2.1- Structure Of Cells

Question 1

1) List three ways eukaryotic cells differ from prokaryotic cells.

Answer 1

• membrane bound organelles
• DNA is enclosed in a nucleus
• Have larger ribosomes (80S) than prokaryotic cells

Question 2

2) Prokaryotic cells include bacterial cells. List the differences between Prokaryotic and Eukaryotic cells.

Answer 2

• No membrane bound organelles (e.g. mitochondria, endoplasmic reticulum, golgi)
• No nucleus - instead they have a single circular DNA molecule that is free in the cytoplasm and is not associated with proteins
• Smaller ribosomes
• Has a murein cell wall, a capsule and plasmids

Question 3

Nucleus

Answer 3

ccontaining chromosomes, consisting of protein-bound, linear DNA within the nucleoplasm. Contains one (nucleolus) or more nucleoli
EA, EP

Question 4

Nuclear Envelope

Answer 4

Double membrane surrounding the nucleus with nuclear pores to let molecules in and out
EA, EP

Question 5

Plasma Membrane

Answer 5

Description of structure: A phospholipid bilayer with hydrophllic phosphate heads facing outwards and hydrophobic fatty acid tails facing inwards.
Function: Involved in the transport of substances via diffusion or facilitated diffusion, active transport (with the aid of proteins) EA, EP, P

Question 6

Cytoplasm

Answer 6

Where chemical reactions take place EA, EP, P

Question 7

Ribosome

Answer 7

Where amino acids are joined together to make proteins (i.e. protein synthesis)
They have no membrane • EA, EP = have 80s ribosomes
• P = have 70s ribosomes

Question 8

Mitochondria

Answer 8

Site of AEROBIC respiration
Produces ATP (releases energy)
Contains short, circular DNA (not associated with proteins)
Has a double membrane. The inner membrane is folded forming cristae. 
Contains smaller (70S) ribosomes (thought to have evolved from prokaryotic cells.) 	EA, EP

Question 9

Rough Endoplasmic Reticulum (RER)

Answer 9

Has ribosomes on their surface which are involved in protein synthesis
Proteins are also folded up inside the RER
EA, EP

Question 10

Smooth endoplasmic reticulum

Answer 10

Synthesis and storage of molecules such as steroids and sterols EA, EP

Question 11

Golgi Apparatus and golgi vesicles

Answer 11

Modifies proteins (e.g. by adding carbohydrate groups to form a glycoprotein or lipid groups to make a lipoprotein)
Stores proteins
Packages proteins into vesicles
Transport vesicles to cell surface EA, EP

Question 12

Lysosomes

Answer 12

A vesicle that contains hydrolytic enzymes (lysozymes) which are used to digest molecules EA, EP

Question 13

Chloroplast

Answer 13

Chlorophyll absorb light for photosynthesis to produce carbohydrates
Has a double membrane. Inside there are thylakoid membranes which can form a stack called a granum (pl. grana). The grana are linked by lamellae.
Contain starch grains EP and ALGAE

Question 14

Cell Wall

Answer 14

For strength and support EP (cellulose) and P (Murein) Algae and fungi Vacuole

Question 15

Vacuole

Answer 15

Tonoplast – a membrane surrounding the vacuole
When the vacuole is full of fluid the cell becomes turgid.
Temporary food store containing sugars and amino acids
Contains pigments (colour petals)

Question 16

Flagella

Answer 16

For movement

Question 17

Plasmids

Answer 17

Circular rings of DNA

(can reproduce independently and may give resistance to harmful chemicals such as antibiotics) P

Question 18

Capsule

Answer 18

A ‘slime’ layer on the outside of the cell, made of polysaccharides
It provides protection for the cell and might help cells stick together P

Question 19

Circular DNA

Answer 19

Genetic material in a prokaryote (not enclosed in a nucleus) P

Question 20

1. Define magnification

Answer 20

Magnification is how much bigger the image is than the specimen

Question 21

2. Define resolution

Answer 21

Resolution is the minimum distance apart that the two objects can be in order for them to appear as separate items.

Question 22

3. Describe the procedure to prepare a slide

Answer 22

• Add a drop of water to the slide
• Remove a thin section of tissue and place it onto the slide (flat as possible)
• Add 1 drop of iodine dissolved in potassium iodide to stain the sample (This is only correct if it is plant tissue)
• Lower a coverslip on top using a mounting needle

Question 23

4. Explain why it was important that the sections of tissue were thin

Answer 23

• A thin section allows more light through;

* allows a single layer of cells to be viewed.

Question 24

5. Explain why you should push down hard on the cover slip, but should not push the cover slip sideways.

Answer 24

• Push hard to squash the tissue to create a single layer of cells.
• Do not push sideways as this will cause the cells to roll together.

Question 25

6. A plant cell was observed with an optical microscope. Describe how the length of the cell could be estimated.

Answer 25

* Use a stage micrometer to help us calibrate the size of the eyepiece graticule. * Measure the length of the plant cell with an eyepiece graticule.  

Question 26

Calibrating a stage micrometer

Answer 26

1) Find out the length of the stage micrometer? e.g. 10mm long = 10000m 2) Find out how many divisions there are? e.g 100 divisions 3) Work out how big each division is in m? e.g 10000/100 = 100m = 1 division 4) Place the stage micrometer on the stage 5) Line up the divisions on the eyepiece graticule with those of the micrometer 6) Lets say 100 units on the eyepiece graticule fit 30 units of the stage micrometer 7) Now work out the length of one eyepiece graticule unit in m

Question 27

If the question is asking what is the actual length of the cell

Answer 27

8) Place your slide on the stage 9) Measure the length of the cell using eyepiece graticule units 10) Multiply the number of eyepiece graticule units by the length of one eyepiece graticule unit in m (part 7 – see above)

Question 28

7. Fill in the table below comparing light (optical) and electron microscopes

Answer 28

Scanning electron microscope Transmission electron microscope Light microscope Images are black and white   Can use living specimens  Specimens are always dead   Highest resolution  Lowest resolution  Allows you to see the structures and details within cells  Have to cut cross sections of samples  Do not need thin sections to be cut  Shows the surface of the specimen  Electrons pass through the object to create an image on the screen below (the denser the sample, the darker it appears)  Electrons bounce off the surface of the object and create an image  There has to be a vacuum maintained inside the microscope (so that air particles do not interfere with the electrons)   Resolution of 20nm  Resolution of 0.1nm  Resolution of 0.2m  Can see the natural colours of the object  The microscopes are relatively small and mobile  These microscopes are very big and cannot be moved   2-D image  3-D image produced   False colour can be added   Use magnets to focus the beam of electrons   Use lenses to focus the light 

Question 29

8. Why are electron microscopes used to view cells?

Answer 29

• They have a HIGH resolution • because electrons have a shorter wavelength than light. • This allows you to view internal structures/organelles of a cell. (Remember using this – Light microscope = Low resolution = Longer wavelength)

Question 30

9. Name two structures in a eukaryotic cell that cannot be identified using an optical microscope.

Answer 30

• Mitochondrion / ribosome / endoplasmic reticulum / lysosome / cell-surface membrane

Question 31

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

Answer 31

. 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; Limitations: 5. Cannot look at living material / Must be in a vacuum; 6. Specimen must be (very) thin; 7. Artefacts present; 8. Complex staining method / complex / long preparation time; 9. Image not in 3D / only 2D images produced.

Question 32

11. What is the advantage of TEM (transmission) compared to SEM (scanning) is

Answer 32

* Higher resolution | * Allows internal structures within cells to be seen

Question 33

12. What is the advantages of SEM (scanning) compared to TEM is

Answer 33

* Thin sections do not need to be prepared * shows surface of specimen * can have 3-D images

Question 34

13. Write an equation to calculate magnification and draw a formula triangle

Answer 34

Magnification = size of image | size of object

Question 35

14. How many micrometers in a millimeter?

Answer 35

1mm = 1000m

Question 36

1. What is meant by ‘cell fractionation’ and why would scientists want to do it?

Answer 36

* Separating out the contents of a cell into the different ‘fractions’ (i.e. different parts). * This usually means separating out the different organelles. * This is useful for scientists because it allows them to study individual organelles

Question 37

2. What is homogenisation? What is the point of it?

Answer 37

• Using a blender (or a homogenizer) to break open the cell membrane to release all of the organelles inside

Question 38

3. Why is the solution filtered?

Answer 38

• To remove any whole cells or large cell debris before carrying out centrifugation

Question 39

4. The cells are kept in a specific type of solution: (you must memorise the whole sentence!)

Answer 39

A. Why is it cold? To slow down enzyme activity to prevent digestion of organelles B. Why is it isotonic? To prevent osmosis, so that the ORGANELLES do not shrivel or burst C. Why is it buffered? So that the pH is kept constant, so that proteins are not denatured

Question 40

5. What does ultracentrifugation mean? Describe what happens:

Answer 40

* The sample is put into a tube * It is spun at a low speed (low Revolutions Per Minute = RPM) * The most dense organelle forms a sediment/pellet at the bottom of the test tube * The other organelles remain suspended in the supernatant * The supernatant is removed and put into a clean test tube and spun at higher RPM * The second most dense organelle forms a sediment at the bottom etc