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Biology ReviewCell Biology

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Much of the text material is from, “Essential Biology with Physiology” by Neil A. Campbell, Jane B. Reece, and Eric J. Simon

(2004 and 2008). I don’t claim authorship. Other sources were also used and are noted.

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Outline

• Cell theory• Microscopy• Cells and their components

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Cell Theory

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Cell Theory

• Cells were first described in 1665 by the British scientist, Robert Hooke, from microscopic examination of thin slices of cork from Mediterranean oak trees.

• Over the next two centuries, cells have been found in all organisms that were examined.

• The accumulation of evidence led to cell theory: all organisms are com-posed of cells.

• Cell theory was later expanded to encompass observations that all cells arise from previously existing cells—that is, cells do not spontaneously form.

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Cellular Structure of Cork

http://farm1.static.flickr.com

Robert Hooke’s drawing.

http://www.isa.utl.pt

http://cache.eb.com

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Microscopy

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Microscopic World of Cells

• Each and every cell in a living organism is intricate and extremely complex.

• The most elaborate machine, if it were reduced to the size of a cell, would seem simple in comparison.

• Cells must be very small for materials to to move in and out to meet their metabolic needs.

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Microscopic World of Cells (continued)

• Organisms are single-cellular (bacteria, archaea, and some pro-tista) or multi-cellular (other protista, fungi, plants, and animals).

• The human body has many trillions of cells that work cooperatively to perform their functions, which is a focus of this course on human physiology.

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Light Microscope

• The light microscope was invented during the Renaissance, about 400 years ago.

• Visible light passes through a specimen—the lens enlarges the image and projects it onto the human eye, film camera, or electronic sensing device.

• Modern light microscopes have compound lenses to reduce chromatic (color) aberration and spherical aberration to improve the quality of the viewed image.

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Light Microscope (continued)

http

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A modern lab and classroom version.

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Magnification and Resolving Power

• Two key aspects of microscopes are their magnification and resolving power.

• Magnification is the increase in an object’s apparent size compared to its actual size.

• Resolving power is the ability to show two or more objects as distinct entities.

• Due to limitations in resolving power (about 0.2 m), the maximum useful magnification of a light microscope ranges between X400 and X1000.

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Red blood cells and a stained white blood cell.

http

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Light Micrograph

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Another Light Micrograph

Coronal cross section of a rat brain under low magnification (about X10).

http://www.emsdiasum.com

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Electron Microscope

• Electron microscopes use beams of electrons rather than light to access the very small world.

• Resolving power is much higher than for light microscopes, allowing for much higher useful magnifications.

• Electron micrographs can be produced at magnifications of X100,000 or higher.

• The study of cells advanced rapidly when electron microscopes were developed in the 1950s.

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Electron Microscope (continued)

http

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An electron microscopy laboratory.

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Types of Electron Microscopes

• Scanning electron microscopes are used to study the surfaces of cells.

• Transmission electron microscopes are used to explore the inter-nal structures of cells.

• Electron microscopes are used with prepared (dead) specimens, while light microscopes are suited for either live or prepared speci-mens.

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Electron Micrograph

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A collection of pollens.

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Another Electron Micrograph

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Escherichia coli (E. coli).

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Cells and Their Components

http://www.steve.gb.com

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Prokaryotic and Eukaryotic Cells

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Eukaryotic cellPlants, animals, and fungi

(animal cell shown)

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Prokaryotic cellBacteria and archaea

(bacterium shown)

Not to scale: A prokaryotic cell is about 1000 times smaller in volume

than a eukaryotic cell.

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Cell Comparisons

• Prokaryotic cells first appeared about 3.5 billion years ago; eukary-otic cells appeared about 1.8 billion years later.

• Eukaryotic cells are much larger—they are about ten times the dia-meter of prokaryotic cells and their volume is even greater (by about a thousand times).

• The DNA in eukaryotic cells is in the nucleus surrounded by a mem-brane, while the DNA in prokaryotic cells is in an unenclosed central region.

• The two cell types use different processes for the replication of their DNA.

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Cell Comparisons (continued)

• Eukaryotic cells have several types of membrane enclosed organ-elles with specialized functions, while prokaryotic cells have many fewer.

• Eukaryotic cells use aerobic respiration and anaerobic respiration for chemical energy production, while prokaryotic cells only use an-aerobic respiration.

Organelle = a compartment within a cell that has a specialized function, for example, ribosome, lysosome, Golgi apparatus, or

mitochondrion. (http://sis.nlm.nih.gov/enviro/iupacglossary/glossaryo.html)

Most of these membrane-enclosed organelles, as we will discuss, are unique to eukaryotic cells.

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A Few Animal Cell Types

1. Blood

2. Purkinje (cerebellum)

Images 1 and 2, http://upload.wikimedia.orgImage 3, http://www.proteinpower.comImage 4, http://focus.harvard.edu

3. Adipose

4. Intestinal

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Components of Eukaryotic Cells

Component Animal cell Plant cell

Plasma membrane x xNucleus x xChromosomes x xCytoplasm x x �Ribosomes x xEndoplasmic reticulum x xGolgi apparatus x xLysosomes x RareMitochondria x xCytoskeleton x xVacuoles and vesicles x xFlagella and cilia x RareCentrioles x xCell wall -- xChloroplasts -- xCentral vacuole -- x

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Plasma Membrane

• A plasma membrane separates the intracellular space in a cell from its surrounding extracellular space.

• The plasma membrane defines the cell boundary.

• The membrane is a double layer (bilayer) of phospholipid molecules.

Computer generated graphichttp://www.sci-design.com

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Plasma Membrane (continued)

• The glycerol heads with their attached phosphate group orient toward the fluids in the intracellular and extracellular spaces because they are hydrophilic.

• The two lipid tails attached to the glycerol molecule orient inward since they are hydrophobic.

• Phospholipid membranes can be self-organizing due to their hydrophilic and hydrophobic properties.

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Plasma Membrane (continued)

• Receptors and other protein molecules are embedded in the plasma membrane.

• The plasma membrane is not a static structure of molecules fixed in place.

• Phospholipids and most proteins are drift about in the plane of the membrane, much like icebergs floating in the high-latitude oceans.

• The plasma membrane of a cell is often described as a fluid mosaic.

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Selective Permeability, Membrane Transport

• The plasma membrane and membranes that enclose organelles are selectively permeable.

• The membranes allow some substances to pass while blocking others.

• Some substances can diffuse across the plasma membrane including O2, CO2, and some nutrients.

• The passage of other substances requires transport proteins in the plasma membrane.

• Glucose, a major source for cellular energy, is attached to a transport protein to enter cells.

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Nucleus

• The nucleus of a cell contains the genetic code of life in the form of DNA.

• The nucleus is enclosed in a double membrane known as the nuclear envelope—it is similar in molecular structure to the cell’s plasma mem-brane.

• Pores in the nuclear envelope permit the passage of material between the nucleus and cytoplasm (specifically, messenger RNA and compon-ents of ribosomes).

Electron micrographhttp:.//www.science.org.au

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Nucleus (continued)

• DNA molecules and proteins in the nucleus form long fibers called chromatin.

• Each fiber makes-up one chromosome—humans usually have 46 chromosomes in the somatic cells of their bodies.

• A ball-like mass in the nucleus, called the nucleolus, produces the components of ribosomes.

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Cytoplasm

• The cytoplasm is the region between the cell’s plasma membrane and its nucleus.

• It contains organelles suspended in a fluid known as cytosol.

• Each type of organelle performs specific functions, as we will discuss.

• Most organelles in eukaryotic cells have their own phospholipid mem-branes.

Electron micrographhttp://www.danforthcenter.org

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Ribosomes

• Ribosomes are found in the cytoplasm, and often in close proximity to the cell nucleus.

• These organelles synthesize the polypeptides that make up proteins from amino acids.

Computer generated graphichttp://rna.ucsc.edu

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Ribosomes (continued)

• The genetic information of DNA is transferred via messenger RNA (mRNA) to the ribosomes to provide instructions for the synthesis of polypeptides that form proteins.

• Some ribosomes synthesize proteins for use in the cytoplasm or plasma membrane.

• Other ribosomes make proteins for secretion by the cell for use by other cells.

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Endoplasmic Reticulum

• The endoplasmic reticulum (ER) produces many types of biological molecules.

• ER is made-up of an elaborate system of tubes and sacs in the cyto-plasm.

• The two types of endoplasmic reticulum are rough ER and smooth ER.

Electron micrographhttp://www.bu.edu

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Rough ER

• Rough ER has the appearance of roughness due to the ribosomes that stud its surface.

• Rough ER produces proteins for the plasma membranes of cells and organelles, and for secretion by the cell.

• Cells that secrete substantial amounts of proteins, such as salivary glands of the mouth, are rich in rough ER.

• Products are sent to other locations in the cell in membrane covered packages called transport vesicles—the vesicles bud and separate from the ER.

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Smooth ER

• Smooth ER lacks the ribosomes that stud the surface of rough ER.

• One function is the synthesis of steroids in the testes, ovaries, and adrenal glands.

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Smooth ER (continued)

• Smooth ER in liver cells (known as hepatocytes) produce enzymes to detoxify drugs and poisons in the blood.

• The amount of smooth ER increases (up-regulates) with exposure to certain drugs.

• The body increases its tolerance to the drug, which requires higher dosages to achieve the same physiological effect.

• One hallmark of addiction is increased drug tolerance (along with a psychological component).

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Golgi Apparatus

• The Golgi apparatus works with the endoplasmic reticulum to refine, store, and distribute molecules synthesized by the cell.

• Products synthesized in the ER reach the Golgi apparatus via trans-port vesicles.

• Enzymes in the Golgi apparatus modify many of the products synthe-sized by the ER.

Electron micrographhttp://www.bu.edu

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Golgi Apparatus (continued)

• The Golgi apparatus is named for its discoverer, Camillo Golgi.

• It works with the ER to refine, store, and distribute chemical products.

• The Golgi apparatus tags proteins with addresses of their destinations within the cell.

• Vesicles that bud from the Golgi apparatus distribute products to other organelles.

• Other products are distributed to the plasma membrane for secretion by the cell.

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Lysosomes

• Lysosomes are membrane-enclosed sacs of enzymes for digestion to prevent self destruction of the cell.

• Enzymes in the lysosomes breakdown macromolecules including pro-teins, glycogen, fats, and nucleic acids.

• Molecules from this process nourish the cell.

Electron micrographhttp://biology.unm.edu

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Lysosomes (continued)

• Other lysosomes function as recycling centers by engulfing and digest-ing damaged organelles and making some of these molecules available for the synthesis of new organelles.

• Lysosomes in white blood cells ingest bacteria—their enzymes destroy bacterial cell walls.

• Another type of lysosome destroys the webbing that joins the fingers in human embryos.

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Mitochondria

• Mitochondria perform cellular respiration to harvest chemical energy for cellular work.

• The processes, known as the Krebs or citric acid cycle and the elec-tron transport chain, are aerobic since as require a continuing supply of oxygen molecules.

• Sugars and other types of food molecules are converted to a form of energy known as ATP.

Electron micrographhttp://is2.okcupid.com

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Mitochondria (continued)

• The inner membrane of a mitochondrion contains enzymes and other molecules for cellular respiration.

• The membrane has many folds to increase its surface area and maxi-mize ATP output.

• Mitochondria may have been invaders in the earliest eukaryotic cells.

• Now, a symbiotic relationship exists between mitochondria and eukary-otic cells.

Mitochondria—plural; mitochondrion—singular.Symbiotic = an interaction between dissimilar organisms,

especially when they benefit each other.

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Mitochondria (continued)

• Mitochondrial DNA is passed through maternal lineage from mother to daughter.

• The DNA maintains remarkable stability from generation to generation.

• These aspects enable mitochondrial DNA to be used in tracing popula-tion groups 10,000 years or more back in time as they have migrated.

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Cytoskeleton

• Microtubules of several different types of proteins form a network of fibers known as the cytoskeleton.

• The cytoskeleton is found in the cytoplasm.

• It provides structural support for a cell and a means for specialized movements.

Color enhanced electron micrographhttp://www.bcsb.org

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Cytoskeleton (continued)

• Microtubules also hold the organelles in place in the cytoplasm and guide the movement of vesicles.

• Other microtubules guide the movement of individual chromosomes when cells divide.

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Cytoskeleton (continued)

• Unlike a bony skeleton, a cytoskeleton can be rapidly dismantled in one location of the cell to reform in a new location.

• This process occurs through the removal and replacement of its pro-tein units.

• It contributes to the crawling motion of single-cell amoeba and move-ment of white blood cells.

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Vacuoles and Vesicles

• Vacuoles and vesicles are membrane enclosed sacs that bud from a cell’s plasma membrane, endoplasmic reticulum, and Golgi appar-atus.

• They differ in their functions—for example, vacuoles in the plasma membrane of a cell engulf food molecules for transportation to the lysosomes.

Electron micrographhttp://www.pharmacology.com

In neurons, vacuoles known as synaptic vesicles store neurotransmitters to com-municate with other neurons, muscles,

and glands.

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Flagella

• Some eukaryotic cells have appendages and specialized microtub-ules that enable movement.

• Flagella (singular, flagellum) propel cells by an undulating, whip-like motion.

• Flagella usually occur singly—for example, in sperm that must travel the length of the female reproductive tract to fertilize an egg released by an ovary.

Human sperm, Electron micrographhttp://www2.sunysuffolck.edu

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Cilia

• Cilia, which are usually shorter and more numerous than flagella, pro-duce motion through rhythmic back-and-forth movements (think of the rows of oars on an ancient galley ship).

• Cilia of cells in the oviducts (fallopian tubes) can sweep a fertilized egg along the reproductive path for implantation in the uterus.

http://www.gibinquirer.net Electron micrographhttp://www.talbotcentral.ucr.edu

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Cilia (continued)

• Tobacco smoke can damage or destroy the cilia in the bronchial tubes, which interferes with the body’s normal means for removing pollutants from the lungs.

• Smoker’s cough is the body’s compensatory attempt to cleanse the respiratory system.

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Extracellular Matrix

• Most animal cells secrete a thick, sticky substance known as the extracellular matrix.

• It helps hold cells together in tissues, and provides protective and supportive functions.

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Cell Junctions

• The cells in many animal tissues are often connected by cell junctions.

• Tight junctions bind cells to form a leak-proof sheet of tissue such as in the small intestine and large intestine to prevent fluids from leaking into the abdominal cavity.

• Anchoring junctions bind cells together while allowing some molecules to pass among the spaces between them.

• Communicating junctions have channels that permit water and other small molecules to flow among neighboring cells.

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Centrioles

• Centrioles are can-shaped structures of microtubules in the cyto-plasm that support cell division.

• We will cover their functions when we discuss mitosis and meiosis in the next lecture.

Computer generated graphichttp://www.sparkleberrysprings.com