The cell – structure, function and metabolism

A cell (Latin Cellula = small chamber, cell) is the elementary unit of all living beings. The fundamental metabolic processes take place in it, without which humans cannot exist. In this chapter, you will learn which small organs, so-called organelles, occur in the cell and their function.

Cytoplasm and Organelles of the Cell


The cytoplasm fills the cell like a “soup”. The cytoplasm consists of water, salts, protein, fat droplets, sugar … The so-called glycolysis also takes place here.


Organelles are the functional units of a cell (see Fig.1). Here is a list of their most important functions.

    1. cell nucleus – Latin = nucleus with genetic information (blueprints) stored in long chemical molecules = DNS/DNA
    2. Mitochondria – are the “power plants of the cell”. Energy production from sugar using the citric acid cycle with O2 consumption.

    1. Endoplasmic reticulum – the endoplasmic reticulum (= ER) is responsible for membrane formation and mass transport within the cell (intracellular), mass storage (e.g. Ca++ in the muscle cell).
    2. ribosomes – are the “protein factories” of our cells. Here protein chains are assembled according to a blueprint (= DNA from the cell nucleus).

  1. Golgi apparatus – is the “packaging factory”; secretions are packed here in small bubbles (= vesicles) and brought to the plasma membrane. There the contents of the vesicles are released (= exocytosis).
  2. lysosomes – with protein-degrading enzymes. Enzyme = protein that initiates a biochemical reaction, comparable to a “soldering iron” or “cutting torch”. The lysosomes fuse with the incoming food vacuoles and the enzymes of the lysosomes degrade the “food”. The food vacuoles are formed by endocytosis.


The cytoskeleton has microfilaments of protein threads that can shimmy along each other. This causes a change in shape of the cell, possibly movement.


Microtubules are protein tubes (see also microfilaments), e.g. in cilia = flagella = cilia, with the help of which unicellular organisms such as slipper animals move.


Microvilli are finger-shaped membrane protrusions, e.g. for surface enlargement in the intestine.

Material exchange between extracellular space and intracellular space


Diffusion is the exchange of substances along a concentration gradient (principle of maximum disorder).

Multiple cellsBiological membranes (see Fig.2)) are only permeable for certain substances (=semipermeable), e.g. small, uncharged particles like O2, CO2, H2O but also fat-like substances. Many other substances pass through substance-specific pore proteins, otherwise the membrane is impermeable to them, e.g. charged particles (= ions) such as Na+, Cl etc. as in the nerve cell.

But also large molecules, e.g. sugar (glucose), insulin (= key for sugar pore proteins). If there is a lack of insulin, the sugar cannot enter the cells and remains in the blood – Diabetes mellitus.


Osmosis refers to the diffusion of H2O to balance concentrations, e.g. red blood cell “Ery” in concentrated salt solution.


As the water tries to equalize the salt content of the red blood cell (ery) with that of the salt solution, it shrinks more and more.

Example: Hypernatremia


Now it’s the exact opposite: As the water tries to match the salt content of the red blood cell (ery) with that of the distilled water, it swells more and more. This can go so far that the “ery” bursts.

All liquids with the same concentration as the cell plasma are isotonic, e.g. 0.9 percent NaCl solution.

  • All liquids with higher concentration are hypertonic.
  • All liquids with lower concentration are hypotonic.

Active transport is the exchange of substances against the concentration gradient under energy consumption. E.g. Na+ – K+ – Pump in the membrane of nerve cells. After excitation it pumps Na+ out of the cell again -> establishing the order of origin. E.g. iodine pump in thyroid cells.


Golgivesicles migrate to the plasma membrane, merge with it and release their contents to the outside; e.g. glands, excitation transmission at the synapses from nerve cell to nerve cell.


Flowing around of “food” and absorption into a vesicle. An example of this is phagocytosis.

Cell metabolism

  • Metabolism – Energy metabolism. Used for the uptake, transport and chemical conversion of nutrients and the release of metabolic waste products in an organism.
  • Anabolism – also called assimilation, denotes the growth, composition, and retention of endogenous components.
  • catabolism – also called dissimilation, refers to the production of building materials and energy of an organism.


Close-up of a cell
Close-up of a cell

Glycolysis describes the process of anaerobic energy production from glucose in the cytoplasm.

  • The so-called glycolysis takes place permanently in the cytoplasm of the (muscle) cells.
  • Glucose (blood sugar) is metabolized there in 10 steps to produce energy.
  • The energy released is low.

End products of glycolysis are per glucose molecule:

  • 2 pyruvate molecules
  • 2 ATP molecules
  • 4 hydrogen atoms

At rest or at low load, practically the entire pyruvate produced is introduced into the citric acid cycle for aerobic energy generation. At high loads, however, the increased pyruvate can no longer be completely processed via the citric acid cycle. This excess pyruvate is converted into lactate within the (muscle) cell.

Citric acid cycle (citrate cycle)

Citric acid cycle (citrate cycle) describes the process of aerobic energy production from glucose in the mitochondria.

The glycolysis resulting from the

  • pyruvate releases 2 hydrogen atoms within the mitochondrion and decays to:
  • Acetyl coenzyme A (Ac-CoA) see Fig.8a and Fig.13
  • The Ac-CoA enters the citric acid cycle and is burned further and further (oxidized) there.
  • Within the citric acid cycle only 1 ATP is formed, but 8 hydrogen atoms, of course not in gaseous form. This would immediately leave the cell by diffusion. The hydrogen in the cell is stored with the help of coenzymes. This is chemically bound hydrogen (e.g. in the form of NADH/H+or FADH2).
  • All hydrogen atoms obtained so far are released by the coenzymes and burned under oxygen with high energy gain in several partial steps to H2O:
    NADH/H+ + 1/2 O2 => H2O + NAD+ + Energy
  • The energy released is packed into the energy transport molecule ATP in order to be transported in portions to the sites of energy consumption (e.g. membrane, Golgi apparatus, ribosome, cytoskeleton). In the optimum case, up to 38 ADP per glucose molecule are loaded into ATP.

The breakdown of fat (lipolysis)

Fat is comminuted to monoglyceride and fatty acids.

The above-mentioned fragments are absorbed into the blood or lymph (=absorption) and transported to the cells. Further degradation takes place in the cell.

Aerobic energy production from fat

Body fat can also be converted to acetyl-CoA and introduced into the citric acid cycle, but the chemical reaction is very slow, so that this form of energy delivery provides a decreasing relative share of the energy delivered as the load increases. A calculation example of the distribution of carbohydrates and fat with increasing physical exertion can be found in the article myth fat burning.

Anaerobic energy production from fat

Fat can also be used to generate energy in cases of oxygen deficiency. The energy released in the process produces CO2 and H2O as well as keto bodies.

Protein degradation

Protein chains are crushed into amino acids.

Anaerobic energy generation from protein

Protein can be degraded in the absence of oxygen to produce energy (Fig.12). The released energy produces glucose and urea.

  • The released energy is used to generate ATP.
  • The resulting urea is excreted via the kidneys.
  • The glucose produced enters into the carbohydrate metabolism (glycolysis or citric acid cycle).

Anabolism = substance composition (assimilation)

Carbohydrate structure

Glucose is built up into sugar chains (membrane building blocks) or stored in the liver as glycogen (energy store). See also below: carbohydrates

Fat structure

Among other things, grease is used as a membrane building block and is also excellently suited as an energy store. For this purpose ,the fat is stored in depots under the skin. See also below: Fats and oils

Protein structure

Done at the ribosomes: composition of the AS chains according to a blueprint (DNA) from the cell nucleus (= protein biosynthesis). Protein is a building block for microfilaments (muscle), membranes and enzymes.

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William C. Hilberg
As an author, Mr. Hilberg has published several papers on health issues that have gained international recognition. He is close to nature and loves the seclusion and activity as a freelance journalist. In his function as editor William C. Hilberg manages the entire content of PENP. Our team greatly appreciates his expertise and is proud to have him on board.