Cells and Their Organelles (Biomedicine)

Question 1

Describe the differences between eukaryotic and prokaryotic cells

Answer 1

Procaryotic cells

• capsules for protections
• Inside the cytoplasm, there are free ribosomes

Eukaryotic cells

• Unlike prokaryotes, eukaryotic cells contain extensive internal membranes.
• Cytoskeleton present to give structure

difference

• prokaryotes are smaller (2um vs 10-100um)
• Eukaryotes have more DNA (990mm vs 1.36mm)
• Eukaryotes have more genes (30-38000 vs 4377)
• Eukaryotes have 2 or more chromosomes in membrane-bound nucleus vs Prokaryote have 1 circular chromosome in nucleoid

Question 2

Explain the structure and function of the cell membrane

Answer 2

The plasma membrane

• A phospholipid bilayer
• Hydrophilic phosphate head groups orientate toward the aqueous internal / external environments
• Hydrophobic lipid tails orientate towards each other.
• One of the most common lipids in the PM is phosphatidylcholine
• Cholesterol buffers the fluidity of the plasma membrane, interacts with 2 hydrocarbons, increases the fluidity in cooler temperature. At higher temperatures, 40 degrees, cholesterol can reduce the fluidity - helping the cell to maintain its integrity
• Phosphatidylcholine’s hydrocarbon tail, contains a cis-double bond which allows the plasma membrane to be more fluid

Selective permeability of the plasma membrane

• Gases = permeable
• Small uncharged polar molecules, e.g. ethanol = permeable, water = slightly permeable (takes too long so aquaporin is needed)
• Large uncharged polar molecules, e.g. glucose, fructose = impermeable
• Ions = impermeable
• charged polar molecules; amino acids, ATP, glucose 6-phosphate = impermeable
• Charged molecules will not cross the PM very well. Due to the charge this inhibits diffusion through the very hydrophobic interior of the membrane.
• Specific proteins help to regulate the concentration of molecules inside the cell

Question 3

Recognise the main role of organelles and vesicles in the cell cytoplasm

Answer 3

ER

• Rough ER: studded with ribosomes (rough appearance).
• Smooth ER: site of fatty acid and phospholipid synthesis

Golgi apparatus

• The Golgi sorts proteins and lipids so that they end up at their correct cellular destinations by recognising ‘tags’ encoded within the protein.
• Golgi has 3 defined regions:
• Cis (same)
• Medial (middle)
• Trans (away)
• Site of post-translational modifications
• Post-translational modifications are very important for the function of proteins. Some of the most common modifications are

The Lysosome

• Contain a battery of degradative enzymes.
• Acidic; pH = 5
• activated at a lower pH, reduces the risk of degrading self
• Help to break down complex molecules into their components.
• Primary lysosome buds off from the Golgi apparatus, fuse with phagosome to from a secondary lysosome.

Peroxisome

• Contain enzymes that break down fatty acids and amino acids and as a by-product generate hydrogen peroxide (H2O2).
• This potentially damaging chemical is neutralized by large amounts of catalase within the peroxisome.
• Many peroxisomes contain a crystalline array of catalase as it is in such a high concentration, forming the lattice.
• Getting rid of hydrogen peroxide as it could cause free radicals

The mitochondrian

Completes the aerobic degradation of glucose:

• Most eukaryotic cells contain many mitochondria
• Contain a double membrane:
• Inner: impermeable and has a large number of foldings called cristae. Increases surface area
• Outer: permeable due to presence of porins (proteins that allow the passage of small molecules).

The proton gradient and ATP synthase

Electrochemical gradient, allows protons to go down ATP synthase

Stage 1: Electron transport drives pump that pumps protons across membrane

• Stage 2: Proton gradient is harnessed by ATP synthase to make ATP

Question 4

Understand the role of the cytoskeleton

Answer 4

Simplified:

• Is involved in the processes such as mitosis (spindle apparatus)
• Acts as a highway for intracellular vesicles
• Provides support to the plasma membrane
• Enables cellular locomotion
• Controls the shape of the cell

Microfilaments:

• made up of actin monomer

Intermediate filaments:

• fibrous subunit

Microtubule:

• tubulin dimer
• b-tubulin

a-tubulin

Actin

• A dynamic filament that assembles at the minus and plus ends
• Interaction with myosin generates muscle contraction
• cellular motion utilises actin

Intermediate filaments

• More scaffolding-like
• Composed from a group of fibrous proteins:
• Keratin
• Vimentin
• Lamins
• Neurofilament protein

Microtubules

• Long tube-like structures composed of a and b tublin
• Utilised as a transport medium for structural motility, and cell division
• They are dynamic structures that are regulated by numerous binding partners, proteins that can stabilise or destabilise.
• Drag vesicles along microtubules
• mitosis division, spindle of microtubules shrinks to pull physically.

Question 5

Recognise that some cells such as stem cells are immortal and are involved in the replacement of tissue

Answer 5

Definition

• They can divide indefinitely
• Are not terminally differentiated
• Daughters have a choice: differentiate or remain a stem cell
• Symmetric division or asymmetric division, progenitor division, progenitor differentiation

Question 6

Describe the Cell Theory

Answer 6

Cells: a structural compartment separate from the external environment in which macromolecules can perform unique functions in a relatively constant internal environment. These ‘living compartments’ are cells.

• Cells are the fundamental units of life
• All organisms are composed of cells
• All cells come from pre-existing cells
• Metabolism occurs in cells
• Cells contain genetic material
• All cells have a similar chemical composition