BIO 10 Lecture 4 A SEPARATE SELF: THE CELL What is a Cell? • “The functional unit of all life forms” (Krogh) • The smallest enclosed system whereby the information molecule (DNA) can protect and perpetuate itself (Selfish Gene model) • “The smallest thing that can be said to be alive” (Dr. Ballard) – Direct a stream of negative entropy upon itself – Separate self from non-self – Reproduce – Evolve Common Characteristics of all Cells on Earth • Use DNA as their information molecule • Use proteins as their “workhorses” – Use the same 20 amino acids in their proteins – Use only “left-handed” amino acids • Have a phospholipid bilayer cell membrane • Produce and use ATP as their “energy currency” • Except for some notable exceptions, are between 1 micron and 100 microns in diameter Why are Cells the Size they Are? Learn the Metric System of measurement • 1 m = about 3 feet • 1 mm = 1/1000 of a meter (= 1/10th of a cm; Can be seen on a ruler but just barely!) • 1 um = 1/1,000,000 of a meter (much too small to see with the naked eye) • 1 nm = 1/1,000,000,000 of a meter – A DNA molecule is 2 nm wide Prokaryotes: 1 – 10 um in diameter Eukaryotes: 10-100 uM in diameter 1/10th of the distance between the smallest divisions on a metric ruler Plant cell: 100 um diameter Note that this scale is logarithmic! • The size of cells is influenced by trade-offs involving: – Predation considerations • Larger cells can engulf smaller cells more easily and are less likely to be eaten themselves • BUT ... larger cells must eat (or photosynthesize) more to sustain themselves – Surface:volume ratio • Cells must take in nutrients and dispose of waste through their cell membranes • As the diameter of a cell doubles, its volume increases 8-fold (2-fold in length, width, and height) • Thus, as a cell’s diameter increases, the cell membrane must become ever more efficient in order to still do its job – Diffusion • Molecules in the same biochemical pathways must be able to find one another • Prokaryotic cells have remained very small, like the earliest cells on Earth • Eukaryotic cells have been able to grow 100-1,000 times larger than prokaryotic cells – 10-fold bigger in all 3 dimensions = 1,000x the volume – Have a system of internal membranes that acts to: • Increase the total membranous surface available for cell metabolism • Separate the components of chemical reactions into different internal “bags” Prokaryotes – “Prokaryote” means “before a nucleus” – No internal membrane-bound organelles – just one little bag of cytoplasm – No nucleus – Usually single-celled (may form simple colonies) – May or may not require oxygen for survival. – Earliest types of cells on Earth – Cell type of all bacteria and Archaea • Much tougher than eukaryotes – Can survive almost anywhere – and do! • Have much greater genetic diversity than eukaryotes • Have a cell wall surrounding the cell membrane (different chemistry from plant cell wall) Eukaryotes – Means “true nucleus” – The nucleus is a membrane-bound organelle that encloses and protects the DNA – Contains many other membrane-bound organelles that have a variety of functions – Many eukaryotesare multi-cellular and highly complex - Usually require oxygen for survival - Cell type of all animals, plants, fungi, and protists (e.g. amoeba, diatoms) Common Components of Eukaryotic Cells • The five main components shared by all eukaryotic cells are: a nucleus, other membrane-bound organelles, a cytosol, a cytoskeleton, and a plasma membrane. Touring A Eukaryotic Cell • Cells can be viewed as tiny factories for the production of the proteins that sustain it (and the precious DNA cargo it holds) – This is simplistic, since other biomolecules are also important for life – But proteins do most of the work in the cell • Therefore, following the process of protein production is one of the best ways to tour a eukaryotic cell for the first time – The code for the amino acid sequence of each protein is carried by regions of the DNA called genes – Therefore, our tour of the cell will begin in the nucleus • In the nucleus: – DNA is enclosed in a double-thick membrane called the nuclear envelope • Humans have 46 DNA molecules per cell • Some organisms have more, some fewer – One strand of each gene is copied into an RNA molecule, which exits the nucleus (through nuclear pores in the nuclear envelope) and travels to where proteins are made, the cytoplasm. • Average size of a gene in bacteria: 2,000 bp DNA • Average size of a human gene: 20,000 bp DNA – A subset of RNAs, called messenger RNAs (mRNAs), carry the codes for making polypeptide chains (proteins) – In the cytoplasm: • mRNAs are recognized and bound by large protein/RNA complexes called ribosomes • Ribosomes “read” the messenger RNA molecules to produce the correct polypeptides • Most polypeptides are then released into the cytoplasm, where they fold into functional proteins polypeptide protein • Some of the messenger RNAs code for proteins that need to be secreted from the cell or be modified in some way before they can function properly – These mRNAs, along with their attached ribosomes, are transported to the endoplasmic reticulum (ER) • Consists of a series of flattened membrane sacs called cisternae. – Once at the ER membrane, the ribosomes continue “reading” the mRNAs to produce the encoded polypeptides • These polypeptides are then inserted through pores in the ER membrane into the interior of the ER for sorting, modification, and distribution • Some of the proteins remain in the ER at this point – They are “resident ER proteins” that function and belong there • Others continue on to the next membranebound organelle, the Golgi Apparatus – To facilitate this transfer, the ER membrane buds off to form balls of membranous sacs called transport vesicles – These sacs then travel (with their polypeptide cargo) to the Golgi Apparatus and merge with its membrane, dumping the polypeptides inside • The membrane of the Golgi Apparatus can also bud off. • Transport vesicles can then merge with the cell membrane to release polypeptides from the cell in a process called exocytosis Other Important Organelles/Cell Structures Animal Cells Include: – Lysosomes • The cell’s recycling organelles • Break down large molecules from food, defective organelles, or old proteins into their monomers for reuse – Mitochondria • Extract carbon and energy from food, using oxygen • Are themselves ancient bacteria that invaded an early “proeukaryote” • Contain their own bacterial DNA • Have bacterial-type ribosomes – The Cytoskeleton The Lysosome The Mitochondrion The Cytoskeleton Summary: The Cell as a Factory Plant and Animal cells contain many of the same structures and organelles • However, plant cells also contain these additional components: – Cell wall: functions include: structural strength, limiting water absorption, and protection. Composition is cellulose and lignin. – Central vacuole: functions include: storing nutrients and water, retains and degrades wastes – Plastids: functions include: gathering and storing nutrients, and photosynthesis (chloroplasts) • Chloroplasts also have a bacterial origin The Plant Cell The Chloroplast Short Review of Lecture 4 • What is the smallest functional unit that can be said to be “alive”? • How big are cells and why? • How are prokaryotic and eukaryotic cells the same? In what ways are they different? • What do plant and animal cells have in common? In what ways do they differ and why? • What is the purpose of DNA? For what does it encode and by what pathway? • How does each of the following molecules or organelles contribute to one of the four primary functions of life? DNA, ribosome, mitochondria, chloroplast, cell membrane?