All Living Organisms are Made Up of Units Called Cells
Cell Theory
All living creatures are made from 1 or more cells.
All cells are produced from previously existing cells (no spontaneous generation).
All cells appear to be descended from the first cell, which existed about 4 billion years ago.
For a species to exist its reproductive cells must be potentially immortal (no aging).
Our bodies start from a single cell and contain about 100,000,000,000,000 (1013) cells at maturity (Fig. 1.1).
There are 2 Basic Types of Cells: Prokaryotic and Eukaryotic.
Prokaryotic cells are more primitive, small and without organelles, e.g. bacteria, blue-green algae.
Eukaryotic cells are more advanced, larger, and contain organelles, e.g. all higher species: animals, plants, and fungi. Our cells are of the eukaryotic type.
Prokaryotic Cells are Small and Simple but Fast
Size: Mycoplasmas are 0.1–1 micron diameter, other bacteria are 1–10 micron diameter.
A few much larger exceptions are known.
High surface to volume (S/V) ratio controls metabolic rate.
Volume is proportional to cell radius cubed.
Surface is proportional to cell radius squared.
Small cells such as bacteria divide fast (~ 20 min). Have no nucleus hence DNA less protected, and mutates faster. One compartment, that is like a chemistry lab with a single beaker (all reactions taking place within a single compartment). Our bodies are not made of prokaryotic cells, but there are approximately 2.5 lbs (~1.3 kg) of bacteria living within our guts. Because these cells are small in size they actually outnumber our body cells by a factor of 10 to 1. Intestinal bacteria are believed to be beneficial- produce vitamins, stimulate the immune system.
Eukaryotic Cells are Large and Versatile but Slow
Size: typically 10–100 micron diameter, volumes typically 1000 to 1 million times larger than prokaryotes. Cell division is very slow (~ 20 hours). Have nucleus and DNA better protected hence slow mutation rate. Organelles allow many activities to take place within the same cell—like a chemistry lab with many beakers.
Visualizing and Isolating Organelles
Light microscope gives magnification of about 1000–1500 X.
Stains used to make structures stand out
Electron microscope allows magnification of around 400,000 X- excellent for seeing small organelles
Organelles can be isolated for study by centrifuge techniques.
Some of the Organelles Found in Eukaryotic Cells Come from Endosymbiosis
Cells often ingest other cells and digest them for food. Sometimes the ingested cell is not digested, but the 2 cells learn to live together for mutual benefit (endosymbiosis). Mitochondria and plant chloroplasts are believed to have originated in this way. These organelles have their own DNA and double membranes (2 bilayers).
The Cell Organelles are Found within the Cytosol
Cytosol is the liquid matrix of the cell mostly water (cytosol + organelles except nucleus = cytoplasm) that contains salts, dissolved molecules, enzymes, etc. Glycolysis (energy metabolism: anaerobic) takes place in cytoplasm.
The Cell Membrane Separates the Cytoplasm from the External World
Cell membrane is made of phospholipid and protein, which forms barrier to movement of things in and out of the cell. Hydrophobic molecules pass through it more readily than hydrophilic ones. Specialized transport mechanisms, selectively move materials across the membrane. Cell membrane is supported on inside by protein filaments (cytoskeleton).
The Cytoskeleton Determines the Shape of the Cell
Tubules imbedded in the cytosol form a meshwork of fibers that give the cell shape and are also used to transport structures within the cell (i.e. chromosomes in mitosis) and movement of the whole cell.
Three basic types of fibers are embedded in the cytosol, microtubules (made of tubulin, 25 nm dia), intermediate 4filaments (made of several proteins, 8–12 nm dia), and microfilaments (made of actin, 7 nm dia).
The Nucleus Contains the Molecule of Heredity: DNA
DNA (genetic information) is an archival copy of the genes organized into chromosomes that does not leave nucleus.
Genes are encoded in the DNA and each chromosome has many genes.
DNA is associated with proteins, which are involved in repair mechanism of DNA and turns genes on and off.
Nucleus may contain 1 or more nucleoli, which make ribosomes.
RNA is copy of gene; messenger RNA (mRNA) is made in nucleus. Nucleus, surrounded by 2 membranes (the nuclear envelope) with special pores, allows RNA out.
Most cells contain 1 nucleus, but a few have more, e.g. some liver cells and muscle cells. Mature red blood cell has lost its nucleus.
The Centrioles Organize the Mitotic Spindle for Cell Division
Centrioles are a pair of small structures found in the centrosome near the nucleus. Structure is similar to that of cilia that contain a set of 9 triplet tubules. In animal cells centrioles divide before cell division and help to organize the mitotic spindle (made of tubulin).
The Mitochondria are the Powerhouses of the Cell
Mitochondria are the site of cell respiration (Krebs cycle and electron transport) that requires oxygen to produce ATPs, the major source of cell energy. Mitochondria are covered by 2 bi-layer membranes. Typical cells have about 1000 mitochondria, but active cells like muscles will have more. Have small amounts of DNA, all your mitochondria come from your mother (very few in sperm) (Fig. 1.2).5
The above diagram shows the internal structure of a mitochondrion.
Krebs cycle is located in the internal matrix. NADH and FADH2 (produced by glycolysis and the Krebs cycle) deliver their hydrogens and electrons to the electron transport chain (ETC). The ETC pumps hydrogen ions into the intramembrane space (yellow); this sets up a pH gradient- pH 8 in the matrix and pH 7 in the intramembrane space. Hydrogen ions flow through a channel in the enzyme ATP synthase from the intramembrane space to the matrix (see arrows). This causes a shaft to rotate, and generates ATP in the matrix.
Proteins are Made on the Ribosomes
Ribosomes are made of RNA and proteins in the nucleolus, then leave nucleus and enter cytoplasm. Each ribosome has 2 subunits that decode the genetic code on mRNA (messenger RNA) and make protein (translation). Some ribosomes are free, but others attach to the endoplasmic reticulum, producing the rough endoplasmic reticulum (RER).
Proteins that are secreted by the cell or which go to other organelles are made on the rough endoplasmic reticulum. RER is prominent in cells that are secreting hormones and enzymes, e.g. pancreas cells.6
The Smooth ER is Involved in Synthesis of Lipids and Detoxification of Drugs
Smooth endoplasmic reticulum (SER) is made up of lipid membranes and free of ribosomes. They are involved in lipid metabolism- makes cell membranes, steroid hormones, etc.
Liver smooth ER has enzymes that detoxify drugs (cytochrome P450 system). In muscle special smooth ER (sarcoplasmic reticulum) accumulates Ca+ ions (trigger for muscle contraction). Smooth ER can be seen best in cells that make lipid hormones (ovary, testes, adrenal cortex) and in cells that detoxify drugs (liver).
Proteins are Processed and Routed in the Golgi Apparatus
Golgi apparatus is a set of stacked membranes compartments found near the nucleus. Compartments have different functions. Golgi is found in all cells but is especially well developed in cells that secrete proteins, e.g. plasma cells (secrete antibodies), and pancreatic acinar cells (secrete digestive enzymes). Proteins are processed in golgi by adding sugar molecules to side groups to protect proteins from breakdown, packages proteins into vesicles for secretion or internal use, and sorts proteins and routes them to the right destination: some go to mitochondria, others to lysosomes, some to cell membranes, etc.
Lysosomes Digest Materials within the Cell
These are small vesicles surrounded by membranes. They contain digestive enzymes that break down proteins, lipids, etc at an acidic pH (~4.5).
Defective cell parts are broken down so that they can be recycled. Also digest food brought into cell by phagocytosis and involved in apoptosis (programmed cell death).
Peroxisomes Deal with Reactive Oxygen Molecules Such as Peroxides
Contain the enzyme catalase, which converts hydrogen peroxide to O2 and water. They also contain another enzyme, urate oxidase. It plays an important role in fat metabolism.7
Cilia and Flagella Allow Cells to Move
Eukaryotic cilia and flagella are whip like projections from the cell. Both have same internal structure: 9 pairs of tubules arranged in circle, surrounding a central pair of tubules (called the 9 + 2 structure) (Fig. 1.3).
Beat repetitively (a bending motion) and cause cell to move (or move fluids along a surface of cells). Bending caused by a contractile protein, dynein, made of at least 200 different proteins enclosed within the cell membrane. Some biologists call them undulipodia.
Differences between Cilia and Flagella
Flagella much longer (50–200 microns length) than cilia: cells with flagella usually have only 1 or 2. Cilia are short (5–10 microns length); ciliated cells usually have hundreds. Example of flagellated cells in the body is sperm. Examples of ciliated cells in the body are cells lining the respiratory tract that moves mucus, fallopian tube that move egg cells, and spinal canal that help move cerebrospinal fluid.
Microvilli Increase the Surface Area of Cells
The projections on cell surface form the brush borders of cells that are sometimes confused with cilia, but are much smaller (1 micron length) and with a different structure. Projections are supported by cytoskeletal filaments- mostly the protein actin.
Microvilli increase the surface area of cells for faster absorption or secretion of materials.
Example of absorptive cells with microvilli is intestinal epithelium.
Example of secretary cells with microvilli is choroid plexus cells of brain that secrete cerebrospinal fluid.8
Specialized microvilli, called stereo cilia (misnamed), are found on the surface of the hair cells of the inner ear that respond to sound vibrations and are involved in hearing.
Defective Cell Organelles are Responsible for Some Diseases
Examples:
- Lysosomal storage diseases such as Tay-Sachs: lipids accumulate in lysosomes because they cannot be broken down.
- Cilia paralyzed by tobacco smoke and other pollutants cannot move mucus. Mucus accumulates in the lungs, impairing respiration
- Lactic acidosis can be produced by abnormal mitochondria with defective aerobic metabolism. In this situation lactic acid can accumulate in the blood.
There are About 250 Types of Specialized Cells in the Body
Cells specialize by turning genes on and off and by structural modifications. Examples of specialized cells:
Red blood cells: Specialized for carrying O2 to the tissues, they are loaded with hemoglobin (O2 carrying protein) and have no nuclei and mitochondria.
Nerve cells: Specialized for transmitting electrical impulses, have long axons- may be a meter or more in length. They have specialized Na+ and K+ channels for generating electricity. They require a special axonal transport mechanism to deliver proteins made in the cell body to the ends of the cell.
Muscle cells: Specialized for producing force by contraction, have special contractile proteins (actin and myosin), arranged in a sarcomere. These are very long cells: often attached to 2 bones.
Insulin-secreting cells (beta cells of pancreas): Gene for making insulin hormone is turned on in these cells. They have large amounts of rough endoplasmic reticulum, needed for synthesis and secretion of insulin (protein).