3.1 Diffusion & Osmosis (Cambridge (CIE) O Level Biology): Flashcards
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Define diffusion.
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Define diffusion.
The net movement of molecules from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of random movement.
In diffusion, molecules move down a gradient.
In diffusion, molecules move down a concentration gradient.
Where does the energy for diffusion come from?
The kinetic energy of the random movement of molecules and ions.
Give three processes in living organisms that rely on diffusion.
Obtaining requirements (e.g. glucose)
Removing waste products
Gas exchange for respiration
In the lungs, in which direction does oxygen diffuse?
From the alveolar air space into the blood in the alveoli capillaries.
Define a partially permeable membrane.
A membrane that allows some molecules (e.g. small ones like water) to pass through easily but restricts or blocks others.
The products of digestion, such as glucose, diffuse from the small intestine into the .
The products of digestion, such as glucose, diffuse from the small intestine into the blood (in the villi capillaries).
Why does carbon dioxide diffuse into the chloroplasts of a leaf?
It is used up in photosynthesis, keeping its concentration low inside the chloroplasts, so it diffuses in down a concentration gradient.
True or False?
Diffusion requires energy from respiration.
False.
Diffusion is passive — it uses the kinetic energy of the random movement of particles, not energy from respiration.
True or False?
The cell membrane can restrict the free movement of some molecules during diffusion.
True.
The cell membrane is partially permeable, letting some molecules cross easily and others with difficulty or not at all.
Name four factors that affect the rate of diffusion.
Surface area to volume ratio
Distance
Temperature
Concentration gradient
How does temperature affect the rate of diffusion?
A higher temperature gives molecules more kinetic energy so they move faster, increasing the rate of diffusion.
How does the concentration gradient affect the rate of diffusion?
The greater the difference in concentration across the membrane, the faster the rate of diffusion.
How does distance affect the rate of diffusion?
The shorter the distance, the faster diffusion occurs — which is why capillary and alveolus walls are only one cell thick.
A larger cell has a surface area to volume ratio, which slows diffusion.
A larger cell has a smaller surface area to volume ratio, which slows diffusion.
How are root hair cells and the cells lining the small intestine adapted for diffusion?
They have an increased surface area (root hairs / villi) to speed up the absorption of substances.
True or False?
Increasing the temperature slows down the rate of diffusion.
False.
Increasing the temperature speeds up diffusion, because molecules gain more kinetic energy and move faster.
Capillary and alveolus walls are only thick to give a short diffusion distance.
Capillary and alveolus walls are only one cell thick to give a short diffusion distance.
Why does a larger concentration gradient increase the rate of diffusion?
There are more random collisions against the membrane on the side with the higher concentration.
Define surface area to volume ratio.
The comparison of a cell's surface area to its volume; a larger ratio allows faster movement of substances across the surface.
Define a solvent.
A liquid in which substances dissolve to form a solution; water is the main solvent in living organisms.
Why is water important as a solvent in organisms?
Many substances dissolve in it, allowing them to be transported around the body and to take part in reactions.
Give two examples of dissolved substances being transported in organisms.
Dissolved substances in the xylem and phloem of plants
Dissolved food molecules in the blood
Toxic waste such as urea dissolves in water so it can be removed from the body in .
Toxic waste such as urea dissolves in water so it can be removed from the body in urine.
Why is water needed for digested food to reach body cells?
Digested food molecules dissolve in water so they can be absorbed and transported to cells all over the body.
True or False?
Water plays a role in allowing metabolic reactions to occur in cells.
True.
Water is a major part of the cytoplasm and is needed for metabolic reactions to take place.
Name two waste or excess substances that dissolve in water for removal from the body.
Urea
Excess salts
A substance that can dissolve many others is described as a good .
A substance that can dissolve many others is described as a good solvent.
How does water help transport substances in plants?
Dissolved substances are carried in water through the xylem and phloem.
True or False?
Only a few substances are able to dissolve in water.
False.
Many substances dissolve in water, which is what makes it such a useful solvent for life.
Define osmosis.
The net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), across a partially permeable membrane.
In osmosis, water moves from a region of high water potential to a region of water potential.
In osmosis, water moves from a region of high water potential to a region of low water potential.
What type of membrane does osmosis occur across?
A partially permeable membrane, which lets small water molecules through but not larger solute molecules.
Does a dilute solution have a high or low water potential?
A high water potential. A concentrated solution has a low water potential.
The cell membrane is permeable, letting water through but not larger solute molecules.
The cell membrane is partially permeable, letting water through but not larger solute molecules.
True or False?
Osmosis is the movement of water down its concentration gradient.
True.
Water moves down its concentration gradient, from a high to a low water potential.
Define water potential.
A measure of the tendency of water to move; a dilute solution has a high water potential and a concentrated solution has a low water potential.
Which molecules can pass through a partially permeable membrane during osmosis?
Small molecules such as water; larger solute molecules cannot pass through.
True or False?
During osmosis, solute molecules move across the membrane.
False.
Only water molecules move; solute molecules are too large to cross the partially permeable membrane.
Osmosis is the net movement of molecules across a partially permeable membrane.
Osmosis is the net movement of water molecules across a partially permeable membrane.
Describe the common potato osmosis experiment.
Cut cylinders of potato, weigh them, place them in distilled water and sucrose solutions of different concentrations for 20–30 minutes, then dry and reweigh them to find the change in mass.
What does it mean if a potato cylinder gains mass?
Water moved into the tissue by osmosis, so the surrounding solution had a higher water potential (more dilute) than the tissue; the cells become turgid.
What does it mean if a potato cylinder loses mass?
Water moved out of the tissue by osmosis, so the surrounding solution had a lower water potential (more concentrated); the cells become flaccid.
What does no change in mass tell you about the surrounding solution?
There was no net movement of water, so the solution and the tissue had the same water potential.
Define dialysis tubing.
A non-living partially permeable membrane (made of cellulose) used to model osmosis outside cells; also called visking tubing.
Before and after immersing the potato cylinders, you must them to find the change in mass.
Before and after immersing the potato cylinders, you must weigh them to find the change in mass.
In a dialysis tubing experiment, which molecules can pass through the membrane and which cannot?
Water and glucose (small molecules) can pass through; sucrose (a large molecule) cannot.
True or False?
Potato cylinders should be reweighed without drying them first.
False.
They must be dried to remove excess surface liquid before reweighing, otherwise the mass reading is inaccurate.
Why are the potato cylinders weighed at the start and the end of the experiment?
To measure the change in mass, which shows the net movement of water into or out of the tissue by osmosis.
Dialysis tubing filled with sucrose solution and placed in distilled water gains water by .
Dialysis tubing filled with sucrose solution and placed in distilled water gains water by osmosis.
Define turgid.
A plant cell that is firm and rigid because water has entered by osmosis, pushing the cell membrane against the cell wall.
Define turgor pressure.
The outward pressure of the cell contents against the cell wall when a plant cell is full of water.
Define plasmolysis.
When a plant cell loses so much water that the cell membrane pulls away from the cell wall.
Define flaccid.
A plant cell that has lost water by osmosis and is no longer firm, reducing its ability to support the plant.
Why is turgor important to plants?
Turgid cells make the plant rigid and firm, providing support so it stands upright and holds its leaves out to catch sunlight.
What happens to a plant cell placed in distilled water?
Water enters by osmosis and the cell becomes turgid, as turgor pressure pushes the membrane against the cell wall.
What happens to a plant cell placed in a concentrated solution?
Water leaves by osmosis and the cell becomes flaccid; it may become plasmolysed as the membrane pulls away from the cell wall.
Why does a plant wilt if it does not get enough water?
The cells lose water and can no longer stay turgid, so they cannot support the plant.
What happens to an animal cell placed in distilled water?
Water enters by osmosis and, with no cell wall to create turgor pressure, the cell swells and eventually bursts.
What happens to an animal cell placed in a concentrated sugar solution?
It loses water by osmosis and becomes crenated (shrivelled).
True or False?
The cell wall stops a plant cell from bursting when it takes in water.
True.
The pressure created by the cell wall stops too much water entering and prevents the cell from bursting.
An animal cell placed in a concentrated solution loses water and becomes .
An animal cell placed in a concentrated solution loses water and becomes crenated.
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