CELLS Chapter 3
Learning Objectives • Describe what a cell is and the two general types of cells. • Describe the structure and functions of cell membranes. • Describe several ways in which molecules move across membranes. • Describe how cells are connected and how they communicate with each other. • Describe nine important landmarks in eukaryotic cells.
What is a Cell? cell: smallest unit of life that can function independently and perform all the necessary functions of life, including reproducing itself
Cell • A cell is a three-dimensional structure, like a fluid-filled balloon, in which many of the essential chemical reactions of life take place. • Nearly all cells contain DNA (deoxyribonucleic acid).
Cell • Although most cells are too small to see with the naked eye, there are a few exceptions, including hens’ eggs from the supermarket, each of which is an individual cell. • The ostrich egg, weighing about three pounds, is the largest of all cells.
Early Cell Biologists • Van Leeuwenhoek • Hooke • Schleiden • Schwann • Virchow
Anton Van Leeuwenhoek • first to microscopically observe microorganisms in water • first to see bacteria
Robert Hooke • discovered cell in 1655 • examined (under a coarse, compound microscope) very thin slices of cork and saw a multitude of tiny pores that he remarked looked like the walled compartments a monk would live in • did not know the real structure or function of a cell
What Hooke saw… Illustration from Robert Hooke's Micrographia, in which he proposed that living things were composed of minute structures called "cells."
Cell Theory Matthias Schleiden German botanist Theodor Schwann German physiologist Rudolf Virchow German pathologist
Cell Theory • All living things or organisms are made of cells and their products. • Cells are the basic building units of life. • New cells are created by old cells dividing into two. Schleiden & Schwann 1839 Virchow 1858
Landmarks in Cell Biology • 1595: Jansen (Holland) - 1st compound microscope (>1 lens) • 1626: Redi - argue against spontaneousgeneration • 1655: Hooke - “cells” in cork • 1674: Leeuwenhoek - protozoans • 1833: Brown - cell nucleus • 1839: Cell Theory (S & S) • 1840: von Roelliker - sperm/egg are also cells • 1857: Kolliker - mitochondria
Landmarks in Cell Biology • 1858: Virchow – cells from pre-existing cells • 1869: Miescher – DNA • 1879: Flemming – chromosome behavior during mitosis • 1898: Golgi – golgi apparatus • 1939: 1st TEM • 1953: Watson, Crick, Wilkins – DNA double helix • 1965: 1st commercial SEM
Prokaryotic cells are structurally simple, but there are many types of them
Every cell on earth is either… • a eukaryotic cell (from the Greek for “true nucleus”) • central control structure called a nucleus (contains cell’s DNA) • organisms composed of eukaryotic cells are called eukaryotes
…OR a prokaryote • no nucleus; its DNA simply resides in the middle of the cell • smaller, evolutionarily older, structurally more simple, unicelllular • metabolically diverse • some fuel activities in presence/absence of oxygen using almost any energy source on earth • sulfur in deep sea hydrothermal vents • hydrogen • sun
Eukaryotes… • 1.5 billion years after prokaryotes • not all eukaryotes are multicellular • Protista: nearly all are single-celled organisms visible only with a microscope
Which feature below is only found in eukaryotic cells? Ribosomes DNA Cell membrane Mitochondria
Endosymbiosis Theory • explains presence of two organelles in eukaryotes • chloroplasts (plants/algae) • mitochondria (plants/animals) • 2 different types of prokaryotes close partnerships w/each other • small prokaryotes capable of performing photosynthesis may have come to live inside a larger “host” prokaryote • photosynthetic “boarder” may have made some of energy from photosynthesis available to host
Endosymbiosis Theory • 2 cells become more and more dependent on each other until neither cell could live without the other, and they became a single, more complex organism • eventually, photosynthetic prokaryote chloroplast • similarly, prokaryote unusually efficient @ converting food and oxygen easily usable energy might take up residence in another prokaryote mitochondrion
Which evidence below supports the theory that mitochondria and chloroplasts were originally bacteria? Circular DNA is present in both organelles. Both organelles are larger than other organelles in the cell. Both organelles are surrounded by a single lipid bilayer. All of the above.
Plasma Membranes • lipid bilayer - phospholipids • head: • glycerol linked to a molecule containing phosphorous • polar - has an electrical charge (water is also a polar molecule and for this reason, other polar molecules mix easily with water.) • hydrophilic (“water loving”) • 2 legs are long chains of carbon and hydrogen atoms. • no electrical charge non-polar molecules • do not mix with water hydrophobic (“water fearing”) • hydrophilic head region mixes easily with water, whereas their hydrophobic tail region does not mix with water
Plasma Membranes • phospholipid bilayer: in a cell’s plasma membrane, 2 of these sheets of phospholipids are arranged so that the hydrophobic tails are all in contact with each other and the hydrophilic heads are in contact with the watery solution outside and inside the cell
What determines whether a protein resides on the surface or extends through the bilayer? • all the amino acids that make up each protein have side chains that differ from one another chemically • some side chains are hydrophilic • some are hydrophobic • as protein is assembled final shape, side chains can cause parts of protein to be attracted to hydrophobic or hydrophilic regions
What determines whether a protein resides on the surface or extends through the bilayer? • transmembrane protein • both hydrophobic and hydrophilic regions • peripheral membrane proteins • entirely hydrophilic structure • bind only to head regions of phospholipids • can be positioned on either outer or inner side of membrane
There are four primary types of membrane proteins, each of which performs a different function.
Lipids that make up the cell membrane are hydrophobic. Hydrophilic molecules, like glucose, cannot cross this barrier. What major component of the plasma membrane helps glucose get into and out of the cell? Phospholipids Carbohydrates Nucleic acids Proteins Fatty acids
The Plasma Membrane – “Fluid Mosaic” In addition to proteins, two other molecules are found in the plasma membrane: • Short, branched carbohydrate chains • serve as part of a membrane’s fingerprint along with recognition proteins • Cholesterol • helps the membrane maintain its flexibility, even at low temps • some cells ~25%, others (bacteria, plants) - none
Smallmouth Bass and Yellow Perch are considered warm water fish, whereas certain species of char are found in the arctic. Which fish do you think might have the most cholesterol in the membranes of its cells? Smallmouth Bass Yellow Perch Arctic char Both 1 and 2 are correct.
Faulty membranes can cause disease. • What is Cystic Fibrosis?
Membrane surfaces have a “fingerprint” that identifies the cell. Cells with an improper fingerprint are recognized as foreign and are attacked by your body’s defenses (antibodies).
Which of the cell membrane functions below is possible due to the presence of proteins in the plasma membrane? The cell membrane determines what molecules will enter or leave the cell. The cell membrane contains proteins that can bind to messages sent from other cells and relay the message to the interior of the cell. The cell membrane contains proteins that act as a fingerprint for distinguishing between self and foreign cells. Both 1 and 2 are correct. All of the above.
Passive transport is the spontaneous diffusion of molecules across a membrane. • Two kinds of passive transport • Diffusion • Osmosis
Diffusion and Concentration Gradients • Diffusion • passive transport (no energy needed) • Solute particle is dissolved in a gas or liquid (solvent) • moves from an area of high solute concentration to an area of lower concentration • molecules tend to move down their concentration gradient
Simple & Facilitated Diffusion • simple diffusion • molecules s/a O2, CO2 (small, no charge) pass directly through lipid bilayer of membrane w/o assistance • facilitated diffusion • spontaneous diffusion across a plasma membrane requires a transport protein • may be electrically charged repelled by hydrophobic middle region of phospholipid bilayer • OR may be too big to squeeze through the membrane • In either case, there must be a concentration gradient from one side of the membrane to the other
Defects in Transport Proteins • Can reduce or even bring facilitated diffusion to a complete stop • Serious health consequences • Many genetic diseases s/a cystinuria • when structured and functioning properly, this transport protein facilitates the diffusion of some amino acids (including cystine, from which the disease gets its name) out of the kidneys • when proteins are malformed, they cannot facilitate diffusion • amino acids build up in the kidneys, forming painful and dangerous kidney stones