movement in and out of cells

Question 1

Diffusion is

Answer 1

the movement of molecules from a region of its higher concentration to a region of its lower concentration

Molecules move down a concentration gradient, as a result of their random movement

Question 2

the cell is surrounded by … which can restrict the free movement of the molecules

Answer 2

cell membrane

Question 3

The cell membrane is a

Answer 3

a partially permeable membrane – this means it allows some molecules to cross easily, but others with difficulty or not at all

Question 4

Diffusion helps living organisms to

Answer 4

obtain many of their requirements

get rid of many of their waste products

carry out gas exchange for respiration

Question 5

Brownian Motion

Answer 5

All particles move randomly at all times

This is known as Brownian motion

The energy for diffusion comes from the kinetic energy of this random movement of molecules and ions

Question 6

Factors that Influence Diffusion

Answer 6

Surface Area to Volume Ratio

The bigger a cell or structure is, the smaller its surface area to volume ratio is, slowing down the rate at which substances can move across its surface

Many cells which are adapted for diffusion have increased surface area in some way – eg root hair cells in plants (which absorb water and mineral ions) and cells lining the ileum in animals (which absorb the products of digestion)

Diagram showing the large surface area: volume of a root hair cell

The highly folded surface of the small intestine increases its surface area

Distance

The smaller the distance molecules have to travel the faster transport will occur

This is why blood capillaries and alveoli have walls which are only one cell thick, ensure the rate of diffusion across them is as fast as possible

Temperature

The higher the temperature, the faster molecules move as they have more energy

This results in more collisions against the cell membrane and therefore a faster rate of movement across them

Concentration Gradient

The greater the difference in concentration either side of the membrane, the faster movement across it will occur

This is because on the side with the higher concentration, more random collisions against the membrane will occur

Question 7

Water can move in and out of cells by

Answer 7

osmosis

Question 8

Osmosis is

Answer 8

the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane

Question 9

The cell membrane is partially permeable which means

Answer 9

it allows small molecules (like water) through but not larger molecules (like solute molecules)

Question 10

a diluted solution has a

Answer 10

high water potential

Question 11

a concentrated solution has a

Answer 11

low water potential

Question 12

Osmosis Experiment potato

Answer 12

The most common osmosis practical involves cutting cylinders of potato and placing them into distilled water and sucrose solutions of increasing concentration

The potato cylinders are weighed before placing into the solutions

They are left in the solutions for 20 – 30 minutes and then removed, dried to remove excess liquid and reweighed

The potato cylinder in the distilled water will have increased its mass the most as there is a greater concentration gradient in this tube between the distilled water (high water concentration) and the potato cells (lower water concentration)

This means more water molecules will move into the potato cells by osmosis, pushing the cell membrane against the cell wall and so increasing the turgor pressure in the cells which makes them turgid – the potato cylinders will feel hard

A turgid plant cell

The potato cylinder in the strongest sucrose concentration will have decreased its mass the most as there is a greater concentration gradient in this tube between the potato cells (higher water concentration) and the sucrose solution (lower water concentration)

This means more water molecules will move out of the potato cells by osmosis, making them flaccid and decreasing the mass of the cylinder – the potato cylinders will feel floppy

If looked at underneath the microscope, cells from this potato cylinder might be plasmolysed, meaning the cell membrane has pulled away from the cell wall

A plasmolysed plant cell

If there is a potato cylinder that has not increased or decreased in mass, it means there was no overall movement of water into or out of the potato cells

This is because the solution that cylinder was in was the same concentration as the solution found in the cytoplasm of the potato cells, so there was no concentration gradient

Question 13

A turgid plant cell

Answer 13

more water molecules will move into the potato cells by osmosis, pushing the cell membrane against the cell wall and so increasing the turgor pressure in the cells which makes them turgid – the potato cylinders will feel hard

Question 14

A plasmolysed plant cell

Answer 14

This means more water molecules will move out of the potato cells by osmosis, making them flaccid and decreasing the mass of the cylinder – the potato cylinders will feel floppy

If looked at underneath the microscope, cells from this potato cylinder might be plasmolysed, meaning the cell membrane has pulled away from the cell wall

Question 15

Importance of Osmosis in Tissues

Answer 15

When water moves into a plant cell, the vacuole gets bigger, pushing the cell membrane against the cell wall

Water entering the cell by osmosis makes the cell rigid and firm

This is important for plants as the effect of all the cells in a plant being firm is to provide support and strength for the plant – making the plant stand upright with its leaves held out to catch sunlight

The pressure created by the cell wall stops too much water entering and prevents the cell from bursting

If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant wilts

Question 16

Osmosis in plant tissues

Answer 16

Plant cells that are turgid are full of water and contain a high turgor pressure (the pressure of the cytoplasm pushing against the cell wall)

This pressure prevents any more water entering the cell by osmosis, even if it is in a solution that has a higher water potential than inside the cytoplasm of the cells

This prevents the plant cells from taking in too much water and bursting

Plant roots are surrounded by soil water and the cytoplasm of root cells has a lower water potential than the soil water

This means water will move across the cell membrane of root hair cells into the root by osmosis

The water moves across the root from cell to cell by osmosis until it reaches the xylem

Once they enter the xylem they are transported away from the root by the transpiration stream, helping to maintain a concentration gradient between the root cells and the xylem vessels

Question 17

osmosis in animal tissues

Answer 17

Animal cells also lose and gain water as a result of osmosis

As animal cells do not have a supporting cell wall, the results on the cell are more severe

If an animal cell is placed into a strong sugar solution (with a lower water potential than the cell), it will lose water by osmosis and become crenated (shrivelled up)

If an animal cell is placed into distilled water (with a higher water potential than the cell), it will gain water by osmosis and, as it has no cell wall to create turgor pressure, will continue to do so until the cell membrane is stretched too far and it bursts

Question 18

What is Active Transport?

Answer 18

Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration

Question 19

what is needed and why in active transport?

Answer 19

Energy is needed because particles are being moved against a concentration gradient, in the opposite direction from which they would naturally move (by diffusion)

Question 20

Examples of active transport include

Answer 20

uptake of glucose by epithelial cells in the villi of the small intestine and by kidney tubules in the nephron

uptake of ions from soil water by root hair cells in plants

Question 21

How Protein Molecules Move Particles

Answer 21

Active transport works by using carrier proteins embedded in the cell membrane to pick up specific molecules and take them through the cell membrane against their concentration gradient:

Substance combines with carrier protein molecule in the cell membrane

Carrier transports substances across membrane using energy from respiration to give them the kinetic energy needed to change shape and move the substance through the cell membrane

Substance released into cell

Question 22

Answer 22