Cells Flashcards
List the organelles of an animal cell
Plasma membrane
RER
SER
Nucleolus
Nucleus
Nuclear envelope
Mitochondria
Cytoplasm
Golgi apparatus
Nuclear envelope
Ribosome
Lysosome
What does a plant cell have, that an animal cell does not?
Cellulose cell wall with plasmodesmata
A vacuole
Chloroplasts
What is plasmodesmata?
Channels for exchanging substances between adjacent cells
What else might you see in a plant cell?
Starch grains
What are starch grains used for?
To store excess sugars
What makes an algal cell different to a plant cell?
The chloroplasts in algal cells are a different shape and size to plant chloroplasts
e.g. some algae have one large chloroplast rather than several small
What makes fungal cells different to plant cells?
Cell walls made of chitin, not cellulose
Don’t have chloroplasts
What is the structure of the plasma membrane?
Found on surface of animal cells and just inside cell wall of other cells. Mainly made of lipids and protein
What is the function of plasma membrane?
Regulates the movement of substances into and out of cell
Also has receptor molecules on it, which allow it to respond to chemicals like hormones
What is the structure of the nucleus?
Large organelle
Surrounded by nuclear envelope (double membrane) which contains many pores
Nucleus contains chromosomes (made from protein-bound linear DNA)
Contains a nucleolus
What is the function of the nucleus?
Controls cell’s activities (by controlling transcription of DNA)
DNA contains instructions to make proteins
The pores allow substances (e.g. RNA) to move between nucleus and cytoplasm
The nucleolus makes ribosomes
What is the structure of mitochondrion?
Oval-shaped
Double membrane
Inner membrane folded to form cristae
Inside is matrix, contains enzymes involved in respiration
What is the function of mitochondrion?
Site of aerobic respiration
Produces ATP - common energy source in cell
Found in large numbers in cells that are very active and require a lot of energy
What is the structure of chloroplasts?
Small, flattened structure found in plant cells and algal cells
Surrounded by double membrane
Has membranes inside called thylakoid membranes
Membranes are stacked to form grana
Grana linked my lamellae - thin, flat pieces of thylakoid membrane
What is the function of chloroplasts?
Site of photosynthesis
Some parts happen in grana, other in stroma (thick fluid)
What is the structure of the golgi apparatus?
Group of fluid-filled membrane-bound flattened sacs
Vesicles are often seen at the edges of the sacs
What is the structure of the golgi apparatus?
It process and packages new lipids and proteins
Makes lysosomes
What is the structure of the golgi vesicle?
Small fluid-filled sac in cytoplasm
Surrounded by membrane
Produced by golgi apparatus
What is the function of the golgi vesicle?
Stores lipids and proteins made by the golgi apparatus and transports them out of the cell (via plasma membrane)
What is the structure of lysosomes?
Round organelle surrounded by a membrane with no clear internal structure
Type of golgi vesicle
What is the function of lysosomes?
Contains digestive enzymes called lysozymes which are kept separate from cytoplasm by surrounding membrane
Can be used to digest invading cells or to break down worn out components of the cell
What is the structure of ribosomes?
Very small organelle that floats free in cytoplasm or is attached to RER
Made up of protein and RNA
Not surrounded by membrane
What is the function of ribosomes?
Site where proteins are made
What is the structure of the RER?
A system of membranes enclosing a fluid-filled space
Surface is covered in ribosomes
What is the function of the RER?
Folds and processes proteins that have been made at the ribosomes
What is the structure of the SER?
Similar to RER but with no ribosomes
What is the function of the SER?
Synthesises and processes lipids
What is the structure of the cell wall?
Rigid structure that surrounds cells in plants, algae and fungi
In plants and algae it’s made of mainly cellulose
In fungi it’s made of chitin
What is the function of the cell wall?
Supports cell and prevents them from changing shape
What is the structure of the vacuole?
Membrane-bound organelle found in the cytoplasm
Contains cell sap - a weak solution of sugar and salts
Surrounding membrane is called the tonoplast
What is the function of the cell vacuole?
Helps to maintain pressure inside the cell and keep it rigid
Helps stop plant wilting
Involved in isolation of unwanted chemicals inside the cell
Why do cells become specialised?
To carry out specific functions
How are epithelial cells in the small intestine adapted to its function?
-Wall of small intestine have lots of villi which increase surface area
-Epithelial cells on the surface of the villi have folds in their plasma membranes, called microvilli. Microvilli increase the surface area more
-Lots of mitochondria - to provide energy for the transport of digested food molecules into the cell
How are red blood cells adapted to their function?
-Adapted to carry oxygen around body
-No nucleus to make more room for the oxygen- carrying compound haemoglobin
How are sperm cells adapted to their function?
Contain lots of mitochondria to provide the large amounts of energy they need to propel themselves towards an egg
What is a tissue?
A group of cells working together to perform a particular function
What are organs?
Different tissues working together
What is an organ system?
Different organs working together
Are prokaryotic cells bigger or smaller than eukaryotes?
Smaller
What is the cell wall of a prokaryote made of?
Murein - glycoprotein (protein with carbohydrate attached)
What is the plasma membrane made of in eukaryotic cells?
Lipids and proteins
What is the the difference between eukaryotic ribosomes and prokaryotic ribosomes?
Eukaryotic are 80s
Prokaryotic are 70s
What is the slime capsule for on a prokaryotic cell?
Made of secreted slime
Sticky outer coating that protects cell from drying out
Protects bacteria from attack by cells of immune system
What are plasmids?
small loops of DNA that aren’t part of the main circular DNA molecule.
Contain genes for things like antibiotic resistance, and can be passed between prokaryotes
Not always present in prokaryotic cells, some have several
What is the prokaryotic version of a nucleus?
Circular DNA
Describe circular DNA
Present as one coiled-up strand
Not attached to histone proteins
What is the flagella?
Long, hair-like structure that rotates to make the prokaryotic cell move
Not all prokaryotes have flagellum, some have more than one
What is the size of prokaryotes compared to eukaryotes?
Prokaryotes less than 2 micrometres in diameter
Eukaryotes up to 50 times bigger (around 0.1mm)
Describe the process of binary fission?
Step 1:
Circular DNA and plasmids replicate. Main DNA loop is only replicated once, but plasmids can be replicated loads of times
Step 2:
The cell gets bigger and the DNA loops move to opposite ‘poles’ (ends) of the cell
Step 3:
The cytoplasm begins to divide (and new cell walls begin to form)
Step 4:
Cytoplasm divides and two daughter cells are produced. Each daughter cell has one copy of one circular DNA, but can have a variable number of copies of the plasmids
What are viruses?
Nucleic acids surrounded by protein
What is a host cell?
The cells that viruses invade and reproduce inside
What is the structure of a virus?
Capsid
Attachment proteins sticking out of it
What are the attachment proteins for?
Lets virus cling onto suitable host cell
Describe the process of viral replication?
1) Virus attaches to host cell receptor proteins
2) Genetic material is released into host cell
3) Genetic material and proteins are replicated by host cell
4) Viral components assemble
5) Replicated viruses released from host cell
What is the equation for magnification?
Image size / actual size
What is resolution?
How well a microscope distinguishes between two points that are close together
What does an optical microscope do?
Use light to form an image
Max resolution of 0.2 micrometres
Can see ribosomes, ER, lysosomes, and nucleus
Max magnification is about x1500
What does an electron microscope do?
Use electrons to form an image
Higher resolution than optical microscopes so give more detailed image
Max resolution of 0.0002 micrometres
Maximum useful magnification of an electron microscope is about x1500000
Electron microscopes produce black and white images, but these are often coloured by a computer
What are the two types of electron microscope?
Transmission
Scanning
How does a TEM work?
Use electromagnets to focus a beam of electrons, which is then transmitted through the specimen.
Denser parts of the specimen absorb more electrons, making them look darker on final image
What is a positive to using TEMs?
High resolution images, so you see the internal structure of organelles like chloroplasts
What is a negative to using TEMs?
Have to view specimen in vacuum, so they’re no good for looking at living organisms
They can also only be used on thin specimens
How does a SEM work?
Scan a beam of electrons across specimen
This knocks off electrons from the specimen, which are gathered in a cathode ray tube to form an image
The images you end up with show the surface of the specimen
Can be 3D
What is a positive to using SEMs?
Can be used on thick specimens
What is a negative to using SEMs?
Give lower resolution images than TEMs
How do you prepare a microscope slide?
- Start by pipetting a small drop of water onto the centre of the slide
- Then use tweezers to place a thin section of your specimen on top of the water drop
- Add a drop of stain
- Add cover slip. To do so, stand the slip upright on the slide, next to the water droplet. Then carefully tilt and lower it so it covers the specimen
What is a temporary mount?
Where the specimen is suspended in a drop of liquid (e.g. water, oil) on the slide
Why does the specimen section need to be thin?
Need to let light through to be able to see it through microscope
What is the stain used for?
Highlight objects in a cell
What will air bubbles do to the specimen?
Obstruct the view of specimen
What is a microscope artefact?
Things you can see down a microscope that aren’t part of the cell or specimen you’re looking at
Explain why it wouldn’t be possible to n determine the identity of a structure in an image using an optical microscope
Resolution too low
Wavelength of light too long
What is cell fractionation used for?
To look at organelles under an electron microscope, separating them from the rest of the cell
What are the three steps of cell fractionation?
Homogenisation
Filtration
Ultracentrifugation
Describe homogenisation
-Can be done in several ways e.g. vibrating cells or by grinding the cells up in a blender
-This breaks up the plasma membrane and releases organelles into solution
What should the solution be during homogenisation?
Ice-cold, isotonic, buffer
Why does the homogenisation solution need to be ice-cold?
Reduce activity of enzymes that break down organelles
Why does the homogenisation solution need to be isotonic?
Solution should have same concentration of chemicals as the cells being broken down, to prevent damage to the organelles through osmosis
Why does the homogenisation solution need a buffer added?
To maintain pH
Describe filtration
Homogenised cell solution is filtered through a gauze to separate any large cell debris or tissue debris, like connective tissue, from the organelles.
The organelles are much smaller than the debris, so they pass through the gauze
Describe ultracentrifugation
Cell fragments are poured into a tube. The tube is put into a centrifuge and spun at low speed.
Heaviest organelles, like nuclei, get flung to the bottom - the pellet. The rest of the organelles stay suspended in the fluid above the sediment - the supernatant
This happens again and again, where centrifuge is spun at a higher speed each time
This happens until all organelles are separated out. Each time the pellet at bottom of tube is made up of light organelles
The organelles are separated in order of mass (heaviest to lightest) - nuclei, chloroplasts, mitochondria, lysosomes, ER, ribosomes
Describe how a sample of chloroplasts could be isolated from leaves
Break open cells by blending and filter
In cold, isotonic buffer solution
Centrifuge and remove nuclei
Centrifuge at higher speed, chloroplasts settle out
Explain why an organelle may not be seen in an image?
A section/slice so organelle in another part of cell
Nucleus not stained
What does a longer wavelength of light mean in terms of microscopes?
Light has longer wavelength so lower resolution
What is the criteria for drawing a biological image (organelle)?
No sketched/hanging/crossing lines/shading
Must look similar
Key components (e.g. matrix and crista correctly labelled)
Correct scale stated (e.g. x 62 800)
What does the cell cycle consist of?
Interphase
Mitosis
What are the three growth stages of interphase?
G1
S
G3
What happens during G1 of interphase?
cell grows and new organelles and proteins are made
What happens during S phase of interphase?
Synthesis
-Cell replicates its DNA, ready to divide by mitosis
What happens during G2 phase of interphase?
Cell keeps growing and proteins needed for cell division are made
Describe what happens in interphase
Cell’s DNA is unravelled and replicated, to double its genetic content. The organelles are also replicated so it has spare ones, and its ATP content is increased
What is mitosis?
Parent cell divides to produce two identically identical daughter cells
Why is mitosis needed?
Growth of multicellular organisms and for repairing damaged tissues
What are the stages of mitosis?
Prophase
Metaphase
Anaphase
Telophase
Describe what happens in prophase
The chromosomes condense, getting shorter and fatter.
Centrioles start moving to opposite ends of the cell, forming a network of protein fibres across it called the spindle
Nuclear envelope breaks down and chromosomes lie free in the cytoplasm
Describe what happens in metaphase
Chromosomes line up along the middle of the cell and become attached to the spindle by their centromere
Describe what happens in anaphase?
Centromeres divide, separating each pair of sister chromatids
Spindles contract, pulling chromatids to opposite poles of the spindle, centromere first
This made the chromatids appear v-shaped
Describe what happens in telophase
Chromatids reach opposite poles on the spindle
They uncoil and becomes long and thin again
They’re now called chromosomes again
A nuclear envelope forms around each group of chromosomes, so there are now two nuclei
The cytoplasm divides (cytokinesis) and there are now two daughter cells that are genetically identical the the original cell and to each other
Mitosis is finished and each daughter cell starts the interphase part of the cell cycle to get ready for next round of mitosis
What can uncontrolled cell division lead to?
tumours
cancer
How do you prepare a root tip cell squash?
- Add 1M HCl to boiling tube
Should be enough acid in tube to cover root tip - few mm deep
Put the tube in a water bath that has been allowed to reach 60°C - Use scalpel to cut 1c, from tip from growing root
It needs to be the tip as that’s where growth occurs and so that’s where mitosis takes place - Carefully transfer root tip into boiling tube containing the acid
Incubate for 5 mins - Use tweezers to remove root tip from tube and use pipette to rinse it well with cold water
Leave to dry on paper towel - Place root tip on a microscope slide and cut 2mm from tip
- Use mounted needle to break tip open and spread cells out thinly
- Add few drops of stain and leave it for a few minutes
- Place cover slip over cells and out a piece of folded filter paper on top
Push firmly to squash tissue
What will squashing the tissue do to it?
Makes tissue thinner and allow light to pass through it
What may happen if you smear the cover slip sideways?
Damage chromosomes
What microscope can you look through squashed root tip with?
Optical
How do you use an optical microscope?
Clip slide to stage
Select lowest-powered objective lens
Use coarse adjustment knob to move the objective lens down to just above slide
Look down eyepiece (contains ocular lens) and adjust with fine adjustment knob, until you get a clear image of what’s on the slide
What is the equation for mitotic index?
Number of cells with visible chromosomes / total number of cells observed
What may a high mitotic index indicate?
Constant growing (e.g. in root tip)
Tissue repair taking place
Cancerous growth in the tissue
What is an eyepiece graticule?
Fitted onto eyepiece
Transparent ruler
Number, no units
What is a stage micrometer?
Placed on stage - microscope slide with accurate scale
Has units
Used to work out the value of the divisions on the eyepiece graticule with a particular magnification
What is a plasma membrane?
Barrier between cell and environment, controlling which substances enter and leave the cell
Partially permeable
What does partially permeable mean?
Let some molecules through, not others
What are the three ways substances can diffuse across membranes?
Diffusion
Osmosis
Active transport
What do membranes within cells do?
Membranes around organelles divide the cell into different compartments - they act as a barrier between the organelle and the cytoplasm
They are also partially permeable and control what substances enter and leave the organelle
What is the fluid mosaic model?
Phospholipid molecules form a continuous, double layer (called a bilayer). The bilayer is ‘fluid’ as the phospholipids are constantly moving
Proteins are scattered through the bilayer, like tiles in a mosaic. These include channel and carrier proteins, which allow large molecules and ions to pass through the membrane
Receptor proteins on cell-surface membrane allow cell to detect chemicals released from other cells
Chemicals signal to cell to respond in some way
Also glycoproteins and glycolipids
Also cholesterol
What are proteins with a carbohydrate attached?
Glycoproteins
What are lipids with a carbohydrate attached?
Glycolipids
What is a phospholipid?
They form a barrier to dissolved substances
Hydrophilic head
Hydrophobic tail
Heads face outwards
Centre of bilayer is hydrophobic so membrane doesn’t allow water-soluble substances through it
Small, non-polar substances and water can still diffuse through membrane
Describe cholesterol as part of the plasma membrane
Gives membrane stability
Type of lipid that’s present in all cell membranes
Cholesterol fits between phospholipids, causing them the pack more closely together
This restricts the movement of the phospholipids, making the membrane less fluid and more rigid
Helps maintain shape of animal cells. Important for cells not supported by others e.g. red blood cells
Also has hydrophobic regions, so able to create further barrier to polar substances moving through the membrane
What is the plasma membrane like at temperatures below 0°C?
Phospholipids don’t have much energy, so can’t move very much
Packed closely together and membrane is rigid
But channel proteins and carrier proteins in the membrane denature, increasing permeability of membrane
Ice crystals may form and pierce the membrane, making it highly permeable when it thaws
What is the plasma membrane like at temperatures between 0 and 45°C?
Phospholipids can move around and aren’t packed as tightly together - membrane is partially permeable
As the temperature increases the phospholipids move more because they have more energy - this increases the permeability of the membrane
What is the plasma membrane like at temperatures above 45°C?
Phospholipid bilayer starts to melt and the membrane becomes more permeable
Water inside the cell expands, putting pressure on the membrane
Channel proteins and carrier proteins in the membrane denature so they can’t control what enter or leaves the cell - this increases permeability of the membrane
How do you investigate cell membrane permeability?
- Use a scalpel to carefully cut five equal sized pieces of beetroot
Rinse the pieces to remove any pigment released during cutting - Add the five pieces to five different test tubes, each containing 5cm3 of water. Use a measuring cylinder or pipette to measure the water
- Place each test tube in a water bath at a different temperature for the same length of time
- Remove the pieces of beetroot from the tubes, leaving just the coloured liquid
- Use colorimeter. Switch on and allow 5 mins for it to stabilise. Then set up to use blue filter
- Add distilled water to cuvette’s so it is three quarters full. Put cuvette in colorimeter. Calibrate machine to zero
- Use pipette to transfer a sample of liquid from the first test tube to clean cuvette
- Put the cuvette in the colorimeter and read and record the absorbance of the solution
- Repeat for liquids in remaining test tubes
What should you do when putting the cuvette in the colorimeter?
Two sides are rigid or frosted, so put in correct way so light passes through the clear sides
What is a colorimeter?
A machine that passes light of a specific wavelength through a liquid and measures how much of that light is absorbed
Why may a filter be used in a colorimeter?
To make sure the light passing through the liquid is at the desired wavelength
What does a higher absorbance reading indicate?
More pigment released, so higher permeability of the membrane
How could you investigate the effect of solvents?
Beetroot to investigate effect of solvents on permeability of plasma membranes
Place beetroot cubes in different concentrations of a particular solvent
Surrounding cells in an increasing concentration of a solvent increases membrane permeability because the solvent dissolves the lipids in the plasma membrane, causing it to lose structure
What is diffusion?
Net movement of particles from an area of higher concentration to an area of lower concentration
What is concentration gradient in diffusion?
The path from an area of higher concentration to an area of lower concentration
What type of process is diffusion?
Passive - no energy required
What is simple diffusion?
When molecules diffuse directly through a plasma membrane
What are three factors affecting the rate of diffusion?
-Concentration gradient: the higher it is, the faster the rate of diffusion. As diffusion takes place, the difference in conc. between the two sides of the membrane decreases until it reaches an equilibrium. This means that diffusion slows down over time
-Thickness of exchange surface: the thinner the exchange surface (i.e. the shorter the distance the particles have to travel), the faster the rate of diffusion
-The surface area: the larger the surface area, the faster the rate of diffusion
What is facilitated diffusion?
Large or charged particles diffuse through carrier proteins or channel proteins in the cell membrane
What type of process is facilitated diffusion?
Passive - doesn’t use energy
What are the two types of protein involved in facilitated diffusion?
Carrier and channel
What is a carrier protein?
Move large molecules across the membrane, down their concentration gradient
How do carrier proteins work?
Large molecule attaches to carrier protein in the membrane
Protein changes shape
Releases molecule on opposite side of membrane
What is a channel protein?
Form pores in the membrane for charged particles to diffuse through (down their concentration gradient). Different channel proteins facilitate the diffusion for different charged particles
What are the factors affecting the rate of facilitated diffusion?
-Concentration gradient - higher conc. gradient, the faster the rate of facilitated diffusion (up to a point). As equilibrium is reached, the rate of facilitated diffusion will level off
-Number of channel or carrier proteins: Once all the proteins in a membrane are in use, facilitated diffusion can’t happen any faster, even if you increase the concentration gradient
What is osmosis?
Diffusion of water molecules across a partially permeable membrane, from an area of higher water potential to an area of lower water potential
What is water potential?
The potential of water molecules to diffuse out of or into a solution
What is the water potential of pure water?
Zero
What does adding solute do to the water potential?
Lowers the water potential - always negative
What does a more negative water potential mean?
The more negative the water potential, the stronger the concentration of solutes in the solution
What does isotonic mean?
When two solutions have the same water potential
What does hypotonic mean?
Solutions with a higher water potential compared to inside of the cell
What does hypertonic mean?
Solutions with a lower water potential than the cell
What are factors affecting the rate of osmosis?
-The water potential gradient: the higher the water potential gradient, the fast the rate of osmosis. As osmosis takes place, the difference in water potential on either side of the membrane decreases, so the rate of osmosis levels off over time
-Thickness of exchange surface: the thinner the exchange surface, the faster the rate of osmosis
-The surface area of the exchange surface: the larger the surface area, the faster the rate of osmosis
How do you make serial dilutions?
For example making five serial dilutions of sucrose solution, starting with an initial sucrose conc. of 2 M and diluting each solution by a factor of 2
1. Line up five test tubes in a rack
2. Add 10cm3 of the initial 2 M sucrose solution to the first test tube and 5cm3 of distilled water to the other four test tubes
3. Then use pipette to draw 5cm3 of the solution from the first test tube, add it to distilled water int he second test tube and mix solution thoroughly. You now have 10cm3 of solution that’s half as concentrated as the solution in the first test tube (1 M)
4. Repeat the process three more times to create solutions of 0.5 M, 0.25 M, and 0.125 M
How do you use serial dilutions to measure change in mass?
- Use a cork borer to cut potatoes into identically sized chips, around 1cm in diameter. Divide the chips into groups of three and measure the mass of each group using a mass balance
- Place one group into each of sucrose solutions and leave chips in solution for at least 20 minutes
- Remove chips and pat dry gently with a paper towel. Weigh each group again and record your results. Calc percentage change in mass for each group
What is a serial dilution?
Create a set of solutions that decrease in concentration by the same factor each time
What is active transport?
Uses energy to move molecules and ions across plasma membrane, against a concentration gradient. Carrier proteins and co-transporters are involved in active transport
What are the differences between active transport and facilitated diffusion?
-Active transport moves solutes from low to high concentration
-Active transport requires energy
How is ATP used in active transport?
Undergoes hydrolysis, splitting into ADP and Pi (inorganic phosphate)
This releases energy so the the solutes can be transported
What are co-transporters?
Type of carrier protein
Bind two molecules at a time
The conc. gradient of one of the molecules is used to move the other molecule against its own concentration gradient
Describe how co-transport enable the absorption of glucose
STEP 1. Sodium ions are actively transported out of the epithelial cells in the ileum, into the blood, by the sodium potassium pump
This creates a concentration gradient - there’s now a higher concentration of sodium ions in the lumen of the ileum than inside the cell
STEP 2. This causes sodium ions to diffuse from the lumen of the ileum into the epithelial cell, down their conc. gradient. They do this via the sodium-glucose co-transporter proteins. The co-transporter carries glucose into cell with sodium. This makes conc. of glucose inside cell increase
STEP 3. Glucose diffuses out of cell, into blood, down conc. gradient through a protein channel, by facilitated diffusion
What are the factors affecting the rate of active transport?
-Speed of individual carrier proteins - faster they work, faster rate
-Number of carrier proteins present - more proteins, faster rate
-Rate of respiration in cell and availability of ATP. If respiration inhibited, active transport can’t take place
What are antigens?
Molecules that can generate an immune response when detected by body
Usually found on surface of cells, including all body cells
Antigens that aren’t normally found in body are foreign antigens
What are pathogens?
Organisms that cause disease, e.g. bacteria, viruses and fungi
All pathogens have antigens on their surface - which are identified as foreign by immune system cells
What are abnormal body cells?
Cancerous or pathogen-infected cells have abnormal antigens on their surface, which trigger an immune response
What are toxins?
Poisons
Molecules, not cells
Some produced by bacteria
Why do some transplanted organs or blood transfusions get rejected sleep?
The cells will have some antigens that are different to own. The foreign antigens trigger an immune response. This response leads to rejection if drugs aren’t taken to suppress the recipient’s immune system
What is a phagocyte?
Type of white blood cell that carries out phagocytosis. Found in blood and tissues and are first cells to respond to an immune system trigger inside the body
How does phagocytosis work?
Phagocyte recognises foreign antigens on pathogen
Cytoplasm of phagocyte moves round the pathogen, engulfing it
Pathogen is now contained in a phagocytic vacuole in cytoplasm of phagocyte
Lysosome fuses with phagocytic vacuole. Lysozymes break down pathogen
Phagocyte presents the pathogen’s antigens - sticks antigens on its surface to activate other immune system cells. Phagocyte acts as antigen-presenting cell
What is a T-cell?
Another type of WBC
Has receptor proteins on its surface that bind to complementary antigens presented to it by phagocytes. This activates the T-cell
What are the ways in which a T-cell can respond?
Helper T-cells release chemical signals that activate and stimulate phagocytes and cytotoxic T-cells, which kills abnormal and foreign cells. Helper T-cells also activate B-cells, which secrete antibodies
What are B-cells and what do they do?
Type of WBC
Covered with antibodies - proteins that bind to antigens to form an antigen-antibody complex
Each B-cell has a different shaped antibody on its membrane, so different ones bind to different shaped antigens
When the antibody on the surface of a B-cell meets a complementary shaped antigen, it binds to it. This, with substances released from helper T-cells, activates the B-cell
This provides clonal selection
The activated B-cell divides into plasma cells
What is antibody production?
Plasma cells are identical to B-cells (clones).
They secrete loads of antibodies specific to the antigen. These are called monoclonal antibodies. They bind to the antigens on the surface of the pathogen to form lots of antigen-antibody complexes
What is agglutination?
Antibody has two binding sites, so can bind two pathogens at the same time
This means pathogens become clumped together
How is agglutination useful?
Phagocytes bind to the antibodies and phagocytose many pathogens at once
This process leads to the destruction of pathogens carrying this antigen in the body
What are antibodies?
Proteins
Specificity of an antibody depends on its variable regions, which form the antigen binding sites
Each antibody has a variable region with a unique tertiary structure (due to different amino acid sequences) that’s complementary to one specific antigen. All antibodies have the same constant regions
What is a cellular response?
T-cells and other immune system cells that they interact with, e.g. phagocytes, form the cellular response
What is the humoral response?
B-cells, clonal selection and the production of monoclonal antibodies form the humoral response
What is the primary response?
When an antigen enters the body for the first time it activates the immune system
Slow as there aren’t many B-cells that can make the antibody needed to bind to it. Eventually the body will produce enough of the right antibody to overcome the infection
Meanwhile, infected person shows symptoms of disease
After being exposed to antigen, T and B-cells produce memory cells which remain in body for long time
Memory T-cells remember specific antigen and will recognise it a second time round
Memory B-cells record the specific antibodies needed to bind the antigen
Person now immune
What is the secondary response?
If the same pathogen enters the body again, immune system will produce a quicker, stronger immune response - secondary
Clonal selection happens faster
Memory B-cells activated and divide into plasma cells that produce the right antibody to the antigen
Memory T-cells are activated and divide into correct type of T-cells to kill cell carrying antigen
Secondary response often gets rid of pathogen before you begin to show symptoms
What is natural active immunity?
When you become immune after catching a disease
What is artificial active immunity?
When you become immune after you’ve been given a vaccination containing a harmless dose of antigen
What is natural passive immunity?
When a baby becomes immune due to antibodies it receives from its mother, through the placenta and in breast milk
What is artificial passive immunity?
When you become immune after being injected with antibodies from someone else
Contrast active and passive immunity?
-Active requires exposure to antigen, passive does not require exposure to antigen
-With active takes a while for protection to develop, with passive protection is immediate
-With active, memory cells are produced. Whereas with passive, memory cells are not produced
-With active, protection is long-term because the antibody is produced in response to complementary antigen being present in the body. With passive, protection is short-term as the antibodies given are broken down
What are vaccines?
Contain antigens that cause your body to produce memory cells against a particular pathogen, without the pathogen causing disease
What are the ethical issues surrounding the use of vaccines?
-Tested on animals before on humans: some people disagree with animal testing. Also, animal based substances may be used to produce a vaccine, which some people disagree with
-Testing vaccines on humans can be risky: e.g. volunteers may put themselves at unnecessary risk of contracting the disease because they think they’re fully protected (e.g. may have unprotected sex because they have had new HIV vaccine and think they’re protected - vaccine may not work)
-S0me people don’t want to take vaccine due to risk of side effects, but they are still protected due to herd immunity - other people think it’s unfair
-If there was an epidemic of a new disease, there would be a rush to receive a vaccine and difficult decisions would have to be made about who would be the first to receive it
What is antigenic variation?
Different antigens are formed due to changes in genes of a pathogen
How does antigen variation affect the immune response?
When infected a second time, memory cells produced from the first infection will not recognise the different antigens
So immune system has to start again and carry out primary response against new antigens
What are monoclonal antibodies?
Antibodies produced from a single group of genetically identical B-cells (plasma cells). This means they’re all identical in structure
How can monoclonal antibodies be used in pregnancy tests?
Detect the hormone human chorionic gonadotropin (hCG) that’s found in the urine of pregnant women:
-Application area contains antibodies that are complementary to the hCG protein, bound to a coloured bead (blue)
-When urine is applied to the application area any hCG will bind to the antibody on the beads, forming an antigen-antibody complex
-Urine moves up the stick to the test strip, carrying any beads with it
-Test strip contains antibodies to hCG that are stuck in place (immobilised)
-If there is hCG present the test strip turns blue because the immobilised antibody binds to any hCG - concentrating the hCG-antibody complex with the blue beads attached. If no hCG is present, the beads will pass through the test area without binding to anything, and so it won’t go blue
What does ELISA stand for?
Enzyme-linked immunosorbent assay
What does ELISA allow you to do?
To see if a patient has any antibodies to a certain antigen or any antigen to a certain antibody
What can ELISA be used in?
Medical diagnosis to test for pathogenic infections
Allergies
Or anything you can make an antibody for
What does ELISA do?
An antibody is used which has an enzyme attached to it
This enzyme can react with a substrate to produce a coloured product
This causes the solution in the reaction vessel to change colour
If colour change, antigen or antibody in interest is present in tested sample
In some types of ELISA, the quantity of this antigen/antibody can be worked out from intensity of colour change
What is direct ELISA?
Uses single antibody that is complementary to antigen you’re testing for
How does direct ELISA work?
Antigens from a patient sample are bound to the inside of a well in a well plate (a plastic tray with loads of circular pits in it)
A detection antibody (with an attached enzyme) that is complementary to the antigen of interest is added
If the antigen of interest is present in the patient sample, it will be immobilised on the inside surface of the well and the detection antibody will bind to it
The well is then washed out to remove any unbound antibody
Substrate solution added
If the detection antibody is present, the enzyme reacts with the substrate to give a colour change
This is a positive result for presence of the antigen
How is indirect ELISA different to direct ELISA?
Uses two different antibodies
How does indirect ELISA work?
HIV antigen is bound to bottom of well plate
Sample of patient’s blood plasma is added to well
If there are any HIV-specific antibodies in the plasma these will bind to the HIV antigen stuck to the bottom of the well
The well is then washed out to remove any unbound antibodies
A secondary antibody, that has a specific enzyme attached to it, is added to the well
This secondary antibody can bind to the HIV-specific antibody (primary antibody)
The well is washed out again to remove any unbound secondary antibody
If no primary antibody in sample, all of secondary antibody will be washed away
A solution is added to well
Solution contains substrate which is able to react with the enzyme attached to the secondary antibody and produce a coloured product
If solution changes colour it indicates that the patient has HIV-specific antibodies in their blood and is infected with HIV
What are the ethical issues surrounding monoclonal antibodies?
Animal rights issues
Animals are used to produces the cells from which the monoclonal antibodies are produced
Some people disagree with the use of animals in this way
What does HIV stand for?
Human immunodeficiency virus
What does HIV do?
Affects the human immune system
What does HIV lead to and what does that do?
AIDS
AIDS is a condition where the immune system deteriorates and eventually fails
This makes someone with AIDS more vulnerable to other infections, like pneumonia
What does HIV infect and kill?
Helper T-cells which act as host cells for the virus
What does helper T-cells do?
Send chemical signals that activate phagocytes, cytotoxic T-cells and B-cells so they’re hugely important cells in the immune response
When does AIDS develop?
When people infected with HIV’s helper T-cells reach a critically low level
What happens in the initial infection period?
HIV replicates rapidly and the infected person may experience severe flu-like symptoms
What happens after initial infection?
Latency period: HIV replication drops to a lower level
What does the infected person feel during the latency period and how long can this last?
HIV replication drops to a lower level
Can last years
What are the initial symptoms of AIDS?
Minor infections of mucous membranes (e.g. inside nose, ears and genitals)
Recurring respiratory infections
What happens as AIDS progresses?
Number of immune system cells decreases further
Patients become susceptible to more serious infections including chronic diarrhoea, severe bacterial infections and tuberculosis
What happens during the late stages of AIDS?
Patients have low number of immune system cells and can develop a range of serious infections such as toxoplasmosis of the brain (parasite infection) and candidiasis of the respiratory system (fungal infection)
What is the structure of HIV?
Spherical structure
Core containing RNA
and some proteins (including the enzyme reverse transcriptase)
Outer coating of protein called a capsid and an extra outer layer called and envelope, which is made of membrane stolen from the cell membrane of a previous host cell. Sticking out from the envelope are loads of copies of an attachment proteins that help HIV attach to the host helper T-cell
Describe the process of HIV replication
- Attachment protein attaches to a receptor molecule on the cell membrane of the host helper T-cell
- The capsid is released in the cell, where it uncoats and releases the genetic material into the cell’s cytoplasm
- Inside the cell, reverse transcriptase is used to make a complementary strand of DNA from the viral RNA template
- From this, double-stranded DNA is made and inserted into the human DNA
- Host cell enzymes are used to make viral proteins from the viral DNA found within the human DNA
- The viral proteins are assembled into new viruses, which bud from the cell and go on to infect other cells
