Starter Paramecium is a common freshwater Protista, found in ponds or slow-moving streams. 1. Discuss the relationship between osmosis and contractile vacuole activity in a Paramecium, in its freshwater environment.

Answer – Question 1 AchievedMeritExcellence Description of process / osmosis: Water moves along a concentration gradient Or (from a high internal water concentration to a lower external water concentration) Or (from a solution with a lower solute concentration to a solution with a higher solute concentration) Explanation of why gradient exists. Water concentration gradient occurs because the solute concentrations on both sides of the cell membrane differ. Comparison of cytoplasm / vacuole i.e. the external solution has higher solute concentrations and thus lower water concentration. These ideas linked to the role of the cell membrane. Cell membrane acts as a semi- permeable membrane (this needs to be elaborated) i.e smaller molecules (water), can pass through but larger ones (solute), cannot. Or As the cytoplasm loses water it becomes more concentrated and water moves into it from the vacuole (this can be observed in the photo of Cell B). i.e. role of vacuoles membrane is discussed rather than the plasma / cell membrane.

Starter 2. Osmosis and active transport are both processes used by cells. Compare and contrast these two processes.

Answer – Question 2

Enzymes Enzymes are biological catalysts – a molecule made by a cell which speeds up a specific chemical reaction. Almost all processes in a cell need enzymes in order to occur at a significant rate.

Enzymes Enzymes lower the energy required for a reaction to occur (activation energy) thus increasing the reaction rate. Most enzyme reaction rates are millions of times faster than those of comparable un-catalyzed reactions.

Induced Fit Model Enzymes consist of one or more amino acid chains folded to form a specially shaped pocket called an active site. The specific molecule that fits into the enzymes active site is called the substrate molecule.

Induced Fit Model The shape of the enzyme will change when the substrate(s) bond with the active site. Only one type of substrate molecule will fit into the active site of an enzyme.

Anabolic Reactions Substrates Bond to Active Site Products Enzyme

Catabolic Reactions Substrate Bond to Active Site Products Enzyme

Catabolic Reactions SubstrateEnzymeProduct Hydrogen peroxide CatalaseOxygen and Water StarchAmylaseMaltose MaltaseGlucose ProteinProteaseAmino acids FatsLipaseFatty Acids and Glycerol

Enzymes Since enzymes are extremely selective for their substrates and speed up only a few reactions from among many possibilities, the set of enzymes made in a cell determines which metabolic pathways occur in that cell. Temperature and pH affect the activity of an enzyme.

Hydrogen Peroxide Hydrogen peroxide (H 2 O 2 ) is a reactive molecule that can potentially damage proteins, lipids and DNA. To prevent cellular damage, it must be quickly converted into other, less dangerous substances.

Catalase The enzymes catalase breaks down hydrogen peroxide into water and gaseous oxygen. 2 H 2 O 2 catalase 2 H 2 O + O 2 Animals use catalase in every organ, with particularly high concentrations occurring in the liver.

Enzyme Activity