Movement Of Molecules Flashcards
Definition of diffusion
The net movement of particles from a region of high concentration to a region of lower concentration
What is meant by the term net movement?
The overall movement in one direction
Net movement = Particles moving in - Particles moving out
What does diffusion allow dissolved substances to do? Which dissolved substances?
Dissolved substances such as oxygen, nutrients and carbon dioxide have to pass through a cell membrane to get into or out of a cell. Diffusion allows this to happen
Why do living things rely on diffusion to do what? 2 things
They use it to get raw materials to use in vital life processes for example respiration and photosynthesis (in green plants).
They use it to remove waste products for example carbon dioxide from respiration.
What states does diffusion work in? How does it work?
Liquids and gases
The molecules in the liquids and gases are constantly moving and bumping into each other. This means that they tend to spread out until an equilibrium is reached. Particles diffuse randomly but the overall movement (net movement) is from a region of higher concentration to a region of lower concentration.
When two substances are put together in a container what happens?
- The particles are separate when first put together.
- They slowly start to mix due to their random motion
- The particles bump into each other which helps them to keep spreading out.
- Eventually, the particles are completely mixed and an equilibrium is reached.
Definition of equilibrium
The even distribution of particles over time
Definition of concentration gradient
The difference between the concentration in two areas
It goes from a higher concentration to a lower concentration remember!
What type of a process is diffusion? What does it mean?
Diffusion is a passive process which means it needs no energy
Does diffusion occur down a concentration or along?
Down
EXAM QUESTION: When dye molecules are added to water molecules what happens? Explain in your own words what is happening
Do once marked
Name the four factors which affect diffusion
How steep the concentration gradient is
Surface area over which the particles are moving
Temperature
Distance
How does the steepness of the concentration gradient affect diffusion?
The steeper the concentration gradient, the faster the rate of diffusion
Give an example in biology of how the concentration gradient is kept steep to ensure fast diffusion
In blood flow
Blood flow next the alveoli continuously takes away oxygen to respiring cells. This means that the concentration of oxygen is low in the blood compared to the concentration of oxygen in the alveoli to maintain a steep concentration gradient.
How does surface area affect the rate of diffusion?
The larger the surface area, the faster the rate of diffusion because the more surface area available, the more molecules can cross over at any given time.
Give examples where there are adaptations to increase surface area to ensure fast diffusion
In the lungs
The alveoli is spherical in shape and there are millions of them to provide a huge surface area for gas exchange
How does temperature affect the rate of diffusion?
The higher the temperature, the faster the rate of diffusion as the particles have more kinetic energy and so move more quickly.
How does distance affect the rate of diffusion?
The shorter the diffusion distance, the faster diffusion occurs because the molecules reach their destination more quickly. Surfaces where fast diffusion is necessary often keeps the diffusion distances short by having walls which are only one cell thick.
Examples where there are adaptations to decrease diffusion distance to ensure fast diffusion- examples of an exchange surface where the walls are only one cell thick?
In the lungs - both the wall of the capillary and the alveoli are only once cell thick which decreases the diffusion distance which ensures fast diffusion.
In the small intestine - the walls of each villus are one cell thick which decreases the diffusion distance which ensures fast diffusion.
Examples in living organisms where diffusion is important
Placenta and mother - diffusion enables nutrients and oxygen to pass from the mother to the fétus and waste substances to diffuse from the fétus to the mother.
Understanding surface area to volume ratio
An object’s surface area to volume ratio is like a way of describing how close every internal part of it is to its surface.
It is worked out by dividing the surface area by the volume.
A high surface area to volume ratio shows that every part of an object is quite close to the edge. This means that diffusion can occur quickly as there is only a short distance from the edge to the middle.
For example, if the side length of a small cube was 1cm, we can work out the ratio. To work out the volume you do the side length x itself three times. So 1 x 1 x 1 = 1cm^3. The surface area is worked out by 6 x (the side length x itself two times. So 6 x (1 x 1) = 6cm ^2. And so the ratio is 6/1 = 6
A low ratio means that there are parts of the object that are a long way from the edge which means diffusion takes longer to move molecules to the middle as there is a longer distance.
What is osmosis?
What is meant by the term selectively permeable?
Osmosis - the net movement of water across a selectively permeable membrane from a higher concentration of water molecules to a lower concentration of water molecules.
Selectively permeable - the cell membrane has very small holes in it. Small molecules can pass through but larger ones can’t. This is selectively permeable.
Osmosis only occurs across a selectively permeable membrane
Osmosis is a passive process
Concentrated and dilute solutions
Solutions inside and outside cells may have lots of different substances dissolved in them, we call these substances the solute. Depending on the amount of solute, a solution can be described as concentrated or dilute.
A concentrated solution has a lower concentration of water molecules and a higher concentration of solute molecules.
A dilute solution has a higher concentration of water molecules and a lower concentration of solute molecules.
Water will always move from a dilute solution (which has a higher concentration of water molecules) to a concentrated solution (where there is a lower concentration of lower water molecules).
If the two solutions are the same, there will be no net movement of water as it is already at an equilibrium.
When answering an exam question on what’s happening in the picture to do with osmosis….
Say that is happens across a selectively permeable membrane
Clearly state it is osmosis
Higher concentration —> lower concentration
PRACTICAL: Investigating the effect of concentration of black currant squash on osmosis in chipped potatoes.
Safety - do not drink the squash in the lab and take care when cutting the potato.
Investigation
d Dry the chips on a paper towel.
e Weigh the chips using a balance and record masses in an appropriately designed table.
f Place chips in a beaker/ boiling tube. The solutions should completely cover the chips.
g Leave for at least 15 minutes.
h Remove chips one at a time from the beakers/ boiling tubes.
i Dry the chips on paper towel.
j Reweigh chips and record results in the table.
k Calculate the change in mass and then the percentage change in mass of each set of potato chips.
% change in mass = change in mass / initial mass x 100
Why is it a good idea to remove all the skin? - In case the movement of solution through the skin is at a different rate than through freshly cut tissue.
Why do we dry the chips on a paper towel before weighing them? - so that we are sure that we are measuring the change in mass of the potato and not the solution it is in.
Explain what is happening to the potato chips in the different black currant squash solutions - In dilute solutions, the potato chips are drawing water in by osmosis as their cell contents are more concentrated than the outside solution. In concentrated solutions, the potato chips lose water by osmosis as their cell contents are less concentrated than the outside solution. At one concentration the cell contents are in equilibrium with the outside and so there will be no change in the mass of the potato chips.
What do you think is the approximate concentration of the cell contents of the potatoes? - The approximate concentration of the cell contents of the potatoes is the same as the solution with no change in mass.
Active transport
The process by which dissolved molecules move across a cell membrane from a lower to higher concentration. As the molecules moves against the concentration gradient, they require an input of energy from the cell.
Osmosis will move only across through a selectively permeable membrane so this is no good for moving other molecules. Both diffusion and osmosis can only move substances from high to low concentration and the substances need to be small enough to pass through the membrane but sometimes cells need to be able to move substances against a concentration gradient meaning lower to higher.
Give an example of when both an animal cell and a plant cell may need to use active transport instead of diffusion
Animal: The lower concentration of glucose in the small intestine compared to the high concentration in the blood.
Plant: Moving mineral ions from a lower concentration in the soul to a high concentration in the root cells.
Energy needed.
Active transport requires a great amour of energy.
How do cells get the energy they need for active transport? - Adenosine triphosphate which is porches during cellular respiration.
What adaptation might you find in cells that carry out of active transport such as root hair cells? - Mitochondria
As the rate of active transport increases, the rate of respiration increases.
How would you determine whether a mineral was being moved by active transport or diffusion in a root hair cell?
Treat it with a metabolic poison that prevents respiration and observe the results. If the movement of molecules, stopped then it must be active transport, if it continues then they must have been moved by diffusion
How does active transport work?
Within the cell membrane are special protein carrier molecules which can pick up particles on one side of the membrane and transport them to the other side.
Energy changes the shape of the carrier protein or causes it to rotate enabling it to move the desired solute from one side of the membrane to the other against the concentration gradient.
