Cell Biology Interview Questions And Answers

Plasma membrane and cell wall is not the same thing. 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.

Cell Structure Review - Image Diversity: cell wall

The main constituents of the plasma membrane are phospholipids, proteins and carbohydrates. The phospholipds, 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.

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

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

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 for the passage of substances inwards or outwards.

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Cells can be classified as eukaryotic or prokaryotic.

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

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The walls of the cavities observed by Hooke were the walls of the plant cells that form the tissue. The observation leaded to the 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.

Cell Structure Review - Image Diversity: Hooke's cell

The virus is considered the only alive beings that do not have cells. Virus are constituted by genetic material (DNA or RNA) enwrapped by a protein capsule. They do not have membrane and cell organelles neither self-metabolism.

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

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

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

In bacteria cell wall is made of peptidoglycans; among protists algae have cell wall 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.

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.

Cell Structure Review - Image Diversity: cell nucleus

This is an interesting problem of biological evolution. The most accepted hypothesis asserts that the more simple cell, the prokaryotic cell, appeared early 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.

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 nucleus and so they are prokaryotic cells and their genetic material is found dispersed in the cytosol.

The eukaryotic cell can be divided into two main portions: the cell membrane that separates the intracellular space from the outer space phisically 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.

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.

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

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 the nuclear metabolism.

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

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

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

Cell Structure Review - Image Diversity: cell organelles

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

Fat drops and glycogen granules are examples of cytoplasmic inclusions.

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

Cell Structure Review - Image Diversity: ribosomes

The endoplasmic reticulum is a delicate membranous structure contiguous to the nuclear membrane and present in the cytoplasm. It forms an extense net of channels throughout the cell and it is divided in rough and smooth types.

The rough endoplasmic reticulum has great amount of ribosomes adhered 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 importante in the conduction of the contraction stimulus.

Cell Structure Review - Image Diversity: endoplasmic reticulum

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 in 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.

Cell Structure Review - Image Diversity: Golgi apparatus

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.