Cellular Metabolism and Transport

Metabolism

• Refers to the buildup and breakdown of nutrients within a cell.
• These activities enable the cell to thrive in its environment.
• Molecules must move in/out of the cell for it to thrive.
• Molecule transport occurs via passive and active transport.
• Passive transport does not require energy, while active transport does.

Passive Transport (Simple Diffusion)

• Overall movement of molecules or ions goes from an area of high concentration to an area of low concentration.
• Movement will continue until equilibrium is met.
• Examples include CO2 and oxygen movement.

Molecule Transport

• Bacteria may need a molecule too large to be transported into the cell via simple diffusion.
• Bacteria will release extracellular enzymes to break down the molecule, which transporters can then transport.
• No energy is required, as molecules are moving down the concentration gradient (facilitated diffusion).

Passive Transport (Facilitated Diffusion)

• Used mostly by small, hydrophilic ions needing to cross the hydrophobic interior of the bilayer.
• Mainly seen in prokaryotes.
• Large, bulky molecules, such as sugars and vitamins, are transported via facilitated diffusion.
• Molecules bind to transporters and undergo a conformational change to release molecules inside the cell.
• Mostly seen in eukaryotes.

Active Transport

• Active transport is a process that requires energy.

Molecule Transport (Phagocytosis)

• The phases include chemotaxis & adherence, ingestion (endocytosis), formation of phagosome, fusion between phagosome & lysosome, digestion, formation of residual body, and discharge of waste (exocytosis).
• Macrophages and WBC use this to destroy foreign substances and is a type of active transport.
• Endocytosis happens when a cell membrane surrounds a particle or molecule.
• Phagocytosis is a specific type of endocytosis in which solid objects like bacteria are engulfed.

3 Variations of Endocytosis

• Endocytosis has variations in the mechanism of particle engulfment.

Passive Transport (Osmosis)

• Is water movement across a membrane.
• Water moving across the membrane produces osmotic pressure.
• Osmotic pressure is required in order to prevent the flow of pure water (no solutes) across the membrane.
• The water concentration is opposite the solute concentration (things dissolved in water).
• Diffusion (osmosis) and aquaporins are 2 types of water movement.
• Although water is polar (hydrophilic) and expected to diffuse at low rates across membranes, the concentration of water molecules is very high, and the surface area of the membrane is large relative to volume.
• The low water permeability is counteracted by the high concentration and surface area and the presence of porins and other protein channels.

Tonicity

• The ability of an extracellular solution determines if water moves in or out of a cell by osmosis.
• A solution's tonicity relates to the total concentration of all solutes in the solution.
• Results in three types of solutions.
  • Isotonic - water moves in and out of the cell at an equal rate.
  • Hypotonic -water moves into the cell, causing it to swell ("hippo").
  • Hypertonic - water leaves the cell, causing it to shrink.

Principle of Osmosis

• The basic process determines water movement based on differences in solute concentration.
• Distilled H2O has low osmotic pressure and no salt and is an example of hypotonic solution.
• The addition of salt or sugar causes high osmotic pressure, which helps preserve food, and causes a hypertonic solution

Tonicity Cont.

• Bacterial cells can find themselves in isotonic, hypotonic, or hypertonic solutions.
• Cell walls can slow down or prevent lysis from hypotonic solutions.
• Cell wall-less bacteria like Mycoplasmas have a protective mechanism based on their cell membrane, which contains sterols that push water away.

Osmosis in Bacterial Cells

• External conditions impact a cell.

Osmotic Pressure & Bacteria

• Halophiles are organisms that grow in high salt environments.
• Extreme or obligate (restricted) halophiles require high osmotic pressure.
• Facultative halophiles tolerate high osmotic pressure.
• Marine algae and organisms from the Dead Sea are examples.

Enzymes

• Major players in metabolism that catalyze (speed up) reactions without permanently altering themselves.
• Optimally functioning enzymes are critical to microbes (and all living organisms).
• Physical and chemical agents that control microbial growth aim to ruin enzymes.
• Recycled and work by lowering the activation energy and end in -ase (ex. Helicase).
• E+S -> E/S Complex and allow reactions to just take longer to achieve their products.

Factors Affecting Enzyme Activity

• Includes temperature and pH.

Enzymes & Temperature

• Enzymes are efficient and can operate at low temperatures.
• High temperatures destroy protein's 3-D shape (denaturation).
• It is important that the temperature is compatible with the normal functioning of the cell.
• Bacteria producing disease in the human body optimally function between 35°C and 40°C.
• Minimal to no activity occurs if the temperature is lower or higher.

Enzymes & pH

• Most enzymes have an optimum pH.
• The enzyme loses activity below or above that pH.
• A drastic change in H+ concentration can affect a protein's 3-D shape (denaturation) and disrupt ionic bonds.

Outcome of Metabolism

• Is microbial growth.
• Increases in cell number, not cell size.
• Involves populations and colonies.

The Requirements for Growth

• Physical requirements include temperature, pH, and osmotic pressure.
• Chemical requirements include carbon, nitrogen, sulfur, phosphorous, trace elements (I, Zn, CU), oxygen, and organic growth factors (AA, Vitamins and Nitrogenous bases).

Physical Requirements

• Temperature is a key physical requirement.
• There are three important temperature points including the minimum growth temperature (lowest temperature growth is possible), the optimum growth temperature (temperature species grows the best), and the maximum growth temperature (highest temperature in which growth is possible).

Typical Growth Rates and Temperature

• Temperatures affect relative growth rates.

Psychrotrophs

• Grow between are capable of growing between 0°C and 20–30°C.
• Causes them to induce food spoilage because and grow well at refrigerator temperatures
• It makes them appear as mold, slime or off-taste or off-color in foods

pH & Growth

• Most bacteria grow between pH 6.5 and 7.5.
• Molds and yeasts grow between pH 5 and 6.
• Few bacteria can grow at a pH below 4 (this is why food like pickles, sauerkraut and some cheeses is preserved from spoilage by bacterial fermentation).
• Bacteria, acidophiles, grow in acidic environments.

Chemical Requirements & Growth

• Carbon is the structural backbone of living matter and an energy source.
• Chemoheterotrophs use organic carbon sources (proteins, carbs and lipids).
• Autotrophs use CO2.

Chemical Requirements & Growth Cont. (2)

• Nitrogen is needed for the synthesis of amino acids and proteins.
• Most bacteria decompose proteins.
• Some bacteria use NH4+ (ammonium ions) or NO3- (nitrates).
• Few bacteria use N2 (from atmosphere) in nitrogen fixation.
• Nitrogen fixation is a chemical process that converts nitrogen gas (fix) into ammonia and other nitrogenous compounds to aid in growth since organisms cannot utilize raw nitrogen gas

Chemical Requirements & Growth Cont.(3)

• Sulfur is needed for the synthesis of amino acids, thiamine, and biotin (vitamins).
• Most bacteria decompose proteins.
• Some bacteria use SO42- (sulfate ions) or H2S (hydrogen sulfide).
• Phosphorus is in DNA, RNA, ATP, and membranes.
• PO43- (phosphate ion) is a source of phosphorus.

Chemical Requirements & Growth Cont. (4)

• Trace elements are inorganic elements required in small amounts.
• Bacteria require small amounts of iron, copper and zinc (often found in tap water).
• They are essential for the function of some enzymes and act as enzyme cofactors (helpers).

The Effect of Oxygen (O2) on Growth

• Varying levels of oxygen are suitable for different species of bacteria.

Oxygen Can Be Toxic to Microbes

• It is formed during normal respiration.
• It is one of the body's most important defenses against a pathogen.
• Toxic oxygen is used in phagocytosis and ingested pathogens are killed in the phagolysosome stage.
• Microbes produces enzymes to neutralize this.

Toxic Oxygen (Molecular O2)

• Singlet oxygen occurs when O2 is boosted to a higher-energy state (extremely reactive).
• Superoxide free radicals: O2¯.
• Peroxide anion: 022-.
• Hydroxyl radical (OH•) is the most reactive and is formed by ionizing radiation.