Chapter 4, How Cells Work, part 2 of 2
Up to this point, we have attempted to give an overview of life at the level of the cell. Cells are made of chemicals, many of which (the organic ones at least, not Na+, Cl-, or other ions) can be classified into four broad categories or organic molecules (carbohydrates, lipids, proteins, nucleic acids). Organic molecules define both the structure of the cell and, together with inorganic molecules and ions, its chemical environment. We have also begun to consider common chemical processes cells perform, such as exchanging chemicals with extra-cellular fluid across membranes. Chemicals, in this case proteins, in the cell membrane also allow cells to react to physical contact with certain other chemicals, e.g. hormones and antigens were used to illustrate this.
We now
come to another process performed by cells, energy transformations.
Cells perform energy transformations via chemical reactions (metabolism)
organized in sequential order (metabolic pathways). Chapter 4,
sections 4.1 through 4.3, covers two important concepts: energy and
metabolic pathways.
“Metabolism” refers to the total or sum of all chemical reactions in
cells; metabolism requires energy
for building organic molecules and for breaking them down, and for
creating movement within the cell (the basis of all bodily movement).
Ask yourself, What is energy?
Coal, gas, and electricity? Foods
we eat are fuels for internal reactions but is the food itself energy?.
I. Energy – the capacity to do work; bring about change.
A rather uninspiring definition, & we should acknowledge that the concept of energy is difficult to fully grasp. Energy is intangible, takes up no space, has no mass, it is not material, not hard and concrete. Nevertheless, energy takes various forms, e.g. electrical, heat, light, & motion. Energy can also be stored as in a battery or within chemical bonds between atoms as in the covalent bonds of organic molecules (think carbohydrates and fats - high energy foods).
From studies on energy we know that energy flows through a system and must always be replaced from an outside source if current useable energy levels are to be sustained. Consider the Universe. There is no outside energy source, thus the Universe has all the energy it will ever have.
Two Laws of Energy:
1) ENERGY CANNOT BE CREATED OR DESTROYED BUT IT CAN CHANGE FORM.
Ex. Light is converted to Heat, Heat is converted to Electricity, Electricity is converted to Light, or heat, or motion.
Chemical bond energy is converted
to light, heat & motion [the breaking bonds of burning coal
yield light and heat; a firefly lights up from the release of bond energy
in the food it ate; you
convert the chemical bond energy of food into bodily motion and body
heat.]
Light energy is converted to chemical bond energy by means of PHOTOSYNTHESIS
The second law of energy states:
2) WITH EACH ENERGY CONVERSION SOME ENERGY IS "LOST" AS HEAT.
HEAT = RANDOM MOTION, WHICH CAN BE SEEN AS DISORDER OR ENERGY THAT CAN'T BE USED. ENTROPY IS A MEASURE OF A SYSTEM'S DISORDER
ENTROPY OF THE UNIVERSE IN INCREASING.
(Recall a bookshelf or dorm room – when in use these naturally
become messy without energy to keep them organized)
Earth is a special place where the process of life fueled by a seemingly endless supply of sunlight energy has lowered the entropy of the biosphere. Maintaining this lowered state of entropy, or the higher state of order, requires a constant energy input from an outside energy source (the sun).
In light of the 2nd law of energy, it is useful to acknowledge that CELLS HAVE LOW ENTROPY and to keep it that way, requires a constant input of energy from an outside source- maintaining the organized state required for life processes requires constant energy input from outside energy source
Please read and ponder: Ultimately that outside energy source is the Sun (you've heard that before). The sun’s energy is channeled through plants into chemical bonds (perhaps that concept is new to you). Chemical bonds store energy that was once light energy. Chemical bonds are the basis of food energy. Plants make food from nonfood (i.e. CO2) using energy of light. All animal life and all decomposers depend directly or indirectly on energy in food made by plants. Plants constantly convert light energy into food energy thus supplying the living world its energy needs. Without the action of plants converting light energy into food energy (via photosynthesis) the living world wood soon run out of useable energy and life would cease. We convert some of the energy of plants into our own bodily energy stores. Unlike recyclable aluminum cans, this food energy can’t be recycled indefinitely because each time we convert energy from its source (food) some is “lost” as heat (random molecular motion)—randomness is in contrast to the highly ordered structure of living organisms. If the world were a closed system, it would soon run out of useful energy and life would end. The Sun of course provides the energy life needs. Light energy from the sun is converted into the chemical energy of organic molecules (PHOTOSYNTHESIS accomplishes this). Carbohydrate molecules created by photosynthesis supply the energy needed to assemble amino acids during protein construction (a ribosomal, ER event). Energy conversions by living beings are accomplished via metabolism, and metabolism is accomplished by chemical reactions.
II. METABOLIC PATHWAYS – organized series of chemical reactions. Our examples to illustrate metabolic pathways in detail will be photosynthesis (Chapter 5) and respiration (Chapter 6). For now, a few basics about metabolism are to be covered.
A
given metabolic pathway (organized series of reactions) is often housed within a
membrane-bound cellular compartment (i.e. organelle). Thus, the
chemicals involved in photosynthesis, for example, aren't mixed up with
the chemicals involved in respiration (chloroplasts house the reactions of
photosynthesis; mitochondria house the reactions for most of respiration).
Enzymes - control each reaction in a metabolic pathway.
Enzymes are globular proteins w/ specific shape.
“Substrates” are the
reactants in an enzyme facilitated reaction
“Active site” is the "crevice" part of an enzyme whose shape
fits that of the substrates
(lock & key – the active site is the lock’s key hole and
the key is the substrate)
The Induced-fit model explains enzyme action. Upon binding with a substrate at the active site, an induced shape change occurs that forces the substrate to undergo a reaction. The product of this reaction is released and the active site is left vacant, waiting another substrate or substrates. By aligning substrates and forcing them to react, enzymes reduce the amount of energy and time it may otherwise take for reactions to occur (i.e., enzymes lower the activation energy).
Enzymes
are:
·
Reaction specific
(a lock operates with only a specific key, thus active sites operate with
only specific substrates) an enzyme will
catalyze only one specific reaction (usually).
· Reusable (each
enzyme can repeatedly catalyze the same type reaction)
· Can be inhibited by
chemicals that block enzyme action. Ex,. excess end products in metabolic pathways may interfere with enzymes of a metabolic
pathway.
enz.
1
enz. 2
enz. 3
A
--------> B
--------> C --------> D (D is final product)
Excess product “D” interferes with enzyme 1; this interference
helps maintain chemical equilibrium (homeostasis). Only so much of
“D” is needed, so if excess D begins to accumulate, the excess D will
inhibit its own production by interfering with the active site of enzyme
#1. Thus, chemical reactions that support life do not occur
helter-skelter but are controlled and mediated.
· Can be helped
(cofactors such as vitamins complete the shape of some enzymes)
· Can have structure
detrimentally altered = denatured. ex. extreme heat, or changes in pH
ATP
– the energy currency of the cell; ATP is needed for many enzyme catalyzed
reactions.
ATP stores energy when ATP is formed and ATP releases energy when
ATP is broken down. The
stored ATP energy comes from the food we breakdown during cellular
respiration. The released ATP
energy provides the quantity of energy needed for reactions that require
energy input, ex. muscle contractions & building new molecules for
cell growth (neither process can use food energy directly, the energy
of food must first be transferred to ATP during ATP synthesis).
Keep in mind that some of the food we eat is not for energy needs,
but rather for materials to build our bodies--“you are what you
eat”---at least that’s where your material body comes from, a
restructuring of the food molecules ingested. The energy from food combusted to CO2 through
cellular respiration yields the energy in the form of ATP needed to
perform this molecular reconstruction that forms your body out of the food
eaten. Thus
“food,” a non-technical term, has two purposes:
1) some food is destroyed as its chemical bond energy is
transferred to ATP (waste CO2 is released as the remains of
food eaten and used in this process); 2)
some food is molecularly rearranged keeping its organic structure but
reformed to fill the molecular needs of your body.
Most
ATP production is produced from Electron Transport Chains (ETC)
ETC’s are located on the inner membranes in chloroplasts and
mitochondria.
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