Topic 1b - Transport in Cells

Question 1

What is diffusion?

Answer 1

Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration.

Question 2

How do small particles/ molecules move in and out of cells?

Answer 2

Very small particles/ molecules can diffuse through the cell membrane.

Question 3

How do particles move in diffusion?

Answer 3

Freely.

Question 4

What happens to big molecules?

Answer 4

Big molecules like starch and protein can’t fit through the membrane.

Question 5

What are some examples of substances being transported in and out of cells by diffusion? (5)

Answer 5

• oxygen and carbon dioxide in gas exchange
• the waste product urea from cells into the blood plasma for excretion in the kidney
• water
• amino acids
• glucose

Question 6

What are the factors that affect the rate of diffusion? (3)

Answer 6

• concentration gradient (difference in concentrations)
• temperature
• surface area (of the membrane)

Question 7

How does the concentration gradient affect the rate of diffusion?

Answer 7

The bigger the concentration gradient (difference in concentrations), the faster the rate of diffusion because the net movement from one side is greater.

Question 8

How does the temperature affect the rate of diffusion?

Answer 8

The higher the temperature, the faster the rate of diffusion because the particles have more energy and therefore move faster.

Question 9

How does the surface area affect the rate of diffusion?

Answer 9

The larger the surface area (of the membrane), the faster the rate of diffusion because more particles can pass through at once.

Question 10

Do single-celled organisms have a large SA:Vol? What does this allow?

Answer 10

A single-celled organism has a relatively large surface area to volume ratio. This allows sufficient transport of molecules into and out of the cell to meet the needs of the organism.

Question 11

Do multicellular/larger organisms have a smaller or larger SA:Vol than smaller organisms?

Answer 11

• larger organisms = smaller SA:Vol
• smaller organisms = larger SA:Vol

Question 12

How do you calculate an organisms Volume and Surface area?

Answer 12

Estimate the size of an organisms in the form of a block;

• Volume = length x width x height
• Area of one side = length x width
• Surface area = area of all the sides added together

Question 13

What is the need for exchange surfaces and a transport system in multicellular organisms?

Answer 13

Multicellular organisms have a smaller SA:Vol, so not enough substances are able to diffuse from their outside surface to supply them with everything they need, therefore they need an exchange surface for diffusion.

Question 14

How is the effectiveness of an exchange surface is increased? (4 adaptations)

Answer 14

• having a large surface area so lots of a substance can diffuse at once
• a membrane that is thin, to provide a short diffusion pathway
• (in animals) having an efficient blood supply, by having lots of blood vessels to get things in and out of the blood quickly
• (in animals, for gaseous exchange) being ventilated - air moves in and out

Question 15

Where are villi found? What do they do? How are they adapted (3)?

Answer 15

• found in small intestine - mammals
• absorb food molecules (amino acids, glucose) into the blood stream

Adaptations;
- many villi increase the SA:Vol of the small intestine
- walls of the villi = 1 cell thick/single layer of surface cells - to ensure a short diffusion pathway
- a good blood supply maintains a steep concentration gradient

Question 16

Where are alveoli found? What do they do? How are they adapted (3 adaptions)?

Answer 16

• found in the lungs - mammals
• the alveoli carry out gas exchange (oxygen absorbed into bloodstream + carbon dioxide released)

adaptations;
- millions of alveoli in each lung, providing large SA for diffusion
- alveoli has a moist surface for gases to dissolve
- a large capillary network ensures that blood constantly moves oxygen away from the lungs - maintaining a steep concentration gradient

Question 17

What do the gills of a fish do? How are they adapted? (3 adaptations)

Answer 17

• the gills carry out gas exchange under the water (oxygen = absorbed into the blood + carbon dioxide is released)

adaptations;
- many gill filaments provide a large SA for diffusion (each gill filament has folds in the surface, lamellae, to further increase the SA)
- the membrane of the gill filaments = 1 cell thick, providing a short diffusion pathway
- flow of blood = in opposite direction to the flow of water across the gill filaments, so there is a steep concentration gradient

Question 18

What do the leaves do - in terms of diffusion? How are they adapted (3)?

Answer 18

• leaves carry out gas exchange (carbon dioxide is taken into the leaf + oxygen is released)

adaptations
- broad, flat shape provides a large SA for diffusion
- leaf = thin, providing a short diffusion pathway
- many air spaces to allow gases to easily diffuse through the leaf, maintaining a steep concentration gradient

Question 19

What is osmosis?

Answer 19

Water may move across cell membranes via osmosis. Osmosis is the diffusion of water from a dilute solution (region of higher water concentration) to a concentrated solution (region of low water concentration) through a partially permeable membrane.

Question 20

What is osmosis a type of and what does it do?

Answer 20

Diffusion - passive movement of water molecules down the concentration gradient.

Question 21

Which side of the membrane does water go during osmosis?

Answer 21

The water molecules pass through to both sides of the membrane during osmosis, but because of the concentration gradient there is a net movement from high to low.

Question 22

What does Hypertonic mean? And what does this cause the cell to look like in animals and plants?

Answer 22

• Hypertonic = The solution is concentrated/contains very little water.
• Shrivelled in animal cells
• Plasmolyzed in plant cells

Question 23

What does Hypotonic mean? And what does this cause the cell to look like in animals and plants?

Answer 23

• Hypotonic = The solution is dilute/contains a lot of water.
• Lysed in animal cells
• Turgid (normal) in plant cells

Question 24

What does Isotonic mean? And what does this cause the cell to look like in animals and plants?

Answer 24

• Isotonic = There is no difference in concentration between the solute and water, and the movement of water remains constant.
• Normal in animal cells
• Flaccid in plant cells

Question 25

What is the 12 step method for required practical 3 - diffusion practical with potatoes?

Answer 25

1. Use a cork borer to cut five potato cylinders of the same diameter.
2. Use the knife to trim off any potato skin on each potato cylinder. Then trim each potato cylinder so that they are all the same length.
3. Accurately measure the mass of each potato cylinder.
4. Accurately measure the length of each cylinder.
5. Record your measurements in a table.
6. Measure 10 cm(3) of each concentration of sugar or salt solution and put into boling tubes. Label each boiling tube clearly.
7. Measure 10 cm(3) of the distilled water and put into the fifth boiling tube. Label the bolling tube cleady.
8. Add one potato cylinder to each boiling tube.
9. Leave the potato cylinders in the boiling tubes for a chosen amount of time.
10. Remove the potato cylinders from the boiling tubes and carefully blot them dry with the paper towels.
11. Measure the new mass and length of each potato cylinder again. Record your measurements for each concentration in your table.
12. Calculate the percentage change of mass and length for each cylinder.

Question 26

How do you calculate the percentage change?

Answer 26

(final mass - initial mass) / initial mass x 100

Question 27

What does a positive and negative percentage change mean in regard to the potatoe practical?

Answer 27

• Positive indicates that the potato has gained water by osmosis, meaning the solution is more dilute.
• Negative percentage change suggests that the potato has lost water by osmosis, meaning the solution is more concentrated.

Question 28

What should you do after Required practical 3 - potato practical?

Answer 28

Plot a graph for percentage change in mass or length against sugar/salt solution concentration. (should be curved LBF)

Question 29

What conclusion can you make from the potato practical graph?

Answer 29

The point at which the line of best fit crosses the x-axis is the concentration of sugar/salt inside the potato, as this is where there would be no change in the mass of the potato.

Question 30

What is active transport?

Answer 30

Active transport moves substances from a more dilute solution to a more concentrated solution (against/up a concentration gradient). This requires energy from respiration.

Question 31

What is active transport used for?

Answer 31

Its used to move substances in and out of cells - allows cells to absorb ions from very dilute solutions.

Question 32

How is active transport used in plant roots? (3)

Answer 32

• Active transport allows mineral ions to be absorbed into plant root hairs from very dilute solutions in the soil.
• They can’t use diffusion, as there is usually more mineral ions in the root hair cells than the surrounding soil.
• Plants require mineral ions for healthy growth.

Question 33

How is active transport used in the gut? (2)

Answer 33

• It also allows sugar molecules/nutrients (glucose + amino acids) to be absorbed from lower concentrations in the gut into the blood which has a higher sugar concentration.
• Sugar molecules (glucose) can then be transported and used for cell respiration.