CellsThe Cell TheoryBasic Cell OrganizationWhy Cells?Cell SizeEukaryotic CellsCell MembraneCell Membrane, pt. 2Transport Across the Cell MembranePassive and Active TransportBulk TransportWater in the CellResponse to Osmotic PressureNucleusNucleus, pt. 2Cytomembrane SystemEndoplasmic ReticulumGolgi Body and SecretionLysosomes and PeroxisomesMitochondriaEndosymbiont HypothesisCytoskeletonCytoskeleton, pt. 2More CytoskeletonPlant Cell OrganellesMore Plant OrganellesProkaryotic CellsCells•Anton Leeuwenhoek invented the microscope in the late 1600’s, which first showed that all living things are composed of cells. Also, he was the first to see microorganisms. •Light microscopes have a limited resolution: magnification of more than about 2000-fold does not improve what you can see.•Electron microscopes use electrons instead of light. The short wavelength of electrons allows magnifications much better than visible light.The Cell Theory•Use of the microscope for 150 years or so led to these basic beliefs about cells:•1. All living things are made of cells.•2. The cell is the smallest unit of life.•3. All cells arise from pre-existing cells.Basic Cell Organization•All cells contain:–1. cell membrane that keeps the inside and outside separate.–2. DNA-containing region that holds the instructions to run the processes of life.–3. Cytoplasm: a semi-fluid region containing the rest of the cell’s machinery.•Prokaryotes: (bacteria): simple cells with no internal membrane-bound structures. DNA is in a special region of the cytoplasm.•Eukaryotes: complex cells with internal membranes. DNA is in a nucleus separated from the cytoplasm by a membrane.Why Cells?•The basic problem is surface-to-volume ratio. All food and oxygen has to come in through the cell’s surface. As size increases, you get less surface area to support a given volume of cell contents. •For example, if the cell’s diameter increases: 1-2-3-4-5, its surface area increases 1-4-9-16-25, and its volume increases 1-8-27-64-125. A 5-fold increase in diameter cuts the amount of surface area per volume to 1/5 of the original: the cell starves.•Also, consider how a cell responds to a change in the environment: the signal must travel from the surface of the cell to the nucleus, then the nucleus issues new instructions to deal with the situation. The instructions must reach all parts of the cell. The bigger the cell, the longer it takes to respond to the environment.•Thus, cells are limited to small sizes, and multicellular organisms are composed of many cells.•Some minor exceptions: long thin cells, like nerve cells, can be several feet long. Also, cells can increase their surface area by using frilly membranes, like the cells that absorb food in the small intestine.Cell SizeEukaryotic Cells•Eukaryotic cells contain internal membranes and organelles. An organelle is an internal membrane bound structure that serves some specialized function within the cell. •Organelles we will discuss:–Cell membrane–Nucleus–Cytomembrane system, including endoplasmic reticulum, Golgi apparatus, vesicles, lysosomes, and peroxisomes–Mitochondria–Cytoskeleton–Special plant organelles: chloroplast, central vacuole, cell wallCell Membrane•Composed of phospholipids, with a polar (and therefore hydrophilic) head group, and 2 non-polar (hydrophobic) tails. A bilayer with the polar heads on the outsides and hydrophobic tails inside satisfies all of the molecule. The membrane is a “phospholipid bilayer”.•The membrane also contains cholesterol and various proteins. The proteins act as sensors, attachment points, cell recognition, or they transport small molecules through the membrane. •Membrane proteins and membrane lipids often have sugars attached to their outside edges: glycoproteins and glycolipids. For example, the differences between the ABO blood groups are due to differences in sugars attached to the outer membranes of red blood cells.Cell Membrane, pt. 2•The molecules in the membrane can move about like ships floating on the sea: the membrane is a two-dimensional fluid•In some cells, the membrane proteins are held in fixed positions by a network of proteins just under the membrane, a cytoskeleton.•Only water, a few gasses, and a few other small non-polar molecules can move freely through a pure phospholipid membrane. Everything else must be transported into the cell by protein channels in the membrane.Transport Across the Cell Membrane•Basic rule: things spontaneously move from high concentration to low concentration (downhill). This process is called diffusion.•To get things to move from low to high (uphill), you need to add energy. In the cell, energy is kept in the form of ATP.•Three basic transport mechanisms: passive transport for downhill, active transport for uphill, and bulk transport for large amounts of material in either direction.•Also need to deal with excess water entering the cell.Passive and Active Transport•Passive transport uses protein channels through the membrane that allow a particular molecule to go through it, down the concentration gradient. The speed and direction of movement depends on the relative concentrations inside and outside. Glucose is a good example: since cells burn glucose for energy, the concentration inside is less than the concentration outside.•Active transport uses proteins as pumps to concentrate molecules against the concentration gradient. The pumps use ATP for energy. One example is the calcium pump, which keeps the level of calcium ions in the cell 1000 times lower than outside, by constantly pumping calcium ions out. The balance of sodium and potassium ions is maintained with potassium high inside and sodium low inside, using a pump. Up to 1/3 of all energy used by the cell goes into maintaining the sodium/potassium balance.Bulk Transport•In bulk transport, materials can move into the cell (called endocytosis) or out of the cell (called exocytosis). The two processes are reverses of each other.•In endocytosis, an area of the cell membrane forms an indentation that gradually pinches off into a small, self-contained membrane-bound sphere called a vesicle. The vesicle contains material that used to be outside the cell. An example is white blood cells engulfing and killing bacteria that have invaded the body. •In exocytosis, material the cell wishes to remove is contained in a vesicle. The