Cell Membranes and Transport

Question 1

What is the structure of phospholipids?

Answer 1

1. The heads of phospholipids are polar (hydrophilic)
2. Their tails are non-polar (hydrophobic)
3. If phospholipids are shaken up with water, they can form stable ball-like structures in the water called micelles
4. Alternatively , two-layered structures, called bilayers can from in sheets
5. This phospholipid bilayer is the basic structure of membranes

Question 2

What is the fluid mosaic model?

Answer 2

1. (Fluid because) Both the phospholipids and the proteins can move about by diffusion
2. The phospholipids move sideways, mainly in their own layers and the protein molecules also move about within the phospholipid bilayer
3. Mosaic because of scattered appearance of protein molecules

Question 3

What is the membrane?

Answer 3

1. The membrane is a bilayer of phospholipid molecules and the individual phospholipids molecules move about by diffusion within their own monolayers
2. Their phospholipid tails point inwards, facing each other forming a non-polar hydrophobic interior
3. The phospholipids heads face the aqueous medium that surrounds the membranes

Question 4

What does it mean if the tail of a phospholipid is saturated?

Answer 4

1. The more unsaturated a phospholipid tails is the more fluid the membrane
2. The unsaturated fatty acid tails are bent and therefore fit together more loosely
   - Fluidity is also affected by tail length as the longer the tail, the less fluid the membrane
   - As temperature decreases, membranes become less fluid

Question 5

What are intrinsic proteins?

Answer 5

• Proteins found embedded within the membrane (or integral proteins)
• They can be found in the inner layer, the outer layer or if they are spanning the whole membrane they are called transmembrane proteins
• In transmembrane proteins, the hydrophobic regions which cross the membrane are often made up of one or more alpha helical chains

Question 6

Describe cholesterol in the cell surface membrane

Answer 6

1. Relatively small molecule wth hydrophilic heads and hydrophobic tails and as a result fit neatly between the phospholipid molecules with their heads at the membrane surface
2. More common in animal cells than plant cells and are absent from prokaryotes

Question 7

What is the role of cholesterol in the cell surface membrane at low temperatures?

Answer 7

• At low temperatures, cholesterol increases the fluidity of the membrane, preventing it from becoming too rigid since it prevents close packing of the phospholipid tails
• This increased fluidity means cells can survive colder temperatures

Question 8

What is the role of cholesterol in the cell surface membrane at higher temperatures?

Answer 8

-The interaction of the phospholipid tails with the cholesterol molecules also helps to stabilise cells at higher temperatures when the membrane could otherwise become too fluid

Question 9

What is the role of cholesterol in the cell surface membrane in stability?

Answer 9

• Without cholesterol membranes would quickly break and cells burst open as it helps the mechanical stability of membranes
• The hydrophobic regions of cholesterol molecules help to prevent ions or polar molecules from passing through the membrane
• This is particularly important in the myelin sheath (made up of many layers of cell surface membrane) around never cells, where leakage of ions would slow down never impulses

Question 10

What is the role of phospholipids in the cell surface membrane?

Answer 10

1. Form the bilayer
2. They act as a barrier to most water-soluble substances since the tails of phospholipids are non-polar, it is difficult for polar molecules, or ions, to pass through membranes
3. Some can be modified chemically to act as signalling molecules and they may move about in the phospholipid bilayer , activating other molecules such as enzymes
4. May be hydrolysed to release small, water-soluble glycerol-related molecules and these diffuse through the cytoplasm and bind to specific receptors

Question 11

What are glycolipids and glycoproteins?

Answer 11

1. Many of the lipid molecules uses on the outer surfaces of cell surface membrane, and probably all of the protein molecules have shortchanged carbohydrate chains attached to them
2. The carbohydrate chains project like antennae into the watery fluids surrounding the cell, where they form hydrogen bonds with the water molecules and so help to stabilise the membrane structure
3. The carbohydrate chains form a sugary coating to the cell, known as the glycocalyx (animal cells the glycocalyx is formed mainly from glycoproteins and mainly glycolipids in plant cells)

Question 12

What is the role of glycolipids and glycoproteins? What are the three major groups of receptors and what do they do?

Answer 12

1. The carbohydrate chains also help the gylcoproteins and glycolipids to act as receptor molecules which bind with particular substances a the cell surface
2. The three major groups of receptor:
   - Signalling receptors; they are part of a signalling system the coordinates the activities of cells
   - Involved in endocytosis, bind to molecules that are parts of structures to be engulfed by the CSM
   - Involved in cell adhesion (binding cells to other cells in tissues and organs of animals)

Question 13

Describe the role of the cell surface membrane with antigens

Answer 13

1. Some glycolipids and glycoproteins act as cell markers or antigens, allowing cell-cell recognition and each type of cell has its own type of antigen
2. EXAMPLE: the ABO blood group antigens are glycoplipids and glycoproteins which have small differences in their carbohydrate portions

Question 14

Describe the role of proteins in the cell surface membrane

Answer 14

1. Many proteins act as transport proteins
2. These provide hydrophilic channels or passageways for ions and play molecules to pass through the membrane
   - There are two types; channel proteins and carrier proteins and each transport protein is specific for a particular kind of ion or molecule
   - Therefore the types of substances that enter or leave the cell can be controlled
3. Other types of membrane proteins are enzymes e.g. digestive enzymes in CSM of cells lining small intestine which catalyse the hydrolysis of molecules such as disaccharides
4. Some proteins on the inside of the CSM are attached to cytoskeleton and help to maintain and decide shape of cell
   - They may also be involved in phages of shape when cells move
5. Proteins play important role in membranes of organelles

Question 15

What is cell signalling?

Answer 15

1. Receiving a stimulus or signal at the receptor
2. Transmitting the message and making an appropriate response
3. Conversion of the original single to a message that is then transmitted is called transduction
4. Transmitting the message involved crossing barriers such as cell surface membranes
5. Signalling molecules are usually very small for easy transport
   - The release of chemicals that combine with cell surface receptors on target cells, leading to specific responses
   - Distances may be short (diffusion within one cell) or long distance in blood or phloem
   - release of chemicals that combine with cell surface receptors on target cells, leading to specific responses

Question 16

Describe cell signalling when the signalling molecule is hydrophobic

Answer 16

They can diffuse directly across the cell surface membrane and bind to receptors in the cytoplasm or nucleus e.g. the steroid hormones (e.g. oestrogen)

Question 17

Describe cell signalling when the signalling molecule is water-soluble

Answer 17

1. A typical signalling pathways starts with the single arriving at a protein receptor in a cell surface membrane
2. The receptor is a specific shape which recognises the signal and only cells with this receptor can recognise the signal
3. The signal brings about a change in the shape of the receptor, and since this spans the membrane, the message is in effect passed to the inside of the cell (signal transduction)
4. Changing the shape of the receptor allows it to interact with the next component of the pathway, so the message gets transmitted

Question 18

What is a ‘G-protein’?

Answer 18

1. Acts as a switch to bring about the release of a ‘second messenger’ which is a small molecule which diffuses through the cell relaying the message
2. Many second messenger molecules can be made in response to one receptor molecule being stimulated and this represents an amplification (magnification) of the original signal
3. The second messenger typically activates an enzyme, which in turn activates further enzymes, increasing the amplification at each stage
4. Finally, an enzyme is produced which brings about the required change in cell metabolism
   - The sequence of events triggered by the G-protein is called a signalling cascade

Question 19

What are other ways, not involving second messengers, that a receptor can alter the activity of a cell?

Answer 19

1. Opening an ion channel, resulting in a change of membrane potential
2. Acting directly as a membrane-bound enzyme e.g. insulin receptor
3. Acting as an intracellular receptor when the initial signal passes straight through the cell surface membrane e.g. the oestrogen receptor is in the nucleus and directly controls gene expression when combined with oestrogen

Question 20

What is diffusion?

Answer 20

• The net movement of molecules or ions from a region of higher concentration to a region or lower concentration, down a gradient, as a result of the random movements of particles
• The molecules or ions move down a concentration gradient
• The random movement is caused by the natural kinetic energy of the molecules or ions
• As a result of diffusion, molecules or ions tend to reach an equilibrium situation

Question 21

What affects the rate of diffusion?

Answer 21

1. Concentration gradient: the greater the difference in the concentration, the greater the difference in the number of molecules passing in the two directions, and hence the faster the rate of diffusion
2. Temperature: higher, faster
3. Surface Area (across which diffusion is taking place): the surface area to volume ratio decreases as the size of any three-dimensional object increases. Greater surface area, the more molecules or ions can cross it at any one moment, and therefore the faster diffusion can occur
4. The nature of the molecules or ions

Question 22

How does the nature of the molecules or ions affect the rate of diffusion?

Answer 22

1. Larger molecules, require more energy to get them moving than small ones do, so large molecules tend to diffuse more slowly than small molecules
2. Non-polar molecules such as glycerol, alcohol and steroid hormones diffuse much more easily through cell membranes than polar ones because they are soluble in the non-polar phospholipid tails
3. The respiratory gases, oxygen and carbon dioxide cross membranes by diffusion, they are unchanged and non-polar, and so can cross through the phospholipid bilayer directly between the phospholipid molecules
4. Water molecules, despite being very polar, can diffuse rapidly across the phospholipid bilayer because they are small enough

Question 23

What is facilitated diffusion?

Answer 23

It is the diffusion of a substance through transport proteins in a cell membrane; the proteins provide hydrophilic areas Tay allow the molecules or ions to pass through the membrane which would otherwise be less permeable to them

Question 24

What is osmosis?

Answer 24

Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane, as a result of their random motion (diffusion)

Question 25

What is active transport?

Answer 25

It is the movement of molecules or ions through transport proteins across a cell membrane, against their concentration gradient, using energy from ATP

Question 26

What is endocytosis?

Answer 26

It is the bulk movement of liquids (pinocytosis) or solids (phagocytosis) into a cell, by the unfolding of cell surface membrane to form vesicles containing the substance; endocytosis is an active process requiring ATP

Question 27

What is exocytosis?

Answer 27

It is the bulk movement of liquids or solids out of a cell, by the fusion of vesicles containing the substance with the cell surface membrane; exocytosis is an active process requiring ATP

Question 28

Describe facilitated diffusion

Answer 28

1. Large polar molecules such as glucose and amino acids or ions such as Na+ and Cl- cannot diffusion through the phospholipid bilayer
2. They can only do so with the help of channel proteins and carrier proteins
   (Diagram)

Question 29

What determines the water potential of a substance?

Answer 29

1. The concentration of the solution (dilute then high water potential)
2. The pressure applied to it (increase increase)

Question 30

What is an example of a carrier protein used for active transport? What is it’s role?

Answer 30

• Sodium potassium pump (Na+-K+ pump)
• Its role is to pump 3 sodium ions out of the cell at the same time as allowing two potassium ions into the cell for each ATP molecule used
• The ions are both positively charged, so the net result is that the inside of cell becomes more negative than the outside and a pd is created across the membrane
• The pump has a receptor site for ATP on its inner surface. It acts as an ATPase enzyme in bringing about the hydrolysis of ATP to ADP and phosphate to release energy

Question 31

What is the rule with surface area to volume ratio?

Answer 31

The surface area to volume ration= DECREASES as the size of any three dimensional object INCREASES

Question 32

What is a channel protein?

Answer 32

• Facilitated diffusion
  1. A water filled pore
  2. They look charged substances, usually ions to diffuse through the membrane
  3. They are ‘gated’
• This means that part of the protein molecule on the inside surface of the membrane can move to close or open the pore, like a gate
• This allows control of ion exchange

Question 33

What is a carrier protein?

Answer 33

• Facilitated diffusion
  1. They can flip between two shapes
  2. As a result the binding site is alternately open to one side of the membrane and then the other
  3. If molecules are diffusing across the membrane, then the direction of movement will normal depending on their relative concentration on each side of the membrane (they will move for high conc to low conc)

Question 34

What does the rate of facilitated diffusion depend on?

Answer 34

1. How many channel or carrier proteins molecules are in the membrane
2. Whether the channel proteins are open or not

Question 35

How is active transport achieved?

Answer 35

1. Carrier proteins which are specific for a particular type of molecule or ion
2. Requires energy supplied by ATP
3. The energy is used to make the carrier protein change it shape, transferring the molecules or ions across the membrane in the process

Question 36

What is bulk transport?

Answer 36

-For transport of large quantities of materials into cells (endocytosis) and out of cells (exocytosis) large molecules e.f. proteins or polysaccharides

Question 37

Describe endocytosis

Answer 37

• Involves the engulfing of the material by the cell surface embrace to form a small sac or ‘endocytic vacuole’ and takes two forms
  1. Phagocytosis
  2. Pinocytosis

Question 38

What is phagocytosis?

Answer 38

• The bulk uptake of solid material, cells specialising in this are called phagocytes
• The process is called phagocytosis and the vacuoles are called phagocytic vacuoles

Question 39

What is pinocytosis?

Answer 39

• The bulk uptake of liquid

- The vacuoles (vesicles) formed are often extremely small, in which case the process is called micropinocytosis

Question 40

Describe exocytosis

Answer 40

1. It happens, for example in the secretion of digestive enzymes from cells of the pancreas
2. Secretory vesicles from the Golgi body carry the enzymes to the cell surface and release their contents
3. Plant cells use exocytosis to get their cell wall building materials to the outside of the cell surface membrane

Question 41

What is an example of when active transport occurs?

Answer 41

1. Reabsorption in the kidneys, where certain useful molecules and ions have to be reabsorbed into the blood after filtration into the kidney tubules
2. Absorption of some products of digestion from the gut
3. In plants, active transport is used to loads sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant, and to load inorganic ions from the soil into root hairs

Question 42

What is solute potential?

Answer 42

1. The two factors that determine the water potential of a solution: the concentration of the solution, and the pressure applied to it
2. The contribution of the solution to water potential is called the solute potential. We can think of solute potential as being the extent to which the solute molecules decrease the water potential of the solution
   - The more the solute there is, the lower the tendency for water to move out of the solution
3. Just like water potential, solute potential is 0 for pure water and has a negative value for a solution
4. Adding more solute to a solution decreases its water potential, so the greater the concentration of the solute, the more negative the value of solute potential
   - The psi symbol can be used to show the solute potential but this time with the subscript s

Question 43

What is pressure potential?

Answer 43

1. The contribution of pressure to the water potential of a solution is called pressure potential
2. We can see that increasing the pressure on B increases the tendency for water to move out of it, that is increases its water potential
3. Pressure potential can be shown using the symbol psi little p

Question 44

What is water potential?

Answer 44

1. The term water potential is very useful when considering osmosis, the greek letter psi can be used to mean water potential
2. You can think of water potential as being the tendency of water to move out of a solution and this depends on two factors
   - How much water the solution contains in relation to solutes, and
   - How much pressure is being applied to it
3. Water always moves from a region of high water potential to a region of low water potential
4. We say water always moves down. water potential gradient
5. This will happen until the water potential is the same throughout the system, at which point we can say that equilibrium has been reached
   - For example, a solution containing a lot of water (a dilute solution) has. higher water potential that a solution containing only a little water (a concentrated solution)

Question 45

What is osmosis in animal cells like?

Answer 45

Red blood cells burst in pure water or dilute solution, and remains normal in a solution with same concentration as red cell and shrinks in a concentrated solution

Question 46

What is osmosis in plants cells like?

Answer 46

1. Unlike animal cells, plant cells are surrounded by cell walls, which a very strong and rigid
2. Imagine a plant cell being placed in pure water or a dilute solution
3. The water or solution has a higher water potential than the plants cell, and wattle therefore enters the cell through it partially permeable cell surface membrane by osmosis
4. Just like int he animal cell, the volume of the cell increases, but in the plant cell the cell wall pushes back against the expanding protoplast (the living part of the cell inside the cell wall), and the pressure starts to build up rapidly
5. This is the pressure potential, and it increases the water potential of the cell until the water potential inside the cell equals the water potential outside the cell, and equilibrium is reached
   - The cell wall is so inelastic that it takes very little water to enter the cell to achive this

Question 47

What is the width of the membrane?

Answer 47

7nm

Question 48

Why are aqua porins necessary?

Answer 48

1. water is a polar molecule
2. Few polar molecules pass through phospholipid bilateral
   3 phospholipid raid are hydrophobic
3. Channels through aquaporin is hydrophilic
4. They increase permeability of membrane to water
5. Eg in nephron
6. Water is a solvent in cells (role of water in cell)

Question 49

What is cell signalling?

Answer 49

The molecular mechanisms by which cells detect and respond to external stimuli, including communication between cells

Question 50

What are antigens?

Answer 50

A substance that is forgein to the body and stimulates an immune response

Question 51

What are channel proteins?

Answer 51

A membrane protein of fixed shape which has a water-filler pore through which selected hydrophilic ions or molecules can pass

Question 52

What are carrier proteins?

Answer 52

A type of membrane protein which changes shape to allow the passage Ito or out of the cell of specific ions or molecules by facilitated diffusion or active transport

Question 53

What does a cell wall do?

Answer 53

1. Prevents the cell from bursting and when plant cell fully inflated with water it is fully turgid
2. Water potential in plant cells is combination of solute potential and pressure potential

Question 54

What happens when a plant cell is placed in a solution of lower water potential?

Answer 54

• Water leave the cell by osmosis
• Protoplast gradually shrinks until it is exerting no pressure on the cell wall
• At this point the pressure potential is zero so water potential is equal to solute potential
• Both solute molecules and water molecules of external solution cans pass through freely permeable cell wall so the external solution stays in contact with the shrinking protoplast
• As protoplast continues to shrunk it begins to move away from the cell wall
• This is plasmolysis and a cell in which it has happened is said to plasmolysed
• Eventually as with animal cell, an equilibrium is reached when the water potential of the cell has decreased until it equals that of the external solution
• Seen by light microscope using strips of epidermis peeled from rhubarb petiole or from the swollen storage leaves of onion bulbs and placed in a range of sucrose solutions of different concentration

Question 55

What is incident plasmolysis?

Answer 55

The point at which pressure potential has just reached zero and plasmolysis is about to occur