AS1 Chapter 4 - Cell Physiology Flashcards
Give a definition of diffusion
Diffusion can be defined as the net movement of a substance from where it is in higher concentration to where it is in a lower concentration, down the concentration gradient.
What factors affect diffusion across membranes?
- The concentration gradient
- The size of the molecule
- The temperature
- The thickness of the exchange surface
- The surface area of the membrane
What substances can easily pass through the cell membrane by simple diffusion?
Non- polar molecules (such as lipid soluble oxygen and carbon dioxide) Very small polar molecules such as water can pass through (due to their small size) Water soluble (polar) molecules generally cannot pass through the phospholipid bilayer due to the hydrophobic centre.
Definition of osmosis
Osmosis can be defined as the net movement of water through a selectively (differentially) permeable membrane, from a solution of high water potential to a solution of lower (more negative) water potential, down the water potential gradient.
Definition of 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.
Water potential is measured in …
Kilopascals (kPa)
Pure water has a water potential of …
0 kPa
What symbol denotes ‘water potential’?
Psi Cell
What symbol denotes ‘solute potential’?
Psi S
What symbol denotes ‘pressure potential’?
Psi P
Definition of solute potential
The solute potential of a solution may be regarded as its potential to take in water by osmosis from pure water across a selectively permeable membrane.
Extra: 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 a cell. For example, a turgid cell will still have the potential to take in water, as it is still more concentrated than pure water, but because it is turgid it may be unable to take in water, as there is simply no space.
Definition of pressure potential
This is the effect of pressure on the solution.
Extra: A plant cell that is turgid will exert considerable pressure on its cell wall, whereas one that is not will exert much less pressure. 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).
What is the equation for water potential?
Water potential = Solute potential + Pressure potential
Psi Cell = Psi S + Psi P
Sugar molecules and water molecules in solution will form …
Hydration shells
In solutions, some of the water molecules are not ‘free’, as they form (blank) around the solutes.
Hydration shells
What are the two ways in which water can move through the selectively or differentially permeable phospholipid membrane?
- Simple diffusion (osmosis)
2. Facilitated diffusion (osmosis) - special channel proteins called aquaporins
When animal cells swell up and burst this is known as …
Cell lysis
When animal cells lose water, shrink and shrivel up this is known as …
Cell crenation
How and why does cell lysis occur in animal cells?
Animal cells do not have a cell wall and therefore there is noting to stop the expansion of the cell membrane until it bursts (due to excess water moving into the cell by osmosis).
Crenation (in animal cells) and plasmolysis (in plant cells) occurs when cells are placed in (blank) solutions.
Hypertonic
Lysis (in animal cells) and turgidity (in plant cells) occurs when cells are placed in (blank) solutions.
Hypotonic
The point at which the cell membrane just begins to lose contact with the cell wall in plants is called the point of …
Incipient plasmolysis
What is the point of incipient plasmolysis?
The point at which the cell membrane just begins to lose contact with the cell wall.
What is plasmolysis?
Plasmolysis occurs when water moves out of the plant cell in excess. The vacuole will shrink and the cell membrane will pull away from the cell wall. (i.e. The protoplast will pull away from the cell wall, except at points where plasmodesmata connect adjacent protoplasts). The cell will lose all turgor (pressure).