OSMOSIS AND DIFFUSION

Objectives 2. Explain how the processes of diffusion, active transport, photosynthesis, and respiration are accomplished in a cell. 2.1 Identify the factors which influence the rate and direction of diffusion. 2.2 Examine the mechanisms of active transport by identifying and explaining the two processes. (Process one involves the expenditure of energy where a carrier molecule takes a substance from one side of a membrane to the other side of the membrane. Process two involves the inpocketing of material by a membrance -- pinocytosis and exocytosis.) 2.7 Identify how osmosis is related to diffusion and the value of osmosis to living organisms 2.8 Compare the similarities and differences between active and passive transport.

DIFFUSION OSMOSIS PASSIVE TRANSPORT Passive transport occurs without expenditure of energy. Molecules move using their own kinetic energy. Diffusion and osmosis are examples of passive transport. Passive transport allows cells to get water, oxygen and other small molecules that they need. It also allows the cell to get rid of waste such as carbon dioxide.

Diffusion -Molecules spread out over a large area -Everything but water Osmosis -Molecules go through a semipermeable membrane - Just water Similarities - Molecules move around to create equilibrium Osmosis and Diffusion

Definitions Concentration Gradient - change in the concentration of a substance from one area to another. Diffusion - Molecules in solution tend to slowly spread apart over time. Permeable – a membrane that allows a substance to pass through

Diffusion Movement of molecules from an area of high concentration to an area of lower concentration. Factors that affect the rate of diffusion: size of molecules, size of pores in membrane, temperature, pressure, and concentration. Although molecules move in every direction, the overall direction is outward towards areas of lower concentration [High] [Low] concentrated, high energy molecules diffuse, low energy molecules

Factors that affect rates of diffusion 1. Concentration gradient – differences in concentrations in two areas. The steeper the concentration gradient the faster the rate of diffusion 2. Temperature – the higher the temperature the faster the molecules will move and the faster diffusion proceeds 3. Size of the molecule – larger molecules move slower and diffuse slower. 4. Pressure – increasing pressure makes diffusion go faster

Diffusion will continue until equilibrium is reached. This means there will be an equal distribution of molecules throughout the space. This is why food coloring moves throughout a beaker of water; why odors smell strong at first and then disappear over time. Equilibrium, a result of diffusion, shows the uniform distribution of molecules of different substances over time as indicated in the above diagram.

Which molecules will diffuse in each of the figures below?

Answers No Movement

Osmosis Diffusion of water across a selectively permeable membrane (a barrier that allows some substances to pass but not others). The cell membrane is such a barrier. At first the concentration of solute is very high on the left. But over time, the water moves across the semi-permeable membrane and dilutes the particles. Small molecules pass through – ex: water Large molecules can’t pass through – ex: proteins and complex carbohydrates

Hypotonic and Hypertonic Hypotonic – The solution on one side of a membrane where the solute concentration is less than on the other side. Hypotonic Solutions contain a low concentration of solute relative to another solution. Hypertonic – The solution on one side of a membrane where the solute concentration is greater than on the other side. Hypertonic Solutions contain a high concentration of solute relative to another solution.

Isotonic Over time molecules will move across the membrane until the concentration of solutes is equal on both sides.

Osmosis in cells Cytoplasm is a solution of water and solids (solutes dissolved in the water). Water moves into and out of cells because of the different concentrations of the solutes. Different kinds of cells react differently depending on the solution they are in. Below are examples of red blood cells in different types of solutions and shows what happened to the red blood cells.

Osmosis in cells In the hypertonic solution the red blood cells are undergoing plasmolysis. The cytoplasm is shrinking away from the cell wall as the water is leaking the cell In the isotonic solution the cell has a good amount of turgor pressure. There is enough water in the cell to keep pressure against the cell wall In the hypotonic solution the cell is almost to a point of cytolysis. A point of the cell bursting due to water coming into the cell

Turgor Pressure builds in the cell and causes osmosis to stop because of the rigid cell wall. Plants will wilt when cells lose water through osmosis. Hypotonic Solution Hypertonic Solution PLANT CELLS

ELODEA CELLS As viewed under the microscope

Active transport

Endocytosis Endocytosis is the process by which cells take in materials. The cellular membrane folds around the desired materials outside the cell. The ingested particle becomes trapped within a pouch, vacuole or inside the cytoplasm. Often enzymes from lysosomes are then used to digest the molecules absorbed by this process. Biologists distinguish two main types of endocytosis: pinocytosis and phagocytosis. In pinocytosis (cell drinking), cells engulf liquid particles (in humans this process occurs in the small intestine, cells there engulf fat droplets). In phagocytosis (cell eating), cells engulf solid particles.

Pinocytosis and Phagocytosis

Exocytosis Exocytosis is the durable process by which a cell directs the contents of secretory vesicles out of the cell membrane. These membrane-bound vesicles contain soluble proteins to be secreted to the extracellular environment, as well as membrane proteins and lipids that are sent to become components of the cell membrane.

Transporter Proteins Active transport usually happens across the cell membrane. There are thousands of proteins embedded in the cell's lipid bilayer. Those proteins do much of the work in active transport. They are positioned to cross the membrane so one part is on the inside of the cell and one part is on the outside. Only when they cross the bilayer are they able to move molecules and ions in and out of the cell. The membrane proteins are very specific. One protein that moves glucose will not move calcium (Ca) ions. There are hundreds of types of these membrane proteins in the many cells of your body. Many times, proteins have to work against a concentration gradient. That term means they are pumping something (usually ions) from areas of lower to higher concentration. This happens a lot in neurons. The membrane proteins are constantly pumping ions in and out to get the membrane of the neuron ready to transmit electrical impulses

Inhibitors Even though these proteins are working to keep the cell alive, their activity can be stopped. There are poisons that stop the membrane proteins from transporting their molecules. Those poisons are called inhibitors. Sometimes the proteins are destroyed and other times they are just plugged up. Imagine that you are a cell and have ten proteins working to pump calcium into the cell. What if a poison came along and blocked eight of them? You could not survive with just two pumps working and would slowly die. It would be like expecting you to breathe with your mouth and nose plugged up.

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