Biology Chapter 2: Movement of substances
diffusion
defined as the net movement of particles from a region of higher concentration to a region where the particles are of a lower concentration, which is down a concentration gradient
molecules diffuse DOWN a concentration gradient, a concentration gradient is needed in order for the diffusion of molecules and particles
diffusion is affected by 4 factors, thickness of membrane if present, concentration gradient, surface to volume ratio, temperature and size of particles
concentration gradient: the steeper the concentration gradient, the faster the rate of diffusion.
surface to volume ratio: the larger the surface area to volume ratio, the faster the rate of diffusion, when the surface area to volume ratio decreases, the rate of movement of substances decrease as well
thickness of cell membrane (if present) : the thicker the cell membrane, the slower the rate of diffusion
temperature: an increase in temperature would lead to an increase in the rate of diffusion
particle size: smaller molecules or ions diffuse faster than larger molecules
diffusion also occurs in respiration and breathing
during respiration cells use oxygen continuously during aerobic respiration, this leads to a fall in oxygen concentration in the cell, as a result, oxygen molecules diffuse into the cell until the concentration level of oxygen inside and outside the cell are the same
during breathing, the air sacs in the lungs have a high concentration of carbon dioxide, and outside the air sacs, the lungs have a high concentration of oxygen. When carbon dioxide diffuses from the air sacs to the lungs due to the different levels of concentration of carbon dioxide, oxygen diffuses from the lungs to the air sacs due to a difference in concentration levels of oxygen
when oxygen is taken in by the cells, carbon dioxide is produced by the cells during respiration. The concentration of carbon dioxide in the cells rises and creates a concentration gradient, thus enabling carbon dioxide to diffuse out of the cell and allowing oxygen to diffuse into the cell
osmosis
osmosis is defined as the movement of water molecules from a solution with a high water potential to a solution of low water potential, through a partially permeable membrane
osmosis only takes place when a PARTIALLY PERMEABLE MEMBRANE is present and only works for WATER MOLECULES.
plant cell
in a solution of high water potential, water moves from a high concentration of water potential to a low concentration of water potential, as the water enters the cell via osmosis. The vacuole in creases in size and pushes against the cytoplasm against the cell wall, the cell does not burst due to the cell wall, and the cell has become turgid. The pressure exerted by the water in the vacuole on the cell wall is called turgor pressure
occurs when there is a difference in water potential in 2 places
in a solution of low water potential, water from the vacuole leaves the cell via osmosis as water moves from a high water potential to a low water potential. As the cell loses water, the vacuole decreases in size, and the cytoplasm shrinks away from the cell wall. This process of shrinking is called plasmolysis and the cell has become plasmolysed.
turgor pressure is important for the cell as it plays an important role in maintaining the shape of the soft tissues in the plant, keeps non-woody plants upright and firm, if the plant loses its turgidity, the plant will wilt
animal cell
in a solution of high water potential, water enters the cell via osmosis as water moves from a high water potential to a low water potential, causing the cell to swell and even burst as it has no cell wall to maintain the structure of the cell
in a solution of low water potential, water leaves the cytoplasm of the cell via the partially permeable cell membrane via osmosis, the cell then shrinks and spikes appear, thus the cell becomes crenated and over time, the cell will be dehydrated and die
active transport
active transport is defined as the process where energy is needed to move particles across a membrane AGAINST the concentration gradient, in other words, from a region where the particles are of lower concentration to a region where the particles are of high concentration
active transport requires energy, thus it can occur only in living cells. Some examples would be dissolved mineral salts are taken up by root hair cells via active transport, and glucose and amino acids are taken up by cells in the small intestines of humans by active transport