Lecture 3 - Monday, January 27, 2001 R. Jones Chapters 4 and 16
More on Origins of Life and the Cellular Basis of Life
1. Prokaryotic organisms relatively simple and small and are about 1-5 micrometers in length/diameter
2. Eukaryotic cells larger and can be up to 100 micrometers in length/diameter. Increasing cell size was allowed because cytoplasm became divided into subcompartments called ORGANELLES. This occurred about 1.5 bya.
3. Eukaryotic cells all contain a nucleus surrounded by nuclear envelope or nuclear membrane. Enclosed in the nucleus is the nucleolus (makes ribosomes) and chromtin (DNA and protein complex). Mitochondria are found in all eukaryotic cells. These are double-membrane bound organelles with infodings of the inner membrane referred to as cristae. Mitochondria have a circular chromosome and 70S ribosomes. Mitochondria are important in the conversion of food to usable chemical energy. Endoplasmic reticulum (ER) synthesizes proteins on 80S ribosomes attached to the ER membrane. Golgi bodies are involved in packaging of the cellular materials synthesized by the ER and moving these materials to the cell exterior, the process of secretion. ER and Golgi like lysosmes/vacuoles are single membrane bound organelles. Lysosomes/vacuoles contain enzymes that are involved in cell death.
4. In addition to these named membrane-bound organelles, photosynthetic eukaryotes (protists and plants) contain chloroplasts. These are double-membrane bound organelles that have extensive internal membranes called thylakoids. Thylakoids have photosynthetic pigments inserted within them and they act to harvest light for photosynthesis. Chloroplasts also contain a circular chromosome and 70S ribosomes.
5. Other, non-membrane bound strcutures are found in the cytoplasm, including the cytoskeleton (microtubules , microfilaments and intermediate filaments), a system of cables or wires made from proteins that allow things to move within the cell. Ribosomes of the 80S type are also found floating around within the cytosol.
6. Evolution of the eukaryotic cell is thought to have occurred by the process of endosymbiosis. In this process it it thought that early prokaryotic cells engulfed each other causing cells within cells to be formed. Evidence that mitochondria and chloroplasts arose by such a process comes from the observation that these organelles are surrounded by double membranes (this would have happened as they were engulfed by a host cell) and have 70S ribosomes and a cicular chromosome typical only of prokaryotic cells. Endosymbiosis is also a plausible idea because it happens on a frequent basis today. Sea slugs and snails that feed on unicellular protists and cyanobacteria incorporate these organisms into their cells where they can carry out photosynthesis and provide the host with food. Membranes of the endoplasmic reticulum and nuclear envelope couls have formed as a result of the invagibations of the cell membrane, also known as the plasmalemma.
7. Multicellular organisms evolved from single-celled organisms. Materials (oxygen, carbon dioxide, sugars, amino acids etc.) move into and out of cells by the process of diffusion. This is a physical process occurring by random movement of molecules along concentration gradients. Diffusion across small distances (e.g. micromemters) can be relatively rapid (seconds to minutes) but movement of small moeclues over large distances (e.g meters) takes a very long time (years!). Cells therefore can not be too large otherwise they will be unable to exchange materials with their environment. To get bigger, an organism must divide its cells--that is, keep cell volume constant while increasing cell number.