CELL STRUCTURE
A Review of All Cell Organelles Through Q&As
Cell Theory
1. What is cell theory?
Cell theory states that the cell is the basic unit of living organisms.
Before the discovery of the cell, people were unaware that living organisms were made of building blocks like cells.
2. Are there living organisms without cells?
Viruses are considered the only living organisms that do not have cells. Viruses are made up of genetic material (DNA or RNA) enclosed in a protein capsule. They do not have membranes, cell organelles, or own metabolism.
3. In 1665, Robert Hooke, an English scientist, published his book Micrographia, in which he described that pieces of cork viewed under a microscope presented small cavities, similar to pores and filled with air. Based on knowledge discovered later on, what do you think those cavities were composed of? What is the historical importance of this observation?
The walls of the cavities observed by Hooke were the walls of the plant cells that form the tissue. This observation led to the discovery of cells, a fact only possible after the invention of the microscope. In that book, Hooke established the term “cell", which is now widely used in biology, to designate those cavities seen under the microscope.
Eukaryotic and Prokaryotic Cells
4. What are the two main groups into which cells are classified?
Cells can be classified as eukaryotic or prokaryotic.
Prokaryotic cells are those that do not have an enclosed nucleus. Eukaryotic cells are those with a nucleus enclosed by a membrane.
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5. Do the cells of bacteria have a nucleus?
In bacteria, genetic material is contained in the cytosol and there is no internal membrane that encloses a nucleus.
6. Are any bacteria made of more than one cell?
There are no pluricellular bacteria. All bacteria are unicellular and prokaryotic.
Plasma Membrane
7. What is the plasma membrane of the cell? What are its main functions?
The plasma membrane is the outer membrane of a cell, it encloses the cell itself, maintaining specific conditions for cellular function within the cell. Since it is selectively permeable, the plasma membrane plays an important role in the entrance and exit of substances
8. What chemical substances compose the plasma membrane?
The main components of the plasma membrane are phospholipids, proteins and carbohydrates. Phospholipids are amphipathic molecules that are regularly organized in the membrane according to their polarity: two layers of phospholipids form the lipid bilayer, with the polar part of the phospholipids pointing to the exterior part of the layer and the non-polar phospholipid chains toward the interior. Proteins can be found embedded in the lipid bilayer. In addition, there are also some carbohydrates bound to proteins and to phospholipids in the outer surface of the membrane.
9. What is the difference between a plasma membrane and a cell wall?
A plasma membrane and a cell wall are not the same thing. The plasma membrane, also called the cell membrane, is the outer membrane common to all living cells, made of a phospholipid bilayer, embedded proteins and some bound carbohydrates.
Because cell membranes are fragile, in some types of cells, there are also external structures to support and protect the membrane, like the cellulose wall of plant cells and the chitin wall of some fungi cells. Most bacteria also have an outer cell wall made of peptidoglycans and other organic substances.
Cell Structure Review - Image Diversity: cell wall
10. What are the main respective components of cell walls in bacteria, protists, fungi and plants?
In bacteria, the cell wall is made of peptidoglycans; among protists, algae have cell walls made of cellulose; in fungi, the cell wall is made of chitin (the same substance that makes the exoskeleton of arthropods); and in plants, the cell wall is also made of cellulose.
11. Are membranes only present as the outside of cells?
Lipid membranes do not only form the outer layer of cells. Cell organelles, such as the Golgi complex, mitochondria, chloroplasts, lysosomes, the endoplasmic reticula and the nucleus, are also enclosed by membranes.
Cell Structure Review - Image Diversity: cell nucleus
Cell Nucleus
12. Which type of cell evolved first, the eukaryotic cell or the prokaryotic cell?
This is an interesting problem of biological evolution. The most accepted hypothesis claims that the simpler cell, the prokaryotic cell, appeared earlier in evolution than the more complex eukaryotic cell. The endosymbiotic hypothesis, for example, claims that aerobic eukaryotic cells appeared from the mutualistic ecological interaction between aerobic prokaryotes and primitive anaerobic eukaryotes.
13. Regarding the presence of the nucleus, what is the difference between animal and bacterial cells?
Animal cells (the cells of organisms of the kingdom Animalia) have an interior membrane that encloses a cell nucleus and are therefore eukaryotic cells. In these cells, the genetic material is located within the nucleus. Bacterial cells (the cells of living organisms of the kingdom Monera) do not have organized cellular nuclei and are therefore prokaryotic cells. Their genetic material is found in the cytosol.
14. What are the three main parts of a eukaryotic cell?
Eukaryotic cells can be divided into three main parts: the cell membrane that physically separates the intracellular space from the outer space by enclosing the cell; the cytoplasm, the interior portion filled with cytosol (the aqueous fluid inside the cell); and the nucleus, the membrane-enclosed internal region that contains genetic material.
15. What are the main structures within the nucleus of a cell?
Within the nucleus of a cell, the main structures are: the nucleolus, an optically dense region, sphere shaped region, which contains concentrated ribosomal RNA (rRNA) bound to proteins (there may be more than one nucleolus in a nucleus); the chromatin, made of DNA molecules released into the nuclear matrix during cell interphase; and the karyotheca, or nuclear membrane, which is the membrane that encloses the nucleus.
16. What substances is chromatin made up of? What is the difference between chromatin and a chromosome?
Chromatin, dispersed in the nucleus, is a set of filamentous DNA molecules attached to nuclear proteins called histones. Each DNA filament is a double helix of DNA and therefore a chromosome.
17. What is the fluid that fills the nucleus called?
The aqueous fluid that fills the nuclear region is called karyolymph, or the nucleoplasm. This fluid contains proteins, enzymes and other important substances for nuclear metabolism.
18. What substances make up the nucleolus? Is there a membrane around the nucleolus?
The nucleolus is a region within the nucleus made of ribosomal RNA (rRNA) and proteins. It is not enclosed by a membrane.
19. What is the name of the membrane that encloses the nucleus? Which component of cell structure is contiguous to this membrane?
The nuclear membrane is also called the karyotheca. The nuclear membrane is contiguous to the endoplasmic reticulum membrane.
The Cytoplasm
20. What are the main structures of the cytoplasm present in animal cells?
The main structures of the cytoplasm of a cell are centrioles, the cytoskeleton, lysosomes, mitochondria, peroxisomes, the Golgi apparatus, the endoplasmic reticula and ribosomes.
- Cell Structure Review - Image Diversity: cell organelles
21. What are cytoplasmic inclusions?
Cytoplasmic inclusions are foreign molecules added to the cytoplasm, such as pigments, organic polymers and crystals. They are not considered cell organelles.
Fat droplets and glycogen granules are examples of cytoplasmic inclusions.
- Cell Structure Review - Image Diversity: cytoplasmic inclusions
Ribosomes
22. Where in the cell can ribosomes be found? What is the main biological function of ribosomes?
Ribosomes can be found unbound in the cytoplasm, attached to the outer side of the nuclear membrane or attached to the endoplasmic reticulum membrane that encloses the rough endoplasmic reticulum. Ribosomes are the structures in which protein synthesis takes place.
- Cell Structure Review - Image Diversity: ribosomes
The Endoplasmic Reticulum
23. What is the difference between the smooth and rough endoplasmic reticulum?
The endoplasmic reticulum is a delicate membrane structure that is contiguous to the nuclear membrane and which is present in the cytoplasm. It forms an extensive net of channels throughout the cell and is classified into rough or smooth types.
The rough endoplasmic reticulum has a large number of ribosomes attached to the external side of its membrane. The smooth endoplasmic reticulum does not have ribosomes attached to its membrane.
The main functions of the rough endoplasmic reticulum are the synthesis and storage of proteins made in the ribosomes. The smooth endoplasmic reticulum plays a role in lipid synthesis and, in muscle cells, it is important in carrying out of contraction stimuli.
- Cell Structure Review - Image Diversity: endoplasmic reticulum
The Golgi Apparatus
24. A netlike membrane complex of superposed flat saccules with vesicles detaching from its extremities seen is observed during electron microscopy. What is the observed structure called? What is its biological function?
What is being observed is the Golgi complex, or Golgi apparatus. This cytoplasmic organelle is associated with chemical processing and the modification of proteins made by the cell as well as with the storage and marking of these proteins for later use or secretion. Vesicles seen under an electronic microscope contain materials already processed, and which are ready to be exported (secreted) by the cell. The vesicles detach from the Golgi apparatus, travel across the cytoplasm and fuse with the plasma membrane, secreting their substances to the exterior.
- Cell Structure Review - Image Diversity: Golgi apparatus
Lysosomes and Peroxysomes
25. Which organelle of the cell structure is responsible for intracellular digestion? What is the chemical content of those organelles?
Intracellular digestion occurs through the action of lysosomes. Lysosomes contain digestive enzymes (hydrolases) that are produced in the rough endoplasmic reticulum and stored in the Golgi apparatus. Lysosomes are hydrolase-containing vesicles that detach from the Golgi apparatus.
26. Why are lysosomes known as “the cleaners” of cell waste?
Lysosomes carry out autophagic and heterophagic digestion. Autophagic digestion occurs when residual substances of the cellular metabolism are digested. Heterophagic digestion takes place when substances that enter the cell are digested. Lysosomes enfold the substances to be broken down, forming digestive vacuoles or residual vacuoles, which later migrate toward the plasma membrane, fusing with it and releasing (exocytosis) the digested material to the exterior.
- Cell Structure Review - Image Diversity: lysosomes
27. What are the morphological, chemical and functional similarities and differences between lysosomes and peroxisomes?
Similarities: lysosomes and peroxisomes are small membranous vesicles that contain enzymes and enclose residual substances of an internal or external origin to break them down. Differences: lysosomes have digestive enzymes (hydrolases) that break down substances to be digested into smaller molecules whereas peroxisomes contain enzymes that mainly break down long-chain fatty acids and amino acids, and which inactivate toxic agents including ethanol. In addition, within peroxisomes, the enzyme catalase is present. It is responsible for the oxidation of organic compounds by hydrogen peroxide (H₂O₂) and, when this substance is present in excess, it is responsible for the breaking down of the peroxide into water and molecular oxygen.
Centrioles
28. Which cell organelles participate in cell division and in the formation of the cilia and flagella of some eukaryotic cells?
The organelles that participate in cell division and in the formation of the cilia and flagella of some eukaryotic cells are centrioles. Some cells have cilia (paramecium, the bronchial ciliated epithelium, etc.) or flagella (flagellate protists, sperm cells, etc.). These cell structures are composed of microtubules that originate from the centrioles. Centrioles also produce the aster microtubules that are very important for cell division.
- Cell Structure Review - Image Diversity: centrioles
Mitochondria
29. What are mitochondria? What is the basic morphology of these organelles and in which cells can they be found?
Mitochondria are the organelles in which the most important part of cellular respiration occurs: ATP production.
Mitochondria are organelles enclosed by two lipid membranes. The inner membrane invaginates to the interior of the organelle, forming the cristae that enclose the internal space known as the mitochondrial matrix, in which mitochondrial DNA (mtDNA), mitochondrial RNA (mt RNA), mitochondrial ribosomes and respiratory enzymes can be found. Mitochondria are numerous in eukaryotic cells and they are even more abundant in cells that use more energy, such as muscle cells. Because they have their own DNA, RNA and ribosomes, mitochondria can self-replicate.
- Cell Structure Review - Image Diversity: mitochondria
30. Why can mitochondria be considered the "power plants" of aerobic cells?
Mitochondria are the “power plants” of aerobic cells because, within them, the final stages of the cellular respiration process occur. Cellular respiration is the process of using an organic molecule (mainly glucose) and oxygen to produce carbon dioxide and energy. The energy is stored in the form of ATP (adenosine triphosphate) molecules and is later used in other cellular metabolic reactions. In mitochondria, the two last steps of cellular respiration take place: the Krebs cycle and the respiratory chain.
31. What is the endosymbiotic hypothesis regarding the origin of mitochondria? What molecular facts support this hypothesis? To which other cellular organelles can the hypothesis also be applied?
It is presumed that mitochondria were primitive aerobic prokaryotes that were engaged in mutualism with primitive anaerobic eukaryotes, receiving protection from these organisms and providing them with energy in return. This hypothesis is called the endosymbiotic hypothesis of the origin of mitochondria.
This hypothesis is strengthened by some molecular evidence, such as the fact that mitochondria have their own independent DNA and protein synthesis machinery, as well as their own RNA and ribosomes, and that they can self-replicate.
The endosymbiotic theory can also be applied to chloroplasts. It is assumed that these organelles were primitive photosynthetic prokaryotes because they have their own DNA, RNA and ribosomes, and can also self-replicate.
The Cytoskeleton
32. What are the main components of the cytoskeleton?
The cytoskeleton is a network of very small tubules and filaments distributed throughout the cytoplasm of eukaryotic cells. It is made of microtubules, microfilaments and intermediate filaments.
Microtubules are formed by molecules of a protein called tubulin. Microfilaments are made of actin, the same protein that is involved in the contraction of muscle cells. Intermediate filaments are also made of protein.
- Cell Structure Review - Image Diversity: cytoskeleton
33. What are the functions of the cytoskeleton?
As the name indicates, the cytoskeleton is responsible for maintaining of the normal shape of the cell. It also facilitates the transport of substances across the cell and the movement of cellular organelles. For example, the interaction between actin-containing filaments and the protein myosin creates pseudopods. In the cells of the phagocytic defense system, such as macrophages, the cytoskeleton is responsible for the plasma membrane projections that engulf the external material to be interiorized and attacked by the cell.
Chloroplasts
34. What are chloroplasts? What is the main function of chloroplasts?
Chloroplasts are organelles present in the cytoplasm of plant and algae cells. Like mitochondria, chloroplasts have two boundary membranes and many internal membranous sacs. Within the organelle, DNA, RNA ribosomes and also the pigment chlorophyll are present. The latter is responsible for the absorption of the light photic energy used in photosynthesis.
The main function of chloroplasts is photosynthesis: the production of highly energetic organic molecules (glucose) from carbon dioxide, water and light.
- Cell Structure Review - Image Diversity: chloroplasts
35. What is the molecule responsible for the absorption of light energy during photosynthesis? Where is that molecule located in photosynthetic cells?
Chlorophyll molecules are responsible for the absorption of light energy during photosynthesis. These molecules are found in the internal membranes of chloroplasts.
36. What colors (of the electromagnetic spectrum) are absorbed by plants? What would happen to photosynthesis if the green light waves that reach a plant were blocked?
Chlorophyll absorbs all other colors of the electromagnetic spectrum, but it does not absorb green. Green is reflected and such reflection is the reason for that characteristic color of plants. If the green light that reaches a plant was blocked and exposure of the plant to other colors was maintained, there would be no harm to the photosynthesis process. This appears to be a paradox: green light is not important for photosynthesis.
There is a difference between the optimum color frequency for the two main types of chlorophyll, chlorophyll A and the chlorophyll B. Chlorophyll A has an absorption peak at a wavelength of approximately 420 nm (indigo) and chlorophyll B has its major absorption at a wavelength of 450 nm (blue).
- Cell Structure Review - Image Diversity: electromagnetic spectrum
37. What path is followed by the energy absorbed by plants to be used in photosynthesis?
The energy source of photosynthesis is the sun, the unique and central star of our solar system. In photosynthesis, solar energy is transformed into chemical energy, the energy of the chemical bonds of the produced glucose molecules (and of the molecular oxygen released). The energy of glucose is then stored as starch (a glucose polymer) or it is used in the cellular respiration process and transferred to ATP molecules. ATP is consumed during metabolic processes that require energy (for example, in active transport across membranes).
Plant Cell Wall and Vacuoles
38. What substance are plant cell walls made of? Which monomer is this substance made of?
Plant cell walls are made of cellulose. Cellulose is a polymer whose monomer is glucose. There are other polymers of glucose, such as glycogen and starch.
- Cell Structure Review - Image Diversity: plant cell wall
39. What is the function of plant cell walls?
Plant cell walls have structural and protective functions. They play an important role in limiting cell size, and stopping cells from bursting, when they absorb a lot of water.
40. What are plant cell vacuoles? What are their functions? What is the covering membrane of vacuoles called?
Plant cell vacuoles are cell structures enclosed by membranes within which there is an aqueous solution made of various substances such as carbohydrates and proteins. In young plant cells, many small vacuoles can be seen; within adult cells, the majority of the internal area of the cell is occupied by a central vacuole.
The main function of vacuoles is the osmotic balance of the intracellular space. They act as “an external space” inside the cell. Vacuoles absorb or release water in response to cellular metabolic necessities by increasing or lowering the concentration of osmotic particles dissolved in the cytosol. Vacuoles also serve as a place for the storage of some substances.
The membrane that encloses vacuoles is called the tonoplast, named after the osmotic function of the structure.
- Cell Structure Review - Image Diversity: plant cell vacuoles
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