Why are cells important

Cytology: structure and functions of the cell

The cell membrane, also called plasmalemm, delimits the cytoplasm and serves to delimit the intra- and extracellular space. It consists of a phospholipid bilayer, the hydrophilic components of the phospholipids after intra- or. show extracellularly and the hydrophobic parts are centrally located in the membrane.

On the outside you can find the Glycocalyx, consisting of sugar chains (polysaccharides) that are covalently bound to the membrane proteins (glycoproteins) and membrane lipids (glycolipids). It is individual and cell type specific, i. H. it determines, for example, the blood group properties on the erythrocytes.

The cell membrane is stable and flexible at the same time, guaranteed by the fluidity of the membrane, which can change with temperature and lipid composition. she is semi-permeable (also called selectively permeable), which means that it is well permeable to small-molecule substances such as water, so that they can migrate osmotically. However, larger molecular substances such as proteins require specific transport systems in order to be able to pass through the membrane.

The functionality of the cell membrane is largely determined by its Membrane proteins These include: ion channels, cell adhesion molecules, aquaporins, membrane pumps, carrier and receptor proteins.

Structure and function of the cell nucleus

The nucleus contains the DNA, packed in chromosomes and can be variable in size and structure depending on the state of activity. The karyoplasm is separated from the cytoplasm by the pore-containing nuclear membrane (karyolemm).

The Nuclear membrane consists of an inner and outer nuclear membrane and the space in between, the perinuclear cistern. The outer nuclear membrane merges smoothly into the rough endoplasmic reticulum and is covered with ribosomes. The inner nuclear membrane is surrounded by the felt-like nuclear lamina (lamina nuclearis), which is formed from a 30-100 nm thick layer of intermediate filaments.

The approx. 1000-4000 nuclear pores serve the exchange of substances between the cyto- and karyoplasm, whereby molecules <5 kDa diffuse freely and larger molecules such as proteins are bound in and out of the receptor.

The nucleus contains a small spherical core (nucleolus) in which the ribosomal RNA is created. Transcription as a prerequisite for translation and replication as a prerequisite for mitosis are also regulated in the cell nucleus.

Function of the cytoplasm

The cytoplasm - also called cytosol - is bounded by the cell membrane and represents the basic liquid substance of every cell The cytoskeleton, the cell organelles and cell inclusions are embedded in it.

It is the place of protein biosynthesis and the transport space for ion currents and the transport of vesicles between the Golgi apparatus, endoplasmic reticulum and the cell membrane. It makes up 50% of the cell volume and the pH is 7.2.

Classification of cell organelles

Cell organelles are embedded in the cytoplasm and are divided into:

  • Membrane-bounded organelles (rough and smooth endoplasmic reticulum, Golgi apparatus, mitochondria, lysosomes, peroxisomes)
  • Organelles that are not membrane-bounded such as ribosomes or centrioles

Structure and function of the individual cell organelles

Image: “Prototypical Human Cell” by philschatz. License: CC BY 4.0

Endoplasmic Reticulum (ER)

This is a tubular membrane system.

Image: “Endoplasmic Reticulum (ER)” by Phil Schatz. License: CC BY 4.0

The rough ER is occupied with ribosomes and is used for protein synthesis for endosomes, transmembrane proteins or secretory granules.

The smooth ER is not occupied by ribosomes and has a wide variety of tasks:

  • Calcium storage in smooth and striated muscle cells (here called sarcoplasmic reticulum)
  • Synthesis of lipids and steroid hormones
  • Detoxification of the body's own and foreign substances within the liver epithelial cells

Golgi apparat

This membrane stack of dictyosomes (a stack of four to ten membrane-covered, plate-shaped cavities) has a convex cis region and a concave trans regionfacing each other.

The proteins formed by the rough ER reach the Cis-Golgi with the help of transport vesicles and are then modified and processed within the Golgi apparatus (phosphorylation, sulfation, glycolysis) and sorted according to the destination.

At the Trans side the packaging in secretion granules or vesicles then takes place again. For enzymes that e.g. B. are required in the ER, a retrograde (trans to cis) transport could also be detected.

Mitochondria

These “power plants” of the cell, which are popular in the test serve to provide energy through oxidative phosphorylation. Mitochondria are found in almost all cells, with the exception of mature erythrocytes.

Mitochondria have two membranes and the intermembrane space in between. The smooth outer membrane contains porins that can pass molecules up to 10 kDa and the inner mitochondrial membrane is strongly folded according to the principle of surface enlargement, delimits the matrix space and carries the enzymes of the respiratory chain and ATP synthesis.

A basic distinction is made based on the folding of the inner membrane two types:

  1. Crista type: in metabolically active cells such as heart muscle cells
  2. Tubular type: in steroid producing cells

in the Matrix space the enzymes of ß-oxidation and the citric acid cycle are localized.

Mitochondria are semi-autonomous, because they have their own circular DNA (mtDNA). The endosymbiotic theory states that mitochondria were phylogenetically prokaryotes that were absorbed by eukaryotes in the sense of a symbiosis. This hypothesis is also confirmed by the fact that mitochondria 70S have ribosomes (subunits 50S and 30S) and the bacterial lipid cardiolipin is involved in the structure of the inner membrane.

Lysosomes

Lysosomes are characterized by their acidic pH (4.5-5) and high content of acidic hydrolases, including among others. Proteases, lipases, esterases, elastases, collagenases and acid phosphatases. Their functions are auto- and heterophagy, the breakdown of cellular and foreign substances. As soon as the primary lysosome (still inactive) fuses with the material to be broken down, it is called a secondary lysosome.

Peroxisomes

The peroxisomes found mainly in the liver and kidneys contain the enzymes peroxidase as well as catalase and serve to break down fatty acids by means of oxidation. The by-product hydrogen peroxide produced in this way can lead to cell damage and must therefore be broken down into water and oxygen by means of the catalase.

Ribosomes

The 80S ribosomes of the eukaryotes consist of two subunits(60S and 40S)which are composed of one third of proteins and two thirds of rRNA. They can be freely present in the cytosol for the synthesis of cytoplasmic or nuclear proteins or membrane-bound on the rough ER for the synthesis of lysosomal proteins, export or membrane proteins.

Centrioles

This Cell organelle made up of microtubules appears cylindrical. A pair of centrioles arranged at right angles to each other forms the centrosome. This in turn is the place where microtubules are formed and is therefore also called MTOC (microtubule-organizing center).

Cell inclusions

Are cell inclusions Metabolic by-products, stored nutrients or accumulations of exogenous or endogenous substances that are freely present in the cytoplasm. These include glycogen particles, intracellular fat droplets, pigmented cell structures (hemosiderin, lipofuscin, carbon dust) and virus particles.

In certain diseases such as hemochromatosis or glycogen storage disease, cell inclusions are present to a pathological extent.

Components and functions of the cytoskeleton

The cytoskeleton is located within the cytoplasm and is responsible for the Stabilization, intracellular mass transport and migration (Latin: migrare = to wander) responsible for the cell. This three-dimensional network is formed by microtubules, intermediate and actin filaments. These components are subject to constant build-up and breakdown, known as polymerisation and depolymerisation.

Exam tip: The components and tasks of the cytoskeleton are a popular exam topic.

Image: “The Three Components of the Cytoskeleton” by Phil Schatz. License: CC BY 4.0

Actin filaments (F-actin)

You are the smallest building blocks of the cytoskeleton with a diameter of 7nm and are also called microfilaments.

F-actin consists of two actin chains wound around each other in a helical manner, which result from the polymerisation of many globular actin monomers (G-actin). Often - but not always - actin filaments are associated with myosin, the motor protein of the actin system. Together they form the basis for the filament gliding mechanism of the muscles.

In addition to changing the cell shape, actin filaments also have a stabilizing role in that they form the basic structure of the microvilli or the anchorage point for desmosomes.

Intermediate filaments

With a diameter of 10nm, they form that passive support structure of the cell. The expression of the intermediate filaments varies among the tissue types, so that one can use this to determine the origin of a malignant tumor, for example:

Intermediate filamentTissue typefunction
CytokeratinEpitheliaMechanical protection
VimentinTissue of mesenchymal origin e.g. B. cartilage or connective tissuenot yet fully known
DesminMuscle tissueCohesion of myofibrils
Glial Fibril Acid Protein (GFAP)Astrocytes of the CNSstructure
NeurofilamentNeuronsStructure of the axons

Microtubules

With a diameter of 25nm they are that largest building blocks of the cytoskeleton and are formed from the centrosome (see above). They are made up of dimers of α- and β-tubulin and visually resemble a hollow cylinder, with one end being negatively charged and the other positively charged.

Microtubules determine the position of the cell organelles within the cell and form the road network for the directed transport of substances. They are also the basic structure of kinocilia and the spindle apparatus in mitosis and meiosis.

Structure and function of cell contacts

Based on the function, three types of cell contacts are distinguished:

  1. Communication contacts
  2. Detention contacts
  3. Barrier contacts as impermeable connections

Communication contacts

This includes the Gap junctions, also known as Nexus, are an absolute test favorite.

They are made up of transmembrane proteins called connexin. Six connexins form a connexon and two connexons then form a nexus. you enable electrical and metabolic communication between two neighboring cells. A particularly large number of gap junctions can be found e.g. B. in the shiny strip of the myocardium.

Adhesive contacts

They are used for mechanical anchoring and consist of three essential building blocks: transmembrane proteins, plaque proteins and cytoskeletal components. Desmosomes are found between neighboring cells as a cell-to-cell contact. Hemidesmosomes, on the other hand, connect the cell with the extracellular matrix as a cell-matrix contact.

The following table gives an overview of the Types of adhesive contacts, because they are not only used in histology but z. Sometimes also in biochemistry relevant to the exam are.

ArtOccurrenceFilamentsAdhesion MoleculePlaque protein
Spot desmosomes= Maculae adhaerensMyocardium, epitheliumIntermediate filamentsCadherine (Desmocollin, Desmoglein)Plakoglobin, desmoplakin
Point desmosomes= Adhesive punctaubiquitousActin filamentsCadherine
Belt desmosomes= Zonula adhaerenscubic and highly prismatic epitheliumActin filamentsCadherine (mostly E-Cadherine)Α-actinin, vinculin, catenin
Stripe desmosomes= Fasciae adhaerensShiny streak myocardiumActin filamentsIntegrinTalin, vinculin, α-actinin
Hemidesmosomesbetween epithelial cell and basal laminaIntermediate filamentsIntegrin, collagenPick, dystonin

Barrier / locking contacts

you will be Tight junctions or zonula occludens called and arise from a fusion of the outer membranes of neighboring cells. The intercellular gap is thus closed in a belt-like manner in this area, so that the paracellular molecule flow is hindered (diffusion barrier). Important transmembrane proteins involved are occludin and claudin.

Closing bar complex

This adhesive complex serves as a selective permeability barrier and From apical to basal, consists of: Zonula occludens, Zonula adhaerens, Macula adhaerens.

Popular cell exam questions

The solutions can be found below the references.

1. Which statement about mitochondria is incorrect?

  1. They have their own circular DNA.
  2. They occur in all cells.
  3. The matrix space is the place of β-oxidation.
  4. The bacterial lipid cardiolipin is a component of the inner mitochondrial membrane.
  5. They have 70S ribosomes.

2. Which assignment of tissue types and intermediate filaments is wrong?

  1. Neurocytes - Neurofilament
  2. Fibrocytes - Vimentin
  3. Myocytes - Desmin
  4. Osteocytes - GFAP
  5. Epidermis - cytokeratin

3. What is the correct size order of the building blocks of the cytoskeleton?

  1. Microtubules
  2. Actin filament
  3. Intermediate filament
  4. Actin filament
  5. Intermediate filament
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