AS Biology Term 1

Question 1

Define the terms: Polymer, Monomer and Macromolecule

Answer 1

Polymer: A chain of repeating, similar subunits.

Monomer: Subunits that join together to form a polymer.

Macromolecule: A giant molecule. E.g. polysaccharides, proteins

Question 2

What is a monosaccharide?

Answer 2

A monosaccharide is a sugar. Monosaccharides have a general formula of (CH2O)n and consist of a single sugar molecule. The main types of monosaccharides are trioses (3C), pentose (5C) and hexose (6C).

Question 3

Describe the structure of pentoses and hexoses

Answer 3

The chain of carbons is long enough to close up on itself and form a stable ring structure. Glucose is an important example as it is the most common monosaccharide. In glucose, the carbon atom 1 joins to carbon atom 5. The ring contains an oxygen and carbon atom 6 is not a part of the ring.

Question 4

Describe the difference between alpha and beta glucose.

Answer 4

In a glucose molecule, there is a hydroxyl (OH) group present on carbon atom 1. This hydroxyl group can be above or below the plane of the ring. If the group is below it is an alpha-glucose. If it is above it is a beta-glucose. The same molecule can switch between the two forms.

Question 5

Describe the roles of monosaccharides in living organisms

Answer 5

1. Common source of energy in respiration. This is due to the large amounts of carbon-hydrogen bonds, which when broken release large amounts of energy, which is transferred to help make ATP from ADP.
2. Monosaccharides are important as building blocks for larger molecules.

Question 6

Describe a disaccharide

Answer 6

They are sugars which are formed by two monosaccharides joined together by a glycosidic bond. Common disaccharides are maltose (glucose + glucose), sucrose (glucose + fructose) & lactose (glucose + galactose). Sucrose is the transport sugar in plants and is what we know as sugar. Lactose is found in milk.

Question 7

How are monosaccharides joined to form disaccharides?

Answer 7

The joining takes place by a process known as condensation. In such a reaction, two hydroxyl groups line up alongside each other. One hydroxyl group combines with a hydrogen atom from another hydroxyl group to form a water molecule. This forms an oxygen bridge between the two monosaccharides, thus forming a dissacharide. This bridge is called a glycosidic bond. The reverse reaction is hydrolysis, which is the addition of water. This reaction takes place when digesting polysaccharides and disaccharides to break the down into monosaccharides.

Question 8

What is a reducing sugar and how are they tested for?

Answer 8

A reducing sugar is a sugar that can reduce copper(II) ions in soluble blue copper sulfate to form an insoluble red copper oxide, containing copper(I), becoming oxidised in the process.

1. Add a known volume of the solution you are testing into a test tube
2. Add same volume of Benedict’s solution into test tube
3. Place in water bath for 5 mins
4. Observe colour change, based on the concentration of the reducing sugar the solution will turn from blue, to green, to yellow, to orange and finally, red.
5. Compare colour to previous tests done, with known concentrations of the reducing sugar, to estimate the concentration.

Question 9

What is a non-reducing sugar and what is the test for it?

Answer 9

Do not reduce copper(II) and would give a negative result for the Benedict’s test.

1. Add few drops of HCl to the solution being tested and place in water bath and heat to light boil for a minute. This will release free monosaccharides.
2. Benedict’s requires alkaline conditions and so, once the test tube has cooled, add few drops of sodium bicarbonate (NaHCO3) to neutralise the solution
3. Add Benedict’s reagent and heat in water bath for 5 minutes
4. Colour change should be the same as with reducing sugars.

Question 10

Describe a polysaccharide

Answer 10

A polysaccharide is a polymer which is made up of monosaccharide monomers through condensation reactions forming glycosidic bonds. Polysaccharides are not sugars and are ususally used for storage or strength. The storage polysaccharide in plants is starch, while in animals, it is glycogen.

Question 11

Describe differences in the structure of starch and glycogen

Answer 11

Starch: Mixture of two substances
- Amylose: Made up by condensation reactions which
occur between carbon atom 1 and carbon atom 4 of
successive glucose units creating 1,4 linked glucose
molecules. The chains are curved and coil up into
helical structures, making the final molecule more
compact.
- Amylopectin: Also made up of 1,4 linked alpha-glucose
molecules. However, chains are shorter than amylose
and branch out to the side through 1,6 linkages.

Glycogen: Made up of both 1,4 and 1,6 linkages, just like amylopectin. It is in fact more branched than amylopectin.

Question 12

Describe the structure of cellulose and cell walls

Answer 12

Cellulose is a polymer of beta-glucose. In beta-glucose, the hydroxyl group is above meaning one glucose molecule has to be turned upside down to link with one another. This creates straight chains of molecules. -OH groups in one molecule are weakly attracted to oxygen atoms in other molecules due to hydrogen bonds being formed. 60-70 cellulose molecules become tightly cross-linked to form bundles called microfibrils. Microfibrils are held together by hydrogen bonding, forming fibres. Cell walls have several layers of fibres running in different directions to increase strength.

Question 13

What is the difference between saturated and unsaturated in fatty acids?

Answer 13

Saturated: A fatty acid that contains the maximum number of hydrogen atoms. This means they only contain C-C bonds.

Unsaturated: A fatty acid that does not have the maximum possible number of hydrogen atoms. This means they contain double or triple bonds. Such fatty acids will have kinks in them cause by these bonds.

Question 14

What is a triglyceride?

Answer 14

The most common lipid. It is an ester formed between three fatty acids and an alcohol glycerol head. A condensation reaction takes place between the fatty acids and the glycerol whereby H2O is derived from the -COOH in the fatty acid and the -OH in the glycerol. The bond between the glycerol and the fatty acid is called an ester bond. These fatty acids are hydrophobic and therefore, triglycerides are insoluble in water.

Question 15

What are the roles of a triglyceride?

Answer 15

• Energy Reserves: Are even richer in carbon-hydrogen bonds than carbohydrates meaning they will yield more energy.
• Protection/Insulation: Covers areas such as the kidney and under the dermis, giving protection to vital organs and insulating warmth. Also insulate myelin sheath allowing for faster nerve transmission.
• Metabolic source of water: When oxidised, they are converted to carbon dioxide and water. Can be used by animals in dry habitats.

Question 16

Describe a phospholipid

Answer 16

One of the fatty acids that would be present in a triglyceride is replaced by a phosphate group. This phosphate group is polar and therefore, dissolves in water. However, the fatty acids are still hydrophobic. This is useful in the cell membrane where the phosphate head lies outside the membrane, in the watery solution, while the hydrophobic tails form an impermeable layer to hydrophilic substances. This forms a bilayer, of which the cell membrane is made of.

Question 17

Describe the test for lipids

Answer 17

1. 1 cm3 ethanol into the substance being tested. Lipids will dissolve into the ethanol.
2. Shake the test tube vigorously
3. If lipids are present, solution will turn milky white, if not, it will remain transparent.
   Such a colour change occurs due to the lipid molecules forming droplets throughout the liquid. These droplets reflect and scatter light causing a white, cloudy colour.

Question 18

What is an amino acid?

Answer 18

An amino acid is the monomer for proteins. There are 20 amino acids, 12 of which are non-essential (made in the body) and 8 essential (intaken in diet). Amino acids have a basic amine group (NH2), an alkyl or variable R group, and an acidic carboxylic acid group (-COOH).

Question 19

How is a dipeptide formed?

Answer 19

The -OH from the -COOH of one amino acid is lost with a -H from the -NH2 from another amino acid. A peptide bond is formed between the two amino acids.

Question 20

Describe the primary structure of a protein

Answer 20

The particular amino acids contained in the chain, and the sequence in which they are joined is called the primary structure.

Question 21

Describe the secondary structure of a protein

Answer 21

This is the effect different amino acids in a chain, that are not directly next to each other, have on each other. Different shapes of polypeptide chains are the result:

Alpha-Helix - A corkscrew shape to which a polypeptide chain may coil. This is due to the hydrogen bonding between the oxygen of the -CO- group of one amino to the hydrogen in -NH- four amino acids in front of it.

Beta-Pleated Sheet - A much looser, straighter shape. `

Question 22

Describe the tertiary structure of a protein

Answer 22

The folding of the secondary structures that take place on a polypeptide chains. It is a 3-dimensional shape.

Question 23

Describe the types of bonds that keep the exact shape of proteins

Answer 23

Hydrogen Bonds - form between strongly polar groups e.g. -NH, -CO and -OH groups

Disulfide bonds - form between cysteine molecules. These are strong, covalent bonds that occur between sulfur atoms. Can be broken by reducing agents.

Ionic Bonds - form between ionised amine (NH3+) and ionised carboxylic acid (COO-) groups. Can be broken by pH changes.

Hydrophobic Interactions - occur between non-polar R groups and are very weak. Groups stay together due to being surrounded by their watery environment.

Question 24

Describe the quaternary structure of a protein

Answer 24

The association of different polypeptide chains. The chains are held together by the same 4 bonds as the tertiary structure.

Question 25

What is the difference between globular and fibrous proteins?

Answer 25

Globular:
- Transportable, soluble
- 3D structure
Fibrous
- Structural Role
- Usually insoluble
- Protein consisting of long parallel chains with cross links (Hydrogen Bonding)

Question 26

Describe the test for Proteins

Answer 26

1. Add 2 cm3 sodium hydroxide

2. Add 2 cm3 copper sulfate, if protein is present, will turn lilac, if not, will turn blue.

Question 27

Describe the test for starch

Answer 27

1. Add 5 drops of iodine solution to substance being tested.
   - If starch is present, will turn blue-black. If not, will turn orange-brown.

Question 28

Define polarity and electronegativity

Answer 28

Electronegativity: A measure of the tendency of an atom to attract a bonding pair of electrons.

Polarity: The separation of electric charge leading to a molecule with positively and negatively charged ends.

Question 29

Describe the properties of water

Answer 29

Solvent: Excellent solvent as water is polar meaning other polar substances and ions can dissolve in it. This is due to the polarity of water meaning it surrounds the charged particles, thus, dissolving them. This makes water an important transport medium in organisms.

Specific Heat Capacity: Amount of energy required to increase the temperature of 1kg of water by 1°C. Water has high heat capacity allowing body temp. to remain constant and restrict fluctuation of water body temps.

Latent Heat of Vaporisation: Measure of heat energy needed to vaporise a liquid. High due to strong hydrogen bonds. Acts as a cooling mechanism in organisms as large amounts of heat energy is transferred from body to sweat allowing sweat to evaporate and remove the heat energy.

Cohesion/Adhesion: Strong hydrogen bonds and polarity means that water sticks to other substances as well as to other water molecules. This is important in transpiration in plants and in creating surface tension in the water for organisms to live on.

Metabolism/Reagent: Water is crucial in hydrolysis reactions in which water is added to break up substances. Used in digestion. Also used as a reagent in photosynthesis.

Density: Water acts anomalously as it is most dense at 4°C as a liquid and then expands from 4°C to 0°C. This means water does not freeze from the bottom up and rather, forms on the surface first, creating an insulating layer below. This also allows for ocean currents as warmer water is lighter and colder water is heavier, meaning water circulates the Earth which allows nutrients to circulate the ocean.

Question 30

Describe the structure of a nucleotide

Answer 30

A nucleotide contains a phosphate group, a pentose sugar, and a nitrogen base. In DNA, this sugar is a deoxyribose while in RNA, it is a ribose sugar. The difference between the two is that ribose has an -OH on the 2nd Carbon while deoxyribose only has an -O

Question 31

List and explain the differences between DNA and RNA

Answer 31

Number of Strands: DNA has 2 strands that are anti-parallel. RNA only has one strand.

Length: DNA is long, RNA is short

Bases: DNA has A,T,G,C while RNA has A,U,G,C

Question 32

Which bases bond to which?

Answer 32

In DNA: A=T
G≡C
In RNA: A=U
G≡C

Question 33

Describe the types of nitrogen bases

Answer 33

• There are equal amounts of purines and pyrimidines
  Purines bases: have two closed carbon rings - Adenine, Guanine
  Pyrimidine bases: have one closed carbon ring - Thymine, Uracil, Cytosine

Question 34

Describe an enzyme

Answer 34

Enzymes are protein molecules which have specific roles in the body. They are biological catalysts, chemical agents that accelerate a reaction without being used up. Enzymes decrease the activation energy required for a reaction to occur. Enzymes are selective as there are only certain reactions which each enzyme will catalyse. Enzymes that operate inside the cell are intracellular enzymes while enzymes that operate outside the cell are extracellular enzymes.

Question 35

Describe the different theories of enzyme-substrate bonding

Answer 35

Lock and Key theory: The cleft in the enzyme called the active site has a complementary shape to a substrate, the reactant an enzyme acts upon. The substrate is held in place by temporary bonds which form between the substrate and some of the R groups in the enzyme’s amino acids. The combined structure is the enzyme-substrate complex. In this theory, the enzyme is specific to the substrate.

Induced Fit Theory: Adds the idea that the enzyme and sometimes the substrate can change shape slightly as the substrate enters the enzyme, allowing for a perfect fit. This makes the catalysis more efficient.

Question 36

Describe factors that affect enzyme action

Answer 36

Enzyme Concentration: Greater enzyme concentration means more active sites available for substrates to react with. Substrate concentration will be a limiting factor as eventually, increasing enzyme concentration will have no effect to the rate of reaction as all substrates are reacting in the enzymes.

Substrate Concentration: Greater substrate concentration means the enzyme’s active site can bind with a substrate more often. Eventually, however, all enzymes would be filled up with substrates and so increasing substrate concentration would have no effect.

Temperature: At low temperatures, molecules move slowly as they have low kinetic energy. This means enzyme substrate collisions are less frequent. As temp. increases, frequency of collisions increase as well as the number of successful reactions as more enzymes and substrate collide with the required activation energy. Above a certain, optimum temperature, the structure of the enzyme molecule vibrates so energetically that some of the bonds holding the enzyme molecule in its shape begin to break. The enzyme eventually loses its shape and activity and is denatured.

pH: Hydrogen ions can interact with R groups in the enzyme by affecting ionisation and the ionic bonding between groups which affect the three-dimensional arrangement of the enzyme. This means the substrate would no longer be able to fit into the active site. A pH very different from the optimum can cause denaturation.

Question 37

Describe the types of inhibitors

Answer 37

Competitive:

• Inhibitors have similar shape to substrate molecules
• Inhibitors compete with substrate to bind to the active site. No reaction takes place.
• Can be reversed by increasing substrate concentration

Non-competitive:

• Bind to enzyme away from active site (bind to allosteric site)
• Causes active site to change shape so substrate molecules can no longer bind to it.
• Different shape to active site and so are not competing with the substrate
• Increasing substrate concentration will have no effect.

Question 38

Describe Vmax and Km

Answer 38

Vmax: The point at which all enzyme molecules are bound to substrate molecules. In theory, this value is at an infinite substrate concentration and so to measure it, a double reciprocal graph is used (1/V vs 1/S) as the Y-intercept would be 1/Vmax

Km: The Michaelis-Menten constant or Km is the substrate concentration where half of the active sites are occupied by substrate. Km is a measure of the enzyme’s affinity (the enzyme’s ability to join with the substrate) as at higher affinites, substrate concentration required is lower. Therefore, the lower the Km, the higher the affinity and vice versa. Km can be measured on the double reciprocal graph as the x-intercept would be -1/Km

Question 39

Describe the way in which enzymes are immobilised

Answer 39

• Enzyme is mixed with a solution of sodium alginate. Little droplets of this mixture are added to a solution of calcium chloride which acts as a hardening agent. Jelly beads containing the enzyme are formed.

Question 40

List advantages and disadvantages of using immobilised enzymes

Answer 40

Adv:
- Enzymes are kept separate from product meaning purification is not required, means cost is decreased.
- Enzymes are immediately available for reuse, no need for complicated extraction process.
- More stable as enzymes are now more tolerant to temperature and pH changes.
Disadv:
- Initially more expensive
- Slower reaction rates as molecules do not mix freely with substrate molecules.
- Contamination is difficult to deal with
- Enzymes may detach
- May affect shape and function of enzyme.

Question 41

Define resolution and magnification

Answer 41

Resolution: The ability to distinguish between two points on an image i.e. the clarity

Magnification: The enlargement factor (eye lens x objective lens)

Question 42

What is the formula for magnification?

Answer 42

Magnification = Observed size of image / actual size

M = I/A

Question 43

Describe TEM and SEM

Answer 43

Transmission Electron Microscope:

• Beam of electrons pass through specimen being viewed
• Can only use thin sections to allow electrons to pass through allowing to see inside of the cell.

SEM:

• Electron beam used to scan surfaces of structures
• Only reflected beam is observed
• Low resolution (3-20 nm)

Question 44

List differences between light microscope and electron microscope

Answer 44

Light microscope:

• Simple preparation
• Vacuum not required
• Specimen can be alive or dead
• Low magnification x2000
• Stains such as iodine needed to make specimens visible
• Lower resolution

Electron Microscope:

• Complex preparation
• Vacuum required
• Dead specimen required
• Electron dense chemical staining (usually heavy metals such as osmium, gold or lead)
• Better magnification x500 000
• Better resolution

Question 45

Describe the role of the nucleus

Answer 45

Chromosomes are in a loosely coiled state known as chromatin within the nucleus. Chromosomes contain DNA which is organised into functional units called genes. The nucleus is surrounded by a membrane known as the nuclear envelope. The nuclear envelope’s outer membrane is continuous with endoplasmic reticulum. It also has small pores called nuclear pores which allow and control exchange between the nucleus and the cytoplasm. There is also a nucleolus within the nucleus which produces ribosomes.

Question 46

Describe the endoplasmic reticulum

Answer 46

Membranes which form a system of flattened compartments called sacs. There are two types of endoplasmic reticulums:

1. Rough ER: Covered with ribosomes which are the site of protein synthesis. Proteins made by ribosomes on the rough ER move through the sacs and are often modified. Small sacs called vesicles break off from the ER and join to form the Golgi Body.
2. Smooth ER: Makes lipids and steroids

Question 47

Describe the Golgi Body

Answer 47

Stack of flattened sacs which is being formed at one end from vesicles which bud off from the ER and broken down on the other end to form Golgi vesicles. The Golgi apparatus processes and sorts molecules ready for transport in Golgi vesicles either to other parts of the cell or out of the cell.

Question 48

Describe lysosomes

Answer 48

Spherical sacs surrounded by a single membrane and has no internal structure. They are about 0.1 - 0.5 μm in diameter. They contain digestive (hydrolytic) enzymes which must be kept separate to prevent damage to the cell. They are responsible for the breakdown of unwanted structures such as old organelles.

Question 49

Describe the structure of mitochondria

Answer 49

About 1μm in diameter. The mitochondria are surrounded by two membranes, the inner of which fold to form finger-like cristae which project into the matrix (inner fluid). The space between the membranes is the intermembrane space.

Question 50

Describe the function of mitochondria

Answer 50

They carry out aerobic respiration. The energy which is produced during respiration is transferred to molecules of ATP (adenosine triphosphate). Once ATP is produced, it spreads rapidly to all parts of the cell where energy is needed, and so it is a small, water-soluble molecule. Energy is released by breaking ATP to ADP (adenosine diphosphate).

Question 51

Describe endosymbiont theory

Answer 51

This is the idea that mitochondria and chloroplast are in fact ancient bacteria which have a symbiotic relationship with larger cells such as animal and plant cells.

Question 52

Describe the cell surface membrane

Answer 52

Extremely thin (about 7 nm). It is partially permeable and controls exchange between the cell and its environment

Question 53

Describe microvilli

Answer 53

Finger like extensions of the cell surface membrane which increase surface area of the membrane. Useful in processes such as absorption in the gut.

Question 54

Describe microtubules and microtubule organising centres

Answer 54

• Long, rigid and hollow tubes found in cytoplasm about 25 nm in diameter. They make up the cytoskeleton which determine the shape of the cell. Made up of tubulin which has two forms: alpha-tubulin and beta-tubulin. These tubulin molecules combine to form dimers which are joined up end to end to form protofilaments. 13 protofilaments line up alongside each other in a ring to form a hollow cylinder which is a microtubule.
• The assembly of microtubules from tubulin molecules are controlled in special locations in the cell called microtubule organising centres or MTOC’s.

Question 55

Describe a chloroplast

Answer 55

Chloroplasts carry out photosynthesis. Chloroplasts use light energy from the sun to convert water and carbon dioxide to glucose and oxygen. Chloroplasts have a complex system of membranes which consists of fluid filled sacs called thylakoids which spread out like sheets in three dimensions. In places, the thylakoids form flat, disc-like structures that stack up many layers deep. They are structures known as grana. The fluid in the chloroplast in which sugars may be stored is the stroma. The chloroplasts also have 70s ribosomes and a circular DNA.

Question 56

List and describe structures always present in a prokaryote

Answer 56

• Cell Wall: contains murein, a peptidoglycan
• Cell Surface Membrane
• Cytoplasm
• Circular DNA
• Ribosomes

Question 57

List and describe structures sometimes present in a prokaryote

Answer 57

• Flagellum: Used for locomotion
• Capsule: Additional protection
• Plasmid: Small circle of DNA, several may be present
• Pili: For attachment to other cells or surfaces and used in sexual reproduction.

Question 58

Describe differences between eukaryotes and prokaryotes

Answer 58

Prokaryotes:

• Average cell diameter: 0.5 - 5 μm
• DNA is circular and lies free in cytoplasm, it is naked
• 70s ribosomes
• no ER
• Very few cell organelles and no separate membrane bound compartments unless formed by infolding of cell membrane
• Cell wall present, contains murein, a peptidoglycan.

Eukaryotes:

• Cells up to 40 μm in diameter
• DNA is not naked and not circular. It is contained in the nucleus
• 80s ribsomes
• ER present
• Many organelles present, most have at least one membrane
• Cell wall sometimes present, cellulose or lignin in plants, chitin in fungi.

Question 59

Describe a virus

Answer 59

A very simple structure only consisting of a self replicating molecule of DNA or RNA which acts as a genetic code, and a protective coat of protein molecules called a capsid. Each separate protein molecule is called a capsomere. All viruses are parasitic as they can only reproduce by infecting and taking over living cells. Viruses are very small at about 20 - 300 nm.

Question 60

Describe the structure of membranes

Answer 60

• About 7nm wide
• Modeled by the fluid-mosaic model: Fluid as phospholipids and proteins can move about through diffusion, mosaic due to the pattern produced by the scattered protein molecules when the membrane is viewed from above.

Question 61

Describe the features of the fluid mosaic model

Answer 61

• Kinks in unsaturated fatty acid tails mean the membrane is more fluid as phospholipids fit together more loosely
• The longer the fatty acid tail, the less fluid the membrane becomes
• Increasing temperature increases fluidity. Organisms that cannot regulate temperature increase the proportion of unsaturated fatty acid tails if temperature decreases.

Question 62

Describe the types of proteins found in or around cell membranes

Answer 62

Intrinsic/Integral Proteins: Embedded within membrane. Can be found on inner or outer layer of membrane, or most commonly, spanning the entire membrane (transmembrane proteins). Hydrophobic regions which are found within the cell membrane contain many alpha-helical chains.

Extrinsic/Peripheral Proteins: Found on inner or outer surface of membrane. Many are bound to intrinsic proteins or phospholipids and other molecules.

Question 63

Describe the roles of phospholipids in membranes

Answer 63

• Tails are non-polar so it is difficult for polar molecules or ions to pass through
• Can be chemically modified to act as signalling molecules which activate other molecules.
• Can be hydrolysed to release glycerol related molecules.

Question 64

Describe the role of cholesterol in membranes

Answer 64

• At low temperatures, cholesterol increases fluidity by preventing close packing of phospholipids. At high temperatures, interactions between cholesterol and phospholipids stabilises the membrane that would otherwise be too fluid.
• Provides mechanical stability, as without it, the membrane would quickly break and burst open.

Question 65

Describe the role of glycoproteins and glycolipids

Answer 65

• molecules combined with carbohydrates that project into fluid surrounding cell where H-bonds are formed between water and carbohydrates which stabilise the cell structure.
• Carbohydrate chains form a sugary coating known as the glycocalyx which help the glycoproteins and glycolipids to act as receptor molecules.

Question 66

Describe the major types of receptors on the cell membrane

Answer 66

Those that are affected by signalling Molecules: Co-ordinates activities of a cell. This receptor recognises messenger molecules such as hormones or neurotransmitters.

Those that are involved in endocytosis: Bind to molecules which are structures to be engulfed by cell surface membranes.

Those that are involved in cell adhesion: Act as antigens allowing cell-cell recognition. Each cell has a specific, individual antigen.

Question 67

Describe the roles of the proteins within cells and cell membranes

Answer 67

• Transport Proteins: Provide hydrophilic channels or passageways for ions and polar molecules to pass through the cell membrane (Channel Proteins, Carrier Proteins).
• Enzymes
• Some proteins bind to cytoskeleton and maintain the shape of the cell.

Question 68

Define cell signalling and state the process of cell signalling.

Answer 68

The process of getting a message from one place to another.
Stimulus/Signal → Receptor (Transduction or conversion of signal to a message that is transmitted occurs here) → (Transmission of message/signal) → Target/Effector → Response.

Question 69

Define a signalling cascade and describe the cell signalling pathway

Answer 69

A signalling cascade is a sequence of events triggered by a G protein which acts as a switch to bring about the release of a second messenger.

Signalling Pathway: Signal reaches a protein receptor causing the receptor to change shape and pass the message to the inside of the cell. This often activates a G protein which acts as a switch, releasing a second messenger. The second messenger activates an enzyme which activates further enzymes increasing amplification at each stage until there is a change in cell metabolism.

Question 70

Describe alternative ways in which receptors can alter cell activity other than using second messengers

Answer 70

• Opening an ion channel which results in a change of membrane potential.
• Acting directly on a membrane-bound enzyme

Question 71

Define diffusion and explain factors that affect diffusion

Answer 71

• It is the net movement of molecules or ions from a region of high concentration to a region of lower concentration down a concentration gradient as a result of the random motion of particles.

Factors:

• Steepness of concentration gradient: Particles move to find equilibrium more quickly if gradient is steeper.
• Temperature: Particles move faster so diffusion is faster
• Surface Area: Greater surface area means more particles can cross at any moment so diffusion is faster.
• Nature of particles: Heavier particles diffuse slower.

Question 72

Define facilitated diffusion and describe the proteins involved

Answer 72

• The diffusion of substances through transport proteins in a cell membrane.
• Channel proteins and carrier proteins are used in facilitated diffusion.
• Channel Proteins: Water filled pores that are gated (protein on inside surface of membrane can move to open or close part of protein on inside surface of membrane)
• Carrier Proteins: Can flip between two shapes and binding site is alternately open to side inside membrane.

Question 73

Define osmosis, water potential, and describe the factors that affect water potential

Answer 73

• Osmosis is the net movement of water molecules from a region of high water potential to a region of lower water potential, down a water potential gradient, through a partially permeable membrane as a result of random motion.
• Water potential: The tendency of water to move out of a solution. Pure water has a water potential of 0 so water potential of a solution is negative.
• Factors:
  1. Solute potential: extent to which solute molecules decrease the water potential of the solution
  2. Pressure potential: the contribution of pressure to water potential.

Question 74

Describe osmosis in plant and animal cells

Answer 74

Animal Cell:
Hypertonic Solution (Water potential inside cell is higher than outside) - Cell shrinks

Isotonic Solution (Water potential is same inside cell and outside) - Cell remains normal

Hypotonic Solution (Water potential is higher outside the cell than inside) - Cell swells and bursts

Plant Cell:
Hypotonic Solution: Water enters cell until cell is turgid. Doesn’t burst due to cell wall which matches the pressure of the water.

Isotonic Solution: Cell remains normal

Hypertonic Solution: Cell is plasmolysed (protoplast has torn away from cell wall.) Point at which pressure potential is 0 and plasmolysis is about to occur is incipient plasmolysis.

Question 75

Define active transport

Answer 75

• It is the movement of molecules or ions through transport proteins across a cell membrane against the concentration gradient.
• The energy required for this is supplied by the conversion of ATP to ADP. This is usually done on the cell membrane through receptor sites on proteins which act as an ATPase enzyme which bring about the hydrolysis of ATP to ADP.

Question 76

Explain the sodium-potassium pump

Answer 76

The Na+ - K+ pump use about 30% of the cells energy and 70% of the energy in nerve cells. The pump uses a single molecules of ATP to pump 3 sodium ions out of the cell while allowing 2 potassium ions into the cell. This causes the inside of the cell to be more negative than the outside creating a potential difference.

Question 77

Describe bulk transport

Answer 77

It is the transport of large quantities of materials into or out of cells as opposed to individual molecules.
- Endocytosis: bulk movement of liquids or solids into the cell by the infolding of the cell surface membrane to form vesicles containing the substance. (Phagocytosis: Bulk uptake of solid material, Pinocytosis: Bulk uptake of liquid )

• Exocytosis: Bulk movement of solids or liquids out of a cell by fusion of vesicles containing the substance with the cell surface membrane.

Both are active processes and require energy in the form of ATP.