Key Concepts in Biology: Evolution, Digestion, and More
Darwin: The Theory of Evolution by Natural Selection
Darwin’s theory of evolution by natural selection is based on the following principles:
- There are heritable differences between individuals in a living population.
- More individuals are born than can survive.
- Some hereditary variations provide individuals with advantages in survival.
The Synthetic Theory of Evolution
This theory incorporates the idea of evolution by natural selection. The evolutionary unit is not the individual but the population. Differential reproduction is the mechanism resulting from the joint action of genetic variation and environmental pressure. Evolution is caused by a gradual change in the genetic makeup of species.
Adaptation Explained by Natural Selection
Adaptation is any feature that improves the body’s ability to use environmental resources in order to survive and reproduce.
Incorporation of Water and Mineral Salts
- Via A or Symplast: One part of the water and most of the salts circulate inside the root to the wood vessels through the cytoplasm of the cells that form the cortical parenchyma. From one cell to another, they pass through the plasma membrane and plasmodesmata; minerals do so by active transport and water by osmosis.
- Via B or Apoplast: Most of the water and minerals circulate inside the root through the cell walls and intercellular spaces, reaching the endodermis. From here, water crosses the membrane and cytoplasm of the cells of the Casparian band through osmosis, while mineral salts enter cells by active transport.
Parts of the Human Digestive System
The human digestive system consists of the following parts:
- Mouth: Mechanical and chemical digestion
- Pharynx
- Esophagus: Peristalsis
- Stomach: Mechanical and chemical digestion, forming chyme
- Small intestine: Digestion, forming chyle
- Large intestine: Absorption of water and nutrients
- Anus
Types of Circulatory Systems
Circulatory systems can be classified as:
- Simple/Complete
- Double/Incomplete
- Double and Complete
Tension-Adhesion-Cohesion
The rise of crude sap against gravity is produced by several phenomena that depend on the internal structure of the plant and the physical properties of water (TAC):
- Root Pressure: The cells of the root have a higher solute concentration than the soil water, so water penetrates the root’s interior by osmosis. The water inlet produces a root pressure that allows the rise of crude sap.
- Transpiration: This occurs in the leaves and is the loss of water through evaporation. Water loss produces a suction force that exerts pressure, called tension, and literally pulls the water molecules upward. This is effective thanks to the great cohesion of water molecules.
- Tension-Cohesion: This is the tension that supports an elevated column of water. Capillary action of water is also involved in this mechanism.
Flow Pressure
- The source elaborated sap moves by active transport to the vessel cells.
- The increase of solute in the vessel produces an increase in concentration, the water potential decreases, and water enters by osmosis.
- The entry of water causes increased pressure inside, which makes the sap move towards the sinks.
- When the sap reaches the sinks, the solutes leave by active transport, and the water leaves by osmosis and enters the surrounding cells.
Synapse
Once the nerve impulse reaches the end zone of the axon, it is transmitted to another neuron or effector organ by means of the synapse. When the impulse reaches the presynaptic element, neurotransmitters are released into the synaptic cleft and bind to receptors on the membrane, causing an increased permeability of Na+. The entry of sodium depolarizes the membrane and makes the impulse spread again through the second neuron.