Biology A-level (module 2)

What is cell theory?

• Both plant and animal tissues are made of cells

• Cells are the basic unit of all life

• Cells form from existing cells

What was the importance of microscopes in the study of living organisms?

• Disproved the theory of spontaneous generation

• Cell theory

• Helped to understand genetics and the cause of disease

Why wasn’t cell theory fully developed before the mid -19th century?

• Lack of evidence

• Didn’t fit existing theories

• Lack of good quality microscope lenses

• Not enough magnification

• Slow spread of ideas (books)

Why does the specimen have to be thin when using a microscope?

So the light can pass through it

What are the 4 ways to prepare samples for a microscope?

• Dry mount

• Wet mount

• Squash slides

• Smear slides

What is a dry mount?

Where solid specimens are viewed whole or cut into very thin slices (sectioning) and placed on a slide with a cover slip on top

What is it called when you cut very thin slices off of a specimen?

Sectioning

What are the advantages of dry mount slides?

• quick

• Little to no preparation needed

• Able to view living cells

What are the disadvantages of dry mount slides?

• only outer layer visible of the organism

• hard to cut very thin sections (especially if opaque)

• Some cell types are unsuitable

What is a wet mount slide?

Specimens are suspended in a liquid and a cover slip is placed on an angle on top

Why must the cover slip be placed on an angle when using a wet mount?

To avoid air bubbles

Why must the liquid have the same refractive index as the glass in a wet mount?

If they are not the same it becomes hard to focus as the light changes directions multiple times

What is a squash slide?

A wet mount is first prepared and lens tissue is used to gently press down the cover slip

What are wet mounts good for?

Soft samples

What is the danger of using a wet mount?

Could damage the specimen or distort the image of the specimen

What is a smear slide?

The edge of a slide is used to smear the sample to create a thin even coating on another slide (blood)

Why is a smear slide used for viewing a drop of blood?

Cells in a drop of blood are densely packed together. A smear spreads the cells out so it is easier to view individual cells

What things must you consider when producing slides or microscopes?

• specimens must be thin

• Refractive index of specimens must be the same as the glass slide

• Cover slip is placed to avoid air bubbles (wet mount)

• Density of cells (e.g blood)

What is the difference between positive and negative stain technique?

• Some dyes are negatively charged so are repelled by the negatively charged cytosol, so the dyes stay outside the cell walls and stain the backgrounds (negative)

• Some dyes are positively charged so are attracted to the negatively charged cytosol, so the dyes stain the cell components (positive)

What is differential staining and why is it useful?

Using 2 different stains to distinguish between 2 different organisms/ organelles that would be otherwise hard to identify

What is the difference between gram positive and gram negative bacteria?

• Gram positive can retain crystal violet dye when it’s washed away so it appears blue/ violet under a microscope

• Gram negative can’t retain crystal violet dye as the walls are too thin so a counter stain is used and it appears red under a microscope

What is magnification?

How many times larger the image is than the actual size of the specimen

What is resolution?

The ability to distinguish between 2 close together objects

What is the limit of the resolution of a light microscope?

No less than half a wavelength of visible light (250 nm)

How is magnification calculated?

Magnification = image size/ actual size

How do you convert between metres to millimetres?

X 1000

How do you convert between micrometers to millimetres?

/ 1000

Which is larger; micrometers or nanometres?

Micrometers

What is the order of units from largest to smallest?

Metres, centimetres, millimetres, micrometers, nanometres

What do you have to do every time you change magnification or microscope when using an eyepiece graticule?

Re-calibrate / re do calculations using scale micrometer

What are the features of an animal cell?

• Mitochondria

• Ribosome

• Cell membrane

• Lysosome

• Nucleolus

• Nucleus

• Nuclear envelope

• Golgi apparatus

• Rough endoplasmic reticulum

• Smooth endoplasmic reticulum

What are the features of a plant cell?

• Mitochondria

• Ribosome

• Cell membrane

• Nucleolus

• Nucleus

• Nuclear envelope

• Golgi apparatus

• Rough endoplasmic reticulum

• Smooth endoplasmic reticulum

• Amyloplast containing starch grains

• Permanent vacuole

What is a Eukaryote?

Any organism containing or or more cells that contains DNA in a membrane bound nucleus, separate from the cytoplasm

What is the structure of a nucleus?

• Largest organelle

• Has pores in the nuclear envelope

• Contains chromatin

What is the function of a nucleus?

• Protein synthesis

• Makes ribosomes

• Houses nearly all the genetic material

What is chromatin and why cant it leave the nucleus?

• Something that contains DNA and proteins. It makes up chromosomes.

• It is too big to fit through the pores so mRNA is transcribed which is small enough to leave the nucleus

What is the structure of the mitochondria?

• 2 membranes, inner and outer layer

• Inner layer folds to form cristae, the cristae project into a liquid called the matrix

• Inner membrane is coated in enzymes to catalyse the reactions of aerobic respiration to make ATP

What is the function of the mitochondria?

• generates energy

• Respiration - large surface area increases rate

What is the structure and function of the lysosome?

• spherical sacs surrounded by a single membrane (due to the enzymes)

• Specialised vesicles

• Contains powerful digestive enzymes

• Their role is to break down things (dead cells/ waste/ pathogens)

What is a vesicle?

Membrane bound sac of fluid

What is the Golgi apparatus?

• creates lysosomes vesicles

• Processes/ modified proteins & lipids before packaging them into Golgi vesicles (for transport)

What 3 things make up the cytoskeleton (and what do they do)?

• microfilament (movement)

• Microtubules (shape & structure)

• Intermediate fibres (mechanical strength)

What are centrioles?

• only found in animal cells

• Found in pairs

• Responsible for spindle fibres in cell division (pulls apart cell components)

• Responsible for movement of flagella and cilia

What do flagella and cilia do in a single celled organism?

Essential for locomotion of individual organisms

What do flagella and cilia do in multicellular organism?

Move fluid or materials past immobile objects

What is the structure and function of ribosomes?

• tiny organelles in cytoplasm or bound to rough endoplasmic reticulum

• Consists of 2 sub-units

• Site of protein synthesis

• Made of small proteins & mRNA

What is the function and structure of Rough Endoplasmic Reticulum?

• Series of flattened membrane bound sacs called cisternae

• Studded with ribosomes and transports the proteins made on them

• Some proteins are secreted, some placed on surface of the cell

What is the structure and function of Smooth Endoplasmic Reticulum?

• Series of flatted membrane bound sacs called cisternae

• Involved in the making of lipids that the cells need

What is the structure of the cell wall in a plant cell?

• Made of cellulose (polysaccharide) (permeable to water)

• Mesh like (made of microfibres - hydrogen bonds between the cellulose particles)

• Pores between cells allows for exchange of substances and can connect 2 cells together by their cytoplasm (plasmodesmata)

• Lamalae between cells (gap) - acts as glue

What is the functions of a cell wall in a plant cell?

• strength & support

• Protect from pathogens

• Stop cell from bursting when turgid

What other types of cells also have cell walls but what is the difference between these and plant cells?

• algae and fungi

• Cell walls aren’t made of cellulose

What is the structure of a chloroplast?

• double membrane

• Thylakoids (membrane bound discs that contains chlorophyll and enzymes) stacked like plates called grana (plural) or granum (single) which are linked together by a flattened membrane to each other. The membrane is the site of photosynthesis

• Filled with a liquid called stroma

• Starch grains - stored glucose made photosynthesis

• Has own loops of DNA & ribosomes to make proteins

What are 2 types of reactions that take place in the chloroplast?

• light dependent (chlorophyll harvests sunlight)

• Not light dependent ( production of glucose)

What is the structure of a vacuole in a plant cell?

• Membrane (called the tonoplast) controls the entry and exit of chemicals

• Filled with sap

• Important in keeping cell firm

What does sap contain?

• water

• Sugar

• Enzymes

• Pigments

What does it mean when a cell is said to be ‘turgid’?

The vacuole is full of sap (swollen)

Why do plants wilt?

The vacuole lacks water and minerals for sap

What is a peptidoglycan?

A material that most bacterial cells walls contain.

Polymers of modified sugars cross linked by a short chained polypeptide

What is a mesosome?

Invaginations formed by the plasma membrane to provide internal membrane surfaces for special purposes

E.g supposedly the site of respiration

What is a plasmid?

Short circular strand of DNA that carry as few genes as possible for special metabolic path ways & resistance to antibiotics

Can transfer between bacteria which makes it important for genetic engineering

What are some differences between prokaryotes and eukaryotes?

• P have no membrane bound organelles, E do

• P have no nuclear membrane, E do

• P have a nuclear mass, E have a nucleus

• P have one long chromosome, E have many

• P are usually single, E are usually part of an organism

• P don’t have mitochondria, E do

• P always have a cell wall, E only has them in plant cells (cellulose)

What are some similarities between chloroplasts and mitochondria?

• double membrane

• Compartmentalisation

• Both contain DNA

• Both contain ribosomes

• Both capable of protein synthesis

• Both have readily permeable outer membrane and selectively permeable inner membrane

• Permeability is brought about by proteins which span the membrane

Where is the DNA located in prokaryotic cell?

In direct contact with the cytoplasm in a singular continuous thread of DNA located in a region called the nucleoid and plasmids. There is no true nucleus.

What is a systematic error?

An error that will occur every time, due to a mistake in setting up the equipment or the way the particle is ran

What is a random error?

An error that occurs once by random chance. Different amounts every time/ different errors

E.g. human mistakes

What are the microtubules made from?

• The protein called tubulin

• Made from an alpha tubulin subunit and a beta tubulin subunit join to make a tubulin dimer

• 25 nm in diameter

What do tubulin diners join to form? (+ how many of these need to join to make a microtubule?)

• Protofilaments

• 13 of these make microtubules

What are the difference between electron and light microscopes?

• light are cheaper to buy and operate

• Light are small and portable, electron are large and have to be installed

• Light have simple sample preparation, electron have complex sample preparation

• Electron sample preparation can lead to distortion of materials

• Vacuum is not require for light but is for electron

• You can see the natural colour using a light microscope but it’s black and white with electron (can be coloured digitally)

• Electron has a greater magnification than light (500,000 x compared to 2000 x)

• Electron has a higher resolving power (electron can see from 0.5nm or 3-10nm whereas light can see 200nm)

• Light microscopes can view living organisms whereas electron cant

What are the advantages and disadvantages of using an electron microscope?

Advantages:

• the resolution is 0.2nm

• See structured detailed images of organelles inside the cell

• SEM (scanning) can produce 3D images of the arrangement of cells

Disadvantages:

• electron beams are affected by air molecules so requires a vacuum

• Very expensive

• Preparing slides and samples requires high level of training and skill (slides have to be very thin)

What is an artefact?

Visible object or distorted cell structure present due to the sample preparation

Why can you see more details using an electron microscope than with a light microscope?

Electron wavelengths are smaller than the half wavelength of light so has a higher resolution and you can see more detailed structures

What are the advantages and disadvantages of a Transmission electron microscope (TEM)?

Advantages:

• Greater magnification

• Greater resolution

Disadvantages:

• smaller samples (more preparation)

What are the advantages and disadvantages of a Scanning electron microscope (SEM)?

Advantages:

• 3D images

• Sample doesn’t need to be thin

Disadvantages:

• lower magnification

• Lower resolution

What organelles do plant cells not have?

• centrioles

• Lysosomes

• Flagella

• Cilia

• Glycogen granules

What are the cell wells made from in plant, bacterium and yeast cells?

Plant: cellulose

Bacterium: peptidoglycan

Yeast: chitin

What kind of cell division occurs in plant, animal, bacterium and yeast cells?

Plant: cytokinesis

Animal: cytokinesis

Bacterium: binary fission

Yeast: budding

Do plant, animal, bacterium and yeast cells contain nucleus?

Yes - animal, plant and yeast

No - bacterium

What is an ultra structure?

Detailed structure of cells visible only with electron microscope