Biological Membranes

Question 1

What are some roles of membranes?

Answer 1

1. Site of chemical reactions
2. Site of cell communication (cell signalling)
3. partially permeable barriers between cells and its environment, organelles and the cytoplasm and within organelles
4. Compartmentalisation: when membranes enclose and isolate organelles enabling them to maintain specific environments for chemical reactions

Question 2

What is cell signalling?

Answer 2

It is how cells communicate with each other

Question 3

How do cells communicate with each other?

Answer 3

They communicate using messenger molecules- hormones. One cell releases a hormone which travels to another cell and binds to the receptor on its cell membrane

Question 4

How does the fluid mosaic model describe the structure of plasma membranes?

Answer 4

“Fluid” is used to describe the movement of the phospholipids in membrane and the “Mosaic” is used to describe how the membranes are studded with proteins that are randomly arranged.

Question 5

What is the role of cholesterol in membranes?

Answer 5

1. It regulates the fluidity of the membranes
2. It also reduces the movement of water soluble chemicals across the cell surface membrane

Question 6

What are the 2 categories of membrane proteins?

Answer 6

1. Intrinsic or integral proteins
2. Extrinsic or peripheral proteins

Question 7

What are intrinsic proteins?

Answer 7

They are transmembrane proteins that are fully embedded through the layers of the membrane

Question 8

What are the types of intrinsic proteins?

Answer 8

Protein channels and;
Carrier proteins

Question 9

What are protein channels?

Answer 9

They are IP that are lined with hydrophilic R-groups that allow polar, soluble and hydrophilic molecules to diffuse across the membrane

Question 10

Structure of protein channels

Answer 10

They have hydrophobic R-groups on the outside surface of the proteins and this interacts with the hydrophobic core of the membrane, keeping it in place

Question 11

What are carrier proteins?

Answer 11

They are proteins that can change their shape and positions to transfer molecules through the layers of the membrane

Question 12

What are extrinsic proteins?

Answer 12

They are proteins that are found on the side of the membrane

Question 13

Function of extrinsic proteins

Answer 13

1. They act as receptors for chemicals eg hormones
2. They play structural roles as well

Question 14

What are glycoproteins?

Answer 14

They are proteins that are attached to carbohydrate chains

Question 15

Function of glycoprotein

Answer 15

1. Acts as receptors for chemical signals eg hormones
2. Plays a role In cell adhesion (cells joining together) to form tissues

Question 16

What are glycolipids?

Answer 16

They are lipids that are attached to carbohydrate chains

Question 17

Function of glycolipids

Answer 17

They act as antigens

Question 18

Define the term compartmentalisation (2 marks)

Answer 18

Membranes form cells and separate areas within cells (1) isolating each area from its external environment (1)

Question 19

Membranes, particularly those present within mitochondria, are often highly folded. Suggest what advantages this folding provides. (6 marks)

Answer 19

Processes occur within/across, membranes (1) this process is enzyme controlled (1).
Folding gives increased surface area (1), so there are more enzymes (1), meaning an increased rate of reactions (1) and therefore an increased rate of ATP production (1).

Question 20

What are factors affecting membrane permeability?

Answer 20

1. Temperature
2. Organic solvents
3. pH

Question 21

How does temperature affect MP?

Answer 21

As temperature ⬆ kinetic energy ⬆. This means that the phospholipid molecules move more making the membrane more fluid and so more molecules can diffuse at a faster rate

Question 22

What happens if temperature continues to increase?

Answer 22

If temperature continues to increase, the proteins in the membrane will start to denature and so they can no longer function affecting MP

Question 23

How does organic solvents affect MP?

Answer 23

Organic solvents like alcohol can dissolve membranes as they are less polar than water. Because of this, they can disrupt the structure of the membrane making it more fluid and so the cells in the membrane cannot perform their function

Question 24

How does pH affect MP?

Answer 24

It disrupts the ionic bonds and if the pH is too high, the proteins get denatured

Question 25

Explain why solvents like water do not disrupt cell membranes (2 marks)

Answer 25

Water is a polar solvent(1) and phospholipids cannot dissolve in water(1)

Question 26

Suggest how an excessive consumption of alcohol could lead to liver cell death and ultimately be fatal (6 marks)

Answer 26

Alcohol is lipid soluble and dissolves a membrane bilayer(1). This disrupts the bilayer and stops/reduces transport of materials(1) preventing the normal functioning (1) and may cause cell death. The liver is particularly affected due to its role in filtering substances from the blood (1). This may ultimately be fatal if the liver function is destroyed (1) or if impulse transmission is depressed, prevent involuntary reflexes (1) such as breathing.

Question 27

Practical on membrane permeability- Steps

Answer 27

1. Using a cork borer, cut 5 equal strips of the beetroot and rinse them to remove any surface pigments released during cutting 2. Place the five pieces in five different test tubes, each with 5cm^3 of water 3. Place each test tube in a water bath at different temperatures with 10 ° C interval 4. Remove the pieces of beetroot from the tubes, leaving just the coloured liquid 5. Use a colorimeter to measure how much light is absorbed from each sample with a blue filter 6. Repeat 3 times with fresh beetroot pieces and calculate a mean

Question 28

Why is the experiment repeated three times and a mean is calculated?

Answer 28

This reduces the effect of random errors which makes our result more precise.

Question 29

Why are beetroot cells used?

Answer 29

Beetroot cells contain betalain which is released when the beetroot cell membranes are disrupted.

Question 30

Suggest how you could carry out an investigation to see the effects of organic solvents on membrane permeability

Answer 30

Carry out the same procedure- steps 1&2 except keep temperature constant (1) and use different organic solvents (1) e.g. ethanol (1)

Question 31

What is diffusion?

Answer 31

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration

Question 32

What process is diffusion?

Answer 32

It is a passive process which means it doesn't require metabolic energy, rather it utilises the energy from the natural movement of the particles

Question 33

What are some factors affecting diffusion?

Answer 33

Temperature Concentration gradient Presence of channel proteins (FD only) Surface area Thickness of the membrane

Question 34

How does temperature affect diffusion?

Answer 34

The higher the temperature, the higher the rate of diffusion. This is because the particles gain more kinetic energy and diffuse at a faster rate

Question 35

How does concentration gradient affect diffusion?

Answer 35

The greater the difference in concentration between the two regions, the higher the rate of diffusion. This is because the overall movement from the higher conc to the lower conce will be larger. OR you could just say the steeper it is, the faster the rate of diffusion.

Question 36

How does surface area affect diffusion?

Answer 36

The larger the SA, the greater the rate of diffusion

Question 37

How does the thickness of the membrane affect diffusion?

Answer 37

The thinner the membrane, the shorter the diffusion distance so the greater the rate of diffusion

Question 38

How does the presence of protein channel affect diffusion?

Answer 38

The more channel proteins, the higher the rate of diffusion. This is because more polar and hydrophilic molecules can diffuse across more

Question 39

What is facilitated diffusion?

Answer 39

It is the movement of polar and hydrophilic molecules through channel proteins in the membrane from an area of higher conc to an area of lower conc

Question 40

Is facilitated diffusion a passive or active process?

Answer 40

Passive process and it also moves substances DOWN the concentration gradient

Question 41

What are protein channels?

Answer 41

They are proteins that are lined with hydrophilic amino acid that allow hydrophilic, polar and soluble molecules to diffuse through

Question 42

What are carrier proteins?

Answer 42

They are proteins that have receptor sites for certain chemicals to bind to, changing the tertiary shape and this causes the molecule to be transported to the other side of the membrane

Question 43

Movement requires energy and yet the movement of molecules in diffusion is described as passive(not requiring energy). Explain this statement and state the source of the energy involved in diffusion (2 marks)

Answer 43

Diffusion is described as passive because it does not require any metabolic energy source(1) rather it utilises the energy from the natural random movement of particles (1)

Question 44

What is active transport?

Answer 44

Active transport is the movement of molecules from a region of low concentration to a region of high water concentration (AGAINST the concentration gradient) using energy/ATP involving carrier proteins

Question 45

Process of active process

Answer 45

1. The molecule that needs to be transported binds to the carrier protein 2. A molecule of ATP binds to the protein and is hydrolysed to produce phosphate and ADP 3. The phosphate attaches to the carrier protein and this causes the shape of the carrier protein to change allowing the molecule to pass through the other side 4. The phosphate then leaves the carrier protein and reforms with ADP to form ATP during respiration

Question 46

What is bulk transport?

Answer 46

It is the movement of extremely large substances across the cell membrane using ATP

Question 47

What are the types of bulk transport?

Answer 47

Endocytosis and Exocytosis

Question 48

What is Endocytosis?

Answer 48

It is the movement of substances INTO the cell

Question 49

Types of Endocytosis?

Answer 49

Phagocytosis- movement of SOLID materials into the cell pinocytosis- movement of LIQUID substances into the cell

Question 50

Process of Endocytosis

Answer 50

1. The cell surface membrane folds inwards and forms a cavity around the molecule 2. The membrane enfolds the material until the membrane fuses forming a vesicle 3. The vesicle moves into the cytoplasm to transfer the material for further processing within the cells

Question 51

What is exocytosis?

Answer 51

It is the movement of substances OUTSIDE of the cell

Question 52

Process of Exocytosis?

Answer 52

Vesicles formed from the Golgi Apparatus, fuses with the CSPM and releases the content outside of the cell

Question 53

Plants root take up mineral ions from the soil. The concentration of mineral ions in the soil water is very low. Suggest why active transport is very important in root hair cells (4 marks)

Answer 53

AT is important because plants need mineral ions(1) and there's a higher concentration of mineral ions inside the RHC (1) and so the mineral ions will diffuse out of the RHC (1) and so energy is needed to move the mineral ions against the concentration gradient(1)

Question 54

What is osmosis?

Answer 54

It is the movement of water particles from a region to higher water potential to a region of lower water potential through a partially permeable membrane

Question 55

What is water potential?

Answer 55

It is the pressure exerted by water molecules as they collide with the membrane or container

Question 56

What is water potential measured in?

Answer 56

It is measured in pascals(Pa) or kilopascals(kpa)

Question 57

What substance has the highest water potential?

Answer 57

Pure water has the highest water potential of 0 kPa

Question 58

How does water potential relate with the amount of solute?

Answer 58

The more solute there is in a solution, the lower the water potential (the more negative the value is)

Question 59

What is a hypertonic solution?

Answer 59

It is a solution that has a high concentration of solutes therefore it has a low water potential

Question 60

What is an isotonic solution?

Answer 60

It is a solution in which there is an equal concentration of water in both internal and external solutions so water moves in and out at an equal rate

Question 61

What is a hypotonic solution?

Answer 61

It is a solution that has a low concentration of solutes, therefore it has a high water potential

Question 62

Effect of osmosis on animal cells in a **HYPERTONIC** solution

Answer 62

In a more concentrated (has more solutes) solution, water will leave the RBC into the solution causing it to shrink

Question 63

Effect of osmosis on animal cells in a **HYPOTONIC** solution

Answer 63

In a diluted solution, water will enter the RBC causing it to rupture and burst as the hydrostatic pressure increases

Question 64

Effect of osmosis on animal cells in an **ISOTONIC** solution

Answer 64

In a solution that has the same water potential, osmosis will not take place as there is no net movement of water molecules

Question 65

Effect of osmosis on plant cells in a **HYPERTONIC** solution

Answer 65

In a more concentrated solution, water will leave the plant cell and this reduces the hydrostatic pressure causing the cell surface membrane to pull away from the cell- the cell is said to be plasmolysed

Question 66

Effect of osmosis on plant cells in a **HYPOTONIC** solution

Answer 66

In a diluted solution, water will enter the plant cell and this increases the hydrostatic pressure causing the water to push against the cell wall, becoming turgid

Question 67

Osmosis practical on potato

Answer 67

1. prepare sucrose solutions with different concentrations 2. Use a cork borer to cut potatoes into the same sized pieces 3. Divide the chips into groups of three and use a mass balance to measure the starting mass of each group 4. Place one group in each solution 5. Leave the chips in the solution for the same length of time. (Try to leave them for at least 20 minutes) 6. Remove the chips and pat dry gently with a paper towel 7. Reweigh each group and record the results 8. Calculate % change in mass for each group

Question 68

Explain why it is not possible to have a positive water potential (2 marks)

Answer 68

It is not possible because pure water is the only substance that has a positive water potential of 0 kPa (1). Addition of a solute decreases water potential (1) therefore all solutions have a negative water potential

Question 69

Explain why it is important to keep the concentrations of electrolytes (solutes)in body tissues at the correct level to ensure proper hydration (3 marks)

Answer 69

Electrolytes/solutes are necessary for many body processes (1) and help prevent excess water loss by osmosis (1) to help maintain correct fluid balance for reactions (1)