StarterParamecium 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 1Achieved Merit Excellence

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.OrAs 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.

Starter2. Osmosis and active transport

are both processes used by cells. Compare and contrast these two processes.

Answer – Question 2

Enzymes

EnzymesEnzymes are biological catalysts

– a molecule made by a cell which speeds up a specific chemical reaction.

Almost all processes in a cell need enzymesin order to occur at a significant rate.

EnzymesEnzymes 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 ModelEnzymes 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 ModelThe 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 ReactionsSubstrates

Bond to Active SiteProducts

Enzyme

Catabolic Reactions

Substrate

Bond to Active Site

Products

Enzyme

Catabolic ReactionsSubstrate Enzyme Product

Hydrogen peroxide

Catalase Oxygen and Water

Starch Amylase Maltose

Maltose Maltase Glucose

Protein Protease Amino acids

Fats Lipase Fatty Acids and Glycerol

EnzymesSince 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 PeroxideHydrogen peroxide (H2O2) 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.

CatalaseThe enzymes catalase breaks

down hydrogen peroxide into water and gaseous oxygen.

2 H2O2 catalase 2 H2O + O2

Animals use catalase in every organ, with particularly high concentrations occurring in the liver.

Enzyme Activity