Membrane Structure and Cell transport (SCR)

Question 1

Why do lipids dissolve in non-polar substances?

Answer 1

Lipids are only sparingly soluble in aqueous (water based) solvents. For this reason they are said to be hydrophobic. Lipids are however not repelled by water- they are just more attracted to non-polar substances

Question 2

What are amphipathic substances?

Answer 2

Substances that are both hydrophobic (water-hating), and hydrophilic (water-loving)

Question 3

Why are phospholipids amphipathic?

Answer 3

They have a hydrophilic phosphate head and hydrophobic hydrocarbon tails.
When phospholipids are mixed with water, the phosphate heads are attracted to the water, but the hydrocarbon tails are attracted to each other more than the water. This leads to the formation of the phospholipid bilayer (head facing the water and tails facing inwards)

Question 4

What are triglycerides?

Answer 4

Component parts: 3x fatty acids
1x glycerol

Bonds between components: Ester bonds, formed by condensation reactions (under enzyme control)

Properties: Non polar, insoluble in water (no osmotic effect on cells)
Soluble in organic solvents
more compact than carbohydrates

Function: Thermal insulation, Energy storage, Protection, Bouyancy

Question 5

What are phospholipids?

Answer 5

Component parts: 2x fatty acid molecules
1x glycerol molecule
1x phosphate group

Bonds between components: 2x ester bonds
1x phosphate bond
Formed by condensation reactions

Properties: Amphipathic
Soluble in water + oil
More compact than carbohydrates

Function: Basic structure of cell membrane by forming phospholipid bilayer
Association with oligosaccharides to form glycolipids (which help in cell-cell recognition+ cell-cell adhesion)

Question 6

What is cholesterol?

Answer 6

Component parts: Carbon skeleton consisting of:
- 4 fused carbon rings
- 1x hydrocarbon tail
- 1x -OH group

Bonds between components: carbon-carbon single bonds
carbon- hydrogen single bonds

Properties: Virtually non polar
Almost insoluble in water
Soluble in organic solvents
More compact than carbohydrates

Function: Component of cell membrane
Regulates membrane fluidity
Maintains mechanical stability
Prevents leakage of small polar molecules
Precursor for synthesis of steroid hormones

Question 7

How can you identify steroid molecules?

Answer 7

• four fused rings of carbon atoms
• three cyclohexane rings, one cyclopentane ring
• 17 carbon atoms in total in the rings

Question 8

why can steroids pass through the lipid bilayer?

Answer 8

Steroids are mostly hydrocarbon and therefore hydrophobic.

Question 9

How to identify Oestradiol vs Testosterone (steroid molecules)

Answer 9

Testosterone has additional =O, and -CH3 groups
Oestradiol has an extra -OH group

They are both however made from the same steroid, cholesterol
They are both anabolic steroids

Question 9

What is Oestradiol?

Answer 9

• steroid molecule
• involved in the coordination of the menstrual cycle
• development of secondary sexual characteristics in females

Question 10

What is testosterone

Answer 10

• steroid molecule
• development of male secondary sexual characteristics

Question 11

How do lipid bilayers act as the basis of cell membranes?

Answer 11

Membranes are an essential component of cells. The plasma membrane forms the border between a cell and its environment. Membranes inside eukaryotic cells divide the cytoplasm into compartments. The basic structure of all biological membranes is the same. A bilayer of phospholipids and other amphipathic molecules forms a continuous sheet that controls the passage of substances despite being 10nm or less across.

Question 12

How do lipid bilayers act as barriers?

Answer 12

• Due to the amphipathic nature of the phospholipid membrane and how it has a hydrophobic core, It has a low permeability to hydrophilic particles
• Molecular size also influences membrane permeability. The trend is that the larger the molecule, the lower the permeability. For example, water molecules which are only slightly larger than single oxygen atoms, pass through membranes more easily than large molecules such as glycogen or protein

Question 13

What are integral proteins?

Answer 13

• Integral proteins are hydrophobic on at least part of their surface and are therefore embedded in the hydrocarbon chains in the centre of the membrane. They may fit in one of the two phospholipid layers or extend across both. Many integral proteins are transmembrane proteins- they extend across the membrane, with hydrophilic parts projecting through the regions of phosphate heads on either side.

Question 14

What are peripheral proteins?

Answer 14

Peripheral proteins are hydrophilic on their surface, so are not embedded in the membrane. Most of them are attached to the surface of integral proteins and this attachment is often reversible. Some have single hydrocarbon chain attached to them which is inserted into the membrane, anchoring the protein to the surface.

Question 15

How does the protein content of plasma membranes vary?

Answer 15

The protein content of the membranes is very variable because the function of the membranes varies. The more active a membrane, the higher its protein content. Membranes in the myelin sheath around nerve fibres just act as insulators and have protein content of about 18%. Most plasma membranes on the outside of the cell have a protein content of about 50%. The highest protein content- about 75%- is found in the membranes of chloroplasts and mitochondria, which are active in photosynthesis and respiration.

Question 16

What are glycoproteins?

Answer 16

Glycoproteins are conjugated proteins with carbohydrate as the non-polypeptide component. They are a component of the plasma membrane of cells, with the protein part embedded in the membrane and the carbohydrate part projecting out into the exterior cell environment.

Question 17

What are glycolipids?

Answer 17

Glycolipids are molecules consisting of carbohydrates linked to lipids. The carbohydrate part is usually a single monosaccharide or a short chain of between two and four sugar units. The lipid part usually contains one or two hydrocarbon chains, which naturally fit into the hydrophobic core of membranes. Glycolipids occur in the plasma membranes of all eukaryotic cells, with the attached carbohydrate projecting outwards into the extracellular environment of the cell.

Question 18

What is the role of glycoproteins and glycolipids?

Answer 18

The role of glycoproteins is in cell-to-cell recognition, glycolipids also have a role in cell recognition. They help the immune system to distinguish between self and non-self cells, so pathogens and foreign tissue can be recognised and be destroyed.
- they also form a carbohydrate rich layer on the outer face of the plasma membrane, called the glycocalyx which helps with cell binding

Question 19

Characteristics of saturated and unsaturated fatty acids.

Answer 19

Saturated fatty aids have straight chains and therefore pack together tightly in bilayers, giving a high density of phospholipids. This reduces the fluidity of the membrane and therefore its flexibility and permeability by simple diffusion. In contrast, unsaturated fatty acids have one or more kinks in their hydrocarbon chain, so they pack together more loosely . This makes membranes more fluid, flexible and permeable.
- the ratio of unsaturated to saturated depends on the environment

Question 20

How does cholesterol affect membrane fluidity in animal cells?

Answer 20

Cholesterol stabilises membranes at higher temperatures, maintaining impermeability to hydrophilic particles such as sodium ions and hydrogen ions. Cholesterol also helps to ensure that saturated fatty acid tails do not solidify at low temperatures, so preventing a stiffening of the membrane.

Question 21

Process of endocytosis leading to the formation of vesicles.

Answer 21

A small region of membrane of a membrane is pulled from the rest of the membrane, taking in a molecule (e.g. protein/hormone/enzyme) from outside of the cell and is pinched off, forming a vesicle

common types of endocytosis:
- phagocytosis- the process by which solid substances are ingested
- pinocytosis- the process by which liquids/dissolved substances are ingested

Question 22

Process of exocytosis

Answer 22

When vesicles meet their destination, vesicles fuse and bind with the target membrane and disappear in the process. This transfers the contents of the vesicle across the plasma membrane.
- it can be used to expel waste products (e.g. waste water from the contractile vacuole)
- Also used for adding proteins to the plasma membrane: In a growing cell, the area of the plasma membrane needs to increase. Phospholipids are synthesised and then inserted into the rER membrane. Ribosomes on the rER synthesise membrane proteins which are added to the membrane. Vesicles bud off the rER and move to the plasma membrane. They fuse with it, each increasing the area of the plasma membrane by a very small amount.

Question 23

Properties of Water and what they are useful for?

Answer 23

• transparency of water allows for photosynthesis of aquatic plants
• it has a high specific heat capacity, meaning the temperature remains stable, providing a thermally stable environment as lots of energy is required to break the hydrogen bonds to cause small increases in temperature
• it has a high boiling point (liquid from 0-100 degrees celsius) so has a high availability for all organisms
• it is cohesive , meaning it binds with other water molecules- this is caused by hydrogen bonding between water molecules. This leads to a high surface tension, benefiting aquatic animals as it means the water can support them without the surface tension breaking (providing a habitat)
• it is adhesive , so it is used to carry mineral ions- useful capillary action in the xylem

Question 24

How do aquaporins help with osmosis?

Answer 24

Some cells have water channels called aquaporins, which greatly increase membrane permeability to water. At its narrowest point, the channel in an aquaporin is only slightly wider than water molecules, which therefore pass through in single file. Positive charges at this point in the channel prevent protons (H+) from passing through.

Question 25

What is solvation?

Answer 25

Solvation is the combination of a solvent with the molecules or ions of a solute. They depend on waters polarity, with a partial negative charge at the oxygen pole and partial positive charge at the hydrogen pole of the molecule.

Question 26

How does solvation happen?

Answer 26

• Polar solutes dissolve due to attraction between the partial positive and partial negative charges on water molecules and solute molecules
• Positively charged ions are attracted to the partial negative oxygen pole of water
• Negatively charged ions are attracted to the partial positive hydrogen pole of water
  Because of these attractions, water molecules form shells around many types of ion and charged molecule, preventing them from precipitating by clumping together. Cytoplasm is a complex mixture of dissolved substances in which chemical reactions of metabolism occur.

Question 27

What is osmosis?

Answer 27

Osmosis is the movement of water molecules from an area of high water potential to an area of lower water potential across a partially permeable membrane until equilibrium is reached.

Question 28

What is a hypertonic solution

Answer 28

Hypertonic solution= when the external solution is more concentrated (i.e. has a higher solute potential/lower water potential) than the inside of the cell (which will have a lower solute potential/higher water potential) and there is a net movement of water out of a cell by osmosis

Question 29

What is a hypotonic solution?

Answer 29

Hypotonic solution= where the external solution is less concentrated (i.e. has a lower solute potential/higher water potential) than the inside of the cell (which will have a higher solute potential/lower water potential) and there is a net movement of water into the cell by osmosis

Question 30

What is an isotonic solution

Answer 30

There is no net movement of water between two isotonic solutions because there is no difference between the concentrations of osmotically active solutes, so equal numbers of water molecules move between them. This is known as dynamic equilibrium.

Question 31

Effects of hypotonic solutions on cells without cell walls.

Answer 31

If an animal cell is bathed in a hypotonic solution, water enters the cell by osmosis, making it swell. Because it lacks the support that a wall would provide, the cell easily bursts. This can be demonstrated by placing a small droplet of blood in pure water and then examining it using a microscope. The blood cells swell up to form a spherical shape and then burst, leaving ruptured plasma membranes called red ghosts. (lysing of red blood cells is called heamolysis)

Question 32

Effects of hypertonic solutions on animal cells

Answer 32

If an animal cell is bathed in a hypertonic solution, water leaves the cell by osmosis, so the cytoplasm shrinks in volume. The area of plasma membrane does not change, so the cell develops indentations, which are sometimes called crenations.

Question 33

Effects of hypotonic solutions on plant cells (with cell wall)

Answer 33

High pressures due to entry of water by osmosis can build up inside plant cells because the cell wall is strong enough to prevent bursting. A cell that has become pressurised in this way is said to have become turgid. This means it is swollen. This is the normal state of healthy plant cells. Turgid plant tissue can provide support because of its strength under compression. The stems and leaves of non-woody plants resist gravity in this way.

Question 34

Effects of hypertonic solutions on plant cells

Answer 34

If plant cells lose water, the pressure of the cytoplasm decreases. If it drops to atmospheric pressure, the plasma membrane no longer pushes against the cell wall, and the cell is not turgid. Further water loss causes plant cells to become flaccid, meaning limp or floppy. Leaves and stems bend downwards. This is called wilting. The cell wall is permeable to water and does not move. This means that as the volume of cytoplasm decreases, the plasma membrane eventually pulls away from the cell wall. This is called plasmolysis.

Question 35

Medical applications of isotonic solutions

Answer 35

isotonic solutions to be used during medical procedures. Usually, an isotonic sodium chloride solution is used, which is called “normal saline”

Normal saline can be used for:
- safely introduced to a patients blood system via an intravenous (IV) drip
- used to rinse wounds and skin abrasions
- used to keep areas of damaged skin moistened prior to skin grafts
- used as the basis for eye drops
- frozen to the consistency of slush for cooling hearts, kidneys and other donor organs, to be transported to the hospital where the transplant operation is to be done

Question 36

why can’t hypertonic/hypotonic solutions be used in medicine

Answer 36

Hypertonic solutions can damage human cells by dehydrating them. Hypotonic solutions can cause human cells to swell and lyse (burst). If extracellular fluid is isotonic, water molecules can pass in and out through the plasma membrane at the same rate, so cells remain healthy.

Question 37

What is water potential

Answer 37

The concept of water potential helps to understand movement of water in living systems, especially plants. It is a measure of the potential energy per unit volume. The symbol for water potential is Ψ (psi) and the units for measurement are kilopascals (kPa) or megapascals (MPa). The absolute quantity of potential energy cannot be determined, so all values are relative. Pure water at standard atmospheric pressure and 20°C has been assigned a water potential of zero.

Question 38

How do you calculate water potential?

Answer 38

water potential (Ψw)= solute potential (Ψs)+ pressure potential (Ψp)

Question 39

How does solute potential affect water potential?

Answer 39

The potential energy of water changes if solute dissolves in it- this component is solute potential (Ψs). When solutes dissolve, the potential energy of water is reduced (this is because Intermolecular attractions between solutes and water are even stronger, which explains how solutions form. These attractions restrict the movement of water molecules, so solutions are more viscous than pure water. ) . With no solutes dissolved, the solute potential is zero. Because it is impossible for water to hold less than no solutes, the only possible solute potentials are zero or negative.

Question 40

How does pressure potential affect water potential?

Answer 40

Rises or falls in hydrostatic pressure also change the potential energy of water- this component is the pressure potential (Ψp). The higher the pressure, the more potential energy water has. Pressure potential can be negative or positive because it can be greater or less than atmospheric pressure

Question 41

what is simple diffusion

Answer 41

Diffusion is the spreading out of particles in liquids that happens because the particles are in continuous random motion. More particles move from an area of higher concentration to an area of lower concentration than move in the opposite direction. There is therefore a net movement of particles from an area of high concentration to an area of lower concentration- down the concentration gradient. Living organisms do not have to use energy for this to occur; it is a passive process.

Question 42

what is facilitated diffusion

Answer 42

the passive movement of molecules or ions across a membrane from a region of high concentration to a region of low concentration across a membrane through specific protein channels or carrier proteins.

Question 43

Why is facilitated diffusion necessary?

Answer 43

Ions and polar molecules cannot easily pass between phospholipids, but diffusion of these substances is still possible with the help of proteins acting as channels.

Question 44

Protein channels and facilitated diffusion

Answer 44

A channel protein is an integral, transmembrane protein with a pore that connects the cytoplasm to the aqueous solution outside the cell. The diameter of a pore and the chemical properties of its sides ensure that only one type of particle passes through- for example, sodium ions or potassium ions (but not both) This makes them highly specific.

Question 45

Why are the ion channel proteins highly specific

Answer 45

• the binding sites of the hydrophilic amino acid side chains lining the ion channel being highly specific
• the size of the pore acting as a size filter

Question 46

How do ions channel that open and close work?

Answer 46

Most Ion channels open in response to specific stimuli, such as:

• changes in voltage across the membrane or voltage gated channels
• binding of small molecules to the channel proteins or ligand gated channels
• mechanical forces like pressure

Question 47

How do ion channels help with facilitated diffusion?

Answer 47

• ion channels in facilitated diffusion are gated
• when the gates are open, the ions pass through the pore down a concentration gradient
• on the other hand, in a the closed state, the pore is plugged, preventing the passage of the ion

Question 48

How do pump proteins (for active transport) differ from channel proteins (for facilitated diffusion)?

Answer 48

• pump proteins use energy so they carry out active transport, whereas diffusion though channel proteins is passive
• pump proteins only move particles across the membrane in one direction, whereas particles can move in either direction through a channel protein
• pump proteins usually move particles against the concentration gradient, whereas facilitated diffusion through channel proteins is always down the concentration gradient

Question 49

How do pump proteins work?

Answer 49

Pump proteins are interconvertible between two different conformations. In one conformation, the transported particle can enter the pump from one side of the membrane to reach a central chamber or a binding site. The pump protein then changes to the other conformation, which allows the ion or molecule to pass out on the opposite side of the membrane. The pump protein returns to its original conformation. Energy is used to change the protein from one of the conformations (the more stable) to the other (the less stable), but the reverse change does not require energy. Most pump proteins use ATP to supply the energy required for active transport (making them ATPase pumps). Every cell produces its own ATP by cell respiration.

Question 50

How do carrier proteins work?

Answer 50

• Like channel proteins, carrier proteins are transmembrane proteins that play an important role in facilitated diffusion.
• The carrier protein binds to the solute molecules (molecules to be transported), undergoes a conformational change (change in secondary and tertiary structure) and transfers the molecule to the other side of the membrane
• Carrier proteins have sites specific for the solute or class of solutes to be transported and hence are highly specific

Question 51

What is an example for a type of pump protein?

Answer 51

One example is the GLUT (e.g. insulin(from pancreas to receptors on muscle and liver cells)) or glucose transporter, a carrier protein that helps in the transport of glucose into the red blood cell (RBC) down its concentration gradient

Question 52

Why are sodium potassium pumps necessary in the nervous system?

Answer 52

For a neuron to convey a nerve impulse, there must be concentration gradients of sodium and potassium ions across the membrane.

Question 53

How is the concentration gradient for sodium-potassium nerve impulses generated?

Answer 53

generated by active transport, using a sodium-potassium pump protein. This pump follows a repeating cycle of steps that result in three sodium ions being pumped out of the axon and two potassium ions being pumped in. Each time the pump goes around this cycle it uses one ATP to supply energy.

Question 54

How does the sodium potassium pump work?

Answer 54

1. The pump is open to the inside of the cell, so three Na+ ions can enter and attach to their binding sites, reducing the Na+ concentration inside.
2. ATP transfers a phosphate group to the pump, which causes a conformational change and closes the pump
3. The pump opens to the outside and the Na+ ions can exit, increasing the Na+ concentration outside the neuron
4. Two K+ ions from outside enter and attach their binding sites in the pump, reducing the K+ concentration outside
5. Binding of K+ causes release of the phosphate group, causing a conformational change, closing the pump
6. The pump opens to the inside and the K+ ions exit, increasing the K+ concentration inside; more Na+ ions can then enter

(THIS RESULTS IN A LOWER MEMBRANE POTENTIAL INSIDE THE AXON THAN OUTSIDE THE AXON)

Question 55

What is indirect active transport?

Answer 55

Indirect active transport or co-transport, where the movement of one solute down its concentration gradient drives the movement of the second solute against its concentration gradient

Question 56

How do sodium dependant glucose transporters work and why are they an example of indirect active transport?

Answer 56

Glucose absorption in cells depends on the Na+ ion concentration being greater outside than inside. The concentration gradient is maintained by active transport of Na+ ions out of the cell. Sodium-potassium pumps in the plasma membrane on the inner (basal) side of the cell transfer Na+ out of the cell towards nearby blood capillaries and transfer K+ ions in. Sodium-dependent glucose co-transport depends on energy from ATP, so is not passive. However it is not typical active transport because energy is not used directly by the co-transporter. This is called secondary or indirect active transport.

Question 57

What is direct active transport?

Answer 57

direct active transport is where the energy released by an exergonic reaction like the hydrolysis of ATP is used directly to transport molecules across the cell membrane (using ATPase pumps)

Question 58

What does active transport help with?

Answer 58

• take up essential nutrients- uptake of glucose from the lumen of the intestine to the epithelial cells lining the small intestine
• remove secretory or waste materials from the cells into the extracellular fluid
• maintain the right concentrations of ions in the cells; for example, active transport helps red blood cells maintain their internal sodium and potassium levels