Chapter 3 - Cell Structure

Question 1

What is a eukaryotic cell?

Answer 1

Defined as cells that contain membrane bound organelles.

Question 2

Describe the structure and function of the nucleus.

Answer 2

1. Has a nuclear envelop - double membrane that surrounds it.
2. Has nuclear pores - allow molecules (ribosomes/mRNA) out
3. Nucleolus - where ribosomal RNA is made

• contains DNA which codes for protein synthesis
• site of DNA REPLICATION in eukaryotes.
• histone proteins: wrap linear DNA around these to make chromosomes.

Question 3

RIBOSOMES

What are they made of, what are they used for and is there size differences?

Answer 3

• made of ribosomal RNA and protein - assembled in nucleolus
• HAVE NO MEMBRANE
• used for protein synthesis
• ribosomes in eukaryotes are LARGER than in prokaryotes.

Question 4

How are ribosomes made? [4]

Answer 4

1. Nucleolus makes ribosomal RNA
2. Ribosomal proteins are made in cytoplasm, then imported into nucleus
3. Nucleolus then assembles complete ribosomes using protein and ribosomal RNA
4. The completed ribosomes then move from the nucleus into the cytoplasm via nuclear pores.
   - some attach to endoplasmic reticulum, others remain free in cytoplasm

Question 5

Describe the structure and functions of the rough endoplasmic reticulum

Answer 5

• consists of membrane - bound flattened sacs called cisternae

rER:
1. Has ribosomes on outer surface
2. transports proteins made by ribosomes
3. Present in large amounts of cells that make enzymes that may be secreted out of the cell.

Question 6

1. Name the structure that the smooth endoplasmic reticulum doesn’t have
2. What does the sER do?

Answer 6

• Has membranes that LACK ribosomes
• assist in the synthesis and transport of lipids

Question 7

Describe the process of protein secretion. [7]

Answer 7

1. DNA in nucleus codes for a protein
2. Ribosomes on rER produce protein
3. Mitochondria produce ATP for this protein synthesis
4. Vesicles bud off the rER and transport the protein to the Golgi apparatus
5. The Golgi apparatus MODIFIES the protein (add carbohydrate to make glycoprotein)
6. Vesicles then bud off the Golgi apparatus, carrying the protein to the cell surface membrane
7. The vesicles FUSE with the cell surface membrane, releasing their protein contents to the outside of the cell

Question 8

1. Where will the protein be made if it is going to end up in the cell cytoplasm

AND

2. Where will it be made if going to be transported to the Golgi apparatus?

Answer 8

1. Made by the ribosomes free in cytoplasm
2. Made by ribosomes bound to rER

Question 9

Describe the structure and function of the Golgi apparatus
Also mention about Golgi vesicles.

Answer 9

Golgi apparatus: stack of flattened sacs = cisternae

1. Enzymes + proteins packaged into Golgi vesicles for secretion
2. Some combined with carbohydrate to form glycoproteins
3. Work in transporting and storing lipids
4. Form lysosomes

Question 10

Describe what lysosomes are, what they contain and what these do.

Answer 10

• they are a special type of Golgi vesicle
• contain LYSOZYMES that are enzymes which carry out hydrolysis.
• Lysozymes can be used to digest worn out organelles, digest food particles taken up by phagocytosis
• white blood cells (phagocytes) use lysozymes to digest pathogens.

Question 11

Describe the function of mitochondria and it structure.

Answer 11

• produces ATP via aerobic respiration = energy for many cell processes
• has folds called cristae which provide a large SA to hold proteins involved in aerobic respiration
• the matrix (fluid filling the mitochondria) contains enzymes for aerobic respiration.

Question 12

EXAM TECHNIQUE

Why do some mitochondria look different under a microscope?

Answer 12

They have been cut in different planes

Question 13

What is the word equation for photosynthesis?

Answer 13

Carbon dioxide + water —> oxygen + glucose

Question 14

PLANTS ONLY

Describe the structure of chloroplasts.

Answer 14

• surrounded by double membrane = envelope
• contain thylakoids - contain chlorophyll which absorbs light energy
• thylakoids stack up into grana
• some of the glucose gets stored up as starch grains
• has stroma = fluid that fills chloroplast (contains enzymes for photosynthesis)

Question 15

PLANTS ONLY

Describe the function of chloroplast

Answer 15

• site of photosynthesis - absorb light energy to make sugar

Question 16

PLANTS ONLY

What is the cell wall made of

Answer 16

Cellulose

Question 17

Describe the functions of the cell wall. [3]

Answer 17

1. Provides mechanical strength to prevent OSMOTIC LYSIS
2. Provides mechanical strength to plant as a whole
3. Permeable to water so allows water to travel through the plant

Question 18

What is different about the cell walls of fungi?

Answer 18

Possess cell walls made of a polymer called chitin - not plants so do not photosynthesise

Question 19

PLANTS ONLY

What is the cell vacuole?

Answer 19

Large, permanent fluid filled sac surrounded by a single membrane called a TONOPLAST

Question 20

PLANTS ONLY

Describe the functions of the cell vacuole [3]

Answer 20

1. inflates with water to make cells turgid
2. Stores sugar and amino acids as food
3. Contain pigments to colour petals of flowers and therefore attract insects for pollination

Question 21

How is a prokaryotic cell different to eukaryotic cells? [5]

Answer 21

• much smaller
• no membrane bound organelles
• smaller ribosomes
• no nucleus - instead have single circular DNA molecule free in cytoplasm that is not associated with histones
• a cell wall that contains MUREIN = a glycoprotein.

E.g. BACTERIA

Question 22

What are some other things that prokaryotes have? [3]

Answer 22

• plasmids: small circular piece of DNA
• capsule around cell: mucilaginous layer of slime
• flagella: used for locomotion

Question 23

Why might have mitochondria and chloroplast evolved from free living bacteria?

Answer 23

• contain circular DNA
• contain smaller ribosomes
  (Both features are shared with bacteria)

Question 24

What are the 2 key things about viruses?

What are the 3 key structures on a virus?

Answer 24

• acellular (not made of cells) and non living

1. Genetic material: can be DNA or RNA
2. Capsid: made of proteins and contains genetic material
3. Attachment proteins: help the virus identify and attach to host cell

Question 25

What does the term resolution refer to?

Answer 25

- a measure of the ability of a microscope to distinguish detail E.g. the minimum distance at which two distinct points of a specimen can still be seen as SEPARATE ENTITIES

Question 26

Which microscope has a better resolution, optical or electron and why?

Answer 26

Electron has better resolution because: - a beam of electrons has a shorter wavelength than light

Question 27

What are the differences between electron and optical microscopes? [7]

Answer 27

1. ELECTRON uses electrons whereas OPTICAL uses light. 2. ELECTRON has greater resolution whereas OPTICAL has lower resolution 3. SO… ELECTRON can observe smaller organelles in more details whereas OPTICAL can only observe larger organelles in less detail 4. In ELECTRON the specimen has to be dead whereas OPTICAL can use living or dead 5. ELECTRON does not show colour whereas OPTICAL can show colour 6. ELECTRON prep is more complex and more time consuming whereas OPTICAL is less complex and time consuming 7. ELECTRON focuses using magnets whereas OPTICAL focuses using glass lenses

Question 28

How would you mount a specimen for the optical microscope? [3]

Answer 28

1. Take a thin slice/ section; 2. Put on slide and add stain as required; 3. Add cover slip using a mounted needle;

Question 29

Why must electron microscope specimens be dead?

Answer 29

- electron microscopes must use a vacuum (completely empty space) to ensure the electron beam isn’t disrupted - vacuum would kill any specimen

Question 30

What stops electron microscopes achieving max resolution?

Answer 30

1. Changes to specimen during prep can limit resolution 2. Higher energy electron beams give even greater resolution, but can damage specimen

Question 31

What are the two types of electron microscopes and what’s the differences?

Answer 31

1. Transmission electron microscope: - very thin specimen - electron beam passed through specimen - 2D image at BEST RESOLUTION 2. Scanning electron microscope: - thicker specimen - electron beam scans across specimen and is scattered off the surface of sample - pattern of scatter used to determine 3D shape of sample - resolution not so good

Question 32

How would you measure the size of an object viewed with an optical microscope?

Answer 32

1. Measure structure using eyepiece graticule; 2. Calibrate eyepiece graticule against stage micrometer; 3. Take a number of measurements to calculate mean;

Question 33

What is the formula for magnification?

Answer 33

Magnification = size of image / size of real object

Question 34

What is an artefact?

Answer 34

When a specimen is damaged during preparation

Question 35

Describe how you would carry out cell fractionation and ultracentrifugation.

Answer 35

1. Chop up fresh liver tissue in ice-cold isotonic buffer solution 2. Put tissue in homogeniser (blender) to break open cells and release organelles 3. Filter mixture to remove debris 4. Spin mixture in a centrifuge so that denser parts get moved to the bottom forming a bullet 5. The supernatant is poured into a tube leaving a sediment (which contains nuclei) 6. Supernatent can then be spun again at a faster speed to produce a sediment containing LESS DENSE organelles etc.

Question 36

What are the 3 stages of centrifugation? List the organelles that are filtered out at each one.

Answer 36

1. Spun at low speed = more dense = nuclei 2. Spun at medium speed = bit less dense = mitochondria 3. Spun at high speed = least dense = ribsomes

Question 37

What are the conditions for homogenisation solution and what do they do?

Answer 37

1. ICE COLD - slows/ stops enzyme activity to prevent digestion of organelles 2. BUFFERED - maintains pH so that enzymes/ other proteins are not denatured 3. ISOTONIC - same water potential prevents osmosis so no lysis/ shrinkage of organelles

Question 38

How is eukaryotic DNA packaged?

Answer 38

- packaged into chromosomes DNA + HISTONE PROTEINS = CHROMATIN

Question 39

Stage 1 of mitosis: What occurs during prophase?

Answer 39

- chromosomes condense; - chromosomes appear as 2 sister chromatids joined at the centromere; - nuclear envelope breaks down;

Question 40

Stage 2 of mitosis: What occurs during metaphase?

Answer 40

- chromosomes line up on the equator/ centre of the cell; - chromosomes attached to spindle fibres by their centromere;

Question 41

Stage 3 of mitosis: What occurs during anaphase?

Answer 41

- the centromere splits; - sister chromatids, now chromosomes, are pulled to opposite poles of the cell by the SHORTENING of the spindle fibres;

Question 42

Stage 4 of mitosis: What occurs during telophase?

Answer 42

- chromosomes uncoil and de-condense; - nuclear envelop reforms around each group of chromosomes;

Question 43

What occurs during cytokinesis?

Answer 43

- cytoplasm divides producing two genetically identical daughter cells.

Question 44

Why is mitosis important?

Answer 44

1. Increases cell number to allow growth; 2. Replace cells to allow repair of tissues; 3. Produces genetically identical cells; 4. Can sometimes allow asexual reproduction;

Question 45

How do prokaryotic cells divide and what does it involve?

Answer 45

Use binary fission: 1. Replication of circular DNA and of plasmids 2. Division of cytoplasm to produce two daughter cells, each with a single copy of the circular DNA and a variable number of plasmids.