Q & A

Q & A on Cell Structure

Q: What is cell theory?

Cell theory asserts that the cell is the constituent unit of living beings.

Before the discovery of the cell, it was not recognized that living beings were made of building blocks like cells.

The cell theory is one of the basic theories of Biology.

Q:  Are there living beings without cells?

Viruses are considered the only living beings that do not have cells. Viruses are constituted by genetic material (DNA or RNA) enwrapped by a protein capsule. They do not have membranes and cell organelles nor do they have self-metabolism.

Q:  In 1665 Robert Hooke, an English scientist, published his book Micrographia, in which he described that pieces of cork viewed under the microscope presented small cavities similar to pores which were filled with air. Based on later knowledge, of what were the walls of those cavities constituted? What is the historical importance of that observation?

The walls of the cavities observed by Hooke were the walls of the plant cells that form the tissue. The observation led to the discovery of the cells, a fact only possible after the invention of the microscope. In that work, Hooke established the term “cell”, now widely used in Biology, to designate those cavities seen under the microscope.

Q: What are the two big groups into which cells are classified?

Cells can be classified as eukaryotic or prokaryotic.

A prokaryotic cell is that without a delimited nucleus. Eukaryotic cells are those with nucleus delimited by a membrane.

Q:  Do bacteria cells have a nucleus?

In bacteria, the genetic material is dispersed in the cytosol and there is no internal membrane that delimits a nucleus.

Q: Are there any bacteria made of more than one cell?

There are no pluricellular bacteria. All bacteria are unicellular prokaryotic.

Q: What is the plasma membrane of the cell? What are its main functions?

The plasma membrane is the outer membrane of the cell, it delimits the cell itself and a cell interior with specific conditions for the cellular function.

Since it is selectively permeable, the plasma membrane has an important role in the passage of substances inwards or outwards.

Q: What are the chemical substances that compose the plasma membrane?

The main constituents of the plasma membrane are phospholipids, proteins, and carbohydrates. The phospholipids, amphipathic molecules, 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 of the layer and the non-polar phospholipid chains in the interior. Proteins can be found embedded in the lipid bilayer and there are also some carbohydrates bound to proteins and to phospholipids in the outer face of the membrane.

Q: What is the difference between the plasma membrane and cell wall?

Plasma membrane and cell wall are not the same things. The plasma membrane, also called cell membrane, is the outer membrane common to all living cells and it is made of a phospholipid bilayer, embedded proteins, and some appended carbohydrates.

Because cell membranes are fragile, in some types of cells there are even outer structures that support and protect the membrane, like the cellulose wall of plant cells and the chitin wall of some fungi cells. Most bacteria also present an outer cell wall made of peptidoglycans and other organic substances.

Q: What are the main respective constituents 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); in plants, the cell wall is made of cellulose too.

Q: Do membranes form only the outer wrapping of cells?

Lipid membranes do not form only the outer cover of cells. Cell organelles, such as the Golgi complex, mitochondria, chloroplasts, lysosomes, the endoplasmic reticula and the nucleus, are delimited by membranes too.

Q: Which type of cell came first in evolution – the eukaryotic cell or the prokaryotic cell?

This is an interesting problem with biological evolution. The most accepted hypothesis asserts that the more simple cell, the prokaryotic cell, appeared earlier in evolution than the more complex eukaryotic cell. The endosymbiotic hypothesis, for example, affirms that aerobic eukaryotic cells appeared from the mutualist ecological interaction between aerobic prokaryotes and primitive anaerobic eukaryotes.

Q: Concerning the presence of the nucleus what is the difference between animal and bacterial cells?

Animal cells (cells of living beings of the kingdom Animalia) have an interior membrane that delimits a cell nucleus and thus they are eukaryotic cells; in these cells, the genetic material is located within the nucleus. Bacterial cells (cells of living beings of the kingdom Monera) do not have organized cellular nuclei and so they are prokaryotic cells and their genetic material is found dispersed in the cytosol.

Q: What are the three main parts of a eukaryotic cell?

The eukaryotic cell can be divided into two main portions: the cell membrane that separates the intracellular space from the outer space physically delimiting the cell; the cytoplasm, the interior portion filled with cytosol (the aqueous fluid inside the cell); and the nucleus, the membrane-delimited internal region that contains the genetic material.

Q: What are the main structures within the cell nucleus?

Within the cell nucleus the main structures are: the nucleolus, an optically dense region, spherical-shaped, where there are concentrated ribosomal RNA (rRNA) associated to proteins (there may be more than one nucleolus in a nucleus); the chromatin, made of DNA molecules dispersed in the nuclear matrix during the cell interphase; the karyotecha, or nuclear membrane, the membrane that delimits the nucleus.

Q: What are the substances that constitute the chromatin? What is the difference between chromatin and chromosome?

The chromatin, dispersed in the nucleus, is a set of filamentous DNA molecules associated with nuclear proteins called histones. Each DNA filament is a double helix of DNA and thus a chromosome.

Q: What is the fluid that fills the nucleus called?

The aqueous fluid that fills the nuclear region is called karyolymph, or nucleoplasm. In the fluid there are proteins, enzymes, and other important substances for nuclear metabolism.

Q: Of what substances is the nucleolus made? Is there a membrane around the nucleolus?

Nucleolus is a region within the nucleus made of ribosomal RNA (rRNA) and proteins. It is not delimited by membrane.

Q: What is the name of the membrane that delimits the nucleus? To which component of the cell structure is that membrane contiguous?

The nuclear membrane is also called karyotheca. The nuclear membrane is continuous to the endoplasmic reticulum membrane.

Q: What are the main cytoplasmic structures present in animal cells?

The main cytoplasmic structures of the cell are the centrioles, the cytoskeleton, lysosomes, mitochondria, peroxisomes, the Golgi apparatus, the endoplasmic reticula, and ribosomes.

Q: What are cytoplasmic inclusions?

Cytoplasmic inclusions are cytoplasmic molecular aggregates, such as pigments, organic polymers, and crystals. They are not considered cell organelles.

Fat droplets and glycogen granules are examples of cytoplasmic inclusions.

Q: Where in the cell can ribosomes be found? What is the main biological function of ribosomes?

Ribosomes can be found free in the cytoplasm, adhered to the outer side of the nuclear membrane or associated with the endoplasmic reticulum membrane defining the rough endoplasmic reticulum. Ribosomes are the structures where protein synthesis takes place.

Q: What is the difference between smooth and rough endoplasmic reticulum?

The endoplasmic reticulum is a delicate membranous structure contiguous to the nuclear membrane and 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 great 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 synthesis and storage of proteins made in the ribosomes. The smooth endoplasmic reticulum plays a role in the lipid synthesis and, in muscle cells, it is important in the conduction of the contraction stimulus.

Q: A netlike membranous complex of superposed flat saccules with vesicles detaching from the extremities seen in electronic microscopy. What is the observed structure? 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 modification of proteins made by the cell and with storage and branding of these proteins for posterior use or secretion. Vesicles seen under the electronic microscope contain material already processed, 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 then secreting their substances to the exterior.

Q: On which organelle of the cell structure does intracellular digestion depend? What is the chemical content of those organelles?

Intracellular digestion occurs by the action of lysosomes. Lysosomes have digestive enzymes (hydrolases) that are made in the rough endoplasmic reticulum and stored in the Golgi apparatus. Lysosomes are hydrolase- containing vesicles that detach from the Golgi apparatus.

Q: Why are lysosomes known as “the cleaners” of the cell waste?

Lysosomes carry out autophagic and heterophagic digestion: autophagic digestion by digesting residual substances from the cellular metabolism; heterophagic digestion by digesting substances that enter the cell. Lysosomes enfold the substances to be degraded forming digestive vacuoles, or residual vacuoles, that later migrate toward the plasma membrane fusing with it and liberating (exocytosis) the digested material to the exterior.

Q: Which are the cell organelles that participate in cell division and in the formation of cilia and flagella of some eukaryotic cells?

The organelles that participate in the cell division and in the formation of cilia and flagella of some eukaryotic cells are the 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 originated from the centrioles. Centrioles also make the aster microtubules that are very important for cell division.

Q: 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 from internal or external origin degrading them. Differences: lysosomes have digestive enzymes (hydrolases) that break substances to be digested into small molecules; peroxisomes contain enzymes that degrade mainly long-chained fatty acids and amino acids and that inactivate toxic agents including ethanol; within peroxisomes there is the enzyme catalase, responsible for the oxidation of organic compounds by hydrogen peroxide (H2O2) and, when this substance is in excess, by the degradation of the peroxide into water and molecular oxygen.

Q: 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 the cellular respiration occurs: the ATP production.

Mitochondria are organelles delimited by two lipid membranes. The inner membrane invaginates to the interior of the organelle forming cristae that delimitate the internal space known as mitochondrial matrix and where 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 those cells that use more energy, like muscle cells. Because they have their own DNA, RNA and ribosomes, mitochondria can self- replicate.

Q: Why can mitochondria be considered the power plants of the aerobic cells?

Mitochondria are the “power plants” of aerobic cells because within them the final stages of the cellular respiration process occurs. Cellular respiration is the process of using 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 later used in other cellular metabolic reactions. In mitochondria the two last steps of the cellular respiration take place: the Krebs cycle and the respiratory chain.

Q: What is the endosymbiotic hypothesis about the origin of mitochondria? What are the molecular facts that support the hypothesis? To which other cellular organelles can the hypothesis also be applied?

It is presumed that mitochondria were primitive aerobic prokaryotes that were engulfed in mutualism by primitive anaerobic eukaryotes, receiving protection from these beings and

offering energy to them. This hypothesis is called the endosymbiotic hypothesis on the origin of mitochondria.

The hypothesis is strengthened by some molecular evidence such as the fact that mitochondria have their own independent DNA and protein synthesis machinery, with their own RNA and ribosomes, and that they can self- replicate.

The endosymbiotic theory can be applied to chloroplasts too. It is supposed that these organelles were primitive photosynthetic prokaryotes because they have their own DNA, RNA and ribosomes and they can self- replicate too.

Q: 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 participates in the contraction of muscle cells. Intermediate filaments are made of protein too.

Q: What are the functions of the cytoskeleton?

As the name indicates, the cytoskeleton is responsible for the support of the normal shape of the cell; it also acts as a facilitator for substance transport across the cell and for the movement of cellular organelles. For example, the sliding between actin-containing filaments and the protein myosin creates pseudopods. In cells of the phagocytic defense system, like macrophages, cytoskeleton is responsible for the plasma membrane projections that engulf the external material to be interiorized and attacked by the cell.

Q: What are chloroplasts? What are 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 there are DNA, RNA and ribosomes and also the pigment chlorophyll, responsible for absorption of photic energy that is used in photosynthesis.

The main function of chloroplasts is photosynthesis: the production of highly energetic organic molecules (glucose) from carbon dioxide, water and light.

Q: What is the molecule responsible for the absorption of photic energy for photosynthesis? Where is that molecule located in photosynthetic cells?

The chlorophyll molecules are responsible for the absorption of light energy for photosynthesis. These molecules are found in the internal membranes of chloroplasts.

Q: What are the colors (of the electromagnetic spectrum) absorbed by plants? What would happen to photosynthesis if the green light waves that reach a vegetable were blocked?

Chlorophyll absorbs all other colors of the electromagnetic spectrum but it practically does not absorb the green. The green color is reflected and such reflection provides the characteristic color of plants. If the green light that reaches a plant is blocked and exposure of the plant to other colors is maintained there would be no harm to the photosynthesis process. Apparent paradox: the green light is not important for photosynthesis.

There is a difference between the optimum color frequency for the two main types of chlorophyll, the chlorophyll A and the chlorophyll B. Chlorophyll A has an absorption peak at approximately 420 nm wavelength (anil) and chlorophyll B has its major

absorption in 450 nm wavelength (blue).

Q: What is the path 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 planetary system. In photosynthesis the solar energy is transformed into chemical energy, the energy of the chemical bonds of the produced glucose molecules (and of the released molecular oxygen). 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 in metabolic processes that spend energy (for example, in active transport across membranes).

Q: Of what substance is the plant cell wall made? Of which monomer is it made?

The plant cell wall is made of cellulose. Cellulose is a polymer whose monomer is glucose. There are other polymers of glucose, like glycogen and starch.

Q: What is the function of the plant cell wall?

The plant cell wall has structural and protective functions. It plays an important role in the constraint of the

cell size, preventing the cell to break when it absorbs a lot of water.

Q: What are plant cell vacuoles? What are their functions? What is the covering membrane of the vacuoles called?

Plant cell vacuoles are cell structures delimited by membranes within which there is an aqueous solution made of various substances like carbohydrates and proteins. In young plant cells many small vacuoles can be seen; within adult cells, the most part of the internal area of the cell is occupied by a central vacuole.

The main function of the 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 the cellular metabolic necessities by increasing or lowering the concentration of osmotic particles dissolved in the cytosol.

Vacuoles also serve as a storage place for some substances.

The membrane that delimits the vacuoles is called tonoplast, named after the osmotic function of the structure

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