Topic 2: Cells, viruses and reproduction of living things

what makes a good biological drawing

a to scale, clean, accurate and unshaded line drawing that shows structural features

methylene blue

stains the nucleus of animal cells blue

iodine

turns starch (usually in plant cells) blue/black

eosin

stains cytoplasm in plant cells pink

acetic orcein

stains DNA chromosomes red/ purple

how many micrometers in a milimeter

1mm= 1000µm

advantages of a light microscope

can see living plant and animal cells

relatively cheap and accessible

light and portable

disadvantages of a light microscope

preservation and staining can impact results

limited magnifying and resolving powers

Scanning electron microscopes (SEM)

electron microscope that bounces a beam of electrons across the surface to create a detailed 3D image of its external features

transmission electron microscope (TEM)

a high-resolution microscope that passes a beam of electrons through a specimen to create a 2D image

TEMs vs SEMs microscopes

Transmission electron microscopes	Scanning electron microscopes
Electrons pass through the specimen	Electrons scan the surface
2D flat image	3D surface image
Shows sub-cellular organelles	External surface shape
Higher resolution	Lower resolution

Why do electron microscopes have a greater resolution than light microscopes

electron beams have much smaller wavelength than light waves

how are cells organised in complex organisms

cells ->tissues ->organs -> organ systems -> organism

how to adjust the condenser

1- get a sharp pencil and place it over the light source (lamp or mirror)

2-look through the slide; if the pencil is clear the condenser is set right

condenser is adjusted by a small lever

how to calculate the magnification of a microscope image

multiply eyepiece power by objective lens power

how to measure the size of an object under a microscope

1- place stage micrometer on stage and bring line into focus

2- move stage micrometer until it collides with lines on eyepiece graticule

3- see how many graticule lines fit in the stage micrometer fit in 1mm eg. 40

4- 1mm=1000µm so 1000/40= 25µm

5- take stage micrometer away and look at specimen again. Eg if it covers 2 graticles its 50µm.

how to prepare a microscope slide

1- use a scalpel to remove a thin layer of tissue (so light can pass through)

2- pipette a drop of water on the clean glass slide to mount the sample

3- use clean forceps to place the sample flat on the water

4- add a drop of suitable stain to the specimen to increase contrast

5- place the coverslip on at a 45 degree angle using forceps

light microscope

beam of light (lamp or mirror source) through the object. Objective lens + eyepiece lens magnify and focus the specimen to be observed. course focus is adjusted to focus smaller magnification fine focus focuses higher magnification

magnification

how much bigger the image is compared to the actual size

magnification equation

magnification= image size/ actual size

resolution

how far apart 2 points can be before they are seen as 1

cell theory

all living things are made of cells formed by the division of pre-existing cells all chemical reactions of life take place in cells

Eukaryote

a cell with a nucleus and membrane bound organisms (except ribosomes)

Prokaryotes

a cell with no nucleus and non-membrane bound organelles

structure of the nucleus

double membrane, nuclear envelope, nuclear pores

function of the nucleus

stores DNA and allows substances and mRNA in/out

structure of the nucleolus

dense non-membrane area in the nucleus

function of the nucleolus

produces rRNA and makes ribosomes

structure of 80s ribosomes

40s and 60s sub-units, free or attached to RER

function on 80s ribosomes

site of protein synthesis/ translation

structure of rough endoplasmic reticulum (RER)

membrane bound flattened sacs with ribosomes attached

funtion of rough endoplasmic reticulum (RER)

synthesises folds and transports proteins for secretion, membrane/ lysosomes

structure of smooth endoplasmic reticulum (SER)

membrane bound flattened sacs, no ribosomes

function of smooth endoplasmic reticulum (SER)

lipid synthesis

structure of mitochondria

double membrane- inner folded in cristae. matrix has enzymes, mitochondrial DNA and 70s ribosomes

function of mitochondria

aerobic respiration and ATP production

structure of cetnrtioles

pair of cylindrical structures composed of microtubules

function of centrioles

organises micro tubules for spindle during cell division

structure of lysosomes

single membrane containing hydrolytic/ digestive enzymes

function of lysosomes

digests worn out organelles

structure of golgi apparatus

stack of flattened membrane bound sacs with vesicles

function of golgi apparatus

modifies, sorts and packages proteins and lipids into vesicles to be transported to and from the apparatus

structure of plant cell wall

fully permeable. made of cellulose, middle lamella binds cellulose layers together

function of a plant cell wall

supports cell, maintains shape. strong cellulose fibres prevents bursting

structure of chloroplast

double membrane, thykaloids stacked into grana, has chloroplast DNA and 70s ribosomes

function of chloroplast

grana has large surface area to take in light for photosynthesis

structure of vacuole

membrane bound sac contains water and cell sap

function of vacuole

maintains rigidity and stores water and sap

structure of tonoplast

membrane surrounding the vacuole

function of tonoplast

controls movement of water and cell sap in/out the vacuole

what is ultracentrification

spinning a sample fast to separate organelles (slow= biggest organelles, fast= smallest organelles)

what is supernatant

the fluid left over after a sample is obtained from ultracentrification ready to be spun again at a higher speed to separate the next organelle

structure of a nucleoid

region of cytoplasm in prokaryote where DNA resides. no membrane

function of a nucleoid

prokaryote- contains main genetic information, controls cell activity

structure of plasmids

small circular DNA

function of plasmids

carry additional genes giving bacteria its characteristics

structure of 70s ribosomes

made of rRNA and protein. 30s and 50s subunits

function of 70s ribosomes

site of protein synthesis/ translation

structure of prokaryote cell wall

prokaryote- rigid layer outside the cell

function of a prokaryotic cell wall

maintains cell shape and prevents bursting due to osmosis

fuction of slime coat

prevents desiccation and protects from hosts immune system

structure of slime coat/ protein capsid

sits outside cell wall

Gram positive

thick peptidoglycon layer. no outer membrane. holds crystal violet

gram negative

thin peptidoglycon layer. doesn’t hold crystal violet. does hold sasfranin (red)

plasmodesmata

small channels passing through cell walls joining plant cytoplasm together

gram stain process

1- flood sample with crystal purple

2- wash sample with alcohol

3- counter stain with safranin (red)

gram positive takes up purple- thick peptidoglycon layer absorbs it.

gram negative membrane lets in alcohol doesn’t absorb crystal violet, does absorb safranin

how are viruses classified

structure

type of nucleic acid (DNA or RNA)

DNA nucleic acid

DNA acts as a template for new DNA and mRNA to make viral proteins in host cell. Eg. lambda phage

RNA nucleic acid

contains RNA to make new RNA, no DNA

Positive single stranded RNA viruses

RNA acts as mRNA and is translated at ribosomes of the host. Eg. tobacco mosaic

Negative single stranded RNA viruses

RNA is transcribed into mRNA and translated at the ribosomes of the host. Eg. ebola

RNA retroviruses

converts RNA into double DNA using reverse transcriptase enzyme. DNA then integrated in the cells DNA. Eg. HIV

what is a virus

• infectious particle made up of nucleic acid surrounded by a protein coat

• viruses are not cells but are classes as living organisms because they change and evolve.

lytic cycle

1- virus attaches itself to host

2-virus uses protein and enzymes in host to replicate viral DNA

3-assembles all viral components to form virus

4- host cell bursts (lysis) and virus spreads

lysogenic/ latency cycle

1- viral DNA in the genome to form a provirus

2-viral DNA replicated when host cell divides (virus is dormant)

3-trigger eg. stress can activate viral DNA

4- then enters lytic cycle and bursts releasing virus

antivirals

inhibit virus replication:

• blocking binder receptor prevents the virus from entering the host

• inhibiting assembly viral components cannot reassemble

how and why to control viruses

its important to control viruses as they’re hard to treat.

control by:

preventing transmission

identify cases

isolation

appropriate infection control

Ebola 2014 outbreak

RNA virus (needs a host)

spread through guinea, liberia, sierra leone

28000 cases, 11000 deaths

How is ebola transmitted

bodily fluids:

saliva

blood

diarrhoea

why did ebola spread so much in 2014

• infected people went to uninfected places

• healthcare workers transmit between patients

• limited heathcare resources

Ethical implications of using untested drugs during an outbreak

Strengths

• may save lives

• allows treatment to be tested quickly

Weaknesses

• unknown risks

• may feel forced to consent

what features of a virus attach to the host

spikes, bacteriophages, tail fibres, glycoproteins

function of slime coat/ protein capsid

prevents desiccation

helps bacteria stick to surfaces

protects the virus from the host´s immune system

why are antibiotics not used to treat a virus

because antibiotics inhibit bacterium´s cell wall and viruses do not have cell walls