pediagenosis: Cell
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Showing posts with label Cell. Show all posts
Showing posts with label Cell. Show all posts

Saturday, October 3, 2020

VERTEBRATES

VERTEBRATES

VERTEBRATES

The most complex animals are the vertebrates, including humans and other mammals. The human genome is approximately 3 billion base pairs—about 20 to 30 times larger than the genomes of C. elegans, Drosophila, or Arabidopsis—and contains about 20,000 protein-coding genes. Moreover, the human body is composed of more than 200 different kinds of specialized cell types. This complexity makes the vertebrates difficult to study from the standpoint of cell and molecular biology, but much of the interest in biological sciences nonetheless stems from the desire to understand the human organism. Moreover, an understanding of many questions of immediate practical importance (e.g., in medicine) must be based directly on studies of human (or closely related) cell types.

ARABIDOPSIS THALIANA

ARABIDOPSIS THALIANA

ARABIDOPSIS THALIANA

The study of plant molecular biology and development is an active and expanding field of considerable economic importance as well as intellectual interest. Since the genomes of plants cover a range of complexity comparable to that of animal genomes (see Table 1.2), an optimal model for studies of plant development would be a relatively simple organism with some of the advantageous properties of C. elegans and Drosophila. The small flowering plant Arabidopsis thaliana (mouse-ear cress) (Figure 1.19) meets these criteria and is therefore widely used as a model to study the molecular biology of plants.

Thursday, September 24, 2020

CAENORHABDITIS ELEGANS AND DROSOPHILA MELANOGASTER

CAENORHABDITIS ELEGANS AND DROSOPHILA MELANOGASTER

CAENORHABDITIS ELEGANS AND DROSOPHILA MELANOGASTER

The unicellular yeasts are important models for studies of eukaryotic cells, but understanding the development of multicellular organisms requires the experimental analysis of plants and animals organisms that are more complex. The nematode Caenorhabditis elegans (Figure 1.17) possesses several notable features that make it one of the most widely used models for studies of animal development and cell differentiation.

YEASTS

YEASTS

YEASTS

Although bacteria have been an invaluable model for studies of many con- served properties of cells, they obviously cannot be used to study aspects of cell structure and function that are unique to eukaryotes. Yeasts, the simplest eukaryotes, have a number of experimental advantages similar to those of E. coli. Consequently, yeasts have provided a crucial model for studies of many fundamental aspects of eukaryotic cell biology.

E. COLI

E. COLI

E. COLI

Because of their comparative simplicity, bacteria are ideal models for studying many fundamental aspects of biochemistry and molecular biology. The most thoroughly studied species of bacteria is Escherichia coli (E. coli), which has long been the favored organism for investigation of the basic mechanisms of molecular genetics. Most of our present concepts of molecular biology including our understanding of DNA replication, the genetic code, gene expression, and protein synthesis derive from studies of this humble bacterium.

THE DEVELOPMENT OF MULTICELLULAR ORGANISMS

THE DEVELOPMENT OF MULTICELLULAR ORGANISMS

THE DEVELOPMENT OF MULTICELLULAR ORGANISMS

Many eukaryotes are unicellular organisms that, like bacteria, consist of only single cells capable of self-replication. The simplest eukaryotes are the yeasts, which contain only slightly more genes than many bacteria (Table acteria, they are much smaller and simpler than most cells of animals or plants. For example, the commonly studied yeast Saccharomyces cerevisiae is about 6 μm in diameter and contains 12 million base pairs of DNA (Figure 1.9). Other unicellular eukaryotes, however, are far more complex cells, with substantially larger and more complex genomes. They include organisms specialized to perform a variety of tasks, including photosynthesis, movement, and the capture and ingestion of other organisms as food. The ciliated protozoan Paramecium, for example, is a large, complex cell that can be up to 350 μm in length and is specialized for movement and feeding on bacteria and yeast (Figure 1.10). Surprisingly, the Paramecium genome contains almost twice as many genes as humans (see Table 1.2), illustrating the fact that neither genome size nor gene number is directly related to the complexity of an organism an unexpected result of genome sequencing projects that will be discussed further in Chapters 5 and 6. Other unicellular eukaryotes, such as the green alga Chlamydomonas (see Figure 1.10), contain chloroplasts and are able to carry out photosynthesis.

THE ORIGIN OF EUKARYOTES

THE ORIGIN OF EUKARYOTES

THE ORIGIN OF EUKARYOTES

Eukaryotic cells are the third domain of life, called the Eukarya, which arose as a branch from the Archaea (Figure 1.7). A critical step in the evolution of eukaryotic cells was the acquisition of membrane-enclosed subcellular organelles, allowing the development of the complexity characteristic of these cells. It is likely that some organelles evolved from invaginations of the plasma membrane.

Monday, September 21, 2020

EUKARYOTIC CELLS

EUKARYOTIC CELLS

EUKARYOTIC CELLS

Like prokaryotic cells, all eukaryotic cells are surrounded by a plasma membrane and contain ribosomes. However, eukaryotic cells are much more complex and contain a nucleus and a variety of cytoplasmic organelles (Figure 1.6). The largest and most prominent organelle of eukaryotic cells is the nucleus, with a diameter of approximately 5 μm. The nucleus contains the genetic information of the cell, which in eukaryotes is organized as linear rather than circular DNA molecules. The nucleus is the site of DNA replication and of RNA synthesis; the translation of RNA into proteins takes place on ribosomes in the cytoplasm.

PROKARYOTES

PROKARYOTES

PROKARYOTES

Prokaryotes include cells of two domains, the Archaea and the Bacteria, which diverged early in evolution. The Archaea include cells that live in extreme environments that are unusual today but may have been preva lent in primitive Earth. For example, thermoacidophiles live in hot sulfur springs with temperatures as high as 80°C and pH values as low as 2. The Bacteria include the common forms of present-day prokaryotes a large group of organisms that live in a wide range of environments, including soil, water, and other organisms (e.g., human pathogens).

THE EVOLUTION OF METABOLISM

THE EVOLUTION OF METABOLISM

THE EVOLUTION OF METABOLISM

Because cells originated in a sea of organic molecules, they were able to obtain food and energy directly from their environment. But such a situation is self-limiting, so cells needed to evolve their own mechanisms for generating energy and synthesizing the molecules necessary for their replication. The generation and controlled utilization of metabolic energy is central to all cell activities, and the principal pathways of energy metabolism (discussed in detail in Chapter 3) are highly conserved in present-day cells. All cells use adenosine 5-triphosphate (ATP) as their source of metabolic energy to drive the synthesis of cell constituents and carry out other energy-requiring activities, such as movement (e.g., muscle contraction). The mechanisms used by cells for the generation of ATP are thought to have evolved in three stages, corresponding to the evolution of glycolysis, photosynthesis, and oxidative metabolism (Figure 1.4). The development of these metabolic pathways changed Earth’s atmosphere, thereby altering the course of further evolution. In the initially anaerobic atmosphere of Earth, the first energy-generating reactions presumably involved the breakdown of organic molecules in the absence of oxygen. These reactions are likely to have been a form of presentday glycolysis the anaerobic breakdown of glucose to lactic acid, with the net energy gain of two molecules of ATP. In addition to using ATP as their source of intracellular chemical energy, all present-day cells carry out glycolysis, consistent with the notion that these reactions arose very early in evolution. Glycolysis provided a mechanism by which the energy in preformed organic molecules (e.g., glucose) could be converted to ATP, which could then be used as a source of energy to drive other metabolic reactions. The development of photosynthesis is generally thought to have been the next major evolutionary step, which allowed the cell to harness energy from sunlight and provided independence from the utilization of preformed organic molecules. The first photosynthetic bacteria probably utilized H2S to convert CO2 molecules a pathway of photosynthesis still used by some bacteria. The use of H2O as a donor of electrons and hydrogen for the conversion of CO2 to organic compounds evolved later and had the important consequence of changing Earth’s atmosphere. The use of H2O in photosynthetic reactions produces the by-product free O2; this mechanism is thought to have been responsible for making O2 abundant in Earth’s atmosphere, which occurred about 2.4 billion years ago.

HOW DID THE FIRST CELL ARISE?

HOW DID THE FIRST CELL ARISE?

HOW DID THE FIRST CELL ARISE?

It appears that life first emerged at least 3.8 billion years ago, approximately 750 million years after Earth was formed. How life originated and how the first cell came into being are matters of speculation, since these events cannot be reproduced in the laboratory. Nonetheless, several types of experiments provide important evidence bearing on some steps of the process.

Monday, September 14, 2020

What Are Stem Cells?

What Are Stem Cells?


What Are Stem Cells?
Stem cells are incredibly special because they have the potential to become any kind of cell in the body, from red blood cells to brain cells. They are essential to life and growth, as they repair tissues and replace dead cells. Skin, for example, is constantly replenished by skin stem cells.
Inside a Nucleus

Inside a Nucleus

Inside a Nucleus

Surrounded by cytoplasm, the nucleus contains a cell’s DNA and controls all of its functions and processes such as movement and reproduction. There are two main types of cell: eukaryotic and prokaryotic. Eukaryotic cells contain a nucleus while prokaryotic do not. Some eukaryotic cells have more than one nucleus – called multinucleate cells – occurring when fusion or division creates two or more nuclei.

Monday, June 1, 2020

TYPES OF HUMAN CELL

TYPES OF HUMAN CELL


TYPES OF HUMAN CELL
NERVE CELLS
The cells that make up the nervous system and the brain are nerve cells or neurons. Electrical messages pass between nerve cells along long filaments called axons. To cross the gaps between nerve cells (the synapse) that electrical signal is converted into a chemical signal. These cells enable us to feel sensations, such as pain, and they also enable us to move.
CELL STRUCTURE EXPLAINED

CELL STRUCTURE EXPLAINED


CELL STRUCTURE EXPLAINED
There are around 75 trillion cells in the human body, but what are they and how do they work?
Cells are life and cells are alive. You are here because every cell inside your body has a specific function and a very specialised job to do. There are many different types of cell, each one working to keep the body’s various systems operating. A single cell is the smallest unit of living material in the body capable of life. When grouped together in layers or clusters, however, cells with similar jobs to do form tissue, such as skin or muscle. To keep these cells working, there are thousands of chemical reactions going on all the time.

Thursday, January 3, 2019

Membrane Transport Proteins And Ion Channels

Membrane Transport Proteins And Ion Channels


Membrane Transport Proteins And Ion Channels
Proteins provide several routes for the movement of materials across membranes: (i) large pores, constructed of several protein subunits, that allow the bulk flow of water, ions and sometimes larger molecules (e.g. aquaporin, Chapter 34; and the connexins, that combine on the connexons to form gap junctions between cells); (ii) transporter molecules, some of which use metabolic energy (either direct or indirect) to move molecules against chemical and/or electrical gradients; and (iii) ion channels, specialized to allow the passage of particular ion species across the membrane under defined conditions.
Body water compartments and physiological fluids

Body water compartments and physiological fluids


Body water compartments and physiological fluids
Osmosis
Osmosis is the passive movement of water across a semi-permeable membrane from regions of low solute concentration to those of higher solute concentration. Biological membranes are semi-permeable in that they usually allow the free movement of water but restrict the movement of solutes. The creation of osmotic gradients is the primary method for the movement of water in biological systems. This is why the osmotic potential (osmolality) of body fluids is closely regulated by a number of homeostatic mechanisms (Chapter 35).
Homeostasis And The Physiology Of Proteins

Homeostasis And The Physiology Of Proteins


Homeostasis And The Physiology Of Proteins
Claude Bernard (1813–1878) first described ‘le mileau intérieur’ and observed that the internal environment of the body remained remark- ably constant (or in equilibrium) despite the ever changing external environment. The term homeostasis was not used until 1929 when Walter Cannon first used it to describe this ability of physiological systems to maintain conditions within the body in a relatively constant state of equilibrium. It is arguably the most important concept in physiology.
Cells, Membranes And Organelles

Cells, Membranes And Organelles


Cells, Membranes And Organelles
The aqueous internal environment of the cell is separated from the aqueous external medium by an envelope of fat molecules (lipids) known as the plasma membrane. About half the cell is filled with cytosol, a viscous, protein-rich fluid between the internal structures. These consist of organelles which are themselves enclosed by lipid membranes, and components of the cytoskeleton such as microtubules and actin filaments which provide structural stability. The reticular appearance of the cell interior is due to organelles whose membranes are folded to maximize surface area. These include the rough endo- plasmic reticulum and Golgi apparatus, which are involved in protein assembly, and the smooth endoplasmic reticulum which serves as a store for intracellular Ca2+ and is the major site of lipid production (Fig. 3a).
Biological Electricity

Biological Electricity


Biological Electricity
Electrical events in biological tissues are caused by the movement of ions across the membrane. A potential difference exists across the membranes of all cells (membrane potential, Em), but only excitable tissues can generate action potentials (transient depolarization of a cell as a result of ion channel activity). Action potentials transmit information in nerve cells (Chapter 6) and trigger contractions in muscle cells (Chapter 12). Cell membranes are electrically polarized so that the inside is negative relative to the outside. In excitable tissues, resting Em  is usually between –60 and –90 mV.

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