Make your own free website on Tripod.com

THE CELLULAR LEVEL OF ORGANIZATION

INTRODUCTION

Cytological Techniques

1.An understanding of cellular structure has been possible only because impressive technical difficulties have been overcome.

CELLULAR ANATOMY

1. The micrometer (um) or micron (u) is the standard unit used to express dimensions at the cellular level.

2. A cell consists of cytoplasm surrounded by a cell membrane. The cytoplasm contains organelles, and the cytoskeleton provides structural support.

The Cell Membrane

1. The cell membrane is a lipid bilayer containing proteins, phospholipids, and cholesterol. Other lipids and carbohydrates make a small contribution.

2. The cell membrane performs the functions of physical isolation, metabolic isolation, and sensitivity. In organized tissues, the cell membranes provide structural support.

The Cytoplasm

1. The cytoplasm is a colloid containing water, organic and inorganic substances, and organelles.

Inclusions

1. Inclusions may be present within the cytoplasm. Inclusions may be stored nutrients, wastes, pigments, minerals, or secretory products.

Organelles

1. The cytoskeleton consists of microfilaments and microtubules that provide support and move intracellular materials and structures.

2. Centrioles, cilia, and flagella are composed of microtubules. Centrioles move chromosomes during cell division, cilia move materials past the cell surface, and flagella move the cells to which they are attached.

3. Mitochondria are responsible for 95 percent of the ATP production within a typical cell.

4. The nucleus contains most of the DNA within the cell. The DNA is complexed with histones to form chromatin strands.

5. The nucleus is surrounded by a nuclear envelope that communicates with the cytoplasm through nuclear pores.

6. Ribosomes are combinations of proteins and RNA. There are fixed ribosomes and free ribosomes. Ribosomes are sites of protein synthesis.

7. The endoplasmic reticulum extends throughout the cytoplasm.

8. The granular, or rough endoplasmic, reticulum is bound to ribosomes, and may accumulate synthesized proteins.

9. The agranular, or smooth endoplasmic, reticulum lacks ribosomes and may accumulate synthesized lipids.

10. The Golgi apparatus packages lysosomes, peroxisomes, and secretory vesicles. The Golgi apparatus may add carbohydrate components to secretions prior to their release.

Membrane Interactions

1. Through membrane flow, membrane components are continually exchanged, modified, and recycled.

CELLULAR PHYSIOLOGY

Passive Membrane Processes

1. Cell membranes are selectively permeable, and materials may enter through passive or active mechanisms.

Diffusion

1. Diffusion is a passive process that results from molecular movement. Diffusion will tend to eliminate regional differences in the concentration of dissolved materials.

2. Diffusion across a cell membrane depends upon the size, charge, and lipid solubility of the molecule.

3.Facilitated diffusion requires the presence of a carrier protein.

4. Cotransport involves the passive transport of two molecules at the same time.

Osmosis

1. Osmosis is the movement of water across a semipermeable membrane in response to differences in solute concentration. The force of movement is the osmotic pressure.

Filtration

1. In filtration, water and solutes are pushed across a membrane barrier.

Active Membrane Processes

Active Transport

1. Active transport mechanisms consume ATP and ignore

concentration gradients.

Endocytosis

1. Endocytosis is an active process that includes pinocytosis and phagocytosis.

The Transmembrane Potential

1. The transmembrane potential results from the uneven distribution of electrolytes across the cell membrane.

2. Potassium is the major cation inside the cell; sodium is most abundant outside. The primary anions within the cell are proteins; chloride ions are most abundant in the extracellular fluids.

3. Active transport mechanisms maintain the transmembrane potential and the differences between the intracellular and extracellular fluids.

CELLULAR METABOLISM

1. The study of how living things acquire and utilize energy

constitutes the study of metabolism.

2. Intermediary metabolism refers to the biochemical reactions that occur within individual cells.

3. General categories of cellular activities include anabolism (metabolic turnover, growth, secretion) and work (locomotion, transport, exocytosis and endocytosis, cell division).

4. Cells are continually breaking down organic molecules and synthesizing new ones.This process of cellular renewal is called metabolic turnover. Metabolic turnover ensures that a cell can adapt to meet changing environmental conditions.

5. Catabolism involves the breakdown of complex molecules into smaller fragments, and the release of energy.

Catabolic Processes

Phosphorylation

1. Energy is stored in high-energy bonds. ATP is the most important high-energy compound, but GTP, CTP, CP and UTP may be required for specific biochemical processes.

2. The creation of a high-energy bond is called phosphorylation. In substrate-linked phosphorylation the energy released by the breaking of a covalent bond is directly linked to the creation of a high-energy bond on another molecule.

3. Oxidative phosphorylation occurs in a series of steps. Hydrogen atoms are removed from a substrate, and their electrons passed along the electron transport system. An oxygen atom ultimately receives the electrons and combines with the hydrogen ions to form a water molecule.

4. Oxidation involves the donation of an electron with or without a hydrogen ion; reduction occurs when these items are accepted by some other molecule.

5. Coenzymes initiate oxidative phosphorylation by reducing substrate molecules. The coenzymes involved are NAD, FAD, FMN, and Coenzyme Q.

6. The electron transport chain consists of metalloproteins called cytochromes.

7. Every time a hydrogen or electron transfer occurs, energy is lost. If the loss is great enough, part of the energy will be trapped through the creation of an ATP molecule.

Mitochondria and Energy Production

1. The electron transport system is bound to the inner mitochondrial membranes. The enzymes and coenzymes of the Krebs cycle are found within the mitochondrial matrix.

Carbohydrate Catabolism

1. Aerobic glycolysis involves the complete breakdown of glucose to carbon dioxide and water.

2. Anaerobic glycolysis occurs when oxygen supplies are reduced. Glucose molecules are only partially broken down, and actate dehydrogenase converts the pyruvic acid to lactic acid.

Lipid Catabolism

1. Through beta oxidation fatty acids undergo lipolysis, producing two carbon fragments that can be plugged into the Krebs cycle.

Protein Catabolism

1. Proteins are broken down into amino acids, and the amine groups removed by ransamination or deamination reactions. This leaves a small carbon chain that can be catabolized further.

2. Most of the ammonia generated by deamination is neutralized by conversion to urea in the liver.

Nucleic Acid Catabolism

1. Pyrimidines are broken down to form amino acids that can be broken down further in the Krebs cycle.

2. Purines are deaminated and excreted as uric acid, another example of a nitrogenous waste.

Anabolic Processes

Carbohydrate Synthesis

1. Gluconeogenesis can occur using any component of the anaerobic glycolytic pathway. Acetyl-CoA cannot be utilized for glucose manufacture, so many amino acids and fatty acids are not suitable as substrates.

Lipid Synthesis

1. Lipogenesis involves several different pathways. Glycerol can be derived from the three-carbon molecules in the anaerobic glycolytic pathway. Other lipids can be manufactured from acetyl- CoA, with the notable exceptions of the essential fatty acids.

Protein Synthesis

1. A dietary supply of amino acids is essential because our cells cannot manufacture adequate quantities of ten essential amino acids.

2. The chromosomes in a single human cell contain all of the information required to manufacture over 100,000 different proteins.

3. The information is stored in the nucleotides of DNA strands. The message is preserved in the triplet code of individual genes.

4. The manufacture of a particular protein involves: activation of the gene, transcription of mRNA, and translation at a ribosome.

5. Ribosomal RNA forms part of the structure of the ribosomes. Molecules of tRNA bring amino acids to the active site of the mRNA/rRNA complex.

Nucleic Acid Synthesis

1. RNA is turned over rapidly, and RNA synthesis occurs at the nucleoli of the nucleus.

2. DNA replication resembles transcription, but the enzyme is a DNA polymerase, it attaches DNA nucleotides, and the entire strand is duplicated at one time.

The Regulation of Cellular Metabolism

1. Cells show adaptability, in changing their metabolic operations in response to internal or external variations.

2. Many reaction sequences are self-regulating, as local environmental factors alter the behavior of key enzymes. Hormones affect cells throughout the body by modifying enzyme activities and concentrations.

The Control of Genetic Activity

1. Individual genes may be controlled by negative feedback, inducers, or repressors.

CELL REPRODUCTION AND DIVERSITY

Mitosis

1. Mitosis refers to the division of somatic cells. Reproductive cells (sperm and eggs) are produced by meiosis.

2. In preparation for mitosis, a cell duplicates its DNA and increases in size. Over the same period the major organelles are replicated.

3. There are four phases of mitosis: prophase, metaphase, anaphase, and telophase. The cell then enters interphase.

4. The duration of interphase varies depending on the type of cell involved.Stem cells undergo frequent mitoses to replace other, more specialized cells.

The Origin of Cellular Diversity

1. differentiation involves the long-term inhibition of genes that may be active in other cells.

GO TO TISSUE LEVEL OF ORGANIZATION

RETURN