AN INTRODUCTION TO METABOLISM 2008 Biology Olympiad

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2008 Biology Olympiad Preparation Program

2008 Biology Olympiad Training Program 2

AN INTRODUCTION TO METABOLISM


Metabolism The totality of an organism’s chemical reactions.

Catabolic pathways – degradative processes, breaking complex molecules into simpler ones, releasing energy.

“Downhill”.

Anabolic pathways – constructive processes, forming complex molecules from simpler ones, consuming energy.

“Uphill”.


Energy Energy is the capacity to do work.

Kinetic energy – the energy of motion.

Potential energy – the energy stored in matter because of its location or structure.

Law of conservation of energy – energy can neither be created nor destroyed. It can only be converted from one

form to another.


Work

A cell has to perform 3 main types of work:

1. Mechanical work 2. Transport work 3. Chemical work

Cells use a special energy molecule to do work.


Adenosine triphosphate (ATP) The phosphate bonds are

relatively unstable, and their hydrolysis releases large amounts of energy.

The hydrolysis of ATP releases energy. ATP + H2O ADP + Pi + ENERGY

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Exergonic reactions Exergonic reactions proceed with a net release of energy. These

reactions occur spontaneously.


Endergonic reactions Endergonic reactions proceed by absorbing energy from the

environment. These reactions are not spontaneous.


Activation energy (EA) The initial investment of energy for starting a reaction is the free energy of activation, or the activation energy. It is like

an ‘energy barrier’ which must be overcome.

Enzymes lower the activation energy of a reaction.


ENZYMES


Enzymes as catalysts A catalyst is an agent that increases the rate of a reaction

without being consumed by the reaction.

Enzymes are catalytic proteins.

The reactant an enzyme acts on is referred to as the enzyme’s substrate.

An enzyme is substrate specific.

Enzyme Substrate(s) Product(s)


The active site This is the region of the enzyme which actually binds to

the substrate and within which the enzyme-catalysed reaction takes place.

Substrate entry induces a conformation change in the enzyme, making the active site wrap even tighter around the substrate in

an induced fit.

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Enzymatic action Due to the specificity of enzymes, each enzyme only

catalyses one specific reaction.

Substrate(s) enter the active site, the enzyme acts, and products are formed and released. The enzyme emerges unchanged.


Lowering the EA

Produce mechanical stress on substrate. Distorts substrate so that less energy is required to break bonds.

Provide favourable microenvironment. Eg provide acidic microenvironment for transfer of H+.

EA proportional to difficulty of breaking bonds. Easier bond breaking lower EA.

Participation in the reaction. Eg R groups temporarily form covalent bonds with substrate.

Enzymes lower the EA by:


Factors affecting enzyme activity Temperature

Higher temperatures faster moving molecules more frequent collisions

between substrate and active site.

Too high a temperature and the enzyme denatures.

pH affects bonds (eg ionic, hydrogen) present in the enzyme molecule.


Factors affecting enzyme activity Substrate concentration plays a vital role.

More substrate molecules more frequent active site access higher rate of reaction

However there is a limit. The enzyme is saturated at this concentration of substrate.


Factors affecting enzyme activity Cofactors – non-protein helpers required for enzymatic activity.

Coenzymes are organic cofactors.

Competitive inhibitors – mimics the substrate and

competes for the active site.

Non-competitive inhibitors – binds to the enzyme at site away from the active site,

altering active site’s conformation so substrate

cannot bind. 2008 Biology Olympiad Training Program 18

Control of enzymes Allosteric regulation – process where the reversible binding of either an inhibitor or activator molecule to an allosteric site

on an enzyme changes the enzyme’s operation.

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Control of enzymes Feedback inhibition – the switching off of a metabolic pathway by

one of its products, which acts as an allosteric (or otherwise) inhibitor of an enzyme within the pathway.


Control of enzymes Cooperativity – one substrate binds and induces a

conformation change in the enzyme which makes it more readily accept additional substrate molecules.


CELL MEMBRANES


Phospholipid bilayer Phospholipids are amphipathic molecules, having both

hydrophilic and hydrophobic regions.

Phospholipids are arranged in membranes as a phospholipid bilayer.


Fluid-mosaic model The cell membrane is a mosaic of protein molecules bobbing

in a fluid layer of phospholipids.

Phospholipids Protein

Cell membrane


Membrane fluidity Phospholipids, proteins and other molecules making up membranes are not static – they

frequently move laterally within the membrane.

As temperature decreases, molecular motion decreases and so membranes

become less fluid; and vice versa.

Membranes high in unsaturated phospholipids remain fluid at a lower temperature due to the kinks in the hydrocarbon tails.

Cholesterol also hinders close packing of phospholipids at low temperatures. It also lowers membrane fluidity at high

temperatures.

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Membrane proteins

Integral proteins – penetrate to the

hydrophobic core of the phospholipid

bilayer.

Peripheral proteins – not embedded in the

bilayer at all.


The cell membrane

Cytoplasm

Extracellular fluid

Phospholipid bilayer

Protein Cholesterol

Oligosaccharide


Functions of membranes Compartmentalisation – keeping substances on each side of

the membrane separate from each other.

Cell membranes keep the interstitial

(extracellular) fluid separate from the

cytoplasm.


TRAFFIC ACROSS MEMBRANES


Selective membrane permeability Selective permeability – allows some substances

through but not others.

Can pass easily

• Hydrophobic molecules • Small molecules

• Uncharged molecules

• Eg CO2, O2

Cannot pass easily

• Hydrophilic molecules • Large molecules

• Charged molecules

• Eg H2O, glucose, ions


Transport proteins Transport proteins span the bilayer and provide a means of transport across the membrane for molecules that otherwise

would not easily pass.

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Transport across membranes Passive transport – diffusion of a substance

across a biological membrane.

Any substance will diffuse down its concentration gradient – diffusion always

occurs down a concentration gradient.

Active transport – the pumping of substances across a membrane against its

concentration gradient, using cellular energy.


Passive transport In passive transport, substances diffuse spontaneously down their concentration gradients, crossing the membrane with

no expenditure of energy by the cell.

Diffusion Facilitated diffusion


Active transport

The uphill transport of substances against their concentration gradient, with the use of cellular energy.


Cotransport The coupling of the downhill diffusion of one substance to the

uphill transport of another against its own concentration gradient.


Osmosis Osmosis – the diffusion of water across a selectively permeable

membrane. A form of passive transport.

Water moves from a place of lower total solute concentration (hypotonic solution) to a place of higher total solute concentration

(hypertonic solution).

Solutions of equal total solute concentration are said to be isotonic.


Water balance of cells

Animal

Plant

Hypotonic solution

Hypertonic solution

Isotonic solution

Shrivelled Normal

Lysed

Turgid (normal) Flaccid Plasmolysed

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Traffic of large substances

Exocytosis – process by which a cell secretes macromolecules, etc, by the fusion of vesicles with the

plasma membrane.

Endocytosis – process by which a cell takes in macromolecules, etc, by forming new vesicles from the

plasma membrane.


Exocytosis


Endocytosis Phagocytosis – pseudopodia

engulf a particle, which is packaged inside a food

vacuole and taken inside the cell.

Pinocytosis – droplets of extracellular fluid are

incorporated into the cell in small vesicles.


Endocytosis

Receptor-mediated endocytosis – vesicles form when specific molecules (ligands) bind to

receptor molecules on the cell surface.


Question Time

• What does activation energy mean? • How do our bodies control enzyme activity? • What is the structure of the plasma

membrane. • Name some ways in which we can transport

substances across a membrane.


Homework

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Homework

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AN INTRODUCTION TO METABOLISM 2008 Biology Olympiad