1. cell activities 2 © zanichelli editore 2015 cell metabolism and atp 3 © zanichelli editore 2015

Cell activities

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© Zanichelli editore 2015

Cell metabolism and ATP

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Cell metabolism:anabolic and catabolic reactions

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Cell metabolism is the sum total of the reactions carried out by a cell.

It involves anabolic reactions, which result in the synthesisof biomolecules and require energy, as well as catabolic reactions, which result in the degradation of biomolecules and produce energy.


Cells and the laws of thermodynamics

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Cells are ordered systems and they need energy in order to function.

They use the chemical energy stored in bonds between molecules of nutrients.

Energy metabolism is the sum of reactions that allow cells to produce energy.


H CChemical energy

ATP and energy storage

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Cells convert chemical energy into ATP; high-energy molecules composed of adenine, ribose and three phosphate units linked together by covalent bonds.

P P P

Adenine

Ribose

Phosphate groups


ATP hydrolysis /1

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ATP hydrolysis releases free energy that can be used for cell metabolism:

ATP + H2O ADP + P + Energy


ATP hydrolysis /2

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P P P

P P

Pi + ENERGY


ATP cycle

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ATP molecules can be “recharged”: energy can be released and acquired in a cyclic manner.

ATP

ADP + Pi

ENERGYENERGY


The role of cell membranes

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Functions of biological membranes

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Biological membranes play an important role in cell metabolism.

From the functional point of view, all biological membranes:•are selective filters;•are flexible and dynamic;•take part in metabolic processes, thanks to proteins.


Structure of the plasma membrane

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Its flexible and dynamic structure is called fluid mosaic.


The plasma membrane is made up of a phospholipid bilayer with proteins immersed in it.

Phospholipids have a hydrophilic head, containing a phosphate group, and two hydrophobic tails.

Proteins in cell membranes

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Proteins represent around 50% of cell membranes.

They can be:• integral, when they cross the double layer and protrude on both sides of the membrane;

• peripheral, when they are associated with the internal or external side of the membrane.


Active and passive transport

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Molecules can cross the membrane by two types of transport:•passive transport – it is spontaneous;•active transport – it requires energy from ATP.

Larger molecules can be transported across the membraneby vesicles.


Passive transport /1

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Passive transport /2

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In passive transport, molecules can cross the membrane by diffusion due to the concentration gradient. This process does not require energy.

Simple diffusion is the movement of small and apolar molecules across the double layer of the membrane.

In facilitated diffusion, polar molecules or ions cross the membrane using specific transport proteins.

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Osmosis and water diffusion

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Osmosis is the movement of water across a semipermeable membrane; it is a type of diffusion.

Water leaves the cell if the extracellular fluid is hypertonic (higher concentration of solutes), it enters the cell if it is hypotonic (lower concentration of solutes).


Active transport

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Active transport requires energy from ATP and specific transport proteins.

Transport can happen using a uniport, symport or antiport mechanism.


Endocytosis and exocytosis

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In endocytosis and exocytosis, macromolecules are engulfed in transport vesicles made of membrane fragments.


endocytosis

exocytosis

From nutrients to ATP

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Metabolic pathways

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Energy metabolism is organized in metabolic pathways; sequences of reactions that gradually transform initial reagents into final products through a series of ordered steps.


Initial reagent

Step A

Step B

Step C

Final product

The role of NAD in metabolism

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The molecule NAD is a co-enzyme that intervenes in many metabolic pathways. It exists in two forms: NAD+ and NADH.In the presence of hydrogen and an electron donor, NAD+ becomes NADH which contains more energy.

NAD+

NADH + H+

acquires e– acquires e–loses e–loses e–

RH2

R R’

R’H2


The most common nutrient is glucose

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The most common nutrient for obtaining energy is glucose.

C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy

The breakdown of the molecule occurs in two phases: glycolysis and cellular respiration.

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Glycolysis is a universal metabolic pathway

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Glycolysis occurs in cytoplasm and transforms glucose into 2 molecules of pyruvate, producing 2 molecules of ATP and 2 of NADH.


glucose(C6H12O6)

2 pyruvate(C3H4O3)

2 ATP

4 ATP

2 NADH

Cell respiration /1

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Cell respiration takes place in the mitochondria and uses O2 molecules as electron acceptors.


Its preparatory phase is called pyruvate oxidation which degrades pyruvate to CO2 to produce acetyl-CoA.

PYRUVATE OXIDATION

2 pyruvate(C3H4O3)

2 CoA

2 CO2

2 NADH

2 acetyl-CoA(CH3CO-S-CoA)

2 NAD+

Cell respiration /2

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The second phase of cell respiration is the Krebs cycle, which degrades acetyl-CoA to CO2 to produce electron carriers (NADH and FADH2) and ATP.

KREBS CYCLE

2 acetyl-CoA(CH3CO-S-CoA)

4 CO2

2 ATP

6 NADH

2 FADH2

6 NAD+ 2 FAD

Cell respiration /3

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The final step of cell respiration is the electron transport chain: it oxidizes carriers and produces H2O and ATP.

ELECTRON TRANSPORT CHAIN

10 NADH +2 FADH2

6 O2

6 H2O

28 ATP

10 NAD+

2 FAD

Fermentation

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In the absence of oxygen, fermentation occurs, which regenerates NAD+ from NADH, by an alternative path.


Fermentation is used by anaerobic organisms (for example Lactobacillus bulgaricus or yeasts) and by muscle cells during intense exercise.

Solar energyand the productionof nutrients

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Autotrophs produce glucose

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Autotrophs produce glucose from inorganic molecules andan external energy source - usually the Sun.

The most important process used by autotrophs is photosynthesis, done by plants, algae and cyanobacteria.

Photosynthesis transforms CO2 and H2O into glucose and oxygen:

6 CO2 + 6 H2O + Energy C6H12O6 + 6 O2


Photosynthesis: different phases /1

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The process of photosynthesis can be divided into two major phases:

• during the light-dependent reactions, chlorophyll and other photosynthetic pigments capture energy from the Sun and use it to produce ATP and NADPH;

• in the light-independent reactions (or Calvin cycle), the high-energy molecules are used to fix carbon and transform CO2 in sugars.


Photosynthesis: different phases /1

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Photosynthesis takes place in the chloroplasts /1

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In the thylakoids inside the chloroplasts there are molecules of chlorophyll.

They are necessary to transfer electrons from water to NADPH during the light-dependent phase.

Calvin cycle occurs in the stroma - the synthesis phase in which carbon is fixed in sugars.


Photosynthesis takes place in the chloroplasts /2

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1. cell activities 2 © zanichelli editore 2015 cell metabolism and atp 3 © zanichelli editore 2015

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