Chapter
5
The Working Cell
Energy
Energy is defined as the capacity to cause change
Some forms of energy are used to perform work, such as moving an object against an
opposing force
Conceptually, we can classify energy as either of two forms
Potential - stored energy
Kinetic - energy of motion
Energy in living systems
All living organisms require energy to survive
The sun is the ultimate source of energy for most living organisms
Photosynthesizers (producers) convert sunlight energy to chemical energy in sugars
Sunlight energy is only really useful to producers; chemical energy is useful to all
organisms
Metabolism is the sum of all of the chemical reactions that occur in the body
Metabolism transfers and converts energy and follows the laws of thermodynamics
Laws of thermodynamics
1st law - Energy is neither created nor destroyed
It is conserved and can only be converted from one form to another
2nd law - When reactions occur, they lead to more disorder
Entropy is a measure of disorder
The greater the entropy, the greater the disorder
New order (increased complexity) can only be achieved with the input of energy
Heat is a type of kinetic energy contained in the random motion of atoms and
molecules
Heat is the least useful form of energy for biological organisms
Chemical Energy
The molecules of food, gasoline, and other fuels have a form of potential energy
called chemical energy, which arises from the arrangement of atoms and can be
released by a chemical reaction
But every chemical reaction leads to the conversion of some energy as heat
Cellular Respiration
Cellular respiration is
the energy-releasing chemical breakdown of fuel molecules and
the storage of that energy in the form of ATP (energy currency of the cell)
Living organisms can bring about local increases in order (within themselves)
through their metabolic processes - as long as there is a supply of energy
Food Calories
A calorie (cal) is the amount of energy that can raise the temperature of 1 gram (g) of
water by 1°C
Food calories are actually kilocalories, equal to 1,000 calories
Calories in food are used to fuel the activities of life
ATP and Cellular Work
Chemical energy released by the breakdown of organic molecules (food, fuel) during
cellular respiration drives the production of molecules of ATP (adenosine
triphosphate)
ATP acts as an energy intermediary, produced to store energy so it can be released
later to do work
ATP energizes other molecules in cells by transferring phosphate groups to those
molecules
This energy
helps cells change shape,
enables the transport of ions and other substances across membranes, and
drives the production of a cells macromolecules
Cells constantly use an enormous amount of ATP (as much as 10 million per second
in a single muscle cell)
If cells had to make all this ATP from scratch, it would never be able to do
anything else
Instead, it uses the ATP/ADP cycle
Endergonic chemical reactions require and store energy
Living organisms store energy in the products of endergonic reactions
Exergonic chemical reactions release energy
Living organisms use the released energy to
run endergonic reactions
carry on work in cells or the body
The key for living organisms is that endergonic and exergonic reactions are linked in
coupled reactions
The energy released from exergonic reactions drives the endergonic reactions
Metabolism
Metabolism consists of two processes
Anabolism - biosynthesis of complex molecules from simpler building blocks
(polymers from monomers)
Requires energy
Catabolism - breakdown of complex molecules for energy and monomers
Releases energy
Chemical reactions
Chemical reactions are crucial for life
Oddly, most chemical reactions would occur extremely slowly under normal
conditions for life - temperature, concentrations
Therefore, life needs help to carry on all of the essential reactions
Major chemical changes require a series of small changes
These small changes make up a metabolic pathway
Metabolic pathways usually have multiple steps requiring multiple enzymes
Enzymes do not actively seek out their substrates; they require random collisions
and then binding
Enzymes in the same metabolic pathway may be localized near each other,
maximizing the likelihood of these random collisions
Enzymes
Enzymes lower the activation energy of chemical reactions thereby speeding up the
reactions
They are catalytic
Almost all enzymes are proteins
They are highly specific for their reactants and therefore the reactions they catalyze
3-D shape is crucial because that forms the active site where the substrate binds
Induced fit is when the active site changes upon binding the substrate
Almost all chemical reactions in a cell require enzymes
Enzymes put smaller molecules together as well as pull them apart
Regulating Enzyme Activity
Since cells (and organisms) are constantly dealing with changing conditions, it is
crucial that enzyme activity be finely regulated
Enzyme activity can be controlled in several ways
Competitive inhibition
A substance other than the enzyme’s usual substrate binds at the active site substrate imposter
Allosteric regulation
A molecule binds at a second site affecting the enzyme’s active site
The effect can be either positive or negative
Plasma membrane
The plasma membrane is thin and flexible
Yet it is stable enough to maintain its integrity despite being continually remade due
to the constant movement of materials moving through it
All biological membranes are composed of a phospholipid bilayer
It is selectively permeable
Small, uncharged substances pass freely through the membrane
Other substances may pass freely with the help of proteins
Embedded and associated with the cell membrane are many proteins
Many of these proteins are involved in transport across the membrane
Other proteins have different functions
Some membrane proteins are integral - embedded in and spanning the membrane
Others are peripheral - not actually embedded in the membrane but usually
associated with an integral protein
Movement of substances across the membrane can occur passively (without any
energy expended) or actively (with energy expended)
Passive Transport
Small, uncharged substances can simply diffuse across the membrane
Simple diffusion
Net movement down a concentration gradient
From an area of higher concentration to an area of lower concentration
Osmosis is the special case of diffusion of water across the membrane
Net movement is still from an area of higher concentration to an area of lower
concentration
For those substances that do not simply diffuse across the membrane, sometimes
there are proteins involved as well
Sometimes, only protein channels are required to allow the material to move
down its concentration gradient - facilitated diffusion
We can use three comparative terms to describe solutions
They only make sense when we are comparing two solutions
Isotonic - [solute] are equal
Hypertonic - solution with higher [solute]
Hypotonic - solution with lower [solute]
Because of the cell membrane, we consider the [solute] inside and outside of the cell
Osmosis will occur to try to equalize the [solute]
In animal cells, the cells can shrink from drying out or expand and even burst
Plant cells are protected from this to some degree by the cell wall
Plant cells are healthiest in a hypotonic environment with a net inflow of water,
which expands their cell walls without bursting
As a plant cell loses water,
it shrivels and
its plasma membrane may pull away from the cell wall, usually killing the
cell
Active Transport
For some substances, energy is required and carrier proteins are involved - active
transport
ATP is the energy source
Can produce a concentration gradient
It can move substances from an area of lower concentration to one of higher
concentration
Moving Larger Molecules
Exocytosis
Release of materials to the outside of the cell through fusion of the transport
vesicle membrane with the cell membrane
Endocytosis moves material in
In a process called phagocytosis (“cellular eating”), a cell engulfs a particle and
packages it within a food vacuole
Some cells are specialized for engulfing and destroying invading microbes