Cell Structure And Microscopy

Question 1

Cell membrane

Answer 1

Semi-permeable barrier that controls the entry and exit of substances.

Question 2

Cytosol

Answer 2

Fluid portion of the cytoplasm (doesn’t include the organelles or other soluble materials)

Question 3

Nucleus

Answer 3

Contains hereditary material so controls cell activity (via transcription) and mitosis (via DNA replication)

Question 4

Nucleolus

Answer 4

Site of production and assembly of ribosome components.

Question 5

Ribosome

Answer 5

Complexes of RNA and protein that are responsible for polypeptide synthesis (80s in eukaryotic)

Question 6

Mitochondria

Answer 6

Site of aerobic respiration. Produces ATP from organic compounds.

Question 7

Golgi apparatus

Answer 7

An assembly of vesicles and folded membranes involved in the sorting, storing and modification of secretory products.

Question 8

Lysosome

Answer 8

Site of hydrolysis/ digestion/ breakdown of macromolecules

Question 9

Peroxisome

Answer 9

Catalyses breakdown of toxic substances like hydrogen peroxide and other metabolites.

Question 10

Centrioles

Answer 10

Microtubule-organising centres involved in cell division (mitosis/meiosis and cytokinesis)

Question 11

Endoplasmic reticulum
Rough
Smooth

Answer 11

A system of membranes involved in the transport of materials between organelles.
R-studded with ribosomes and involved in the synthesis and transport of proteins destined for secretion.
S- involved in the synthesis and transport of lipids and steroids, as well as metabolism of carbohydrates.

Question 12

The cellular levels of organisation in a multicellular organism

Answer 12

Stem cells (undifferentiated)
Specialised cells (differentiated- certain genes on/ off)
Tissues- a group of different cells working together to perform a specific function
Organ- groups of tissues aggregated together to perform a function
Organ system- organs working together as a single unit to perform a function.

Question 13

Prokaryotic vs eukaryotic cells

Answer 13

Prokaryotes:
No true nucleus, only a spread out area of nuclear material with no nuclear membrane.
DNA not associated with histones.
Circular strands of DNA (plasmids) but no chromosomes.
No membrane bound organelles
No chloroplast 
70s ribosomes 
No ER, Golgi apparatus and lysosomes
Murein cell wall

Eukaryotes:
Distinct nucleus with a nuclear envelope.
Nucleolus present
Chromosomes present in which DNA is located
Membrane bound organelles
Chloroplasts present in plants and algae
80s ribosomes
ER Golgi apparatus and lysosomes
Cell wall made mostly of cellulose (chitin in fungi)

Question 14

Additional features of prokaryotic cells and their functions.

Answer 14

Murein cell wall- protecting against osmotic lysis
External capsule - protection and helps bacteria stick together
Plasmids- small circular pieces of DNA which have additional genes e.g. antibiotic resistance
Flagella- motile ‘tail’
Pili- microscopic tube extensions to allow transfer of plasmid DNA between individual bacteria (transjugation)

Question 15

Key features of viruses

Answer 15

Acellular 
Non-living 
20-300nm
Contain DNA or RNA nucleic acids (genome)
Enclosed within a capsid 
Sometimes enveloped (e.g.HIV)
Has surface attachment proteins
Require a specific host cell to enter and replicate

Question 16

Advantages and disadvantages of the light microscope and resolving power

Answer 16

Advantages:
Can see living specimens
Easier specimen preparation
Variety of coloured stains

Disadvantages:
Low resolution so organelle detain/ smaller components not visible.

Resolving power: limited 0.2 micrometres/ 200nm

Question 17

Transmission electron microscope (TEM) advantages, disadvantages and resolving power.

Answer 17

Advantages:
very high resolution at high magnification.
Detailed organelle/ sub-organelle structure (inside)

Disadvantages:
Specimens are not alive (are in a vacuum)
Difficult preparation because very thin specimens/ complex staining process.
Black and white image.
Artefacts caused by staining process can ruin image.

Resolving power: 0.1 nm but not always achieved as difficult specimen prep/ high energy beam can destroy specimen.

Question 18

Scanning electron microscope advantages, disadvantages and resolving power.

Answer 18

Advantages:
3D images show structural formation

Disadvantages:
Specimens are not alive because the system is in a vacuum
Difficult prep
Black and white image.

Resolving power: 20nm

Question 19

Magnification equation

Answer 19

Magnification= size of image (divided by) actual size of object

Question 20

Principles of cell fractionation and ultracentrifugation

Answer 20

1. Homogenisation
   - This breaks open the cells
   - Done by vibrating the cells or grinding them up in a blender
   - It is done with a cold, isotonic buffer: cold to slow down and stop organelle activity, particularly the hydrolytic enzymes in lysosomes. Isotonic to prevent movement of water in and out of organelles by osmosis (no net flow) so cells don’t shrink or burst.
   - A buffer solution to prevent changes in PH levels (and enzyme activity)
2. Filtration
   Filter the solution through a gauze to remove debris e.g. large cell debris or tissue debris
3. Ultracentrifugation
   - Spin the solution in a centrifuge at low speed
   - The heaviest organelles (nuclei, chloroplast) fall to the bottom (pellets). The rest of the organelles stay suspended in the fluid above the sediment. This is the supernatant.
   - The supernatant is drained off, poured into another tube, and spun again at a higher speed. Mitochondria and lysosomes fall to the bottom.
   - The supernatant is drained off again, poured into another tube and spun at a higher speed so the lightest organelles remain (ribosomes).

Question 21

Stages of mitosis in eukaryotic cells

Answer 21

Interphase:
Decondensed chromatin in the nucleus.
Nucleolus visible
Nuclear membrane visible
Centrioles present in animal cells. Centrioles replicate.
DNA replication takes place before mitosis starts

Prophase:
Chromatin condenses and chromosomes become visible
Nucleoli disappear
Nuclear membrane disappears
Centrioles move to poles of the cell and form spindle fibres (spindle apparatus)
Chromosomes attach to the spindle at their centromeres

Metaphase:
Chromosomes (made up of 2 sister chromatids) line up on the equator of the spindle apparatus.

Anaphase:
The centromeres divide and the spindle fibres begin to shorten.
Sister chromatids are pulled to opposite poles by the spindle fibres using energy from ATP,
The separated chromatids are now referred to as separate chromosomes.

Telophase:
The chromosomes reach the poles
The chromosomes decondense and are no longer visible
The spindle breaks down and the centrioles reappear
The nuclear membrane reappears and the nucleoli appear.

Cytokinesis:
Division of the cytoplasm to form 2 genetically identical daughter cells.

Question 22

How would you prepare a slide showing stages of mitosis in plant cells?

Answer 22

• Use a sharp scalpel to remove the last 1mm of an actively growing root tip (cells at tip will be actively going through the cell cycle/ dividing).
• Place the tip in some 1M hydrochloric acid for 10 minutes- breaks down the plant tissue so it’s easier to separate the cells for viewing.
• Rinse the root tip in water
• Crush the root tip on your slide (one cell thick)
• Add the stain- darkens the chromosomal DNA so phases of mitosis are easier to distinguish
• Place on a cover slip
• Cell preparation is to be one cell thick so light can come through and individual cells can be counted.

Question 23

Mitotic index

Answer 23

Mitotic index = cells in mitosis (divided by) the total number of cells.
Cells in mitosis can be distinguished by them having visible chromosomes.

Question 24

How may tumours and cancer develop

Answer 24

Tumours (cancer) develops when cells lose control over the cell cycle and divide in an uncontrollable manner.
This results in a mass of rapidly dividing, abnormal cells.
Cancers result from mutations of key genes that control the cell cycle.
Treatments involve:
-Preventing DNA from replicating
-Inhibiting the metaphase stage of mitosis by interfering with spindle formation.

Question 25

Cell division in prokaryotic cells

Answer 25

Binary fission

• The circular DNA molecule replicates and both copies attach to the cell membrane.
• The plasmids also replicate
• The cell membrane begins to grow between the two DNA molecules and begins to pinch inward, dividing the cytoplasm into two.
• A new cell wall forms between the two molecules of DNA, dividing the original cells into two identical daughter cells, each with a single copy of the circular DNA and a variable number of copies of the plasmids.

Question 26

Simplified virus replication

Answer 26

As viruses are non-living, they can’t undergo cell division. They replicate by attaching to their host cell with the attachment proteins on their surface. They inject their nucleic acids into the host cell. The genetic information on the injected viral nucleic acid (on the mRNA) then provides the instructions for the host cells metabolic processes to start producing the viral components, nucleic acid, enzymes and structural proteins which are assembled into new viruses.