The cell represents the fundamental structural and functional unit of all living beings. The cell is the smallest unit capable of manifesting the properties of a living being. It is able to synthesize all of its constituents using molecules from the extracellular medium. The cell is capable of developing and multiplying.There are a multitude of cell types, each performing a specific function.
There are two types of cells:
The prokaryotic cell
characterized by :
– The absence of a nucleus.
– The presence of a single chromosome, formed of deoxyribonucleic acid (DNA), and not surrounded by an envelope.
The eukaryotic cell
possesses:
– A nucleus, bounded by the nuclear envelope, and which contains genetic material in the form of DNA.
– Many specific organelles.
The eukaryotic cell constitutes unicellular organisms and multicellular organisms
Structure of the eukaryotic cell
Cytoplasm
is located inside the cell and is made up of two elements:
– The cytosol called hyaloplasm: which is a solution rich in water (about 80%), proteins, sugars and ions. It appears homogeneous and transparent.
– Organelles (the morphoplasm): are cellular elements responsible for a function and delimited by a membrane.
The plasma membrane
-consists of a double lipid layer of proteins, and carbohydrates.
-Constitutes a fluid and tight barrier, which will isolate the cell from the outside environment.
It plays a selective barrier role and in the regulation of water and ion concentrations.
The Nucleus
Is a more or less rounded element delimited by the nuclear envelope. It contains almost all of the cell’s DNA. Represents the most visible organelle under the microscope.
The nuclear envelope
consists of two membranes, an outer membrane in contact with the cytoplasm and an inner membrane related to the nucleoplasm. The two membranes are separated by peri-nuclear space. The nuclear envelope is pierced by orifices, the nuclear pores, which allow exchanges between the nucleus and the cytoplasm.
Chromatin
is a substance highly stainable by basic dyes, consisting of DNA and proteins. It is present only during the interphase and will condense into a chromosome at the start of cell division.
The nucleolus
is a dense, anastomosed mesh network, which contains ribosomal RNA, and is involved in the formation of ribosomes.
The endoplasmic reticulum
Membrane system composed of flattened cavities
communicating with each other and possibly carrying ribosomes. In the cell, there are two types of reticulum:
The rough endoplasmic reticulum: characterized by the presence of ribosomes, attached to the external face of the membrane. In some cells, it is highly developed and is called an ergoplasm.
The smooth endoplasmic reticulum: is a network of branched tubules connected to each other. Unlike the rough endoplasmic reticulum, no ribosomes are attached to the surface. It has a function in the synthesis of lipids, in the detoxification of drugs, and in the storage of calcium.
The golgi apparatus
is a membrane system formed by a stack of flattened poches.
These poches are called cisternae. The ends of the cisternae have swellings that can detach to form secretory vesicles. A stack of saccules is called a dictyosome. The set of dictyosomes forms the Golgi apparatus. Each dictyosome has a concave face and a convex face. It receives vesicles from the rough endoplasmic reticulum containing proteins. Golgi apparatus will modify proteins before packaging them outside the cell. It plays a small role in protein synthesis and elimination of waste.
Lysosomes
are membrane-bound organelles that contain enzymes necessary for cellular digestion.
Peroxisomes
are spherical or oval organelles, which have the role of
detoxify the cell by degrading certain molecules (fatty acids, alcohol, etc.) thanks to β- oxidation. This reaction produces hydrogen peroxide (H2O2)
The cytoskeleton
is a network of fibers of protein nature, organized in three dimensions in the cytoplasm, and being composed of 3 types of fibers:
– The microtubules.
– Intermediate filaments.
-Actin microfilaments.
They play the following roles:
-A structural role: it is responsible for the shape of the cells.
-An organizing role: it manages cell division.
-A role in the movement of the cell and organelles.
The centrosome
Located near the nucleus formed by two centrioles which are perpendicular to each other. From this centrosome, all the microtubules leave. During cell division,
the centrosome splits. Each centrosome will migrate to a pole of the cell.
The mitochondria
is an organelle in the form of rods (0.5 to 1 µm). Delimited by two
membranes: one external which is smooth and one internal which forms ridges.
The inter-membrane space has a chemical composition close to the cytoplasm. The mitochondria contain mitochondrial DNA.
The number of mitochondria varies depending on the cell type.
Role: Production of ATP