Transport across membranes

Question 1

Define the term ‘‘simple diffusion’’.

Answer 1

Diffusion is the net movement of particles from an area of high concentration to an area of low concentration down the concentration gradient.

Question 2

Define the term ‘‘facilitated diffusion’’.

Answer 2

The diffusion of particles through carrier or channel proteins in the plasma membrane

Question 3

• Define the term “osmosis”.

Answer 3

The diffusion of water across a partially permeable membrane down a water potential gradient. A passive process (does not require energy).

Question 4

• Define the term “passive transport”.

Answer 4

The movements of substances across cell membranes without the requirement of energy.

Question 5

• Define the term “active transport”.

Answer 5

The movement of particles across the plasma membrane against a concentration gradient. Requires energy and carrier proteins.

Question 6

• Define the term“endocytosis”.

Answer 6

The process by which a cell surrounds substances with a section of its plasma membrane and takes them into the cell. An active process requiring ATP.

Question 7

• Define the term “exocytosis”.

Answer 7

The bulk transport of materials out of cells. Vesicles containing the material fuse with the cell surface membrane and the contents are released to the outside of the cell.

Question 8

• Define the term “pinocytosis”.

Answer 8

The endocytosis of liquid materials.

Question 9

• Define the term “phagocytosis”.

Answer 9

The process by which white blood cells called phagocytes recognise non-self cells and engulf them and digest them within a vesicle called a phagolysosome.

Question 10

Explain the difference between a channel protein and a carrier protein and link this to their functions.

Answer 10

Channel proteins are only involved in passive transportion. They provide a hydrophilic channel for polar molecules and ions.
Carrier proteins are involved in both active and passive transportation. It often involves the shape of the carrier protein itself changing.

Question 11

State 3 particular examples of channel proteins and 3 carrier proteins and for each describe the role that they perform.

Answer 11

Carrier proteins:

- GLUT1 is found in almost all animal cells and specifically helps to transport glucose across the plasma membrane

Question 12

Explain the importance of the structure of ATP in active transport.

Answer 12

1) Small - it can move easily acroos and within cells.
2) Water soluable - energy requiring processes happen in aqueous environments .
3) Contains bonds between phosphate with immediate energy; large enough to be useful for cellular reactions but not so large that energy is wasted as heat.
4) Releases in small quantities - small quantities are suitable to most cellular needs so that energy is not wasted as heat
5) Easily regenerated - can be recharged with energy.

Question 13

List the factors affecting the rate of diffusion of a molecule.

Answer 13

• Temperature: the higher the temp the higher the rate of diffusion as the particles have more kinetic energy.
• Concentration difference: the greater the difference the faster the rate of diffusion because the overall movement from the higher concentration to lower concentration will be larger. There is a steeper concentration gradient.
• Surface area: a big surface area means a higher rate of diffusion.
• Thickness of membrane: thinner membrane means faster rate.

Question 14

How can substances move across a membrane by simple diffusion.

Answer 14

Diffusion happens because gas or liquid particles have kinetic energy. The movement is random and an uneven distribution of particles will eventually become an equal distribution. It is a passive process and it will continue until there is a concentration equilibrium between the two areas.

Question 15

State which types of molecule can move across a membrane by simple diffusion.

Answer 15

Non-polar molecules like oxygen freely diffuse through, down a concentration gradient.
The hydrophobic interior of the membrane repels ions so they can’t pass through easily.
Polar molecules like water can diffuse through membranes but only at a very slow rate. Small ones pass through more easily.

Question 16

Explain, in a paragraph and with a diagram, how substances can move across a membrane by facilitated diffusion (include protein specificity).

Answer 16

Membranes contain channel proteins through which polar molecules and ions can pass. Membranes with protein channels are selectively permeable as protein channels are specific to one molecule of ion. It can also involve carrier proteins which change shape when a specific molecules binds to it. It doesn’t require energy and is down the concentration gradient.
Same factors affecting it as simple diffusion but also the number of channel proteins.

Question 17

Explain, in a paragraph and with a diagram, how substances can move across a membrane by active transport.

Answer 17

Active transport is the movement of molecules or ions into or out of a cell from a region of low concentration to a region of high concentration against the conc gradient.
Requires energy and carrier proteins, metabolic energy is supplied by ATP.
A molecules attaches to a carrier protein, the protein changes shape and this moves the molecule across the membrane, releasing it on the other side.

Question 18

Explain, in a paragraph and with a diagram, how substances can move into a cell by endocytosis (using phagocytosis as an example).

Answer 18

Some molecules are way too large (proteins, lipids, some carbs and some white blood cells i.e. phagocytes) to be taken into a cell by carrier proteins. Instead a cell can surround a substance with a section of its plasma membrane, the membrane pinches off to form to a vesicle inside the cell containing the ingested substance.
This uses ATP for energy.

Question 19

Explain, in a paragraph and with a diagram, how substances can move out of a cell by exocytosis (using secretion as an example).

Answer 19

Some substances produced by the cell like hormones or digestive enzymes or lipids need to be released from the cell. Vesicles containing these substances pinch off from the sacks of the golgi apparatus and move toward the plasma membrane. The vesicles fuse with the plasma membrane and release their contents outside the cell. It uses ATP.

Question 20

Identify which transport mechanisms require an input of energy from ATP.

Answer 20

Active transport, endocytosis and exocytosis.

Question 21

Explain why it is easier for an oxygen molecule to diffuse across a membrane than a water molecule.

Answer 21

Because an oxgen molecules is non-polar so it doesn’t get repelled by the hydrophobic interior of the membrane.

Question 22

Explain why steroid hormones can easily cross a membrane by simple diffusion.

Answer 22

They are lipid derived hormones so they can pass through every cell in our body.

Question 23

Explain why ions are only able to cross a membrane through a protein channel.

Answer 23

Because they are repelled by the hydrophobic interior of the membrane. So they need to be transported in a protein channel.

Question 24

Describe how to carry out an investigation to investigate how the rate of diffusion is affected by surface area.

Answer 24

1) Cut different sizes of agar jelly with phenolphthalein indicator in it.
2) Put into a beaker of NaOH and time and long it takes for the the agar to turn pink.

Question 25

Describe and explain the results you would expect to see in an investigation into how the rate of diffusion is affected by surface area.

Answer 25

The higher the surface area to volume ratio, the faster the rate of diffusion.

Question 26

Describe how to carry out an experiment to investigate how the rate of diffusion is affected by temperature.

Answer 26

1) Prepare agar jelly and cut into equal sized cubes
2) Prepare several different boiling tubes containing the same concentration of HCL and put them in separate water baths of different temperatures
3) When the HCL has reached desired temp put the agr jelly in and time how long it takes the cube to go colourless.

Question 27

Describe and explain the results you would expect to see in an investigation into how the rate of diffusion is affected by temperature.

Answer 27

The higher the temp, the faster the rate of diffusion.

Question 28

Define the term “solvent”.

Answer 28

A liquid in which substances (or solutes) are dissolved forming a solution.

Question 29

Define the term “solute”

Answer 29

A substance dissolved in a solvent to form a component of a solution.

Question 30

Define the term “solution”.

Answer 30

A substance formed from a solute, dissolved into a solvent.

Question 31

Define the term “water potential”, state the symbol for water potential, state the water potential of pure water

Answer 31

Water potential: A measure of the quantity of water compared to solutes. Measured as the pressure created by water molecules in Kpa.
The likelihood of water molecules to diffuse into or out of a solution.
Symbol: weird trident thingy
Water potential of pure water: Zero
Water potential cannot have a positive vaule because pure water has the highest water potential and that is zero. If you added anymore solutes to pure water, the water potential would decrease, making a negative number.

Question 32

Define the terms “water potential gradient”.

Answer 32

Quantifies the potential of water to move from one area to another.

Question 33

Define the term “net movement of water”.

Answer 33

Overall movement of water from area to another.

Question 34

Draw a table to compare the effects of external solutions, with different water potentials in comparison to the water potential in a cell on animal cells (include a description of relative water potentials, osmosis and the overall effect on the cell).

Answer 34

Water potential of external solution compared to cell solution: higher (hypotonic) - water enters the cell - swells and burst (cytolysis)
Equal (isotonic) - no net movement of water - no change
Lower (hypertonic) - water leaves the cell - shrinks (crenated)

Question 35

Draw a table to compare the effects of external solutions, with different water potentials in comparison to the water potential in a cell on plant cells (include a description of relative water potentials, osmosis and the overall effect on the cell).

Answer 35

Water potential of external solution compared to cell solution:
Higher (hypotonic) - water enters the cell - becomes turgid
Equal (isotonic) - no net movement of water molecules - no change
Lower (hypertonic) - water moves out of the cell - cell becomes flaccid/ plasmolysed

Question 36

Define the term “plasmolysis”.

Answer 36

The pulling away of the cytoplasm and plasma membrane from the cell wall due to lack of water.

Question 37

Define the term “protoplast”.

Answer 37

The whole cell excluding the cell wall.

Question 38

Define the term “crenation”.

Answer 38

Cells shrink by osmosis as water leaves the cells.Only in animal cells.

Question 39

Define the term “turgid”.

Answer 39

A cell which is swollen with water.

Question 40

Define the term “cytolysis”.

Answer 40

The bursting of an animal cell caused by increasing hydrostatic pressure as water enters by osmosis.

Question 41

Define the term “haemolysis”.

Answer 41

The rupture or destruction of red blood cells.

Question 42

6. Describe how to carry out an experiment to investigate the effects of solutions of different water potential on plant and animal cells.

Answer 42

1) Make up NaCl solutions of different concentrations e.g. 0.2, 0.4 and poor an equal volume into separate beakers.
2) Take de-shelled eggs and carefully pat them with absorbent to absorb excess moisture.
3) Use a mass balance to weigh each egg and record the masses in a suitable table.
4) Place each egg in a different beaker and leave for the same amount of time.
5) Remove eggs, dry them, weigh again and use results to calculate percentage change in mass.
6) Plot a graph
Repeat - with potato cylinders of equal size in sucrose solutions.

Question 43

Describe and explain the results you would expect to see from an experiment to investigate the effects of solutions of different water potential on plant and animal cells.

Answer 43

Animal cell - higher concentration of surround sucrose solution means bigger mass loss.
Plant cell - Cylinders will gain water in solutions with a higher water potential.

Question 44

8. Describe the meaning of the symbols: =, >, >, ∝ , ~

Answer 44

= : equal to 
< : less than 
< : greater than 
>>: much greater 
∝ : proportional to 
~ : approximate

Question 45

Explain why plant cells don’t burst by osmosis by referring to solute potential and pressure potential.

Answer 45

They have strong cellulose walls, when water enters by osmosis the increased hydrostatic pressure pushes the membrane against the rigid cell wall, preventing it from bursting. This is called turgor pressure. As the pressure increases it resists the entry of further water and the cell becomes turgid.

Question 46

Write an equation to link the water potential of a cell with its pressure potential and its solute potential.

Answer 46

Water potential = pressure potential + solute potential (measured in kPa)

Question 47

Define the terms “absolute uncertainty” and “relative uncertainty or percentage error”.

Answer 47

Absolute uncertainty: the percent by which the measurement could be out.
Relative uncertainty: the ratio of the absolute uncertainty to the original value multiplied by 100.

Question 48

Explain how to calculate the percentage error of a measurement (and when data are combined e.g. to obtain data on the change of a particular value over time).

Answer 48

1) Take the absolute uncertainty e.g. +- 0.5
2) divide it by the amount measured (e.g. 100ml)
3) multiply by 100
4) if measuring the change in something or have two measurements you add the absolute uncertainties together.

Question 49

Explain the significance of where a line of best fit crosses the x-axis in a graph showing how external concentration affects mass change in plant tissues.

Answer 49

The point where the line crosses the x-axis is the point where there was no osmosis so that means that it has the same water potential in and out of the cell (isotonic). From this, you can work out the concentration inside the cell.

Question 50

Define the term “anomaly” and explain how to identify anomalies in experimental measurements (also describe how anomalies can be dealt with).

Answer 50

Anomaly - a value in a set of results that is judged not to be part of the inherent variation.
If it is far away from all the other results it’s probably an anomaly.

Question 51

Define the term “precision”.

Answer 51

The closeness of agreement between independent measurements obtained under the same conditions. It depends only on the distribution of random errors and does not relate to the true value.

Question 52

Define the term “accuracy”.

Answer 52

The measure of the closeness of agreement between an individual test result and the true value. If a test result is accurate it is in close agreement with the true value.

Question 53

Define the term “repeatability”.

Answer 53

The precision obtained when measurement results are produced over a short time scale by one person, using the same equipment in the same place.

Question 54

Define the term “reproducibility”.

Answer 54

The precision obtained when measurement results are produced over a wider time scale by different people using equivalent equipment in different but equivalent places.

Question 55

Define the term “resolution”.

Answer 55

The smallest change in quantity being measured that can be detected by an instrument.

Question 56

Define the term “validity”.

Answer 56

A measurement is valid if it measures what it is supposed to be measuring. An investigation procedure is valid if it suitable to answer the question being asked.