What are the simplest physiological requirements that challenges all organisms?
- Establishment of self and non-self and the control of the passage of substances across that boundary to maintain homeostasis in the organism’s internal environment, which includes:
- The distribution of nutrients across the organism’s body.
- The removal of waste products.
- Internal signalling system.
- Locomotion of either self or gametes. (sedentary lineages cause competition between parents and offspring)
- Major differences in the anatomical implementation of these requirements are present in the tree of life. Nevertheless, the challenges are the same.
Why comparative physiology?
- Understanding the differences and commonalities in how different organisms evolved solutions to common physiological challenges.
- It reveals divergence and convergence patterns across the tree of life
What is the “August Krogh Principle”?
If you are interested in studying a physiological process, even though all organisms may have some version of that process, there will be one or a few organisms that will perform it especially efficiently, or live in an environment where the process is especially critical to survival, or the organ system that mediates the process may be large etc.
What factors are considered when choosing a study organism?
- Whether that physiological process of interest is particularly developed or important according to the August Krogh Principle.
- Rate of development
- Ability to breed in the lab
Cons of working with model organisms?
- Only a couple of organisms are expected to be representative of thousands (millions!) of species.
- The model organism may not be the most optimal for studying the specific physiological trait.
- Model organisms may have critical physiological differences (such as scruvy and humans not being able to produce their own vitamin C whereas model organism mice and rats are)
Pros of working with model organisms?
- You’re more likely to get funding from the NIH.
- Many people research these organisms, thus knowledge advances quickly.
- Biomedical supply companies make equipment specifically for working with these organisms.
Movement of nutrients, signaling molecules and wastes in the bodies of single celled eukaryotes:
- Chaos carolinensis is a relatively large (up to 3 mm in diameter) single celled eukaryote. Because it’s large, diffusion is inefficient. There’s also mild compartmentalization in the organism, with ribosomes on a section of the cell, mitochondria in another section etc.
- Thus, the organism moves nutrients around its body using a mixture of active and passive processes called cytoplasmic streaming.
- Certain compounds or organelles attached to myosin molecules are shuffled around the disorderly array of actin filaments that form the cytoskeleton of the amoeboid. (active movement)
- The movement of the actin filament passively stirs the cytoplasm, leading to the passive diffusion of molecules.
- (This is also the mechanism through which locomotion is achieved. Actin molecules are continuously produced and broken down.)
Movement of nutrients, signaling molecules and wastes in the bodies of simple plants:
- Even though the cells are at most one cell away from the aquatic environment and each have their own chloroplasts to produce sugars, nutrient movement is required to move nutrients within the limited cytoplasm due to the presence of large vacuoles.
- The movement of the cytoplasm is facilitated by actin-myosin filaments (ATP required) that constitute the cytoskeleton. However, this sort of movement can be considered facilitated rather than active, since the aim is the dissolution of the nutrients in the cytoplasm that are NOT attached to the cytoskeleton.
Movement of nutrients, signaling molecules and wastes in the bodies of flowering plants:
- As plants got taller in competition for sunlight, they required structural support and specialized transport: xylem and phloem, collectively called the stele.
- Vascular bundles, outer cells are phloems, inner cells are xylems.
- Phloem movement is both active and passive. Movement of water to sinks to create hydrostatic pressure (bulk flow) is active. The movement of the phloem sap is passive.
- Movement from roots through xylem is passive.
Movement of nutrients, signaling molecules and wastes in the bodies of Vertebrate animals:
- Most animals have “flow through” alimentary tracts in which food is digested in the stomach and nutrients are moved across an epithelium and into the body.
- In larger animals, diffusion is simply impossible. Thus, in vertebrates, closed tubes with a muscular pump that actively contracts to move the fluid in the tube through bulk flow exist as special transport systems.
- In the circulatory systems of amphibians such as frogs, there aren’t two complete loops for oxygenated and poorly oxygenated blood. The single ventricle pumps mixed blood to the tissues. This isn’t very efficient, but it’s sufficient, as many amphibians take a large percentage of their oxygen through diffusion (skin).
- Nutrients etc. are moved out of the lumen (non-living tissue) through bulk flow and are taken up by metabolically active sink cells through passive diffusion down the concentration gradient.