Cell structure and function

[box type=”download”] A basic understanding of the roles of:  Mitochondria  Nucleus  Endoplasmic reticulum and Golgi apparatus  Lysosomes [/box]


Glucose is first converted to pyruvate in the cytosol by glycolysis, producing a small net amount of ATP
and reduced nicotinic adenine dinucleotide (NADH).
Glycolysis does not require O2 and when there’s no O2, NADH gets reoxidized to NAD+ by metabolism of the pyruvate to lactate.

When there is sufficient O2, oxidative phosphorylation in the mitochondria produces ∼15-fold more ATP for each glucose molecule than does glycolysis.
Pyruvate and fatty acids transported into the mitochondrial matrix for citric acid (Krebs’) cycle, which generates NADH and the waste product CO2.
The electron transport chain, a series of enzymes in the inner mitochondrial membrane, then uses molecular O2 to re-oxidize NADH to NAD+.
In doing so, it generates a H+ ion gradient across the inner membrane which drives the ATP synthase.
Mitochondria are also involved in Ca2+ homeostasis and signalling.
The mitochondria are also the major source of body heat production.

Endoplasmic reticulum and Golgi apparatus

In plasma membrane, about half the cell is filled with cytosol, a viscous, protein-rich fluid between the internal structures. These organelles themselves enclosed by lipid membranes, and components of the cytoskeleton such as microtubules and actin filaments which provide structural stability and the ability of the cell to change shape or move.
Rough endoplasmic reticulum and Golgi apparatus are involved in protein assembly.
Smooth endoplasmic reticulum serves as a store for intracellular Ca2+ and is the major site of lipid production.

The nucleus contains the chromosomes and nucleolus(membrane-less).
Nucleolus is responsible for production of ribosomes.
Ribosomes translocate to the rough endoplasmic reticulum (giving it its appearance), where they are responsible for protein assembly.
The endoplasmic reticulum and Golgi apparatus perform post-translational processing of new proteins.
This includes trimming amino acid chains to the right length, protein folding, addition of polysaccharide chains (glycosylation) and identification of improperly folded proteins.
Bad proteins are tagged for recycling with multiple ubiquitin molecules.
Proteasomes (proteolytic protein complexes) recognise the tags and recycle/destroy bad proteins.
Proteins are delivered from the Golgi apparatus to specific intracellular destinations (For example, receptor and structural proteins are sent to the membrane and digestive enzymes to lysosomes, and molecules for extracellular action are packaged into secretory vesicles.


Lysosomes are small vesicles containing acid hydrolase enzymes which catabolize macromolecules.
They work optimally at pH 5.0, and as cytosolic pH is ∼7.2, a protective mechanism.
Lysosomes digest endocytosed, unwanted and defective proteins, thereby recycling raw materials.



Lesson tags: acid hydrolase, Citric acid cycle, glycolysis, glycosylation, golgi apparatus, Kreb's cycle, lactate, Lysosomes, Mitochondria, NAD+, NADH, nucleolus, nucleus, oxidative phosphorylation, proteasomes, pyruvate, ribosomes, rough endoplasmic reticulum, smooth endoplasmic reticulum, ubiquitin
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