Cells: Structure & Function

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 (10^13) cells at maturity

There Are 2 Basic Types of Cells: Prokaryotic and Eukaryotic

• Prokaryotic cells are more primitive, small and without organelles
  • Bacteria, blue-green algae
• Eukaryotic cells are more advanced, larger, contain organelles
  • All higher species: animals, plants, fungi, protozoa
• Our cells are of the eukaryotic type

Small Prokaryotic Cells are Simple but Fast

• Size: mycoplasmas : 0.1-1 micron dia.; other bacteria 1-10 micron dia.
  • A few much larger exceptions are known
• High surface to volume (S/V) ratio
  • 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: DNA less protected, mutates faster
• One compartment- like a chem 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

Large Eukaryotic Cells are Slow but Versatile

• Size: typically 10-100 micron dia.; volumes typically 1000 to 1 million times larger than prokaryotes
• Low S/V ratio
• Cell division very slow (~ 20 hours)
• Have nucleus: DNA better protected, slow mutation rate
• Organelles allow many activities to take place within the same cell- like a chem lab with many beakers
• Visualizing & isolating organelles:
  • Light microscope gives magnification of about 1000-1500 X
    • Cannot see structures smaller than about 0.1 microns in diameter
    • 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)
• 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 & protein
• 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
• Supported on inside by protein filaments (cytoskeleton)

The Cytoskeleton Determines the Shape of the Cell

• Tubules are imbedded in the cytosol
• Form a meshwork of fibers that:
  • Give the cell shape
  • Are used to transport structures within the cell (i.e., chromosomes in mitosis)
  • Are involved in movement of the whole cell
• Three basic types of fibers:
  • Microtubules (made of tubulin, 25 nm dia)
  • Intermediate filaments (made of several proteins, 8-12 nm dia)
  • Microfilaments (made of actin, 7 nm dia)

The Nucleus Contains the Molecule of Heredity: DNA

• Contains the DNA (genetic information)– DNA does not leave nucleus- it is an archival copy of the genes
• DNA is organized into chromosomes
• Genes are encoded in the DNA; many genes on each chromosome
• DNA associated with protein- protein turns genes on and off
• Many repair mechanisms for DNA
• Nucleus may contain 1 or more nucleoli (for making ribosomes)
• RNA copy of gene is made in nucleus (transcription): messenger RNA
• Nucleus is surrounded by 2 membranes (the nuclear envelope) with special pores that let RNA out
• Most cells contain 1 nucleus, but a few have more
  • Some liver cells have multiple nuclei (polyploidy)
  • Muscle cells are very long and have hundreds of nuclei
  • 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 (see below): contain a set of 9 triplet tubules
• In animal cells they divide before cell division and help to organize the mitotic spindle (made of tubulin)
• Related to the basal bodies that are involved in organizing flagella

The Mitochondria are the Powerhouses of the Cell

• Sites of cell respiration (Krebs cycle & electron transport)
  • Require oxygen
  • Produce 36 ATPs/glucose molecule- major source of cell energy
• Covered by 2 bilayer membranes
• Probably produced from bacteria by endosymbiosis
• Typical cells have about 1000 mitochondria, but active cells like muscles will have more.
• Have small amounts of DNA (left over from when they were independent microorganisms?)
• All your mitochondria come from your mother (very few in sperm)
• The diagram shows the internal structure of a mitochondrion
  • Krebs cycle is located in the internal matrix (blue)
  • 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 in nucleolus, then leave nucleus and enter cytoplasm
• Made of RNA and protein
• Each has 2 subunits
• Decode the genetic code and make protein (translation)
• Some 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 ER
  • RER
• RER is prominent in cells that are secreting hormones and enzymes: i.e., pancreas cells

The Smooth ER Makes Lipids and Detoxifies Drugs

• Smooth endoplasmic reticulum, SER, is made up of lipid membranes, has no ribosomes
  • Involved in lipid metabolism- makes cell membranes, steroid hormones, etc.
  • Liver smooth SER 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 Finished off and Routed in the Golgi Apparatus

• Golgi apparatus is a set of stacked membranes compartments found near the nucleus
  • Compartments have different functions
• Golgi finishes proteins: adds sugar molecules to side groups, protects proteins from breakdown
• Packages proteins into vesicles for secretion or internal use
• Sorts proteins & routes them to the right destination: some go to mitochondria, others to lysosomes , some to cell membranes, etc.
• Golgi is found in all cells but is especially well developed in cells that secrete materials:
  • Plasma cells: secrete antibodies
• Pancreatic acinar cells: secrete digestive enzymes

Lysosomes Digest Materials within the Cell

• Small vesicles surrounded by membranes
• Lysosomes contain digestive enzymes that break down proteins, lipids, etc
• Break down defective cell parts so they can be recycled
• Also digest food brought into cell by phagocytosis
• Involved in apoptosis (programmed cell death)
• Require an acid pH inside (~4.5)

Peroxisomes Deal with Reactive Oxygen Molecules Such as Peroxides

• Contain the enzyme, catalase, which converts hydrogen peroxide to O2 and water
• Another enzyme, urate oxidase, sometimes forms crystals within the peroxisome
• Important in fat metabolism

Cilia and Flagella Allow Cells to Move

• Eukaryotic cilia & flagella are whiplike projections from the cell
  • Have same internal structure: 9 pairs of tubules arranged in circle, surrounding a central pair of tubules (called the 9 + 2 structure)
  • 
  • 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
  • Enclosed within the cell membrane
  • Made of at least 200 different proteins
  • Some biologists call them undulipodia
  • Differences between cilia & 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
  • Flagella in the body: sperm
  • Cilia in the body:
    • Respiratory tract lining: move mucus
    • Fallopian tube lining: move egg cells
  • Spinal canal lining: help move cerebrospinal fluid

Microvilli Increase the Surface Area of Cells

• Projections of cell surface: form the brush borders of cells
• Sometimes confused with cilia, but much smaller (1 micron length) and with a different structure
• Projections are supported by cytoskeletal filaments- mostly the protein actin
• Used to increase the surface are for faster absorption or secretion of materials
  • Absorptive cells with microvilli: intestinal epithelium
  • Secretory cells with microvilli: choroid plexus cells of brain- secrete cerebrospinal fluid
• Specialized microvilli, called stereocilia (misnamed), are found on the surface of the hair cells of the inner ear
  • The stereocilia respond to sound vibrations and are involved in hearing

Defective Cell Organelles are Responsible for Some Diseases

• Examples:
  • a) Lysosomal storage diseases such as Tay-Sachs: lipids accumulate in lysosomes because they cannot be broken down
  • b) Cilia paralyzed by tobacco smoke and other pollutants cannot move mucus. Mucus accumulates in the lungs, impairing respiration
  • c) Lacticacidosis- 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
    • Loaded with hemoglobin- O2 carrying protein
    • Have lost their nuclei and mitochondria
  • Nerve cells
    • Specialized for transmitting electrical impulses
    • Have long axons- may be a meter or more in length
    • Have specialized Na and K channels for generating electricity
    • Only a single nucleus in the cell body- requires 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 & myosin, arranged in a sarcomere
    • Very long cells: often attached to 2 bones
    • Formed by fusion of many smaller cells; contain many nuclei
  • Insulin-secreting cells (beta cells of pancreas)
    • Gene for making the insulin hormone is turned on
    • Contain large amounts of rough endoplasmic reticulum- needed for secretion

Cell Diagram:

This picture is modified from one published by Peter Cull, editor, in the copyright-free collection, The Sourcebook of Medical Illustration. Park Ridge, NJ: Parthenon, 1989.

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