B1 L10 Active transport and exchange surfaces | AQA GCSE Biology

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Screen 1

A fish gill: an exchange surface

A fish takes in oxygen from the water through its gills. Gills are an exchange surface.
Each gill has a curved, bony gill arch that holds two rows of thin, feathery gill filaments.
The filaments give a huge surface area. Real gills are deep red because they are full of blood.

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Screen 2

A real gill and a diagram

A real gill has hundreds of deep red filaments, in two rows.
A diagram is simplified on purpose: it shows a few filaments and draws the tiny plates much bigger.
Real gills are deep red because they are full of blood. The diagram is simplified.

Tap a difference to see it on both pictures.

A painting of one fish gill on a white tile.

Screen 3

Active transport: against the gradient

Active transport moves substances from a more dilute solution to a more concentrated solution: against a concentration gradient.
It needs energy from respiration. Diffusion and osmosis do not.
Root hair cells use it to take in mineral ions, such as nitrate, from the soil.

Mineral ions in the soil water: low

Mineral ions in the cell: high16

mineral ionwaterenergy

Screen 4

Active transport in the gut

Just after a meal, there is lots of sugar (glucose) in the gut. It diffuses into the blood.
Later, the gut has less sugar than the blood. Active transport keeps moving sugar into the blood.
The blood carries the sugar to the body's cells, where it is used for respiration.

Just after a meal, the gut has more sugar than the blood. Sugar diffuses into the blood, from a higher to a lower concentration. No energy is needed.

Sugar in the gut: high · Sugar in the blood: low

The wall of the small intestine is folded into villi (B2 L2 The digestive system).

sugar (glucose)energy

Screen 5

Checkpoint 1 of 3

Quick check on screens 1 to 4. Tap an answer.

1.Why are real fish gills deep red?

2.Which of these describes active transport?

3.Some hours after a meal, the gut has less sugar than the blood. How is sugar still absorbed into the blood?

Screen 6

Diffusion, osmosis and active transport

Diffusion: particles move from a higher to a lower concentration. No energy is needed.
Osmosis: water moves from a dilute solution to a more concentrated solution, through a partially permeable membrane. No energy is needed.
Active transport: substances move from a lower to a higher concentration. It needs energy from respiration.

Only active transport moves substances against a concentration gradient and needs energy. In osmosis, water moves. In active transport, a dissolved substance moves.

oxygenwatersugarmineral ionenergy

Table: three ways substances move into and out of cells
	Diffusion	Osmosis	Active transport
What moves	particles of a gas or a dissolved substance	water only	substances such as mineral ions and sugar
Which way	higher → lower concentration	dilute → more concentrated solution	lower → higher concentration (against the gradient)
Energy from respiration?	no	no	yes
Membrane	not always needed (in cells: the cell membrane)	partially permeable	a cell membrane
Example	oxygen into the blood in the gills	water into a root hair cell	mineral ions into a root hair cell

Screen 7

Surface area to volume ratio

Surface area is the total area of the outside of an object, in cm². Volume is the space inside it, in cm³.
The surface area to volume ratio compares them. Simplify it to "something : 1".
As a cube gets bigger, its surface area to volume ratio gets smaller.

Try it: a cube calculator

Side of the cube: 1 cm

Surface area = 6 × side × side = 6 × 1 × 1 = 6 cm²

Volume = side × side × side = 1 × 1 × 1 = 1 cm³

Surface area : volume = 6 : 1

Check: for any cube, R = 6 ÷ side. 6 ÷ 1 = 6.

surface area for each cm³ of volume

6 cm²

For example: a 3 cm cube has 54 cm² of surface and 27 cm³ of volume. 54 : 27 = 2 : 1.

Screen 8

Checkpoint 2 of 3

Quick check on screens 6 and 7. Tap an answer.

1.Which of these needs energy from respiration?

2.A cuboid has a surface area of 40 cm² and a volume of 16 cm³. What is its surface area to volume ratio?

3.A cube gets bigger. What happens to its surface area to volume ratio?

Screen 9

Why big organisms need exchange surfaces

A single-celled organism has a large surface area to volume ratio. Diffusion across its surface meets its needs.
A multicellular organism has a smaller ratio, and most of its cells are far from the surface.
So it needs exchange surfaces to take substances in, and a transport system to carry them to every cell.

Surface area to volume ratio: large

oxygen

Screen 10

Gills: built for exchange

Many thin gill filaments, each covered in tiny plates, give a large surface area.
The plates are thin, so oxygen has a short diffusion path into the blood.
A good blood supply carries oxygen away, and water flowing over the gills brings more. Both keep the concentration gradient steep.

Hundreds of filaments × many tiny plates on each = a huge surface area.

oxygenwater flow.Red: blood in the capillaries.

Screen 11

Exchange surfaces in animals and plants

Every exchange surface has a large surface area and a short diffusion path.
In animals there is also an efficient blood supply, and gas exchange surfaces are ventilated.
Plants have no blood. Root hair cells give a large surface area; leaves are flat and thin, with stomata and air spaces.

Gills (fish): oxygen in, carbon dioxide out. Many thin filaments covered in tiny plates, a good blood supply, and water flowing over them.

Every exchange surface has a large surface area and a short diffusion path.

Screen 12

Checkpoint 3 of 3

Quick check on screens 9 to 11. Tap an answer.

1.Why does a single-celled organism not need an exchange surface such as gills?

2.How does water flowing over a fish's gills help the fish take in oxygen?

3.Which feature helps a leaf to exchange gases?

Quiz

Quiz: B1 L10 Active transport and exchange surfaces

10 questions. Tap an answer. Your first answer counts towards your score.

What is an exchange surface?

Which of these is true of a real gill, but NOT of a diagram of a gill?

The water in the soil has very few mineral ions. How do mineral ions get into a root hair cell?

Why do root hair cells have many mitochondria?

Which of these is true of osmosis, but NOT of active transport?

A cube has sides of 10 cm. Its surface area is 600 cm² and its volume is 1000 cm³. What is its surface area to volume ratio?

Why does a large animal need a transport system?

Which of these is NOT a feature of an effective exchange surface?

Which exchange surface takes in water and mineral ions from the soil?

Villi in the small intestine have a good blood supply. How does this help?

Exam questions

The same five questions as the B1 L10 Exam questions worksheet. 20 marks.

Write your answer, then open the mark scheme and tick each point you made.

01Figure 1 shows a diagram of part of a fish gill.

Figure 1

01.1Complete Figure 1.

Choose answers from the box.

[3 marks]

blood vesselgill archgill filamentstomavillus

1 2 3

01.2Which feature of the gill filaments gives them a large surface area?

Tick (✓) one box.

[1 mark]

01.2 Which feature of the gill filaments gives them a large surface area? They are covered in tiny plates. They are deep red. They are held by a bony arch. They are under a gill cover.

Mark scheme

01.1 line 1 (to the arch): gill arch(1)
01.1 line 2 (to the top filament): gill filament(1)
01.1 line 3 (to the vessel): blood vessel(1)
01.2 They are covered in tiny plates.(1)

01.1: 1 mark for each correct label. allow capillary for line 3. If more than one word from the box is written on one line, do not give that mark. 01.2: If more than one box is ticked, 0 marks.

Model answer01.1 1 gill arch; 2 gill filament; 3 blood vessel. 01.2 They are covered in tiny plates.

02A teacher made a model to show how size affects diffusion.

She cut three cubes of agar jelly of different sizes, shown in Figure 2. The jelly contained an indicator that is pink in alkali and colourless in acid.
She put the cubes in acid and timed how long each cube took to become colourless all the way through.

Figure 2

Table 1 shows the results.

Table 1
Side of cube in cm	Surface area in cm²	Volume in cm³	Surface area : volume	Time to become colourless in minutes
0.5	X	0.125	Y	1.5
1	6	1	6 : 1	6
2	24	8	3 : 1	23

02.1Calculate the surface area X of the 0.5 cm cube.

Use the equation:

surface area of a cube = 6 × side × side

[2 marks]

X =cm²

02.2Calculate the surface area to volume ratio Y of the 0.5 cm cube.

Give your answer in the form ___ : 1

[1 mark]

Y =: 1

02.3Describe the pattern in Table 1.

[1 mark]

02.4The cubes are a model of organisms of different sizes.

Use Table 1 to suggest why a large organism needs exchange surfaces and a transport system.

[2 marks]

Mark scheme

02.1 6 × 0.5 × 0.5(1)
02.1 = 1.5 (cm²)(1)
02.2 12 : 1(1)
02.3 the bigger the cube, the longer it takes to become colourless(1)
02.4 a larger cube / organism has a smaller surface area to volume ratio, so it takes much longer for substances to diffuse to its middle(1)
02.4 so a large organism needs exchange surfaces (to take in enough) and a transport system (to carry substances to every cell)(1)

02.1: Correct answer with no working = 2 marks. 02.2: allow their 02.1 answer ÷ 0.125, written as ___ : 1. do not accept 1 : 12. 02.3: allow "the smaller the surface area to volume ratio, the longer it takes". "it takes longer" alone = 0: the answer must link size or ratio to time. 02.4: allow data used for the first mark, for example "the 2 cm cube took about 4 times as long as the 1 cm cube".

Model answer02.1 6 × 0.5 × 0.5 = 1.5 cm². 02.2 1.5 : 0.125 = 12 : 1. 02.3 The bigger the cube, the longer it takes to become colourless. 02.4 Bigger cubes have a smaller surface area to volume ratio and take much longer for the acid to diffuse to the middle. So a large organism needs exchange surfaces to take in enough, and a transport system to carry substances to every cell.

03Root hair cells take in nitrate ions from the soil.

Table 2 shows the concentration of nitrate ions in the soil water and in a root hair cell.

Table 2
Where	Concentration of nitrate ions in arbitrary units
soil water	0.5
root hair cell	25

Explain why the root hair cell must use active transport to take in nitrate ions.

Use data from Table 2.

[2 marks]

Mark scheme

the concentration of nitrate ions is higher in the root hair cell (25) than in the soil water (0.5) / 50 times higher(1)
so the ions must move against the concentration gradient, which needs energy from respiration(1)

the first mark needs data or a comparison from Table 2. allow "diffusion would move nitrate ions out of the cell" for the second mark. do not accept "energy is made / produced".

Model answerThe concentration of nitrate ions is higher in the root hair cell (25) than in the soil water (0.5). So the ions must be moved against the concentration gradient, and this needs energy from respiration.

04Table 3 shows three exchange surfaces.

Complete Table 3.

Tick (✓) the features that each exchange surface has.

The first row has been done for you.

[2 marks]

Mark scheme

villi row: ✓ large surface area, ✓ efficient blood supply, no tick for ventilated(1)
leaf row: ✓ large surface area only(1)

each row must be fully correct for its mark.

Model answervilli: large surface area and efficient blood supply; leaf: large surface area only.

05Diffusion, osmosis and active transport are three ways that substances move into and out of cells.

Compare diffusion, osmosis and active transport.

[6 marks]

Mark scheme

Level of response. Read your answer, choose the level that fits best, then pick your mark.

Level	Marks	Descriptor
3	5–6	A detailed and coherent comparison of all three processes. What moves, which way it moves (relative to the concentration gradient) and whether energy is needed are given for each, with clear similarities and differences, and examples.
2	3–4	A comparison of at least two of the processes, with some similarities or differences correctly stated. There may be an omission or one error.
1	1–2	Simple statements about one or more of the processes, with little or no comparison.
0	0	No relevant content.

Indicative content

all three move substances into or out of cells / across membranes
diffusion: the net movement of particles from a higher to a lower concentration / down a concentration gradient
diffusion happens in gases and in solutions; example: oxygen into the blood / carbon dioxide out of a respiring cell
osmosis: the movement of water only
osmosis: from a dilute solution to a more concentrated solution, through a partially permeable membrane
osmosis is the diffusion of water
in osmosis, water moves; in active transport, a dissolved substance (such as mineral ions or sugar) moves
example of osmosis: water into a root hair cell
active transport: from a more dilute to a more concentrated solution / from a lower to a higher concentration / against a concentration gradient
active transport needs energy from respiration; diffusion and osmosis do not
examples of active transport: mineral ions into root hair cells / sugar from the gut into the blood
cells that do a lot of active transport have many mitochondria
do not accept "energy is made / produced"; for osmosis, "from a high concentration to a low concentration" is credited only if it says "of water"

Your mark:

Model answer (Level 3)All three move substances into or out of cells. In diffusion, particles of a gas or a dissolved substance move from a higher to a lower concentration, for example oxygen into the blood. Osmosis is the diffusion of water only, from a dilute solution to a more concentrated solution through a partially permeable membrane, for example water into a root hair cell. Active transport is different: it moves substances from a lower to a higher concentration, against the concentration gradient, for example mineral ions into a root hair cell. Active transport needs energy from respiration, so cells that do a lot of it have many mitochondria. Diffusion and osmosis need no energy.

Total: 0 out of 20

Printable copy: the B1 L10 Exam questions worksheet and its mark scheme (PDF) are in Downloads.

B1 L10notes

Active transport

Active transport: the movement of substances from a more dilute solution to a more concentrated solution (against a concentration gradient), which requires energy from respiration.
Root hair cells take in mineral ions by active transport, and water by osmosis.
When the gut has less sugar than the blood, active transport still moves sugar into the blood.
Cells that do a lot of active transport have many mitochondria.

Surface area to volume ratio

Cube: surface area = 6 × side × side; volume = side × side × side.
A 3 cm cube: 54 : 27 = 2 : 1. The bigger the object, the smaller the ratio.
Single-celled organisms rely on diffusion. Large organisms need exchange surfaces and a transport system.

Effective exchange surfaces

A large surface area, and a thin membrane giving a short diffusion path.
In animals: an efficient blood supply and, for gas exchange, ventilation.
Gills: many thin filaments covered in tiny plates, a good blood supply, and water flowing over them.
Plants have no blood: root hair cells give a large surface area; leaves are flat and thin, with stomata.

Watch out

Only active transport goes against the concentration gradient, and only active transport needs energy.

Full revision notes (PDF)

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Worksheet answers are in the PowerPoint, in green, after each worksheet.

All B1 lessons Daily 5 Exam practice Revision sheet (PDF)

Exchange surface	Large surface area	Efficient blood supply	Ventilated
Alveoli in the lungs (done for you)
Villi in the small intestine
A leaf

B1 L10Active transport and exchange surfaces

Exam questions

01Figure 1 shows a diagram of part of a fish gill.

02A teacher made a model to show how size affects diffusion.

03Root hair cells take in nitrate ions from the soil.

04Table 3 shows three exchange surfaces.

05Diffusion, osmosis and active transport are three ways that substances move into and out of cells.

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