Topic 3 Transport in Cells

Question 1

Substances may move into and out of cells across the cell membranes via————
which is the:

Answer 1

diffusion
spreading out of particles resulting in a net movement from an area of higher concentration to an area of lower concentration

Question 2

Some of the substances transported in and out of cells by diffusion are

Answer 2

oxygen and carbon dioxide in gas exchange, and of the waste product urea from cells into the blood plasma for excretion in the kidney

Question 3

A single-celled organism has a relatively large surface area to

This allows:

Answer 3

volume ratio

sufficient transport of molecules into and out of the cell to meet the needs of the organism.

Question 4

In multicellular organisms, surfaces and organ systems are specialised for exchanging materials. The effectiveness of an exchange surface is increased by having:

Answer 4

A large surface area
A membrane that is thin to provide a short diffusion pathway
An efficient blood supply (in animals)
Efficient ventilation (in animals for gaseous exchange)

Question 5

Osmosis is the

Answer 5

the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.

Question 6

Active transport moves substances from a

Answer 6

more dilute solution to a more concentrated solution, against a concentration gradient. This requires energy from respiration.

Question 7

Active transport allows mineral ions
what do ions do to plants?

Answer 7

mineral ions to be absorbed into plant root hairs from very dilute solutions in the soil. Plants require ions for healthy growth

Question 8

Active transport allows sugar molecules to be
what are sugar molecules used for?

Answer 8

absorbed from lower concentrations in the gut into the blood which has a higher sugar concentration. Sugar molecules are used for cell respiration.

Question 9

Investigate the effect of a range of sugar concentrations on the mass of plant tissue
Describe and explain the method you would use for this practical

Answer 9

Set up 5 boiling tubes with different concentrations of sugar solution
Remove potato skin and cut 5 equal sized pieces of potato - using a scalpel / core borer
Record the mass of each piece of potato - using a mass balance
Place each potato piece in a different concentration of sugar solution
Leave for 1 hour
Take out potato pieces and dry them - using blotting paper
Record the end mass of each piece of potato - using a mass balance
Calculate the percentage change in mass
Plot a graph of sugar solution concentration against percentage change in mass
Where the line of best fit crosses the X axis is the concentration of cytoplasm inside the potato tissue

Question 10

Explain how different factors affect the rate of diffusion:

Answer 10

the bigger the conc gradient the faster the diffusion rate
A high temp also does this because the particles have more energy so move around faster
the larger the surface area of the membrane, the faster the diffusion rate because more particles can pass through at once

Question 11

Explain the need for exchange surfaces and transport systems in multicellular organisms in terms of surface area to volume ratio

Answer 11

they have a smaller surface area to volume ratio not enough substances can diffuse from their outside surface to supply their entire volume. This means that they need some sort of exchange surface for efficient diffusion.

Question 12

Adaptations of:
The small intestine in mammals

Answer 12

the inside of the small intestine is covered in millions and millions of these tiny little projections called villi. they increase the surface area in a big way so that digested food is absorbed much more quickly into the blood. they have - a single layer of surface cells
- a very good blood supply to assist quick absorption

Question 13

Adaptations of: The lungs in mammals (trachea, bronchi, alveoli, capillary network surrounding alveoli)- job and adaptations of alveoli

Answer 13

the job of the lungs is to transfer oxygen to the blood and to remove waste carbon dioixide from it. To do this the lungs contain millions of little air sacs called alveoli where gas exchange takes place. The alveoli are specialised to maximise diffusion of oxygen and carbon dioxide they have:
- An enormous surface area
- A moist lining for dissolving gases
- very thin walls
- a good blood supply

Question 14

Adaptations of: Gills in fish -

Answer 14

each gill is made of lots of thin plates called gill filaments which give a big surface area for exchange of gases.

These filaments are covered in lots of tiny structures called lamellae, which increase the surface area even more. The lamellae have lots of blood capillaries to speed up diffusion.

They also have a thin surface layer of cells to minimise the distance that gases have to diffuse.

Blood flows through the lamellae in one direction and water flows over in the opposite direction. This maintains a large conc gradient between the water and the blood.

The conc of oxygen in the water is always higher than that in the blood so as much oxygen as possible diffuses from the water into the blood.

Question 15

Adaptations of root in plants

Answer 15

Root hair cells are adapted for taking up water and mineral ions by having a large surface area to increase the rate of absorption. because the hairs create this. They also contain lots of
mitochondria, which release energy from glucose during
respiration in order to provide the energy needed for active transport.

Question 16

Adaptations of leaves in plants

Answer 16

carbon dioxide diffuses into the air spaces within the leaf, then it diffuses into the cells where photosynthesis. The leaf’s structure is adapted so that this can happen easily:
- The underneath of the leaf is an exchange surface. It’s covered in little holes called stomata which carbon dioxide diffuses in through
-Oxygen and water vapour also diffuse out through the stomata
- the size of the stomata is controlled by guard cells. these close the stomata if the plant is losing water faster than it is being replaced by the roots. Without these guard cells the plant would soon wilt
- The flattened shape of the leaf increases the area of this exchange surface so that it’s more effective
- The wall of the cells inside the leaf form another exchange surface. The air spaces inside the leaf increase the area of this surface so there’s more chance for carbon dioxide to get into the cells

Question 17

Explain the differences between diffusion, osmosis and active transport

Answer 17

Key Differences
What moves: Diffusion involves particles like oxygen or carbon dioxide, osmosis involves water, and active transport involves substances like ions or glucose.
Direction of movement: Diffusion and osmosis occur down a concentration gradient (high to low), while active transport moves substances against the gradient (low to high).
Energy: Only active transport requires energy, while diffusion and osmosis are passive processes.

Question 18

Describe the main sources of error

Answer 18

Not removing all excess water from the potato chips (dependent variable)
Not setting up the concentrations of sucrose accurately (Independent variable)

Question 19

Describe how to improve the accuracy of the experiment

Answer 19

Repeat the experiment at intermediate values of sucrose solution (can draw a more accurate line of best fit)
Repeat the experiment and calculate a mean

Question 20

Suggest an uncontrolled variable

Answer 20

The starting mass of the chips
The part of the potato the chip came from

Question 21

Describe how to improve the validity of the experiment

Answer 21

Cut all chips from the same potato
Repeat and calculate a mean

Question 22

Describe how you could change the experiment to investigate something different

Answer 22

You could change the temperature of the solution but keep the concentration of sucrose solution the same
You could change the surface area of the potato chip but keep the concentration of sucrose solution the same
You could change the type of potatoes used but keep the concentration of sucrose solution the same

Question 23

Explain the effects of osmosis on plant and animal cells

Answer 23

As water moves into the plant cell via osmosis, the plant cell becomes turgid. This provides the plant cell with support. However, if too much water were to enter, the plant cell could burst

Animal cells can lose and gain water as a result of osmosis
As animal cells do not have a supporting cell wall (unlike plant cells), the results of this loss or gain of water on the cell are severe
Animal cells losing water
If an animal cell is placed in a solution with a lower water potential than the cell (such as a concentrated sucrose solution)
Water will leave the cell through its partially permeable cell surface membrane by osmosis and the cell will shrink and shrivel up