Building Eukaryotic Cells: Organelles

What are the key similarities between prokaryotic and eukaryotic cells?

Plasma membrane

DNA as genetic material

General biochemical processes:

• Metabolism

• Protein synthesis (at ribosomes)

What are the key differences between prokaryotic and eukaryotic cells?

Nuclear membrane?

• Eukaryotes: yes – have a nucleus

• Prokaryotes: no – only have nucleoid

Segregation of cellular processes?

• Eukaryotes: yes – in membrane-bound organelles

• Prokaryotes: no – in cytosol

Cytoskeleton?

• Eukaryotes: yes – intricate, provides shape/motility

• Prokaryotes: much simpler than eukaryotes 

Why are cells limited in size, and what features allow eukaryotic cells to overcome these limits and grow larger?

Cell size is limited by surface area/volume ratio

• If a cell wants to grow bigger, it must increase its surface area sufficiently relative to the increase in its volume to ensure adequate exchange of nutrients and waste.

• Eukaryotic solution = cytoskeleton supports specialised shapes to have high surface area/volume ratio

Cell size is limited by diffusion rates of molecules inside the cell

• Eukaryotic solution = cytoskeleton to support sophisticated intracellular transport systems

 

Cell size is limited by the need for sufficient substrate and enzyme concentrations

• Cells must have reaction rates that are compatible with life

• Eukaryotic solution = organelles that can compartmentalise reactions

Describe the structure of the nuclear envelope. What determines whether a molecule can freely pass between the nucleus and cytosol?

There is an inner and outer membrane which are connected at nuclear pores.

Nuclear pore complexes (proteins in nuclear pores) filter based on size what can pass between nucleus and cytosol by free diffusion.

Which cellular components make up the endomembrane system, and how do they exchange materials with one another?

Endomembrane system is made up of endoplasmic reticulum (ER), golgi, lysosomes, and vesicles 

Materials (e.g. proteins) are exchanged through vesicles

What functional roles are common to both the rough and smooth ER, and what structural or functional features distinguish their specialized roles?

Shared function:

• Membrane lipid & cholesterol synthesis

Specific to smooth ER:

• Steroid hormone synthesis

• Sequester Ca²⁺ for controlled release (e.g. in muscles)

• Drug detoxification

Specific to rough ER:

• Contains ribosomes (that’s why termed ‘rough’)

• Synthesis and processing of membrane and secreted proteins as well as soluble proteins that function in endomembrane system  

Describe the structure and primary function of the Golgi.

• Golgi receives proteins from ER by vesicles

• Main function: protein processing (e.g. adding sugars to proteins)

• Structure: stacks of flat membrane-bound cisternae (sacs) with functionally different compartments

What is the primary function of lysosomes, and why is maintaining a low internal pH essential both for their digestive enzymes and for the safety of the cell?

Primary function: intracellular digestion of macromolecules via action of hydrolytic enzymes (hydrolases)

Lysosome contains acid hydrolases and so function optimally at a low pH (4-5). This means that if the lysosome membrane broke and acid hydrolases were in cytosol, which has a pH of 7, these acid hydrolases would not function protecting against all macromolecules being degraded in the cytosol.

Why is the lysosome better described as a recycling center for the cell, while the peroxisome functions more like a garbage disposal? 

Lysosome breaks down macromolecules into their monomers to be used again in the cell. This is more like recycling.

Peroxisome is more like garbage disposal as a main role is to break down many toxic compounds. It can also break down very long chain fatty acids.

According to the Endosymbiotic Theory, how did ancestral eukaryotic cells acquire mitochondria and chloroplasts, and which was acquired first?

• Mitochondria and chloroplasts were acquired via endosymbiosis

• Ancestral eukaryotic cell engulfed a bacterium capable of respiration which became the mitochondrion

• Then some ancestral eukaryotic cells also engulfed a photosynthetic bacterium which became the chloroplast

• The mitochondrion was acquired first

Describe the basic structural membrane setup of mitochondria and chloroplasts.

Mitochondrion

• Two membranes with inner membrane being heavily folded to increase surface area for greater ATP production 

• Two compartments

Chloroplast

• Three membranes with one membrane (thylakoid) having large surface area to enable efficient photosynthesis

• Three compartments