CELLS PRESENTATION

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definitions of cells

Each living organism present on Earth is united to the other by cells, microscopic structures capable of performing all vital functions All cells on Earth have three common fundamental structures: ⁃ the plasma membrane, an envelope useful for the transport and communication of the cell, which separates it from the surrounding environment, allowing the passage of substances necessary for the cell to the inside and outside of it; ⁃ the cytoplasm, the internal environment, gelatinous and rich in water, mineral salts and organic substances, in which most cellular activities take place; ⁃ the genetic program, which is localized in DNA and guides the activities of the cell, and the determination of its characteristics. According to the organization of the genetic material and the presence or absence of “organelles”, it is possible to differentiate two main kinds of cells: prokaryotic cells, present in Bacteria and Archea, and eukaryotic cells, present in all other living organisms.

PROKARYOTIC CELLS

bacteria

prokaryotic plasma membrane

reproduction of bacteria

index

prokaryotic cytoplasm and ribosomes

heterotrophic and autotrophic bacteria

PRocariotic genetic material

prokaryotic cells

The word “Prokaryote” comes from two Greek words: pro, meaning before, and karion, meaning nucleus. This name refers to the fact that prokaryotic cells are much smaller than eukaryotic cells, are presented as single or unicellular cells, with a diameter of about 10microns and a simple structure. In fact, they lack membrane, organelles and nucleus, but they have the genetic material, namely the DNA or bacterial chromosome, contained in an area of the cytoplasm called “nucleoid”, where ribosomes are also found. All prokaryotic cells are surrounded by a cell wall and many also have a capsule, that is made of polysaccharides, and some appendages on their surface. Flagella and some pili are used for cells locomotion towards nutrients and other chemical attractants or away from the repellent ones, fimbriae help the cell stick to a surface, and sex pili are used for DNA exchange.

prokaryotic plasma membrane

Both prokaryotic and eukaryotic cells have a plasma membrane, a double layer of phosholipids that separates the cell interior from the outside environment. These phospholipids spontaneously arrange themselves in a double-layered structure with their hydrophobic tails pointing inward and their hydrophilic heads facing outward. Inside this two-layer structure, called a phospholipid bilayer, are inserted numerous proteins that can pass all the way through the membrane, serving as channels or signal receptors, or just remain attached at the edge. As the last component of the plasma membrane we find a steroid, cholesterol, which helps to stabilize the double layer of phospholipids.

Some prokaryotic cells, know as “gram-negative bacteria” can have multiple plasma membranes, with which cells assimilate the substances they need and eliminate waste products in the external environment; this border can also controls the passage of various small or larger molecules - including sugars, amino acids, ions, and water – into and out of the cell. Some proteins stuck in the membrane also aid prokaryotic cells in communicating and interacting with the surrounding environment, sending and receiving chemical signals from other cells Prokaryotic cells are also surrounded by a rigid cell wall, located outside the cell membrane and consisting of cellulose, which provides cells support and gives them shape and protection against mechanical stress or damage from osmotic rupture and lysis. Its major component is peptidoglycan, a huge polymer consisting of sugars and amino acids.

PROKARYOTIC CYTOPLASM

The cytoplasm in prokaryotic cells is a gel-like, fluid substance in which all of the other cellular components are suspended. Respect to the eukaryotic cytoplasm, it doesn’t contain organelles. Most prokaryotic cells contain filamentous proteins and filament systems that are known as a cytoskeleton. It is a sort of framework which is made up of proteins, ribosomes, and small rings of DNA called plasmids moving around within the cell and that helps a prokaryotic cell divide and maintain its shape.

PROKARYOTIC RIBOSOMES

Prokaryotic ribosomes are smaller and have a slightly different shape and composition than those found in eukaryotic cells but they have the identical function: they build proteins by translating messages sent from DNA.

PROKARYOTIC GENETIC MATERIAL

All prokaryotic cells don’t have a nucleus and, for this reason, the genes needed for the cell growth, survival, and reproduction is found in the cytoplasm. This strand of DNA forms the circular bacterial chromosome, that usually, can be transcribed into messenger RNA, that is immediately translated into proteins in the ribosomes, capable of maintaining the chromosomal structure and regulating the gene expression. In addition to the bacterial chromosome, many prokaryotic cells also contain small pieces of DNA, called plasmids, that can be transferred from one prokaryotic cell to another through pili, which are small projections of the cell membrane that form physical channels with the pili of adjacent cells. This transfer is called ”bacterial sex”, as it is considered an original form of sexuality.

bacteria

In some ways, prokaryotic cells usually have fewer visible structures, smaller than those seen in eukaryotic cells. One of the most common example of prokaryotic cells are Bacteria. Bacteria are classified into 5 groups according to their basic shapes: spherical (cocci), rod (bacilli), spiral (spirilla), comma (vibrios), corkscrew (spirochaetes). They can exist as single cells of a few micrometres, in pairs, chains or cluster. Bacteria are found in every habitat on Earth: soil, rock, ocean and even arctic snow. Some live in or on other organisms, such as plants and animals, including humans. Some live on dead plant matter, where they aid the cycling of nutrients. Some are useful in the production of fermented foods, others are parasites or pathogens. In these cases they cause disease in animals and plants.

reproduction of bacteria

Bacteria commonly reproduce through binary fission, that is a process where 1 cell divides to form 2 identical daughter cells. It is a type of asexual reproduction that involves only 1 parent and produces offspring that are identical to the parent. In case of this process, when conditions are favorable, for example when temperature is right and nutrients are available, some bacteria, like Escherichia coli, can divide every 20 minutes. That’s why we can quickly become ill when pathogenic microbes invade our bodies. The form of sexual reproduction where 1 bacterium gives some of its DNA to another bacterium through a pilum, is called conjugation.

HETEROTROPHIC AND AUTOTROPHIC BACTERIA

Some bacteria are heterotrophic, others are autotrophic. Heterotrophic bacteria don’t synthesize their own food but they get food from other organisms or from dead organic matter. They can be divided in: - parasites, which live on other organisms called “hosts”, from which they obtain food. Disease causing parasites are known as pathogens. - saprophytes, which are decomposers that obtain their food from dead organic matter. - symbiontes, which live in association with other organisms for mutual benefit. Autotrophic organisms take inorganic substances into their bodies and transform them into organic nourishment. They are essential to life because they are the primary producers at the base of all food chains. Autotrophic bacteria are divided into two types: photoautotrophic bacteria and chemioautotrophic bacteria. Photoautotrophic bacteria, or better known as Cyanobacteria, make use of light energy to synthesize food. They are often called "blue-green algae", due to the presence of pigments used for photosynthesis and because they are photosynthetic in both aquatic and terrestrial ecosystems. All cyanobacteria contain chlorophyll a and most other pigments, giving the cells their characteristic color, depending on the balance of pigments within the cells. Chemoautotrophic bacteria use chemical energy to synthesize their food and to kill other organisms, classified in their own new taxonomic kingdom.

EUKARYOTIC CELL

eukaryotic cell

Eukaryotic cells are a fundamental unit of life, making up the building blocks of complex organisms, including plants, animals, fungi, and protists. Eukaryotes have multiple, linear chromosomes enclosed in a nuclear membrane, and many kinds of organelles Eukaryotic cells have a much greater diversity in their structure than prokaryotic cells: for instance, plant cells are very different from animal cells.

CYTOPLASM NUCLEUS

Cell nucleus is the cell brain that control all cellular activities. It contains the genetic material/DNA, a nucleotide polymer arranged in chromosomes, which contains the genetic information necessary to direct cell functions; it is surrounded by a nuclear envelope, a double membrane that encloses the entire organelle and isolates its contents from the cytoplasm. It is crossed by nuclear pores, that let pass the larger molecules,transported by carrier proteins across some channels and also allow free movement of small molecules and ions. Within the nucleus it is possible to observe a densely stained structure, not surrounded by a membrane, called nucleolus. His main roles are to synthesize rRNA and assemble ribosomes.In fact,after being produced in the nucleolus, ribosomes are exported to the cytoplasm where they translate mRNA and synthesize proteins.

The clear liquid that fills all kind of cells, both prokaryotic and eukaryotic, is called “cytoplasm”. It contains three parts: cytosol, organelles and cytoplasmic inclusions. - Cytosol is the liquid containing the organelles; it is mainly formed of water with some protein strands that help support the organelles. - Organelles are specialized parts of the cell, each having its own function including cellular respiration, creation of new proteins and destruction of waste material. - The cytoplasmic inclusions are formed during metabolic activities and perform various functions, which can be classified into three main groups: reserved products, secretory products, excretory products.

CELL MEMBRANE AND STRUCTURE

All cells are surrounded by a cell membrane, performing many functions. In particular, cell membranes: a) protect the integrity of cells, from the harmful chemicals in its external environment and from losing useful biological macromolecules.; b) control the movement of ions and molecules into and out of the cells; c) help support the cell and maintain its shape; d) interact with the membranes of adjacent cells to form plant and animal tissues. All the cells in our body have a membrane, called the plasma membrane, that separates the external environment from the internal one, called cytoplasm, a gelatinous substance in which a series of insoluble particles are immersed. Through the plasma membrane the cells assimilate the substances they need and eliminate waste products in the external environment. The currently accepted model for the plasma membrane structure, called the fluid mosaic model, is a mosaic of components, like phospholipids, cholesterol, and proteins, that can move about freely and fluidly in the plane of the membrane,and carbohydrates, that give membranes their flexibility.

RIBOSOMES AND ENDOPLASMATIC RETICULUM

Ribosomes are small organelles formed of molecules of RNA and proteins, found within the cytoplasm and on or in some other organelles. Prokaryotic cells can have tens of thousands of ribosomes. Eukaryotic cells can have hundreds of thousands of them, all making proteins. Their only function is to assemble proteins, translating the genetic information of DNA.

Once proteins are synthesized, they need to be moved toward different parts of the cell. The organelle that works to move proteins from one area of the cell to another is the endoplasmic reticulum (E.R.) which comes in two different forms: rough and smooth. The rough E.R. has ribosomes attached to it. The smooth E.R. does not have any ribosome on its surface. The primary job of the rough E.R. is the synthesis and modification of proteins destined for the cell membrane or for export from the cell. Typically, a protein is synthesized within a ribosome, attached on the membranes of the rough E.R.. Later, it is released inside the channels of the E.R., sometimes bonded to other proteins, forming glycoproteins, and finally transported within a vesicle, formed from the reticulum to the Golgi apparatus. One of the main functions of the smooth E.R. is the synthesis of lipids and steroid hormons. The smooth E.R. also metabolizes some carbohydrates and performs a detoxification role, breaking down certain toxins.

LYSOSOMES

Inside a cell, one of the key organelles involved in digestion and waste removal is the lysosome. They contain about 50 different enzymes that can hydrolyze proteins, DNA, RNA, polysaccharides and lipids, but also digest worn out organelles, food particles, foreign bodies like bacteria or viruses, and many substances that would become toxic if they were left in the cell. Lysosomes are spherical organelles, formed from tiny vesicles surrounded by a membrane having the same structure as the cell membrane. Tiny vesicles separate the cisternae from the Golgi apparatus; here, the various digestive enzymes produced in the endoplasmic reticulum and that are present in the lysosomes, are transported to the Golgi apparatus and finally distributed to the lysosomes.

GOLGI APPARATUS

The Golgi Apparatus is formed of flattened discs, known as cisternae. Their external membrane has the same structure as the plasma membrane and they contain specific enzymes that carry out the functions of the organelle. The Golgi apparatus is a part of a complex internal sorting and distributing system in the cell, working closely with the rough E.R.. When a protein is formed in the rough E.R., it passes through the cytoplasm within a vesicle and reaches the Golgi apparatus; there, it is combined with other molecules, forming more complex substances, and finally it is packaged in new vesicles and stored for later use or sent out of the cell, through a process called secretion. The Golgi apparatus is always found near the cell membrane, for the expulsions of some molecules out of the cell. The side of the apparatus that receives substances coming from the E.R. is called cis face. The side that sends out these substances, is called trans face. Sometimes, the molecules modified by the Golgi apparatus can become part of the cell membrane of the same cell or they can form other organelles, as lysosomes.

MITOCHONDRIA

All living things need energy in order to perform their daily functions: mitochondria are the organelles that produce energy through a process called cellular respiration; they are found both in animal and in plant cells. This term refers to the biochemical pathway through which cells, in the presence of oxygen (aerobic conditions), release energy from the chemical bonds of food molecules and provide that energy for the essential processes of life. Prokaryotic cells carry out cellular respiration within the cytoplasm or on the inner cell membrane surface. In eukaryotic cells , cellular respiration occurs in mitochondria. Mitochondria have an oval shape and are bounded by a double membrane. Each of these membranes is formed of a phospholipid bilayer with proteins. The outermost membrane is smooth, while the inner membrane has many folds called cristae. Mitochondria have a circular molecule of DNA, their ribosomes, make their own proteins, and have some control over their reproduction. Similar to bacteria, they replicate by a process known as binary fission.

plant cells and animal cells

Animal cells and plant cells have many features in common, such as nucleus, cytoplasm, cell membrane, E.R., mitochondria and ribosomes. Generally, plant cells are larger than animal cells and are often cuboid in shape. Plant cells also have extra parts: - cell wall provides plant cells rigidity, structural support, and cell to cell interaction; - chloroplast contains chlorophyll, which absorbs light energy for photosynthesis; - large vacuole stores water, metabolites, nutrients, and even waste.

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D'Anna Vittoria Ievoli Antonetta Monte Asia Rosa Musone Conisiglia Sagliano Marika

PROTEINS

Membrane proteins may extend partway into the plasma membrane, cross the membrane entirely, or be attached to its inside or outside surface. They perform differents functions dividing in: -Structural proteins help give the cell support and shape. -Receptor proteins help cells communicate with their external environment . -Transport proteins transport molecules across cell membranes through a process known as facilitated diffusion.

CELL WALL

Besides the presence of chloroplasts, another major difference between plant and animal cells is the presence of a cell wall. Cell walls are mainly composed of cellulose. They surround plasma membranes and provid protection against mechanical and osmotic stress. Cellulose fibers are long, linear polymers of hundreds of glucose molecules. Other components of cell walls are pectins, glycoproteins, hemicellulose and lignin.

MEMBRANE LIPIDS

Phospholipids are the major components of plasma membranes. They form a bilayer in which their hydrophilic (attracted by water) heads spontaneously arrange to face the aqueous cytosol and the extracellular fluid, while their hydrophobic (repelled by water) tails face each other. The lipid bilayer is semi-permeable, allowing only certain molecules to diffuse across the membrane. As the last component of the plasma membrane we find another lipid component, a steroid called cholesterol, that helps to stabilize the fluidity of the cell membrane.

ORGANELLES

There are tiny structures performing very specific functions within the cells, called organelles, that include nucleus, ribosomes, endoplasmic reticulum, Golgi apparatus, vacuoles, lysosomes, mitochondria, and, in plants, chloroplasts.

CARBOHYDRATES

Carbohydrates are present only on the outer surface, where they are attached to proteins, forming glycoproteins, or to lipids, forming glycolipids. Along with membrane proteins, these carbohydrates allow cells to recognize each other.

CHLOROPLAST

Plants are characterized by the unique ability of making photosynthesis, the synthesis of food molecules in the presence of sunlight. This ability is due to the presence of specialized organelles called chloroplasts. Chloroplasts are oval-shaped organelles surrounded by two membranes, made up of a phospholipid bilayer plus proteins: an outer and an inner membrane. The internal fluid substance that fills chloroplasts is called stroma. There are tiny membranes, called thylakoids, that contain chlorophyll and other pigments used in photosynthesis. To make this process, plants need carbon dioxide, water and sunlight. During photosynthesis: chlorophyll absorbs the sun’s energy, used to split water molecules into hydrogen and oxygen atoms. Oxygen is released from plant leaves into the atmosphere. Hydrogen and carbon dioxide are used to form glucose, which is the main source of food for plants.

LARGE VACUOLE

Plants use vacuoles to store water. Those tiny water bags, much larger than in animal cells, help support plant cells. Sometimes in these cells, some vacuoles contain pigments that give certain flowers their typical colors. The plant cell central vacuole also contains waste that tastes bitter to insects and animals.