Active Transport

Active transport is the movement of molecules across a cell membrane in the direction against their concentration gradient, i.e. moving from an area of lower concentration to an area of higher concentration. Active transport is usually associated with accumulating high concentrations of molecules that the cell needs, such as ions, glucose and amino acids. If the process uses chemical energy, such as from adenosine triphosphate (ATP), it is termed primary active transport. Secondary active transport involves the use of an electrochemical gradient. Active transport uses cellular energy, unlike passive transport, which does not use cellular energy. Active transport is a good example of a process for which cells require energy. Examples of active transport include the uptake of glucose in the intestines in humans and the uptake of mineral ions into root hair cells of plants.


Specialized transmembrane proteins recognize the substance and allow it access(or, in the case of secondary transport, expend energy on forcing it) to cross the membrane when it otherwise would not, either because it is one to which the phospholipid bilayer of the membrane is impermeable or because it is moved against the direction of the concentration gradient. The last case, known as primary active transport, and the proteins involved in it as pumps, normally uses the chemical energy of ATP. The other cases, which usually derive their energy through exploitation of an electrochemical gradient, are known as secondary active transport and involve pore-forming proteins that form channels through the cell membrane.

Sometimes the system transports one substance in one direction at the same time as cotransporting another substance in the other direction. This is called antiport. Symport is the name if two substrates are being transported in the same direction across the membrane. Antiport and symport are associated with secondary active transport, meaning that one of the two substances is transported in the direction of its concentration gradient utilizing the energy derived from the transport of second substance (mostly Na+, K+ or H+) down its concentration gradient.

Particles moving from areas of lower concentration to areas of higher concentration(i.e., in the opposite direction as, or against, the concentration gradient) require specific trans-membrane carrier proteins. These proteins have receptors that bind to specific molecules (e.g., glucose) and thus transport them into the cell. Because energy is required for this process, it is known as 'active' transport. Examples of active transport include the transportation of sodium out of the cell and potassium into the cell by the sodium-potassium pump. Active transport often takes place in the internal lining of the small intestine.

Plants need to absorb mineral salts from the soil or other sources, but these salts exist in very dilute solution. Active transport enables these cells to take up salts from this dilute solution against the direction of the concentration gradient.

Primary active transport

The action of the sodium-potassium pump is an example of primary active transport.

Primary active transport, also called direct active transport, directly uses metabolic energy to transport molecules across a membrane.

Most of the enzymes that perform this type of transport are transmembrane ATPases. A primary ATPase universal to all animal life is the sodium-potassium pump, which helps to maintain the cell potential. Other sources of energy for Primary active transport are redox energy and photon energy (light). An example of primary active transport using Redox energy is the mitochondrial electron transport chain that uses the reduction energy of NADH to move protons across the inner mitochondrial membrane against their concentration gradient. An example of primary active transport using light energy are the proteins involved in photosynthesis that use the energy of photons to create a proton gradient across the thylakoid membrane and also to create reduction power in the form of NADPH.

Model of active transport

ATP hydrolysis is used to transport hydrogen ions against the electrochemical gradient (from low to high hydrogen ion concentration). Phosphorylation of the carrier protein and the binding of a hydrogen ion induce a conformational (shape) change that drives the hydrogen ions to transport against the electrochemical gradient. Hydrolysis of the bound phosphate group and release of hydrogen ion then restores the carrier to its original conformation.

Exocytosis Endocytosis

Exocytosis (/ˌɛksoʊsaɪˈtoʊsɪs/; from Greek ἔξω "out" and English cyto- "cell" from Gk. κύτος "receptacle") is the durable, energy-consuming process by which a cell directs the contents of secretory vesicles out of the cell membrane and into the extracellular space. These membrane-bound vesicles contain soluble proteins to be secreted to the extracellular environment, as well as membrane proteins and lipids that are sent to become components of the cell membrane. However, the mechanism of the secretion of intravesicular contents out of the cell is very different from the incorporation in the cell membrane of ion channels, signaling molecules, or receptors. While for membrane recycling and the incorporation in the cell membrane of ion channels, signaling molecules, or receptors complete membrane merger is required, for cell secretion there is transient vesicle fusion with the cell membrane in a process called exocytosis, dumping its contents out of the cell's environment. Examination of cells following secretion using electron microscopy demonstrate increased presence of partially empty vesicles following secretion. This suggested that during the secretory process, only a portion of the vesicular content is able to exit the cell. This could only be possible if the vesicle were to temporarily establish continuity with the cell plasma membrane, expel a portion of its contents, then detach, reseal, and withdraw into the cytosol (endocytose). In this way, the secretory vesicle could be reused for subsequent rounds of exo-endocytosis, until completely empty of its contents.







Endocytosis is an energy-using process by which cells absorb molecules (such as proteins) by engulfing them. It is used by all cells of the body because most substances important to them are large polar molecules that cannot pass through the hydrophobic plasma or cell membrane. The opposite process is exocytosis. 

Energy Conversion

Mitosis

Not to be confused with Meiosis, Miosis, Myositis, or Myosotis.

Mitosis in an animal cell.

Mitosis divides the chromosomes in a cell nucleus.

Onion (Allium) cells in different phases of the cell cycle enlarged 800 diameters.

a. non-dividing cells

b. nuclei preparing for division (spireme-stage)

c. dividing cells showing mitotic figures

e. pair of daughter-cells shortly after division

Mitosis is the cell cycle process by which chromosomes in a cell nucleus are separated into two identical sets of chromosomes, each in its own nucleus. In general, karyokinesis (duplication of the nucleus) is followed by cytokinesis, which divides the cytoplasm, organelles and cell membrane into two new cells containing roughly equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic (M) phase of the cell cycle—the division of the mother cell into two daughter cells, genetically identical to each other and to their parent cell.

The process of mitosis is fast and highly complex. The sequence of events is divided into stages corresponding to the completion of one set of activities and the start of the next. These stages are prophase, prometaphase, metaphase, anaphase, and telophase. During mitosis, the chromosomes, which have already duplicated, condense and attach to fibers that pull one copy of each chromosome to opposite sides of the cell. The cell then divides by cytokinesis to produce two genetically-identical daughter cells. Errors during mitosis can induce apoptosis (programmed cell death) or cause mutations. Certain types of cancer can arise from such mutations.

Mitosis occurs only in eukaryotic cells and the process varies in different organisms.

 For example, animals undergo an "open" mitosis, where the nuclear envelope breaks down before the chromosomes separate, while fungi undergo a "closed" mitosis, where chromosomes divide within an intact cell nucleus.Prokaryotic cells, which lack a nucleus, divide by a different process called binary fission.

Molecular Biology of the Gene Chromosomes, DNA Structure

DNA, Genes, Chromosomes,Nucleic acid structure

The building blocks of inheritance

Nucleic acid structure refers to the structure of nucleic acids such as DNA and RNA. Chemically speaking, DNA and RNA are very similar. Nucleic acid structure is often divided into four different levels: primary, secondary, tertiary and quaternary.DNA (deoxyribonucleic acid) is a biomolecule that holds the blueprint for how living organisms are built.  DNA is made out of two long, twisted strands that contain complementary genetic information (like a picture and its negative). A gene is a segment of DNA that is passed down from parents to children and confers a trait to the offspring. Genes are organized and packaged in units called “chromosomes.”  Humans have 23 pairs of chromosomes. One set of chromosomes for each pair comes from a person’s mother, and the other set of chromosomes comes from the father.                                                                                                

     

Mitosis and Cytokinesis

Mitosis is the cell cycle process by which chromosomes in a cell nucleus are separated into two identical sets of chromosomes, each in its own nucleus. In general, karyokinesis (duplication of the nucleus) is followed by cytokinesis, which divides the cytoplasm, organelles and cell membrane into two new cells containing roughly equal shares of these cellular components.Mitosis and cytokinesis together define the mitotic (M) phase of the cell cycle—the division of the mother cell into two daughter cells, genetically identical to each other and to their parent cell.

The process of mitosis is fast and highly complex. The sequence of events is divided into stages corresponding to the completion of one set of activities and the start of the next. These stages are prophase, prometaphase, metaphase, anaphase, and telophase. During mitosis, the chromosomes, which have already duplicated, condense and attach to fibers that pull one copy of each chromosome to opposite sides of the cell. The cell then divides by cytokinesis to produce two genetically-identical daughter cells. Errors during mitosis can induce apoptosis (programmed cell death) or cause mutations. Certain types of cancer can arise from such mutations.

Mitosis occurs only in eukaryotic cells and the process varies in different organisms. For example, animals undergo an "open" mitosis, where the nuclear envelope breaks down before the chromosomes separate, while fungi undergo a "closed" mitosis, where chromosomes divide within an intact cell nucleus. Prokaryotic cells, which lack a nucleus, divide by a different process called binary fission.

Cytokinesis is not, in the technical sense, a phase of mitosis but rather a separate process, necessary for completing cell division. In animal cells, a cleavage furrow (pinch) containing a contractile ring develops where the metaphase plate used to be, pinching off the separated nuclei. In both animal and plant cells, cell division is also driven by vesicles derived from the Golgi apparatus, which move along microtubules to the middle of the cell. In plants, this structure coalesces into a cell plate at the center of the phragmoplast and develops into a cell wall, separating the two nuclei. The phragmoplast is a microtubule structure typical for higher plants, whereas some green algae use a phycoplast microtubule array during cytokinesis. Each daughter cell has a complete copy of the genome of its parent cell. The end of cytokinesis marks the end of the M-phase.


There are many cells where mitosis and cytokinesis occur separately, forming single cells with multiple nuclei. The most notable occurrence of this is among the fungi and slime molds, but the phenomenon is found in various organisms. Even in animals, cytokinesis and mitosis may occur independently, for instance during certain stages of fruit fly embryonic development.[30]

Significance
Mitosis is important for the maintenance of the chromosomal set; each cell formed receives chromosomes that are alike in composition and equal in number to the chromosomes of the parent cell.

Mitosis occurs in the following circumstances:

Development and growth
The number of cells within an organism increases by mitosis. This is the basis of the development of a multicellular body from a single cell, i.e., zygote and also the basis of the growth of a multicellular body.
Cell replacement
In some parts of body, e.g. skin and digestive tract, cells are constantly sloughed off and replaced by new ones. New cells are formed by mitosis and so are exact copies of the cells being replaced. In like manner, red blood cells have short lifespan (only about 4 months) and new RBCs are formed by mitosis.
Regeneration
Some organisms can regenerate body parts. The production of new cells in such instances is achieved by mitosis. For example, starfish regenerate lost arms through mitosis.
Asexual reproduction
Some organisms produce genetically similar offspring through asexual reproduction. For example, the hydra reproduces asexually by budding. The cells at the surface of hydra undergo mitosis and form a mass called a bud. Mitosis continues in the cells of the bud and this grows into a new individual. The same division happens during asexual reproduction or vegetative propagation in plants.

Instant Notes in Molecular Biology

Instant Notes in Molecular Biology

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Instant Notes in Molecular Biology

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Book Description

The new edition of Instant Notes in Molecular Biology has been revised and updated to include information on micro RNAs, RNA inhibition, functional genomics, proteomics, imaging, stem cells and bioinformatics. Written in an accessible style, the book will be a highly useful tool for studying molecular biology.

Each topic begins with a summary of essential facts-an ideal revision checklist-followed by a description of the subject that focuses on core information, with clear, simple diagrams that are easy for students to understand and recall in essays and exams.

Table of Contents

A. Information Processing and Macromolecules
B. Properties of Nucleic Acids
C. Prokaryotic and Eukaryotic Chromosome Structure
D. DNA Replication
E. DNA Damage, Repair and Recombination
F. Transcription in Prokaryotes
G. Regulation of Transcription in Prokaryotes
H. Transcription in Eukaryotes
I. Regulation of Transcription in Eukaryotes
J. RNA Processing and RNPs
K. The Genetic Code and tRNA
L. Protein Synthesis
M. Bacteriophages and Eukaryotic Viruses
N. Cell Cycle and Cancer
O. Gene Manipulation
P. Cloning Vectors
Q. Gene Libraries and Screening
R. Analysis and uses of Cloned DNA
S. Functional Genomics and the new Technologies

Instant Notes in Molecular Biology

Instant Notes in Molecular Biology

********************************************

Instant Notes in Molecular Biology

Book Description

The new edition of Instant Notes in Molecular Biology has been revised and updated to include information on micro RNAs, RNA inhibition, functional genomics, proteomics, imaging, stem cells and bioinformatics. Written in an accessible style, the book will be a highly useful tool for studying molecular biology.

Each topic begins with a summary of essential facts-an ideal revision checklist-followed by a description of the subject that focuses on core information, with clear, simple diagrams that are easy for students to understand and recall in essays and exams.

Table of Contents

A. Information Processing and Macromolecules
B. Properties of Nucleic Acids
C. Prokaryotic and Eukaryotic Chromosome Structure
D. DNA Replication
E. DNA Damage, Repair and Recombination
F. Transcription in Prokaryotes
G. Regulation of Transcription in Prokaryotes
H. Transcription in Eukaryotes
I. Regulation of Transcription in Eukaryotes
J. RNA Processing and RNPs
K. The Genetic Code and tRNA
L. Protein Synthesis
M. Bacteriophages and Eukaryotic Viruses
N. Cell Cycle and Cancer
O. Gene Manipulation
P. Cloning Vectors
Q. Gene Libraries and Screening
R. Analysis and uses of Cloned DNA
S. Functional Genomics and the new Technologies

Instant Notes in Molecular Biology