Studying Cells

Question 1

Eukaryotic cells:
Eukaryotic means ‘true nucleus’. The DNA of eukaryotes is enclosed by a nuclear membrane

Question 2

Plants, algae, animal, protozoan, and fungi are ALL eukaryotic

Question 3

Animal Cell:
Nucleus
Mitochondria
RER
SER
Ribosome
Golgi
Lysosome
Cell Surface Membrane
Centrioles
Cytoplasm
Microvilli

Question 4

Nucleus

Structure:
1. Nuclear envelope and pores OR Double membrane and pores;
2. Chromosomes/chromatin OR DNA with histones;
3. Nucleolus/nucleoli;

Function:
4. (Holds/stores) genetic information/material for polypeptides (production) OR (Is) code for polypeptides;
5. DNA replication (occurs);
6. Production of mRNA/tRNA OR Transcription (occurs);
7. Production of rRNA/ribosomes

Question 5

Mitochondria

Structure:
1) Double membrane
2) Inner membrane highly folded to form cristae (increase surface area)
3) Matrix (liquid part) containing mitochondrial DNA, ribosomes, proteins and lipids

Function:
4) Site of ATP production by aerobic respiration

Answer 5

Cells that need a lot of ATP have lots of mitochondria

E.g:
Muscle cells (for muscle contraction) and epithelial cells (for active transport of ions)

Question 6

Rough Endoplasmic Reticulum

Structure:
1) Highly folded membranes with 80s ribosomes embedded 2) The membrane is folded into flattened sacks called cisternae
3) Joined to the nucleus

Function:
4) Synthesises and transport of proteins throughout the cell

Answer 6

Cells which make lots of protein i.e. secreting extracellular enzymes or antibodies have lots of Rough Endoplasmic Reticulum
E.g. enzyme secreting gland cells, or antibodies producing plasma cells

Question 7

Smooth Endoplasmic Reticulum

Structure:
1) Highly folded membranes flattened into sacks called cisternae

Function:
2) Recombines glycerol and fatty acids to make triglycerides
3) Packages triglycerides into vesicles and transports them to the Golgi apparatus

Question 8

Ribosome

Structure:
1) Made up of 2 subunits that are made of long strands of rRNA AND ribosomal proteins
2) Eukaryotic cell contains 80s cytoplasmic ribosomes

Function:
3) Site of protein synthesis from amino acids

Question 9

Golgi Apparatus/Body

Structure:
1) Flattened sacs made of membrane filled with fluid
2) Golgi vesicles pinch off from the main membrane

Function:
3) Sorts, modifies, and packages proteins and triglycerides into vesicles
4) Golgi vesicles may be used to form lysosomes

Answer 9

Cells with extensive Golgi, packages lots of molecules for export, e.g. enzyme secreting gland cells, or antibodies producing plasma cells

Question 10

Lysosome

Structure:
1) Membrane-bound organelle that stores and releases (many) hydrolytic enzymes

Function:
2) Contain hydrolytic enzymes

Answer 10

Phagocytes are types of white blood cells that contain many lysosomes, as they hydrolyse invading pathogens

Question 11

Cell Surface Membrane

Structure:
1) Made up of phospholipids, specific transport proteins, and carbohydrates arranged into what is described as a fluid mosaic model

Function:
2) Controls the passage of molecules in and out of the cell

Question 12

Centrioles

Structure
1) Microtubules

Function
2) Form a network of spindle fibres onto which chromosomes attach
3) Pull chromosomes /chromatids apart during mitosis

Answer 12

Not found in plant cells

Question 13

Plant Cell: Palisade
Ribosome
SER
Nucleus
RER
Vacuole
Starch grain
Cell wall
Cell membrane
Golgi
Chloroplast
Vacuole membrane
Mitochondria
Cytoplasm

Question 14

Chloroplasts

Structure: Granum
Function: Stack of thylakoid membranes

Structure: Thylakoid membrane
Function: Contains chlorophyll for photosynthesis & ATP synthase enzyme to produce ATP

Structure: Stroma
Function: Fluid filled part, some of the photosynthetic reactions occur here

Structure: Starch grains
Function: The energy storage molecule in plants

Structure: DNA and Ribosomes
Function: Contain their own DNA and 70s ribosomes for synthesis of enzymes needed for photosynthesis

Question 15

Cellulose cell wall (in plants and algae)

Is very strong (many WEAK hydrogen bonds between cellulose fibrils) which limits the volume of water that can move into the cell and stops osmotic lysis (bursting).

The wall is permeable to most molecules, unlike the membrane (micro fibrils arranged in a matrix).

It also has plasmodesmata these are gaps in the cell walls that connect cell cytoplasm’s together, to allow the easy movement of water-soluble molecules.

Question 16

Fungi also have cell walls, made from chitin

Bacteria also have cell walls, made from muerin

Question 17

Key differences between plant and animal cells

Plant : Animal

P) Cellulose cell wall
A) No cell wall

P) Has chloroplasts
A) No chloroplasts

P) Large vacuole
A) No vacuole

P) Carbohydrates stored a starch
A) Carbohydrates stored as glycogen

P) No centrioles
A) Has centrioles

Question 18

Prokaryotic cells:

Prokaryotes do not have nucleus or other membrane bound organelles.
The DNA of a prokaryotic cell is circular, and it is not associated with histones.

Question 19

Bacteria Cell:

Capsule
Flagellum
Circular DNA free in cytoplasm
Cell membrane
Cell wall
Plasmid
Mesosomes
70s Ribosomes

Question 20

Differences between
Prokaryotic : Eukaryotic

P) Circular DNA not associated with histones
E) Linear DNA associated with histones

P) Contains no membrane bound organelles
E) Contains membrane bound organelles

P) Has no ‘true’ nucleus, DNA is free in cytoplasm
E) Has a nucleus, DNA contained within nuclear membrane

P) Contains smaller ribosomes (70s)
E) Contains larger ribosomes (80s)

P) Some have capsule, one or more flagella, one or more plasmids
E) Do not have capsule

P) Has mesosomes for ATP synthesis
E) Does not have mesosomes (has mitochondria)

P) Cell wall made of muerin or peptidoglycan
E) Plant cell wall made of cellulose

Question 21

Viruses are not cells; they are not alive
They are very small and they require a living cell to replicate inside.

· Contain DNA or RNA, which can be single or double stranded.

· Surrounded by a protein coat called a capsid

· Also a virus has attachment proteins which enable it to bind to host cells

· Has enzymes that is uses to replicate its genetic information and insert it into the host cell DNA

Question 22

Eukaryotic cells produce
and release proteins.
Outline the role of
organelles in the production,
transport and release of
proteins from eukaryotic
cells.

Answer 22

1. DNA in nucleus is code (for protein);
2. Ribosomes/rough endoplasmic reticulum produce (protein);
3. Mitochondria produce ATP (for protein synthesis);
   4 Golgi apparatus package/modify; OR Carbohydrate added/glycoprotein produced by Golgi apparatus;
   5 Vesicles transport OR Rough endoplasmic reticulum transports;
4. (Vesicles) fuse with cell(-surface) membrane;

Question 23

Describe the differences
between Eukaryotic and
Prokaryotic cells

Answer 23

Comparisons
1. Nucleotide structure is identical;
2. Nucleotides joined by phosphodiester bond; OR Deoxyribose joined to phosphate (in sugar, phosphate backbone);
8. DNA in mitochondria / chloroplasts same / similar (structure) to DNA in prokaryotes;

Contrasts
4. Eukaryotic DNA is longer;
5. Eukaryotic DNA contain introns, prokaryotic DNA does not;
6. Eukaryotic DNA is linear, prokaryotic DNA is circular;
7. Eukaryotic DNA is associated with / bound to protein / histones, prokaryotic DNA is not;

Question 24

State three differences between DNA in the nucleus of a plant cell and DNA in a prokaryotic cell

Answer 24

Plant v prokaryote
1. (Associated with) histones/proteins v no histones/proteins;
2. Linear v circular;
3. No plasmids v plasmids;
4. Introns v no introns;
5. Long(er) v short(er)

Question 25

The structure of a cholera
bacterium is different from
the structure of an epithelial
cell from the small intestine.
Describe how the structure
of a cholera bacterium is
different

Answer 25

1. Cholera bacterium is prokaryote;
2. Does not have a nucleus/nuclear envelope/ has DNA free in cytoplasm/has loop of DNA;

3 and 4
Any two from: [No membrane-bound organelles/no mitochondria / no Golgi/no endoplasmic reticulum];

5 Small ribosomes only;

6 and 7
Any two from [Capsule/flagellum/plasmid / cell wall]

Question 26

Name two structures found
in all bacteria that are not
found in plant cells

Answer 26

1. Circular DNA (molecule in cytoplasm);
2. Murein cell wall OR Peptidoglycan cell wall OR Glycoprotein cell wall;
3. Small(er)/70S ribosomes (in cytoplasm);

Question 27

Methods of Studying Cells:
-Light microscopes
-TEM
-SEM

Question 28

Light Microscopes

1) Specimens are illuminated with light, which is focused using glass lenses and viewed using the eye or photographic film

2) At higher magnifications the microscope loses resolution so magnification is limited

3) Specimen can be living or dead

4) Specimen can be stained using dyes

Question 29

Resolution is the ability to distinguish 2 objects that are close together

Question 30

The shorter the wavelength of the light / electrons, the better the resolution

Question 31

The light microscope has a resolution of about 200 nm which is good enough to see cells, but not the details of cell organelles. This means if two objects are greater than the wavelength of light (200nm) apart they are seen as two separate objects using the light microscope. However, if they are less than half the wavelength of light (<200nm) apart they are seen as one object using the light microscope

Question 32

Name 2 structures found within a eukaryotic cell that cannot be identified using an optical microscope

Answer 32

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

Question 33

There are 2 types of electron microscope:

1. Transmission Electron Microscope (TEM)
   Produces 2D images
2. Scanning Electron Microscope (SEM)
   Produces 3D images

Question 34

TEM & SEM use a beam of electron to “illuminate” the specimen
Electrons behave like waves and can easily be produced (using a hot wire), focused using electromagnets and detected using a phosphor screen or photographic film.

Electrons have very small wavelength, so can be used to observe objects as small as ribosomes (20nm), this means the TEM & SEM produce images with HIGHER RESOLUTION

Question 35

TEM (Transmission electron microscope)

1) The electrons pass THROUGH the specimen.

2) Allows you to view organelles/internal structures.

3) Electrons are fired through the specimen, less dense (e.g. cytoplasm) areas absorb less electrons and appear lighter, denser areas/structures (e.g. nucleolus) absorb more electrons and so appear darker

Question 36

SEM (Scanning electron microscope)

1) Specimens are not sliced, and the electrons bounce off the SURFACE of the specimen.

2) Images are always in black and white.

3)These always produce a 3D image and it’s the only way you can tell if the image is made an SEM

Question 37

Electron Microscopes

1) Specimen must be dead, they are fixed in resin and sliced very thinly

2) Specimen must be placed in vacuum

3) Complex staining process as heavy metals are used so can create artefacts (structures not actually there)

Question 38

Describe the principles and the limitations of using a transmission electron microscope to investigate cell structure [5]

Answer 38

Principles:
1. Electrons pass through / enter (very 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

Question 39

Give one advantage of
using a TEM rather than a
SEM

Answer 39

Higher resolution / higher (maximum) magnification / higher detail (of image);
OR
Allows internal details / structures within (cells) to be seen / cross section to be taken;

Question 40

Give one advantage of
using a SEM rather than a
TEM

Answer 40

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

Question 41

Scientists use optical
microscopes and
transmission electron
microscopes to investigate
cell structure. Explain the
advantages and limitations
of using a TEM to
investigate cell structure.

Answer 41

Advantages:
1. Small objects can be seen;
2. TEM has higher resolution;
3. Wavelength of electrons shorter;

Limitations:
4. Cannot look at living cells;
5. Must be in a vacuum;
6. Must cut section / thin specimen;
7. Preparation may create artefacts (false image);

Question 42

Describe how you could
make a temporary mount
of a piece of plant tissue to
observe the position of
starch grains in the cells
when using an optical
(light) microscope

Answer 42

1. Add a drop of water to the microscope slide;
2. Get a thin section of plant tissue and float on the drop of water;
3. Stain with KI solution;
4. Lower the cover slip using a mounted needle to avoid air bubbles;

Question 43

The resolution of an image
obtained using an electron
microscope is higher than
the resolution of an image
obtained using an optical
microscope. Explain why

Answer 43

Shorter wavelength between electrons OR Longer wavelength in light (rays)

Question 44

Calculating the size of cells

When using an optical (light) microscope requires TWO pieces of equipment called:

1) Eyepiece (lens) graticule
2) Stage micro-mete

Question 45

A student viewed impressions of stomata using an optical microscope. She then determined the mean diameter of the stomata. Describe how the student could have used an eyepiece graticule to determine the mean diameter of stomata (3)

Answer 45

1. Measure each stomata using an eye piece graticule
2. Calibrate the eyepiece graticule against a stage micro-meter
3. Take at least 5 measurements and calculate a mean (more measurements = more accurate & more representative

Question 46

ALWAYS measure in mm

1 m = 1000mm
1 mm = 1000µm
1 µm = 1000nm

Question 47

Actual Size = (Image Size) ÷ (Magnification)

I = image (picture on the paper) in mm

A = actual size - µm or nm

M = Magnification - no units

Question 48

Magnification:
How many times bigger the image of a specimen observed is in compared to the actual (real-life) size of the specimen

Question 49

Cell Fractionation

1) Tissue is homogenised in blender to break open the cells releasing the organelles into solution

The solution must be:

· Ice cold: reduce the action of enzymes that would digest organelles

· Isotonic (same water potential): prevents osmosis of water in or out of organelles, so organelles don’t burst (lyse) or shrivel (crenation in animals)

· Buffered: to stop pH changes which could denature proteins.

Question 50

Cell Fractionation

2) Filter the mixture to remove any large pieces of tissue/cells (cellular debris) producing a solution of suspended organelles (supernatant)

Question 51

Differential centrifugation of the supernatant by:

3) Centrifuge at HIGH speed The densest organelles (nucleus) are forced to the bottom of the tube into a pellet which is removed. The supernatant can be spun again to obtain the smaller and lighter organelles.

4) Centrifuge at a HIGHER speed for a longer time. The next densest organelles are forced to the bottom of the tube into a pellet. This pellet is removed and can be re-suspended if required

5) This process can be repeated many times, at higher speeds with each step. This separates the organelles (then molecules) according to their relative densities

Question 52

Describe how you could use cell fractionation to isolate chloroplasts from leaf tissue (3)

Answer 52

1. Break open the cells in a blender/ homogeniser and filter to remove cellular debris;
2. Place in an ice cold, isotonic, buffered solution;
3. Use differential centrifugation;
4. Chloroplasts are in the second pellet;

Question 53

Scientists isolated mitochondria from liver cells. They broke the cells open in an ice-cold, buffered isotonic solution.
Explain why the solution was: a) Isotonic
b) Ice cold
c) buffered

Answer 53

1. Prevents osmosis so no osmotic lysis of organelles/named organelle (e.g. mitochondria);
2. Reduce/prevent enzyme activity so organelles are not digested;
3. Maintain a constant pH so proteins are not denatured;

Question 54

Describe and explain how
cell fractionation and
centrifugation can be used
to isolate mitochondria from
a suspension of animal
cells.

Answer 54

1. Cell homogenisation to break open cells and release organelles;
2. Filter to remove (large) debris/whole cells;
3. Use isotonic solution to prevent osmotic damage to mitochondria / organelles;
4. Keep cold to prevent/reduce damage to organelles by enzyme;
5. Use buffer to maintain pH and prevent protein/enzyme denaturation;
6. Use differential Centrifuge (at high speed) to separate nuclei / cell fragments / heavy organelles;
7. Re-spin (supernatant / after nuclei/pellet removed) at higher speed to get mitochondria in pellet/at bottom;
8. Observe pellet with a microscope to identify mitochondria;