2.1.5b Diffusion

Question 1

see slide 2 for dia of outside/inside the cell membrane

Question 2

5 different ways substances enter/leave cells

Answer 2

1. Simple diffusion (passive process)
2. Facilitated diffusion (passive)
3. Osmosis (passive)
4. Active transport (active, requires ATP energy)
5. Bulk transport (endocytosis & exocytosis) (active)

Question 3

What is a concentration gradient

Answer 3

The difference in concentration of particles between 2 areas. Either steep or shallow

Question 4

Concentration gradient in relation to rate of diffusion

Answer 4

• If concentration gradient is steep, rate of diffusion is faster
• If concentration gradient is shallow, rate of diffusion is slower

Question 5

What is simple diffusion

Answer 5

The net (overall) random movement of particles (atoms, molecules, ions) from a region of higher concentration to lower concentration, down a concentration gradient

Question 6

How are particles able to move in simple diffusion

Answer 6

Due to the random movement & collisions of particles (which have their own kinetic energy)

Question 7

What type of process is simple diffusion

Answer 7

PASSIVE - meaning no ATP energy required

Question 8

When does simple diffusion stop

Answer 8

Continues until there is a concentration equilibrium between both sides

Question 9

Eg of simple diffusion

Answer 9

Oxygen molecules diffuse into cells by simple diffusion

Question 10

What type of molecules can move via simple diffusion

Answer 10

• Small, non polar molecules (eg. oxygen & CO2) can diffuse easily through the cell surface membrane
• This is bc they are very small & can pass through spaces between phospholipids
• Water is also small enough to fit between phospholipids, so it’s able to diffuse across plasma membranes even though it’s polar. This is called osmosis

Question 11

What is facilitated diffusion

Answer 11

The passive movement of molecules down a concentration gradient (higher to lower concentration) across a membrane, & involves a carrier proteins & channel proteins in the membrane

Question 12

What molecules can move in facilitated diffusion

Answer 12

Some larger polar molecules (eg. amino acids, glucose, or ions sa sodium ions & chloride ions) cannot simply diffuse directly through the phospholipid layer of the cell surface membrane

Instead, they diffuse through carrier proteins or channel proteins in the cell membrane

Question 13

What is needed in facilitated diffusion

Answer 13

Substances can only cross the phospholipid bilayer w the help of certain proteins:
- Channel proteins
- Carrier proteins
They are highly specific (they only allow one type of molecule or ion to pass through)
see slide12 for dia

Question 14

What are channel proteins

Answer 14

• Water-filled pores. They allow charged substances (eg. ions) to diffuse through the cell membrane.
  (see slide13)

Question 15

How do channel proteins work

Answer 15

• The diffusion of these ions does not occur freely, most channel proteins are ‘gated’, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore
• This allows the channel proteins to control the exchange of ions

Question 16

What are carrier proteins

Answer 16

• Unlike channel proteins which have a fixed shape, carrier proteins can switch between two shapes
• This causes the binding site of the carrier protein to be open to one side of the membrane first, & then open to the other side of the membrane when the carrier protein switches shape
  (see slide 14 for dia)

Question 17

What does the direction of movement of molecules depend on

Answer 17

Direction of movement of molecules diffusing across the membrane depends on their relative concentration on each side of the membrane

Question 18

Net diffusion of molecules or ions into or out of a cell will occur…

Answer 18

…down a concentration gradient (higher to lower)

Question 19

What does facilitated diffusion depend on

Answer 19

Concentration gradient - the higher the concent gradient, the faster the rate of facilitated diffusion (up to a point). As equilibrium is reached, rate of facilitated diffusion levels off

No. of channel or carrier proteins - once all the proteins in a membrane are in use, facilitated diffusion can’t happen any faster, even if you increase the concent gradient. So the grater the no. of channel or carrier proteins in cell membrane, the faster the rate of facilitated diffusion

Question 20

Why can water diffuse directly through the membrane

Answer 20

Even though its polar, it can diffuse directly through due to its relatively small size

Question 21

see slide 17 for comparison between carrier & channel proteins

Question 22

Where does the energy come from in simple & facilitated diffusion

Answer 22

Kinetic energy of the molecules

Question 23

see slide 21 for comparison between simple & facilitated diffusion

Question 24

Factors affecting the rate of diffusion

Answer 24

1. Concentration gradient (steep/shallow)
2. Temperature
3. SA of membrane
4. Thickness of exchange surface/membrane
5. SA:V
6. Properties of molecules/ions

Question 25

Factors affecting the rate of diffusion: **Concentration gradient**

Answer 25

- If there are **more** molecules on one side of a membrane than on the other, at any one moment more molecules will randomly move across membrane from that side than from the other. - **Steeper** the gradient, **faster** the rate of diffusion

Question 26

Factors affecting the rate of diffusion: **Temperature**

Answer 26

- Molecules & ions have **more kinetic energy** at higher temps - They move **faster**, resulting in **more collisions**, therefore resulting in **higher** rates of diffusion

Question 27

Factors affecting the rate of diffusion: **SA of membrane**

Answer 27

- The **greater the SA** across which diffusion is taking place, the **greater** the no. of molecules or ions that can cross it at any one moment, therefore the **faster** diffusion occurs - SA of membranes can be increased by **folding** (eg. microvilli in the intestine or cristae in mitochondria)

Question 28

Factors affecting the rate of diffusion: **SA:V**

Answer 28

- As a cell **increases** in size, the SA:V **decreases** which slows the rate of diffusion through a cell as the distance required becomes **too great**

Question 29

Factors affecting the rate of diffusion: **Properties of molecules/ions**

Answer 29

- **Large molecules** diffuse more slowly than **smaller ones** as they require **more energy** to move - **Uncharged & non-polar** molecules diffuse directly across the phospholipid bilayer - **Non-polar** molecules diffuse **more quickly** than polar ones as they are soluble in the non-polar phospholipid bilayer

Question 30

How does SA:V affect diffusion

Answer 30

- As the size of a cell **increases**, so does the **SA & volume** - BUT the volume of cell **increases faster** than the SA (bc vol is cubed, whereas SA is squared) - When volume is **too large** relative to the SA of cell, diffusion cannot occur at a high enough rate to supply raw materials to the entire volume of the cell

Question 31

What is surface area

Answer 31

Refers to the outside are of an object unit: cm^2

Question 32

What is volume

Answer 32

Refers to the amount of space inside the object unit: cm^3

Question 33

What is rate

Answer 33

A measure of how much smth is changing over time

Question 34

How to calculate rate on a linear diffusion graph

Answer 34

Gradient = change in y/change in x

Question 35

How to calculate rate on a curved (non-linear) diffusion graph

Answer 35

Draw a tangent Find gradient of the line

Question 36

do SA:V questions on slide 40

Question 37

How are organs specialised to ensure diffusion is efficient

Answer 37

- Have a large SA - Membrane is thin - shot diffusion pathway - Efficient blood supply surrounding exchange surface - Large SA:V ratio - there is more SA relative to the volume inside

Question 38

Adaptations of the intestines

Answer 38

- The inside of the small intestine is covered in millions & millions of these tiny little projections called **villi** - They **increase the SA** so that digested food is absorbed much more quickly into blood - They also have: - a **single layer** of surface cells - a very good **blood supply** to assist quick absorption (see slide 46 for dia)

Question 39

Adaptations of the alveoli (lungs)

Answer 39

- They are **very thin** - walls are only one cell thick - There is a very **efficient blood supply** network of capillaries around the alveoli - The lungs & alveoli have a large **combined SA** - The **walls are moist**, encouraging gas molecules to easily dissolve (see slide 47 for dia)

Question 40

Adaptations for gas exchange in fish

Answer 40

- Fish have gills which are made up of many **gill filaments** - large SA - The gills are covered in tiny structures called **lamellae** - increases SA - Lamellae are surrounded by lots of **blood capillaries** - **Countercurrent exchange system** - **Thin surface layer of cells** - short diffusion pathway (see slide 48-51)

Question 41

Adaptations of roots of a plant

Answer 41

- Plant roots **anchor the plant into the soil** & absorb water & mineral ions from the soil - Have extensions called **root hair cells** - increases SA to absorb MORE water/mineral ions - Root hair cells are **thin** - short diffusion pathway for water/mineral ions to travel into root (see slide 52)

Question 42

Gas exchange at the leaf

Answer 42

Leaves **absorb CO2** from the atmosphere & **release oxygen & water vapour** in the process of photosynthesis

Question 43

Adaptations of leaves

Answer 43

- **Very thin** - short diffusion pathway - **Broad & flat** - gives larger SA so more gases can diffuse into/out of leaf - Have tiny pores called **stomata** - this is where gases are exchanged

Question 44

What is a synapse

Answer 44

A junction between two neurones across which electrical signals must pass (see slide 54 for dia)

Question 45

Diffusion across the synapse of nerve cells

Answer 45

**Neurotransmitter molecules** diffuse from vesicles towards the **neurotransmitter receptors** moving from an area of high to low concentration

Question 46

Practical to investigate diffusion in cells

Answer 46

**Phenolphthalein** is a **pH** indicator - it's pink in alkaline solutions & colourless in acidic solutions. You can use it to investigate diffusion in **agar jelly**

Question 47

Method of agar jelly diffusion experiment

Answer 47

1. Make up some agar jelly with **phenolphthalein** & dilute **sodium hydroxide**. This will make the jelly pink 2. Fill a **beaker** w some dilute **hydrochloric acid**. Using a **scalpel**, cut out a few **cubes** from the jelly & put them in the beaker 3. Leave the cubes for a while - eventually, they'll turn **colourless** as the **acid diffuses into** the agar jelly & **neutralises** the sodium hydroxide Using this exp, you can investigate factors that affect rate of diffusion (SA, concent gradient, temp)

Question 48

Agar jelly diffusion experiment (SA)

Answer 48

- Cut the agar jelly into **different sized** (side lengths: 0.5cm, 1cm, 2cm) cubes & work out their **SA:V ratio** - Time how long it takes each cube to turn **colourless** when placed in the **same concentration** of hydrochloric acid. You would expect the cubes with the **largest SA:V ratio** to go colourless **fastest**

Question 49

Agar jelly diffusion experiment (Concentration gradient)

Answer 49

- Prepare the test tubes containing **different concentrations** of hydrochloric acid. - Put an **equal sized cube** of agar jelly in each test tube & time how long it takes each one to turn **colourless** You would expect the cubes in the **highest concentration** of hydrochloric acid to turn colourless **fastest**

Question 50

Agar jelly diffusion experiment (Temperature)

Answer 50

- Prepare several boiling tubes containing the **same concentration** of hydrochloric acid & put the tubes into **water baths** of **varying temps** - Put an **equal sized cube** in each boiling tube & time how long it takes each cube to turn **colourless** You would expect the cubes in the **highest temperature** to turn colourless **fastest**

Question 51

What do Carrier proteins do (pg56)

Answer 51

- Move **large molecules** into or out of the cell, down their concentration gradient - **Different carrier proteins** facilitate the diffusion of **different molecules**

Question 52

Step by step of carrier proteins in action (pg56)

Answer 52

1. First, a large molecule **attaches** to a carrier protein in the membrane 2. Then, the protein **changes shape** 3. This **releases** the molecule on the **opposite side** of the membrane

Question 53

What do Channel proteins do (pg56)

Answer 53

- Form **pores** in the membrane for **charged particles** to diffuse through, down their concentration gradient - **Different channel proteins** facilitate the diffusion of **different charged particles**