Introduction to Biology: Cell Ultrastructure and Biochemical Organization

Course Overview and Evaluation Framework

BIOLOGIE I, Course 1, is led by Ș.L. Dr. Biol. Adela Banciu. The curriculum for this course is extensive, covering the ultrastructure of both animal and vegetable cells, the biochemical organization of the cell, and the organization of cells into tissues. Specific animal tissues studied include epithelial, connective, glandular, muscular, and nervous tissues. Vegetable tissues covered include protective (defensive), fundamental, vascular (conducting), support, and secretory tissues. The course further examines macroscopic structures, microscopic aspects, functions, and principal pathologies along with associated microscopic modifications for various physiological systems, including the circulatory, digestive, respiratory, locomotor, excretory, and nervous systems (including sense organs).

Laboratory and practical applications are an integral part of the curriculum. These sessions involve labor protection and good laboratory practice rules, the principles of optical and electronic microscopy, the utilization of optical microscopes, and the use of micropipettes. Practical skills developed include the microscopic examination of animal cells, the separation and examination of vegetable cells from tissues, and techniques for harvesting, preparing, and staining tissues. Students are expected to realize correlations between tissue structure and function and to identify the primary structures of animal and plant tissues under an optical microscope. This includes the examination of microscopic preparations associated with the circulatory, digestive, respiratory, excretory, and nervous systems, as well as identifying the functions of different structures and any pathological changes.

Evaluation follows a dual structure. The overall grade is split equally between the theoretical course examination ($50\,\%$) and laboratory performance ($50\,\%$). Within the laboratory portion, $25\,\%$ is derived from continuous evaluation and active participation, while the remaining $25\,\%$ comes from a practical exam focused on the microscopic evaluation of histological preparations.

Animal Cell Ultrastructure and Morphology

The study of animal cell ultrastructure involves examining sub-cellular structures and their corresponding functions. A generalized animal cell is often conceptualized around a size threshold of approximately $200\,\mu m$ (referenced from Lodish, 2008). The course provides specific examples of cell types and sizes through imaging. One example includes isolated neurons from the rat dorsal spinal ganglia, which are noted at a scale of $75\,\mu m$. Another specific example is the SH-SY5Y neuroblastoma cell line, also examined at the $75\,\mu m$ scale (Source: Carl Zeiss MicroImaging GmbH 2010).

The Plasma Membrane: Lipid and Carbohydrate Composition

The plasma membrane is a complex bilayer primarily composed of phospholipids, cholesterol, and glycolipids. The general structure of a phospholipid includes a polar hydrophilic head group and nonpolar hydrophobic fatty acid chains. Chemically, a phospholipid consists of two fatty acid chains, a glycerol backbone, a phosphate group, and a polar group which may include choline, ethanolamine, or serine. Fatty acid chains can be saturated (straight) or unsaturated (bent).

Cholesterol is another vital component of the membrane. It is characterized by a polar hydrophilic end (a hydroxyl group), a rigid region provided by the steroid ring, and a more fluid nonpolar tail. In the lipid bilayer, cholesterol molecules orient themselves with their hydroxyl groups positioned near the polar heads of the phospholipids. Glycolipids also form part of the membrane structure, specifically mentioning galactocerebrosides and gangliosides. These molecules contain sugar chains. Key components in glycolipids include Galactose (Gal), Glucose (Glc), N-acetylgalactosamine (GalNAc), and N-acetylneuraminic acid (NANA), also known as sialic acid.

Biochemical Classification of Carbohydrates and Lipids

Carbohydrates are classified based on their complexity. Monosaccharides follow the general formula $(CH_2O)_n$ and include molecules such as Glucose, Fructose, Ribose, Deoxyribose, Mannose, and Galactose. Disaccharides are dimers of monosaccharides; examples include Lactose (composed of glucose and galactose), Maltose (composed of two glucose molecules), and Sucrose (composed of glucose and fructose). Polysaccharides are polymers of mono- or disaccharides. These include Starch (comprising amylose and amylopectin, which are $\alpha$-glucose monomers), Cellulose (made of $\beta$-glucose), and Glycogen (made of $\alpha$-glucose), which is similar in structure to amylopectin.

Carbohydrates serve multiple functions: they are a primary energy source (with glucose being the most significant), provide building blocks for larger molecules like nucleic acids, and form structural components and energy reserves (starch, glycogen, cellulose). Lipids serve as energy reserves, thermal insulators, and a metabolic source of water. They have a crucial structural role, primarily as phospholipids in membranes. Furthermore, lipids act as solvents for fat-soluble vitamins; a lack of lipids in the diet can lead to deficiencies in these vitamins.

Protein Structure and Membrane Functions

The structure of proteins is categorized into four levels. The primary structure is the linear sequence of amino acids, determined by the genetic material that encodes it. The secondary structure consists of repetitive structural organizations such as the $\alpha$-helix and the $\beta$-sheet. The tertiary structure refers to the three-dimensional folding of these secondary structures into specific shapes. Finally, the quaternary structure involves the assembly and packaging of multiple polypeptide chains.

Proteins within the plasma membrane perform a variety of essential duties. They act as enzymes to catalyze chemical reactions, serve as marker proteins, and function as receptors. They provide structural support and facilitate the transmembrane transport of molecules and ions. Membrane proteins are responsible for establishing intercellular connections and anchoring cells to the extracellular matrix. They control the flow of information through the recognition, binding, and transmission of signal molecules, and they play a critical role in cellular immunity.

Transmembrane Transport and Permeability

The phospholipid bilayer acts as a selective barrier. It does not allow the passage of ions (such as $H^+$ or $Na^+$) or charged polar molecules like amino acids and nucleic acids. It has very low permeability for large molecules like glucose and sucrose, and low permeability for small polar molecules such as water, urea, and glycerol. However, it has high permeability for hydrophobic molecules like $O_2$, $CO_2$, and benzene.

Water-soluble molecules must be moved via specific transport proteins. Carrier proteins (transporters) facilitate diffusion by moving substances (like carbohydrates and amino acids) along their concentration gradient. Channel proteins, such as ion channels for sodium ($Na^+$), potassium ($K^+$), and calcium ($Ca^{2+}$), are integral proteins that recognize and select specific ions. These channels are activated by chemical, mechanical, or electrical stimuli and form a narrow hydrophilic pore specific to different ions.

Signal Transduction and Membrane Receptors

Receptors in the plasma membrane are involved in the transduction of various signals and control complex cellular processes such as cell division, growth, proliferation, differentiation, and migration. The signal transduction process typically follows a specific sequence. First, a ligand binds to a receptor, causing a conformational change that activates the receptor. Then, a G-protein binds to the activated receptor. GTP binds to the G-protein, causing it to become active and detach from the receptor. This activated G-protein then attaches to and activates an effector protein, which might be an ion channel or an enzyme.

Once the effector protein is active, it makes a second messenger available within the cell. This second messenger leads to the activation of a protein kinase enzyme. The active protein kinase then phosphorylates other enzymes, which results in specific cellular enzymes being turned on or off to produce a physiological response.

The Nucleus: Organization and Genetic Material

The nucleus is the site for the storage and replication of DNA, as well as the synthesis and processing of RNA. It is delimited by a double phospholipid layer known as the nuclear envelope. The outer nuclear membrane is continuous with the rough endoplasmic reticulum (RE), and the space between the inner and outer membranes (the perinuclear space) is continuous with the lumen of the rough RE. The inner membrane is in contact with the nuclear lamina, a network supporting the nucleoplasm.

The nuclear envelope is perforated by nuclear pores made of specific proteins that ensure bidirectional transport into and out of the nucleus. Inside, DNA is packaged as chromatin, which consists of DNA, histones, and non-histone proteins that play roles in structure, regulation of gene expression, and enzymatic activity. Chromatin is organized into two forms: heterochromatin, which has a high degree of condensation and is less prone to transcription, and euchromatin, which is less condensed and functional for active transcription. The nucleolus is another prominent structure within the nucleus, as seen in electron microscopy images of stem cells isolated from bone marrow.

Questions & Discussion

Identification Task: Identify the natural polymers presented in this course! (This refers to molecules like DNA, RNA, starch, cellulose, glycogen, and proteins discussed throughout the lecture).