CELL ORGANELLES
Learning objective: Alberts et al. Essential Cell biology. Chapter 1+2
- Compare different types of cells.
- Explain how common properties of cells suggest a single evolutionary origin
- Describe basic properties of prokaryotic cells using E. Coli as an example
- Give examples of single cell and multicellular eukaryotes
- Brief description of major eukaryotic organelles and its functions including:
Nucleus, rough and smooth ER, GA, lysosomes and peroxisomes, mitochondria
Cell theory:
1. All living organisms are made up of one or more cells. Viruses and mitochondria are
2. The cell is the basic unit of all living organisms. not cells bc they can’t
3. Cells arise from existing cells by division. reproduce independently.
They require host cells for
Common features of a cell: replication.
- Surrounded by plasma membrane
- Majority of all cells contain genetic material in the form of DNA. Every cell needs
DNA but some cells such as erythrocytes kick out their nucleus and cannot
differentiate any further.
- All cells perform metabolism.
Functions of plasma membrane:
- Acts as a barrier maintaining different composition of external and internal
compartments.
- Mediate communication by limiting and allowing communication.
- Import and export of nutrients, signalling molecules etc.
Types of cells:
- Prokaryote: no nucleus, single cell (bacteria)
- Eukaryotes: Nucleus present, single or multicellular
Prokaryotes:
1. Bacteria: formerly known as eubacteria, this family includes number of
pathogens such as Escherichia coli (E. coli) bacteria in the gut, E.coli
Streptococcus. Also, cyanobacteria (recently discovered, formerly known as
blue-green algae). Size range from 1um to a few um. They come in varying
shapes and can reproduce in 20 mins (they start replicating for next
replication while not fully developed.)
E. coli structure:
- plasma membrane - cytosol contains ribosomes
- peptidoglycan (protein and oligosaccharide complex) cell wall, also allows
bacteria to stick to surfaces.
- DNA, singular mainly packaged w proteins
2. Archaea: formerly known as archaebacteria due to recent phylogenetic
analysis. Sufficiently different to bacteria to be a separate class. They
, usually live in unusual environments such as Halophiles in high salt concentrations,
Thermoacidophiles in hot sulphur springs.
Eukaryotes:
- Plants - animals - fungi - protozoans -algae -yeast
- Their DNA is enclosed in a membrane.
- They have membrane bound organelles and a cytoskeleton.
- Size ranges from 5-50um but size and shape are very variable.
Nucleus:
- Diameter: few um, but varies. Almost same size as a bacterium.
- Surrounded by a double membrane, nuclear envelope, enclosing the nucleoplasm.
- Contains DNA, packaged w proteins as chromosomes (histones and DNA)
- Site of RNA synthesis (transcription).
Rough Endoplasmic reticulum:
- Covered in ribosomes.
- Site of synthesis of certain proteins, includes most proteins to be
secreted.
- As protein is synthesised it passes into the lumen through the
membrane to be transported further.
Smooth ER:
- No ribosomes. The abundance and functions depend on cell type:
- Liver → detoxifications, release of glucose from glycogen in liver.
Golgi complex:
- Stack of flattened membrane vesicles.
- Proteins come here after the ER to be modified for secretion or
membrane proteins.
Lysosomes:
- Membrane bound in animals’ cells only.
- Contain enzymes which degrade unwanted proteins, membranes,
organelles.
- PH slightly acidic = 5 compared to PH= 7 of cytoplasm. The
difference in PH allows inhibition of degrading proteins working in
case of a bursting lysosome.
Peroxisomes:
- Degrade fatty acids and toxic compounds by oxidation.
, - Oxidation produces hydrogen peroxide which is corrosive.
- Fatty acid oxidation produces heat and precursors for biosynthetic
pathways.
- 2H2O2 → O2 + 2H2O, using catalase enzyme (protective mechanism).
Mitochondria:
- Oval shape, 0.5-1um in diameter and 1-2 um in length.
- Main site of ATP production through oxidative phosphorylation (OP).
- Converting glucose into energy.
- They take up to 25% of cytoplasm.
- They have a double membrane. The outer membrane has a 50:50
of lipid to protein and the inner has a ratio of 20:80 lipid to
proteins. This is because the inner membrane is packed with
proteins which are clustered in the inner membrane and these
proteins carry out OP. The double membrane ensures there is a
flow of electrons needed for OP.
- The inner membrane is folded (cristae) that protrude into the central matrix and
provide a large surface area for the Krebs cycle in OP.
- Mitochondria may derive from symbiotic bacteria as they have their own DNA and
encode their own genes, RNA and ribosomes. So some proteins found in
mitochondria is produced insitu. They reproduce like dividing like normal cells.
- Mitochondria can be seen as the grand children of very. Ancient prokaryotic cells
which invaded eukarotic cells
Cytosol:
- This is the area outside the membrane bound organelles.
- They contain a network of protein fibres, which maintains shape of the cell, is
involved in moving organelles within the cell and the actual cell movement.
- Contain inclusion bodies e.g. Glycogen granules as energy source
- Contains metabolic enzyme
Essential
cell biology
chapter 4.
,Sub- cellular fractionation
- This allows us to separate organelles in large numbers.
- You can get a pure nucleus from cells
Method:
1. Cells need to be disrupted and held in solution with similar PH, salt content and
tonicity to interior of cell (sucrose often makes isotonic solutions)
2. Keep it cold
3. Centrifugation at successively increased speeds to separate pellet at different
velocities. Differential- velocity centrifugation.
4. Alternatively, you can carry out equilibrium-density centrifugation. Initially a solution
with different densities is used e.g. sucrose and you put cell fractionation solution on
top and start spinning and if you’re careful not to disrupt the gradient layers
eventually the different types of organelles will end up in those layers on sucrose
layers equivalent to their density.
5. We can test molecular markers to ensure reaction has occurred properly. E.g.
Catalase → peroxisomes cytochrome C → mitochondria
Properties influencing the behaviour of cell components in centrifugal field:
- For each component its behaviour is given by a value s= sedimentation constant,
expressed in Svedberg units.
- Behaviour of cell components depends on its: volume, density and shape.
CYTOSKELETON
Cytoskeleton is very important component of the cell because it is responsible to almost all
cell processes. If it’s not present or damaged then it can lead to diseases such as infection,
cancer, chronic inflammation. The cytoskeleton is the muscle and bones of the cells and its
dynamic allowing cells to rapidly change shape.
Components of cytoskeleton:
1. Actin filaments/microfilaments
2. Microtubules Mitochondria radiator centre
, 3. Intermediate filaments
Functions of cytoskeleton:
1. Gives structure to the cell
2. Provides mechanical force allowing cells to change shape and move
3. Moves organelles inside the cells.
4. Provides mechanical force for cell division.
ACTIN FILAMENTS
- Monomer: globular actin monomer
- Actin filaments/microfilaments are made by twisting of two strands.
- Flexible and strong; diameter about 7 nm.
- Dispersed throughout the cell, they are most highly concentrated in the
cortex, the layer of cytoplasm just beneath the plasma membrane.
- Different types:
A = microvilli
B = contractile bundle in cytoplasm
C = protruding filopodia
D = contractile ring in cell division
- Actin filaments have
structural polarity with a
plus end and minus end.
- The positive end is the barbed end and the negative end is the pointy end.
- Actin filaments are polarised.
- Actin binds ATP or ADP. Growth is faster on “barbed end”. ATP-actin binds more
favourably at the barbed end.
- Net loss at the pointed end leads to the treadmilling effect.
- Naked actin filaments are unstable and require actin-binding proteins for stability.
Nucleation and disassembly:
- free actin monomers carry a tightly bound ATP.
- Actin filaments assemble by nucleation. Three actin
monomers need to associate which is difficult because the
formation of a G-actin trimer is energetically unfavourable.
- This is bc rate of dissociation is v high. When G-actin trimer is
added binding becomes easier.
- Hydrolysis of ATP to ADP in an actin filament reduces the
strength of binding between the monomers, decreasing its
stability.
- ATP hydrolysis doesn’t drive polymerisation but acts as a
timer for filament stability and disassembly.
