AS Biology, topic 2

  • Created by: Abbie
  • Created on: 01-08-19 15:43

Eukaryotic cells

Cells can either be prokaryotic (e.g. bacteria) or eukaryotic (more complex, including all animal and plant cells and algal and fungal cells). Both of these cells contain organelle, which are parts of cells that each have a specific function

Image result for animal cell diagram
Algal cells have all the same organelles as plant cells. Fungal cells have 2 key differences- their cell walls are made of chitin (not cellulose) and they don't have chloroplasts as they don't photosynthesise

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Eukaryotic organelles

Cell-surface (plasma) membrane: Found on surface of animal cells and just inside cell walls of others, mainly made of lipids and proteins, regulates movement of substances coming in/out of cells. Has receptor molecules so can respond to chemicals, e.g. hormones Nucleus: 

  • Nuclear envelope (double membrane)- surrouns nucleus, with the outer membrane often joining onto RER. Controls entry and exit of substances in nucleus and contains the reactions within in
  • Nuclear poes- allow substances (e.g. RNA) to move between nucelus/cytoplasm
  • Nucleoplasm- jelly like, makes up most of nucleus
  • Chromatin (chromosomes)- protein bound, linear DNA- hold's cell's genetic material, controls cell activity by controlling the transcription of DNA to make tRNA and mRNA
  • Nucleolus-  makes and assembles ribosomes/ribosomal RNA

Mitochondrion: has a doubke membrane that controls what enters/exits, with the inner membrane folding to for, cristae, which provide a large SA for proteins to attach for respiration. Inside this is the matrix- contains enzymes involved in respiration and males own proteins during the Krebs cycle. Site of aerobic respiration (produces ATP)- so lots found in active cells that need more energy

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Eukaryotic organelles (2)

Chloroplasts: found in plant and algal cells as they're the site of photosynthesis (1st stage in the grana, 2nd stage in the stroma) surrounded by the chloroplast envelope- double plasma membrane that controls what enters/exits. Also has thylakoid membranes, which are stacked up to form the grana, which contain chlorophyll, and are linked together by lamellae. The stroma is a thick fluid found in the choloroplast

Endoplasmic reticulum: system of membranes enclosing a fluid-filled space:

  • Rough Endoplasmic Reticulum- surface covered with ribsomes to fold and process the proteins made at ribosomes and provide a large SA for them to be synthesised. Also provides a pathway for the transport of materials (esp proteins) through the cell
  • Smooth Endoplasmic Reticulum- has no ribosomes; synthesises, stores and transports lipids and carbs. Cells that manufacture any of these things tend to have more ER

Golgi apparatus: group of fluid-filled, membrane bound flattened sacs, with vesicles often at the edge (which are surrounded by a membrane and produced by the GA to store lipids/proteins made by GA and transport them out the cell). The GA processes and packages new lipids and proteins, also making lysosomes

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Eukaryotic organelles (3)

Lysosomes: type of golgi vesicle that is surrounded by a membrane that keeps lysozymes seperate from the cytoplasm. Contain digestive enzymes called lysozymes that digest invading cells and break down the worn out components of a cell: autolysis- breaking down dead cells and exocytosis- destroying material around the cell by releasing lysozymes.

Ribosomes: either floats free in cytoplasm ir attached to RER- made of proteins and RNA and isn't surrounded by a membrane. 2 types- 80s (in eukaryotic cells) and 70s (in prokaryotic cells). Site where proteins are made

Cell wall: In plants and alage, it's made mainly of the pollysaccharide cellulose and in fungi, is chitin. It supports cells and stops them from changing shape during osmosis. Allows water to pass through- contributes to movement of water through the plant

Cell vacuole: membrane-bound (surrounding membrane is the 'tonoplast'), found in the cytoplasm of plant cells. Contains cell sap- weak solution of sugar and salts. It helps to maintain pressure inside the cell and keep it rigid. It stops plants wilting and also helps isolate unwanted chemicals inside the cell. The sugars and amino acids may act as a temporary food store

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Prokaryotic cells

Image result for prokaryotic cell diagram
 Plasma membrane- mainly made of lipids and proteins, controls what enters/exits cell. Cell wall- made of a glycoprotein called murein, supports cell and stops it changing shape. Some, e.g. bacteria have a capsule made of secreted slime to protect it from attack by immune system cells. Plasmids- not essential for survival, so not always present, but can be an advantage in stressful environments, so some cells have several. They're small loops of DNA not part pf the main DNA molecule, and contain genes for things like antibiotic resistance. Can be passed between prokaryotes. No nucleus, so DNA floats free in the cytoplasm. It's circular and coiled up in a strand. Cytoplasm ahd ni membrane-bound organelles, but does have 70s ribosomes. The Flagellum rotates to allow the cell to move; not all prokaryotes have one, and some have more than one.  

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Binary fission

This is how prokaryotic cells replictae- during this, the cell replicates its genetic material before spiltting into 2 daughter cells.

1. Circular DNA and plasmid(s) replicate- main DNA, only once, but plasmids can replicate many times

2.Cell gets bigger, and the 2 DNA loops move to opposite poles of the cell

3. The cytoplasm starts to divide and new cell walls begin to form at the divide

4. Once the cytoplasm has divided, 2 daughter cells are produced, each one wiyj a copy of the circular DNA, but potentially varying numbers of plasmids

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Magnification and resolution

Magnification is how much bigger the image is than the specimen is in reality. It can be caluclated by dividing the size of the image by the size of the real object. To convert from micrometers to millimeters, divide by 1000

Resolution is how detailed the image is in terms of how well a microscope can distingish between 2 points that are close together. If the resolution is poor, then increasing the magnification won't help. 

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Optical (light) and electron microscopes

Optical- use light to form an image, max resolution of around 0.2 um, so can't see organelles smaller than this, e.g. ribosomes and ER. Can't see slightly larger ones like mitochondria in much detail. Can observe living specimens directly. Max useful magnification is about x1500. Smaller and cheaper, 2D and colour images

Electron- use electrons to form images, higher resolution, so produces more detailed images- can look at more organelles. Max resolution of about 0.0002 um (much higher) and max magnification of about x 15000000. Can be 3D and produce black and white images. Expensive and very large, can only view specimens in a vacuum, so they must be dead. Can be either 'scanning' or 'transmission':

  • Scanning- scans beam of electrons across the specimen, knocking off elctrons from the specimen onto a cathode ray tube to from a 3D image that shows the surface of the specimenn- can be used with thick specimens, but lower resolution than TEMs
  • Transmission- use electromagnets to focus a beam of electrons that is transmitted through the sorcimen, denser parts of it absorb more elctrons- looks darker on the image. Higher resolution images- can see the internal structure of organelles, but can only be used on thin specimens. 
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Preparing a slide

Slides are used to view specimens underneath an optical microscope. Preparing a temporary mount:

  • Pipette small drop of water on and use tweesers to put a thin section of the specimen on top
  • Add a drop of stain to highlight the organelles within the cell
  • Add a cover slip, carefully lowering in onto the specimen until it covers it to avoid air bubbles that could obstruct the view of the specimen
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Cell fractionation

This separates organelles from the rest of the cell so they can be observed under a microscope:

  • Homogenisation (breaking up cells)- multiple methods, e.g. vibrating cells to break the plasma membrane and releasing the organelles into solution (must be ice cold to reduce enzyme activity that could break down organelles and isotonic to prevent damage to the organelles via osmosis). Need to add buffer solution to maintain pH
  • Filtration (getting rid of big bits)- solution is filtered through a gauze to seprate any debris from the organelles (which are much smaller)
  • Ultracentrifugatin (separating the organelles)- does this in order of mass- heaviest to lightest: the cell fragments are put in a tube, which is put in a centrifuge and spun at a low speed; the heaviest organelles go to the bottom, forming a thick pellet. The rest remain suspended in the fluid above this. This fluid is drained and poured into another tube to be spun again at a higher speed. This is repeated until all the organelles are separated.
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Mitosis- Interphase

During mitosis, a parent cell divides into two genetically identical daughter cells; it's needed for the growth of multicellular organisms and for repairing damaged tissues. Mitosis occurs during the 'cell cycle' following interphase (a period of cell growth and DNA replication). Interphase is subdivided into 3 stages:

  • G1 (growth/gap phase 1): the cell grows and new organelles and proteins are made
  • Synthesis: cell replicates its DNA in preperation for division during mitosis
  • G2 (growth/gap phase 2): the cell continues to grow whilst the proteins needed for cell division are made

Whilst mitosis is technically one continuous process, it can be subdivided into a series of division stages: prophase, metaphase, anaphase and telophase/cytokinesis. Interphase occurs before mitosis in the cell cycle

Interphase- Whilst carrying out normal processes, the cell prepares to divide by unravelling its DNA and replicating this. The organelles are also replicated so it has spare and ATP content is increased in order to provide enough energy for cell division

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Mitosis- division stages

Prophase: chromosomes coil up, getting shorter and thicker. A bundle of proteins called a centriole divides, with the 2 different bundles beginning to move to opposite poles of the cell. This forms a network of protein fibres across the cell called the spindle. The cell's nuclear envelope breaks down, allowing the choromsomes to float free within the cytoplasm

Metaphase: the chromosomes (which each contain 2 chromatids) line up across the middle of the cell on the spindle equator and become attached to the spindle by their centromere

Anaphase: centromeres divide, separating each pair of sister chromatids. The spindle fibres begin to contract, which then pulls the chromatids to opposite piles of the spindle, bringing their centromere, making the chromatids appear v-shaped

Telophase: the chromatids reach opposite poles of the cell, where they begin to uncoil and become longer and thinner again, becoming chromosomes again. As the nuclear envelopse begins to form around each group of chromosomes, 2 nuclei begin to appear

Cytokinesis: the cytoplasm divides, producing 2 genetically identical daughter cells. Mitosis is now finished and each daughter cell begins interphase

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Cancer and the cell cycle

  • Cancer is the result of uncontrolled cell division; the cell cycle is controlled by genes, which tell the cycle to stop when it has produced enough new cells. If there is a mutation in such genes, the cells can grow out of control- they keep on dividing to eventually form a tumor. Cancer is a tumor that invades surrounding tissue
  • There are checkpoints at each stage of the cell cycle, where control genes can decide if a cell can continue to the next stage of the cycle- if these are mutated, they can't control the checkpoints. E.g. tumour-suppressor genes- encode proteins that block checkpoints- mutations stop the correct proteins being formed, so the cell cycle continues unchecked.
  • Some cancer treatments target specific parts of the cell cycle, designed to control the rate of cell division in tumours by blocking the cell cycle in order to kill the tumour cells. BUT, can't distinguish between normal and tumour cells, so kills normal cells too. But, as they don't divide as frequently as tumour cells, they're less likely to be targeted. Examples inc:
  • G1 (protein production)- chemotheraphy prevents the synthesis of the enzymes needed for DNA replication, meaning the cell will die as it can't enter the synthesis phase
  • Synthesis phase (DNA replication)- radiation and some drugs damage DNA, meaning that at the check points (inc just before the S phase), when the DNA is checked for damage the cell will kill itself, preventing further tumour growth
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Cell Membrane sructure

All cells are surrounded by membranes, as well as many organelles in eukaryotic cells. Cell surface membranes surround cells, acting as a barrier between cell/environment, controlling what enters/exits; so they're partially permeable- let some molecules pass but not others. Substances can cross this via diffusion, osmosis or active transport. Membranes around organelles divide the cell into different sections, becoming a barrier between organelle/cytoplasm. Also pp. All cell membranes have a similar basic structure; composed of lipids (mainly phospholipids), proteins and carbs (attached to proteins or lipids):

  • The 'fluid mosaic model' explains the arrangement of molecules in a membrane- the phospholipid molecules form a continuous double layer (bilayer), which is 'fluid' as the phospholipids are constantly moving. Cholestrol molecules also present in the bilayer and proteins are scattered through it, inc channel and carrier proteins, which allow larger molecules/ions to pass through. 
  • Receptor proteins on the cell-surface membrane let the cell detect chemicals released from other cells and respond appropriately. Some proteins move sideways through bilayer, some in a fixed position. Some are glucoproteins as they have a polysaccharide chain attached. Some lipids are glycolipids as they too have an attached polysaccharide chain
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Components in cell membranes

Phospholipids form a barrier to dissolved substances- they have a hydrophilic head and a hydrophobic tail, meaning the molecules automatically arrange themselves into a bilayer with heads facing out to the water on either side of the membrane. The hydrophobic centre means the membrane doesn't allow water-soluble substances through (like ions), acting as a barrier to these dissolved substances

Cholesterol gives the membrane stability- it's a type of lipid that's present in all but bacteria cell membranes. These molecules fit between the phospholipids, binding to their hydrophobic tails and causing them to pack more closely together- makes the membrane more rigid by restricting the movement of the phospholipids. Cholesterol is important in maintaing the shape of animal cells (no cell walls), which is esp important for cells that aren't supported by other cells- e.g. red blood cells which float free in blood

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Simple diffusion

  • Net movement of particles from high to low concentration- molecules can diffuse both ways, but the net movement is to the lower concentration and will continue until the particles are evenly distributed in the liquid/gas.
  • Particles diffuse down a concentration gradient
  • Passive process-requires no energy
  • Particles are able to diffuse across cell membranes as long as they can move freely through the membrane; if they're able to diffuse directly through the cell membrane, the process is known as simple diffusion
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Facilitated diffusion

  • It'd take larger molecules like amino acids too long to diffuse through the phospholipid bilyaer due to their size. Also true of charged particles (ions, polar molecules) as they're water soluble and the bilayer's centre is hydrophobic. So, these particles diffuse through the membrane via carrier/channel proteins to speed up the process- facilitated diffusion
  • This is still a passive process that involves particles moving down a concentration gradient

Carrier proteins- move large molecules acrss the membrane down their concentration gradient- different proteins move different molecules. The molecule attaches to the binding site of the protein in the membrane, causing the protein to change shape and release the molecule on the opposite side of the membrane.

Channel proteins- forms pores in the membrane that allow charged particles to diffuse through down a concentration gradient. Again, different proteins facilitate the diffusion of different particles

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Factors affecting rate of diffusion

Simple diffusion depends on:

  • Concentration gradient- higher is a faster rate. Diffusion slows down over time as both sides of the membrane come closer to reaching an equilibrium
  • Thickness of exchange surface- thinner is faster rate as shorter distance for particles to travel
  • Surface area- larger surface area is faster rate- the microvilli on some cells increase the surface area, in humans- up to 600 times larger in humans- more particles can be exchanged in the same amount of time

Facilitated diffusion depends on:

  • Concentration gradient- higher is a faster rate up to a point
  • No. of channel/carrier proteins- more is a faster rate- once they're all used up, facilitated diffusion can't occur any faster- e.g some kidney cells are adapted to have lots of aquaporions in order to reabsorb lots of water that would otherwise be lost
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Osmosis

  • It's the diffusion of water molecules across a partially permeable membrane from an area of higher to lower water potential. Water potential is the likelihood water molecules will diffuse in or out of a solution. Pure water has the highest potential (0)- all other values are negative
  • 2 solutions with the same water potential are 'isotonic'

Factors affecting the rate of osmosis:

  • water potential gradient- higher is a faster rate. As osmosis continues, the difference between either side of the membrane will decrease, meaning osmosis rate levels off over time
  • thickness of exchange surface- thinner is a faster rate
  • the surface area of the exchange surface- larger SA, faster rate of diffusion
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Active transport

  • Requires energy to move ions and molecules across membranes, usually against a concentration gradient. Uses carrier proteins to transport the molecules across the membrane in the same way as in facilitated diffusion- BUT, here solutes are usually being moved from low to high concentration, and it's an energy-requiring process, using the ATP produced during respiration. 
  • During a hydrolysis reaction, ATP splits into ADP/Pi, releasing energy that allows the solutes to be transported
  • One type of carrier proteins are co-transporters, which bind to 2 molecules at one time- the concentration gradient of one of these is used to allow the other to move against its concentration gradient. E.g. sodium ions and glucose are co-transported- as the sodium ions move DOWN their concentration gradient, glucose moves AGAINST theirs
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Factors affecting rate of active transport

When the molecules are being moved against their concentration gradient, decreasing this gradient doesn't impact the rate of active transport. These factors do though:

  • speed of individual carrier proteins- the faster they work, the faster active transport is
  • no. of carrier proteins- more=faster rate
  • rate of respiration in the cell/availability of ATP- without this, active transport is inhibited
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Active transport- sodium-potassium pump

An example of co-transport is the way that glucose is absorbed in the mammalian ileum. Glucose is absorbed into the bloodstream in the small intestine, but is unable to diffuse into the blood in the ileum (final part of the small intestine), as the concentration of glucose is too low. So, it's absorbed from the lumen (middle) of the ileum via co-transport:

  • Sodium ions actively transported out of ileum epithelial cells into the blood via a sodium potassium pump- creates a concentration gradient- higher sodium concentration in the lumen of the ileum that in the epithelial cell, causing the sodium to diffuse down the concentration gradient into the epithelial cell from the lumen via sodium-glucos co-transporter proteins
  • The co-transporter carrier glucose into the cell with the sodiym, increasing glucose concentration inside the cell
  • This allows glucose to diffuse out of the cell and into the blood (down the concentration gradient) through a channel protein during facilitated diffusion
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The Immune System- defence mechanisms

Antigens are molecules (usually proteins) that can generate the immune response when detected in the bosy- usually present on the surface of cells and are used by the immune system to identify:

  • pathogens: disease-causing microorganisms
  • abnormal body cells: cancerous cells/tumours, pathofen-infected cells- have abnormal antigens on their surface
  • toxins: which are released by bacteria
  • cells from other individuals of the smae species- e.g. organ transplants

In order to protect itself from pathogens/infection, the body has a range of defence mechanisms:

  • Non specific (immediate response, same for all pathogens)- physical barriers (e.g. skin) and phagocytosis
  • Specific (slower response, pathogen specific)- cell-mediated/cellular reponse (T lymphocytes and the other immune system cells they interact with, e.g. phagocytes) and Humoural response (B lymphocytes, clonal selection and production of monoclonal antibodies). Both cellular and humoral are needed to remove a pathogen, and the two interact with each other
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The Immune Response

1. Phagocytes engulf pathogens (phagocytosis)- a phagocyte is a type of white blood cell found in the blood and tissues, that carries out phagocytosis. They're the first cells to respond to the immune system being triggered in the body:

  • phagoctye recognises foreign antigens on the pathogen, the cytoplasm of the phagoctye moving around the pathogen to engulf it- pathogen is now inside a phagocytoc vacuole in the cytoplasm
  • A lysosome (contains enzymes called lysozymes) fuses with the vacuole, akkowing the lysozymes to break down the pathogen
  • The phagoctye then sticks the antigens onto its surface in order to present them to the other immune system cells, allowing them to be activated

2. Phagoctyes active T-cells (T-lymphocytes)- this is another type of white blood cell that has receptor proteins on its surface that bind to the complementary antigens on the phagoctye in order to activate the T-cell. Different types of T-cells respond differently: Helper T-cells (Th cells) release chemical signals that activate phagoctyes and cytotoxic T-cells (Tc cells), which kill abnormal and forgein cells. Th cells also activate B-cells, which secrete antibodies and allow the reponse to continue

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The Immune Response (2)

3. T-cells activate B-cells (B-lymphoctyes), which divide into plasma cells- B cells are a type of white blood cell covered in antibodies (proteins that bond to antigens to from an antigen-antibody complex). Specific- each has different shaped antibodies on its membrane-binds to different antigens:

  • when complementary shapes meet, the antibody on the B-cell binds to the antigen
  • This (and substances released from Th cells) activates the B-cell. This is known as clonal selection
  • The activated B-cell divides into plasma cells

4.  Plasma cells make more antibodies to a specific antigen- plasma cells are clones of B-cells that secrete lots of antigen-specific (monoclonal) antibodies, which bind to the antigens on the pathogen's surface to form an antigen-antibody complex. Antibodies have 2 binding sites, so can bind to 2 pathogens at the same time- this causes pathogens to become clumped together (agglutination)- this allows pagocytes to bind to the antibodies and phagocytose many pathogens at once, leading to the destruction of the pathogen carrying to antigen in the body

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Antibodies

Antibodies are proteins that're made of amino acids that're produced by plasma cells. The specificty of each one depends on its variable regions, which form the antigen binding sites. Each anyibody has 2 unique variable regions that bind to specific antigens, allowing agglutination to clump rhe antigens together. These varivale regions have unique tertiary structure due to the differing amino acid sequences; each structure is complementary to one specific antigen. All antibodies have the same constant regions, which are made of 2 heavy and 2 light chains, connected by disulfide bridges. Hinge proteins connect the variable and constant regions.

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Cell recognition- lymphocytes

Lymphocytes- (B/T): types of white blood cells that're activated in the presence of antigens. Distinguish between body's own and foreign material through the fact each cell has specific surface molecules with highly specific proteins (tertiary structure) that allows cells to be distinguished from each other. These proteins allow the immune system to identify pathogens, foreign materual, toxinsn and abnormal body cells.

  • This can pose an issue for organ donor cells though, as lymphocytes attempt to destroy them- minimised by donors being matched as close as poss in terms of genetics and immunosuppressant drugs
  • Specific lymphoctyes (around 10 million types) already exist before an infection, but there are so few of each one that they must divide during clonal selection to build up enough numbers yo destroy the pathogen
  • In the fetus, lymphocytes collide with other cells constantly and as infection is rare here, they collide almost exclusively with the body's own material. The lymphocytes that have receptors for the body's own cells either die or are suppressed, leaving only those that fit foregin material. Adult lymphoctyes are produced in the bone marrow, initially only encoutering self-antigens. The same process occurs leaving only those that respond to non-self antigens in the blood
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Primary/secondary immune responses

The immune response for antigens can be memorised:

Primary immune response: occurs whrn an antigen enters body for the first time and activates the immune response- this is slow as there aren't many B-vells that make the antibody needed to bind to it. Eventually enough are made to overcome the infection/symptoms. When T/B cells are exposed to an antigen, they produce memory cells that remain in the body for a long time- they remember the antigen and can recognise it the second time around. Memory B-cells remember the specific antibodies needed to bind to the antigen, meaning the person is now immune- the immune system can respond far more quickly in the future

Secondary immune response: quicker if the same pathogen enters again- quicker clonal selectiom; memory B-cells are activated and divide into plasma cells that can produce the right antibodies. Memory T-cells are activated and divide into the correct types of T cells to kill the cell carrying the antigen. The secondary response is often able to destroy the pathogen before any symptoms appear

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Vaccines

  • They contain antigens, causing memory cells to be produced without causing the disease. Allows the person to become immune without getting any symptoms. This protects individuals who have the vaccine, reducing the occurance of the disease in the population, meaning that non-vaccinated people are also less likely to catch it. This is known as 'herd immunity', and can help to protect the most vulnerable in the population, e.g. babies. Over 90% of the population needs to be vaccinated if it's highly contagious, and 80% if not. 
  • Vaccines contain antigens which may be free or attached to a dead/weakened pathogen
  • Can be injected or taken orally- but oral vaccibes could be broken down by enzymes in the gut, or molecules could be too large to be absorbed in the blood
  • Sometimes booster vaccines are given later to ensure memory cells are produced
  • Continuous trial: some vaccinations are continuously being changed because of the rapid evolution of antigens- e.g. HIV
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Antigenic variation

  • Some pathogens change their surface antigens due to changes in the genes of the pathogen, meaning that memory cells won't recognise it and the primary reponse must occur again against the new antigens if the person is infected a second time.
  • Antigenic variation makes it hard to develop vaccines against such pathogens, e.g. HIV and the influenza virus; the flu vaccine changes yearly due to changing surface antigens resoluting in new, immunologically distinct strains. These circulate in the population each year, so a new vaccine must be developed and the most suitable one is chosen and implemented
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Active and passive immunity

Active immunity: Immune system creates its own antibodies after being stimulated by an antigen:

  • Natural- antibodies creates in response catching a disease
  • Artificial- antibodies created in response to a vaccination of antigens

Takes time for protection to develop, produces memory cells, long-term protection, requires exposure to the antigen

Passive immunity: Individual recieves antibodies from an external source:

  • Natural: antibodies transferred to a baby from the mother's milk
  • Artificial: you become injected with antibodies from someone else

Immediate protection, memory cells aren't produed, protection is short-term as the given antibodies are broken down, doesn't require exposure to the antigen

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Antibodies in medicine- cancer cells

  • Monoclonal antibodies are produced from plasma cells, meaning they're all identical in structure
  • The unique tertiary structure of antibodies means that they'll only bind to a complementary antigen- you can make monoclonal antibodies bind to anything (e.g. a cell antigen) and they will only bind to/target this particular molecule. This means that monoclonal antibodies can be used in medicine to target specific substances of cells

E.g. targeting drugs to a particular cell type- cancer cells:

  • Cancer cells have antigens called tumour markers that aren't found on normal cells- can make monoclonal antibodies bind to these and attach anti-cancer drugs to these antibodies
  • So, when the antibodies come into contact with the cancer cells, they bind to the tumour markers, meaning the drug only accumulates in the body where the cancer cells are, minimising the side effects of such drugs
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Antibodies in medicine- pregnancy testing

E.g. testing a particular substance for medical diagnosis- pregnancy testing:

  • pregnancy tests detect the hormone hCG that's found in the urine of pregnant women. The application area contains antibodies for hCH bound to a blue bead
  • when urine is applied to this rea, any hCG will bind to the antibodies, forming an antigen-antibody complex
  • the urine moves up the test *****, carrying any beads with it. The test ***** contains immbolisied hCG antibodies
  • if hCG is present, the test ***** will turn blue as the immobilised antibody will bind the the hCG, concentrating the antibody-antigen complec with the blue beads attached. If none is present, the beads will pass through the test area without binding to anything, so it won't turn blue
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The ELISA test

Lets us see if a patient has any antibodies to a certain antigen or vice-versa. Can be used to test for pathogenic infections, allergies etc. During the test, an antibody is used with an enzyme attached to it- this enzyme can react witha substrate to form a coloured product, causing the solution in the reaction vessel to change colour. A colour change demonstrates the presence of the antigen/antibody, some tests allowing the quantity of this to be determined by the intensity of the colour. Direct ELISAs use a single antibody that's complementary to the antigen you're testing for. Indirect ELISAs use 2 different antibodies and can be used to test for HIV:

  • HIV antigen is bound to the bottom of a well plate and a sample of the patient's blood (may contain multiple antibodies) is added. If any HIV-specific antibodies are present, they'll bind to the HIV antigen stuck to the bottom of the well. Well is washed to remove unbound antibodies
  • A secondary antibody with a specific enzyme attached is added and can bind to the HIV-specific antibody (primary antibody)- well washed out again. If no primary antibodies are present, all secondary ones will wash away
  • A solution is added to the well that contains a substrate which can react with the enzyme attached to the secondary antibody to produce a coloured product. A colour change in the solution indicates the patient is infected with HIV
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HIV

  • HIV is a virus that weakens the immune systems and eventually leads to AIDS, a condition in which the immune system deteriorates and eventually fails
  • HIV infects and eventually kills helper T-cells, which act as host cells for the virus- without these, the immune system can't effectively respond to infections as it's responsible for activating the other immune system cells
  • People with HIV develop AIDS when their helper T-cell numbers have reached a critically low level

HIV has a spherical structure:

  • A core that contains the gentic material (RNA) and some proteins (inc the enzyme reverse ranscriptase, which is needed for virus replication). An outer coating of protein called a capsid, an extra outer layer called an envelope made of membrane stolen from the cell membrane of a previous host cell. Sticking out from the envelope are lots of copies of the attachment protein that allow HIV to attach to the host helper T-cell
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HIV replication

Viruses like HIV reproduce inside of their host cells as it doesn't have the ribosomes and enzymes of its own- so HIV uses that of the helper T-cell:

  • the attachment protein attaches to a receptor molecule on the host cell's cell membrane
  • the capsid is released into the cell, where it uncoats and releases its RNA into the cell's cytoplasm
  • inside the cell, the reverse transcriptase makes a complementary strand of DNA frim the viral RNA template- this forms a double stranded DNA that it inserted into the human DNA
  • host cell enzymes are used to make viral proteins from the viral DNA found within the DNA. The viral proteins are assembled into new viruses that can go and infect new cells

During the infection period, HIV rapidly replicates and the infected person can experience severe flu-like symptoms. After this, HIV replication drops to a lower level; the 'latency period'. This can last for years, and during this time the infected person won't experience any symptoms

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AIDS

  • HIv sufferes are classed as having AIDS when symptoms of their failing immune system appear or their helper T-cell count drops below a certain level. AIDS sufferers generally develop diseases that wouldnt normally cause serious problems to healthy people. The length of time between HIV infection and developing AIDS varies, but is usually around 10 years.
  • Initial AIDS symptoms inc minor infections of mucous membranes and reccuring respiratory infections
  • The immune system further deteriorates as AIDS progresses and people become susceptible to more serious infections, inc TB
  • During the late stages of AIDS, patients have a very low number of immune system cells and can develop a range of very serious illnesses. These are what kill the patient, not AIDS/HIV itself
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HIV treatment

Antibiotics don't work against viruses as they kill bacteria by interfering with their metabolic reactions, targeting the bacterial enzymes and ribosomes involved in these. But as antiboitics target bacterial enzymes/ribosomes and viruses use human cells to reproduce, antibiotics can't inhibit them as they don't target human processes. Most antiviral drugs are designed to target the few virus-sepcific enzymes that exist- HIV uses reverse transcriptase (which human cells don't have), so the reverse-transcriptase inhibitors can target the virus without damaging the host cell

There's no cure for HIV but antiviral drugs can be used to slow down its progression. The best way to control HIV in a population is to reduce its spread by having protected sex to reduce infection via shared (infected) bodily fluids. HIV can also be spread from a HIV- positive mother to her fetus. Not all of these babies are born infected, and taking antiviral drugs during pregnancy can reduce this risk

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