2.1 Cell Structure

Define the terms eukaryotic and
prokaryotic cell

Eukaryotic: DNA is contained in a
nucleus, contains membrane-bound
specialised organelles.
Prokaryotic: DNA is 'free' in cytoplasm,
no organelles e.g. bacteria & archaea.

State the relationship between a system
and specialised cells.

Specialised cells → tissues that perform
specific function → organs made of
several tissue types → organ systems

Describe the structure and function of
the cell-surface membrane.

'Fluid mosaic' phospholipid bilayer with extrinsic &
intrinsic proteins embedded
● Isolates cytoplasm from extracellular environment.
● Selectively permeable to regulate transport of
substances.
● Involved in cell signalling / cell recognition.

Explain the role of cholesterol,
glycoproteins & glycolipids in the cell-
surface membrane.

Cholesterol: steroid molecule connects
phospholipids & reduces fluidity.
Glycoproteins: cell signalling, cell recognition
(antigens) & binding cells together.
Glycolipids: cell signalling & cell recognition.

Describe the structure of the nucleus.

● Surrounded by nuclear envelope, a
semi-permeable double membrane.
● Nuclear pores allow substances to
enter/exit.
● Dense nucleolus made of RNA & proteins
assembles ribosomes.

Describe the function of the nucleus.

● Contains DNA coiled around chromatin into
chromosomes.
● Controls cellular processes: gene
expression determines specialisation & site
of mRNA transcription, mitosis,
semiconservative replication.

Describe the structure of a
mitochondrion.

● Surrounded by double membrane folded
inner membrane forms cristae: site of
electron transport chain
● Fluid matrix: contains mitochondrial DNA,
respiratory enzymes, lipids, proteins

Describe the structure of a chloroplast.

● Vesicular plastid with double membrane.
● Thylakoids: flattened discs stack to form
grana

contain photosystems with chlorophyll.
● Intergranal lamellae: tubes attach thylakoids
in adjacent grana.
● Stroma: fluid-filled matrix.

State the function of mitochondria and
chloroplasts

● Mitochondria: site of aerobic
respiration to produce ATP.
● Chloroplasts: site of photosynthesis
to convert solar energy to chemical
energy.

Describe the structure and function of
the Golgi apparatus.

Planar stack of membrane-bound, flattened sacs
cis face aligns with rER.
Molecules are processed in cisternae
vesicles bud off trans face via exocytosis:
● modifies & packages proteins for export
● synthesises glycoproteins

Describe the structure and function of a
lysosome

Sac surrounded by single membrane
embedded H+ pump maintains acidic conditions
contains digestive hydrolase enzymes
glycoprotein coat protects cell interior:
● digests contents of phagosome
● exocytosis of digestive enzymes

Describe the structure and function of a
ribosome.

Formed of protein & rRNA
free in cytoplasm or attached to ER.
● Site of protein synthesis via translation:
large subunit: joins amino acids
small subunit: contains mRNA binding site

Describe the structure and function of
the endoplasmic reticulum (ER).

Cisternae: network of tubules & flattened sacs
extends from cell membrane through cytoplasm &
connects to nuclear envelope:
● Rough ER: many ribosomes attached for protein
synthesis & transport.
● Smooth ER: lipid synthesis.

Describe the structure of the cell wall

● Bacteria:
Made of the polysaccharide murein.
● Plants:
Made of cellulose microfibrils
plasmodesmata allow molecules to pass between
cells, middle lamella acts as boundary between
adjacent cell walls.

State the functions of the cell wall.

● Mechanical strength and support.
● Physical barrier against pathogens.
● Part of apoplast pathway (plants) to
enable easy diffusion of water.

Describe the structure and function of
the cell vacuole in plants.

Surrounded by single membrane: tonoplast
contains cell sap: mineral ions, water, enzymes,
soluble pigments.
● Controls turgor pressure.
● Absorbs and hydrolyses potentially harmful
substances to detoxify cytoplasm.

Explain some common cell adaptations.

● Folded membrane or microvilli increase
surface area e.g. for diffusion.
● Many mitochondria = large amounts of ATP for
active transport.
● Walls one cell thick to reduce distance of
diffusion pathway.

State the role of plasmids in prokaryotes.

● Small ring of DNA that carries
non-essential genes.
● Can be exchanged between bacterial
cells via conjugation.

State the role of flagella in prokaryotes.

Rotating tail propels (usually unicellular)
organism.

State the role of the capsule in
prokaryotes.

polysaccharide layer:
● Prevents desiccation.
● Acts as food reserve.
● Provides mechanical protection against
phagocytosis & external chemicals.
● Sticks cells together.

Compare eukaryotic and prokaryotic
cells.

● Cell membrane.
● Cytoplasm.
● Ribosomes (don't count as an
organelle since not membrane-bound).

Contrast eukaryotic and prokaryotic
cells.

Why are viruses referred to as 'particles'
instead of cells?

Acellular & non-living: no cytoplasm,
cannot self-reproduce, no metabolism.

Describe the structure of a viral particle.

● Linear genetic material (DNA or RNA) &
viral enzymes e.g. reverse transcriptase.
● Surrounded by capsid (protein coat
made of capsomeres).
● No cytoplasm.

Describe the structure of an enveloped
virus.

● Simple virus surrounded by matrix
protein.
● Matrix protein surrounded by envelope
derived from cell membrane of host cell.
● Attachment proteins on surface.

State the role of the capsid on viral
particles.

● Protect nucleic acid from degradation
by restriction endonucleases.
● Surface sites enable viral particle to
bind to & enter host cells or inject their
genetic material.

State the role of attachment proteins on
viral particles.

Enable viral particle to bind to
complementary sites on host cell : entry
via endosymbiosis.

Describe how optical microscopes work.

1. Lenses focus rays of light and magnify the
   view of a thin slice of specimen.
2. Different structures absorb different amounts
   and wavelengths of light.
3. Reflected light is transmitted to the observer
   via the objective lens and eyepiece.

Outline how a student could prepare a
temporary mount of tissue for an optical
microscope.

1. Obtain thin section of tissue e.g. using ultratome or
   by maceration.
2. Place plant tissue in a drop of water.
3. Stain tissue on a slide to make structures visible.
4. Add coverslip using mounted needle at 45° to
   avoid trapping air bubbles.

Suggest the advantages and limitations
of using an optical microscope.

• colour image
• can show living structures
• affordable apparatus
• 2D image
• lower resolution than electron microscopes =
  cannot see ultrastructure

Describe how a transmission electron
microscope (TEM) works.

1. Pass a high energy beam of electrons through
   thin slice of specimen.
2. More dense structures appear darker since they
   absorb more electrons.
3. Focus image onto fluorescent screen or
   photographic plate using magnetic lenses.

Suggest the advantages and limitations
of using a TEM.

• electrons have shorter wavelength than light = high
  resolution, so ultrastructure visible
• high magnification (x 500000)
• 2D image
• requires a vacuum = cannot show living structures
• extensive preparation may introduce artefacts
• no colour imagehttps://bit.ly/pmt-cc
  https://bit.ly/pmt-cch

Describe how a scanning electron
microscope (SEM) works.

1. Focus a beam of electrons onto a specimen's
   surface using electromagnetic lenses.
2. Reflected electrons hit a collecting device and
   are amplified to produce an image on a
   photographic plate.

Suggest the advantages and limitations
of using an SEM.

• 3D image
• electrons have shorter wavelength than light = high
  resolution
• requires a vacuum = cannot show living structures
• no colour image
• only shows outer surface

Define magnification and resolution.

Magnification: factor by which the
image is larger than the actual specimen.
Resolution: smallest separation
distance at which 2 separate structures
can be distinguished from one another.

Explain how to use an eyepiece graticule
and stage micrometer to measure the
size of a structure.

1. Place micrometer on stage to calibrate eyepiece graticule.
2. Line up scales on graticule and micrometer. Count how
   many graticule divisions are in 100μm on the micrometer.
3. Length of 1 eyepiece division = 100μm / number of
   divisions
4. Use calibrated values to calculate actual length of
   structures.

State an equation to calculate the actual
size of a structure from microscopy.

actual size

image size
/
magnification

Outline what happens during cell
fractionation and ultracentrifugation.

1. Mince and homogenize tissue to break open cells &
   release organelles.
2. Filter homogenate to remove debris.
3. Perform differential centrifugation:
   a) Spin homogenate in centrifuge.
   b) The most dense organelles in the mixture form a pellet.
   c) Filter off the supernatant and spin again at a higher speed

State the order of sedimentation of
organelles during differential
centrifugation.

most dense → least dense
nucleus → mitochondria → lysosomes →
RER → plasma membrane → SER →
ribosomes

Explain why fractionated cells are kept in
a cold, buffered, isotonic solution.

cold: slow action of hydrolase enzymes.
buffered: maintain constant pH.
isotonic: prevent osmotic lysis/ shrinking
of organelles.