2.1.1 Cell structure

Question 1

State the key aspects of cell theory

Answer 1

1. Both plant and animal tissue is composed of cells
2. Cells are the basic unit of all life
3. Cells only develop from existing cells

Question 2

Describe the role of microscopy in the development of biological understanding

Answer 2

1. Microscopes produce magnified images of biological material
2. It gives us information about the structure of living organisms
3. Which can be related to functions at various levels (ultrastructure (organelles), cell structure, tissues, organs, organisms)

Question 3

List the types of microscopes

Answer 3

1. Light microscope
2. Transmission electron microscope
3. Scanning electron microscope
4. Laser scanning confocal microscope

Question 4

Describe light microscopy and the images it produces

Answer 4

1. Light passing through samples is passed through objective and eyepiece lenses to produce a magnified image
2. Natural colours of samples can be observed
3. Up to 2000x magnification possible
4. Resolution of 200 nm (organelles apart from nucleus not visible)
5. Specimens can be living or dead

Question 5

Describe electron microscopy and the images it produces

Answer 5

1. Electrons are used to illuminate the specimens
2. In transmission electron microscopy (TEM), electrons passing through the specimen are detected and used to produce the magnified image.
3. TEM has a resolution of 0.5 nm (more detailed images)
4. TEM provides detailed but 2-D images
5. Internal cellular detail such as organelles are visible in TEM
6. In scanning electron microscopy (SEM), electrons that are reflected off the surface of the sample are detected.
7. SEM has a resolution of 3-10 nm
8. SEM provides less detail, but gives 3-D information as well, outer details of organisms should be visible, but not necessarily organelles.
9. For both TEM and SEM sample preparation is complex and samples always dead
10. Images produced are black and white (but could be false coloured)

Question 6

Describe laser scanning confocal microscopy and the images it produces

Answer 6

1. A modified light microscopy technique that used high intensity laser to illuminate the sample, and detects fluorescence emitted from specifically labelled cellular components
2. Resolution is higher than light microscopy because only fluorescence from a single focal plane is detected
3. Images are false colour and depend on the fluorescence wavelength of the labels
4. Samples can be fixed (dead) or living

Question 7

Describe the preparation of a slide for light microscopy

Answer 7

1. Samples can be: dry mounted (whole samples or thin sections), wet mounted (aquatic organisms), squash slides (tissue squashed to thin it out) or smear slides (liquid biological samples, eg blood)
2. In all cases samples but be spread thinly to allow light to pass through
3. Must be covered with a glass coverslip

Question 8

Describe the purpose of differential stains

Answer 8

1. Biological materials/components usually don’t absorb a lot of light, this means contrast is low, and structures difficult to differentiate
2. Chemical stains bind to cellular components and increase their visibility/image contrast
3. Different stains, with different chemical properties, bind to different cellular components
4. Which allows their structure to observed..
5. ..And the structures to be identified (and related to function)

Question 9

State the types of differential stain and what they are used to identify

Answer 9

1. Eosin red is basophilic and will bind to positively charged components (usually in the cytoplasm)
2. Methylene blue is acidophilic and will bind to negatively charged components such as DNA (identifies nucleus)
3. Crystal violet is a dye taken up and retained by bacterial cells with thick cell walls, and washed out of bacterial cells with thinner cells walls. This stain helps to identify different types of bacteria.

Question 10

Explain the need for different subcellular structures within cells

Answer 10

1. Cells carry out a range of different metabolic processes in order to function
2. The enzymes, substrates and conditions for these processes can be very different
3. Membranes allow compartmentalisation, separation and concentrations required for these processes to proceed optimally
4. Membranes and specialised proteins within membranes ensure selective transport of substances into and out of each compartment
5. Additionally, certain functions of a cell may require very specific and specialised structural arrangements and components, for example for support, transport or motility.
6. The structure of each organelle is related to its function

Question 11

List the components of the ultrastructure of eukaryotic cells (key organelles)

Answer 11

1. Nucleus
2. Nucleolus
3. Nuclear envelope
4. Rough endoplasmic reticulum
5. Smooth endoplasmic reticulum
6. Golgi apparatus
7. Ribosomes
8. Mitochondria
9. Lysosomes
10. Chloroplasts
11. Plasma membrane
12. Centrioles
13. Cell wall
14. Flagella
15. Cilia

Question 12

Describe how the structure is related to the function of: Nucleus

Answer 12

S: Contains the genomic information of the organism

S: in the form of DNA (chromosomes)

S: Chromosomes consist of DNA associated with histone proteins

F: genes code for proteins, which carry out the metabolic and other functions of the cell

F: nucleus thus control the activities of the cell

Question 13

Describe how the structure is related to the function of: Nucleolus

Answer 13

S: composed of protein and RNA

F: this is where genes for ribosomal protein and ribosomal RNA are transcribed

Question 14

Describe how the structure is related to the function of: Nuclear envelope

Answer 14

S: a double membrane structure (two membrane bilayers)

F: separating the chromosomal DNA from the cytoplasm

F: where it may be damaged / digested by enzymes

S: it contains nuclear pores

F: which selectively allow molecules to move in and out of the nucleus

Question 15

Describe how the structure is related to the function of: Mitochondria

Answer 15

F: carries out link reaction, Kreb’s cycle and oxidative phosphorylation for aerobic ATP generation during respiration.

S: Enveloped (double-membraned) organelle in all eukaryotic cells

S: Outer mitochondrial membrane and inner mitochondrial membrane

S: inner mitochondrial membrane is folded to form cristae to…

F: …increase surface area and components for oxidative phosphorylation

S: Stalked particles on cristae are part of ATP synthase

S: inner mitochondrial membrane contains the matrix

F: which contains enzymes required for link reaction and Kreb’s cycle

S: The matrix also contains circular DNA with some mitochondria-specific genes

Question 16

Describe how the structure is related to the function of: vesicles

Answer 16

S: single membrane surrounding fluidic contents

S: contents depend on which organelle it pinched off from

F: transport of substances from one organelle to another

Question 17

Describe how the structure is related to the function of: lysosomes

Answer 17

S: modified vesicles

S: contain hydrolytic enzymes (proteases, carbohydrases, lipases, nucleases)

F: membrane prevents other cellular components being digested

F: digest old organelles, or pathogens

F: especially important in phagocytes (inflammation, immune response)

F: and programmed cell death, apoptosis

Question 18

Describe how the structure is related to the function of: cytoskeleton

Answer 18

S: certain globular proteins can be polymerised to form fibres in the cell.

S: polymerised actin forms actin fibres (microfilaments)

F: microfilaments are involved in cell movement, cytokinesis

S: polymerised tubulin forms microtubules

F: these are used to transport vesicles across the cell

F: and form spindle fibres during cell division

S: intermediate fibres give mechanical strength to the cell

Question 19

Describe how the structure is related to the function of: centrioles

Answer 19

S: cylindrical structures formed of microtubules

F: a pair of centrioles forms the centrosome involved in the separation of chromosomes/chromatids during cell division

Question 20

Describe how the structure is related to the function of: Flagella and cilia

Answer 20

S: hair-like structures made of proteins

F: cilia are more numerous and have sensory function, or they move fluid away from cells (for example in the lungs)

S: flagella are fewer per cell, and function to propel cells

Question 21

Describe how the structure is related to the function of:

Rough endoplasmic reticulum

Answer 21

S: network of interconnected, flattened membrane sacs called cisternae

S: continuous with the nuclear envelope

S: has ribosomes bound to the surface

F: synthesis of membrane proteins (eg channels) and secreted proteins (eg antibodies)

F: contains enzymes for the synthesis of carbohydrates and modification of proteins

F: transport of synthesised proteins to Golgi apparatus

Question 22

Describe how the structure is related to the function of:

Smooth endoplasmic reticulum

Answer 22

S: network of interconnected, flattened membrane sacs called cisternae

S: continuous with the nuclear envelope

F: contains enzymes for lipid and carbohydrate synthesis

Question 23

Describe how the structure is related to the function of:

Ribosomes

Answer 23

S: free-floating in the cytoplasm or attached to endoplasmic reticulum

S: Not membrane-bound structures: composed of protein and ribosomal RNA

F: Site of protein synthesis

F: bind to mRNA and tRNA

F: join amino acids with peptide bonds in condensation reactions, forming polypeptides

Question 24

Describe how the structure is related to the function of:

Golgi apparatus

Answer 24

S: flattened membrane sacs that are not interconnected, called cisternae

S: cis face faces nucleus, trans face faces plasma membrane

F: receives proteins from RER in transport vesicles (utilising cytoskeleton) which fuse to cis face

F: enzymes modify the proteins as they move through the cisternae

F: vesicles containing modified protein are packaged into vesicles leaving the trans face

F: thus, proteins may be delivered to lysosomes, plasma membrane or secreted

Question 25

Describe how organelles in the eukaryotic cell are involved in

protein synthesis

Answer 25

1. A gene in one of the chromosomes may be transcribed (by RNA polymerase)
2. The mRNA formed moves through a nuclear pore to the cytoplasm
3. It will bind a ribosome on the RER
4. The ribosome will carry out the translation of the mRNA and a polypeptide will be formed inside the cisternae of the RER
5. As the polypeptide moves through the cisternae of the RER, it adopts its secondary and tertiary structure, and pinches off the RER in a transport vesicle
6. The vesicle is transported along cytoskeletal filaments
7. The vesicle fuses with the cis face of the Golgi apparatus
8. And within the cisternae of the Golgi apparatus, enzymes modify the protein, for example adding carbohydrate chains
9. The protein pinches off the trans face of the Golgi apparatus in a vesicle
10. Moving to fuse with lysosomes or the plasma membrane using the cytoskeleton

Question 26

Describe how the structure is related to the function of:

cellulose cell wall

Answer 26

S: the cell wall surrounds the cell surface membrane

S: made of bundles of cellulose (insoluble, polysaccharide) composed of ẞ-glucose

F: the cellulose is permeable to water so water and solutes can freely move into and out of cells

F: the cell wall is rigid, so provides support to cells and the whole plant

F: the cell wall exerts pressure back on the cytoplasm in turgid cells, preventing them from bursting, this also supports the plant

Question 27

Describe how the structure is related to the function of:

vacuoles

Answer 27

S: membrane sacs in the cytoplasm of plant cells

S: the membrane of the vacuole is called the tonoplast

F: the fluid content of vacuoles maintain pressure on the cell wall (turgor) which ensures proper support of plant tissues

Question 28

Describe how the structure is related to the function of:

chloroplasts

Answer 28

S: enveloped (double-membraned) organelle (outer membrane, inner membrane)

S: The fluid contained in the chloroplast is called the stroma

S: the stroma contains circular DNA containing chloroplast genes and ribosomes

S: within the stroma there are interconnected, flattened sacs called thylakoids, forming stacks of grana, connected by lamellae, this structure has a high surface area

S: starch grains are found in the stroma

F: the thylakoid membranes contain the photosynthetic pigments and photosystems and ATP synthase for the light-dependent stage (the high surface area ensures there is enough of these components, and maximise light absorbance)

F: the stroma contains the coenzymes, enzymes and substrates for the light-independent stage (Calvin cycle).

F: starch grains store fixed carbon in the form of insoluble polysaccharide

Question 29

Describe the similarities between prokaryotes and eukaryotes

Answer 29

1. Both have genomic information (their genes) contained on molecules of DNA
2. Both have ribosomes in the cytoplasm
3. Both have cytoplasms enclosed by a cell-surface membrane
4. Both prokaryotic and eukaryotic cells (not all) can have flagella

Question 30

Describe the differences between prokaryotes and eukaryotes

Answer 30

1. Prokaryotes have a single molecule of circular DNA, eukaryotes can have multiple molecules of (linear) DNA
2. Prokaryotic DNA is contained in the cytoplasm, eukaryotic DNA is contained in the nucleus
3. Ribosomes in prokaryotes are smaller (70S) than eukaryotic ribosomes (80S)
4. Eukaryotic organisms such as plants and fungi, and prokaryotes all have cell walls, but these are different in composition. Plant cell walls are composed of cellulose, whereas fungi cell walls are made of the polysaccharide chitin. Bacterial cell walls are made of peptidoglycan (a polymer of amino acids and sugars)
5. Prokaryotic flagella have a different structure to eukaryotic flagella
6. Prokaryotes do not have membrane-bound organelles, eukaryotes do
7. Prokaryotic DNA is not associated with histones, eukaryotic DNA is.
8. Prokaryotic cells (up to 1µm) are much smaller than eukaryotic cells (10 µm)
9. Prokaryotic cells have extra DNA called plasmids, only present in eukaryotic mitochondria and chloroplasts
10. Prokaryotes can only reproduce asexually, eukaryotic reproduction can be asexual and/or sexual
11. Prokaryotes are always unicellular, eukaryotes can be multicellular