Cell Biology

In this lecture Professor Zach Murphy will be teaching you about the structure and function of the cell. We review all of the organelles within the cell including the nucleus, nucleolus, rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi apparatus, mitochondria, ribosome, and so much more! 

  1. 00:00 – Intro and Overview
  2. 00:43 – Nucleus
  3. 01:06 – Nuclear Envelope (Inner and Outer Membranes)
  4. 02:50 – Nuclear Pores
  5. 03:52 – Nucleolus
  6. 04:51 – Chromatin
  7. 07:30 – Rough and Smooth Endoplasmic Reticulum (ER)
  8. 18:20 – Golgi Apparatus
  9. 23:03 – Cell Membrane
  10. 27:25 – Lysosomes
  11. 31:20 – Peroxisomes
  12. 35:38 – Mitochondria
  13. 40:04 – Ribosomes (Free and Membrane-Bound)
  14. 43:20 – Cytoskeleton (Actin, Intermediate Filaments, Microtubules)
  15. 54:50 – Wrap up

Cell Biology

Cell biology (also cellular biology or  cytology)  is a branch of biology  that studies  the structure,  function, and behavior of  cells. All living organisms are made of cells. A cell is the basic unit of life that is responsible for the living and functioning of organisms. Cell biology is the study of structural and functional units of cells. Cell biology encompasses  both  prokaryotic  and  eukaryotic cells  and has many subtopics which may include the study of cell metabolism, cell communication, cell cycle, biochemistry, and cell composition.

The study of cells is performed using several  microscopy  techniques,  cell culture,  and  cell fractionation. These have allowed for and are currently being used for discoveries and research pertaining to how cells function, ultimately giving insight into understanding larger organisms. Knowing the components of cells and how cells work is fundamental to all biological sciences while also being essential for research in biomedical fields such as cancerand other diseases. Research in cell biology is interconnected to other fields such as geneticsmolecular geneticsmolecular biologymedical microbiologyimmunology, and cytochemistry.

The cell (from the Latin word cellula meaning ‘small room’) is the basic structural and functional unit of life forms.  Every cell consists of a cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins  and nucleic acids.

Cells can acquire specified function and carry out various tasks within the cell such as replication, DNA repair, protein synthesis, and motility. Cells are capable of specialization and mobility within the cell. Most cells are measured in micrometers due to their small size.

Most plant and animal cells are only visible under a  light microscope,  with dimensions between 1 and 100   micrometres.  Electron microscopy gives a much higher resolution showing greatly detailed cell structure. Organisms can be classified as  unicellular (consisting of a single cell such as bacteria) or  multicellular   (including  plants and animals).  Most  unicellular organisms  are classed as  microorganisms.  The number of cells in plants and animals varies from species to species; it has been approximated that the human body contains an estimated 37 trillion (3.72×1013) cells. The brain accounts for around 80 billion of these cells.

The study of cells and how they work has led to many other studies in the field. Including but not limited to; the discovery of DNA, cancer study development, as well as aging and development.

Cell biology is the study of cells, which were discovered by  Robert Hooke  in 1665, who named them for their resemblance to  cells  inhabited by  Christian monks in a monastery. Cell theory, first developed in 1839 by   Matthias Jakob Schleiden  and  Theodor Schwann,  states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all living organisms, and that all cells come from pre-existing cells.[9] Cells emerged on Earth about 4 billion years ago.

History

Cells were first seen in 17th century Europe with the invention of the  compound microscope.  In 1665,  Robert Hooke  termed the building block of all living organisms as “cells” (published in  Micrographia) after looking at a piece of cork and observing a cell-like structure,  however, the cells were dead and gave no indication to the actual overall components of a cell. A few years later, in 1674,  Anton Van Leeuwenhoek was the first to analyze live cells in his examination of  algae.  All of this preceded the  cell theory  which states that all living things are made up of cells and that cells are the functional and structural unit of organisms. This was ultimately concluded by plant scientist, Matthias Schleiden and animal scientist  Theodor Schwann  in 1838, who viewed live cells in plant and animal tissue, respectively. 19 years later,  Rudolf Virchow  further contributed to the cell theory, adding that all cells come from the division of pre-existing cells. Viruses are not considered in cell biology – they lack the characteristics of a living cell, and instead are studied in the  microbiology  subclass of  virology.

Techniques

Cell biology research looks at different ways to culture and manipulate cells outside of a living body to further research in human anatomy and physiology, and to derive medications. The techniques by which cells are studied have evolved. Due to advancements in microscopy, techniques and technology have allowed scientists to hold a better understanding of the structure and function of cells. Many techniques commonly used to study cell biology are listed below:    Cell culture: Utilizes rapidly growing cells on media which allows for a large amount of a specific cell type and an efficient way to study cells. Cell culture is one of the major tools used in cellular and molecular biology, providing excellent model systems for studying the normal physiology and biochemistry of cells (e.g., metabolic studies, aging), the effects of drugs and toxic compounds on the cells, and mutagenesis and carcinogenesis. It is also used in drug screening and development, and large scale manufacturing of biological compounds (e.g., vaccines, therapeutic proteins).

  • Fluorescence microscopy: Fluorescent markers such as GFP, are used to label a specific component of the cell. Afterwards, a certain light wavelength is used to excite the fluorescent marker which can then be visualized.
  • Phase-contrast microscopy: Uses the optical aspect of light to represent the solid, liquid, and gas-phase changes as brightness differences.
  • Confocal microscopy: Combines fluorescence microscopy with imaging by focusing light and snap shooting instances to form a 3-D image.
  • Transmission electron microscopy: Involves metal staining and the passing of electrons through the cells, which will be deflected upon interaction with metal. This ultimately forms an image of the components being studied.
  • Cytometry: The cells are placed in the machine which uses a beam to scatter the cells based on different aspects and can therefore separate them based on size and content. Cells may also be tagged with GFP-fluorescence and can be separated that way as well.
  • Cell fractionation: This process requires breaking up the cell using high temperature or sonification followed by centrifugation to separate the parts of the cell allowing for them to be studied separately.

Cell types

Cells are of two types: eukaryotic, which contain a nucleus, and prokaryotic cells, which do not have a nucleus, but a nucleoid region is still present. Prokaryotes are single-celled organisms, while eukaryotes may be either single-celled or multicellular.

Prokaryotic cells

Structure of a typical  prokaryotic cell

Prokaryotes  include bacteria  and  archaea,  two of the three domains of life. Prokaryotic cells were the first form of  life  on Earth, characterized  by having  vital biological processes including  cell signaling.  They are simpler and smaller than eukaryotic cells, and lack a  nucleus, and other membrane-bound  organelles. The DNA of a prokaryotic cell consists of a single  circular chromosome  that is in direct contact with the cytoplasm. The nuclear region in the cytoplasm is called the nucleoid. Most prokaryotes are the smallest of all organisms ranging from 0.5 to 2.0 μm in diameter.

A prokaryotic cell has three regions:

  • Enclosing the cell is the  cell envelope  – generally consisting of a  plasma membrane  covered by a  cell wall  which, for some bacteria, may be further covered by a third layer called a  capsule.  Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such as  Mycoplasma  (bacteria) and Thermoplasma  (archaea)  which only possess the cell membrane layer. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. The cell wall consists of  peptidoglycan  in bacteria and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and bursting  (cytolysis)  from  osmotic pressure  due to a hypotonic environment. Some eukaryotic cells (plant cells and fungal cells) also have a cell wall.
  • Inside the cell is the cytoplasmic region that contains the  genome  (DNA), ribosomes and various sorts of inclusions. The genetic material is freely found in the cytoplasm. Prokaryotes can carry  extrachromosomal DNA  elements called  plasmids,  which are usually circular. Linear bacterial plasmids have been identified in several species of  spirochete  bacteria, including members of the genus  Borrelia  notably  Borrelia burgdorferi, which causes Lyme disease.  Though not forming a nucleus, the  DNA  is condensed in a  nucleoid.  Plasmids encode additional genes, such as antibiotic resistance genes.
  • On the outside, flagella  and  pili  project from the cell’s surface. These are structures (not present in all prokaryotes) made of proteins that facilitate movement and communication between cells.

Structure of a typical animal cell

Structure of a typical  plant cell

ProkaryotesEukaryotes
Typical organismsbacteriaarchaeaprotistsfungiplantsanimals
Typical size~ 1–5 μm[21]~ 10–100 μm[21]
Type of nucleusnucleoid region; no true nucleus

true nucleus with double

membrane

DNAcircular (usually)

linear molecules (chromosomes)

with histone proteins

RNA/protein synthesiscoupled in the cytoplasm

RNA synthesis in the nucleus
protein synthesis in the

cytoplasm

Ribosomes50S and 30S60S and 40S
Cytoplasmic structurevery few structures

highly structured by 

endomembranes and a cytoskeleton

Cell movementflagella made of flagellin

flagella and cilia containing microtubules

lamellipodia and filopodia containing actin

Mitochondrianoneone to several thousand
Chloroplastsnonein algae and plants
Organizationusually single cells

single cells, colonies, higher

multicellular organisms with

specialized cells

Cell divisionbinary fission (simple division)mitosis (fission or budding)
meiosis
Chromosomessingle chromosomemore than one chromosome
Membranescell membrane 

Cell Shapes

ProkaryotesEukaryotesTypical organismsbacteriaarchaeaprotistsfungiplantsanimalsTypical size~ 1–5 μm[21]~ 10–100 μm[21]Type of nucleusnucleoid region; no true nucleustrue nucleus with double membraneDNAcircular (usually)

linear molecules (chromosomes)

with histone proteins

RNA/protein synthesiscoupled in the cytoplasmRNA synthesis in the nucleus
protein synthesis in the cytoplasmRibosomes50S and 30S60S and 40SCytoplasmic structurevery few structures

highly structured by endomembranes 

and a cytoskeleton

Cell movementflagella made of flagellin

flagella and cilia containing microtubules; 

lamellipodia and filopodia containing actin

Mitochondrianoneone to several thousand Chloroplastsnonein  algae  and plants  Organizationusually single cells single cells, colonies, higher multicellular organisms with specialized cells. Cell divisionbinary fission (simple division)mitosis (fission or budding)
meiosis Chromosomessingle chromosomemore than one chromosome Membranes  cell  membrane

Cell Shapes

Cell shape also called Cell Morphology has been hypothesized to form from the arrangement and movement of the cytoskeleton.  Many advancements in the study of cell morphology come from studying simple bacteria such as  Staphylococcus aureus,  E. coli,  and  B. subtilis.  Different cell shapes have been found and described but how any why cells form different shapes is still widely unknown.  Cell shapes that have been identified include: rods, cocci, spirochaetes. Cocci have a circular shape, bacilli have an elongated rod-like shape, and spirochaetes have a spiral shape. Although many other shapes have been determined.

Subcellular components

All cells, whether prokaryotic or eukaryotic, have a  membrane  that envelops the cell, regulates what moves in and out (selectively permeable), and maintains the electric potential of the cell. Inside the membrane, the  cytoplasm  takes up most of the cell’s volume.  All cells (except  red blood cells which lack a cell nucleus and most organelles to accommodate maximum space for  hemoglobin)  possess DNA,  the hereditary material of  genes,  and  RNA, containing the information necessary to  build  various  proteins  such as enzymes, the cell’s primary machinery. There are also other kinds of biomolecules in cells. This article lists these primary cellular components, then briefly describes their function.

Cell membrane

Detailed diagram of lipid bilayer of cell membrane

The  cell membrane,  or plasma membrane, is a selectively permeable   biological membrane  that surrounds the cytoplasm of a cell. In animals, the plasma membrane is the outer boundary of the cell, while in plants and prokaryotes it is usually covered by a cell wall. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of phospholipids, which are  amphiphilic  (partly hydrophobic  and partly hydrophilic). Hence, the layer is called a phospholipid bilayer, or sometimes a fluid mosaic membrane. Embedded within this membrane is a macromolecular structure called the  porosome  the universal secretory portal in cells and a variety of  protein  molecules that act as channels and pumps that move different molecules into and out of the cell. 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. Cell surface membranes also contain  receptor  proteins that allow cells to detect external signaling molecules such as hormones.

Cytoskeleton

 

A fluorescent image of an endothelial cell. Nuclei are stained blue, mitochondria are stained red, and microfilaments are stained green.

The cytoskeleton acts to organize and maintain the cell’s shape; anchors organelles in place; helps during  endocytosis, the uptake of external materials by a cell, and  cytokinesis,  the separation of daughter cells after cell division; and moves parts of the cell in processes of growth and mobility. The eukaryotic cytoskeleton is composed of  microtubulesintermediate filaments  and  microfilaments. In the cytoskeleton of a neuron the intermediate filaments are known as neurofilaments. There are a great number of proteins associated with them, each controlling a cell’s structure by directing, bundling, and aligning filaments. The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape,  polarity  and cytokinesis. The subunit protein of microfilaments is a small, monomeric protein called  actin. The subunit of microtubules is a dimeric molecule called  tubulin.  Intermediate filaments are heteropolymers whose subunits vary among the cell types in different tissues. Some of the subunit proteins of intermediate filaments include vimentindesminlamin  (lamins A, B and C),  keratin  (multiple acidic and basic keratins), and neurofilament proteins (NF–L, NF–M).

Genetic material

Two different kinds of genetic material exist:  deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Cells use DNA for their long-term information storage. The biological information contained in an organism is  encoded  in its DNA sequence.  RNA is used for information transport (e.g., mRNA)  and enzymatic  functions  (e.g., ribosomal RNA).  Transfer RNA (tRNA) molecules are used to add amino acids during protein translation.

Prokaryotic genetic material is organized in a simple circular bacterial chromosome in the nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different.  linear molecules called chromosomes inside a discrete nucleus, usually with additional genetic material in some organelles like  mitochondria  and chloroplasts  (see endosymbiotic theory).

human cell has genetic material contained in the  cell nucleus  (the nuclear genome) and in the mitochondria (the mitochondrial genome). In humans, the nuclear genome is divided into 46 linear DNA molecules called  chromosomes, including 22  homologous chromosome  pairs and a pair of  sex chromosomes.  The mitochondrial genome is a circular DNA molecule distinct from nuclear DNA. Although the  mitochondrial DNA  is very small compared to nuclear chromosomes,  it codes for 13 proteins involved in mitochondrial energy production and specific tRNAs.

Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called  transfection.  This can be transient, if the DNA is not inserted into the cell’s genome, or stable, if it is. Certain  viruses also insert their genetic material into the genome.

Organelles

Organelles are parts of the cell that are adapted and/or specialized for carrying out one or more vital functions, analogous to the  organs  of the human body (such as the heart, lung, and kidney, with each organ performing a different function).  Both eukaryotic and prokaryotic cells have organelles, but prokaryotic organelles are generally simpler and are not membrane-bound.

There are several types of organelles in a cell. Some (such as the  nucleus  and  Golgi apparatus) are typically solitary, while others (such as  mitochondriachloroplastsperoxisomes  and  lysosomes) can be numerous (hundreds to thousands). The  cytosol is the gelatinous fluid that fills the cell and surrounds the organelles.

Eukaryotic

Human cancer cells, specifically  HeLa cells, with DNA stained blue. The central and rightmost cell are in  interphase, so their DNA is diffuse and the entire nuclei are labelled.  The cell on the left is going through  mitosis  and its chromosomes have condensed.

  • Cell nucleus: A cell’s information center, the  cell nucleus  is the most conspicuous organelle found in a eukaryotic cell. It houses the cell’s  chromosomes,  and is the place where almost all  DNA  replication and RNA  synthesis  (transcription)  occur. The nucleus is spherical and separated from the cytoplasm by a double membrane called the  nuclear envelope,  space between these two membrane is called perinuclear space. The nuclear envelope isolates and protects a cell’s DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, DNA  is  transcribed, or copied into a special RNA, called  messenger  RNA   (mRNA). This mRNA is then transported  out of the nucleus, where it is translated into a specific protein molecule. The  nucleolus  is a specialized region within the nucleus where ribosome subunits are assembled. In prokaryotes, DNA processing takes place in the  cytoplasm.
  • Mitochondria and chloroplasts: generate energy for the cell.  Mitochondria  are self-replicating double membrane-bound organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells.  Respiration  occurs in the cell mitochondria, which generate the cell’s energy by  oxidative phosphorylation,  using  oxygen to release energy stored in cellular nutrients (typically pertaining to  glucose)  to generate  ATP(aerobic respiration). Mitochondria multiply by binary fission, like prokaryotes. Chloroplasts can only be found in plants and algae, and they capture the sun’s energy to make carbohydrates through  photosynthesis.

Diagram of the endomembrane system

  • Endoplasmic reticulum: The  endoplasmic reticulum  (ER) is a  transport network  for molecules targeted for certain modifications and specific destinations, as compared to molecules that float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface that secrete proteins into the ER, and the smooth ER, which lacks ribosomes. The smooth ER plays a role in calcium sequestration and release and also helps in synthesis of  lipid.
  • Golgi apparatus: The primary function of the Golgi apparatus is to process and package the  macromolecules  such as  proteins  and  lipids that are synthesized by the cell.
  • Lysosomes and peroxisomesLysosomes  contain digestive enzymes  (acid hydrolases).  They digest excess or worn-out  organelles,  food particles, and engulfed  viruses  or  bacteriaPeroxisomes  have enzymes that rid the cell of toxic  peroxides, Lysosomes are optimally active at  acidic pH.  The cell could not house these destructive enzymes if they were not contained in a membrane-bound system. 
  • Centrosome: the cytoskeleton organiser: The centrosome   produces the  microtubules of a cell – a key component of the  cytoskeleton.  It directs the transport through the  ER  and the  Golgi apparatus. Centrosomes are composed of two  centrioles   which lie perpendicular to each other in which each has an organisation like a  cartwheel, which separate during   cell division  and help in the formation of the  mitotic spindle.  A single centrosome is present in the animal cells. They are also found in some fungi and algae cells.
  • VacuolesVacuoles  sequester waste products and in plant cells store water. They are often described as liquid filled spaces and are surrounded by a membrane. Some cells, most notably  Amoeba, have contractile vacuoles, which can pump water out of the cell if there is too much water. The vacuoles of plant cells and fungal cells are usually larger than those of animal cells. Vacuoles of plant cells are surrounded by  tonoplast  which helps in transport of ions and other substances against concentration gradients.

Eukaryotic and prokaryotic

  • Ribosomes: The  ribosome  is a large complex of  RNA  and  protein  molecules.  They each consist of two subunits,  and act as an assembly line where RNA from the nucleus is used to synthesise proteins from amino acids. Ribosomes can be found either floating freely or bound to a membrane (the rough endoplasmatic reticulum in eukaryotes, or the cell membrane in prokaryotes).
  • PlastidsPlastid  are membrane-bound organelle generally found in plant cells and  euglenoids  and contain specific pigments, thus affecting the colour of the plant and organism. And these pigments also helps in food storage and tapping of light energy. There are three types of plastids based upon the specific pigments.  Chloroplasts (contains  chlorophyll  and some carotenoid pigments which helps in the tapping of light energy during photosynthesis), Chromoplasts (contains fat-soluble  carotenoid  pigments like orange carotene and yellow xanthophylls which helps in synthesis and storage), Leucoplasts (are non-pigmented plastids and helps in storage of nutrients).

Structures outside the cell membrane

Many cells also have structures which exist wholly or partially outside the cell membrane. These structures are notable because they are not protected from the external environment by the  semipermeable cell membrane.  In order to assemble these structures, their components must be carried across the cell membrane by export processes.

Cell wall

Many types of prokaryotic and eukaryotic cells have a  cell wall.  The cell wall acts to protect the cell mechanically and chemically from its environment, and is an additional layer of protection to the cell membrane. Different types of cell have cell walls made up of different materials; plant cell walls are primarily made up of  cellulose,  fungi cell walls are made up of  chitin  and bacteria cell walls are made up of peptidoglycan.

Prokaryotic

Capsule

A gelatinous capsule is present in some bacteria outside the cell membrane and cell wall. The capsule may be polysaccharide as in  pneumococcimeningococci  or polypeptide  as  Bacillus anthracis  or  hyaluronic acid  as in streptococci.  Capsules are not marked by normal staining protocols and can be detected by  India ink  or  methyl blue; which allows for higher contrast between the cells for observation.

Flagella

Flagella  are organelles for cellular mobility. The bacterial flagellum stretches from cytoplasm through the cell membrane(s) and extrudes through the cell wall. They are long and thick thread-like appendages, protein in nature. A different type of flagellum is found in archaea and a different type is found in eukaryotes.

Fimbriae

fimbria  (plural fimbriae also known as a  pilus, plural pili) is a short, thin, hair-like filament found on the surface of bacteria. Fimbriae are formed of a protein called  pilin  (antigenic)  and are responsible for the attachment of bacteria to specific receptors on human cells  (cell adhesion). There are special types of pili involved in bacterial conjugation.

Cellular processes

Prokaryotes divide by  binary fission, while  eukaryotes divide by  mitosis  or meiosis.

Replication

Cell division involves a single cell (called a mother cell) dividing into two daughter cells. This leads to growth in multicellular organisms (the growth of tissue) and to procreation (vegetative reproduction) in unicellular organismsProkaryotic cells divide by binary fission, while eukaryotic cells usually undergo a process of nuclear division, called mitosis, followed by division of the cell, called  cytokinesis.  A  diploid  cell may also undergo meiosis  to produce haploid cells, usually four. Haploid cells serve as  gametes in multicellular organisms, fusing to form new diploid cells.

DNA replication,  or the process of duplicating a cell’s genome, always happens when a cell divides through mitosis or binary fission. This occurs during the S phase of the cell cycle.

In meiosis, the DNA is replicated only once, while the cell divides twice. DNA replication only occurs before meiosis I. DNA replication does not occur when the cells divide the second time, in  meiosis II. Replication, like all cellular activities, requires specialized proteins for carrying out the job. An outline of the catabolism of proteinscarbohydrates and fats

DNA repair

In general, cells of all organisms contain enzyme systems that scan their DNA for DNA damage and carry out repair processes when damage is detected.  Diverse repair processes have evolved in organisms ranging from bacteria to humans. The widespread prevalence of these repair processes indicates the importance of maintaining cellular DNA in an undamaged state in order to avoid cell death or errors of replication due to damage that could lead to mutationE. coli  bacteria are a well-studied example of a cellular organism with diverse well-defined  DNA repair  processes. These include: (1)  nucleotide excision repair, (2)  DNA mismatch repair, (3) non-homologous end joining  of double-strand breaks, (4) recombinational repair  and (5) light-dependent repair (photoreactivation).

Growth and metabolism

An overview of protein synthesis. Within the nucleus of the cell (light blue),  genes  (DNA, dark blue) are  transcribed  into  RNA. This RNA is then subject to post-transcriptional modification and control, resulting in a mature  mRNA  (red) that is then transported out of the nucleus and into the  cytoplasm  (peach), where it undergoes  translation  into a protein. mRNA is translated by  ribosomes  (purple)  that match the three-base  codons of the mRNA to the three-base anti-codons of the appropriate  tRNA. Newly synthesized proteins (black) are often further modified, such as by binding to an effector molecule (orange), to become fully active.

Between  successive  cell divisions, cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions:  catabolism,  in which the cell breaks down complex molecules to produce energy and  reducing power, and  anabolism, in which the cell uses energy and reducing power to construct complex molecules and perform other biological functions. Complex sugars consumed by the organism can be broken down into simpler sugar molecules called monosaccharides such as glucose. Once inside the cell, glucose is broken down to make adenosine triphosphate (ATP),  a molecule that possesses readily available energy, through two different pathways.

Protein synthesis

Cells are capable of synthesizing new proteins, which are essential for the modulation and maintenance of cellular activities. This process involves the formation of new protein molecules from  amino acid  building blocks based on information encoded in DNA/RNA. Protein synthesis generally consists of two major steps:  transcription  and  translation.

Transcription is the process where genetic information in DNA is used to produce a complementary RNA strand. This RNA strand is then processed to give  messenger RNA   (mRNA) , which is free to migrate through the cell. mRNA molecules bind to protein-RNA complexes called  ribosomes  located in the  cytosol,  where they are translated into polypeptide sequences. The ribosome mediates the formation of a polypeptide sequence based on the mRNA sequence. The mRNA sequence directly relates to the polypeptide sequence by binding to  transfer RNA  (tRNA) adapter molecules in binding pockets within the ribosome. The new polypeptide then folds into a functional three-dimensional protein molecule.

Motility

Unicellular organisms can move in order to find food or escape predators. Common mechanisms of motion include  flagella  and  cilia.

In multicellular organisms, cells can move during processes such as wound healing, the immune response and  cancer metastasis.  For example, in wound healing in animals, white blood cells move to the wound site to kill the microorganisms that cause infection. Cell motility involves many receptors, crosslinking, bundling, binding, adhesion, motor and other proteins. The process is divided into three steps – protrusion of the leading edge of the cell, adhesion of the leading edge and de-adhesion at the cell body and rear, and cytoskeletal contraction to pull the cell forward. Each step is driven by physical forces generated by unique segments of the cytoskeleton.

Navigation, control and communication

In August 2020, scientists described one way cells – in particular cells of a slime mold and mouse pancreatic cancer–derived cells – are able to navigate efficiently through a body and identify the best routes through complex mazes: generating gradients after breaking down diffused chemoattractants which enable them to sense upcoming maze junctions before reaching them, including around corners.

Multicellularity

Cell specialization/differentiation

 

Staining of a Caenorhabditis elegans highlights the nuclei of its cells. Multicellular organisms are  organisms  that consist of more than one cell, in contrast to  single-celled organisms.

In complex multicellular organisms,  cells specialize into different  cell types  that are adapted to particular functions. In mammals, major cell types include skin cellsmuscle cellsneuronsblood cellsfibroblastsstem cells, and others. Cell types differ both in appearance and function, yet are genetically identical. Cells are able to be of the same genotype but of different cell type due to the differential  expression  of the  genes  they contain.

Most distinct cell types arise from a single  totipotent cell, called a  zygote, that  differentiates into hundreds of different cell types during the course of development. Differentiation of cells is driven by different environmental cues (such as cell–cell interaction) and intrinsic differences (such as those caused by the uneven distribution of molecules during division).

Origin of multicellularity

Multicellularity has evolved independently at least 25 times,[37] including in some prokaryotes, like cyanobacteriamyxobacteriaactinomycetesMagnetoglobus multicellularis, or Methanosarcina. However, complex multicellular organisms evolved only in six eukaryotic groups: animals, fungi, brown algae, red algae, green algae, and plants.[38] It evolved repeatedly for plants (Chloroplastida), once or twice for animals, once for brown algae, and perhaps several times for fungislime molds, and red algae.[39] Multicellularity may have evolved from  colonies  of interdependent organisms, from  cellularization, or from organisms in  symbiotic relationships.

The first evidence of multicellularity is from cyanobacteria-like organisms that lived between 3 and 3.5 billion years ago. Other early fossils of multicellular organisms include the contested  Grypania  spiralis and the fossils of the black shales of the Palaeoproterozoic  Francevillian Group Fossil  B Formation in  Gabon. 

The evolution of multicellularity from unicellular ancestors has been replicated in the laboratory, in evolution experiments using predation as the selective pressure.[37]

Origins

The origin of cells has to do with the  origin of life, which began the  history of life  on Earth.

Origin of the first cell

Stromatolites are left behind  by  cyanobacteria,  also called  blue-green algae. They are the oldest known fossils of life on Earth. This one-billion-year-old fossil is from Glacier National Park  in the United States.

There are several theories about the origin of small molecules that led to life on the  early Earth.  They may have been carried to Earth on meteorites (see  Murchison meteorite), created at  deep-sea vents,  or synthesized by lightning in a reducing atmosphere (see Miller–Urey experiment).  There is little experimental data defining what the first self-replicating forms were.  RNA  is thought to be the earliest self-replicating molecule, as it is capable of both storing genetic information and catalyzing chemical reactions (see  RNA world hypothesis), but some other entity with the potential to self-replicate could have preceded RNA, such as clay or peptide nucleic acid.

Cells emerged at least 3.5 billion years ago. The current belief is that these cells were heterotrophs. The early cell membranes were probably more simple and permeable than modern ones, with only a single fatty acid chain per lipid. Lipids are known to spontaneously form bilayered vesicles in water, and could have preceded RNA, but the first cell membranes could also have been produced by catalytic RNA, or even have required structural proteins before they could form. 

Origin of eukaryotic cells

The  eukaryotic cell seems to have evolved from a  symbiotic community  of prokaryotic cells. DNA-bearing organelles like the  mitochondria and the   chloroplasts  are descended from ancient symbiotic oxygen-breathing Alphaproteobacteria  and ” Cyanobacteria“,  respectively, which were endosymbiosed  by an ancestral  archaean  prokaryote.

There is still considerable debate about whether organelles like the hydrogenosome predated the origin of mitochondria, or vice versa: see the hydrogen hypothesis for the origin of eukaryotic cells.

History of research

Robert Hooke’s drawing of cells in cork, 1665

See also

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Cell types

A drawing of a prokaryotic cell. There are two fundamental classifications of cells:  prokaryotic  and eukaryotic.  Prokaryotic cells are distinguished from eukaryotic cells by the absence of a cell nucleus  or other membrane-bound organelle. Prokaryotic cells are much smaller than eukaryotic cells, making them the smallest form of life. Prokaryotic cells include  Bacteria and  Archaea,  and lack an enclosed cell nucleus.  Eukaryotic cells are found in plants, animals, fungi, and protists. They range from 10–100 μm in diameter, and their DNA is contained within a membrane-bound nucleus. Eukaryotes are organisms containing eukaryotic cells. The four eukaryotic kingdoms are Animalia, Plantae, Fungi, and Protista.

They both reproduce through  binary fission. Bacteria, the most prominent type, have several  different shapes, although most are  spherical or  rod-shaped. Bacteria can be classed as either  gram-positive or  gram-negative depending on the  cell wall composition. Gram-positive bacteria have a thicker  peptidoglycan layer than gram-negative bacteria. Bacterial structural features include a  flagellum  that helps the cell to move,  ribosomes  for the translation of RNA to protein, and a  nucleoid  that holds all the genetic material in a circular structure. There are many processes that occur in prokaryotic cells that allow them to survive.  In prokaryotes, mRNA synthesis is initiated at a promoter sequence on the DNA template comprising two consensus sequences that recruit RNA polymerase. The prokaryotic polymerase consists of a core enzyme of four protein subunits and a σ protein that assists only with initiation. For instance, in a process termed  conjugation,  the fertility factor allows the bacteria to possess a pilus which allows it to transmit DNA to another bacteria which lacks the F factor, permitting the transmittance of resistance allowing it to survive in certain environments. 

Structure and function 

Structure of eukaryotic cells 

A diagram of an animal cell

Eukaryotic cells  are  composed of the following organelles:

  • Nucleus: The nucleus of the cell functions as the  genome  and genetic information storage for the cell, containing all the  DNA  organized in the form of chromosomes.  It is surrounded by a  nuclear envelope,  which includes nuclear pores allowing for the transportation of proteins between the inside and outside of the nucleus.  This is also the site for replication of DNA as well as transcription of DNA to RNA. Afterwards, the RNA is modified and transported out to the cytosol to be translated to protein. 
  • Nucleolus: This structure is within the nucleus, usually dense and spherical in shape. It is the site of ribosomal RNA (rRNA) synthesis, which is needed for ribosomal assembly.
  • Endoplasmic reticulum (ER): This functions to synthesize, store, and secrete proteins to the Golgi apparatus. Structurally, the endoplasmic reticulum is a network of membranes found throughout the cell and connected to the nucleus. The membranes are slightly different from cell to cell and a cell’s function determines the size and structure of the ER. 
  • Mitochondria: Commonly known as the powerhouse of the cell is a double membrane bound cell organelle.  This functions for the production of energy or ATP within the cell. Specifically, this is the place where the Krebs cycle or TCA cycle for the production of NADH and FADH occurs. Afterwards, these products are used within the electron transport chain (ETC) and oxidative phosphorylation for the final production of ATP.   Golgi apparatus:  This functions to further process, package, and secrete the proteins to their destination. The proteins contain a signal sequence that allows the Golgi apparatus to recognize and direct it to the correct place. Golgi apparatus also produce glycoproteins and  glycolipids
  • Lysosome:  The lysosome functions to degrade material brought in from the outside of the cell or old organelles. This contains many acid hydrolases, proteases, nucleases, and lipases, which break down the various molecules. Autophagy is the process of degradation through lysosomes which occurs when a vesicle buds off from the ER and engulfs the material, then, attaches and fuses with the lysosome to allow the material to be degraded. 
  • Ribosomes:  Functions to translate RNA to protein. it serves as a site of protein synthesis. 
  • Cytoskeleton:  Cytoskeleton is a structure that helps to maintain the shape and general organization of the cytoplasm. It anchors organelles within the cells and makes up the structure and stability of the cell. The cytoskeleton is composed of three principal types of protein filaments: actin filaments, intermediate filaments, and microtubules, which are held together and linked to subcellular organelles and the plasma membrane by a variety of accessory proteins. 
  • Cell membrane: The cell membrane can be described as a phospholipid bilayer and is also consisted of lipids and proteins. Because the inside of the bilayer is hydrophobic and in order for molecules to participate in reactions within the cell, they need to be able to cross this membrane layer to get into the cell via  osmotic pressurediffusion,  concentration gradients, and membrane channels. 
  • Centrioles: Function to produce spindle fibers which are used to separate chromosomes during cell division.

Eukaryotic cells may also be composed of the following molecular components:

  • Chromatin: This makes up chromosomes and is a mixture of DNA with various proteins.
  • Cilia: They help to propel substances and can also be used for sensory purposes. 

Cell metabolism 

Cell metabolism is necessary for the production of energy for the cell and therefore its survival and includes many pathways. For  cellular respiration, once glucose is available, glycolysis occurs within the cytosol of the cell to produce pyruvate. Pyruvate undergoes decarboxylation using the multi-enzyme complex to form acetyl coA which can readily be used in the  TCA cycle  to produce NADH and FADH2. These products are involved in the  electron transport chain  to ultimately form a proton gradient across the inner mitochondrial membrane. This gradient can then drive the production of ATP and H2O during  oxidative phosphorylation.  Metabolism in plant cells includes  photosynthesis  which is simply the exact opposite of respiration as it ultimately produces molecules of glucose.

Cell signaling 

Cell signaling or cell communication is important for cell regulation and for cells to process information from the environment and respond accordingly. Signaling can occur through direct cell contact or  endocrineparacrine,  and  autocrine signaling.  Direct cell-cell contact is when a receptor on a cell binds a molecule that is attached to the membrane of another cell. Endocrine signaling occurs through molecules secreted into the bloodstream. Paracrine signaling uses molecules diffusing between two cells to communicate. Autocrine is a cell sending a signal to itself by secreting a molecule that binds to a receptor on its surface. Forms of communication can be through:

  • Ion channels: Can be of different types such as voltage or ligand gated ion channels. They allow for the outflow and inflow of molecules and ions.
  • G-protein coupled receptor (GPCR): Is widely recognized to contain seven transmembrane domains. The ligand binds on the extracellular domain and once the ligand binds, this signals a guanine exchange factor to convert GDP to GTP and activate the G-α subunit. G-α can target other proteins such as adenyl cyclase or phospholipase C, which ultimately produce secondary messengers such as cAMP, Ip3, DAG, and calcium. These secondary messengers function to amplify signals and can target ion channels or other enzymes. One example for amplification of a signal is cAMP binding to and activating PKA by removing the regulatory subunits and releasing the catalytic subunit. The catalytic subunit has a nuclear localization sequence which prompts it to go into the nucleus and phosphorylate other proteins to either repress or activate gene activity. 
  • Receptor tyrosine kinases: Bind growth factors, further promoting the tyrosine on the intracellular portion of the protein to cross phosphorylate. The phosphorylated tyrosine becomes a landing pad for proteins containing an SH2 domain allowing for the activation of Ras and the involvement of the  MAP kinase pathway. 

Growth and development 

Eukaryotic cell cycle 

 

The process of  cell  division in the animal cell cycle

Cells are the foundation of all organisms and are the fundamental units of life. The growth and development of cells are essential for the maintenance of the host and survival of the organism. For this process, the cell goes through the steps of the  cell cycle  and development which involves cell growth,  DNA replicationcell division, regeneration, and  cell death.

The cell cycle is divided into four distinct  phases: G1, S, G2, and M. The G phase – which is the cell growth phase – makes up approximately 95% of the cycle. The proliferation of cells is instigated by progenitors. All cells start out in an identical form and can essentially become any type of cells. Cell signaling such as induction can influence nearby cells to determinate the type of cell it will become. Moreover, this allows cells of the same type to aggregate and form tissues, then organs, and ultimately systems. The G1, G2, and S phase (DNA replication, damage and repair) are considered to be the interphase portion of the cycle, while the M phase (mitosis) is the cell division portion of the cycle. Mitosis is composed of many stages which include, prophase, metaphase, anaphase, telophase, and cytokinesis, respectively. The ultimate result of mitosis is the formation of two identical daughter cells.

The cell cycle is regulated in  cell cycle checkpoints, by a series of signaling factors and complexes such as cyclins,  cyclin-dependent kinase, and  p53. When the cell has completed its growth process and if it is found to be damaged or altered, it undergoes cell death, either by  apoptosis  or  necrosis,  to eliminate the threat it can cause to the organism’s survival.

Cell mortality, cell lineage immortality 

The ancestry of each present day cell presumably traces back, in an unbroken lineage for over 3 billion years to the origin of life. It is not actually cells that are immortal but multi-generational cell lineages. The immortality of a cell lineage depends on the maintenance of  cell division  potential. This potential may be lost in any particular lineage because of cell damage, terminal differentiation as occurs in nerve cells, or programmed cell death (apoptosis) during development. Maintenance of cell division potential over successive generations depends on the avoidance and the accurate repair of cellular damage, particularly  DNA damage. In sexual organisms, continuity of the  germline depends on the effectiveness of processes for avoiding DNA damage and  repairing those DNA damages  that do occur. Sexual processes in eukaryotes, as well as in prokaryotes, provide an opportunity for effective repair of DNA damages in the germ line by  homologous recombination.

Cell cycle phases 

The cell cycle is a four-stage process that a cell goes through as it develops and divides. It includes Gap 1 (G1), synthesis (S), Gap 2 (G2), and mitosis (M).The cell either restarts the cycle from G1 or leaves the cycle through G0 after completing the cycle. The cell can progress from G0 through terminal differentiation.

The interphase refers to the phases of the cell cycle that occur between one mitosis and the next, and includes G1, S, and G2.

G1 phase

The size of the cell grows.

The contents of cells are replicated.

S phase

Replication of DNA

The cell replicates each of the 46 chromosomes (23 pairs).

G2 phase

The cell multiplies.

In preparation for cell division, organelles and proteins form.

M phase 

After mitosis, cytokinesis occurs (cell separation)

Formation of two daughter cells that are identical

G0 phase 

These cells leave G1 and enter G0, a resting stage. A cell in G0 is doing its job without actively preparing to divide.

Pathology 

The scientific branch that studies and diagnoses diseases on the cellular level is called cytopathology. Cytopathology is generally used on samples of free cells or tissue fragments, in contrast to the pathology branch of histopathology, which studies whole tissues. Cytopathology is commonly used to investigate diseases involving a wide range of body sites, often to aid in the diagnosis of cancer but also in the diagnosis of some infectious diseases and other inflammatory conditions. For example, a common application of cytopathology is the Pap smear, a  screening test  used to detect  cervical cancer,  and  precancerous cervical lesions that may lead to cervical cancer. 

Cell cycle checkpoints and DNA damage repair system 

The cell cycle is composed of a number of well-ordered, consecutive stages that result in cellular division. The fact that cells do not begin the next stage until the last one is finished, is a significant element of cell cycle regulation. Cell cycle checkpoints are characteristics that constitute an excellent monitoring strategy for accurate cell cycle and divisions. Cdks, associated cyclin counterparts, protein kinases, and phosphatases regulate cell growth and division from one stage to another.  The cell cycle is controlled by the temporal activation of Cdks, which is governed by cyclin partner interaction, phosphorylation by particular protein kinases, and de-phosphorylation by Cdc25 family phosphatases. In response to DNA damage, a cell’s DNA repair reaction is a cascade of signaling pathways that leads to checkpoint engagement, regulates, the repairing mechanism in DNA, cell cycle alterations, and apoptosis. Numerous biochemical structures, as well as processes that detect damage in DNA, are ATM and ATR, which induce the DNA repair checkpoints[34]

The cell cycle is a sequence of activities in which cell organelles are duplicated and subsequently separated into daughter cells with precision. There are major events that happen during a cell cycle. The processes that happen in the cell cycle include cell development, replication and segregation of chromosomes.  The cell cycle checkpoints are surveillance systems that keep track of the cell cycle’s integrity, accuracy, and chronology. Each checkpoint serves as an alternative cell cycle endpoint, wherein the cell’s parameters are examined and only when desirable characteristics are fulfilled does the cell cycle advance through the distinct steps.The cell cycle’s goal is to precisely copy each organism’s DNA and afterwards equally split the cell and its components between the two new cells. Four main stages occur in the eukaryotes. In G1, the cell is usually active and continues to grow rapidly, while in G2, the cell growth continues while protein molecules become ready for separation. These are not dormant times; they are when cells gain mass, integrate growth factor receptors, establish a replicated genome, and prepare for chromosome segregation. DNA replication is restricted to a separate Synthesis in eukaryotes, which is also known as the S-phase. During mitosis, which is also known as the M-phase, the segregation of the chromosomes occur.  DNA, like every other molecule, is capable of undergoing a wide range of chemical reactions. Modifications in DNA’s sequence, on the other hand, have a considerably bigger impact than modifications in other cellular constituents like RNAs or proteins because DNA acts as a permanent copy of the cell genome. When erroneous nucleotides are incorporated during DNA replication, mutations can occur. The majority of DNA damage is fixed by removing the defective bases and then re-synthesizing the excised area. On the other hand, some DNA lesions can be mended by reversing the damage, which may be a more effective method of coping with common types of DNA damage. Only a few forms of DNA damage are mended in this fashion, including pyrimidine dimers caused by ultraviolet (UV) light changed by the insertion of methyl or ethyl groups at the purine ring’s O6 position. 

Mitochondrial membrane dynamics 

Mitochondria are commonly referred to as the cell’s “powerhouses” because of their capacity to effectively produce ATP which is essential to maintain cellular homeostasis and metabolism. Moreover, researchers have gained a better knowledge of mitochondria’s significance in cell biology because of the discovery of cell signaling pathways by mitochondria which are crucial platforms for cell function regulation such as apoptosis. Its physiological adaptability is strongly linked to the cell mitochondrial channel’s ongoing reconfiguration through a range of mechanisms known as mitochondrial membrane dynamics, which include endomembrane fusion and fragmentation (separation) as well as ultrastructural membrane remodeling. As a result, mitochondrial dynamics regulate and frequently choreograph not only metabolic but also complicated cell signaling processes such as cell pluripotent stem cells, proliferation, maturation, aging, and mortality. Mutually, post-translational alterations of mitochondrial apparatus and the development of transmembrane contact sites among mitochondria and other structures, which both have the potential to link signals from diverse routes that affect mitochondrial membrane dynamics substantially,  Mitochondria are wrapped by two membranes: an inner mitochondrial membrane (IMM) and an outer mitochondrial membrane (OMM), each with a distinctive function and structure, which parallels their dual role as cellular powerhouses and signaling organelles. The inner mitochondrial membrane divides the mitochondrial lumen into two parts: the inner border membrane, which runs parallel to the OMM, and the cristae, which are deeply twisted, multinucleated invaginations that give room for surface area enlargement and house the mitochondrial respiration apparatus. The outer mitochondrial membrane, on the other hand, is soft and permeable. It, therefore, acts as a foundation for cell signaling pathways to congregate, be deciphered, and be transported into mitochondria. Furthermore, the OMM connects to other cellular organelles, such as the endoplasmic reticulum (ER), lysosomes, endosomes, and the plasma membrane. Mitochondria play a wide range of roles in cell biology, which is reflected in their morphological diversity. Ever since the beginning of the mitochondrial study, it has been well documented that mitochondria can have a variety of forms, with both their general and ultra-structural morphology varying greatly among cells, during the cell cycle, and in response to metabolic or cellular cues. Mitochondria can exist as independent organelles or as part of larger systems; they can also be unequally distributed in the cytosol through regulated mitochondrial transport and placement to meet the cell’s localized energy requirements. Mitochondrial dynamics refers to the adaptive and variable aspect of mitochondria, including their shape and subcellular distribution.Autophagy 

Autophagy is a self-degradative mechanism that regulates energy sources during growth and reaction to dietary stress. Autophagy also cleans up after itself, clearing aggregated proteins, cleaning damaged structures including mitochondria and endoplasmic reticulum and eradicating intracellular infections. Additionally, autophagy has antiviral and antibacterial roles within the cell, and it is involved at the beginning of distinctive and adaptive immune responses to viral and bacterial contamination. Some viruses include virulence proteins that prevent autophagy, while others utilize autophagy elements for intracellular development or cellular splitting.  Macro autophagy, micro autophagy, and chaperon-mediated autophagy are the three basic types of autophagy. When macro autophagy is triggered, an exclusion membrane incorporates a section of the cytoplasm, generating the autophagosome, a distinctive double-membraned organelle. The autophagosome then joins the lysosome to create an autolysosome, with lysosomal enzymes degrading the components. In micro autophagy, the lysosome or vacuole engulfs a piece of the cytoplasm by invaginating or protruding the lysosomal membrane to enclose the cytosol or organelles. The chaperone-mediated autophagy  (CMA)  protein  quality  assurance by digesting oxidized and altered proteins under stressful circumstances and supplying amino acids through protein denaturation.  Autophagy is the primary intrinsic degradative system for peptides, fats, carbohydrates, and other cellular structures. In both physiologic and stressful situations, this cellular progression is vital for upholding the correct cellular balance. Autophagy instability leads to a variety of illness symptoms, including inflammation, biochemical disturbances, aging, and neurodegenerative, due to its involvement in controlling cell integrity. The modification of the autophagy-lysosomal networks is a typical hallmark of many neurological and muscular illnesses. As a result, autophagy has been identified as a potential strategy for the prevention and treatment of various disorders. Many of these disorders are prevented or improved by consuming polyphenol in the meal. As a result, natural compounds with the ability to modify the autophagy mechanism are seen as a potential therapeutic option. The creation of the double membrane (phagophore), which would be known as nucleation, is the first step in macro-autophagy. The phagophore approach indicates dysregulated polypeptides or defective organelles that come from the cell membrane, Golgi apparatus, endoplasmic reticulum, and mitochondria. With the conclusion of the autophagocyte, the phagophore’s enlargement comes to an end. The auto-phagosome combines with the lysosomal vesicles to formulate an auto-lysosome that degrades the encapsulated substances, referred to as phagocytosis.

Notable cell biologists 

See also 

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