3.2 Cells 🦠 Flashcards

Question

Identify the differences between Eukaryotic and Prokaryotic cells

Answer 1

• Eukaryotes are larger and often multicellular while prokaryotes are smaller and always unicellular • Eukaryotes have organelles including a nucleus while prokaryotes have a cytoplasm that lacks membrane-bound organelles (no nucleus) • Eukaryotes have linear DNA associated with histones while prokaryotes have circular DNA (plasmids) not associated with proteins • Eukaryotes have larger 80s ribosomes while prokaryotes have smaller 70s ribosomes • Eukaryotes divide my mitosis and meiosis (sexual or asexual reproduction) while prokaryotes divide by binary fission (always asexual) • Eukaryotes have cellulose (plants) and chitin (fungi) cell walls while prokaryotes have murein, a glycoprotein, cell walls • Prokaryotes have capsules, sometimes plasmids/ cytoskeleton/flagella while eukaryotes have no capsule, plasmids or flagella and always have a cytoskeleton

Answer 2

• Contain a cell membrane, cytoplasm and ribosomes (don’t count as an organelle as they aren’t membrane bound)

Answer 3

• Structure: Small ring of DNA • Function: carries non-essential genes and can be exchanged between bacterial cells via conjugation

Answer 4

• Structure: microscopic hair-like organelle • Function: Rotating tails that propels the organism

Answer 5

• Structure: Polysaccharide layer • Function: Prevents desiccation, acts as a food reserve, sticks cells together and provides mechanical protection against phagocytosis and external chemicals

Answer 6

• They’re acellular because they have no cell-surface membrane or cytoplasm • They’re non-living beacuse they have no metabolism and cannot independently respire/move/replicate/excrete without being inside a host cell

Answer 7

• Linear genetic material (DNA/RNA) and viral enzymes (e.g. reverse transcriptase) surrounded by a capsid and no cytoplasm • The capsid’s a protein coat made of capsomeres that protects nucleic acid from degradation by restriction endonucleases. • Attachment proteins enable the viral particle to bind and enter host/inject their genetic material • Enveloped viruses are further surrounded by matrix protein and an envelope derived from the cell membrane of the host cell with attachment proteins on the surface. These enable viral particles to bind to complimentary sites on host cells (entry via endosymbiosis)

Answer 8

• Virus uses attachment proteins on its surface to bind to complementary receptor proteins on the surface of a host cell and injects DNA or RNA into the host cell • The host-cell uses its nucleic acid and ribosomes to produce new viral particles which are released by: 1. The host cell bursts open, releasing all the new viral particles 2. Viral particles leave individually through the host cell membrane via ‘budding’, taking a section of the membrane with them (forming an enveloped virus)

Answer 9

• NEV are more virulent and cause host cell lysis more often • EV have a membrane surrounding the capsid, and can use the host-cells membrane to assemble their own membrane, avoiding lysis and escaping the hosts immune system

Answer 10

A symbiotic relationshop where one organism lives inside the other

Answer 11

A close and long-term biological interaction between two different species, called symbionts that can be mutualistic, commensalistic or parasitic

Answer 12

• Microscopes • Cell fractionisation

Answer 13

• How many times bigger the image of the specimen observed is compared to the actual size of the specimen • Equation: Size of image Magnification = ———————- Actual size

Answer 14

The ability to distinguish between two points

Answer 15

• Something present in a sample that’s been artificially introduced as a result of processing the sample. • They’re more likely to be produced from an electron microscope as there’s more steps involved

Answer 16

• Light Microscope • Transmission Electron Microscope • Scanning Electron Microscope

Answer 17

• Lenses focus rays of light and magnify the view of a thin slice of specimen • Different structures absorb different amounts and wavelengths of light • Reflected light is transmitted from the observer via the objective lens and eyepiece

Answer 18

• Obtain a thin section of tissue (e.g. using ultratome or by maceration) • Place the tissue in a drop of water • Stain the tissue on a slide to make structures visible • Add a code slip using a mounted needle at 45° to avoid trapping air bubbles • Place micrometer on stage to calibrate eyepiece graticule • Line up the scales on the graticule and micrometer and count how many graticule divisions are in 100um on the micrometer • The length of one eyepiece division = 100um divided by the number of divisions • Use calibrated values to calculate actual length of structures

Answer 19

+ Colour image + Can show living structures + Affordable apparatus -2D image -Lower resolution than electron microscopes because light has a longer wavelength than electrons so you can’t see ultrastructure

Answer 20

• Passes a high beam of electrons through a thin slice of specimen • More dense structures appear darker as they absorb more electrons • Focus images onto fluorescent screen or photographic plate using magnetic lenses

Answer 21

+ Electrons have a shorter wavelength than light so it has a higher resolution so ultrastructure is visible + High magnification (X500,000) -2D image -Requires a vacuum, so can’t show living structures -Extensive preparation may cause artefacts -No colour image

Answer 22

• Focuses a beam of electrons into a specimens surface using electromagnetic lenses • Reflected electrons hit a collecting device and are amplified to produce an image on a photographic plate

Answer 23

+ Electrons have a shorter wavelength than light so it has a higher resolution so ultrastructure is visible + 3D image -Requires a vacuum, so can’t show living structures -No colour image -Only shows outer surface

Answer 24

• Cell fractionation is the process of breaking down cells to isolate different organelles that’s used to study the cell’s structure and function Process: • Tissues are cut up and placed in a cold, isotonic, buffer solution • The cell and solution and placed in a homogeniser where tissues are minced and homogenised to break open cells and release organelles, producing a homogenate • Homogenate is filtered to remove whole cells and cell debris • Perform differentiation centrifugation in a centrifuge. The homogenate spins in the centrifuge and fragments are separated according to density by centrifugal force • The most dense organelles form a pellet at the bottom in the order of sedimentation (density): (nucleus, chloroplast, mitochondria, lysosomes, ER, ribosomes) • Filter off the supernatant and spin again at a higher speed

Answer 25

• Cold to denature (protease) enzymes so organelles aren’t broken down • Isotonic so organelles aren’t damaged from gaining or losing water • Buffer solution maintains pH to preserve organelles

Answer 26

• Within multicellular organisms, not all cells retain the ability to divide; Eukaryotic cells that do retain the ability to divide show a cell cycle • DNA replication occurs during interphase of the cell cycle • Mitosis is the part of the cell cycle in which a Eukaryotic cell divides to produce two daughter cells, each with identical copies of DNA produced by the parent cell during DNA replication • Finally, cytokinesis (division of the cytoplasm) usually occurs, producing two new cells

Answer 27

Split into three stages; • Gap phase 1: cell growth and synthesis of protein and organelles • Synthesis phase: DNA synthesis • Gap phase 2: More growth and synthesis of protein and organelles as well as preparation for cell division

Answer 28

• Mitosis is the part of the cell cycle in which a Eukaryotic cell divides to produce two daughter cells, each with identical copies of DNA produced by the parent cell during DNA replication, it’s split into four stages: Prophase, Metaphase, Anaphase and Telophase • Mitosis has multiple roles: • The growth of multicellular organisms • Replacement of cells and repair of tissues • Asexual reproduction

Answer 29

• Chromosomes condense and are visible when stained, consisting of two sister chromatids (containing one DNA molecule each) joined at the centromere • The two centrosomes (replicated in the G2 phase just before prophase) move towards opposite poles of the nucleus and spindle fibres unravel from them • The nuclear envelope (nuclear membrane) breaks down into small vesicles

Answer 30

• Chromosomes line up at the equator of the metaphase plate so they’re equidistant to the two centrosome poles • Spindle fibres (protein microtubules) reach the chromosomes and attach to the centromeres

Answer 31

• The sister chromatids seperate at the centromere • Spindle fibres begin to shorten and pull the separated sister chromatids (now called chromosomes) to opposite poles of the cell

Answer 32

• Chromosomes arrive at opposite poles and begin to decondense • Nuclear envelopes begin to reform around each set of chromosomes and the spindle fibres break down • The cell is splitting in two

Answer 33

• The separation of the parent cell into two genetically identical daughter cells once a new nucleus has completely reformed at each like if the parent cell after telophase • In animals, a ‘cleavage furrow’ forms and separates the daughter cells but in plants a ‘cell plate’ forms at the metaphase plate. Once it reaches the walls of the parent cell, new cell walls are produced, separating the new daughter cells

Answer 34

• The number of cells undergoing mitosis • Mitotic index is useful for: 1. Determining the speed of growth 2. Determining when a tissues becoming cancerous 3. Assessing the efficacy of cancer treatment Number of cells with visible chromosomes Mitosis index = —————————————— Number of cells (total)

Answer 35

(Cells in that phase ➗ total cells in mitosis) ✖️ total time taken for 1 cell cycle

Answer 36

• Mitosis is a controlled process. Uncontrolled cell division can lead to the formation of tumours and cancers • This can be due to mutations of genes that control cell division; almost half of all people with cancer possess a mutated P53 gene that helps control cell growth • Proto-oncogenes stimulate cell division and tumour suppressor genes stop cell division. Mutated genes that cause cancer are called oncogenes

Answer 37

• Benign tumours are slow growing and don’t spread to other parts of the body • Malignant tumours are fast growing, cancerous tumours that invade neighbouring tissue. When a tumour moves from a primary to a secondary location in the body this is known as metastasis

Answer 38

• Many cancer treatments are directed at controlling the rate of cell division by disrupting the cell cycle -> E.g. Chemotherapy drugs such as Methotrexate that inhibits synthesis of DNA nucleotides and Vincristine and Taxol that prevent the formation of the mitotic spindle • However these treatments don’t distinguish between normal cells and tumour cells so they’re both affected. Tumour cells for divide more quickly so they’re more likely to be killed.

Answer 39

• An agent that causes cancer, for example: UV light, X-rays and tar in tobacco smoke

Answer 40

• Cell division in prokaryotic cells that involves: • The replication of the circular DNA and plasmids • The division of the cytoplasm to produce two daughter cells, each with a single copy of the circular DNA and a variable number of copies of plasmids

Answer 41

There’s no nuclear envelope to breakdown and there are no spindle fibres present

Answer 42

Structure: • Phospholipid bilayer composed of two layers of phospholipids where the hydrophilic phosphate head faces outwards and a hydrophobic lipid tail faces inwards described by the Fluid mosaic model Function: • Creates an enclosed space separating the internal cell environment from the external environment in every cell • Intracellular membranes help form organelles, control exchange of materials and act as an interface for communication

Answer 43

• If they’re spread over water they form a phospholipid monolayer • If they’re mixed with water they form spheres with phosphate heads facing outwards called a micelle. Alternatively, two-layered structures may form sheets called phospholipid bilayers

Answer 44

• First outlined in 1972 • Describes cell membranes as fluid because phospholipids and proteins can move around via diffusion. Phospholipids mainly move sideways within their own layer, while proteins move like icebergs in the sea, while some are stationary • Describes cell membranes as mosaics because the scattered pattern produced by proteins within the phospholipid bilayer viewed from above looks like a mosaic • Explains how: 1. Biological molecules are arranged to form cell membranes 2. Passive and active movement between cells and surroundings 3. Cell-to-cell interactions 4. Cell signalling

Answer 45

Structure: • Phospholipids are made up of a polar, hydrophilic phosphate head that’s soluble in water and a non-polar, hydrophobic lipid tail that’s insoluble in water. They can contain glycolipids, glycoproteins, proteins and cholesterol may also be present Function: • Act as a barrier to most water-soluble substances • The non-polar fatty acid tails prevent polar molecules or ions passing across the membrane ensuring water-soluble molecules can’t leak out of the cell and unwanted water- soluble molecules can’t get in • Can be chemically modified to act as signalling molecules. This happens by: • Moving within the bilayer to activate other molecules (e.g. enzymes) • Being hydrolysed which releases smaller water-soluble molecules that bind to specific receptors in the cytoplasm • Form compartments that provide the basic structure of organelles

Answer 46

Structure: • Proteins with carbohydrate chains attached found in the outer phospholipid monolayer Function: • Act as receptor molecules for cell signalling: 1. Signalling receptors for hormones and neurotransmitters 2. Receptors involved in endocytosis 3. Receptors involved in cell adhesion and stabilisation as the carbohydrate part can form hydrogen bonds with water molecules surrounding the cell • Act as cell markers of antigens for cell-to-cell recognition • Binding cells together

Answer 47

Structure: • Lipids with carbohydrate chains attached found in the outer phospholipid monolayer Function: • Act as receptor molecules for cell signalling: 1. Signalling receptors for hormones and neurotransmitters 2. Receptors involved in endocytosis 3. Receptors involved in cell adhesion and stabilisation as the carbohydrate part can form hydrogen bonds with water molecules surrounding the cell • Act as cell markers of antigens for cell-to-cell recognition

Answer 48

• Intrinsic (integral) proteins, embedded in the membrane and most commonly span the entire membrane (transmembrane proteins E.g. transport proteins) • Extrinsic (peripheral) proteins are found on the inner or outer surface of the membrane

Answer 49

• They’re specific to an ion or molecule and allow the cell to control what enters and leaves • They create hydrophilic channels to allow ions and polar molecules to travel through the membrane • Can either be channel or carrier proteins

Answer 50

Structure: • Steroid molecule made up of hydrophobic tails and hydrophilic heads, similarly to the phospholipid bilayer but are absent in prokaryotes Function: • Fits between phospholipid molecules orientated in the same way and regulates the fluidity of the membrane. It does this by stopping them packing too closely together when temperatures are low, preventing freezing and fracturing. They bind to hydrophobic tails of phospholipids at higher temperatures causing them to pack more closely together, stabilising the membrane, stopping it becoming too fluid • Contributes to the impermeability of the membrane to ions • Increases mechanical strength and stability preventing breaking down and bursting

Answer 51

• Membranes become less fluid because: - There’s an increased proportion of saturated fatty acid chains as they pack tightly together creating high intermolecular forces between them. - Temperature decreases as the molecules have less energy and therefore aren’t moving freely, so the structures more closely packed • Membranes become more fluid because: - There’s an increased proportion of unsaturated fatty acid chains that are bent so are less tightly packed, decreasing intermolecular forces - At higher temperatures, molecules have more energy and move more freely increasing membrane fluidity

Answer 52

Cell surface membranes are partially permeable, so substances can cross them by: • Simple diffusion • Facilitated diffusion • Osmosis • Active transport • Co-transport

Answer 53

• The net movement of molecules or ions from a region of high concentration to a region of low concentration down a concentration gradient • The random movement is caused by the natural kinetic energy of the molecules or ions that tend to reach equilibrium where they’re evenly spread in a given volume of space as a result of diffusion • Fick’s law states that the rate of diffusion is proportional to: Concentration gradient x surface area —————————————————— Thickness of the exchange surface

Answer 54

Steepness if the concentration gradient • A greater difference in concentrations means a greater difference in the number of molecules passing from each direction and therefore a faster rate of diffusion Temperature • Molecules and ions have more kinetic energy at higher temperatures so they move faster, resulting in a higher rate of diffusion Surface Area • The greater the surface area, the more molecules that can cross at any one moment, increasing the rate of diffusion • Surface area of cell membranes can be increased by folding (e.g. Microvilli in the intestine/cristae in mitochondria) Properties of molecules or ions • Large molecules diffuse more slowly than small molecules as they require more energy to move • Uncharged and non-polar molecules diffuse directly across the phospholipid bilayer. Non- polar molecules diffuse more quickly than polar ones because they’re soluble in the non-polar phospholipid bilayer

Answer 55

• The movement of molecules or ions from a region of high concentration to a region of low concentration requiring the aid of certain proteins (there are two main types: channel and carrier proteins) • Certain substances can only cross the phospholipid bilayer with the help of certain proteins, including large, polar molecules such as glucose and amino acids and ions such as sodium (Na+) and chloride (Cl-) • Facilitated diffusion is affected by the steepness of the concentration gradient and the number of transport proteins available

Answer 56

Channel proteins • Used to transport charged molecules • Fixed shape • They have water filled pores that allow charged substances (e.g. ions) to diffuse through the cell membrane • Diffusion of these ions doesn’t occur freely, most channel proteins are gated. This means the pores can open and close to control the exchange of ions Carrier Proteins • Used to transport large molecules • Can switch between two shapes. This causes the binding site of the carrier protein to be open on one side of the membrane first, and then open to the other side of the membrane when it switches shape. The direction of movement of molecules depends on the concentration on each side of the membrane and net movement occurs down a concentration gradient

Answer 57

• The passive net movement of water from a region of less negative water potential to a region of more negative water potential, through a partially permeable membrane down a concentration gradient • The partially permeable membrane will allow smaller (e.g. water) but not larger (e.g. solute) molecules through

Answer 58

• The pressure created by free water molecules, measured in Kilopascals (kPa) that describes the tendency of what to move out of solution • The water potential of pure water at atmospheric pressure is 0kPa a concentrated solution would have a water potential of (e.g.) -500kPa • A more negative water potential in the cell than the solution means the cell bursts, which is called haemolysis • A more negative water potential in the cell than the solution means the cell shrinks, which is called crenation • A normal cell has the same water potential as the solution • Osmosis is explained in terms of water potential

Answer 59

It’s possible to investigate the effects of immersing plant tissue in solutions of different water potentials and then use the results to estimate water potential of the plant itself Method: • Use each cork borer to cut 5 potato cylinders of the same diameter and uses a scalpel and ruler to trim each potato to the same length and blot dry to remove excess moisture • Measure using a top pan balance accurate to 0.01g the initial mass of each potato cylinder and record in a table of results as well as a table for initial length (mm) • Measure 10cm3 of each sugar/salt solution (one should be distilled water) and our each into a boiling tube, labelling each one’s clearly including its concentration • Add one potato cylinder to each tube and leave for a specified amount of time • Remove the potatoes, blot dry and record the final mass and length of each Analysis • The percentage change in mass for each potato cylinder is calculated and put a graph for percentage change in mass (Y) against sugar concentration (X) : (Final mass - initial mass) ———————————— X100 Initial mass • A positive change in mass indicates that the potatoes gained water by osmosis so the solution had a higher water potential than the potato. The potato cell is turgid as the water exerts turgor pressure on the cell walls so potatoes feel hard • A negative change in mass indicates that the solution had a lower water potential than the potato, water moves out of the potato cells by osmosis, making them flaccid and feel floppy Conclusion • The point at which the line of best fit crosses the X-axis is the concentration of sugar inside the potato as this is where there would be no change in the mass of the potato

Answer 60

• Water enters the cell through it’s partially permeable membrane by osmosis as the solution has a higher water potential than the plant cell • Water enters the vacuole so its volume increases and the protoplast (entire cell excluding the cell wall) expands and pushes against the cell wall. The inelastic cell wall prevents the cell from bursting, so the cell becomes turgid. • Turgid cells fully inflated with water are firm and provide strength and support allowing the plant to stand upright with its leaves out to catch the sunlight, if plants don’t receive enough water they can’t stay turgid and the plant wilts

Answer 61

The cell surface membrane is partially permeable and the cellulose cell wall is freely permeable

Answer 62

• Water leaves the plant cell through its partially permeable cell surface membrane through osmosis • Water leaves the vacuole so the volume decreases and the protoplast gradually shrinks and pulls away from the cell wall • This is known as plasmolysis (plant cell is plasmolysed)

Answer 63

• Water leaves the cell through the partially permeable cell surface membrane by osmosis and the cell will shrink and shrivel up. •. This occurs in a hypertonic environment where the solution outside the cell has a higher solute concentration than the inside of the cell

Answer 64

• Water enters the cell through its partially permeable cell surface membrane by osmosis as the pure water or dilute solution has a higher water potential than the animal cell • The cell gains water by osmosis until the membranes stretched too far and the cell bursts (cytolysis) because it has no cell wall to withstand the increased pressure • This occurs when the cell is in a hypotonic environment which means the solution outside the cell has a lower solute concentration than inside the cell

Answer 65

• The solution outside cell has the same solute concentration as inside the cell • There is no net movement of water molecules and there is no change to the cells

Answer 66

• The movement of molecules or ions into or out of the cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins Active transport requires: • Carrier proteins: transmembrane proteins that undergo conformational change • Energy: from ATP being hydrolysed to release energy during respiration which causes carrier proteins to change shape

Answer 67

• Reabsorption if useful molecules and ions into the blood after filtration into the kidney tubules • Absorption of products of digestion from the digestive tract • Loading sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant • Loading inorganic ions from the soil into the root hairs

Answer 68

• A molecule or ion bonds to receptor sites on the carrier protein and ATP bonds to the protein (inside the cell) and releases energy when its hydrolysed into ATP + Pi • The carrier protein changes shape, opening up on the other side of the membrane where the molecule or ion is released • The inorganic phosphate (Pi) is released from the protein where it combines with ADP to form ATP • This causes the protein to return to its previous shape

Answer 69

• Facilitated diffusion occurs down a concentration gradient from a high concentration to a low concentration whereas active transport occurs against a concentration gradient from a low concentration to a high concentration • Facilitated diffusion used channel or carrier proteins whereas active transport uses just carrier proteins • Facilitated diffusion uses innate kinetic energy whereas active transport uses energy from ATP

Answer 70

• The coupler movement of substances across a cell surface membrane via a carrier protein, involving a combination of facilitated diffusion and active transport • Co-transporters are a type of carrier protein that bind to two molecules at the same time. The concentration gradient of one molecule is used to move the other molecule against its own concentration gradient • This is illustrated by the absolution of sodium ions and glucose by cells lining the mammalian ileum

Answer 71

• To absorb glucose from the lumen of the gut there must be a higher concentration of glucose in the lumen compared to the epithelial cell for facilitated diffusion • However usually there’s more glucose in the epithelial cells so active transport and co-transport are required. The concentration of glucose in the blood is lower than the epithelial cells because the blood flows and carries away the absorbed glucose 1. Sodium ions are actively transported out of the epithelial cells into the blood 2. This reduces the sodium ion concentration in the epithelial cell 3. Sodium ions can diffuse from the lumen down their concentration gradient into the epithelial cell 4. The protein the sodium ions diffuse through is a co-transporter protein, so either glucose or amino acids also attach and are transported into the epithelial cell against their concentration gradients 5. Glucose then moves by facilitated diffusion from the epithelial cell to the blood 6. Microvilli on the epithelial cell surface increase the surface area for co-transporter proteins. However, protein channels are finite and can become the limiting factor

Answer 72

• Immunity is the ability of a body to be better prepared for fighting a disease than the last time Non-Specific immune responses • Barrier methods • Inflammation • Phagocytosis Specific immune responses • Cell-mediated response (T-cells) • Humoral response (B-cells)

Answer 73

Each type of cell has specific molecules on its surface that identify it called antigens. These molecules include proteins and enable the immune system to identify: • Pathogens • Cells from other organisms of the same species • Abnormal body cells • Toxins

Answer 74

• Antigens are specific macromolecules on the cell-surface membrane that stimulate an immune response and allow cell-to-cell recognition • Antigens produced by the organisms own body cells are known as self antigens and don’t stimulate an immune response • Foreign antigens not produced by the organisms own body cells are known as non-self antigens and stimulate an immune response • Every human body cell, microorganism (both pathogenic and non-pathogenic) such as bacteria and viruses have macromolecules called antigens on their cell surface membranes, bacteria cell walls or the surface of viruses. Some glycolipids and glyco-proteins on the outer surface of cell membranes act as antigens

Answer 75

• Antigenic variability refers to the fact that mutations can change the tertiary structure so the immune system can’t detect the pathogen • Some pathogens exhibit antigenic variability. This is a problem as lymphocytes and memory cells produce a specific immune response complementary in shape to only one antigen. When this shape changes they can no longer bind and there’s no secondary immune response so the host is infected again • E.g. cold and influenza change year after year

Answer 76

• Barrier methods, including physical and chemical defences e.g. skin, mucous membranes, tears (containing lysozyme enzymes) and saliva • Inflammation, involving the swelling and heating of a region invaded by the pathogen • Phagocytosis

Answer 77

• The phagocyte is attracted to the pathogen by it’s chemical products and moves towards it along a concentration gradient by chemotaxis • Receptors in the cell surface membrane of the phagocyte bind to the pathogen. Once attached, the cell surface membrane extends around the pathogen, engulfing it and trapping it within a phagocytic vacuole to form a phagosome. This is called endocytosis. • Lysosomes within the phagocyte migrate towards the phagosome and release digestive enzymes called lysosymes that hydrolyse the pathogen • The lysosome fuses with the phagosome membrane to form a phagolysosome • The hydrolysis products of the bacteria are absorbed by macrophage phagocytes and antigens from the destroyed pathogen can be presented on the surface of the phagocyte due to the major histocompatibility complex. • An antigen presenting cell is formed that presents foreign antigens on their cell surface membrane to activate other immune cells

Answer 78

• Phagocytes • Lymphocytes

Answer 79

• A type of white blood cell (WBC) that are part of a non-specific immune response (the same for all pathogens) and are produced continuously in the bone marrow. • They’re responsible for removing dead cells and identifying and eliminating pathogens by engulfing them and digesting their contents through phagocytosis • There are two types of phagocyte that carry out phagocytosis, Neutrophils and Macrophages

Answer 80

• Neutrophils travel throughout the body and leave the blood by squeezing through capillary walls to patrol body tissues. During infection they’re released in large numbers and are short lived. • Chemicals released from body cells under attack and pathogens attract neutrophils that move towards pathogens (chemotaxis- response to chemical stimuli) • Antibodies attach to receptor proteins on the neutrophils which stimulates them to attack the pathogen, beginning phagocytosis

Answer 81

• Long-lived cells that are larger than neutrophils and travel in the blood as monocytes that then develop into macrophages once they leave the blood into organs such as the lungs and lymph nodes • Macrophages carry out phagocytosis as normal but don’t destroy pathogens completely. They display the antigens on their surface to become an antigen presenting cell through a structure called the major histocompatibility complex. • These antigen presenting cells are recognised by lymphocytes

Answer 82

There are two types of lymphocyte: • T-lymphocytes (T-cells) mature in the thymus gland and are associated with cell-mediated immunity (body cells) • B-lymphocytes (B-cells) mature in bone marrow and are associated with humoral immunity (antibodies in plasma)

Answer 83

• Mediated by T-lymphocytes (T-cells) matured in the thymus that specifically bind to antigens presented on cells. Cell-mediated immunity is more important in eliminating intercellular pathogens • Helper T-cells have receptors on their surface that bind to antigens on the antigen presenting cell from phagocytosis. This stimulates the rapid differentiation of T-cells by mitosis (cloning): 1. Increase phagocytosis which will make more antigen presenting cells and therefore decrease the spread of bacteria 2. Become a helper T-cell that recognise antigens on antigen presenting cells, stimulate more phagocytosis and activate a B-lymphocyte (B-cell) to divide and secrete antibodies 3. Become a memory T-cell which remain in the blood to respond to future infection by the same pathogen 4. Become a cytotoxic T-cell that produces performing which make holes in cells and fills those cells with water so they lyse (burst)

Answer 84

• Initiated by B-cells matured in the bone marrow that secrete antibodies (unlike T-cells). Humoral immunity is more important to eliminating soluble antigens and extracellular microorganisms • Helper T-cells stimulate B-cells to rapidly divide by mitosis to form clones of plasma cells (clonal selection) which produce antibodies specific to the antigen. This means the antibodies attach to the antigen and destroy the pathogen • Some B-cells develop into memory cells that have the specific antibodies attached to their cell surface membranes and remain in the blood and divide by mitosis into more memory cells and plasma cells in response to the same antigen again • The first contact with pathogen is the primary response and the second contact is the secondary response. During the secondary response, once antibodies have destroyed the pathogen, memory B-cells are formed that more rapidly produce more antibodies at a higher intensity of production on re-exposure

Answer 85

Structure: • Proteins that have binding sites complementary to antigens • Quaternary structure of four polypeptide chains (2 light chains and 2 heavy chains) linked by disulphide bonds • They have a constant region that’s present in all antibodies and a variable region that makes antibodies specific to an antigen Function: • Bind to antigens and destroy pathogens. Antibodies destroy pathogens by agglutination: the antibodies that have bound to antigens to form an antigen-antibody complex all stick together, so pathogens can be easily digested by phagocytosis as the larger clumps are more easily detected by the phagocyte

Answer 86

• Vaccinations are made up of dead or inactive pathogen that incite a primary response, learning to the formation of memory B-cells so that, upon re infection, a secondary response is stimulated • This leads to heard immunity where immunising the majority of the population means that unvaccinated people are less likely to come into contact with an infected person. • Active immunity is immunity that a results from antibody production by the immune system in response to the presence of an antigen e.g. vaccination • Ethical issues include: • Whether they should be compulsory (against people’s will for the good of the population) • Who should vaccines be tested on (sick/young/ old) • Are they 100% effective long-term (true for new vaccines e.g. HPV in girls- LT effects aren’t yet seen