN CARRIER MOLECULES (sometimes called PUMPS).
•The substance to be transported binds to the carrier protein.
•As with carriers in facilitated diffusion, the carrier changes shape and releases the transported substance on the other side of the membrane.
ALSO SPECIFIC
active transport vs facilitated diffusion
•Substances are moved AGAINST the concentration gradient
•Metabolic energy in the form of ATP is required.
active transport is affected by what factors?
Their ability to take up molecules or ions against a concentration
gradient is affected by affected by:-
• temperature
• oxygen concentration
• presence of respiratory poisons such as cyanide
Why might a cell benefit from microvilli if carrying out active transport?
•Microvilli present a larger surface area, so more carrier proteins can be supported.
hypotonic
•Hypotonic - a more dilute solution
hypertonic
•Hypertonic – a more concentrated (less
dilute) solution
isotonic
•Isotonic – solutions of equal concentration – osmosis will not occur
DEFINE WATER POTENTIAL
• The WATER POTENTIAL of a solution may be regarded as its TENDENCY TO TAKE IN WATER by osmosis from pure water across a selectively permeable membrane.
•It is measured in kilopascals (kPa)
DEFINE OSMOSIS
net movement of water through a selectively permeable membrane, from a solution of less negative water potential to a solution of more negative water potential.
what is water potential affected by?
water potential is affected by factors such as the space available in a cell i.e The presence of a cell wall prevents a turgid cell taking in any more water
water potential
• The water potential is an indication of the free energy of water molecules.
• The water potential of pure water is 0kPa because all the water molecules are ‘free’- they are not forming associations with other molecules.
• In solutions, some of the water molecules are not ‘free’, as they from HYDRATION SHELLS around the solutes
• The presence of solutes also reduces the ability of water molecules to diffuse throughout the solution. (water molecules are not free)
define solute potential
•SOLUTE POTENTIAL- This is the POTENTIAL of a solution to take IN water
solute potential
• The potential may or may not be the same as the TENDENCY to take in water (water potential).
• The potential relates to the SOLUTE CONCENTRATION ONLY
• but the tendency (water potential) is affected by other factors, such as the space available within the cell.
affects of solute on water potential
• The more solute molecules present, the lower/MORE NEGATIVE the water/solute potential.
• The fewer solute molecules present, the higher/LESS NEGITIVE the water/solute potential
define pressure potential
This is the effect of PRESSURE on the solution.
• A plant cell that is turgid will exert considerable pressure on its cell wall, whereas one that is not will exert much less pressure.
pressure potential
This pressure influences the ability of the cell to take in or lose
water by osmosis.
• The pressure potential is usually positive (although it can be 0)
equation
Water potential = solute potential + pressure potential ycell = ys+ yp
turgidly
Plants rely on tugor for support.
• Herbaceous plants (non-woody) are almost totally reliant on tugor pressure. The plant cell wall has a very important role to play, as its strength limits the expansion of the cell membrane and cell wall on each other help create the tugor itself
Describe what happens when a plant cell is placed in a hypotonic solution
The plant cell will have a MORE NEGATIVE water potential that the solution
• Water will move into the plant cell by osmosis from a a region of LESS NEGATIVE water potential to a region of MORE NEGATIVE water potential through a selectively permeable membrane
• The plant becomes turgid
flaccid
• If the cell was turgid and there is a shortage of water, as water leaves the cell, the cell surface membrane starts to shrink away from the rigid cell wall and is no longer turgid.
• If a large number of cells are flaccid, the phenomenon of wilting occurs
plasmolysis
If a plant cell loses too much water be osmosis, its vacuole can shrink to the extent that the cell membrane (or protoplast) can pull away from the cell wall (except at points where adjacent protoplasts are joined via plasmodesmata).
• This is described as PLASMOLYSIS and the cell is PLASMOLYSED
incipient plasmolysis
The point at which the cell membrane just begins to lose contact with the cell wall is called INCIPIENT PLASMOLYSIS
In nature, plasmolysis seldom occurs, which is just as well as plant cells are unlikely to survive if they have become plasmolysed
give 2 examples
• Plants growing in a field that has been given too much fertilizer
• A seed from a woodland tree being carried to a salt marsh and starting to germinate in this environment
Osmosis in Animal Cells
Animal cells are easily damaged by changes in the external solution because they do not have a cell wall.
•As animal cells have no cell wall… pressure potential = 0
•As animal cells have no cell wall… •pressure potential = 0
•Water potential = solute potential
ycell = ys
•Hypotonic solution
• If a typical animal cell is placed in pure water (hypotonic solution) e.g. red blood cell, it will take in water by osmosis and there is nothing to stop the expansion of the cell membrane therefore the cell will lyse .
crenation
• Hypertonic solution
• - If animal cells are placed in a hypertonic solution, the cells will lose water by osmosis . The cells will shrivel up – this is called crenation.
If the solute concentration of a solution is increased, what would happen to the water potential?
Lowers the water potential ( i.e more negative)
