Chapter 6. The Cell: Basic Unit of Life Adapted from: Kim Foglia

Why do we study cells?

Cell Theory All organisms are made up of cells The cell is the basic living unit of organization for all organisms All cells come from pre-existing cells Today, we take many things in science for granted. Many experiments have been performed and much knowledge has been accumulated that people didn’t always know. For centuries, people based their beliefs on their interpretations of what they saw going on in the world around them without testing their ideas to determine the validity of these theories — in other words, they didn’t use the scientific method to arrive at answers to their questions. Rather, their conclusions were based on untested observations. Among these ideas, for centuries, since at least the time of Aristotle (4th Century BC), people (including scientists) believed that simple living organisms could come into being by spontaneous generation. This was the idea that non-living objects can give rise to living organisms. It was common “knowledge” that simple organisms like worms, beetles, frogs, and salamanders could come from dust, mud, etc., and food left out, quickly “swarmed” with life. For example: Observation: Every year in the spring, the Nile River flooded areas of Egypt along the river, leaving behind nutrient-rich mud that enabled the people to grow that year’s crop of food. However, along with the muddy soil, large numbers of frogs appeared that weren’t around in drier times. Conclusion: It was perfectly obvious to people back then that muddy soil gave rise to the frogs. Observation: In many parts of Europe, medieval farmers stored grain in barns with thatched roofs. As a roof aged, it was not uncommon for it to start leaking. This could lead to spoiled or moldy grain, and of course there were lots of mice around. Conclusion: It was obvious to them that the mice came from the moldy grain. (continued)

Biological diversity & unity Underlying the diversity of life is a striking unity DNA is universal genetic language Cells are the basic units of structure & function lowest level of structure capable of performing all activities of life Observation: Since there were no refrigerators, the mandatory, daily trip to the butcher shop, especially in summer, meant battling the flies around the carcasses. Typically, carcasses were “hung by their heels,” and customers selected which chunk the butcher would carve off for them. Conclusion: Obviously, the rotting meat that had been hanging in the sun all day was the source of the flies. In 1668, Francesco Redi, an Italian physician, did an experiment with flies and wide-mouth jars containing meat. This was a true scientific experiment — many people say this was the first real experiment — containing the following elements: Observation: There are flies around meat carcasses at the butcher shop. Question: Where do the flies come from? Does rotting meat turn into or produce the flies? Hypothesis: Rotten meat does not turn into flies. Only flies can make more flies. Prediction: If meat cannot turn into flies, rotting meat in a sealed (fly-proof) container should not produce flies or maggots. Testing: Wide-mouth jars each containing a piece of meat were subjected to several variations of “openness” while all other variables were kept the same. Control Group:These jars of meat were set out without lids so the meat would be exposed to whatever it might be in the butcher shop. Experimental Group(s): One group of jars were sealed with lids, and another group of jars had gauze placed over them. Replication: Several jars were included in each group. (continued)

Activities of life Most everything you think of a whole organism needing to do, must be done at the cellular level… reproduction growth & development energy utilization response to the environment homeostasis Data: Presence or absence of flies and maggots observed in each jar was recorded. In the control group of jars, flies were seen entering the jars. Later, maggots, then more flies were seen on the meat. In the gauze-covered jars, no flies were seen in the jars, but were observed around and on the gauze, and later a few maggots were seen on the meat. In the sealed jars, no maggots or flies were ever seen on the meat. Conclusion(s): Only flies can make more flies. In the uncovered jars, flies entered and laid eggs on the meat. Maggots hatched from these eggs and grew into more adult flies. Adult flies laid eggs on the gauze on the gauze-covered jars. These eggs or the maggots from them dropped through the gauze onto the meat. In the sealed jars, no flies, maggots, nor eggs could enter, thus none were seen in those jars. Maggots arose only where flies were able to lay eggs. This experiment disproved the idea of spontaneous generation for larger organisms. The cell senses and responds to its environment and exchanges materials & energy with its surroundings. All cells are related through common descent, but evolution has shaped diverse adaptations.

How do we study cells? Microscopes opened up the world of cells Robert Hooke (1665) the 1st cytologist Drawings by Hooke cork flea Examples of Hooke's detailed drawings can be seen in the illustration of cork & flea above. It was in his description of cork that he first used the term "cell" even though he did not kno how important his discovery would become. The cell wasn't really understood until 1839 when scientists began to discover its importance. As it turned out, the flea, Ceratophyllus faciatus in this illustration was the carrier of the Bubonic Plague that was sweeping through Europe at time. However, this was not known by Hooke. Bubonic plague was a bacterial disease carried by fleas of infected Old English rats. At its worst, it killed two million people a year. Those that caught the disease had a 90% chance of dieing from it.

Technology advancing science How do we study cells? Microscopes light microscopes electron microscope transmission electron microscopes (TEM) scanning electron microscopes (SEM) Microscopes provide windows to the world of the cell Technology advancing science

Light microscopes 0.2µm resolution ~size of a bacterium visible light passes through specimen can be used to study live cells While a light microscope can resolve individual cells, it cannot resolve much of the internal anatomy, especially the organelles. To resolve smaller structures we use an electron microscope (EM), which focuses a beam of electrons through the specimen or onto its surface.

Electron microscope 1950s 2.0nm resolution 100 times > light microscope reveals organelles but can only be used on dead cells

Transmission electron microscopes TEM used mainly to study internal structure of cells aims an electron beam through thin section of specimen rabbit trachea cucumber seed leaf to enhance contrast, the thin sections are stained with heavy metals

Scanning electron microscopes SEM studying surface structures sample surface covered with thin film of gold beam excites electrons on surface great depth of field = an image that seems 3-D rabbit trachea

SEM images grasshopper

SEM images spider head

Isolating organelles Cell fractionation separate organelles from cell variable density of organelles ultracentrifuge What organelle would be heaviest? What organelle would be lightest? Make sure you know the difference between supernatant & pellet What organelle would be heaviest? nucleus What organelle would be lightest? ribosome

Ultracentrifuge spins up to 130,000 rpm forces > 1 million X gravity (1,000,000g) Why is it in a BIG thick lead-lined housing? Ultracentrifuge: really fast, but really big. 1 foot thick lead walls Why? And why don’t we have one? Because every now and then one of these things disintegrates while its spinning. When they blow, they really blow!! A real lot of energy in something spinning 100,000 rpm & the rotor is made of titanium… Titanium shrapnel!

Microcentrifuge Biotechnology research study cells at protein & genetic level

Adapted from: Kim Foglia Tour of the Cell Adapted from: Kim Foglia

Cell characteristics All cells: surrounded by a plasma membrane have cytosol semi-fluid substance within the membrane cytoplasm = cytosol + organelles contain chromosomes which have genes in the form of DNA have ribosomes tiny “organelles” that make proteins using instructions contained in genes

Types of cells Prokaryotic vs. eukaryotic cells Location of chromosomes Prokaryotic cell DNA in nucleoid region, without a membrane separating it from rest of cell Eukaryotic cell chromosomes in nucleus, membrane-enclosed organelle

Cell types Prokaryote Eukaryote internal membranes

The prokaryotic cell is much simpler in structure, lacking a nucleus and the other membrane-enclosed organelles of the eukaryotic cell.

Eukaryotic cells Eukaryotic cells are more complex than prokaryotic cells within cytoplasm is a variety of membrane-bounded organelles specialized structures in form & function Eukaryotic cells are generally bigger than prokaryotic cells

Limits to cell size Lower limit Upper limit smallest bacteria, mycoplasmas 0.1 to 1.0 micron (µm = micrometer) most bacteria 1-10 microns Upper limit eukaryotic cells 10-100 microns micron = micrometer = 1/1,000,000 meter diameter of human hair = ~20 microns

What limits cell size? Surface to volume ratio as cell gets bigger its volume increases faster than its surface area smaller objects have greater ratio of surface area to volume What cell organelle governs this? square - cube law As cell gets larger, volume increases cubically, but surface area only increases by the square. The volume of the cell is demanding… it needs exchange. The surface area is the exchange system… as cell gets larger, the surface area cannot keep up with demand. Instead of getting bigger, cell divides -- mitosis. Why is a huge single-cell creature not possible? s:v 6:1 ~1:1 6:1

Limits to cell size Metabolic requirements set upper limit in large cell, cannot move material in & out of cell fast enough to support life aa aa What process is this? CH NH3 aa CHO O2 CH CHO CO2 CHO CO2 CO2 What process is this? diffusion What’s the solution? cell divides make a multi-celled creature = lots of little cell, rather than one BIG cell aa NH3 NH3 O2 O2 CHO NH3 aa CO2 CH aa CH O2 aa O2 What’s the solution?

How to get bigger? Become multi-cellular (cell divides) aa aa aa O2 aa But what challenges do you have to solve now? CO2 CO2 O2 NH3 aa NH3 aa CO2 NH3 O2 CO2 CO2 CH CHO CO2 NH3 Larger organisms do not generally have larger cells than smaller organisms — simply more cells What’s challenges do you have to solve now? how to bathe all cells in fluid that brings nutrients to each & removes wastes from each aa O2 NH3 NH3 CO2 CO2 CO2 CHO aa NH3 NH3 NH3 CH CHO CO2 CO2 O2 aa aa CH

Cell membrane Exchange organelle plasma membrane functions as selective barrier allows passage of O2, nutrients & wastes

Organelles & Internal membranes Eukaryotic cell internal membranes partition cell into compartments create different local environments compartmentalize functions membranes for different compartments are specialized for their function different structures for specific functions unique combination of lipids & proteins To do their job effectively, each organelle has specific resource requirements and need to maintain internal environment distinct from the general cytosol, so each organelle is bound by a membrane… a selectively permeable membrane that allows it to control the local environment