GenBio Q4

• Respiration


  releases chemical energy stored in glucose molecules, it takes in oxygen and releases carbon dioxide as a by-product.
• Gas exchange


  facilitates the movement of gases across and between cells and their external environment
• Diffusion


  the underlying mechanism that governs gas exchange; it is the movement of molecules down their concentration gradient
• Diffusion allows molecules to more from an area with higher concentration to lower concentration
• Fick's law


  developed by Adolf Fick which describes the four main factors affecting the diffusion rate which also affects the efficiency of gas exchange
• When diffusion rate is high, the effectiveness of gas exchange increases, allowing organisms to generate more energy
• The four factors that affect diffusion rate are:

  1. Surface area
  2. concentration gradient
  3. thickness of membrane
  4. distance of diffusion
• greater surface area of the membrane increases diffusion rate, as it accommodates more molecules for diffusion
• larger the difference in concentration increases diffusion rate as the difference allows the downhill movement of molecules
• thicker membrane reduces the diffusion rate
• larger distance of diffusion decreases the diffusion rate, as particles have to travel further
• spongy mesophyll


  this feature of the plant leaf is loosely packed to facilitate gas exchange
• stomata


  microscopic holes in the lower epidermis of leaves of plants to allow the entry and exit of gases, and facilitate the loss of water
• guard cells are bordered by epidermal cells called subsidiary cells
• The stoma opens during the day due to light and warm temperature, and closes during the night due to low temperature and absence of light
• the stoma opens when guard cells absorb water through osmosis and expand due to increase in turgidity
• the stomata closes when guard cells shrink and become flaccid due to water loss and a decrease in turgidity
• in the daytime, when photosynthesis begins, the stomatal opening is triggered by blue light
• Phototropin is the cell photoreceptor protein that acts as a blue light receptor triggering the stomata to open
• upon exposure to blue light, the H+ exporting ATPase in the plasma membrane of guard cells activates and then pumps out H+ ions
• the hyperpolarization (loss of H+ ions) allows the guard cells to take in K+ ions from the subsidiary cells (surrounding cells) through potassium ion channels
• when K+ concentration in guard cells increases, water potential decreases
• Water potential is the tendency of water molecules to move from a solution with less solute to a solution with more solute
• Decrease of water potential allows water molecules to enter the guard cells through osmosis
• through the osmosis of water, turgor pressure increases, then the stomata opens
• the concentration of carbon dioxide in the environment is higher than within the spaces in the spongy mesophyll, while the concentration of oxygen inside the leaf is higher than in the environment
• Abscisic acid is an inhibitor for the uptake of K+ ions to close the stomata in the dark
• in absence of light, the rate of respiration is relatively higher and CO2 accumulates
• When CO2 accumulates and reacts with water, it produces carbonic acid (H2CO3) which increases acidity which activates ABA
• When Ca+ moves outside the cell, it results to membrane depolarization, which cause K+ to go to the subsidiary cells
• After K+ moves out of the guard cells, water potential inside increases, causing the water to move out also
• When water potential increases and turgidity decreases, the stoma closes
• When they recognize these PAMPs, the abscisic acid (ABA) mediates the stoma’s closure to prevent pathogens from entering the leaf. This mechanism is called innate immunity.
• aside from absorbing water from soil, roots of plants also do gas exchange through the single-celled extensions of the root epidermis called root hairs
• plant root extensions that allow gas exchange in plants that are overflooded or water-logged; also called "breathing tubes"

  Pneumatophores
• Suberin


  the waxy waterproof and airproof substance covering mature woody plants
• The layer of dead cells in woody plants are pierced with holes/pores called lenticels
• Lenticels (from the Latin word lenticella, small window) are raised porous tissues consisting of cells with large intercellular spaces in the periderm of plants with woody stems. They are usually round, oval, or elongated in structure.
• Lenticels are analogous to stomata as they permit the exchange of vital gases, such as CO2 , O2 , and water vapor, between the environment and the internal tissue spaces in stems. But unlike stomata, lenticels remain open both day and night as they do not have guard cells to regulate their aperture.
• Direct diffusion


  the movement of gas across membranes that are in close contact with their environment without any transport system