Diffusion, Osmosis and Active Transport Particles like atoms, molecules and ions are always moving Movement increases with temperature (affects phases of matter - solid, liquid, gas) Solids - atoms, molecules and ions vibrate on the spot Liquids - move freely knocking into one another and rebounding Gases - move even more freely, with more space between particles Particles tend to spread out as evenly as possible via diffusion (net movement of particles from a place where they are in high concentration to a place where they are in lower concentration (i.e. down a concentration gradient). Diffusion is important to organisms in the obtaining of metabolic requirements and the excretion of wastes - Photosynthesis Movement of oxygen (out of the leaf) and carbon dioxide (into the leaf) - Respiration Movement of oxygen (into the cells) and carbon dioxide (out of the cells) - Digestion Absorbing of nutrients from the stomach into the ileum Osmosis - The movement of water through a partially permeable membrane - Based on a water molecule concentration gradient - water molecules will move from an area of higher water molecule concentration to an area of lower water molecule concentration in an attempt to reach equilibrium. Remember: solute + solvent = solution Ex. Solutions containing substance dissolved in water (e.g. sugar dissolved in water - sugar is the solute and water is the solvent) has a lower concentration of water molecules relative to pure water in a given amount of space.
So, in a partially permeable membrane, with a substance concentration differential, the water molecules will move from the diluted solution to the concentrated solution. Cell membranes are partially permeable - allowing some substances to pass through, and not allowing other substances to pass through. Concentration differentials between cytoplasm and the fluid outside the cell will cause osmosis across the cell membrane. Ex. Water moving from the soil into the root cells Animal cells burst in pure water - as more water diffuses into the cell it will swell until it bursts! Animal cells shrink in concentrated solutions - as water diffuses out of the cell, it shrivels up. Plant cells do not burst in pure water - due to presence of cell wall which prevents cytolysis (cell bursting) Plant cells plasmolyze in concentrated solutions - as water diffuses out of the cell, the cytoplasm and vacuole shrink into the center of the cell, eventually tearing away from the cell wall. The process by which the internal contents of the plant cell shrink until the cell is permanently damaged is called plasmolysis. Plant cells containing enough water to maintain the upright structure of the plant are turgid. Plant cells not containing enough water to maintain the upright structure of the plant, thereby wilting, are flaccid.
Cell Membrane Structure: The Phospholipid Bilayer Remember the stucture of a phospholipid molecule: The head of the phospholipids is hydrophilic while the fatty acid tails are hydrophobic. Therefore, the hydrophilic heads orient themselves toward watery solutions while the hydrophobic tails orient themselves away from the aqueous solutions and toward themselves. This allows for the formation of a phospholipid bilayer to produce membranes, both for the cell and its organelles. We can see this phospholipids layer at very high magnifications using an electron microscope (total thickness of about 7 nm). Scientists propose the fluid mosaic model as a depiction of the membrane if we could see the individual molecules Some phospholipids tails are more unsaturated than other tails resulting in more fluidity of the membrane. Water, oxygen and carbon dioxide are small molecules and therefore pass through in the air spaces in between the phospholipid molecules very easily. Fluid: The individual phospholipids molecules and proteins move around via diffusion within their layer. Mosaic: The appearance of the proteins scattered about the layer. Other components of the phospholipid bilayer: Proteins - Some proteins are fixed and some float about like icebergs Some proteins are embedded in the outer layer, some in the inner layer and some span the whole membrane. May act as specific transport proteins, as enzymes or in cellular metabolic reactions on membrane surfaces. Many proteins and lipids have short branching carbohydrate chains attached to the external surface known as glycoproteins and glycolipids - they stabilize the membrane structure, act as receptor molecules for hormones and neurotransmitters, or act as antigens for cell recognition. Cholesterol molecules - Found in the membrane, they regulate membrane fluidity, prevent ions and small polar molecules from leaking through, and structurally support the cell membrane (prevent cytolysis).
Diffusion in a cell is affected by several factors: 1) Steepness of concentration gradient - greater differential across the membrane will canse a faster net rate of diffusion 2) Temperature - higher temperatures results higher kinetic energy of molecules and ions which causes faster diffusion 3) Surface Area - greater surface are results in more molecules/ions that can cross at any one moment, therefore, faster diffusion 4) Nature of molecules trying to diffuse - smaller molecules require less energy to move than larger molecules. Non-polar molecules are soluble in the fatty acid tails and therefore diffuse more easily than polar molecules. Cells take in some larger polar molecules by facilitated diffusion (passive transport). Only occurs from high to low concentration gradient and rate is dependent on number of channels and if they are open or not. Glucose and amino acids pass into the cell via diffusion facilitated by hydrophilic protein transport channels. Likewise for ions like Na + and CL -. Each protein channel is specific for the substance it allows to pass through the membrane. Cells take in substances by active transport Cells need to take in substances that are present around them in very small quantities If the concentration of the substance in the cell is greater than the concentration of the same substance outside the cell, diffusion will not happen Cells have developed special carrier transport proteins in the cell membrane to move substances against the concentration gradient by active transport. This requires an input of energy (ATP). Substance to be transported binds into carrier protein. Carrier protein changes configuration (shape) and pushes the substance into the cell. Examples of active transport: Re-absorption of substances from the kidneys into the blood, absorption of digested products in the gut, loading sugar from photosynthesizing leaves into the phloem tissue, loading inorganic ions from soil into root hairs
Cells take in substances by bulk transport Transport of large quantities of substances into cells (endocytosis) or out of cells (exocytosis). Endocytosis - engulfing of the material by the plasma membrane to form an endocytotic vacuole. Occurs 2 ways: Phagocytosis - bulk uptake of solid material - occurs in cells (phagocytes) that specialize in eating something else (e.g. engulfing of bacteria by white blood cells) Pinocytosis - bulk uptake of liquid material - occurs in cells that form vesicles to drink substances from cells around them (e.g. egg cells taking up nutrients from follicle cells in the ovary as it matures prior to ovulation Exocytosis - bulk removal of materials from the cell, via golgi vesicles that transport secretory proteins to the plasma membrane and release their contents (e.g. animals: secretion of digestive enzymes from the pancreas; plants: used to get cell wall building materials outside the plasma membrane)