chapter 5 part 2

Question 1

Facilitated diffusion:

Answer 1

• the phospholipid bilayers of membranes are barriers to polar molecules and ions.
• However, membranes contain channel proteins through which polar molecules and ions can pass.
• -

Question 2

facilitated diffusion.

Answer 2

Diffusion across a membrane through protein channels

Question 3

Investigations into the factors affecting diffusion rates in model cells:

Answer 3

• Cells are too small and cell membranes too thin to use in practical investigations so dialysis tubing is used as a substitute membrane.
• This model enables us to investigate the effects of temperature and concentration on the rate of diffusion across membranes.
• Dialysis tubing is partially permeable, with pores a similar size to those on a partially permeable membrane.
• This means that small molecules like water can pass through it, but larger molecules like starch cannot fit through the pores.
• The tubing is therefore a barrier to large molecules.
• A model cell can be simulated by tying one end of a section of tubing, filling with a solution and then tying the other end.
• The ‘cell’ is then placed into another solution.
• The solutions could contain different sizes, or concentrations, of solute molecules.
• The changes in concentration of solute molecules, both inside and outside the model cells, can be measured over time.
• Rates of diffusion across the tubing can then be calculated.
• Glucose is a small molecule which can cross the tubing.
• Benedict’s solution is used to test for the presence of glucose, and can also be used to estimate concentration.
• Starch molecules are large and will not cross the tubing.
• lodine is used to test for the presence of starch.
• Water is a small molecule which will pass through the tubing while other solutes such as sucrose will not.
• Model cells can be placed in solutions with different solute concentrations.
• The rates of osmosis can be calculated using changes in volume or mass of the model cells over time.
• Rates of diffusion at different temperatures can also be calculated using a water bath to change the temperature of the model cell.
• Other variables such as concentration must be then be kept constant.

Question 4

apparatus

Answer 4

Question 5

Explain why Benedict’s test is both quantitative and qualitative

Answer 5

qualitative detects the presence of, reducing sugar / glucose (1); quantitative colour change is estimate of concentration (of reducing sugar / glucose) (1);

Question 6

Active transport:

Answer 6

• Active transport is the movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration.
• The process requires energy and carrier proteins.
• Energy is needed as the particles are being moved up a concentration gradient, in the opposite direction to diffusion.
• Metabolic energy is supplied by ATP.
• Carrier proteins span the membranes and act as ‘pumps’.
• The general process of active transport is described below - in this example transport is from outside to inside a cell

Question 7

diagram of active transport

Answer 7

Question 8

steps of active transport

Answer 8

• The molecule or ion to be transported binds to receptors in the channel of the carrier protein on the outside of the cell.
• On the inside of the cell ATP binds to the carrier protein and is hydrolysed into ADP and phosphate.
• Binding of the phosphate molecule to the carrier protein causes the protein to change shape - opening up to the inside of the cell.
• The molecule or ion is released to the inside of the cell.
• The phosphate molecule is released from the carrier protein and recombines with ADP to form ATP.
• The carrier protein returns to its original shape.
• The process is selective - specific substances are transported by specific carrier proteins.

Question 9

Bulk transport

Answer 9

Bulk transport is another form of active transport.
Large molecules such as enzymes, hormones, and whole cells like bacteria are too large to move through channel or carrier proteins, so they are moved into and out of cell by bulk transport.

Question 10

• Endocytosis

Answer 10

is the bulk transport of material into cells.
There are two types of endocytosis, phagocytosis for solids and pinocytosis for liquids - the process is the same for both.
The cell-surface membrane first invaginates (bends inwards) when it comes into contact with the material to be transported.
The membrane enfolds the material until eventually the membrane fuses, forming a vesicle.
The vesicle pinches off and moves into the cytoplasm to transfer the material for further processing within the cell.
For example, vesicles containing bacteria are moved towards lysosomes, where the bacteria are digested by enzymes.

Question 11

• Exocytosis

Answer 11

is the reverse of endocytosis.
Vesicles, usually formed by the Golgi apparatus, move towards and fuse with the cell surface membrane.
The contents of the vesicle are then released outside of the cell.
Energy in the form of ATP is required for movement of vesicles along the cytoskeleton, changing the shape of cells to engulf materials, and the fusion of cell membranes as vesicles form or as they meet the cell-surface membrane.

Question 12

Remember that all water potential values are…

Answer 12

negative. Pure water has a water potential of zero.

Question 13

Osmosis

Answer 13

a particular type of diffusion - specifically the diffusion of water across a partially permeable membrane.
As with all types of diffusion it is a passive process and energy is not required.

Question 14

Water potential:

Answer 14

• A solute is a substance dissolved in a solvent (for example water) forming a solution.
• The amount of solute in a certain volume of aqueous solution is the concentration.
• Water potential is the pressure exerted by water molecules as they collide with a membrane or container.
• It is measured in units of pressure pascals (Pa) or kilopascals (kPa).
• The symbol for water potential is the Greek letter psi
• Pure water is defined as having a water potential of 0 kPa (at standard temperature and atmospheric pressure - 25°C and 100 kPa).
• This is the highest possible value for water potential, as the presence of a solute in water lowers the water potential below zero.
• All solutions have negative water potentials - the more concentrated the solution the more negative the water potential.
• When solutions of different concentrations, and therefore different water potentials, are separated by a partially permeable membrane, the water molecules can move between the solutions but the solutes usually cannot.
• There will be a net movement of water from the solution with the higher water potential (less concentrated) to the solution with the lower water potential (more concentrated).
• This will continue until the water potential is equal on both sides of the membrane (equilibrium).

Question 15

Effects of osmosis on plant and animal cells:

Answer 15

The diffusion of water into a solution leads to an increase in volume of this solution.
If the solution is in a closed system, such as a cell, this results in an increase in pressure.
This pressure is called hydrostatic pressure and has the same units as water potential, kPa.
At the cellular level this pressure is relatively large and potentially damaging.

Question 16

Animal cells:

Answer 16

• If an animal cell is placed in a solution with a higher water potential than that of the cytoplasm, water will move into the cell by osmosis, increasing the hydrostatic pressure inside the cell.
• All cells have thin cell-surface membranes (around 7 nm) and no cell walls.
• The cell-surface membrane cannot stretch much and cannot withstand the increased pressure.
• It will break and the cell will burst, an event called cytolysis.
• If an animal cell is placed in a solution that has a lower water potential than the cytoplasm it will lose water to the solution by osmosis down the water potential gradient.
• This will cause a reduction in the volume of the cell and the cell-surface membrane to ‘pucker’, referred to as crenation
• To prevent either cytolysis or crenation, multicellular animals usually have control mechanisms to make sure their cells are continuously surrounded by aqueous solutions with an equal water potential (isotonic).
• In blood the aqueous solution is blood plasma.

Question 17

osmosis on red blood cells

Answer 17

Question 18

Plant cells:

Answer 18

Like animal cells, plant cells contain a variety of solutes, mainly dissolved in a large vacuole.
However, unlike animals, plants are unable to control the water potential of the fluid around them, for example, the roots are usually surrounded by almost pure water.
Plants cells have strong cellulose walls surrounding the cell-surface membrane.
When water enters by osmosis the increased hydrostatic pressure pushes the membrane against the rigid cell walls.
This pressure against the cell wall is called turgor.
As the turgor pressure increases it resists the entry of further water and the cell is said to be turgid.
When plant cells are placed in a solution with a lower water potential than their own, water is lost from the cells by osmosis.
This leads to a reduction in the volume of the cytoplasm, which eventually pulls the cell-surface membrane away from the cell wall - the cell is said to be plasmolysed.

Question 19

osmosis in plant cells

Answer 19

Question 20

Osmosis investigations on plant cells

Answer 20

Pieces of potato or onion can be placed into sugar or salt solutions with different concentrations, and therefore different water potentials.
Water will move into or out of cells depending on the water potential of the solution relative to the water potential of the plant tissue.
As the plant tissue gains or loses water it will increase or decrease in mass and size, and vice versa.
A student used potato cores and their knowledge of osmosis to investigate the water potential of potato cells.

Question 21

Osmosis investigations on Animal cells:

Answer 21

Eggs can be used to demonstrate osmosis in animal cells.
A chicken’s egg is not exactly a single cell, but with the shell removed a single membrane-bound structure remains and it will behave in the same way as a cell when placed in solutions of varying water potentials.
To investigate osmosis, eggs without their shells are placed in different concentrations of sugar syrup.
Over time, osmosis takes place and there will have been a net movement of water into or out of the eggs, depending on the concentration of the syrup they were in.
(Note that if the egg is hard boiled for easier handling that this will damage the membrane.)