Cells Flashcards
• Cell structure • Transport across cell membranes • Cell recognition and the immune system
What is the maximum resolution of a light microscope?
2x10^-7 m
What is the limitation with light microscopes?
The relatively long wavelength of light
What is the resolution of an electron microscope?
1x10^-10 m
What is the material put under the microscope called?
The object
What is the appearance of the object when viewed under a microscope called?
The image
What is the magnification?
How many times bigger the image is when compared to the object
How do you work out magnification?
Magnification = Size of image / Size of real object
What is the resolution of a microscope?
The minimum distance apart that two objects can be in order for them to appear as separate items
What is the resolving power of a microscope?
The minimum distance apart that two objects can be in order for them to appear as separate items
What does greater resolution do to an image?
Greater image clarity
What does greater magnification do to an image?
Increases the size of the image
What happens to the image if you increase magnification beyond the limit of resolution?
The image will be larger, but more blurred
What is cell fractionation?
The process where cells are broken up and the different organelles they contain are separated out
What three things must the solution in which the tissue is placed be for cell fractionation?
Cold
Isotonic
Buffered
Why must you use a cold solution to place the tissue in for cell fractionation?
To reduce enzyme activity that might break down organelles
Why must you use an isotonic solution to place the tissue in for cell fractionation?
To prevent organelles bursting or shrinking as a result of osmotic gain or loss of water
Why must you use a buffered solution to place the tissue in for cell fractionation?
So that pH doesn’t fluctuate
Why must the pH remain constant for cell fractionation?
Any change in pH could alter the structure of organelles or affect the functioning of enzymes
What are the two stages in cell fractionation?
Homogenation
Ultracentrifugation
What happens in homogenation?
Cells broken up by homogeniser Resultant fluid (homogenate) is filtered
Why are cells broken up by a homogeniser in homogenation?
To release the organelles from the cells
Why is the homogenate filtered in homogenation?
To remove any complete cells and large pieces of debris
What is ultracentrifugation?
The process by which fragments in the filtered homogenate are separated in a centrifuge
What does a centrifuge do?
Spins tubes of homogenate at very high speed in order to create a centrifugal force
In ultracentrifugation what speed do you start to spin at?
The lowest, working up to highest after removing the sediment/pellet
After a sample has been spun in a centrifuge, what is the fluid at the top of the tube called?
Supernatant
After a sample has been spun in a centrifuge, what is the layer at the bottom of the tube called?
The sediment or pellet
In ultracentrifugation, which part of the sample is spun again at a higher speed?
Supernatant
In animal cells, what are the first three organelles to form sediment (in order)?
Nuclei
Mitochondria
Lysosomes
In plant cells what are the first three organelles to form sediment (in order)?
Nuclei
Chloroplasts
Mitochondria
What is the speed of centrifugation that will cause nuclei to form the sediment?
1000 revolutions/minute
What is the speed of centrifugation that will cause mitochondria to form the sediment?
3500 revolutions/minute
What is the speed of centrifugation that will cause lysosomes to form the sediment?
16500 revolutions/minute
Why is cell fractionation important?
Allows scientists to study isolated components of cells
What are the two main advantages of an electron microscope?
High resolution
Beam can be focused using electromagnets
Why does an electron microscope have a high resolution?
The electron beam has a very short wavelength
Why can the electron beam in an electron microscope be focused using electromagnets?
Electrons are negatively charged
What has to be created within the chamber of an electron microscope in order for it to work properly?
A vacuum
Why does a vacuum have to be created in order for an electron microscope to work properly?
Electrons can be absorbed or deflected by molecules in air
What are the two types of electron microscope?
Transmission electron microscope (TEM)
Scanning electron microscope (SEM)
What is the resolution of a TEM?
0.1nm
What is the resolution of an SEM?
20nm
Why can a resolution of 0.1nm always be achieved with a TEM?
Difficulties preparing the specimen limit the resolution
A higher energy electron beam is required which could destroy the specimen
What are the main limitations of a TEM?
Whole system must be in a vacuum so living specimens cannot be observed
Complex staining process required (even then image isn’t in colour)
Specimen must be extremely thin
Image may contain artefacts
What are artefacts?
Things that result from the way that the specimen is prepared
What is a photomicrograph?
A picture taken of the image produced by a TEM on a screen
Why does a TEM produce a 2D image?
Specimens must be extremely thin
Why must specimens be extremely thin in order for them to be observed under a TEM?
So electrons can penetrate the specimen
How does a TEM form an image of a specimen?
Parts of the specimen absorb electrons and appear dark
Other parts allow electrons to pass through and so appear bright
What does a TEM consist of?
An electron gun that produces a beam of electrons
A condenser electromagnet that focuses the electron beam onto the specimen
What limitation of the TEM doesn’t apply to the SEM?
Specimens need to be extremely thin (not for SEM)
Why don’t specimens need to be thin for the SEM?
Electrons do not need to penetrate
How does the SEM direct an electron beam on the specimen?
From above onto the surface of the specimen, it is then passes back and forth across a portion of the specimen in a regular pattern
What happens to the electrons in an SEM?
The electrons are scattered by the specimen
What does the pattern of electron scattering in an SEM depend on?
The contours of the specimen’s surface
How can an SEM be used to produce a 3D image of a specimen?
Computer analysis of the pattern of scattered electrons and secondary electrons produced
What is used to measure the size of an object being observed under a light microscope?
Eyepiece graticule
What is needed to calibrate the eyepiece graticule?
Stage micrometer
How do you calculate the scale for different objective lenses?
By dividing the differences in magnifications
Cells have a specific internal structure to suit their functions. What is this known as?
Ultrastructure
What is the nucleus made up of?
Nuclear envelope Nuclear pores Nucleoplasm Chromosomes Nucleolus
What is the function of the nucleus?
Production of mRNA and tRNA (and hence protein synthesis)
Retain genetic information
Manufacture ribosomal RNA and ribosomes
What is the nuclear envelope?
A double membrane that surrounds the nucleus
What is the outer membrane of the nuclear envelope continuous with?
The endoplasmic reticulum
What is often found on the surface of the nuclear envelope?
Ribosomes
What is the purpose of the nuclear envelope?
Controlling entry and exit of materials in and out of the nucleus
Containing reactions taking place within the nucleus
On average how many nuclear pores are there in each nucleus?
Around 3000
Roughly how big are nuclear pores in diameter?
40-100nm in diameter
What is the purpose of nuclear pores?
Allowing the passage of large molecules (e.g. RNA) out of the nucleus
What is the nucleoplasm?
The granular, jelly-like material that makes up the bulk of the nucleus
What do chromosomes consist of?
Protein bound, linear DNA
What is the nucleolus?
A small spherical region within the nucleoplasm
What does the nucleolus do?
Manufactures ribosomal RNA and assembles to ribosomes
How many nucleoli are found in a nucleus?
There may be more than one
Roughly how big is a ‘normal’ nucleus?
10-20 micrometers in diameter
How long is a mitochondrion in length normally?
1-10 micrometers
What is a mitochondrion made up of?
Double membrane
Cristae
Matrix
What is the purpose of the double membrane around a mitochondrion?
To control the entry and exit of material to and from the mitochondrion
What is the inner membrane around a mitochondrion folded to form?
Extensions known as cristae
What are cristae?
Extensions of the inner membrane
What purpose do cristae serve?
Increasing surface area for the attachment of enzymes and other proteins required for respiration
What does the matrix do?
Makes up the rest of the mitochondrion
What is found in the matrix?
Proteins Lipids Ribosomes DNA Enzymes
What are the enzymes found in the matrix involved in?
Respiration
What does the DNA found in the matrix allow the mitochondrion to do?
Control the production of some of the mitochondrion’s own proteins
How many mitochondria are typically found in a cell?
It varies depending on the function of the cell
What cells will contain a large amount of mitochondria?
Cells that have a high level of metabolic activity
What are chloroplasts made up of?
Chloroplast envelope
Grana
Thylakoids
Stroma
What is the function of chloroplasts?
To carry out photosynthesis
What is the chloroplast envelope?
A double plasma membrane that surrounds the chloroplast
What is the function of the chloroplast envelope?
To be highly selective in what it allows to enter and leave the chloroplast
What are grana?
Stacks of up to 100 thylakoids
What takes place in the grana?
The first stages of photosynthesis (light absorption)
What is found in thylakoids?
Chlorophyll
What is chlorophyll?
The photosynthetic pigment
What do some thylakoids have?
Tubular extensions
If a thylakoid has a tubular extension, what does the extension do?
Join up with thylakoids in adjacent grana
What is the stroma?
A fluid filled matrix
What takes place in the stroma?
The second stage of photosynthesis (synthesis of sugars)
What can be found in the stroma?
A number of other structures (e.g. starch grains)
What does the fluid of the stroma possess?
All the enzymes required to make sugars in the second stage of photosynthesis
What do the granal membranes do?
Provide a large surface area
What attaches to the granal membrane?
Chlorophyll
Electron carriers
Enzymes required for the first stage of photosynthesis
How are chemicals attached to the granal membrane?
In a highly ordered fashion
Do all plant cells contain chloroplasts?
No
What do chloroplasts contain?
DNA and ribosomes
Why do chloroplasts contain DNA and ribosomes?
So chloroplasts can quickly and easily manufacture some of the proteins required for photosynthesis
What are the two type of endoplasmic reticulum?
Smooth endoplasmic reticulum (SER)
Rough endoplasmic reticulum (RER)
What is the endoplasmic reticulum?
An elaborate, 3D system of sheet-like membranes
Where is the endoplasmic reticulum found?
Spread through the cytoplasm of cells
What is the endoplasmic reticulum continuous with?
The outer nuclear membrane
What do the membranes in the endoplasmic reticulum enclose?
A network of tubules and flattened sacks called cisternae
What is the general structure of the rough endoplasmic reticulum?
Ribosomes present on outer surfaces of the membranes
What is the general structure of the smooth endoplasmic reticulum?
More tubular in appearance than the RER
No ribosomes on outer surfaces of membranes
What are the functions of the rough endoplasmic reticulum?
Provide a large surface area for the synthesis of proteins and glycoproteins
Provide a pathway for the transport of material (especially proteins) throughout the cell
What are the functions of the smooth endoplasmic reticulum?
Synthesise lipids Synthesise carbohydrates Store lipids Store carbohydrates Transport lipids Transport carbohydrates
What cells have a very extensive endoplasmic reticulum?
Cells that manufacture and store large quantities of lipids, proteins, and carbohydrates
What are cisternae?
Flattened membrane discs/sacks
What is the Golgi apparatus similar in structure to?
Smooth endoplasmic reticulum (but more compact)
What passes through the Golgi apparatus?
Proteins and lipids produced by the endoplasmic reticulum
What does the Golgi apparatus do to the proteins that pass through it?
Modifies the proteins (often by adding a non-protein group)
‘Labels’ the proteins
Why does the Golgi apparatus ‘label’ proteins?
To allow the proteins to be accurately sorted and sent to the correct destinations
What are regularly pinched off from the ends of the Golgi cisternae?
Golgi vesicles
What happens if Golgi vesicles move to the cell surface?
They fuse with the membrane and release their contents to the outside of the cell
Where are glycoproteins formed?
Golgi apparatus
What does the Golgi apparatus add to proteins to make glycoproteins?
Carbohydrates
What are the functions of the Golgi apparatus?
Form glycoproteins Produce secretory enzymes Secrete carbohydrates Transport, modify and store lipids Form lysosomes
Where is the Golgi apparatus especially well developed?
In secretory cells
When are lysosomes formed?
When Golgi vesicles contain enzymes (such as proteases and lipases)
What enzymes do lysosomes isolate from the rest of the cell?
Lysozymes
How many lysozymes can a lysosome contain?
As many as 50
Where do lysosomes release lysozymes?
Either to the outside of the cell or into a phagocytic vesicle within the cell
What are the functions of lysosomes?
Hydrolyse material ingested by phagocytic cells Exocytosis Autolysis Digest warn out organelles Apoptosis
What is exocytosis?
A process by which a cell transports secretory products through the cytoplasm to the plasma membrane
Why do lysosomes carry out exocytosis?
To destroy material around the cell
What is autolysis?
The process by which lysosomes cause a cell to self-digest itself from the inside out (often as a response to illness or injury)
What cells are most susceptible to autolysis?
Unhealthy cells
What is apoptosis?
Programmed cell death that occurs in specific biochemical steps leading to characteristic morphological changes
What are phagocytic cells?
Specialized cells that engulf and ingest other cells or particles
What is an example of a phagocytic cell found in vertebrates?
Phagocytes (specific type of white blood cells)
What is the function of a ribosome?
Protein synthesis
What are the two types of ribosome?
80S and 70S
Where are 80S ribosomes found?
Eukaryotic cells
Where are 70S ribosomes found?
Prokaryotic cells
Mitochondria
Chloroplasts
How big are 80S ribosomes?
About 25nm in diamater
How many subunits do ribosomes have?
2
What sub-units are ribosomes made up of?
One large sub-unit
One small sub-unit
What do the ribosomal sub-units contain?
Ribosomal RNA
Protein
How many types of ribosome are there?
2
What do cell walls consist of?
Cellulose microfibrils embedded in a matrix
What is the purpose of the cellulose microfibrils found in cell walls?
To contribute to the overall strength of the cell wall
What marks the boundary between adjacent cell walls?
The middle lamella
What cements adjacent cells together?
The middle lamella
What is the middle lamella?
A thin layer found in the cell wall
What are the functions of the cell wall?
Provide mechanical strength in order to prevent osmotic rupture
Give mechanical strength to the plant as a whole
Contribute to the movement of water along plants
How do cell walls contribute to the movement of water along plants?
By allowing water to pass along
What are the cell walls of algae made up of?
Cellulose or Glycoproteins or Cellulose & Glycoproteins
Do the cell walls of fungi contain cellulose?
No
What is chitin?
A nitrogen-containing polysaccharide
What are the cell walls of fungi made up of?
Chitin
Glycan
Glycoproteins
What is glycan?
A polysaccharide
What is a vacuole?
A fluid-filled sac bounded by a single membrane
What is the membrane around plant vacuole’s called?
The tonoplast
What does a plant vacuole contain?
A solution of: mineral salts, sugars, amino acids, wastes and pigments
What pigments can be found in a plant vacuole?
Anthocyanins
What functions do plant vacuoles serve?
Support for herbaceous plants and herbaceous parts of woody plants
Temporary food store
Attracting pollinating insects
How do plants vacuoles provide support?
By making cells turgid
How do plant vacuoles act as a temporary food source?
The sugars and amino acids they contain can be used as such
How do plant vacuoles attract pollinating insects?
The pigments they contain may colour petals to attract pollinating insects
What are similar cells grouped together to form?
Tissues
What are similar tissues grouped together to form?
Organs
What are similar organs grouped together to form?
Organ Systems
How do cells become specialised?
Controlled gene expression
Why are similar cells grouped together?
For working efficiency
What is an example of a tissue found in animals?
Epithelial tissue
What is an example of a tissue found in plants?
Xylem tissue
What is an organ?
A combination of tissues that are coordinated to perform various functions, often with a predominant major function
What tissues make up the stomach organ?
Muscle
Epithelium
Connective tissue
What tissues make up the leaf organ?
Palisade mesophyll Spongy mesophyll Epidermis Phloem Xylem
Are blood capillaries organs?
No, they are tissues
Are arteries organs?
Yes
Are veins organs?
Yes
Why are blood capillaries considered a tissue?
They are only made up of one tissue (epithelium)
Why are veins considered to be organs?
They are made up of many tissues (muscle, epithelial, etc)
Why are arteries considered to be organs?
They are made up of many tissues (muscle, epithelial, etc)
What are three examples of organ systems?
Circulatory system
Digestive system
Respiratory system
What organs make up the digestive system?
Salivary Glands Oesophagus Stomach Duodenum Ileum Pancreas Liver Etc
What organs make up the respiratory system?
Trachea
Bronchi
Lungs
Etc
What organs make up the circulatory system?
Heart
Arteries
Veins
Etc
What are the two main types of cell?
Prokaryotic
Eukaryotic
Compare prokaryotic and eukaryotic cells in relation to nuclei
Prokaryotic cells have no true nucleus, only an area where DNA is found
Eukaryotic cells have a distinct nucleus with nuclear envelope
Compare prokaryotic and eukaryotic cells in relation to DNA protein association
The DNA found in prokaryotic cells is not associated with proteins
Eukaryotic cells have DNA that is associated with proteins known as histones
Compare prokaryotic and eukaryotic cells in relation to DNA plasmids
Prokaryotic cells may have DNA in the form of circular strands known as plasmids
Eukaryotic cells have linear DNA and no plasmids
Compare prokaryotic and eukaryotic cells in relation to membrane-bound organelles
Prokaryotic cells have no membrane-bound organelles
Eukaryotic cells have membrane-bound organelles (e.g. mitochondria)
Compare prokaryotic and eukaryotic cells in relation to chloroplasts
Prokaryotic cells have no chloroplasts, but some bacteria may have chlorophyll associated with the cell surface membrane
Eukaryotic cells from plants and algae contain chloroplasts
Compare prokaryotic and eukaryotic cells in relation to ribosomes
Prokaryotic cells have smaller ribosomes (70S)
Eukaryotic cells have larger ribosomes (80S)
Compare prokaryotic and eukaryotic cells in relation to cell walls
Prokaryotic cells have a cell wall made of murein (peptidoglycan)
If present, eukaryotic cells have a cell wall made of cellulose (of chitin in fungi)
Compare prokaryotic and eukaryotic cells in relation to capsules
Prokaryotic cells may have an outer mucilaginous layer known as a capsule
Eukaryotic cells have no capsule
What is a capsule?
An outer mucilaginous layer found around some prokaryotic cells
What is the function of a capsule?
Protects bacterium from other cells and helps groups of bacteria to stick together for further protection
What are viruses?
Acellular, non-living organisms
What is bigger, a virus or a bacterium?
A bacterium is bigger than a virus
How big is the average bacterium?
0.1-10 micrometers
How big is the average virus?
20-300 nanometers
Where is the nucleic acid found in a virus?
In the capsid
What parts make up the human immunodeficiency virus (HIV)?
Genetic material (RNA) Reverse transcriptase (enzyme) Capsid Matrix Lipid envelope Attachment proteins
What are attachment proteins attached to on a virus?
The lipid envelope, or (if this isn’t present) the capsid
What are the purpose of attachment proteins on a virus?
To allow the virus to identify and attach to a host cell
How is genetic material contained in a virus?
As nucleic acids
What nucleic acids might a virus contain?
RNA
DNA
What two processes can cell division occur by?
Mitosis
Meiosis
What is the structure of a chromosome?
Two sister chromatids joined by a centromere
What four stages make up mitosis?
Prophase
Metaphase
Anaphase
Telophase
What happens in prophase? (animal and plant)
Chromosomes become visible
Nuclear envelope breaks down
Nucleolus disappears
Chromosomes free in cytoplasm start to be drawn to cell equator
What happens in prophase (but only in animal cells)?
Spindle fibres develop from each centriole
Spindle fibres attach to centromere and draw chromosomes to equator
What do spindle fibres form?
Spindle apparatus
What happens in metaphase?
Chromosomes seen to be made of two chromatids
Microtubules from poles attach to centromere
Chromosomes pulled along spindle apparatus to arrange across equator
What happens in anaphase?
Centromeres divide into 2
Spindle fibres pull individual chromatids apart
Chromatids move rapidly to opposite poles (now called chromosomes)
What provides the energy for anaphase?
Mitochondria
What happens in telophase?
Chromosomes reach respective poles
Chromosomes become longer and thinner then disappear (leaving chromatin)
Spindle fibres disintegrate
Nuclear envelope and nucleolus reform
What are the three parts of the cell cycle?
Interphase
Mitosis or Meiosis
Cytokinesis
What happens in cytokinesis?
The cell membrane (and cell wall if present) divides
How do prokaryotic cells divide?
Binary fission
Describe the process of binary fission
Circular DNA molecule replicates Both DNA copies attach to cell membrane Plasmids replicate Cell membrane grows between two DNA molecules Cytoplasms begins dividing in two Cell wall forms between DNA molecules Cell divides into two daughter cells
Do daughter cells produced by binary fission have equal quantities of plasmids?
No
Do daughter cells produced by binary fission have equal quantities of the circular DNA?
Yes, each cell has one copy
Can viruses undergo cell division?
No
What is mitosis important for?
Growth
Repair
Reproduction of single celled organisms
What stage occupies most of the cell cycle?
Interphase
Roughly how long is a mammalian cell cycle?
24 hours
Roughly what percent of the cell cycle is spent in interphase in a mammalian cell?
90%
What three stages make up interphase?
G1
S
G2
What happens in G1?
Organelles replicated (excluding nucleus)
What happens in S?
Each of the 46 chromosomes replicate
What happens in G2?
Replicated DNA is checked for errors
What do drugs used to treat cancer normally do?
Disrupt the cell cycle
How to cancer drugs disrupt the cell cycle?
Preventing replication of DNA
Interfering with spindle formation and so inhibiting metaphase
Why are cancer cells damaged more than normal cells by cancer treatment drugs?
Cancer cells divide more rapidly
What body cells are more susceptible to damage from cancer drugs?
Rapidly dividing body cells
Why do cancer drugs often cause hair loss?
Hair cells divide rapidly
What name is given to the membranes around cells?
Plasma membranes
What makes up a plasma membrane?
Phospholipids Proteins Cholesterol Glycolipids Glycoproteins
What do phospholipids form in a plasma membrane?
A hydrophobic bilayer
How are phospholipids arranged in a plasma membrane?
2 layers of phospholipids
Hydrophobic tails point inwards
Hydrophilic heads point outwards
What are the functions of the phospholipids in a plasma membrane?
Allow lipid-soluble substances to enter and leave the cell
Prevent water-soluble substances from entering or leaving the cell
Make the membrane flexible and self-sealing
What are the two main types of protein found in a plasma membrane?
Carrier protein
Channel protein
What do channel proteins do?
Allow the diffusion of water-soluble substances across the membrane
Provide structural support
What do carrier proteins do?
Allow active transport across the membrane
Provide structural support
What is the function of cholesterol in a plasma membrane?
Reduce lateral movement of the membrane
Make membrane less fluid at high temperatures
Prevent water leakage
What are the functions of glycolipids in a plasma membrane?
Act as recognition sites
Maintain stability of membrane
Helps cells adhere together and so form tissues
What are the functions of glycoproteins in a plasma membrane?
Act as recognition sites
Help cells adhere together and so form tissues
Allow cells to recognise one another
Why properties of molecules mean that they can’t freely diffuse across a plasma membrane?
Not soluble in lipids so can’t pass through phospholipid bilayer
Too large to pass through channel proteins
Same charge as charge on protein channel
Polar
What is the arrangement model of a plasma membrane known as?
Fluid-mosaic model
Explain the name ‘fluid-mosaic model’
Fluid: individual phospholipid molecules can move relative to one another
Mosaic: proteins embedded in bilayer vary in shape, size and pattern
What is the benefit of individual phospholipid molecules having the ability to move relative to one another?
Membrane has a flexible structure that is constantly changing in shape
What are the functions of membranes within cells?
Control entry and exit of materials in discrete organelles
Separate organelles from cytoplasm
Provide an internal transport system
Isolate enzymes that might damage the cell
Provide surfaces on which reactions can occur
What is diffusion?
The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until they are evenly distributed
What is diffusion an example of?
Passive transport
What does passive mean in passive diffusion?
That the energy required for diffusion comes from the natural motion of particles rather than from an external source (such as ATP)
What sort of process is facilitated diffusion?
Passive
Can diffusion occur between different concentrations of different molecules?
No, diffusion only occurs between different concentrations of the same substance
What factors impact the rate of diffusion?
Concentration gradient
Area over which diffusion takes place
Thickness of exchange surface
Temperature
What is osmosis?
The passage of water through a semi-permeable membrane from a region where it has a higher water potential to a region where it has a lower water potential
What is the water potential of pure water under standard conditions?
0
What will adding a solute to pure water do to the water potential?
Lower it
What must the water potential of a solution be?
Less than zero (a negative number)
What is the relationship between water potential and the amount of solute added?
The more solute added, the lower the water potential of the solution
What is one method of finding the water potential of cells or tissues?
Place samples of the cells or tissues in a series of solutions of different water potentials. Where there is no net gain of water from the cells or tissues, the water potential inside the samples must be equal to that of the external solution.
What is water potential normally measured in?
kPa (kiloPascals)
What is established when the water potentials on either side of a semi-permeable membrane are equal?
Dynamic equilibrium
What is haemolysis?
The bursting of red blood cells due to osmotic intake
Explain what will happen when an animal cell is placed in a solution of higher water potential
Water will enter the cell, causing it to swell and burst open which will then release the contents of the cell
Explain what will happen to an animal cell when it is placed in a solution of equal water potential?
No change will occur to the cell as there will be no net movement of water in or out of the cell because dynamic equilibrium will be established
Explain what will happen to an animal cell when it is placed in a solution of lower water potential
Water will leave the cell and enter the solution which will cause the cell to become shrunken and shrivelled. (If the cell is a red blood cell it will appear darker as haemoglobin is more concentrated)
Explain what will happen when a plant cell is placed in a solution of higher water potential
Water will enter the cell and cause the protoplast to swell. The swollen protoplast will push up against the cell wall and make the cell turgid.
Explain what will happen to a plant cell if it is placed in a solution of equal water potential
There will be no net movement or water and so the protoplast will not change however may begin to pull away from the cell wall depending on the original water potential of the cell. The cell will be at incipient plasmolysis.
What is incipient plasmolysis?
The stage at which the protoplast no longer presses on the cellulose cell wall in a plant cell
Explain what will happen to a plant cell when it is placed in a solution of lower water potential
Water will leave the cell which will cause the protoplast to shrink and completely pull away from the cell wall. When the protoplast has pulled away from the cell wall, the cell is plasmolysed.
Explain why an animal cell placed in pure water will burst but a plant cell place in pure water will not
Both cell have a lower water potential than pure water so water will enter them by osmosis.
The animal cell is only surrounded by a thin cell-surface membrane so it will swell until it bursts.
The plant cell is surrounded by a rigid cellulose cell wall and once the cell is turgid the cell wall will prevent the cell expanding so water cannot enter and the cell will not burst.
What sort of processes are osmosis and diffusion?
Passive processes
What is active transport?
The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins
In active transport, what is ATP used to do?
Directly move molecules
Co-transport
How does active transport differ from passive forms of transport?
Metabolic energy is required (in the form of ATP)
Substances are moved against the concentration gradient
Carrier protein molecules are involved
The process is very selective
Describe the direct active transport of a single molecule or ion
The carrier proteins span the plasma membrane and bind to the molecule or ion due to be transported on one side of the membrane
The molecule or ion binds to receptor sites on the carrier protein
On the inside of the cell or organelle, ATP binds to the protein causing it to split into ADP and a phosphate molecule
The protein molecule changes shape and opens to the opposite side of the membrane
The molecule or ion is then released to the other side of the membrane
The phosphate molecule is released from the protein and so the protein reverts back to its original shape
During respiration the phosphate molecule will recombine with the ADP molecule to form ATP
Compare the similarities and differences of facilitated diffusion and active transport
Both use carrier proteins
Facilitated confusion occurs down a concentration gradient, active transport occurs up a concentration gradient
Facilitated diffusion does not require metabolic energy and so is a passive process, active transport requires metabolic energy
What is the purpose of mircovilli?
To increase the surface area for the insertion of carrier proteins through which diffusion, facilitated diffusion and active transport can take place
What is one mechanism to increase transport across membranes?
Increase the number of protein channels and carrier proteins in any given area of a membrane
Explain the role of diffusion in the absorption of molecules in the ileum
Carbohydrates and proteins are digested continuously so there is normally a greater concentration of glucose and amino acids in the ileum than in the blood
This creates a concentration gradient for glucose to move into the blood via facilitated diffusion
Blood is constantly circulated by the heart so the glucose absorbed into the blood is constantly moved away from the ileum and used up by respiring cells
This maintains the low concentration of glucose in the bloodoutside the ileum and helps to maintain the favourable concentration gradient between the inside of the ileum and the blood
This means the rate of movement by facilitated diffusion across the epithelial cell surface membrane is increased
What mechanism is used to absorb glucose and amino acids from the small intestine?
Co-transport
Why is the movement of glucose or amino acids out of the small intestine an example of indirect active transport?
The glucose or amino acids are drawn into the cell along with sodium ions
The sodium ion concentration gradient powers the movement of glucose or amino acids into the cell
The glucose or amino acids move against the concentration whilst the sodium ions move along the concentration gradient
The concentration of sodium ions is maintained by active transport
Why is the movement of glucose or amino acids into epithelial cells in the ileum an example of co-transport?
Sodium ions move down a concentration gradient into the cell, carrying the glucose or amino acids with them
Describe and explain the absorption of glucose or amino acids into the blood via co-transport
Sodium ions are actively transported out of epithelial cells, by the sodium-potassium pump, into the blood
This maintains a much higher concentration gradient of sodium ions in the lumen if the intestine than inside the epithelial cells
Sodium ions diffuse into the epithelial cells down the concentration through a co-transport protein in the cell surface membrane
As the sodium ion diffuse in through the carrier protein, they carry either amino acid molecules or glucose molecules into the cell with them
The glucose or amino acids pass into the blood plasma by facilitated diffusion through a carrier protein
What are the two types of defence mechanisms against pathogens?
Specific
Non-specific
What are the differences between specific and non-specific defence mechanisms?
Non-specific responses are immediate and the same for all pathogens
Specific responses are slower and specific to each pathogen
What are non-specific defence mechanisms?
Physical barriers
Phagocytosis
What is an example of a physical barrier?
Skin
What are the two types of specific defence mechanisms?
Humoral response
Cell-mediated response
What do specific defence mechanisms involve?
Lymphocytes
What are lymphocytes?
A type of white blood cell
What are the two types of lymphocyte?
T lymphocytes
B lymphocytes
What do cell-mediated responses involve?
T lymphocytes
What do humoral responses involve?
B lymphocytes
In order to act as a defence mechanism, what do lymphocytes need to be able to do?
Distinguish the body’s own cells and material (self), from those that are foreign (non-self)
What would happen if lymphocytes were unable to distinguish self and non-self material?
The lymphocytes would destroy the organism’s own tissues
What is the immune system able to identify?
Pathogens
Non-self material
Toxins
Abnormal cells
What is an example of non-self material?
Cells from other organisms of the same species
What is an example of abnormal cells?
Cancer cells
How do lymphocytes recognise cells belonging to the body?
In the fetus, lymphocytes are constantly colliding with other cells
Infection in the fetus is rare as it is protected by the placenta
Lymphocytes will therefore almost exclusively collide with the body’s own material (self)
Some of the lymphocytes will have receptors which exactly fit those of the body’s own cells
These lymphocytes will either die or be suppressed
The only remaining lymphocytes are those that might fit foreign material (non-self), and therefore only respond to foreign material
In adults lymphocytes produced in the bone marrow initially only encounter self-antigens
Any lymphocytes that show an immune response to these self-antigens undergo apoptosis before they can differentiate into mature lymphocytes
No copies of any anti-self lymphocytes will appear in the blood, leaving only lymphocytes that might respond to non-self antigens
What are the two types of white blood cell?
Lymphocytes and phagocytes
What type of white blood cell is involved in immune response?
Lymphocytes
What is the process by which phagocytes ingest and destroy pathogens called?
Phagocytosis
Where are phagocytes found?
Some phagocytes travel in the blood but can move out of blood vessels and into other tissues
Describe the process of phagocytosis
Chemical products of pathogens or dead, damaged and abnormal cells act as attractants, causing phagocytes to move towards the pathogen
Phagocytes have several receptors on their cell-surface membrane that recognise and attach to chemical on the surface of the pathogen
The phagocytes then engulf the pathogen to form a vesicles known as a phagosome
Lysosomes move towards the vesicles and fuse with it
Enzymes called lysozyme are present within the lysosome. These enzymes destroy ingested bacteria via hydrolysis of the bacterium cell wall.
The soluble products from the breakdown of the pathogen are absorbed into the cytoplasm of the phagocyte
When a phagocyte is attracted to a pathogen, what does it move along?
A concentration gradient
What is the vesicles formed in phagocytosis known as?
A phagosome
What fuses with the phagosome?
Lysosomes
What is found in a lysosome?
Lysozymes
What are lysozymes?
An enzyme
What is an antigen?
Any part of an organism or substance that is recognised as non-self by the immune system and stimulates an immune response
What does the presence of an antigen do?
Triggers an immune response
Is phagocytosis a specific or non-specific immune response?
Non-specific
Where are lymphocytes produced?
Stem cells in adult bone marrow
Fetus
Where do B lymphocytes mature?
In the bone marrow
What immunity are B lymphocytes associated with?
Humoral immunity
Where do T lymphocytes mature?
Thymus gland
What immunity are T lymphocytes associated with?
Cell-mediated immunity
Why can T lymphocytes distinguish invader cells from normal cells?
Phagocytes that have engulfed and hydrolysed a pathogen present some of a pathogen’s antigens on their own cell-surface membrane
Body cells invaded by a virus present some of the viral antigens on their own cell-surface membrane
Transplanted cells from individuals of the same species have different antigens on their cell-surface membranes
Cancer cells are different from normal body cells and present antigens on their cell-surface membranes
What is the name given to cells that display foreign antigens on their cell-surface membrane?
Antigen-presenting cells
Will T lymphocytes respond to any antigens?
No, T lymphocytes will only respond to antigens that are presented on a body cell and so will not respond to antigens within the body fluids
What do the receptors on T lymphocytes do?
The receptors on each T cell respond to a single antigen
What are the stages in the response of T lymphocytes to infection by a pathogen?
Pathogens invade body cells or are absorbed by phagocytes
The body cells or phagocytes present antigens from the pathogen on their cell-surface membrane
Receptors on a specific helper T cell fit exactly onto these antigens
This attachment activated the T cell to divide rapidly by mitosis and form a clone of genetically identical cells
The cloned T cells will:
-develop into memory cells
-stimulate phagocytes
-stimulate B cells
-activate cytotoxic T cells
What four things can cloned T cells do?
Develop into memory cells that enable a rapid response to future infections by the same pathogen
Stimulate phagocytes to engulf pathogens by phagocytosis
Stimulate B cells to divide and secrete their antibody
Activate cytotoxic T cells
What do cytotoxic T cells do?
Kill abnormal cells and body cells that are infected by pathogens
How do cytotoxic T cells kills abnormal cells and body cells that are infected by pathogens?
By producing a protein called performing that makes holes in the cell-surface membrane
What pathogen are cytotoxic T cells most effective against?
Viruses
Why are cytotoxic T cells to effective against viruses?
Viruses use living cells in which to replicate and so by sacrificing body cells it prevents viruses replicating and infecting more cells
What can cloned B cells develop into?
Plasma cells
Memory cells
How long do plasma cells survive?
A few days
What is the production of antibodies and memory cells known as?
Primary immune response
What do plasma cells do?
Secrete antibodies into the blood plasma
What are memory cells responsible for?
The secondary immune response
Do memory cells live longer than plasma cells?
Yes, memory cells live a lot longer than plasma cells
How long do memory cells often live for?
Several decades
What do memory cells provide?
Long term immunity
Where do memory cells circulate?
In the blood and tissue fluid
What happens when memory cells encounter the same antigen at a later date?
They divide rapidly and develop into plasma cells and more memory cells
How many antibodies can a plasma cell produce?
Around 2000 every second
What is the role of B cells in humoral immunity?
The surface antigens of an invading pathogen are taken up by a B cell
The B cell processes the antigens and presents them on its surface
Helper T cells attach to the processed antigens in the B cell and thereby activate the B cell
The B cell is now activated to divide by mitosis to form clones of identical B cells in a process known as clonal selection
Some of the cloned B cells develop into plasma cells which produce and excrete the specific antibody that exactly fits the antigen on the pathogen’s surface
The antibody attaches to antigens on the pathogen and destroys them
Some B cells develop into memory cells which can respond to future infections by the same pathogen in the secondary immune response
How does the antigen enter the B cell?
By endocytosis
What does clonal selection account for?
The body’s ability to respond rapidly to any of a vast number of antigens
Which is faster, the secondary or primary immune response?
The secondary immune response is faster
Why is the secondary immune response faster than the primary immune response?
In the primary response, the antigens of the pathogen have to be ingested, processed and presented by the B cells.
Helper T cells then need to link with the B cells that then clone, some of the cells then developing into the plasma cells that produce antibodies.
These processes occur consecutively and therefore take time.
In the secondary response memory cells are already present and so the only processes are cloning and development into plasma cells that produce antibodies.
Fewer processes means a quicker response
What are the differences between cell-mediated and humoral immunity?
Cell-mediated immunity involves T cells whilst humoral immunity involves mostly B cells
Humoral immunity produces antibodies but cell-mediated immunity doesn’t
Cell-mediated immunity is the first stage of immune response but humoral immunity is the second stage of immune response and comes after the cell-mediated stage
Cell-mediated immunity is effective through cells whilst humoral immunity is effective through body fluids
How many polypeptide chains are antibodies made up of?
4
Why is agglutination possible?
Each antibody has 2 antigen binding sites
What is the binding site of an antibody known as?
The variable region
What are the shorter chains on an antibody known as?
Light chains
What are the longer chains on an antibody known as?
Heavy chains
When an antigen bonds to he variable region on an antibody, what is formed?
Antigen-antibody complex
Apart from the variable region, what is the rest of the antibody called?
Constant region
How are antibodies different to each other?
The variable region is different in different antibodies
What do antibodies do?
Prepare the antigen for destruction
What is the purpose of agglutination instigated by antibodies?
The non-self material is clumped together, making it easier for phagocytes to locate it as it is less spread out in the body
How do antibodies assist in the destruction of non-self material?
Causing agglutination
Serving as markers that stimulate phagocytes to engulf the non-self material to which they are attached
What are some uses for monoclonal antibodies?
Cancer treatment
Medical diagnosis
Pregnancy testing
What are the two types of monoclonal antibody therapy used to treat cancer?
Direct monoclonal antibody therapy
Indirect monoclonal antibody therapy
What is an advantage of direct monoclonal antibody therapy?
The antibodies are non-toxic and highly specific so they have fewer side effects than other forms of therapy
How does direct monoclonal antibody therapy work?
Monoclonal antibodies are produced that are specific to the antigens on the cancer cells
These antibodies are given to a patient and attach themselves to the receptors on their cancer cells
They attach to the surface of their cancer cells and block the chemical signals that stimulate their uncontrolled growth
How does indirect monoclonal antibody therapy work?
A radioactive or cytotoxic drug is attached to the monoclonal antibody
When the antibody attaches to the cancer cells, it kills them.
Why are indirect monoclonal antibodies used in smaller dosages?
Cheaper
Reduces side effects
What can monoclonal antibodies be used to diagnose?
Influenza
Hepatitis
Chlamydia
Certain cancers
How can monoclonal antibodies be used to diagnose prostate cancer?
Men with prostate cancer often produce more PSA (prostate specific antigen) and so have unusually high levels of it in their blood
By using a monoclonal antibody that interacts with PSA it is possible to obtain a level of PSA from a blood sample
Whilst a higher than normal PSA level is not a full diagnosis it gives an early warning of the possibility of prostate cancer and the need for further tests
What sort of molecules are antibodies?
Proteins
What do home pregnancy testing kits rely on?
During pregnancy the mother produces a hormone called human chorionic gonadatrophin (hCG) which can be found in the urine
What produces the hormone hCG (human chorionic gonadotropin)?
The placenta
What does hCG stand for?
Human chorionic gonadotropin
What are the monoclonal antibodies found on pregnancy test strips linked to?
Coloured particles
What are the common ethical issues with monoclonal antibodies?
The production of monoclonal antibodies involves mice
There are risks and benefits to using monoclonal antibodies
Drug trials present certain dangers
Are monoclonal antibodies a risk free treatment?
No, there have been some deaths associated with the use of monoclonal antibodies to treat sclerosis
What monoclonal antibody drug trial went badly wrong in 2006?
TGN1412
How are monoclonal antibodies produced?
A mouse is exposed to the non-self material against which an antibody is required.
The B cells in the mouse then produce a mixture of antibodies, which are extracted from the spleen of the mouse.
The B cells are mixed with cancer cells to enable them to divide outside the body.
Detergent is added to the mixture to break down the cell-surface membranes of both types of cell and allow them to fuse together. The fused cells are called hybridoma cells.
The hybridoma cells are separated under a microscope and each single cell is cultured to form a clone.
Each clone is tested to see whether it is producing the required antibody.
Any clone producing the required antibody is grown on a large scale and the antibodies are extracted from the growth medium.
Before the monoclonal antibodies produced by mice can be used in human treatment, what has to be done?
The antibodies have to be modified to make them like human cells in a process called humanisation
What are some of the ethical issues of using mice to produce monoclonal antibodies?
Injecting mice with foreign substances
In order to harvest the B cells from the spleen, the mice must be killed
The mice may be used to produce tumour cells, so will be deliberately induced with cancer
Animal testing is very controversial
What are the two forms of immunity?
Passive immunity
Active immunity
How is passive immunity produced?
By the introduction of antibodies into individuals from an outside source
For passive immunity is direct contact with the pathogen or its antigen required?
No
How long does passive immunity take to form once antibodies are introduced into an individual?
Immunity is acquired immediately
How long does passive immunity last?
Not long
Why is passive immunity short-lived?
As the antibodies are not being produced by the individuals themselves, the antibodies are not replaced when they are broken down, no memory cells are formed and so there is no lasting immunity.
What are two examples of passive immunity?
Anti-venom given to the victims of snake bites
The immunity acquired by the Fetus when antibodies pass across the placenta from the mother
How is active immunity produced?
By stimulating the production of antibodies by the individuals’ own immune system
Is direct contact with the pathogen or it’s antigen required for active immunity to form?
Yes
How long does active immunity take to develop?
It is not immediate and takes time
How long does active immunity last?
It is generally long lasting
How many types of active immunity are there?
2
What are the two types of active immunity?
Natural active immunity
Artificial active immunity
What does natural active immunity result from?
An individual becoming infected with a disease under normal circumstances so the body produces its own antibodies
What does artificial active immunity involve?
Inducing an immune response in an individual, without them suffering the symptoms of the disease
What does artificial active immunity form the basis of?
Vaccination (immunisation)
What is vaccination?
The introduction of the appropriate disease antigens into the body, either by injection or by mouth
What is the intention of vaccination?
To stimulate an immune response against a particular disease
How do vaccines work?
Vaccines contain one or more types of antigen from a specific pathogen
These antigens stimulate an immune response
This means that memory cells specific to the antigens are produced
These memory cells remain in the blood and allow a greater, and more immediate, response to a future infection with the pathogen
This means that if the vaccinated individual comes into contact with the pathogen, there will be a rapid production of antibodies and the new infection will be quickly overcome before it can cause any harm
What does the success of a vaccination programme depend on?
A suitable vaccine must be economically available in sufficient quantities to immunise most of the vulnerable population
There must be few side effects, if any, from the vaccination
Means of producing, storing and transporting the vaccine must be available
There must be the means of administering the vaccine properly at the appropriate time
It must be possible to vaccinate the vast majority of the population to produce herd immunity
Why is it important that a vaccine has very few side effects?
Unpleasant side effects may discourage individuals in the population from being vaccinated
What is often involved in producing, storing and transporting a vaccine?
Technologically advanced equipment
Hygienic conditions
Refrigerated transport
What is involved in appropriate vaccine administration?
Training staff with appropriate skills at different centres throughout the population
When does herd immunity arise?
When a sufficiently large proportion of the population has been vaccinated
What is the theory behind herd immunity?
Pathogens are passed from individual to individual when in close contact
When the vast majority of the population is immune, it is very unlikely that an individual without immunity will come in contact with an infected person
In this way individuals who are not immune to the disease are still protected
Why is herd immunity important?
It is never possible to vaccinate everyone in a large population
Why are babies and very young children not vaccinated?
Their immune system is not yet fully functional
What percentage of the population must be vaccinated in order to achieve herd immunity?
It is different for each disease
Why are some diseases not eliminated by vaccination?
Vaccination may fail to induce immunity in certain individuals (e.g. people with defective immune systems)
Individuals may develop the disease immediately after vaccination but before their immunity levels are high enough to prevent the infection
Antigenic variability of the pathogen
There may be so many varieties of a particular pathogen that is almost impossible to develop a vaccine that is effective against them all
Certain pathogens hide from the body’s immune system
Individuals may object to vaccination
Why can antigenic variability make vaccines ineffective?
If a pathogen mutates frequently, its antigens may change suddenly rather than gradually so the new antigens on the pathogens are not recognised by the immune system and so antibodies are not produced to destroy the pathogen. This means that the vaccine is no longer effective
Why is it possible for individuals to sustain multiple infections of the influenza virus?
Antigenic variability happens with the influenza virus and so it changes its antigens frequently. This makes any immunity acquired against the influenza virus relatively short-lived
How can pathogens hide from the immune system?
Concealing themselves inside cells
Existing in places out of reach (such as in the intestines)
What pathogen hides from the immune system by existing in the intestines?
Cholera pathogen
Why might individuals object to vaccination?
Religious reasons
Medical reasons
Ethical reasons
What does HIV stand for?
Human Immunodeficiency Virus
What does AIDS stand for?
Acquired Immune Deficiency Syndrome
What does HIV cause?
AIDS
What causes AIDS?
HIV
What group of viruses does HIV belong to?
Retroviruses
Why is HIV a retrovirus?
The presence of reverse transcriptase within HIV and consequent ability of HIV to make DNA from RNA
Describe the structure of HIV
On the outside is a lipid envelope
Attachment proteins are embedded in the outside of the lipid envelope
Inside the envelope is a protein layer called the capsid
The capsid encloses 2 single strands of RNA and some enzymes
What is found in the capsid of HIV?
2 single strands of RNA
Enzymes (including reverse transcriptase)
What does reverse transcriptase do?
Catalyses the production of DNA from RNA
Where does HIV circulate in the body?
The bloodstream
What protein does an attachment protein on HIV readily bind to?
CD4
What type of cell does HIV most readily attach to?
Helper T cells
Describe the replication of HIV inside a human body
HIV enters the bloodstream and circulates around the body
A protein on HIV binds to a protein called CD4 (readily found on helper T cells)
The protein capsid fuses with the cell surface membrane
The RNA and enzymes of HIV enter the helper T cell
The HIV reverse transcriptase convert the virus’s RNA to DNA
The new DNA is moved into the helper T cell’s nucleus where it is inserted into the cell’s DNA
The HIV DNA inside the nucleus creates mRNA using the cell’s enzymes. The mRNA contains instructions for making new viral proteins and the RNA to go in the new HIV
The mRNA passes out of the nucleus through a nuclear pore and uses the cell’s protein synthesis mechanisms to make HIV particles
The HIV particles break away from the helper T cell with a piece of its cell surface membrane surrounding them which forms the lipid envelope
How does HIV cause AIDS?
By killing or interfering with the normal functioning of helper T cells
How many helper T cells does an uninfected person normally have in each mm^3 of their blood?
800-1200 helper T cells
What happens if a person doesn’t have sufficient amounts of helper T cells?
The immune system cannot stimulate B cells to produce antibodies
The immune system cannot stimulate cytotoxic T cells
Memory cells may become infected and destroyed
The body is unable to produce an adequate immune response and becomes susceptible to other infections and cancers
Does HIV directly kill individuals?
No
What kills individuals infected with HIV?
Secondary infections
What do many AIDS sufferers develop?
Infections in the lungs, intestines, brain and eyes
Weight loss
Diarrhoea
What does ELISA stand for?
Enzyme Linked ImmunoSorbant Assay
How does HIV lead to the death of an individual?
By infecting the immune system and preventing it from functioning normally so the individual is unable to effectively respond to other pathogens
It’s the infections from other pathogens that ultimately cause death
What two things does the ELISA test detect?
The presence of a protein in a sample
The quantity of said protein in the sample
How sensitive is the ELISA test?
Very sensitive
Describe the process of the ELISA test
Apply the sample to the surface of a spotting well to which all the antigens in the sample will attach
Wash the surface several time to remove any unattached antigens
Add the antibody specific to the antigen that is trying to be detected and allow the two to bind
Wash the surface again to remove excess antibody
Add a second antibody to bind with the first antibody (this antibody has an enzyme attached to it)
Wash the surface again so any antibodies not bound are removed
Add the colourless substrate of the enzyme, the enzyme acts on the substrate to change it into a coloured product
The amount of antigen present is relative to the intensity of the colour that develops
Why is it important to wash the surface used in the ELISA test after antibodies and antigens are added?
So that any antibodies not bound won’t react and provide a false positive result
What can the ELISA test be used to detect?
HIV
Tuberculosis
Hepatitis
What is often more important in drug tests; the presence of a drug or the quantity of the drug? Why?
The quantity of the drug as many drugs are found naturally in low concentrations
What two tests is ELISA very useful in (besides pathogen detection)?
Drug testing
Allergen testing
Describe and explain one way that antibiotics such as penicillin work
In bacterial cells, water constantly enters by osmosis
The wall surrounding the bacterium is made of murein which isn’t easily stretched and prevents the cell from bursting via osmotic intake
As water enters the cell via osmosis, the cell expands and pushes against the cell wall
The cell wall is relatively inelastic so reiste expansion and prevents further entry of water
Antibiotics like penicillin inhibit certain enzymes required for the synthesis and assembly of the peptide cross linkages in bacterial cell walls
This weakens the walls and makes them unable to withstand pressure
As water enters the cell naturally by osmosis, the cell bursts and the bacterium dies
Why are antibiotics ineffective against viral diseases?
Viruses rely on the host cells to carry out their metabolic activities and therefore lack their own metabolic pathways and cell structures
As a result antibiotics are ineffective as there are no metabolic mechanisms or cell structures for the, to disrupt
Viruses have a protein coat rather than a murein cell wall and so do not have sites where antibiotics can work
When viruses are within an organism’s own cells, antibiotics cannot reach them
