Chapter 5-5.3- Diffusion Flashcards
Des all movement require energy?
The exchange of substances between cells and their environment or between membrane-bound compartments within cells and their cell cytosol is defined as either active (requiring metabolic energy) or passive.
All movement requires energy. Passive movement, however, utilises energy from the natural motion of particles, rather than energy from another source.
What is diffusion?
Diffusion is the net, or overall, movement of particles (atoms, molecules or ions) form a region of higher concentration to a region of lower concentration.
It is a passive process and it will continue until there is a concentration equilibrium between the two areas.
Equilibrium means a balance or no difference in concentrations.
Diffusion happens because the particles in a gas or liquid have kinetic energy (they are moving). This movement is random and an unequal distribution of particles will eventually become an equal distribution.
Equilibrium doesn’t mean the particles stop moving, just that the movements are equal in both directions.
Particles move at higher speeds and are constantly colliding, which slows down their overall movement. Tis means that over short distances diffusion is fast, but as diffusion distance increases the rate of diffusion slows down because more collisions have taken place.
For this reason cells are generally microscopic- the movement of particles within cells depends on diffusion and a large cell would lead to slow rates of diffusion.
Reactions would not get the substrates they need quickly enough or ATP would be supplied too slowly to energy requiring processes.
What are the factors affecting rate of diffusion?
Temperature- The higher the temperature the higher the rate of diffusion. This is because he particles have more kinetic energy and move at higher speeds.
Concentration difference- The greater the difference in concentration between two regions the faster the rate of diffusion because the overall movement from the higher concentration to lower concentration will be larger.
A concentration difference is said to be a concentration gradient, which goes form high to low concentration.
Diffusion takes place down a concentration gradient. It takes a lot more energy to move substances up a concentration gradient.
So far diffusion in the absence of a barrier or membrane has been considered. This is simple diffusion.
Explain rate of diffusion in reference to surface area.
The rate of diffusion can be calculated in two ways-
by distance travelled/ time and volume filled/time.
Distance travelled/time is not affected by surface area, whilst volume/ time varies depending on the surface area.
A student uses different sized agar blocks to investigate how the rate of diffusion was affected by surface area.
The agar used to make the blocks to investigate how the rate of diffusion was affected by surface area.
The agar used to make the blocks contained the indicator phenolphalein which turns pink in the presence of alkali.
The agar blocks were immersed in a solution of sodium hydroxide for ten minutes. The blocks were removed and the distance the sodium hydroxide had diffused was measured with a ruler.
Explain diffusion across a membrane?
Diffusion across membranes involves particles passing through the phospholipid bilayer.
It can only happen if the membrane is permeable to the particles- non-polar molecules such as oxygen (o(2)) diffuse through freely down a concentration gradient.
The hydrophobic interior of the membrane repels substances with a positive or negative charge (ions), so they cannot easily pass through.
Polar molecules, such as water (H(2)O) with partial positive and negative charges can diffuse through membranes, but only at a very slow rate.
Small polar molecules pass through more easily than larger ones. Membranes are therefore described as partially permeable.
Explain what the rate at which molecules or ions diffuse across membranes is affected by.
- Surface area- the larger the area of an exchange surface, the higher the rate of diffusion.
- Thickness of membrane- the thinner the exchange surface, the higher the rate of diffusion.
What is facilitated diffusion?
The phospholipid bilayers of membranes are barriers to polar molecules and ions. However, membranes contain channel proteins though which polar molecules and ions can pass.
Diffusion across a membrane through protein channels is called facilitated diffusion.
Membranes with protein channels are selectively permeable as most protein channels are specific to one molecule or ion.
Facilitated diffusion can also involve carrier proteins which change shape when a specific molecule binds.
In facilitated diffusion the movement of the molecules is down a concentration gradient and does not require external energy.
The rate of facilitated diffusion is dependent on the temperature, concentration gradient, membrane surface area and thickness, but is also affected by the number of channel proteins present. The more protein channels, the higher the rates of diffusion overall.
