Cells are the basic structural and functional units of all known organisms. It is the smallest unit of an organism classified as a living thing, and is usually called the building material of life. [1] Some organisms, such as most bacteria, are unicellular (composed of single cells). Other organisms, such as humans, are multicellular. (It is estimated that there are 100 trillion or 10 14 cells; What is the typical cell size 10? m； The typical cell mass is 1 nanogram. ) The largest cell known is the unfertilized ostrich egg cell. [2]

1835 before the formation of the final cell theory, Jan Evangelista Prgin observed small "particles" when observing plant tissues through a microscope. The cell theory, first put forward by Matthias Jacob Schleiden and Theodor Schwann in 1839, holds that all living things are composed of one or more cells, all cells come from pre-existing cells, important functions of living things occur in cells, and all cells contain genetic information necessary for regulating cell functions and transmitting information to the next generation of cells. [3]

The word cell comes from the Latin cell, which means a small room. This term describing the smallest biological structure was coined by robert hooke in a book published in 1665, when he compared the cork cells he saw under the microscope to the small room where monks lived. [4]

[Edit] General principles

Mouse cells grown in Petri dishes. These cells grow in large chunks, but each individual cell is about 10 micron. Each cell is at least self-sufficient and self-sustaining: it can absorb nutrients, convert these nutrients into energy, perform specialized functions, and reproduce when necessary. Each cell has its own set of instructions to perform these activities.

All cells have several different abilities: [5]

Reproduction by cell division: (binary fission/mitosis or meiosis).

Enzymes and other protein encoded by DNA genes and produced by messenger RNA intermediates and ribosomes are used.

Metabolism includes ingesting raw materials, building cell components, converting energy, molecules and releasing by-products. The function of a cell depends on its ability to extract and utilize chemical energy stored in organic molecules. This energy is released and then used in metabolic pathways.

Response to external and internal stimuli, such as changes in temperature, pH value or nutritional level.

The cell contents are contained in the cell surface membrane, which consists of a lipid bilayer in which protein is embedded.

Some prokaryotic cells contain important intimal binding compartments, [6] but eukaryotic cells have a set of specialized intimal compartments.

[Editor] Cell Anatomy

There are two types of cells: eukaryotic cells and prokaryotic cells. Prokaryotic cells are usually independent, while eukaryotic cells often exist in multicellular organisms.

Prokaryotic cell

Main article: Prokaryotes.

Typical prokaryotic cell structure diagram Prokaryotic cells are simpler than eukaryotic cells, and have no nucleus and most other organelles of eukaryotic cells. There are two kinds of prokaryotes: bacteria and archaea; These share a similar overall structure.

Prokaryotic cells have three structural regions:

Outside, flagella and fimbriae protrude from the cell surface. These structures are made of protein (not all prokaryotes have them), which are helpful to the movement and communication between cells;

Surrounding cells is the cell envelope-usually consisting of a cell wall covering the plasma membrane, although some bacteria also have a covering called the envelope. Envelope provides rigidity for cells, separates the interior of cells from their environment, and acts as a protective filter. Although most prokaryotes have cell walls, there are exceptions, such as mycoplasma (bacteria) and pyrogen (archaea). In bacteria, the cell wall is composed of peptidoglycan and acts as an additional barrier against external forces. It also prevents cells from swelling and eventually breaking (cell lysis) due to osmotic pressure in a hypotonic environment. Some eukaryotic cells (plant cells and fungal cells) also have cell walls;

Inside a cell is a cytoplasmic region, which contains cell genome (DNA), ribosomes and various inclusions. The chromosomes of prokaryotes are usually circular molecules (Borrelia burgdorferi, which causes Lyme disease, is an exception). Although no nucleus is formed, DNA is concentrated into nucleoid. Prokaryotes can carry extrachromosomal DNA elements, called plasmids, which are usually circular. Plasmids have additional functions such as antibiotic resistance.

Eukaryotic cell

Main article: Eukaryotes.

Typical animal (eukaryotic) cell diagram, showing subcellular components.

Organelle:

(1) nucleus

(2) the nucleus

(3) Ribosomes

(4) Vesicles

(5) Rough endoplasmic reticulum

(6) Golgi apparatus

(7) Cytoskeleton

(8) Smooth endoplasmic reticulum

(9) mitochondria

(10) vacuole

(1 1) cytoplasm

(12) solution

The size of centriole in (13) centrosome eukaryotic cells is about 15 times that of typical prokaryotes, and its volume can be as high as 1000 times. The main difference between prokaryotes and eukaryotes is that eukaryotes contain membrane-bound compartments in which specific metabolic activities occur. The most important thing is the existence of the nucleus, which is a compartment defined by the membrane and contains the DNA of eukaryotic cells. It is this nucleus that gives eukaryotes a name, meaning "true nucleus". Other differences include:

The plasma membrane is similar in function to that of prokaryotes, but slightly different in structure. Cell walls may or may not exist.

Eukaryotic DNA consists of one or more linear molecules called chromosomes, which are associated with histones. All chromosomal DNA is stored in the nucleus and separated from the cytoplasm by a membrane. Some eukaryotic organelles such as mitochondria also contain some DNA.

Many eukaryotic cells have primary cilia. Primary cilia play an important role in chemical sensation, mechanical sensation and thermal sensation. Therefore, cilia can be "regarded as sensory cell antennae that coordinate a large number of cell signal pathways, sometimes coupling signals with cilia movement or cell division and differentiation." [7]

Eukaryotes can use active cilia or flagella to move. Flagella is more complex than that of prokaryotes.

Table 1: comparison of the characteristics of prokaryotic cells and eukaryotic cells.

Typical biological bacteria, archaea protozoa, fungi, plants and animals.

Typical size ~1–10? m ~ 10– 100？ M (sperm cells, except the tail, are relatively small)

The type of nucleoid region of nucleus; There is no eukaryote, but a eukaryote with two membranes.

A circular (usually) linear molecule (chromosome) containing histone.

RNA synthesis in cytoplasm-/protein coupling RNA synthesis in nucleus

Protein synthesis in cytoplasm

Ribosome 50S+30S 60S+40S

Cytoplasmic structure A very few highly structured structures consisting of tunica intima and cytoskeleton.

Cell movement flagella composed of flagellin flagella and cilia containing microtubules; Lamellar and Filamentous Pods Containing Actin

There are not one to thousands of mitochondria (although some have no mitochondria)

There are no chloroplasts in algae and plants.

Tissues are usually higher multicellular organisms with single cells, single cell colonies and specialized cells.

Cell division binary division (simple division) mitosis (division or budding)

meiosis

Table 2: The structures of animal cells and plant cells are typical animal cells and typical plant cells.

Cell organelle nucleus

Nucleolar (nuclear)

Rough endoplasmic reticulum

Smoothing ER

ribosome

Cytoskeleton

Golgi apparatus

cytoplasm

mitochondria

Small vesicle

lysosome

centrosome

centriole

vacuole

nucleus

Nucleolar (nuclear)

rough endoplasmic reticulum, rer

Smoothing ER

ribosome

Cytoskeleton

Gorky body (dictionary body)

cytoplasm

mitochondria

[Edit] Subcellular components

Eukaryotic cells (left) and prokaryotic cells (right) All cells, whether prokaryotic or eukaryotic, have a membrane surrounding the cells, which separates the interior of the cells from the environment, regulates the substances entering and leaving (selective permeation), and maintains the potential of the cells. Inside the cell membrane, the salty cytoplasm occupies most of the cell volume. All cells have DNA (the genetic material of genes) and RNA, which contain the necessary information for making various protein (such as enzymes, the main machine of cells). There are other kinds of biomolecules in cells. This article will list these main components of the unit, and then briefly describe their functions.

[Editor] Cell membrane: the boundary of cells

Main article: Cell membrane

The cytoplasm of a cell is surrounded by a cell membrane or plasma membrane. The plasma membrane of plants and prokaryotes is usually covered by cell walls. This membrane is used to isolate and protect cells from the surrounding environment, and it is mainly composed of bilayer lipids (hydrophobic fatty molecules) and hydrophilic phosphorus molecules. Therefore, this layer is called phospholipid bilayer. It can also be called fluid mosaic membrane. Embedded in this membrane are various protein molecules, which act as channels and pumps to move different molecules into and out of cells. This kind of membrane is called "semi-permeable membrane" because it can let substances (molecules or ions) pass freely, pass within a limited range, or not pass at all. Cell surface membranes also contain receptor proteins, which enable cells to detect external signal molecules, such as hormones.

[Editor] Cytoskeleton: the scaffold of cells

Main article: Cytoskeleton

Bovine pulmonary artery endothelial cells: the nucleus is dyed blue, the mitochondria are dyed red, and F- actin, an important component in microfilaments, is dyed green. The cells were imaged under a fluorescent microscope. The role of cytoskeleton is to organize and maintain the shape of cells; Fixing the organelle in a proper position; It is helpful to endocytosis (cells take in external substances) and cytokinesis (separation of daughter cells after cell division); And move a part of the cell during growth and movement. Eukaryotic cytoskeleton consists of microfilaments, intermediate filaments and microtubules. There are a lot of protein associated with them, and each protein controls the cell structure by guiding, binding and arranging filaments. There is little research on prokaryotic cytoskeleton, but it is related to the maintenance of cell shape, polarity and cytokinesis. [8]

[edit] genetic material

There are two different genetic materials: DNA and RNA. Most organisms use DNA for long-term information storage, but some viruses (such as retroviruses) use RNA as their genetic material. Biological information contained in an organism is encoded in its DNA or RNA sequence. RNA is also used for information transport (such as mRNA) and enzyme function (such as ribosomal RNA) in organisms that use DNA itself as genetic code. Transfer RNA (tRNA) molecules are used to add specific amino acids in protein translation.

The genetic material of prokaryotes consists of a simple circular DNA molecule (bacterial chromosome) in the nucleoid region of cytoplasm. The genetic material of eukaryotes is divided into different linear molecules, called chromosomes in discrete nuclei, and there is usually extra genetic material in some organelles such as mitochondria and chloroplasts (see the theory of endosymbiosis).

Human cells have genetic material in the nucleus (nuclear genome) and mitochondria (mitochondrial genome). The human nuclear genome is divided into 23 pairs of linear DNA molecules called chromosomes. Mitochondrial genome is a circular DNA molecule different from nuclear DNA. Although the mitochondrial DNA is very small compared with the nuclear chromosome, it encodes 13 protein involved in mitochondrial energy production and specific tRNAs.

Foreign genetic material (most commonly DNA) can also be artificially introduced into cells through a process called transfection. If DNA is not inserted into the genome of a cell, it may be temporary, and if it is, it may be stable. Some viruses also insert their genetic material into the genome.

Organelle [edit]

Main articles: organelles

The human body contains many different organs, such as the heart, lungs and kidneys, and each organ performs different functions. Cells also have a group of "small organs", called organelles, which are adapted and/or specially used to perform one or more important functions.

There are several types of organelles in animal cells. Some (such as nucleus and Golgi apparatus) are usually isolated, while others (such as mitochondria, peroxisomes and lysosomes) may be numerous (hundreds to thousands). Cytoplasmic sol is a colloidal liquid filled with cells and surrounding organelles.

Mitochondria and chloroplasts-generators

Mitochondria are self-replicating organelles that exist in the cytoplasm of all eukaryotic cells in various numbers, shapes and sizes. Mitochondria play a key role in generating energy in eukaryotic cells. Mitochondria produce cell energy through oxidative phosphorylation, and use oxygen to release energy stored in cell nutrients (usually glucose) to produce ATP. Mitochondria reproduce by splitting in two.

The organelles of modified chloroplasts are widely called plastids and participate in energy storage through photosynthesis, which uses solar energy to produce carbohydrates and oxygen from carbon dioxide and water. [need to quote]

Mitochondria and chloroplasts contain their own genomes, which are separate and different from the nuclear genome of cells. Both organelles contain this DNA in circular plasmids, much like prokaryotic cells, which strongly supports the evolutionary theory of endosymbiosis. Because these organelles contain their own genomes and have other similarities with prokaryotes, they are considered to have developed through symbiosis after being swallowed by primitive cells. [need to quote]

ribosome

Ribosomes are large complexes of RNA and protein molecules. Each of them consists of two subunits and serves as an assembly line. mRNA from the nucleus is used to synthesize protein from amino acids. Ribosomes can float freely or bind to membranes (rough endoplasmic reticulum of eukaryotes or cell membrane of prokaryotes). [9]

Nucleus-the information center of cells

The nucleus is the most obvious organelle in eukaryotic cells. It contains the chromosomes of cells and is where almost all DNA replication and RNA synthesis (transcription) take place. The nucleus is spherical and separated from the cytoplasm by a double membrane called nuclear membrane. The nuclear membrane isolates and protects the cell's DNA from various molecules, which may accidentally destroy its structure or interfere with its processing. In the process of processing, DNA is transcribed or copied into a special RNA called mRNA. This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. Nucleolar is a special area where ribosome subunits gather in the nucleus. In prokaryotes, DNA processing takes place in the cytoplasm.

Schematic diagram of nucleus

Endoplasmic reticulum-eukaryotes only

Compared with molecules floating freely in cytoplasm, endoplasmic reticulum (ER) is a transport network for molecules with certain modifications and specific destinations. There are two forms of endoplasmic reticulum: rough endoplasmic reticulum and smooth endoplasmic reticulum. The former has ribosomes on its surface and secretes protein into cytoplasm, while the latter has no ribosomes. Smooth endoplasmic reticulum plays a role in calcium isolation and release.

Golgi Body-Eukaryotes Only

The main function of Golgi apparatus is to process and package macromolecules such as protein and lipids synthesized by cells. It is particularly important in the secretion of protein. Golgi apparatus is a part of the endomembrane system of eukaryotic cells. Vesicles entering Golgi apparatus are processed in the forward and reverse directions, which means that they coalesce on the cis side of Golgi apparatus and contract on the opposite (trans) side after processing, forming new vesicles in animal cells. [need to quote]

Schematic diagram of intimal system

Lysosomes and Peroxisomes–Eukaryotes only

Lysosomes contain digestive enzymes (acid hydrolases). They digest redundant or damaged organelles, food particles and swallowed viruses or bacteria. Peroxisome contains enzymes that scavenge toxic peroxides from cells. If these destructive enzymes are not included in the membrane binding system, cells cannot accommodate them. These organelles are often called "suicide bags" because they have the ability to detonate and destroy cells. [need to quote]

Centrosome-the organizer of cytoskeleton

Centrosomes produce microtubules, the key components of cytoskeleton. It directs transportation through the emergency room and Golgi apparatus. Centrosome consists of two centrioles, which separate during cell division and contribute to the formation of mitotic spindle. There is only one centrosome in animal cells. They also exist in some fungi and algae cells. [need to quote]

vacuole

Vacuoles store food and waste. Some vacuoles store extra water. They are usually described as spaces filled with liquid and surrounded by a film. Some cells, most notably amoeba, have contractile vacuoles, which can pump water out of the cells if there is too much water.

[Edit] Structure outside the cell wall

[Editor] Capsule

Some bacteria have a gelatinous coating on the outside of their cell walls. The capsule can be polysaccharide in pneumococcus and meningococcus, polypeptide in Bacillus anthracis or hyaluronic acid in Streptococcus. Capsules that can't be labeled by ordinary dyes but can be detected by special dyes. Capsules are antigenic. Capsule has the function of anti-phagocyte, so it determines the toxicity of many bacteria. It also plays a role in the adhesion of organisms to mucous membranes. [need to quote]