Cell structure and function

About half of the cell is filled with cytosol containing the internal structures (Organelle) which are enclosed by lipid membranes, and components of the cytoskeleton.

Each cell has various organelle each with it’s own function ensuring all cellular processes run properly. The image below shows an overview of them.

The cell structure – original image – By Kelvinsong – Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=22952603

1. Nucleolus
2. Nucleus
3. Ribosomes (dots on rough reticulum walls)
4. Vesicle
5. Rough endoplasmic reticulum
6. Golgi apparatus (or “Golgi body”)
7. Cytoskeleton
8. Smooth endoplasmic reticulum
9. Mitochondrion
10. Vacuole
11. Cytosol
12. Lysosome
13. Centriole
14. Cell membrane

Mitochondria

The mitochondria – image modified from – By Kelvinsong; modified by Sowlos – Own work based on: Mitochondrion mini.svg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=27731882

The mitochondria are mainly concerned with energy production (ATP) – ATP synthase is located along the inner membrane where electron transport chain occurs. The inner membrane has folds called Cristae. Outer membrane is smooth.

Mitochondria also help in Ca²⁺ homeostasis and signaling.

The mitochondria are also the major source of body heat production.

Glucose enters the mitochondria and 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 oxygen and when there’s no O₂, NADH gets re-oxidized to NAD⁺ by metabolism of the pyruvate to Lactate.

When there is sufficient O₂, oxidative phosphorylation (Aerobic) in the mitochondria produces ∼15-fold more ATP for each glucose molecule than does glycolysis (Anaerobic).
Citric acid (Krebs) cycle takes place in mitochondrial matrix and generates NADH and the waste product CO₂.
The electron transport chain taking place in the inner mitochondrial membrane uses molecular O₂ to re-oxidize NADH to NAD⁺ and generates a H⁺ ion gradient that drives the ATP synthase producing ATP.
For each Glucose, eukaryotes produce 36 ATP (38 by prokaryotes) (Krebs cycle gives 12 ATP).

Nucleus

A nucleus is a membrane-bound organelle that contains the cell’s chromosomes (the DNA). It helps maintain the integrity of the cell by allowing transcription and replication as needed.
The interior has Nucleoplasm, chromosomes and Nucleolus.

The nucleolus is the site of transcription and processing of the ribosomal gene (Produces ribosomes). It lacks a membrane.

RBC is a eukaryotic cell but devoid of nucleus.

Bi-lipid nuclear membrane is what differentiates a eukaryotic cell from prokaryote.

Endoplasmic reticulum and Golgi apparatus

Smooth endoplasmic reticulum stores the intracellular Ca²⁺ and releases as needed. It plays a major role in lipid production.

Ribosomes translocate to the rough endoplasmic reticulum (giving it its rough appearance), and help in protein assembly.
The endoplasmic reticulum and Golgi apparatus perform post-translational processing (trimming amino acid chains to desired length, protein folding, addition of polysaccharide chains (glycosylation) and identification of improperly folded proteins) of new proteins.

Bad proteins are tagged for recycling with multiple ubiquitin molecules which are identified by Proteasomes (proteolytic protein complexes), which recycle/destroy bad proteins.

Proteins are delivered from Golgi 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

Small vesicles containing acid hydrolase enzymes which catabolize macromolecules. Their optimal pH is 5.0. As cytosolic pH is ∼7.2, they don’t get activated – a protective mechanism.
Lysosomes digest endocytosed, unwanted and defective proteins, thereby recycling raw materials.