Next lesson - Introduction to DNA
Core
Cells are the units that make up the human body. Each cell has a specialised role. For example, protection is the role of keratinised squamous epithelial cells of the skin (flat cells toughened with the protein keratin), and secretion is the role of mucus-secreting columnar goblet cells of the respiratory tract (tall, thin cells that release mucus). This article covers the common components of cells in the form of the hypothetical "standard" eukaryotic cell (cells with enveloped nuclei).
An organelle is a component of the cell with a specific function.
Diagram - The eukaryotic cell, with its various organelles labelled, each with a specific function
Creative commons source by 3.2 The Cytoplasm and Cellular Organelles by Rice University [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Typically the most prominent organelle in the cell.
- Contains DNA, nucleoproteins (nucleic acids bonded to a protein) & RNA.
- Function: stores DNA and coordinates the cell’s activities, such as growth, metabolism, protein synthesis and cell division.
- Transmission electron microscope images of the nucleus reveal that DNA is found in two forms. Heterochromatin is condensed DNA (chromatin) that appears dark. It is associated with gene silencing, so little RNA is made from it. Euchromatin is uncondensed DNA that appears light and is associated with active RNA synthesis (transcription).
- N.B. Prokaryotic cells have no nucleus, and this absence is what defines them as prokaryotic.
- Found within the nucleus.
- Function: the site of ribosomal RNA (rRNA) synthesis. This is also where the rRNA is combined with proteins to generate immature ribosomes. These ribosomes then mature after being exported from the nucleus into the cytoplasm, where they either attach to rough endoplasmic reticulum or become free-floating ribosomes within the cytoplasm.
- A double layered membrane bounding the nucleus.
- Contains nuclear pores - these allow macromolecules (very large molecules such as proteins and RNA) to pass through when entering or exiting the nucleus.
- Function: controls what enters and exits the nucleus.
Rough Endoplasmic Reticulum (RER)
- A series of interconnecting membranes, vesicles & cisternae (flattened sacs), continuous throughout the cytoplasm.
- Ribosomes are attached to the outer surface of the membranes, making this ER "rough".
- Function: synthesises proteins destined for lysosomes, the cell membrane or extracellular export.
- Pancreatic cells (and other cells which synthesise substances) have abundant RER to synthesise pancreatic enzymes.
Smooth Endoplasmic Reticulum (SER)
- A series of interconnecting membranes, vesicles and cisternae (less flat compared to RER). The lack of embedded ribosomes classifies this ER as smooth. The amount of SER varies with the type of cell: cells that make large amounts of steroid hormones, such as those of the ovaries and testes, contain a lot of it.
- Function: lipid biosynthesis (e.g. steroid hormone production), detoxification of drugs and poisons, and storage of calcium ions.
- Composed of two subunits, each made of ribosomal RNA (rRNA) and proteins: 40S + 60S in eukaryotes (together an 80S ribosome) and 30S + 50S in prokaryotes (together a 70S ribosome). S stands for the Svedberg unit, a measure of how fast a particle settles when spun in a centrifuge, so the subunit values do not simply add up to the total. The subunits wrap around mRNA to begin translation and protein synthesis.
- Function: site of protein synthesis (translation) within the cell.
- Semi-circular shaped stacks of cisternae.
- Vesicles containing proteins bud off from the RER and fuse with the cis face of the Golgi apparatus (the convex, forming face, which is the side closest to the RER).
- The Golgi apparatus has polarity (its two ends differ): proteins move from the convex cis face (entry point) to the concave trans face (exit point) of the stack and are modified as they move.
- Function: sort, concentrate, package and modify proteins synthesised in the RER.
- Vesicles containing the finished proteins leave the maturing, concave trans face of the Golgi apparatus. Some deliver their contents to other parts of the cell, such as lysosomes, and others fuse with the plasma membrane and release their contents outside the cell (secretion).
- Membrane-bound organelles that contain acid hydrolases (enzymes that break molecules down in acid conditions) at pH 5. These include proteases (break down proteins), nucleases (nucleic acids), glycosidases (carbohydrates), lipases (fats) and phosphatases (remove phosphate groups).
- Function: digest macromolecules, break down excess or worn-out organelles (autophagy, the cell digesting its own parts) and destroy pathogens (disease-causing microbes) that enter the cell, such as engulfed bacteria.
- A membrane-bound organelle that contains enzymes that transfer hydrogen atoms from various molecules, including some toxins such as alcohol, to oxygen to produce hydrogen peroxide. Most alcohol is broken down in the liver by the enzyme alcohol dehydrogenase, so peroxisomes contribute only part of this.
- The toxins ingested are neutralised and the hydrogen peroxide produced can be neutralised by the enzymes in peroxisomes to produce water and oxygen.
- Function: chemical detoxification and lipid metabolism.
Diagram - The structure of a mitochondrion
Creative commons source by Mariana Ruiz Villarreal LadyofHats [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- A key component of the cell. Particularly abundant in highly active, energy requiring cells.
- Mitochondria have a double membrane: an outer and an inner lipid bilayer.
- The inner membrane is larger than the outer membrane and is therefore highly folded to fit within the outer membrane. This folding increases the surface area for ATP generation to occur - these folds are called cristae.
- The matrix lies inside the inner membrane and is where the tricarboxylic acid (TCA) cycle, also called the Krebs cycle, takes place. The matrix contains mitochondrial DNA, ribosomes and soluble enzymes.
- The enzymes in the matrix drive the TCA/Krebs cycle, while oxidative phosphorylation and most ATP synthesis occur at the inner mitochondrial membrane.
- Cells such as muscle cells and sperm cells have a higher number of mitochondria due to increased requirements for energy in the form of ATP.
- Function: Responsible for generating most of the cell’s ATP (its major energy currency) through aerobic respiration, and regulates cellular metabolism
Diagram - Labelled image of a mitochondrion under a microscope
Creative commons source by CNX OpenStax [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- The plasma membrane is a bilayer of phospholipids that also contains cholesterol, proteins and carbohydrates (attached to some of the lipids and proteins). The main phospholipids are glycerol-based molecules with two fatty acid chains and a phosphate-containing head group. Phospholipids are amphipathic molecules (have both hydrophobic, or water-repelling, and hydrophilic, or water-attracting, regions)
- Proteins are found in and on the plasma membrane. Integral proteins are embedded in the bilayer (some span only part of it, while others stretch from one side to the other), and peripheral proteins are attached to the inner or outer surface of the membrane.
- Cholesterol is present in plasma membranes and controls the fluidity or rigidity of the membrane at various temperatures.
- Take a look at our Biological membranes article for more detail on structure of membranes
- Functions:
- Acts as a selectively permeable barrier. Small non-polar molecules (which have no charged regions and so dissolve in lipids), such as oxygen, and very small polar molecules, such as water, can cross without the aid of channels, whereas large polar molecules and ions cannot easily cross.
- Compartmentalises cells into discrete units.
- Communication with other molecules and cells.
- Recognition through signalling molecules, adhesion proteins (which stick cells to each other and to their surroundings) and immune surveillance (immune cells checking cell surfaces for signs of infection or damage).
- Electrical or chemical signal generation in response to a stimulus.
- All eukaryotic cells possess a cytoskeleton (a network of protein filaments) which is responsible for maintaining and changing the cell’s shape.
- There are three main structural components of the cytoskeleton: microfilaments, intermediate filaments and microtubules.
- Function:
- Provides structural support for the plasma membrane & cell organelles.
- Provides a means of movement for organelles, plasma membrane and other constituents of the cytosol (the fluid part of the cytoplasm) around the cell.
- Provides the locomotor mechanism for amoeboid movement of cells such as lymphocytes (crawling-like movement) and is the basis of cilia (tiny hair-like projections) and flagella (whip-like tails).
- Provides contractility in cells of specialised tissues (e.g. muscle cells).
- Microfilaments:
- Composed of two intertwined strands of actin (a protein) and about 7 nanometres (nm) in diameter.
- They use ATP, and the filaments act as tracks for the motor protein myosin, which allows contraction (shortening). They can also assemble and dissociate very quickly (dynamic).
- The structure of microvilli (tiny finger-like projections that increase surface area) of the intestinal cells is maintained by a core of actin filaments.
- Intermediate filaments:
- They are more stable than microfilaments, although their assembly is still regulated, and they are about 10 nm in diameter.
- Their function is to form a tough supporting meshwork within the cytoplasm. They are anchored to the plasma membrane at desmosomes (strong junctions between neighbouring cells).
- The filaments are commonly found in nerve cells, neuroglial cells and epithelial cells (made out of cytokeratin).
- Microtubules:
- Subunits of tubulin (a protein) join end to end (polymerise) to form the wall of the hollow microtubule.
- They originate from the centrosome (the cell’s microtubule-organising centre) and are found at sites in the cell where structures are moved: nerve fibres, mitotic spindles (the microtubule fibres that pull chromosomes apart in cell division) and the cores of cilia & flagella.
- Motor proteins (kinesins and dynein) bind to organelles and move them along the microtubules, e.g. vesicles moving through the cytoplasm to the plasma membrane.
Edited by: Dr. Ben Appleby and Dr. Marcus Judge
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