Medical Cell Biology Notes 1: Key Organelles and Their Functions

Cytoplasm is all the region inside the cell membrane and outside the nuclear envelope. It includes organelles and the cytosol.

Under the electron microscope, the cell is divided into membranous structures and non-membranous structures.

Membranous structures include the cell membrane and membrane-bound organelles within the cell, such as the endoplasmic reticulum, Golgi complex, lysosomes, mitochondria, peroxisomes and nuclear envelope. Among them, the endoplasmic reticulum, Golgi complex, lysosomes and peroxisomes are all single-membrane structures; mitochondria and the nuclear envelope are both double-membraned.

Non-membranous structures refer to cell components not wrapped by membranes, such as ribosomes, cytoskeleton, centrosomes, nucleoli and chromatin.

Ribosomes

Their chemical composition is ribosomal RNA (rRNA) and protein. Under the electron microscope, ribosomes are elliptical dense particles about 15 nm × 25 nm. Their structure includes a large subunit, small subunit and central canal; newly synthesized polypeptide chains are released through the central canal.

The function of ribosomes is to synthesize proteins. When carrying out protein synthesis, ribosomes do not work individually; a single mRNA strings several or even dozens of ribosomes together to perform synthesis. This structure performing the special function is called a polyribosome (polysome).

Ribosomes attached to the surface of the rough endoplasmic reticulum are called attached ribosomes; those free in the cytoplasm are called free ribosomes. Ribosomes located inside mitochondria are called mitochondrial ribosomes, which synthesize mitochondrial proteins.

The endomembrane system is a term specific to eukaryotic cytoplasm for membranous organelles that are structurally, functionally and developmentally interrelated. It includes the endoplasmic reticulum, Golgi complex, lysosomes, peroxisomes and transport vesicles. It does not include mitochondria, even though mitochondria are membranous structures.

The emergence of the endomembrane system is one of the important morphological features distinguishing eukaryotic from prokaryotic cells. It also allows different physiological and biochemical reactions within the cell to proceed independently without interfering with one another in specific regions, effectively increasing intracellular surface area and raising the overall metabolic efficiency of the cell.

Marker enzymes of organelles in the endomembrane system

The marker enzyme of the endoplasmic reticulum is glucose-6-phosphatase.

The marker enzyme of the Golgi complex is glycosyltransferase.

The marker enzyme of lysosomes is acid hydrolase.

The marker enzyme of peroxisomes is catalase.

Endoplasmic reticulum (ER)

The endoplasmic reticulum is a vesicular, vacuolar or tubular structure formed by a unit membrane, composed of lipids and proteins. In the ER membrane, lipids account for 30%-40% and proteins 60%-70%. The outer surface of the ER is called the cytosolic face, and the inner surface the luminal face. Depending on whether ribosomes are attached to its surface, the ER is divided into rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). RER has attached ribosomes and functions in the synthesis, modification, processing and transport of secretory proteins. SER has no attached ribosomes and functions in lipid synthesis and metabolism, glycogen metabolism, detoxification, and ion storage and regulation.

Golgi complex

The Golgi complex is formed by a unit membrane and usually consists of three parts: flattened cisternae, small vesicles (also called transport vesicles), and large vacuoles (also called condensing vacuoles or secretory vesicles). Its main components are lipids and proteins. The Golgi complex contains many enzymes; among them, glycosyltransferase is its marker enzyme and can transfer oligosaccharides onto proteins to form glycoproteins.

Functions of the Golgi complex: 1. Further processes, modifies and concentrates proteins from the RER into mature proteins, which are then secreted out of the cell. 2. Participates in the synthesis and modification of glycoproteins; because it has various glycosyltransferases, many proteins are glycosylated here into glycoproteins. 3. Participates in the formation of lysosomes.

Lysosomes

Lysosomes are round or ovoid vesicular structures wrapped by a unit membrane, varying in size and usually 0.2-0.8 µm in diameter. They contain more than 60 acid hydrolases with an optimum pH of 5, capable of breaking down almost all biologically active substances in the body.

Lysosomes are classified into primary, secondary and tertiary lysosomes. Primary lysosomes are large vesicles budded from the mature (trans) face of the Golgi complex; they contain inactive acid hydrolases but no substrate. Secondary lysosomes are formed by fusion of primary lysosomes with various phagosomes containing hydrolyzable substrates; they are larger and morphologically diverse. Tertiary lysosomes are residual structures left after secondary lysosomes have decomposed and digested the substrate, retaining indigestible materials that accumulate in the cell; they are also called residual bodies, such as lipofuscin granules.

Functions of lysosomes: Lysosomes are important intracellular digestive organelles whose main function is decomposition and digestion. Depending on the object being decomposed, they are divided into heterophagy and autophagy. In addition, lysosomes can release hydrolases out of the cell by exocytosis to break down and digest extracellular materials; this is called extracellular digestion, as when osteoclasts resorb and remodel bone tissue. Sometimes rupture of the lysosomal membrane causes the cell itself to be digested; this is called the autolysis of lysosomes, as when the tail disappears during tadpole metamorphosis into a frog.

Heterophagy: Lysosomes decompose and digest exogenous materials taken into the cell. Exogenous materials form phagosomes or pinocytic vacuoles by endocytosis, then fuse with primary lysosomes to form heterophagic lysosomes, where acid hydrolases break down the substrate. This plays an important role in body defense, as when lysosomes decompose bacteria or foreign matter phagocytosed by neutrophils and macrophages.

Autophagy: Lysosomes decompose and digest aged or disintegrated intracellular organelles, or lipids and glycogen granules stored in local cytoplasm.

Peroxisomes

Peroxisomes are small round bodies wrapped by a unit membrane, 0.2-1.7 µm in diameter. Their oxidative reactions are very important in liver and kidney cells, where they perform detoxification; for example, almost half of alcohol is oxidized and detoxified in peroxisomes.

Pathway of protein production, processing and transfer:

Ribosome (production) → Endoplasmic reticulum (primary processing) → Small vesicle (transport) → Golgi complex (processing) → Lysosome.

Mitochondria (the cell's power plant, mainly synthesizing ATP)

Mitochondria appear as granules, rods or filaments, 0.5-1.0 µm in transverse diameter and 2-6 µm long. They are sac-like structures enclosed by two unit membranes nested together. The inner membrane folds inward to form sheet-like or tubular projections called mitochondrial cristae. The space between the outer and inner membranes, about 8 nm, is called the intermembrane space. The space between cristae is called the intercristal space, filled with mitochondrial matrix and hence also called the matrix space. On the inner membrane of the cristae there are many regularly arranged, stalked spherical bodies called elementary particles, also known as ATP synthase complexes, which are the site of cellular aerobic respiration.

The main function of mitochondria is oxidative phosphorylation to synthesize ATP, providing energy for various life activities of the cell. Mitochondria are the site where carbohydrates, fats and proteins are ultimately oxidized to release energy; their elementary particles contain ATP synthase.

Cytoskeleton: mainly includes microtubules, microfilaments and intermediate filaments.

Cell nucleus

The cell nucleus is the largest organelle in eukaryotic cells, the region where genetic material is stored, and the control center for cellular metabolism, growth, differentiation, reproduction, heredity and variation.

The main functions of the nucleus are twofold: 1. Protect the DNA inside the nucleus from mechanical damage, ensuring chromosome replication and cell division, and maintaining the continuity of the species across generations. 2. Compartmentalize the cell into specific functional regions, ensuring gene expression is separated in space and time so that DNA replication, RNA transcription and protein translation proceed in an orderly fashion.

Nuclear envelope is the boundary between cytoplasm and nucleus, comprising the outer nuclear membrane, inner nuclear membrane, perinuclear space and nuclear pore complexes.

Chromatin and chromosomes

Chromatin and chromosomes are the same substance in different forms at different stages of the cell, composed mainly of DNA and histones. The basic unit of chromatin is the nucleosome. The first-order packing is the nucleosome string; the second-order is the solenoid; the third-order is the super-solenoid; the fourth-order is the chromosome. After all levels of packing, chromosomal DNA is compressed about 8400-fold in total.

Nucleolus

The nucleolus is the most conspicuous structure in the interphase nucleus of eukaryotic cells, composed mainly of proteins, rRNA and DNA. It is the site where rDNA is transcribed into rRNA, processed and further assembled into the large and small ribosomal subunits; the nucleolus is closely related to protein synthesis.