3.2 cells Flashcards
State the sub-cellular structures inside a eukaryotic cell
Cell surface membrane
Nucleus
Mitochondria
Chloroplast
Golgi apparatus and Golgi vesicles
Lysosomes
Ribosomes
Rough er (endoplasmic reticulum)
Smooth er (endoplasmic reticulum)
Cell wall
Cell vacuole
Structure of nucleus
Nuclear envelope - double membrane surrounding nucleus
Nuclear pores - large protein complexes that allow molecules in and out of nucleus
Nucleoplasm - granular, jelly like material that is similar to the cytoplasm but inside nucleus
Nucleolus - smaller sphere inside, site of rRNA production and makes ribosomes
Structure of cell wall
3 main sections of cell wall:
Middle Lamella - contains polysaccharides which help bind cell together
Primary cell wall - with cellulose microfibrils that provide strength and flexibility for growth
Secondary cell wall - may contain lignin, which strengthens cell and aids water conductivity
in plants: made of microfibrils of the cellulose polymer
in fungi: made of chitin, a nitrogen containing polysaccharide
Function of nucleus
Controls cell growth and division. (also carries genetic code of the cell when it divides)
site of DNA replication and transcription (making mRNA)
contains genetic code for each cell
Function of cell wall
Provides mechanical strength in order to prevent the cell bursting under the pressure created by the osmosis of water entering the cell
To allow water to pass along it and therefore to contribute to the movement of water through the plant
structure of plasma membrane
found in all cells
phospholipid bilayer - molecules embed within and attached on the outside (proteins, carbohydrates, cholesterol)
function of plasma membrane
- controls the entrance and exit of molecules
Structure of cell vacoule
Fluid filled sac
surrounded by single membrane: tonoplast
Function of cell vacuole
makes cells turgid and therefore provides support
temporary food store of sugars and amino acids
Pigments may colour petals to attract pollinators
structure of mitochondria
double membrane - controls what goes in and out
inner membrane - cristae
fluid centre called mitochondrial matrix - space containing lipids, proteins, enzymes and circular DNA to make own proteins
loop of mitochondria DNA
function of mitochondria
site of aerobic respiration
site of ATP production
DNA to code for enzymes needed in respiration
chloroplasts structure
surrounded by double membrane
thylakoids - folded membranes embedded with pigment
fluid filled stroma - contains enzymes for photosynthesis
found in plants
chloroplasts function
site of photosynthesis
virus facts
non living
have receptor proteins which attach to proteins on other cells and infect them
magnification equation
magnification = size of image/ size of a real object
define magnification
how many times bigger the image is than the specimen
define resolution
minimum distance apart that two objects can be in order for them to appear as separate items
what is cell fractionation
its used to collate different organelles so they can be studied
how does cell fractionation work
cells are broken down to release content and organelles are then separated
must be prepared in a cold, isotonic, buffered solution
why does the cell need to be prepared in a cold isotonic buffered
cold - reduces enzyme activity
isotonic - dont want organelles to be damaged so must be in the same concentration
buffered - has a buffer to prevent damage to the organelles
steps for fractionation
- homogenization
- cells must be broken open so in order to do that we use a blender to blend it into a cold, isotonic buffered solution - ultracentrifiguration
structure of ribosomes
free in cytoplasm or attached to ER
two types : small and large
made up of two subunits of protein and rRNA
large ribosome found in eukaryotic cells
smaller ribosome found in prokaryotic cells, mitochondria and chloroplasts
function of ribosomes
site of protein synthesis (where amino acids are joined together to form a polypeptide)
structure of smooth and rough er
both have folded membranes called cisternae
rough = membranes on the cisternae
rough ER - protein synthesis
smooth ER - synthesis and store lipids and carbohydrates
function of smooth and rough er
synthesis, storage and transport of proteins
function of nuclear pores
allows passages of large molecules e.g RNA
structure of golgi apparatus
- folded membranes making cisternae
- secretary vesicles pinch off from the cisternae
function of nuclear envelope
controls entry and exit of materials in and out of nucleus and reactions taking place within it
function of golgi vesicle
stores lipids and proteins made by golgi apparatus and transports them out of the cell
functions of golgi apparatus
- adds carbohydrates to proteins to form glycoproteins
- produce secretory enzymes
- secrete carbohydrtes
- transport, modify and store lipids
- form lysosomes
- molecules are ‘labelled’ with their destination
- finished products are transported to cell surface in Golgi vesicles where they fuse with the membrane and the contents in released
strucure of lysosome
tiny bag of digestive enzymes, surrounded by single membrane
function of lysosomes
- hydrolyse phagocytic cells
- completely breaks down dead cells
- exocytosis - releases enxymes to outside of cell to destroy material
- digests worn out organelles for reuse of materials
structure of plasmid
small loops of DNA which only carry a few genes
structure of capsule
slimy layer made up of protein
function of capsule
prevents bacteria from desiccating (drying out) and protects the bacteria against the host’s immune system
calculate the length of time in a stage of cell division
length of time in stage= (observed number of cells in this stage/total number of cells) x total length of cell cycle
in what stage of the cell cycle does DNA replication occur
interphase
what part of the cell is mitosis
when a eukaryotic cell divides to produce two daughter cells, each with identical copies of DNA produced by the parent cell during DNA replication
order of stages in mitosis
interphase
prophase
metaphase
anaphase
telophase
(cytokinesis)
what happens in interphase
- cell grows
- DNA replicates
- prepares for cell division
what happens in prophase
- chromosomes become visible by shortening and thickening
- centrioles replicate and move to opposite ends of cells (called poles)
- spindle fibres develop and collectively they’re called spindle apparatus
- nucleolus disappears
-nuclear envelope breaks down - chromosomes now free in cytoplasm
- chromosomes drawn towards equator by spindle fibres that are attached to the centromere
what happens in metaphase
- chromosomes made up of two chromatids
- each chromatid is an identical copy of DNA from the parent cell
- chromatids joined by centromere
- spindle fibres (microtubules) from poles attach to the centromere
- chromosomes pulled along spindle apparatus and arrange themselves across the equator of the cell
what happens in anaphase
- centromeres divide into 2 and spindle fibres pull individual chromatids apart
- chromatids move to their respective sides of the cell
- now referred to as ‘chromosomes’
- energy to separate them comes from ATP (respiration)
what happens in telophase
- chromosomes reach their respective poles and become longer and thinner
- this makes them invisible again
- the spindle fibres disintegrate
- nuclear envelope and nucleolus reform
what happens in cytokinesis
- cytoplasm divides forming two identical cells
- parent cell separate into two genetically identical daughter cells
importance of mitosis
- growth
- repair: if cells become damaged or die, it’s important for the new cells produced to have an identical structure and function to those that have been lost
- reproduction: single celled organisms divide by mitosis to give two new organisms. each new organism is genetically identical to parent organism
difference between mitosis and meiosis
- mitosis: produces 2 daughter cells that have the same number of chromosomes as the parent cell
- meiosis: produces 4 daughter cells, each with half the number of chromosomes of the parent cell.
what can happen if mitosis goes wrong
- mitosis is a controlled process
- if the cell division becomes uncontrolled then it can lead to formation of tumours and cancers. many cancer treatments are directed at controlling the rate of cell division
reproduction in prokaryotic cells
binary fission:
- circular DNA replicates and both copies attach to the cell membrane
- plasmids also replicate
- cell membrane begins to grow between the 2 DNA molecules and begin to pinch inward, which divides the cytoplasm into 2
- new cell wall forms between the molecules of DNA, which divides the original cell wall into two identical daughter cells
- each have a single copy of the circular DNA and variable number of copies of the plasmids
cell replication in viruses `
- viruses don’t undergo cell division because they’re non living.
- if the viruses inject a host cell with their infected nucleic acid, the host cell becomes infected and replicates the virus particles
what is the cell membrane composed of
- transport proteins
- phospholipid heads
- glycolipids
- cholesterol
- cytoskeleton filaments
structure of phospholipds in the cell- surface membrane
- hydrophillic heads of both layers point to the outside of the membrane as it’s attracted to the water on both sides
- hydrophillic tails point to centre of membrane, repelled by water on both sides
function of phospholipids in the cell- surface membrane
- allow lipid soluble substances to enter and leave the cell
- prevents water-soluble substances entering and leaving the cell
- makes the cell flexible and self sealing
structure of proteins in the cell-surface membrane
- interspersed throughout the cell-surface membrane
- some proteins occur in the surface of the bilayer, they act as mechanical support to the membrane
- other proteins completely span the phospholipid bilayer from one side to the other. some are protein channels, which form water-filled tubes to allow water-soluble ions to diffuse across the membrane.
- others are carrier proteins that bind to ions or molecules like glucose etc, then change shape in order to move these molecules across the membrane.
function of proteins in the cell- surface membrane
- provides structural support
- acts as channels transporting water-soluble substances across the membrane
- allow active transport across the membrane through carrier proteins
- form cell-surface receptors for identifying cells
- helps cells adhere together
structure of cholesterol in the cell-surface membrane
-occur within the phospholipid bilayer of the cell-surface membrane
-they add strength to the membranes
- cholesterol molecules are very hydrophobic- play a role in preventing loss of water and dissolved ions from the cell
- also pull together fatty acid tails of the phospholipid molecules, limiting their movement and that of other molecules without making the membrane too rigid
function of cholesterol in the cell-surface membrane
- reduce lateral movement of other molecules including phospholipids
- make the membrane less fluid at high temperatures
- prevent leakage of water and dissolved ions from the cell
structure of glycolipids in the cell-surface membrane
- extends from the phospholipid bilayer into the watery environment outside the cell.
function of gylcolipids in the cell-surface membrane
- acts as recognition sites
- help maintain the stability of the membrane
- helps cells to attach to one another and so form tissues
structure of glycoproteins in the cell-surface membrane
- outer surface of membrane
- acts as cell surface receptors for more specifically hormones and neurotransmitters
function of glycoproteins in the cell-surface membrane
- acts as cell-cell recognition sites
- helps cells to attach to one another and so form tissues
- allows cells to recognise one another, e.g lymphocytes can recognise an organism’s own cells
why is the model for the cell surface membrane called the fluid mosaic
- ‘fluid’ cause the molecules move around each other in the membrane and gives it a flexible structure that is constantly changing in shape
- ‘mosaic’ cause the molecules of different shapes and sizes fit together to form a pattern like a mosaic
functions of membranes within cells
- control entry and exit of materials in discrete organelles e.g mitochondria
- separate organelles from cytoplasm so that specific metabolic reactions can take place within them
- provide an internal transport system e.g E.R
- isolate enzymes that might damage the cell e.g lysosomes
- provide surfaces on which reactions can occur e.g protein synthesis on ribosomes on rough E.R
why don’t some molecules pass through the membrane
- not soluble in lipids and therefore cannot pass though the phospholipid layer
- too large to pass through the channels in membrane
- are the same charge as the charge on the protein channels so they are repelled
- electrically charged (polar) and therefore have difficulty passing through the non-polar tails in the phospholipid bilayer
diffusion definition
the net movement of molecules or ions from a region of high concentration to a region of lower concentration until a dynamic equilibrium is achieved. (down a concentration gradient)
what type of process is facilitated diffusion
passive
why is diffusion a passive process
- cell doesn’t use its ATP to move molecules or ions - they move due to kinetic energy
what type of molecules pass between the phospholipids in simple diffusion
- small, uncharged molecules
e.g oxygen, carbon dioxide
exception = water
why don’t some molecules pass through the cell membrane in facilitated diffusion
- they aren’t lipid soluble
- too large
- have a charge e.g ions and polar molecules
how can molecules pass through the membrane in facilitated diffusion
- they will only pass across the membrane if there is a complementary shaped protein to facilitate its transport e,g channel proteins and carrier proteins
two types of proteins that are in the plasma membrane
- carrier proteins
- channel proteins
role of channel proteins
- have a water filled hydrophilic channel through which ions can pass
- allow specific water-soluble ions to pass through, if specific ion isn’t present, the channel remains closed.
role of carrier proteins
- can change shape when a specific molecule binds and the molecule is then released on the other side of the membrane
osmosis definition
passage of water from a region of higher water potential to a region of lower water potential through a partially permeable membrane until a dynamic equilibrium is reached
pure water, water potential =
0
how will addition of a solute to pure water affect the water potential
lower its water potential (into negatives)
how will water move in osmosis - in terms of water potential
from a region of higher (less negative) water potential to a region of lower (more negative) water potential
e.g -20kPa –> -30kPa
the higher the value of the water potential…
the more pure the solution is (means there is more solute in the solution)
what do antigens allow the immune system to identify:
pathogens
abnormal body cells
toxins
cells from other organisms of the same species e.g organ transplant, blood transplant
antigen definition
a molecule that triggers an immune response by lymphocytes
what is antigenic variation
- when a pathogen mutates frequently, so its antigens change suddenly rather than gradually.
- this means that new antigens on the pathogen are no longer recognised by the immune system and therefore new strains of pathogens become created
how does antigenic variation effect disease prevention
- mutations mean that antigens on the pathogen change suddenly.
- this means that vaccines become ineffective because the new antigens on the pathogens are no longer recognised by the immune system
- so the immune system cannot produce the antibodies needed to destroy the pathogens
examples of a virus that experiences antigenic variability
influenza virus (flu) and HIV
process of phagocytosis
- phagocyte moves towards pathogen along a concentration gradient
- pathogen engulfed by endocytosis
- pathogen now contained within phagosome (phagocytic vacuole)
- lysosomes fuse with phagosome to form a phagolysosome
-lysozymes break down pathogen - waste products excreted by cell via exocytosis
- phagocytes can then present the pathogens antigens by sticking it to its own cell surface becoming an antigen presenting cell.
role of lysozymes in the destruction of pathogens
- lysozymes are present within the lysosome and they destroy ingested bacteria by hydrolysis of their cell walls.
how are antigen presenting cells formed
- when a phagocyte engulfs a pathogen, it doesn’t always fully digest it
-antigen from surface of the pathogen is saved and moved to a special protein complex on surface of cells - cell is now an APC
- exposes antigen on surface so that other cells of immune system recognise the antigen
- special protein complex ensures that APC isn’t mistaken for foreign cell and attacked.
specific immune response definition
specific or adaptive immune response can target a specific pathogen slower than a non specific response
what does the cell mediated (cellular) response involve
- involves specialised T-cells and other immune cells that they react with e.g phagocytes - target pathogens inside cells
process of a cell mediated response
- once a pathogen has been engulfed and destroyed by a lysozmes, the antigens get positioned on the cell surface. this is now an APC
- T helper cells have receptors on their surface which can attach to the antigens on APC
- once attached, this activates the helper T cells to divide by mitosis to replicate and make large numbers of clones
- cloned helper t cells differentiate into different cells. some become cytotoxic T cells
- t helper cells release cytokines to stimulate cytoxic t cells
- cyotoxic t cells detect antigen on surface of infected cell, and once detected, they release perforin which creates pores in the cell membrane
- in addition enzymes are also released which get into the cell and destroy the infected cell
role of cytotoxic T cells
to destroy abnormal or infected cells by releasing perforin. this protein creates pores in the cell surface membrane which causes the cell to shrivel or lyse
purpose of helper t cells
they divide by mitosis to become more helper T cells which stimulate B cells and phagocytosis, or T memory cells or cytotoxic T cells
examples of APCs
- infected body cells will present the viral antigen on their surface
- a macrophage which has engulfed and destroyed a pathogen will present the antigen on their surface.
- cells fo a transplanted organ will have different shaped antigens on their surface compared to your self-cell antigens
- cancer cells will have abnormal shaped self-cell antigens
structure of an antibody
- consists of 4 polypeptide chains - 2 identical light chains and 2 identical heavy chains
- disulphide bonds hold the 4 polypeptide chains of antibody together
- constant region is the long part that isn’t on the end
-antigen binding site enables antibody to bind to specific antigen on a pathogen
-quarternary structure protein
formation of an antigen-antibody complex
- An antigen and its complementary antibody have complementary molecular shapes
- when antibody collides with a foreign cell that posesses ‘non-self’ antigens with a complementary shape, it binds with one of the antigens
- therefore, 2 molecules combine (Antigen + antibody) to form an antigen-antibody complex
what is involved in the humoral response
b cells and antibodies
humoral immune response process
- b cell binds to specific antigen on antigen presenting cell
- t helper cell releases cytokines to stimulate b cells to divide by mitosis ( clonal expansion)
- b cells differentiate into plasma cells which secrete antibodies with complementary variable region to the antigen
- plasma cells develop memory cells
memory b cells
- can live for decades inside body
- memory b cells don’t make antibodies, rather they will divide by mitosis and make plasma cells rapidly if they collide with an antigen they have previously encountered.
- this results in large numbers of antibodies being produced so rapidly that the pathogen is destroyed before any symptoms can occur.
clonal expansion
- APC stimulates helper T cell to produce chemical message. Message activates the B cells - called clonal selection
- activated B cells divide to form plasma cells
- plasma cells undergo rapid division, called clonal expansion then produces required antibody in large quantities
what is a monoclonal antibody
a single type of antibody that can be isolated and cloned
primary immune response
- when an antigen enters the body for the first time, the immune system is activated (primary response)
- primary response is slow and weak cause there aren’t many B-cells that can make the antibody needed to bind to the specific antigen
- after 1st infection, T and B cells produce memory cells that remain in the body
- person is now said to be immune cause immune system has the ability to respond quickly to pathogen
secondary immune response
- if the same pathogen enters the body again, the immune system will produce a quicker and stronger immune response (secondary response)
- clonal selection happens faster
- memory B cells activated, divide into plasma cell to produce specific antibody to antigen
- secondary response often gets rid of pathogen before you display any symptoms
role of cytotoxic t cells
produces perforin that causes pathogen infected cells to lyse
role of plasma cell
generates large quantities of antibodies
agglutination definition
when antigens bind to other antigens on surface of several pathogens, clumping them together
phagocytes can now easily recognise and easily destroy pathogen by phagocytosis
cellular vs humoural immune response
cellular:
- targets pathogens in host cell
- involves cytotoxic T cells
- perforin causes cell to lyse
humoral:
- produces antibodies
- involves plasma cells
- can neutralise pathogens
both:
- coordinated bt helper t cells
- interleukin acts as cell signaller
- involves APC
passive immunity definition
produced by introduction of antibodies into individuals from an outside source
active immunity definition
produced by stimulating the production of antibodies by the individual’s own immune system
examples of natural active and artificial active immunity
natural active: chicken pox
artificial active: vaccinations
examples of natural passive and artifical passive immunity
natural passive: acquiring antibodies from their mother e.g breast milk and across the placenta
artificial passive: injection of rabies antibodies after potential exposure
how can antigens be used to produce a vaccine
- same antigens that are found on the surface of a known pathogen and stimulate the primary response, resulting in production of memory B cells
- may contain attenuated (less virulent) form of the pathogen
how are vaccinations used to protect against diseases caused by particular pathogens
- vaccinations are the introduction of the appropriate disease antigens into the body.
- intention is to stimulate an immune response against a particular disease
- vaccine contains one or more types of antigen from the pathogen
- antigens stimulate immune response
- memory cells are then produced and these remain in the blood and allow a quicker and stronger response to a future pathogen of this same pathogen
- the result is that there is a rapid production of antibodies and the new infection is rapidly overcome before it can cause any harm and with few symptoms or none at all
herd immunity concept
if enough people in a population are vaccinated against a disease, it’s not possible for the disease to spread amongst the population, so everyone is protected
reduced chance of passing it on if more people are vaccinated
importance of herd immunity
it isn’t ever possible to vaccinate the entire population e.g babies and very young children aren’t vaccinated because their immune system aren’t fully functional
could also be dangerous to vaccinate those who are ill or have compromised immune systems.
ethical issues associated with use of vaccines
- production of existing vaccines and development of new ones often involving the use of animals. how acceptable is this
- on whom should vaccines be tested on
structure of HIV
- core of genetic information, RNA, and some proteins
- outer coating of protein called a capsid
- capsid encloses 2 single strands of RNA and some enzymes (one enzyme is reverse transcriptase - catalyses production of DNA to RNA)
- extra outer layer called a (lipid) envelope
- attachment proteins on surface
HIV replication
- Attachment proteins attach to receptors on helper T cell/lymphocyte;
- Nucleic acid/RNA enters cell;
- Reverse transcriptase converts RNA to DNA;
- Viral protein/capsid/enzymes produced;
- Virus (particles) assembled and released (from cell);
difference between being HIV positive and developing AIDS
- once infected with HIV, a person is said to be HIV positive.
- however, the replication of HIV often goes into dormancy and only recommences leading to AIDS many years later
- HIV develops into AIDS when the helper t-cell numbers in their body reach a critically low level
how does HIV cause the symptoms of AIDS
- HIV specifically attacks helper T cells. HIV causes AIDS by killing or interfering with the normal functioning of helper T cells.
- without a sufficient number of helper T cells, the immune system cannot stimulate B cells to produce antibodies or the cytotoxic T cells that kill cells infected by pathogens .
- memory cells also become infected and destroyed
- as a result, the body is unable to produce the adequate immune response and becomes susceptible to other infections or cancers
why are antibiotics ineffective against viruses
- viruses don’t have their own enzymes and ribosomes - they use the ones in the host cells
- antibiotics disrupt cell structures and metabolic mechanisms so because viruses use human enzymes, ribosomes etc to replicate, antibiotics can’t inhibit them
- viruses also have a protein coat rather than a murein cell wall and so do not have the sites where antibiotics can work.
- most antiviral drugs are designed to target the few virus specific enzymes that exist
e.g HIV uses reverse transcriptase to replicate. Human cells don’t have this enzyme so drugs can be designed to inhibit it without affecting human cells - reverse transcriptase inhibitors
how are monoclonal antibodies used in medical diagnosis
cancer drug delivery:
- cancer cells have antigens called tumour markers that aren’t on normal body cells
- monoclonal antibodies can be made that are specific to the antigens on the cancer cells (tumour markers) and will bind to them
- these antibodies are given to a patient and attach themselves to the receptors on their cancer cells
- attach to the surface of their cancel cells and block the chemical signals that stimulate their uncontrolled growth
use of monoclonal antibodies in pregnancy tests
- this test detects the hormone hCG that is found in urine of pregnant women
- application area contains antibodies that are complementary to the hCG protein, which are bound to the coloured bead
- when urine is applied to the application area, hCG will bind to the antibodies on the beads, forming an antigen- antibody complex
- urine moves up the test strip, carrying any beads with it
- test strip contains antibodies to hCG that are immobilised
- if theres hCG present, strip turns blue, immobilised antibody binds to any hCG concentrating the hCG - antibody complex with the blue beads attached
use of ELISA
- allows you to see if a patient has any antibodies to a certain antigen or any antigen to a certain antibody
- can be used in medical diagnosis to test for pathogenic infection e.g HIV or for allergies and anything else
- if a colour change is present in an ELISA test, it demonstrates that the antigen or antibody of interest is present
process of indirect ELISA
- HIV antigen bound to the bottom of the well
- plasma sample is added
- if any HIV specific antibodies in plasma, these will bind to the HIV antigen stuck to the bottom of the well
- well is then washed to remove unbound antibodies
- secondary antibody is added
- secondary antibody can bind to the HIV specific antibody
- well washed again which removes secondary unbound antibody
- if no primary antibody is present, all of the secondary antibody is washed away
- substrate added
- this is able to react with the enzyme attached to the secondary antibody and produce a coloured product
- if solution changes colour, it indicates patient has HIV specific antibodies in blood
ethical considerations of using monoclonal antibodies
- monoclonal antibody therapy, animal rights issue
- animals used to produce cells from which monoclonal antibodies are produced
how does a TEM work
electron beam passed through specimen: absorption/transmission of light creates dark/light sections. Photomicrograph taken of image
how does an SEM work
electron beam passed back & forth across specimen: electrons scattered and pattern used to generate 3D image by computer analysis
how does a light microscope work
uses light and several lenses to magnify a sample
light microscope limitations
poor resolution: organelle ultrastructure can’t be studied
limitations of electron microscopes
- Not in colour
- Artefacts
- System in vacuum so living specimens can’t be studied
limitations of TEM
- 2D image
- Specimens must be very thin (to allow electrons to pass through)
how are prokaryotic cells different to eukaryotic cells
1) The cytoplasm has no membrane bound organelles.
2)The ribosomes are smaller than those in a eukaryotic cell
3)Some prokaryotes have a flagellum, a long hair-like structure that rotates to help the cell move.
4)A prokaryote lacks a nucleus. The DNA floats free in the cytoplasm. It is circular DNA, one long coiled up strand
5)Some prokaryotes have plasmids- small loops of DNA that aren’t part of the main circular DNA molecule. They contain genes for antibiotic resistance.
6)Some prokaryotes have a capsule made up of secreted slime to help protect bacteria from attacks from the immune system.
7)The cell wall supports the cell and is made from a polymer called murein.
why are viruses described as being acellular?
because they are not cells. They are just nucleic acids surrounded by protein and are not even alive.
what is the protein coat around a virus called?
capsid
what allows the virus to attach to the host cells
attachment proteins on edge of capsid
how do viruses use attachment proteins to replicate
use attachment proteins to bind to complimentary receptors proteins on the surface of the host cell
why can some viruses only attach to one type of host cell
cause the receptor proteins have to be complimentary to the attachment proteins on the surface of the virus
