Chapter 2- basic components of living systems

Question 1

How does a light microscope work?

Answer 1

A compound light microscope has two lenses:

• Objective lens - placed near the specimen
• Eyepiece lens – through which the specimen is viewed

The objective lens produces a magnified image which is magnified again by the eyepiece lens.
Illumination is provided by a light under the sample.

Question 1

What are the types of sample preparation?

Answer 1

• Dry mount
• Wet mount
• Squash slides
• Smear slides

Question 2

Explain dry mount and wet mount sample preparation.

Answer 2

Dry mount- solid specimens are viewed whole or cut into very thin slices with a sharp blade (sectioning). A cover slip is placed over the sample.
(Eg. Hair, pollen, muscle tissue)

Wet mount- specimens are suspended in a liquid (water or immersion oil). Cover slip is places from an angle. Used for aquatic samples.

Question 3

Explain squash slides and smear slides sample preparation.

Answer 3

Squash slides- a wet mount is prepared, then a lens tissue is used to gently press down the cover slip. Good for soft samples.

Smear slides- the edge of a slide is used to smear the sample, creating a thin coating on another slide. A cover slip is placed on top. (Cells in blood)

Question 4

Why is staining used?

Answer 4

In light microscopy, the sample is illuminated from below with a white light:

• image has low contrasts as cells don’t absorb much light.
• resolution is limited due to wavelength and diffraction of light.
• cytosol and other cell structures are often transparent.

Stains increase the contrast as different components within a cell take up stains to different levels. It allows components to become more visible and identifyable.

Question 5

How do you prepare a sample to stain?

Answer 5

1. Place on a slide and air dry.

2. Pass it through a flame to heat fix.

Question 6

What is differential staining?

Answer 6

Can distinguish between two types of organisms that would otherwise be hard to identify.
or
Differentiate between organelles of a single organism:

• gram stain technique.
• acid-fast technique.

Question 7

Explain the gram stain technique.

Answer 7

• used to separate bacteria into Gram-positive and Gram-negative.

1. Apply crystal violet to specimen, then iodine, which fixes dye.
2. Wash the slide with alcohol.

• Gram positive bacteria = retain crystal violet stain and appear blue/purple.
• Gram negative bacteria= lose stain due to thinner cell walls. Counterstained with safranin dye and will now appear red.

Gram positive bacteria are susceptible to the antibiotic pencillin, which inhibits the formation of cell walls.

Question 8

Explain the acid-fast technique.

Answer 8

-Used to differentiate species of mycobacterium from other bacteria.

1. Lipid solvent carries carbolfuchsin dye into sample cells.
2. Cells are washed with a dilute acid-alcohol solution

Mycobacterium- retain carbolfuchsin stain = red
Other bacteria- exposed to methylene blue = blue.

Question 9

What are the stages in the production of pre-prepared slides?

Answer 9

1. Fixing- chemicals are used to preserve specimens in as near natural state as possible.
2. Sectioning- specimens are dehydrated with alcohols and placed in mould with wax/resin. Can be sliced thinly.
3. Staining- treated with multiple stains.
4. Mounting- secured to slide with a cover slip on top.

Question 11

What is magnification and how do you calculate it?

Answer 11

How many times larger the image is than the actual size if the object.
Objective lens allows users to adjust magnification.

Magnification = image size / actual size

Question 12

What is an eyepiece graticule?

Answer 12

A glass disc marked with a fine scale of 1-100 (no units).

-the scale remains unchanged if objective lens changes but the relative size of divisions increases with magnification.

The scale on the graticule at each magnification Is calibrated using a stage micrometer.

Question 13

What is a stage micrometer?

Answer 13

A slide with an accurate micro metres (um) scale engraved in it.

The scale marked on the micrometer is usually 100 divisions= 1mm.
So 1 division = 10um

Question 14

What is electron microscopy?

Answer 14

In electron microscopy, a beam of electrons with a wavelength of less than 1nm is used to illuminate the specimen.

• More detail of cell ultrastructure can be seen because electrons have a much smaller wavelength than light waves.
• they can produce images with magnifications of up to x500000 and still have a clear resolution.

Question 15

What are the two types of electron microscopy?

Answer 15

Transmission electron microscope (TEM):
-a beam of electrons is transmitted through the specimen and then focused to produce an image.
similar to light microscopy.
Best resolution with a resolving power of 0.5nm

Scanning electron microscope (SEM):
- beam of electrons sent across the surface of a specimen and the reflected electrons are collected. Three dimensional images of surfaces are produced.
Resolving power is from 3-10nm, so resolution is not as good as TEM.

Question 16

Explain sample preparation for electron microscopes.

Answer 16

Involves:

• Fixation using chemicals/ freezing.
• Staining with heavy metals.
• Dehydration with solvents

Then:
TEM = set in resin and stained again.
SEM= fractured to expose insides and coated with heavy metals.

Question 17

How does a laser scanning confocal microscope work?

Answer 17

1. Moves a single spot of focused light across the specimen. (Point illumination)
2. causes fluorescence ( the absorption and re radiation of light) from the components labelled with a ‘dye’
3. Emitted light is filtered through a pinhole aperture. Only light radiated from close to the focal point is detected.

Question 18

Other key facts about laser scanning confocal microscopes.

Answer 18

• unwanted radiation would cause blurring and reduce the resolution so it doesn’t pass through the pinhole and therefore isn’t detected.
• thin sections of specimen examined = allows high resolution images to be obtained.
• 2D and 3D images can be formed.
• non-invasive: used in the diagnosis of diseases of the eye.
• position of the two pinholes means the light waves from the laser follow the same oath as the radiated light waves from the sample = same focal place (confocal)

Question 19

What is resolution, how is it limited and how can it be increased?

Answer 19

resolution is the ability to see individual objects as separate entities.

• Limited by the diffraction of light as structures are very close and thus causes light to overlap and as a result detail is lost.
• Increased by using beams of electrons as they have a shorter wavelength.

Question 20

nucleus and its function?

Answer 20

• contains coded genetic information in the form of DNA molecules.
• DNA directs the synthesis of all proteins required by the cell.
• DNA associates with proteins called histones to form chromatin which then coils and condenses to form chromosomes.
• Also controls the metabolic activities of the cell.

Question 21

nucleolus and its function?

Answer 21

• It is an area within the nucleus.
• responsible for producing ribosomes.
• composed of proteins and RNA
• RNA is used to produce ribosomal RNA which is then combined with proteins to form the ribosomes necessary for protein synthesis.

Question 22

mitochondria and its function?

Answer 22

• mitochondria have a double membrane. the inner membrane is highly folded to form structures called cristae. This inner membrane contains enzymes used in aerobic respiration.
• the fluid interior is called matrix.
• they are the site of ATP(energy currency) production during aerobic respiration.
• they are self-replicating, so they can reproduce themselves if the cell requires more energy.
• they are abundant in cells where lots of metabolic activity takes place.

Question 23

chloroplasts and its function?

Answer 23

• the organelles responsible for photosynthesis in plant cells.
• found in the cells of the green parts of plants.
• They have a double membrane. the fluid enclosed in the chloroplast is called the stroma.
• Contain DNA and ribosomes so they are able to make their own proteins.
• the internal membranes provide a large SA needed for the enzymes,proteins and other molecules necessary in photosynthesis.

Question 24

Lysosomes and their function?

Answer 24

• Small bags formed from the golgi apparatus. Each is surrounded by a single membrane.
• contain hydrolytic(digestive) enzymes. they are responsible for breaking down waste material in cells, including old organelles.
• important role in the immune system = responsible for breaking down pathogens ingested by phagocytic cells.
• important role in apoptosis(programmed cell death)

Question 25

endoplasmic reticulum (ER) the 2 types and its function?

Answer 25

• the ER is a network of membranes enclosing flattened fluid-filled sacs called cristernae. it is connected to the outer membrane of the nucleus.
• smooth ER is responsible for lipid and carbohydrate synthesis, and storage.
• rough ER has ribosomes bound to the surface and is responsible for the synthesis and transport of proteins.

Question 26

Golgi apparatus and its function?

Answer 26

• a compact structure formed of cristernae and does not contain ribosomes.
• Secretory vesicles bring material to and from the organelle.
• it has a role of modifying proteins and packaging them into vesicles

Question 27

ribosomes and their function?

Answer 27

• can be free floating in the cytoplasm or attached to endoplasmic reticulum (RER).
• not surrounded by a membrane.
• constructed of RNA molecules made in the nucleolus of the cell.
• Ribosomes are the site of protein synthesis.
• mitochondria and chloroplasts contain ribosomes.

Question 28

Vacuole and its function?

Answer 28

• vacuoles are membrane lined sacs in the cytoplasm containing cell sap.
• usually very large in plant cells and are very important in the maintenance of turgor so cells remain rigid.
• the membrane of the vacuole is called the tonoplast which is selectively permeable.
• vacuoles in animal cells are small and temporary.

Question 29

cell wall and its function?

Answer 29

-prokaryotic cells have a cell wall made of peptidoglycan. it is a complex polymer formed from amino acids and sugars.

-cellulose cell wall in plant cells.
helps support the cell by keeping it rigid. Acts as a defence mechanism against invading pathogens.

Question 30

centrioles and its function?

Answer 30

• component of the cytoskeleton composed of of two bundles of microtubules at right angles to eachother.
• involved in cell division.
• involved in the positioning of flagella and cilia.
• look like a flower/sun shape.
• two associated centrioles form the centrosome (involved in the assembly and organisation of spindle fibres during cell division)

Question 31

Vesicles and their function?

Answer 31

Membranous sacs used for storage and transport of materials inside the cell.
Consist of a single membrane with fluid inside.

Question 32

Cytoskeleton and its function?

Answer 32

• Network of fibres necessary for the shape and stability of a cell.
• Holds organelles in place and controls cell movement and movement of organelles inside a cell.
• Found in cytoplasm or eukaryotic cells.

Has 3 main components:
-microfilaments: contractile fibres formed from actin. Responsible for cell movement and cell contraction during cytokinesis.

• microtubules: globular tubulin proteins polymerise to form tubes that determine the shape of a cell. They act as tracks for the movement of organelles around the cell. They make up the spindle fibres used in cell division.
• intermediate fibres: give mechanical strength to cells and help maintain their integrity.

Question 33

Flagella and cilia and their function?

Answer 33

Both are extensions that protrude from some cells.
Flagella are longer but cilia are often present in larger numbers.
Flagella enable cells motility, they are sometimes used as a sensory organelle detecting chemical changes in the cell’s environment.
Stationary cilia is important in sensory organs (nose)
Mobile cilia create a current to move fluids and objects (present in the trachea to move mucus away from lungs + in fallopian tube to move egg cells)
Each cilium has a 9+2 formation (two microtubules with 9 pairs of microtubules around it)

Question 34

Explain the steps of protein production.

Answer 34

1. Proteins are synthesised on the ribosomes bound to the endoplasmic reticulum.
2. They pass into its cisternae and are packaged into transport vesicles.
3. Vesicles containing the newly synthesised proteins move towards the Golgi apparatus via the transport function of the cytoskeleton.
4. Proteins are further processed and modified at the Golgi apparatus.
5. Secretory vesicles carry the proteins to be released from the cell. The vesicles fuse with the cell-surface membrane, releasing their contents by exocytosis.

Question 35

What is the nuclear envelope?

Answer 35

DNA is contained within a double membrane called a nuclear envelope to protect it.

• contains nuclear pores that allow molecules to move into and out of the nucleus.
• DNA itself is too large to leave the nucleus to the site of protein synthesis so it is transcribed into smaller RNA molecules.