Transport in cells

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수업 노트

Diffusion: The Basics

Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement of particles from an area of higher concentration to an area of lower concentration.
Particles in a solution or gas move about randomly; if there are more particles in one region, there is an overall (net) movement to a region where there are fewer particles.
Diffusion is a passive process — it does not require energy from the cell; particles move using their own kinetic energy.
The higher the temperature, the more kinetic energy particles have, so they move and diffuse faster.
The cell membrane is partially permeable: some substances can diffuse across it, while others cannot.
Small molecules such as oxygen and carbon dioxide can diffuse across the cell membrane, but large molecules like starch cannot (they are too big).
Examples of substances entering cells by diffusion: oxygen for aerobic respiration and carbon dioxide for photosynthesis.
Examples of substances leaving cells by diffusion: carbon dioxide from respiration and urea (a toxic waste product) which must be excreted by the kidneys.

Factors that Affect the Rate of Diffusion

Concentration gradient: the greater the difference in concentration between two regions, the faster the rate of diffusion.
Temperature: higher temperature gives particles more kinetic energy, so they move and spread faster.
Surface area of the membrane: a greater surface area gives more 'entry or exit points', increasing the rate of diffusion.
Diffusion distance: the smaller the distance molecules have to travel, the faster transport occurs — this is why capillary and alveoli walls are only one cell thick.
Surface area to volume ratio (SA:V): as the size of an organism increases, its surface area to volume ratio decreases.
Single-celled organisms such as bacteria have a large surface area compared to their volume, so substances do not have to travel far and diffusion alone meets their needs.
Larger organisms with smaller SA:V ratios need exchange surfaces and transport systems to supply their cells.

Diffusion in Multicellular Organisms

Large multicellular organisms have relatively small surface areas compared to their volumes, so the distance from the surface to the centre is large.
This is why they need exchange surfaces and transport systems — diffusion, osmosis and active transport alone cannot meet their needs.
Small intestine: most absorption of digested food molecules into the blood occurs across its wall.
The small intestine has a highly folded surface lined with epithelial cells (which have folded membranes) to increase surface area; it is only one cell thick to decrease diffusion distance; and it has a good blood supply to maintain a concentration gradient.
Lungs (alveoli): millions of alveoli provide a huge surface area; each alveolus wall is one cell thick with a moist lining and excellent blood supply to maintain a concentration gradient.
Fish gills: made of filaments covered in lamellae to increase surface area, with a dense capillary network carrying blood in the opposite direction to the water to maintain a concentration gradient.
Plant roots: highly branched with root hair cells (root hair projections) to increase surface area for absorbing water and mineral ions.
Leaves: stomata (mainly on the lower surface) let air circulate inside the leaf, decreasing the diffusion distance for carbon dioxide and oxygen; spongy mesophyll cells also allow air to circulate.

Osmosis: Principles

Osmosis is the diffusion of water molecules from a dilute solution to a concentrated solution through a partially permeable membrane.
Osmosis is a special type of diffusion, so water molecules move about randomly; it applies only to water.
Water always moves from a more dilute solution to a more concentrated solution to even up the concentration of water molecules on each side of the membrane.
A dilute solution has a high concentration of water molecules, a high water potential, and a low concentration of solute.
A concentrated solution has a low concentration of water molecules, a low water potential, and a high concentration of solute.
Osmosis occurs when two solutions are separated by a partially permeable membrane; always identify which solution is more concentrated and which is more dilute to determine the direction of water movement.

Effects of Osmosis on Plant and Animal Cells

If plant tissue gains mass, water has moved into it by osmosis and the surrounding solution is more dilute than the plant tissue.
If plant tissue loses mass, water has moved out of it by osmosis and the surrounding solution is more concentrated than the plant tissue.
If there is no overall change in mass, there is no net movement of water because the concentration in the tissue and the solution are equal (water still moves in and out, but equally).
In a strong sugar solution (lower water potential than the cell), a plant cell loses water: the vacuole shrinks and the cell membrane pulls away from the cell wall — the cell becomes flaccid or plasmolysed.
In distilled water (higher water potential than the cell), a plant cell gains water: the vacuole gets bigger, pushing the cell membrane against the cell wall — the cell becomes turgid (high turgor pressure).
Turgid plant cells provide support and strength, keeping the plant upright with leaves held out to catch sunlight; if plants do not get enough water, cells cannot stay rigid and the plant wilts.
Animal cells have no supporting cell wall, so osmosis can be severe: in a strong sugar solution they lose water and become crenated (shrivelled); in distilled water they gain water until the membrane stretches too far and the cell bursts.
Plant cells are protected from bursting (cell lysis) by their supporting cell wall.

Effect of osmosis on animal cells

Required Practical: Osmosis

Aim: to investigate the effect of different concentrations of salt or sugar solutions on the mass of plant tissue.
Method: prepare potato cylinders of equal size, measure their mass and length, place one in each concentration of sugar or salt solution, leave for a set time, then remove, blot dry and reweigh.
Independent variable: the concentration of salt or sucrose solution in mol dm⁻³.
Dependent variable: the mass and length of each potato cylinder before and after soaking; the percentage change in mass is calculated from these measurements.
Control variables include: type and volume of solute in solution, temperature, and time.
On a graph of percentage change in mass against concentration, results above 0 mean the potato gained water by osmosis (solution more dilute); results below 0 mean the potato lost water (solution more concentrated).
Where the line crosses the x-axis, there is no net water movement — this is the concentration of sugar/salt inside the potato.
To improve reliability, repeat the investigation with several potato cylinders for each concentration so anomalous results can be identified and a mean calculated.

Active Transport

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.
Energy is needed because particles are moved against a concentration gradient, in the opposite direction from which they would naturally move by diffusion.
Active transport uses protein carrier molecules embedded in the cell membrane, powered by energy released by respiration (ATP).
Plants: root hair cells move mineral ions such as magnesium ions from the dilute solution in the soil into the more concentrated cytoplasm of the cell.
Magnesium ions are needed to make chlorophyll; nitrate ions are needed to make amino acids for protein synthesis and growth.
Animals: active transport allows glucose to be absorbed into the bloodstream from the lumen of the small intestine even when the concentration of sugar in the blood is higher than in the gut.
In the kidney, active transport in the tubules reabsorbs glucose back into the blood so none is lost in the urine.

The process of active transport

Comparing Transport Processes

Diffusion: movement of substances in a fluid down a concentration gradient; no energy from respiration required.
Osmosis: a special type of diffusion of water between two solutions separated by a partially permeable membrane, down a water potential gradient (from dilute to concentrated solution); no energy required.
Active transport: movement of substances actively across a membrane, usually against a concentration gradient; energy from respiration is required.
Substances moved by diffusion include oxygen, carbon dioxide, glucose, amino acids and urea.
Substances moved by active transport include glucose and mineral ions (e.g. sodium, potassium, magnesium, nitrates).
Diffusion occurs in the lungs and leaves (gas exchange) and in the digestive system; osmosis occurs between all cells and their immediate environment; active transport occurs between the gut lumen and intestinal cells and between roots and the soil.

Application: Sports Drinks

During exercise, body temperature rises and water and salts are lost through sweat (and water vapour through the lungs from increased breathing rate).
Muscle activity increases the rate of aerobic respiration, so more glucose is used up in muscle cells and must be replaced.
Sports drinks are designed to replace water, salts (ions/electrolytes) and glucose lost during exercise.
The balance between ions and water must be correct: if there is too little water in the blood or the ion concentration is too high, cells lose too much water by osmosis and shrink/dehydrate.
If the water concentration in the blood is too high or the ion concentration is too low, cells absorb too much water, swelling and possibly bursting.
Isotonic drinks contain similar concentrations of salt and sugar to the human body and are used for hydration and fluid replacement.
Hypertonic drinks contain higher concentrations of salt and sugar than blood, suitable for supplying glucose during intense exercise (e.g. a marathon).
Hypotonic drinks contain lower concentrations of salt and sugar than blood, suitable for rapid rehydration as water is drawn into the bloodstream by osmosis.

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연습 문제

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1.Which process is the spreading out of particles of a gas or of a substance in solution, resulting in a net movement from an area of higher concentration to an area of lower concentration?
Easy
- ADiffusion
- BOsmosis
- CActive transport
- DRespiration
2.Diffusion is an active process that requires energy from respiration.
Easy
True or false?
3.Which factor does NOT affect the rate of diffusion across a cell membrane?
Easy
- AThe colour of the substance diffusing
- BThe concentration gradient
- CThe temperature
- DThe surface area of the membrane
4.A plant cell is placed in a solution with a higher water potential than the cell. What will happen to the cell?
Medium
- AIt will become turgid as water moves in by osmosis.
- BIt will become flaccid as water moves out by osmosis.
- CIt will burst as water moves in by osmosis.
- DIt will shrivel as water moves out by active transport.
5.Which of the following statements about active transport are correct? (Select all that apply)
Medium
- AIt moves substances against a concentration gradient.
- BIt requires energy from respiration.
- CIt is a passive process.
- DIt uses protein carrier molecules in the cell membrane.
- EIt only transports water molecules.
6.Match each transport process with its correct description.
Medium
- Diffusion
- Osmosis
- Active transport
- Movement of substances in a fluid down a concentration gradient.
- Movement of water across a partially permeable membrane from a dilute to a concentrated solution.
- Movement of substances across a membrane against a concentration gradient, using energy.
7.According to Fick's law, what happens to the rate of diffusion if the diffusion distance is halved?
Hard
- AIt doubles.
- BIt halves.
- CIt stays the same.
- DIt quadruples.
8.Arrange the steps of the osmosis required practical in the correct order.
Medium
- Prepare potato cylinders of equal size.
- Blot each cylinder and weigh it.
- Place one cylinder into each sucrose solution.
- After 4 hours, remove cylinders, blot and reweigh.
- Calculate percentage change in mass.

더 많은 문제

9.Which organism would benefit most from a transport system?
Medium
- AOrganism A
- BOrganism B
- COrganism C
- DNone of them
10.The student measured oxygen uptake for the three organisms: 5.0 cm³ g⁻¹ h⁻¹, 0.3 cm³ g⁻¹ h⁻¹, and 12.0 cm³ g⁻¹ h⁻¹. Which value is most likely for organism C?
Hard
- A5.0 cm³ g⁻¹ h⁻¹
- B0.3 cm³ g⁻¹ h⁻¹
- C12.0 cm³ g⁻¹ h⁻¹
- DCannot be determined
11.Which of the following adaptations help to maximise the rate of diffusion across an exchange surface? (Select all that apply)
Medium
- AA large surface area
- BA thick, multicellular barrier
- CA short diffusion distance
- DA steep concentration gradient
- EA low temperature
12.Match each exchange surface with the substance that moves across it.
Medium
- Alveoli in the lungs
- Villi in the small intestine
- Root hair cells in plants
- Oxygen and carbon dioxide
- Digested food molecules such as glucose
- Water and mineral ions
13.A plant cell is placed in a solution with a lower water potential than the cell. What will happen to the cell?
Medium
- AIt will gain water and become turgid.
- BIt will lose water and become plasmolysed.
- CIt will gain water and burst.
- DThere will be no net movement of water.
14.Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.
Easy
True or false?
15.Which of the following is an example of active transport in a plant?
Medium
- AWater moving into root hair cells by osmosis
- BCarbon dioxide diffusing into a leaf through stomata
- CMineral ions moving from the soil into root hair cells against a concentration gradient
- DOxygen diffusing out of a leaf
16.A potato cylinder is placed in a sucrose solution. After 4 hours, its mass has decreased by 5%. What does this indicate?
Hard
- AWater has moved into the potato by osmosis because the solution is more dilute than the potato cells.
- BWater has moved out of the potato by osmosis because the solution is more concentrated than the potato cells.
- CWater has moved out of the potato by diffusion because the solution is more dilute than the potato cells.
- DThere has been no net movement of water because the solution and potato cells are isotonic.
17.In active transport, substances move from a region of lower concentration to a region of higher concentration using energy from respiration.
Easy
True or false?
18.Match each transport process with an example of a substance it moves.
Medium
- Diffusion
- Osmosis
- Active transport
- Oxygen into cells
- Water into root hair cells
- Glucose into the bloodstream from the small intestine when blood glucose is higher
19.A student places a piece of potato tissue into a sucrose solution. After 4 hours, the mass of the potato has increased. What does this indicate about the solution?
Medium
- AThe solution is more dilute than the potato tissue, so water moved into the potato by osmosis.
- BThe solution is more concentrated than the potato tissue, so water moved out of the potato by osmosis.
- CThe solution has the same concentration as the potato tissue, so there was no net movement of water.
- DThe solution is more dilute than the potato tissue, so sugar moved into the potato by diffusion.
20.A potato cylinder is placed in a sucrose solution. Its mass before soaking is 2.0 g, and after soaking it is 1.7 g. What is the percentage change in mass?
Medium
- A-15%
- B-30%
- C-0.3%
- D-85%
21.A student investigates osmosis using potato cylinders in sucrose solutions of different concentrations. The graph of percentage change in mass against sucrose concentration crosses the x-axis at 0.3 mol dm⁻³. What does this point represent?
Medium
- AThe concentration of sucrose inside the potato cells, where there is no net movement of water.
- BThe concentration of sucrose inside the potato cells, where water moves into the potato.
- CThe concentration of sucrose inside the potato cells, where water moves out of the potato.
- DThe concentration of sucrose inside the potato cells, where the potato becomes flaccid.
22.A student places a potato cylinder into a sucrose solution. After 4 hours, the cylinder feels floppy and its length has decreased. Which statement best explains this result?
Medium
- AWater moved out of the potato cells by osmosis because the solution had a lower water potential than the potato cells.
- BWater moved into the potato cells by osmosis because the solution had a higher water potential than the potato cells.
- CWater moved out of the potato cells by active transport because the solution had a lower water potential than the potato cells.
- DWater moved into the potato cells by diffusion because the solution had a higher solute concentration than the potato cells.
23.A student sets up the osmosis required practical using potato cylinders in sucrose solutions from 0 mol dm⁻³ to 1 mol dm⁻³. Which variable must be kept constant to make the results valid?
Medium
- AThe concentration of the sucrose solution
- BThe temperature of the solutions
- CThe change in mass of the potato cylinders
- DThe final length of the potato cylinders
24.A student investigates osmosis using potato cylinders. Which of the following would improve the reliability of the results? (Select all that apply)
Medium
- ARepeat the investigation with several potato cylinders at each sucrose concentration.
- BUse a different type of plant tissue for each concentration.
- CCalculate a mean percentage change in mass for each concentration.
- DBlot each potato cylinder with a paper towel before weighing.
- ELeave one potato cylinder in the solution for only 10 minutes.
25.In the osmosis required practical, a potato cylinder that shows no change in mass has been in a solution with the same concentration as the potato cell contents.
Easy
True or false?
26.Match each part of the osmosis required practical with its correct role.
Medium
- Independent variable
- Dependent variable
- Control variable
- Concentration of sucrose solution
- Percentage change in mass of potato cylinders
- Volume of sucrose solution used
27.Place the steps of the osmosis required practical in the correct order.
Medium
- Cut equally sized potato cylinders using a cork borer and ruler.
- Blot each potato cylinder with a paper towel and weigh it.
- Place one potato cylinder into each sucrose solution.
- After 4 hours, remove the cylinders, blot them and reweigh them.
- Calculate the percentage change in mass for each cylinder.
28.A student places a potato cylinder in a sucrose solution. The graph of percentage change in mass against sucrose concentration crosses the x-axis at 0.3 mol dm⁻³. What does this point represent?
Hard
- AThe concentration of sucrose inside the potato cells
- BThe concentration of sucrose that causes the potato to become plasmolysed
- CThe concentration of sucrose that gives the fastest rate of osmosis
- DThe concentration of sucrose that makes the potato cells burst
29.A student investigates the effect of temperature on the rate of diffusion using a dye in agar. The dye spreads faster at 40 °C than at 20 °C. Which statement best explains this?
Hard
- AAt higher temperatures, particles have more kinetic energy and move faster, so diffusion is faster.
- BAt higher temperatures, particles are actively transported across the agar, so diffusion is faster.
- CAt higher temperatures, the concentration gradient is steeper, so diffusion is faster.
- DAt higher temperatures, the surface area of the agar increases, so diffusion is faster.
30.A cell membrane has a thickness of 10 nm. If the thickness is reduced to 5 nm, what happens to the rate of diffusion of oxygen across the membrane, assuming all other factors remain constant?
Hard
- AThe rate of diffusion doubles.
- BThe rate of diffusion halves.
- CThe rate of diffusion stays the same.
- DThe rate of diffusion increases by a factor of four.
31.A student is investigating factors that affect the rate of diffusion. Which of the following changes would increase the rate of diffusion of a substance across a membrane? (Select all that apply)
Hard
- AIncreasing the concentration gradient across the membrane
- BIncreasing the thickness of the membrane
- CIncreasing the surface area of the membrane
- DDecreasing the temperature
- EIncreasing the temperature
32.A student sets up an experiment to investigate osmosis in potato tissue. They place potato cylinders in sucrose solutions of different concentrations. The graph of percentage change in mass against sucrose concentration is a straight line that crosses the x-axis at 0.25 mol dm⁻³. A potato cylinder is placed in a 0.40 mol dm⁻³ sucrose solution. What would you predict about the change in mass of this cylinder?
Hard
- AIt will decrease in mass because the solution has a lower water potential than the potato cells.
- BIt will increase in mass because the solution has a higher water potential than the potato cells.
- CIt will not change in mass because the solution is isotonic to the potato cells.
- DIt will decrease in mass because the solution has a higher water potential than the potato cells.
33.A student investigates the effect of surface area to volume ratio on diffusion using agar cubes of different sizes. Which cube would have the smallest surface area to volume ratio?
Hard
- AA cube with side length 1 cm
- BA cube with side length 2 cm
- CA cube with side length 3 cm
- DA cube with side length 4 cm
34.A student is investigating the rate of diffusion of a dye into agar cubes. They find that the dye reaches the centre of a 1 cm cube in 10 minutes, but takes 40 minutes to reach the centre of a 2 cm cube. Which statement best explains this?
Hard
- AThe 2 cm cube has a smaller surface area to volume ratio, so the diffusion distance is longer and the rate of diffusion is slower.
- BThe 2 cm cube has a larger surface area to volume ratio, so the diffusion distance is shorter and the rate of diffusion is faster.
- CThe 2 cm cube has a smaller surface area to volume ratio, so the diffusion distance is shorter and the rate of diffusion is faster.
- DThe 2 cm cube has a larger surface area to volume ratio, so the diffusion distance is longer and the rate of diffusion is slower.
35.Which of the following statements best defines diffusion?
Easy
- AThe net movement of particles from an area of higher concentration to an area of lower concentration
- BThe movement of water molecules through a partially permeable membrane
- CThe movement of particles against a concentration gradient using energy from respiration
- DThe movement of particles from an area of lower concentration to an area of higher concentration
36.Which of the following is NOT a factor that affects the rate of diffusion?
Easy
- AThe colour of the substance
- BThe concentration gradient
- CThe temperature
- DThe surface area of the membrane
37.Diffusion is a passive process that does not require energy from respiration.
Easy
True or false?
38.Osmosis is the movement of water molecules from a region of lower water potential to a region of higher water potential.
Easy
True or false?
39.Match each term with its correct description.
Medium
- Diffusion
- Osmosis
- Active transport
- Movement of particles from high to low concentration
- Movement of water through a partially permeable membrane from high to low water potential
- Movement of particles against a concentration gradient using energy
40.A potato cylinder is placed in a sucrose solution. The graph of percentage change in mass against sucrose concentration crosses the x-axis at 0.3 mol dm⁻³. What does this indicate?
Medium
- AThe concentration inside the potato cells is approximately 0.3 mol dm⁻³.
- BThe potato cells have burst at this concentration.
- CThe sucrose solution is isotonic with distilled water.
- DThe rate of osmosis is zero at all concentrations.
41.Complete the sentence about osmosis.
Easy
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a ____ membrane.
42.Name the process by which water moves into and out of cells through a partially permeable membrane.
Medium
43.A cube has a side length of 3 cm. Calculate its surface area to volume ratio. Give your answer in the form X:1.
Medium
44.A cube has a side length of 4 cm. Calculate its surface area to volume ratio. Give your answer in the form X:1.
Medium
45.Complete the sentence about diffusion.
Easy
Diffusion is the net movement of particles from an area of ____ concentration to an area of ____ concentration.
46.Complete the table comparing transport processes.
Medium
Diffusion is a ____ process, whereas active transport requires energy from ____.
47.A cube has a side length of 5 cm. Calculate its surface area to volume ratio. Give your answer in the form X:1.
Medium
48.A cube has a side length of 2 cm. Calculate its surface area to volume ratio. Give your answer in the form X:1.
Medium
49.A potato cylinder has a mass of 2.5 g before soaking and 2.0 g after soaking in a sucrose solution. Calculate the percentage change in mass. Give your answer to 1 decimal place.
Medium
50.Complete the sentence about active transport.
Easy
Active transport moves substances from a region of ____ concentration to a region of ____ concentration, using energy from ____.
51.Complete the sentence about surface area to volume ratio.
Medium
As the size of an organism increases, its surface area to volume ratio ____.
52.Complete the sentence about exchange surfaces.
Medium
A gas exchange surface is more efficient if it has a large ____ and a ____ diffusion distance.
53.Explain how the structure of the small intestine is adapted for efficient absorption of digested food molecules.
Hard
54.Explain why a single-celled organism such as an amoeba does not require a specialist transport system.
Hard
55.Describe how the structure of the small intestine is adapted for efficient absorption of digested food molecules.
Hard
56.Explain how the structure of the lungs is adapted for efficient gas exchange.
Hard
57.Explain how the structure of a root hair cell is adapted for efficient absorption of water and mineral ions.
Hard
58.A cube has a side length of 3 cm. Calculate its surface area to volume ratio. Give your answer in the form X:1.
Medium
- A4 :1
- B1 :1
- C3 :1
- D2 :1
59.A cube has a side length of 4 cm. Calculate its surface area to volume ratio. Give your answer in the form X:1.
Medium
- A3.5 ratio
- B2.5 ratio
- C1.5 ratio
- D0.5 ratio
60.A cube has a side length of 5 cm. Calculate its surface area to volume ratio. Give your answer in the form X:1.
Medium
- A3.2
- B2.2
- C0.2
- D1.2
61.A cube has a side length of 2 cm. Calculate its surface area to volume ratio. Give your answer in the form X:1.
Medium
- A2
- B3
- C5
- D4
62.A potato cylinder has a mass of 2.5 g before soaking and 2.0 g after soaking in a sucrose solution. Calculate the percentage change in mass. Give your answer to 1 decimal place.
Medium
- A-22 %
- B-20 %
- C-18 %
- D-16 %

플래시카드

What is diffusion?

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

Why is diffusion described as a passive process?

Diffusion is passive because it does not require energy from the organism; the particles move using their own kinetic energy.

What is the effect of temperature on the rate of diffusion?

The higher the temperature, the more kinetic energy the particles have, so they move faster and the rate of diffusion increases.

How does the concentration gradient affect the rate of diffusion?

The greater the difference in concentration between two regions, the faster the rate of diffusion.

How does the surface area of a membrane affect the rate of diffusion?

A membrane with a greater surface area has a greater rate of diffusion across it because there are more points for particles to cross.

Why do unicellular organisms not need specialised exchange surfaces?

They have a large surface area to volume ratio, so the distance from the surface to the centre is small and diffusion, osmosis and active transport alone are sufficient to meet their needs.

Why do large multicellular organisms need exchange surfaces and transport systems?

They have a small surface area to volume ratio, so the distance from the surface to the centre is large and simple diffusion would be too slow to meet their needs.

Give two adaptations of the small intestine that increase the efficiency of exchange.

It has a highly folded surface with villi and epithelial cells that have a folded cell membrane, increasing surface area; it also has a good blood supply to maintain a concentration gradient.

How are the alveoli adapted for gas exchange?

There are millions of alveoli providing a large surface area; their walls are one cell thick to give a short diffusion distance; they have a good blood supply and are ventilated to maintain a concentration gradient.

What is osmosis?

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

What is meant by a partially permeable membrane?

A membrane that allows some substances (such as water) to pass through but not others (such as large solute molecules).

What happens to a plant cell placed in a solution with a lower solute concentration than the cell contents?

Water enters the cell by osmosis, the vacuole gets bigger and pushes the cell membrane against the cell wall, making the cell turgid.

What happens to a plant cell placed in a solution with a higher solute concentration than the cell contents?

Water leaves the cell by osmosis, the vacuole shrinks and the cell membrane pulls away from the cell wall, making the cell flaccid or plasmolysed.

What happens to an animal cell placed in distilled water?

Water enters the cell by osmosis; as animal cells have no cell wall, the cell swells and may burst.

What happens to an animal cell placed in a strong sugar solution?

Water leaves the cell by osmosis and the cell becomes crenated (shrivelled).

In an osmosis experiment, what does it mean if a potato cylinder increases in mass?

Water has moved into the potato by osmosis because the solution surrounding it is more dilute than the potato tissue.

In an osmosis experiment, what does it mean if a potato cylinder decreases in mass?

Water has moved out of the potato by osmosis because the solution surrounding it is more concentrated than the potato tissue.

In an osmosis experiment, what does it mean if a potato cylinder shows no change in mass?

There has been no net movement of water because the concentration of the solution is the same as that inside the potato cells.

What is active transport?

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.

Why does active transport require energy?

Energy is needed because particles are moved against a concentration gradient, in the opposite direction to which they would naturally move by diffusion.

What is the role of protein carrier molecules in active transport?

Protein carrier molecules embedded in the cell membrane use energy from respiration to move substances into or out of the cell against a concentration gradient.

Give an example of active transport in plants.

Root hair cells use active transport to move mineral ions such as magnesium ions from the dilute solution in the soil into the cell, where they are at a higher concentration.

Give an example of active transport in animals.

Glucose is absorbed from the lumen of the small intestine into the bloodstream by active transport when the concentration of glucose in the blood is higher than in the gut.

What is the difference between diffusion and active transport in terms of energy?

Diffusion is passive and requires no energy, while active transport requires energy from respiration.

What is the difference between diffusion and osmosis?

Diffusion is the movement of any particles down a concentration gradient, while osmosis is the movement of water molecules only, through a partially permeable membrane.

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