Biological membranes

Question 1

What is compartmentalisation?

Answer 1

Formation of separate membrane bound areas in a cell. Vital to a cell as metabolism includes many different and often incompatible reactions and containing these reactions in different parts of the cell allows the specific conditions required for each reaction.

Question 2

What are the roles of membranes

Answer 2

• Partially permeable barriers between cells and its environment, between organelles and the cytoplasm and within organelles
• Site of cell communication
• Site of chemical reactions

Question 3

What are the roles of membranes within cells

Answer 3

• May contain enzymes involved in metabolic pathways
• May house pigments essential for chemical reactions e.g. chlorophyll on thylakoid membranes of chloroplasts
• Can form vesicles to transport substances e.g. proteins from RER to Golgi
• Compartmentalisation. All components required for specific function are kept in one place. E.g. nuclear membrane surrounding genetic material
• Folded membranes increase SA
• Contain enzymes for a specific chemical reaction

Question 4

What are the roles of membranes at the cell surface?

Answer 4

• Separates the cell’s components from the extracellular environment
• Regulates transport of materials in and out of cells
• May release chemicals to signal other cells
• Contains antigens so that the organism’s immune system recognises the cell as ‘self’ so does not attack it
• Contains receptors for chemical signals, released by other cells e.g. hormone receptors

Question 5

What are glycoproteins

Answer 5

• ntrinsic proteins attached to carbohydrate chains. They play a role in cell adhesion and as receptors for chemical signals.
o Receptors for neurotransmitters: the binding of neurotransmitters triggers or prevents an impulse in the next neurone
o Receptors for peptide hormones: affect the uptake and storage of glucose by cells

Question 6

What are glycolipids?

Answer 6

Lipids with attached carbohydrate chains. These molecules are called cell markers or antigens and can be recognised by cells of the immune system as self or non-self.

Question 7

What are extrinsic proteins

Answer 7

Or peripheral proteins are present on one side of the bilayer. Normally have hydrophobic R groups on their outer surfaces and interact with polar heads of phospholipids or intrinsic proteins. They transport nutrients into the cell and waste out of the cell

Question 8

What is cholesterol?

Answer 8

• It regulates the fluidity of membranes and adds stability to the membrane (by being positioned between the phospholipids in bilayer with hydrophilic end interacting with hydrophobic end pulling them together) without it being too rigid by stopping the phospholipid molecules grouping too closely and crystalizing.

Question 9

What are intrinsic proteins?

Answer 9

• Transmembrane proteins that have amino acids with hydrophobic R groups on their external membranes that interact with hydrophobic cores
o Channel proteins: provide hydrophobic channel that allow passive movement of polar molecules and ions down a concentration gradient through membranes.
o Carrier proteins: have an important role in both passive transport and active transport into cells

Question 10

Whats diffusion and its role in membranes?

Answer 10

• The movement of molecules directly across the phospholipid bilayer from high concentration to low concentration
• Passive process - energy for it comes from kinetic movement of particles (no ATP)
• Molecules must be small (H2/O2/CO2) or lipid soluble (steroid hormones)
• Water – should be repelled by hydrophobic core of bilayer but is so small to slip through gaps in bilayer by simple diffusion

Question 11

What factors affect diffusion?

Answer 11

• Temperature: The higher the temp the higher the KE so diffusion rate is faster
• Conc gradient: The steeper the conc gradient the faster the rate of diffusion
• SA:V Ratio: The larger the ratio the faster the rate of diffusion
• Diffusion distance: The shorter the diffusion distance the faster diffusion
• Size of molecule: The smaller the molecule the faster it diffuses

Question 12

What are channel proteins roles in facilitated diffusion

Answer 12

o Charged ions or polar molecules like water cannot diffuse through the non-polar centre of the phospholipid bilayer
o Channel proteins form pores to allow them to pass through
o Each is specific to one type of ion
o Some can open and close their pores acting like gates

Question 13

What are the carrier proteins roles in facilitated diffusion

Answer 13

o Allow diffusion of larger polar molecules such as sugars and amino acids
o Molecule attaches to specific binding site on carrier protein
o Causes the carrier protein to change shape to ‘deliver’ molecule through membrane

Question 14

Whats the need for facilitated diffusion

Answer 14

• Large and polar molecules can enter the cell by facilitated diffusion down their conc gradient with the aid of proteins

Question 15

Define osmosis

Answer 15

• Diffusion of water molecules across a partially permeable membrane from an area of high water potential (dilute) to an area of low water potential (concentrated)
• Solutions with high water potential have lots of free water molecules
o Pure water = 0 (kPa) water potential and all other solutions negative

Question 16

How does a plant cell become turgid?

Answer 16

o Cell in pure water (high water potential)
o Water potential in cell is lower then water potential outside cell
o Water enters cell by osmosis and cell swells and contents push onto cell wall
o No more water can enter and the cell is turgid

Question 17

How does a plant cell become plasmolyse

Answer 17

o Plant cell in concentrated sugar solution (low water potential)
o Water potential in cell more than water potential outside cell
o Water moves out by osmosis
o Plasma membrane starts to shrink away from cell wall, cell is flaccid.
o More water leaves, contents of cell continues to shrink
o Plasma membrane detaches and pulls away from cell wall and cell is plasmolysed

Question 18

What will happen to an animal cell if placed in a solution with higher water potential than the cytoplasm

Answer 18

o Water will move into the cell by osmosis increasing the hydrostatic pressure inside the cell.
o The cell surface membrane cannot stretch much and cannot withstand this increased pressure.
o It will break and the cell will burst (cytolysis)

Question 19

What will happen to an animal cell if placed in a solution with lower water potential than the cytoplasm

Answer 19

o It will lose water to the solution by osmosis down the water potential gradient
o This will cause a reduction in the volume of the cell
o The cell-surface membrane will ‘pucker’, referred to as crenation

Question 20

How does temp affect membrane structure

Answer 20

• Phospholipids in the membrane are constantly moving so when temperature is increased they gain kinetic energy
• This makes the membrane more fluid and it begins to lose its structure
• If it continues to increase in temperature the cell will eventually break down completely
• This loss of structure increases the permeability of the membrane making it easier for particles to cross it
• Carrier and channel proteins in the membrane will be denatured at higher temperatures. These proteins are involved in transport across the membrane so as they denature, membrane permeability will be affected.

Question 21

How does water affect membrane structure

Answer 21

• Water, a polar solvent, is essential in the formation of the phospholipid bilayer. The non-polar tails of the phospholipid are orientated away from the water, forming a bilayer with a hydrophobic core. The charged phosphate heads interact with water, helping to keep the bilayer intact.

Question 22

How do organic solvents e.g. alcohol affect membrane structure

Answer 22

• Many organic solvents are less polar than water for example alcohols, or are non-polar like benzene Organic solvents will dissolve membranes, disrupting cells. This is why alcohols are used in the antiseptic wipes. The alcohols dissolve the membranes of bacteria in a wound killing them and reducing the risk of infection.

Question 23

How do pure alcohol solutions affect membrane structure?

Answer 23

• Pure or very strong alcohol solutions are toxic as they destroy cells in the body. Less concentrated solutions of alcohols, such as alcoholic drinks, will not dissolve membranes but still cause damage. The non-polar alcoholic molecules can enter the cell membrane and the presence of these molecules between the phospholipids disrupts the membrane.

Question 24

What is cell signalling?

Answer 24

• Cells communications with one another to coordinate body processes and respond to stimuli
• Use messenger molecules
• One cell releases a messenger molecule (e.g. a hormone) which travels to another cell and is detected because it binds to a receptor in it’s plasma membrane (usually glycolipids or glycoproteins) which is complementary. This triggers a response inside the cell.

Question 25

What is active transport?

Answer 25

• Definition: The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration.
• This process requires energy and carrier proteins. Energy is needed as the particles are being move up on a concentration gradient, in the opposite direction to diffusion. Metabolic energy is supplied by ATP.
• Carrier proteins span the membranes and act as ‘pumps’.

Question 26

Describe the process of transport from outside the cell to inside the cell

Answer 26

1. The molecule or ion to be transported binds to receptors in the channel of the carrier protein on the outside of the cell
2. On the inside of the cell ATP binds to the carrier protein and is hydrolysed into ADP and phosphate
3. Binding of the phosphate molecule to the carrier protein to change shape – opening up to the inside of the cell
4. The molecule or ion is released to the inside of the cell.
5. The phosphate molecule is released from the carrier protein and recombines with ADP to form ATP
6. The carrier protein returns to its original shape. This process is selective – specific substances are transported by specific carrier proteins

Question 27

What is bulk transport?

Answer 27

• Another form of active transport
• Large molecules such as enzymes, hormones and whole cells like bacteria are too large to move through channel or carrier proteins, so they are moved into and out of cell by bulk transport

Question 28

Describe endocytosis

Answer 28

• Two types: Phagocytosis (for solids) and Pinocytosis (for liquids), the process is the same for both
• The cell surface membrane first invaginates (bends inwards) when it comes into contact with the material to be transported.
• The membrane enfolds the material until eventually the membrane fuses, forming a vesicle.
• The vesicle pinches off and moves into the cytoplasm to transfer the material for further processing within the cell.
• For example, vesicles containing bacteria are moved towards lysosomes, where the bacteria are digested by lysosomes.

Question 29

Describe exocytosis

Answer 29

• Is the reverse of endocytosis
• Vesicles, usually formed by the Golgi apparatus, more towards and fuse with the cell surface membrane
• The contents of the vesicle are then released outside of the cell
• Energy in the form of ATP is required for movement of vesicles along the cytoskeleton, changing the shape of cells to engulf materials, and the fusion of cell membranes as vesicles form or as they meet cell-surface membrane.

Question 30

Describe how you would investigate the factors that affect diffusion rates in model cells

Answer 30

• Dialysis tubing is used in experiments for this as cells are too small.
• The model enables us to investigate the effects of temperature and concentration on the rate of diffusion across membranes.
• Dialysis tubing: partially permeable with pores the similar size then those on a membrane.
• A model cell can be stimulated by tying one end of a section of tubing filling with a solution then tying the other end and then placing the ‘cell’ into another solution. The solutions could contain different sizes, or concentrations, of solute molecules.
• The changes in conc of solute molecules can be measured over time. Rates of diffusion across the tubing can then be calculated (rates at different temperatures can also be calculated using a water bath to vary the temperature

Question 31

Describe how you would investigate the effects of solutions of diff water potentials on plant and animal cells

Answer 31

• Eggs can be used to demonstrate osmosis in animal cells.
• To investigate osmosis, eggs without their shells are placed in different concentrations of sugar syrup.
• Over time, osmosis takes place and there will have been a net movement of water into or out of the eggs, depending on the conc of the syrup they were in.

Question 32

Describe how you would investigate the factors affecting membrane structure and permeability

Answer 32

• Beetroots are used because of their red pigment (betalain).
• When beetroot cell membranes are disrupted the red pigment is released into a solution making it coloured.
• To investigate the effect of temperature on membranes a student used five small equal sized pieces of beetroot using a cork borer
• The beetroot pieces were cleaned thoroughly in running water and placed in 100ml of distilled water in a water bath
• The temperature of the water baths were at intervals of 10 degrees Celsius
• Samples of the water containing the beetroot were taken five minutes after each temperature was reached.
• Absorbance of each sample was measured using a calorimeter with a blue filter
• The experiment was done three times, each time with fresh beetroot pieces and a mean calculated for each temperature