Evolution cells evolved 2.1 billion years ago as a result of nutrient shortage pressures. These cells are more efficient at using available sources because of the compartmentalization of their life processes. These organelles (the compartments) restrict chemical reactions to certain areas and with that more complex reactions can go on in each organelle, where the required enzymes are concentrated. The more complex organization also requires more chromosomes – chromosomal structure changed to linear chromosomes and there are multiple of them in each eukaryotic cell. The Role of the Nucleus It is a control center of the cell, which is processing inputs from the cytoplasm, stores genetic information and carries out instructions to regulate cell functions. The instructions are coded in DNA. The instructions include genes for making proteins, making RNA and to perform reproduction. Parts of the nucleus: Nuclear envelope – made up of TWO phospholipid bilayer with nuclear pores. These pores are large enough to transport RNA or proteins in and out of the nucleus. Nucleolus – synthesizes rRNA to assemble the large and small subunits of the ribosomes. These subunits only combine later in the cytoplasm. DNA – always remains in the confined and protected area of the nucleus Ribosomes are used to synthesize proteins. If the ribosomes are free floating in the cytoplasm, they synthesize proteins for the cell. If they are suspended on the rough ER, they form proteins that exit the cell. The Endomembrane System A set of organelles, which are functionally interrelated and located in the cytoplasm. Includes the nuclear envelope, rough and smooth endoplasmic reticulum, Golgi apparatus, cell membrane and other organelles that form from these listed ones (vesicles, vacuoles). All other organelles are listed in your book and you can take notes on them on your review sheets. – Harvard cell animation
physiological processes in order to maintain internal equilibrium are called homeostatic mechanisms or homeostasis. Examples of homeostatic mechanisms include temperature regulation, glucose regulation, osmoregulation, pH regulation etc. Animals that manage external environmental changes by adjusting and mostly keeping a constant internal environment are called regulators. They use complex signaling pathways to keep body conditions constant. Birds and mammals are mostly considered regulators. All other animals, like insects, amphibians, fish etc. are considered conformers because they lack complex internal control mechanisms for maintaining their homeostasis in changing environments. Conformers tend to live in environments that are changing less drastically. Many may hibernate when conditions are not ideal. It is beneficial to be a conformer, because it requires less energy to maintain body conditions. However, they tend to be majorly harmed by drastic changes in the environment. Feedback mechanisms Feedback mechanisms – cellular and physiological changes and responses in the body to maintain homeostasis when conditions change from the ideal. Each condition in living organisms fluctuates around a set the ideal condition. However, the fluctuations can be only within a certain range. If fluctuations in the external environment are great enough, internal feedback mechanisms operate to restore balance. Steps of the feedback mechanisms to restore homeostatic balance are the following: Stimulus coming from the environment Receptors to sense the stimulus Control center to process the signal and generate adjustments Effectors that respond to the stimulus Negative feedback mechanisms – This type of feedback reduces the deviation from the set point and reducing the impact of the stimulus. Temperature regulation, osmoregulation, pH regulation and most of other homeostatic processes use negative feedback. Regulation of appetite with leptin hormone is another example – watch the animation in your book Positive feedback mechanisms – they magnify the impact of the stimulus by initiating a series of cascading mechanisms that push the set point beyond normal ranges. Ex. Uterine contractions during childbirth, blood clotting mechanism, pepsin production in the stomach. TEMPERATURE REGULATION Introduction Temperature regulation is a homeostatic mechanism. Organisms require certain temperature for their normal life processes.