Introduction to Cell Biology

Cell Biology of Al-zintan School of Medicine

• The University of Zintan's Department of Biochemistry and Molecular Biology offers this course.
• The presentation is by Osamah S. Alrawab.
• The study notes focus on cells and their organelles, cell theory, different types of cells, and functions of specific organelles.
• Cell theory postulates that organisms are composed of cells, cells originate from other cells, genetic material is passed down through generations, and metabolic processes occur within cells.
• There are two fundamental cell types: prokaryotic and eukaryotic cells.

Prokaryotic Cells

• Prokaryotic cells—including bacteria and cyanobacteria—have minimal internal organization and are smaller than eukaryotic cells.
• They lack membrane-bound organelles.
• Their genetic material (DNA) is not enclosed within a membrane-bound nucleus.
• DNA is not complexed with histones.
• Respiration occurs near the membrane, and reproduction does not involve mitosis or meiosis.

Eukaryotic Cells

• Eukaryotic cells (fungi, plants, and animals) are larger.
• They exhibit a significant degree of internal structure with many membrane-bound organelles and specific functions.
• They contain a defined nucleus, the site of hereditary material, chromatin, where it is housed.
• Reproduction involves both mitosis and meiosis.
• The mitochondria are the site of respiration (energy production).

Cell Organelles: Nucleus

• The nucleus is the primary control center of eukaryotic cells.
• The nucleus houses more than 95% of the cell's DNA.
• It is enclosed within a double membrane called the nuclear envelope.
• Nuclear pores are embedded within the nuclear envelope.
• These complex structures control the movement of proteins and RNA through the envelope.
• Chromatin is the organized DNA within the nucleus.
• The nucleolus is a dense structure within the nucleus involved in ribosome subunit assembly.
• The nucleoplasm is the fluid content of the nucleus and contains DNA polymerases, RNA polymerases, and other enzymes for mRNA and tRNA synthesis.
• Within the nucleus, DNA replication and RNA transcription occur. Transcription is a vital metabolic process.

Cell Organelles: Mitochondria

• Mitochondria are the powerhouses of the cells.
• Mitochondria vary in number, size, and shape depending on the cell and metabolic activity.
• They have a double membrane structure.
• The outer membrane is permeable to small molecules; the inner membrane is highly folded into cristae that increase surface area for respiration.
• The mitochondrial matrix houses enzymes for the citric acid cycle and fatty acid oxidation. The matrix also contains circular DNA, ribosomes, and enzymes for protein synthesis.

Cell Organelles: Endoplasmic Reticulum (ER)

• The ER is a network of membranes spreading throughout the cytoplasm that connects to the cell membrane and the nuclear envelope.
• It's involved in protein synthesis, transport, modification, storage, and secretion.
• The rough ER (RER) is studded with ribosomes.
• RER is involved in protein synthesis, folding and transport, and lipid synthesis.
• The smooth ER (SER) lacks ribosomes.
• SER is involved in lipid synthesis, detoxification of drugs, and calcium storage.
• ER dysfunction links to neurological disorders.

Cell Organelles: Golgi Complexes (Dictyosomes)

• Golgi complexes are associated with the ER.
• They are stacks of flattened, smooth compartments within eukaryotic cells involved in sorting, modification, and packaging proteins and lipids.
• Three compartments: proximal (receiving), medial and distal (shipping)
• Golgi complexes modify proteins with carbohydrates and lipids, package them into vesicles, and transport them to other destinations.

Cell Organelles: Lysosomes

• Lysosomes are membrane-bound organelles containing digestive enzymes. This is where cellular waste is broken down.
• They are abundant in animal cells, except erythrocytes.
• Lysosomes play a role in autophagy (cell's ability to digest its own components) and cellular autolysis (programmed cell death).
• Lysosomal enzymes have an optimum pH of around 5.
• Lysosomal dysfunction leads to accumulation of undegraded material and genetic diseases like I-cell disease.
• Abnormal buildup of complex lipids or polysaccharides can occur due to inherited disorders related to lysosomes.

Cell Organelles: Peroxisomes

• Peroxisomes, also known as microbodies, are small organelles present in eukaryotic cells.
• They lack energy-coupled electron transport systems.
• They carry out oxidation reactions, creating hydrogen peroxide (H2O2).
• Hydrogen peroxide is destroyed by the catalase enzyme.
• Peroxisomes oxidize long-chain fatty acids.
• Zellweger's syndrome is an inherited disorder where peroxisomes might be absent.

Cell Organelles: Cytoplasm (Cytosol)

• Cytosol is the simplest part of the cell.
• It's the 'organelle-free sap'.
• Metabolic processes occur in the cytosol, where substrates and cofactors interact with enzymes.
• It contains proteins, ribosomal RNA, and ribosomes in polysome form.

Biological Membranes

• Plasma membranes are barriers that separate a cell's contents from the external environment.
• These membranes are selectively permeable, allowing the passage of ions and molecules.
• Specialized proteins facilitate molecule/ion transport.
• Membranes are primarily composed of lipids, proteins, and carbohydrates.
• Different membrane components (lipids, proteins) vary between membranes.