细胞解剖

The surface of our planet is populated by living things. Despite their apparent diversity, living things are fundamentally similar inside. Cells are the fundamental units of life. As emphasized by the cell biologist Edmund Beecher Wilson, “The key to every biological problem must finally be sought in the cell; for every living organism is, or at some time has been, a cell.”

Cells can be divided into two groups: prokaryotic and eukaryotic. The DNA in prokaryotes is contained in a central area of the cell called the nucleoid, which is not surrounded by a nuclear membrane. Eukaryotes keep their DNA in a distinct membrane- enclosed intracellular compartment called the nucleus. Eukaryotic cells are much more complicated than those of prokaryotes. They are packed with a fascinating array of subcellular structures that play important roles in energy balance, metabolism, gene expression, etc. Plants, animals, and fungi are eukaryotes; bacteria are prokaryotes, as are archaea, though archaea genetically resembles eukaryotes more than bacteria. Viruses are not living things, they don’t have cells.

Cells are both simple and complicated. On the one hand, the common molecular code in which the specifications for all living organisms are simple and beautiful . DNA structure, replication, transcription, and translation all follow the certain simple rules. On the other hand, complex networks of interactio ns occur between thousands of metabolites, proteins, and DNA in every cell. Professor of biology Michael Denton, in his book entitled Evolution:A Theory in Crisis, explains this complexity with an example: “To grasp the reality of life as it has been revealed by molecular biology, we must magnify a cell a thousand million times until it is twenty kilometers in diameter and resembles a giant airship large enough to cover a great city like London or New York. What we would then see would be an object of unparalleled complexity and adaptive design. On the surface of the cell we would see millions of openings, like port holes of a vast space ship, opening and closing to allow a continual stream of materials to flow in and out. . . (a complexity) beyond our own creative capacities, a reality which is the very antithesis of chance, which excels in every sense anything produced by the intelligence of man…”

Let’s begin from the organelles, the basic structures inside cells that perform various jobs to keep a cell alive.

Cell membranes are crucial to the life of the cell, it encloses the cell, defines its boundaries, and maintains the essential differences between the cytoplasm and the extracellular environment. Inside eukaryotic cells, the membranes of the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, and other membrane-enclosed organelles maintain the characteristic differences between the contents of each organelle and the cytoplasm.

The cell membranes are made mostly f ro m a do u ble laye r of phospholipids molecules, which are amphiphilic (partly hydrophobic and partly hydrophilic). The layer is called a phospholipid bilayer. The membrane is semi-permeable and selectively permeable in that it can either let a substance (molecule or ion) pass through freely, pass through to a limited extent, or not pass through at all. Receptor proteins embedded in the plasma membrane allow cells to detect external signaling molecules such as hormones.

A cell’s information center, the cell nucleus contains genetic material. It is the place where almost all DNA replication and RNA synthesis (transcription) occur. It coordinates cell activities like protein synthesis and cell division . In the nucleus, DNA is transcribed into a special RNA, called messenger RNA (mRNA). This mRNA is then transported out of the nucleus to cytoplasm through the nuclear pore, and translated into a specific protein molecule.

Anatomically, the nucleus is made up of several components, including a nuclear envelope, nuclear lamina, nucleolus, chromosomes, and nucleoplasm . The nucleolus is the site in which new ribosomes are assembled. The nuclear envelope isolates and protects a cell’s DNA from various molecules that could accidentally damage its structure or interfere with its processing.

A ribosome is a large complex of RNA and protein molecules . It consists of two subunits, and acts as an assembly line where mRNA is used to synthesize proteins from amino acids as the template. Ribosomes can be found either floating freely in the cytoplasm or bound to a membrane (the rough endoplasmic reticulum in eukaryotes or the cell membrane in prokaryotes). A typical eukaryotic cell contains millions of ribosomes in its cytoplasm.

After recognizing the structure of the mRNA bound to a transfer RNA (tRNA), the two subunits of the ribosome can combine to start synthesizing protein from the mRNA strand.

The endoplasmic reticulu m (ER) plays a key role in the post – translational modifications (PTMs) of proteins and the synthesis of lipids . PTMs increase the functional diversity of the proteome, which is one of several mechanisms to achieve proteome diversity. It consists of a network of membranous tubules and flattened sacs. They are hollow, and the space inside is called the lumen.

The ER has two forms: the rough ER, which has ribosomes on its surface that secrete proteins into the lumen. Inside the lumen, proteins fold and undergo modifications, and are packaged into vesicles, and shipped to the Golgi apparatus. The smooth ER, which lacks ribosomes. The smooth ER is continuous with the rough ER. It plays a role in the synthesis oflipids and steroid hormones, and the storage of calcium ions.

The Golgi apparatus is a major collection and dispatch station of protein products received from the endoplasmic reticulum . The vesicles that leave the rough endoplasmic reticulum are transported to the cis face of the Golgi apparatus, where they fuse with the Golgi membrane and empty their contents into the lumen. These proteins are modified and placed into different types of vesicles, then delivered to specific intracellular or extracellular locations. In this respect, the Golgi can be thought of a post office: it packages and labels items and then sends them to different destinations.

Lysosomes are spherical vesicles that contain hydrolytic enzymes. The lumen’s pH (~4.5 – 5.0) is the optimum pH for the enzymes involved in cell hydrolysis. Lysosomes digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria. Many of the products of lysosomal digestion are recycled back to the cell for use in the synthesis of new cellular components. Lysosomes have been thought to be involved in necrotic and autophagic cell death and have a role in apoptosis.

Peroxisomes are oxidative organelles. They perform key roles in lipid metabolism and the conversion of reactive oxygen species. In liver and kidney cells, the peroxisomes detoxify various toxic substances that enter the blood.

Mitochondria are often called the energy factories of the cell. Their job is to make a steady supply of adenosine triphosphate (ATP), the cell’s main energy- carrying molecule. The process of cell making ATP using chemical energy from fuels such as sugars is called cellular respiration, and many of its steps happen inside the mitochondria.

Mitochondria are oval-shaped and have two membranes: an outer one, surrounding the whole organelle, and an inner one, with many inward protrusions called cristae that increase surface area. The narrow gap between the inner boundary membrane and the outer membrane is known as the intermembrane space, and the compartment enclosed by the inner membrane is called the mitochondrial matrix.

Mitochondria contain a small amount of their own DNA. Inherited changes in mitochondrial DNA can cause problems with growth, development, and the functioning of the body’s systems.

The cytoskeleton acts to organize and maintain the cell’s shape: it anchors organelles in place; helps the processes of endocytosis, the uptake of external materials by a cell and cytokinesis, the cytoplasmic separation of daughter cells during cell division; and moves parts of the cell during the processes of growth and mobility.

The eukaryotic cytoskeleton is composed of microtubules, intermediate filaments, and microfilaments. Each of them is formed by the polymerization of a distinct type of protein subunit and has its own characteristic shape.

Centrosomes are found in animal cells, but do not exist within most plant cells. In animal cells, centrosome serves as the main microtubule organizing center (MTOC) and assists in the formation and organization of microtubules as well as in self-duplication before cell division.

Embedded in the centrosome are the centrioles, a pair of cylindrical structures arranged at right angles to each other in an L-shaped configuration. A centriole consists of a cylindrical array of short, modified microtubules. The centrosome duplicates and splits into two parts before mitosis. During cell division, two centrosomes move to opposite sides of the nucleus to form the two poles of the mitotic spindle.

In a eukaryotic cell, the cytoplasm is all the substances that are enclosed by a cell membrane except the nucleus. The main components of the cytoplasm are cytosol (a gel-like substance), the organelles, and various cytoplasmic inclusions (such as pigment granules and nutrients like proteins and carbohydrates).

The cytosol, also known as cytoplasmic matrix, is a complex mixture of substances dissolved in water. The concentrations of ions such as sodium and potassium in the cytosol are different to those in the extracellular fluid; these differences in ion levels are important in processes such as osmoregulation, cell signaling, and the generation of action potentials in excitable cells such as endocrine, nerve and muscle cells.

The extracellular matrix (ECM) is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, and glycoproteins. Its main function is to provide an essential scaffold for cells. The extracellular matrix also controls communication between cells and regulates cell processes such as growth, migration, and differentiation. Because multicellularity evolved independently in different lineages, the compositions of ECM vary among multicellular structures.

There is a great diversity of cells in the nature. There are hundreds of different types of cells in the human body. The single largest biological cell found in nature is an ostrich egg, which is about 15cm long and 13cm wide. The mycoplasmas are thought to be the smallest living cells in the biological world. They have a minimal size of approximately 0.2 micrometers. In the human body, the largest cell is the ovum, at about 0.1mm in diameter, and many scientists suggest that the sperm is the smallest cell in the human body. The head of the sperm cell is 4 micrometers long, slightly smaller than the red blood cell.

The cells in complex multicellular organisms like people are organized into tissues which are groups of similar cells that work together on specific tasks. When different types of tissues are organized together to perform a complex function, it’s called an organ. Organs are grouped into organ systems, in which they work together to carry out a particular function for the organism.