Year 11 biology

1. Molecules in Motion Year 11 biology

2. In sciences, a molecule is a group of atoms in a definite arrangement held together by chemical bonds. Molecules move into cells in different ways depending on the size of the molecules, the permeability of a membrane and the concentration of those molecules in the intracellular and extra cellular environment. Molecules in motion notes

3. Diffusion: • In a gas or liquid molecules move freely, bumping and bouncing into one another • This is random movement • Which results in an even spread of molecules throughout the space they occupy • In a liquid the molecules move from a region of high concentration to a region of low concentration, in an attempt to balance the levels or reach-equilibrium. • The difference between these two regions is referred to as a the concentration gradient or diffusion gradient • E.g. • Oxygen moving into cells and carbon dioxide moving out • Needs the molecules to be solution • Membrane to be moist and permeable to them diffusion

4. Diffusion is slow – • It can be sped up in the following ways: • Steeper gradient • Stirring the molecules • Heating • Size is important • Small ones (oxygen) diffuse quicker than large ones (glucose) • Diffusion is related to the surface area to volume ratio • Quicker the greater the surface area is to volume • Key Questions page 67 Contd:

5. Movement of water across a selectively permeable membrane, to balance the levels of solutes. • Water actually moves from a region of low concentration of solutes to a region of high concentration of solutes • It is still diffusion but only of water • This happens when the membrane is permeable to water but not to larger molecules like sugar • Refer to diagram on page 65 fig. 4.13 • Copy if needed OSMOSIS

6. Note: Osmosis in animal cells, especially blood cells could be fatal, as the water would cause the cell to burst • Water entering a red blood cell is endo-osmosis. • If red blood cells are put in salt water the water leaves the cell to balance the solutes and the cell shrivels and dies – exo-osmosis • This means that the fluid part of our body blood cells, the plasma, must be in balance to prevent osmosis and disaster. Osmosis contd:

7. Plants have cellulose in their cells walls • They also have a vacuole inside which contains salts in solution • Membrane of the vacuole is semi permeable • Cell wall is fully permeable to salts and water • If a plant cell is in water it swells, but not bursts because the cell wall is tough: • Eventually it gets full and firm and is said to be turgid which keeps the plant firm and once here, the osmosis stops • If a plant cell is placed into a high solute solution, water is taken out of the vacuole and the cell goes flaccid eventually, cytoplasm pulls away from the cellulose – plasmolysis Osmosis in plant cells

8. This when the cells need molecules of substance which are not in a higher concentration outside the cell, but the cell needs more of them anyway. • Therefore it moves molecules from a region of low concentration to one of high concentration – against the concentration gradient. • It needs energy for this to happen supplied by the process of cellular respiration, which occurs in the mitochondria PAGE 66 NOTE Endocytosis and Exocytosis Active transport