Transport across membranes

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Notas de aula

Big idea: moving substances in and out of cells

Big idea (key concept): Systems. A cell is a system with a boundary. The boundary has to let in the things the cell needs and keep in or remove the things it does not.
Related concept: Transport. Every substance that enters or leaves a cell is transported across the cell membrane by one of three processes: diffusion, osmosis or active transport.
Global context: Scientific and technical innovation. Saline drips, kidney dialysis machines and oral rehydration drinks are all designed using the rules in this lesson.
Cells take in oxygen, glucose, water and mineral ions and get rid of carbon dioxide, urea and other waste. All of it has to cross the cell membrane.
The membrane is a thin double layer of lipids with protein molecules embedded in it. It is partially permeable: small molecules such as oxygen, carbon dioxide and water pass through easily, large molecules such as starch and protein cannot, and ions and glucose usually need channel or carrier proteins.
Diffusion and osmosis are passive: they use the energy the particles already have, not energy from respiration. Active transport needs energy from respiration.

Diffusion, osmosis and active transport compared

Diffusion and what affects its rate

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It happens because particles move randomly.
Particles move in both directions. There is more movement down the gradient than up it because there are more particles on the high concentration side. When the concentrations become equal there is no net movement, but the particles keep moving.
Examples: oxygen moves from the air in the alveoli into the blood, carbon dioxide moves from respiring cells into the blood, and urea moves from liver cells into the blood.
The rate of diffusion is greater when the concentration gradient is steeper, the temperature is higher (particles move faster), the surface area is larger, and the diffusion distance is shorter (a thinner membrane).
As a rule, rate is proportional to (surface area x concentration gradient) / distance. Worked example: the surface area is doubled, the gradient is tripled and the distance is doubled. The rate changes by 2 x 3 / 2 = 3, so diffusion is 3 times faster.
Larger molecules diffuse more slowly than small ones, and a membrane only lets through the substances it is permeable to.

Diffusion across a cell membrane over time

Surface area to volume ratio

The surface area to volume ratio (SA:V) tells you how much surface there is for each unit of volume. Divide the surface area by the volume.
For a cube of side s: surface area = 6 x s x s, and volume = s x s x s. Worked examples: side 1 cm gives 6 cm² and 1 cm³, so SA:V = 6:1. Side 2 cm gives 24 cm² and 8 cm³, so SA:V = 3:1. Side 4 cm gives 96 cm² and 64 cm³, so SA:V = 1.5:1.
For a cuboid, add up the areas of all six faces. A 2 cm by 3 cm by 5 cm block has a surface area of 2 x (6 + 10 + 15) = 62 cm² and a volume of 30 cm³, so SA:V = 62 / 30 = 2.07:1.
As an object gets bigger, its volume grows faster than its surface area. If the side of a cube is tripled, the surface area becomes 9 times larger but the volume becomes 27 times larger, so the SA:V falls to one third.
A tiny single-celled organism has a large SA:V, so diffusion across its surface can supply the whole cell. A large animal has a small SA:V, so it needs exchange surfaces (lungs, villi) and a transport system (blood) to supply all its cells.
Exchange surfaces are adapted to speed up diffusion: they have a large surface area (folds, villi, alveoli), a thin wall for a short diffusion distance, and a good blood supply to keep the concentration gradient steep.

A bigger cube has a smaller surface area to volume ratio

Osmosis in animal and plant cells

Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher water potential (a dilute solution) to a region of lower water potential (a more concentrated solution).
Water potential is a measure of how freely water molecules can move. Pure water has the highest water potential. Dissolving a solute lowers it.
Animal cells have no cell wall. In a dilute solution (lower solute concentration than the cell) water enters, and the cell swells and may burst. In a concentrated solution water leaves and the cell shrivels. In a solution with the same concentration there is no net movement of water.
Plant cells have a strong cell wall. In a dilute solution water enters the vacuole and pushes the membrane against the wall. The cell becomes turgid, which helps to support the plant, and it does not burst.
In a concentrated solution, plant cells lose water, the cytoplasm shrinks and the membrane pulls away from the wall. This is plasmolysis. When the cells of a plant lose turgor the plant wilts.
Hospitals use drips of about 0.9% salt solution because it matches the concentration of body fluids. Pure water would enter blood cells by osmosis and make them burst.

Red blood cells in three different solutions

Active transport

Active transport is the movement of particles across a membrane from a region of lower concentration to a region of higher concentration, against the concentration gradient. It needs energy from respiration and carrier proteins in the membrane.
Root hair cells use active transport to take up mineral ions such as nitrate from the soil water, where the concentration is lower than inside the cell. Without this, plants could not get the ions they need to make proteins.
In the small intestine, glucose is absorbed into the blood by active transport when its concentration in the gut is lower than in the blood. This lets the body absorb all the glucose it can.
Cells that do a lot of active transport have many mitochondria, which release energy by respiration.
If respiration stops, for example because of a poison such as cyanide or a lack of oxygen, active transport stops too. Diffusion and osmosis carry on because they do not use energy from respiration.
Comparing the three: diffusion moves particles down a gradient with no respiration energy. Osmosis moves only water down its water potential gradient, with no respiration energy. Active transport moves particles up a gradient and needs respiration energy.

Root hair cell and intestinal epithelial cell

Think like a scientist: osmosis in potato cylinders

Question: what is the concentration of the cell sap in potato cells? Cut potato cylinders with a cork borer, all the same diameter and length, from one potato. Blot each one dry and record its mass. Place one in each of five sucrose solutions (0.0, 0.2, 0.4, 0.6 and 0.8 mol/dm³) for the same time, for example one hour. Blot them dry and find the new mass.
Variables: the independent variable is the sucrose concentration. The dependent variable is the percentage change in mass. Control variables: the size and surface area of the cylinders, the same potato, the volume of solution, the temperature and the time.
Why use percentage change? The cylinders do not all start with exactly the same mass. Percentage change = (final mass - initial mass) / initial mass x 100. A cylinder that goes from 3.0 g to 3.6 g has changed by +20%.
Results (one class experiment): concentration 0.0, 0.2, 0.4, 0.6, 0.8 mol/dm³ gives percentage change +20%, +10%, 0%, -10%, -20%. A graph of these results crosses zero at 0.4 mol/dm³. At that point there is no net osmosis, so the potato cell sap is about 0.4 mol/dm³.
Safety: cut cylinders on a tile, cut away from your hand, and take care with the cork borer. Wipe up spills.
Inquiry task: explain why the potato in pure water gains mass and the potato in strong sucrose loses mass. Evaluate the method: say why you would use three cylinders for each concentration and calculate a mean, and design a follow-up that tests whether a different vegetable has a different cell sap concentration.

Radish tissue in dilute and concentrated sugar solution

Slides

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Questões de prática

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1.What does it mean to say that a cell membrane is partially permeable?
Easy
- AIt lets every substance pass through freely in both directions
- BIt lets nothing through unless the cell uses energy
- CIt lets some substances through but not others
- DIt lets water through but never any other substance
2.Which molecule can cross a cell membrane most easily without help from a protein?
Easy
- AOxygen
- BStarch
- CA protein molecule
- DGlycogen
3.Which part of the cell membrane lets large molecules such as glucose and ions cross it?
Easy
- AThe cell wall around the membrane
- BThe nucleus beside the membrane
- CThe layer of cytoplasm inside the membrane
- DProtein channels and carriers
4.Match each term to its meaning.
Easy
- Diffusion
- Osmosis
- Active transport
- Partially permeable membrane
- A barrier that lets some substances cross but not others
- Net movement of particles down a concentration gradient
- Net movement of water across a partially permeable membrane
- Movement of particles against a gradient using respiration energy
5.Which statement correctly defines diffusion?
Easy
- AThe movement of particles from a lower to a higher concentration using energy
- BThe net movement of particles from a higher to a lower concentration
- CThe movement of water across a membrane from a concentrated solution to a dilute one
- DThe movement of particles only while the cell is alive and respiring
6.Diffusion needs energy from respiration.
Easy
True or false?
7.A drop of ink spreads through a beaker of still water. Which explanation is correct?
Easy
- AWater molecules push the ink particles towards the sides of the beaker
- BRandom movement carries ink particles from high to low concentration
- CThe ink particles use energy to move towards the clean water
- DThe ink particles are pulled outwards by the glass beaker
8.Which of these movements happen by diffusion? (select all that apply)
Medium
- AGlucose being absorbed into the blood against a concentration gradient
- BOxygen moving from the alveoli into the blood
- CCarbon dioxide moving from respiring muscle cells into the blood
- DNitrate ions entering a root hair cell from soil where the concentration is lower

Mais perguntas

9.Complete the sentence about diffusion.
Easy
Diffusion is the net movement of particles down a ____ gradient.
10.Match each change to the reason it speeds up diffusion.
Medium
- Higher temperature
- Thinner exchange surface
- Steeper concentration gradient
- Larger surface area
- More space for particles to cross at the same time
- Particles have more energy and move faster
- Particles have a shorter distance to travel
- More particles move in one direction than the other
11.Which change would slow diffusion down?
Easy
- ARaising the temperature
- BMaking the concentration gradient steeper
- CMaking the exchange surface thinner
- DMaking the exchange surface smaller
12.Rate of diffusion is proportional to (surface area x concentration gradient) / distance. A surface has its area doubled, its concentration gradient tripled and its diffusion distance doubled. By what factor does the rate of diffusion change?
Medium
13.The walls of the alveoli in the lungs are only one cell thick. How does this help gas exchange?
Medium
- AIt gives a short diffusion distance, so diffusion is faster
- BIt lowers the concentration gradient between air and blood
- CIt gives the oxygen energy to cross by active transport
- DIt stops carbon dioxide from leaving the blood
14.
In one experiment, agar cubes containing alkali and indicator were placed in acid. The time for each cube to turn colourless was recorded:
Side of cube (cm) Time to turn colourless (s)
0.5 30
1.0 120
2.0 480
Which conclusion is best supported by this data?
Hard
- AThe time taken doubled each time the side of the cube doubled
- BAcid diffuses faster into larger cubes because they have more surface
- CLarger cubes took longer because they had a smaller SA:V
- DThe size of the cube made no difference to the time taken
15.Use the agar cube data: a cube has a side of 2.0 cm. Its surface area is 24 cm² and its volume is 8 cm³. Calculate its SA:V ratio (the number before ':1').
Medium
16.Three cubes have sides of 1 cm, 2 cm and 4 cm. Which has the largest surface area to volume ratio?
Easy
- AThe 2 cm cube
- BThe 4 cm cube
- CThey all have the same ratio
- DThe 1 cm cube
17.As an organism gets larger, its surface area to volume ratio usually decreases.
Easy
True or false?
18.A cube has a side of 4 cm. Its surface area is 96 cm² and its volume is 64 cm³. What is its SA:V ratio (the number before ':1')?
Medium
19.A block measures 2 cm by 3 cm by 5 cm. Its surface area is 62 cm² and its volume is 30 cm³. Calculate its SA:V ratio (the number before ':1') to 2 decimal places.
Medium
20.The side of a cube is tripled. What happens to its volume, its surface area and its SA:V ratio?
Hard
- AVolume x9, surface area x27, SA:V rises
- BVolume x27, surface area x9, SA:V falls to one third
- CVolume x27, surface area x27, SA:V stays the same
- DVolume x3, surface area x9, SA:V rises
21.Why do large multicellular organisms need specialised exchange surfaces such as lungs?
Medium
- ATheir cells cannot use oxygen that diffuses in from outside
- BTheir cells have no membranes that let gases cross them
- CTheir SA:V is too small for diffusion alone to supply every cell
- DDiffusion cannot happen at all inside large organisms
22.A root hair cell has a long, thin extension. How does this help the plant?
Easy
- AIt increases the surface area for absorbing water and mineral ions
- BIt makes the cell wall thicker to stop water entering
- CIt carries out photosynthesis near the soil
- DIt reduces the amount of water the plant takes in
23.Complete the sentence about size.
Medium
As a cell gets bigger, its surface area to ____ ratio gets smaller.
24.Which statement correctly defines osmosis?
Easy
- AThe net movement of solute particles across a membrane from a high to a low concentration
- BThe movement of water across a membrane from a concentrated to a dilute solution
- CThe net movement of water across a partially permeable membrane from a dilute to a more concentrated solution
- DThe movement of any particle across a membrane using energy from respiration
25.In osmosis, water moves from a concentrated solution to a dilute solution.
Easy
True or false?
26.Solution A is 5% sugar and solution B is 15% sugar. They are separated by a membrane that lets water through but not sugar. What happens?
Medium
- AWater moves from A to B
- BWater moves from B to A
- CSugar moves from B to A
- DNothing moves because the membrane is in the way
27.A red blood cell is placed in distilled water. What happens and why?
Medium
- AIt shrivels, because water leaves by osmosis
- BIt stays the same, because the membrane is not permeable to water
- CIt swells but never bursts, because the membrane is strong
- DIt swells and may burst, because water enters by osmosis and there is no cell wall
28.Why does a plant cell not burst when it is placed in distilled water?
Easy
- AWater cannot cross the membrane of a plant cell
- BIts strong cell wall resists the swelling
- CThe vacuole removes the water by active transport
- DPlant cells have a higher water potential than distilled water
29.What is the name for a plant cell that is full of water and pushes firmly against its cell wall?
Easy
- APlasmolysed
- BTurgid
- CLysed
- DShrivelled
30.Complete the sentence about plant cells.
Medium
A plant cell that has lost so much water that its membrane pulls away from the cell wall is described as ____.
31.Put these events in order for a plant that wilts because its cells lose water.
Medium
- The pressure of water inside the cells (turgor) falls
- Water leaves the cells faster than the roots can replace it
- The stems and leaves droop and the plant wilts
- The cells become flaccid and soft
32.A red blood cell has a solute concentration equal to 0.9% salt solution. It is placed in 3% salt solution. What happens and why?
Hard
- AWater enters by osmosis because the cell is more dilute, and the cell bursts
- BSalt enters the cell by active transport, and the cell stays the same size
- CNothing happens because both solutions contain salt
- DWater leaves by osmosis because the outside solution is more concentrated, and the cell shrivels
33.Why do hospitals give patients a drip of about 0.9% salt solution rather than pure water?
Medium
- APure water would enter blood cells by osmosis and make them burst
- BPure water would make blood cells shrivel and lose their shape
- CSalt gives blood cells the energy they need to stay alive
- DPure water cannot be carried around the body in blood
34.A plant cell is placed in a strong sugar solution. Which statements are correct? (select all that apply)
Medium
- AThe cell membrane pulls away from the cell wall
- BWater leaves the cell by osmosis
- CThe cell becomes more turgid
- DThe cell becomes plasmolysed
- EThe cell bursts
35.Compare what happens to an animal cell and a plant cell when each is placed in distilled water.
Medium
36.Which statement describes active transport?
Easy
- AParticles move from a higher to a lower concentration without using energy
- BWater moves across a membrane from a dilute to a concentrated solution
- CParticles move from a lower to a higher concentration using energy from respiration
- DParticles move from a higher to a lower concentration using energy
37.Active transport stops if respiration stops.
Easy
True or false?
38.Why do cells that carry out a lot of active transport contain many mitochondria?
Medium
- AMitochondria make the carrier proteins that sit in the membrane
- BMitochondria increase the concentration gradient across the membrane
- CMitochondria store the mineral ions that are taken in
- DMitochondria release the energy from respiration that active transport needs
39.Cyanide stops cells from respiring. Which process in a root hair cell would stop first?
Hard
- ADiffusion of oxygen into the cell
- BActive transport of mineral ions into the cell
- COsmosis of water into the cell
- DDiffusion of carbon dioxide out of the cell
40.The concentration of nitrate ions is 0.5 mmol/dm³ in soil water and 4.0 mmol/dm³ inside a root hair cell. Nitrate ions still move into the cell. Which explanation is best?
Hard
- ADiffusion moves nitrate ions from the low to the high concentration
- BOsmosis moves nitrate ions because the soil has more water
- CActive transport moves nitrate ions against the concentration gradient using energy
- DNitrate ions move in because the soil is at a higher temperature
41.Describe two differences between diffusion and active transport.
Medium
42.Explain why root hair cells are adapted for absorbing water and mineral ions from the soil.
Hard
43.Explain why a large animal such as an elephant cannot rely on diffusion across its skin to supply all its cells with oxygen.
Hard
44.In the potato cylinder investigation, what is the independent variable?
Easy
- AThe concentration of the sucrose solution
- BThe percentage change in mass of each cylinder
- CThe length of time the cylinders are left
- DThe size of each potato cylinder
45.Which variable should be kept the same for every potato cylinder?
Medium
- AThe concentration of the sucrose solution
- BThe final mass of the cylinder
- CThe surface area and size of the cylinder
- DThe percentage change in mass
46.A potato cylinder has an initial mass of 3.0 g. After one hour in a sucrose solution its mass is 3.6 g. Calculate the percentage change in mass.
Medium
47.A potato cylinder has an initial mass of 4.0 g. After one hour in a strong sucrose solution its mass is 3.4 g. Calculate the percentage change in mass (use a minus sign for a decrease).
Medium
48.Why are potato cylinders blotted dry before they are weighed?
Easy
- ATo remove surface solution that would add to the mass
- BTo make the cylinders lose water by osmosis before weighing
- CTo stop the cylinders from respiring during the test
- DTo make the surface area of each cylinder larger
49.
One experiment used potato cylinders with different starting masses:
Sucrose (mol/dm³) Initial mass (g) Final mass (g)
0.0 2.5 2.8
0.4 3.2 3.2
0.8 4.0 3.4
Which conclusion is best supported?
Hard
- AThe potato cells gained water in 0.0 because that solution contained no water at all
- BThe 0.8 solution is the only one in which osmosis took place
- COsmosis only happens when the final mass is greater than the initial mass
- DCells gained water in 0.0, lost water in 0.8, and their sap is close to 0.4 mol/dm³
50.A student used one potato cylinder in each solution and concluded that the potato cell sap is exactly 0.40 mol/dm³. What is the best criticism?
Hard
- APotatoes cannot be used because they have no cell membranes at all
- BThere were no repeats, and steps of 0.2 mol/dm³ only give an estimate
- CPercentage change in mass should never be calculated in such tests
- DThe cylinders should have been left for a different time in each solution
51.A student wants to find out whether thin or thick potato cylinders reach their final mass sooner. Which design is fair?
Hard
- ADifferent lengths and a different solution for each cylinder
- BSame length, different diameters, same solution and same weighing times
- CUse one cylinder and change its solution each time it is weighed
- DDifferent diameters, each left for a different length of time
52.A potato cylinder falls in mass from 3.0 g to 2.4 g over 3 hours in a strong sucrose solution. Calculate its mean rate of change of mass in g per hour (give the size of the rate, not the sign).
Medium
53.
Percentage change in mass for potato cylinders in one class experiment:
Sucrose (mol/dm³) Percentage change
0.0 +14
0.2 +8
0.4 +2
0.6 -6
0.8 -13
Assume the line between 0.4 and 0.6 is straight. Estimate the concentration at which there is no change in mass.
Hard
54.Put the steps of the potato cylinder investigation in order.
Medium
- Record the initial mass of each cylinder
- Record the final mass and calculate the percentage change
- Place each cylinder in a different sucrose solution for the same time
- Cut cylinders of equal size and blot them dry
- Remove the cylinders and blot them dry
55.Placing a plant cell in pure water makes it plasmolysed.
Easy
True or false?

Resumos de revisão

## The big idea - **Systems** and **transport**: the cell membrane controls what enters and leaves a cell, using three processes. ## Diffusion - Net movement of particles **down a concentration gradient**, no respiration energy. - Faster with a steeper gradient, higher temperature, larger surface area, shorter distance. - Rate is proportional to (surface area x gradient) / distance. ## Surface area to volume ratio - SA:V = surface area / volume. Cube: 1 cm is 6:1, 2 cm is 3:1, 4 cm is 1.5:1. - Bigger organisms have a smaller SA:V, so they need exchange surfaces and a transport system. ## Osmosis - Water moves across a partially permeable membrane from dilute (high water potential) to concentrated (low water potential). - Animal cells swell and burst or shrivel. Plant cells become **turgid** or **plasmolysed**. ## Active transport - Moves particles **against** the gradient using carrier proteins and respiration energy. Cells doing it have many mitochondria. ## Think like a scientist - Potato cylinders in sucrose: percentage change = (final - initial) / initial x 100. Where the graph crosses zero, the solution matches the cell sap. Repeat and take a mean.

Partially permeable membrane: A membrane that lets some substances through but not others.
Diffusion: The net movement of particles from a higher to a lower concentration, down a concentration gradient, without energy from respiration.
Concentration gradient: The difference in concentration between two regions.
Surface area to volume ratio: The surface area of an object divided by its volume. It falls as an object gets bigger.
Exchange surface: A surface adapted for fast diffusion, with a large area, a thin wall and a good blood supply.
Osmosis: The net movement of water across a partially permeable membrane from a dilute to a more concentrated solution.
Water potential: A measure of how freely water molecules can move. Pure water has the highest water potential.
Turgid: A plant cell that is full of water and pushes firmly against its cell wall.
Plasmolysis: When a plant cell loses water and the membrane pulls away from the cell wall.
Active transport: The movement of particles from a lower to a higher concentration using carrier proteins and energy from respiration.
Carrier protein: A protein in the cell membrane that moves particles across, for example in active transport.
Mitochondrion: A cell structure that releases energy by respiration. Cells that carry out active transport have many.
Percentage change: (final value - initial value) / initial value x 100.
Isotonic solution: A solution with the same concentration as the cells in it, so there is no net movement of water.

Flashcards

What does partially permeable mean?

A membrane that lets some substances cross but not others.

Define diffusion.

The net movement of particles from a higher to a lower concentration, down a concentration gradient. It needs no energy from respiration.

Give four factors that increase the rate of diffusion.

A steeper concentration gradient, a higher temperature, a larger surface area and a shorter diffusion distance.

What is the rule for the rate of diffusion?

Rate is proportional to (surface area x concentration gradient) / diffusion distance.

How do you calculate SA:V?

Divide the surface area by the volume.

What is the SA:V of a cube with a 2 cm side?

Surface area 24 cm² divided by volume 8 cm³ gives 3:1.

What happens to SA:V as an organism gets larger?

It decreases, because volume increases faster than surface area.

Why do large animals need exchange surfaces and a blood system?

Their SA:V is too small for diffusion across the body surface to supply all their cells.

Name three features of a good exchange surface.

Large surface area, thin wall (short distance) and a good blood supply.

Define osmosis.

The net movement of water across a partially permeable membrane from a dilute to a more concentrated solution.

What is water potential?

A measure of how freely water molecules can move. Pure water has the highest value, and dissolving a solute lowers it.

What happens to an animal cell in pure water?

Water enters by osmosis and the cell swells and may burst, because it has no cell wall.

What does turgid mean?

A plant cell full of water, pushing against its cell wall. It helps support the plant.

What is plasmolysis?

When a plant cell loses water in a concentrated solution and its membrane pulls away from the cell wall.

Define active transport.

Movement of particles against a concentration gradient, from low to high, using carrier proteins and energy from respiration.

Why do root hair cells have many mitochondria?

To release energy by respiration for the active transport of mineral ions.

How do you calculate percentage change in mass?

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

Why use percentage change rather than change in mass in the potato practical?

The cylinders start with slightly different masses, so percentage change allows a fair comparison.

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Em breve

Side of cube (cm)	Time to turn colourless (s)
0.5	30
1.0	120
2.0	480

Sucrose (mol/dm³)	Initial mass (g)	Final mass (g)
0.0	2.5	2.8
0.4	3.2	3.2
0.8	4.0	3.4

Sucrose (mol/dm³)	Percentage change
0.0	+14
0.2	+8
0.4	+2
0.6	-6
0.8	-13