Cell Structure (Chapter 3)

Question 1

Compare Prokaryotic and Eukaryotic Cells

Answer 1

• Bacteria don’t have nucleus/nuclear membrane while eukaryotic do.
• Nucleotide structure is similar in both
• Bacteria don’t have membrane bound organelles while eukaryotic do.
• Bacteria don’t have mitochondria, eukaryotic do.
• Bacteria have 70s/smaller ribosomes, eukaryotic have 80s/larger ribosomes.
• Bacteria have murein/peptidoglycan cell wall but eukaryotes don’t.
• Bacteria cells are smaller than eukaryotes.
• Bacteria have circular DNA while eukaryotes have linear DNA held in chromosomes.
• Bacteria have capsules while eukaryotes don’t
• Bacteria are not associated with histones while eukaryotes are.
• Eukaryotes have introns but bacteria don’t

Question 2

What is centrifuagtion used for?

Answer 2

Helps to separate organelles, most dense organelle is separated first, forming a pellet which is removed. Supernatant contains other organelles which can spin again.

Question 3

Why is centrifugation cold?

Answer 3

Slows down the digestion/stops enzyme activity to prevent digestion of organelles.

Question 4

Why is centrifugation isotonic?

Answer 4

Prevents osmosis - so no lysis/bursting/shrinking of organelles.

Question 5

Why is a buffer used in centrifugation?

Answer 5

Prevents damage to enzymes/proteins (denaturing) and maintains pH.

Question 6

Why are cells Homogenised in Centrifugation?

Answer 6

To break open cell membrane and release contents (organelles). Filter to remove unbroken cells/larger debris.

Question 7

Compare DNA in Chloroplasts vs Nucleus

Answer 7

• Shorter in chloroplast
• Few genes in chloroplast
• DNA is circular in chloroplasts but linear in the nucleus
• DNA is not associated with proteins/histones in chloroplast but are in DNA
• No introns in chloroplast DNA but are in nuclear DNA

Question 8

Describe the process of Centrifugation

Answer 8

1) Chop up cells (e.g. grind with pestle and mortar)
2) Cells are homogenised
3) Place in cold, isotonic, buffer solution
4) Filter and centrifuge filtrate
5) Organelle making up most of the pellet after being spun at low speed.
6) Centrifuge supernatant at higher speed

Question 9

What is the equation for magnification?

Answer 9

Magnification = Image/Object

Question 10

Define Magnification

Answer 10

Being able to enlarge something

Question 11

Define Resolution

Answer 11

Ability to distinguish two points close together

Question 12

What is the difference between TEM and SEM microscopes?

Answer 12

TEM - black and white

SEM - 3D images

Question 13

What are the benefits of an electron vs light microscope?

Answer 13

Electron has higher resolution because electron beam has a shorter wavelength so can see smaller objects.

Light microscopes can produce coloured images and can look at living cells.

Question 14

Why must objects be very thin to be viewed?

Answer 14

They must be a single layer so that light can pass through them.

Question 15

What is the eyepiece graticule?

Answer 15

A glass disc fitted into the eyepiece of the microscope.The scale of this is important as this is what is calibrated.

Question 16

What is the stage micrometer?

Answer 16

Small microscope slide with an engraved fine and accurate scale. Used to calibrate the eyepiece graticule.

Question 17

How do you use an eyepiece graticule?

Answer 17

1) Measure object (e.g. stoma) using eyepiece graticule.
2) Calibrate eyepiece graticule against stage micrometer/ruler/graph paper.
3) Take several measurements to calculate a mean.

Question 18

Define Artefact

Answer 18

Things that results from the preparation of the specimen. Not part of actual specimen.

Question 19

List the Organelles in order of highest to lowest density

Answer 19

Nucleus
Chloroplast
Mitochondria
Lysosome
Endoplasmic Reticulum
Ribosomes

Question 20

Define mitosis

Answer 20

The process which makes 2 identical daughter cells which is used for growth, differentiation and repair.

Question 21

In what order are the stages of mitosis

Answer 21

Interphase, Prophase, Metaphase, Anaphase, Telophase

Question 22

Describe the process of interphase

Answer 22

G1 - Organelles synthesized and cells grow
Synthesis Phase - DNA is replicated and amount of DNA doubles (same no. of chromosomes)
G2 Phase - Organelles grow, energy stores increase

Question 23

Describe the process of prophase

Answer 23

Chromosomes condense and become visible, chromosomes appear as chromatids joined by the centromere, nuclear envelope disappears and the nucleolus disappears

Question 24

Describe the process of metaphase

Answer 24

Spindle formed and chromosomes/chromatids attached to spindle fibre by their centromere.
Chromosomes/chromatids line up at the equator of the cell

Question 25

Describe the process of anaphase

Answer 25

Centromere splits and sister chromatids separate to opposite poles of the cells.

Question 26

Describe the process of telophase

Answer 26

Nuclear envelope reforms and spindle disintegrates and chromosomes uncoil/become thinner.

Question 27

Describe viral replication

Answer 27

Non-living cells cannot undergo cell division so replicate by attaching to host cells with the attachment proteins on their surface.

Inject nucleic acid into a host cell which combines with host cell DNA, starts producing viral components and are assembled into new proteins.

Question 28

What is the process called when bacteria replicate their genetic material?

Answer 28

Binary Fission

Question 29

Describe the process of binary fission

Answer 29

Circular DNA molecules replicate and both copies attach to the cell membrane then plasmids replicate.

Cell membranes grow between the two DNA molecules, the cells pinches inward between the two DNA molecules, dividing the cytoplasm.

New cell wall forms between the two DNA molecules.

Question 30

What is cancer?

Answer 30

Uncontrollable cell division because the genes which regulate mitosis become damaged.

Question 31

What are the roles of the spindle fibres?

Answer 31

To pull the chromatids apart for the centromeres to attach to.

Question 32

Why must root tips be squashed?

Answer 32

To create a layer one cell thick to let light through.

Question 33

How do you make the mitotic index accurate?

Answer 33

Examine lots of cells to get a representative sample and repeat count to ensure that figures are correct.

Question 34

Describe the principles of using a transmission electron microscope to investigate cell structure.

Answer 34

1) Electrons pass through / enter (thin) specimen;
2) Denser parts absorb more electrons;
3) (So) denser parts appear darker;
4) Electrons have short wavelength so give high resolution;