Understanding Cells: The Basic Units of Life
Cells make up the smallest level of a living organism such as yourself and other living things. The cellular level of an organism is where the
metabolic processes occur that keep the organism alive. That is why the cell is called the fundamental unit of life.
Defining cells of living creatures
What exactly are cells? Cells are sacs of fluid surrounded by membranes. Inside the fluid float chemicals and organelles. An organism contains parts
that are smaller than a cell, but the cell is the smallest part of the organism that retains characteristics of the entire organism. For example, a cell
can take in fuel, convert it to energy, and eliminate wastes, just like the organism as a whole can. But, the structures inside the cell cannot perform
these functions on their own, so the cell is considered the lowest level.
Each cell is capable of converting fuel to useable energy. Therefore, cells not only make up living things; they are living things. Cells are found in all
plants, animals, and bacteria. Many of the basic structures found inside all types of cells, as well as the way those structures work, fundamentally
are very similar, so the cell is said to be the fundamental unit of life.
The most important characteristic of a cell is that it can reproduce by dividing. If cells did not reproduce, you or any other living thing would not
continue to live. Cell division is the process by which cells duplicate and replace themselves. If you did not replace your red blood cells, for example,
you would have a life span only as long as that of red blood cells — a mere 120 days.
Increasingly more complex organisms are made up of increasingly more groups of cells (for example, in humans, groups of cells make up each
organ and muscle tissue), and the organisms survive based on products that the cells make. For example, cells in the pancreas make insulin, which
is necessary to ensure that the blood glucose level doesn’t skyrocket. Without insulin, the blood glucose can reach a level that is lethal. So, without
that cellular product, you would die.
Examining eukaryotes and prokaryotes
Cells fall into two major categories: eukaryotes and prokaryotes.
Eukaryotes are organisms that contain chromosomes, including plants and animals, as well as fungi (like mushrooms), protozoa, and most algae.
Eukaryotes have the following characteristics:
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They have a nucleus that stores their genetic information.
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Animal cells have an organelle called a mitochondria that effectively combines oxygen and food to convert energy to a useable form.
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Plant cells have chloroplasts, which use energy from sunlight to create food for the plant.
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Eukaryotic cells have internal membranes, which create compartments inside the cells that have different functions.
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Plants cells have a cell membrane and a cell wall, which is rigid; animal cells have only a cell membrane, which is soft.
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The cytoskeleton, which reinforces the cytoplasm of the cell, controls cellular movements.
Prokaryotes are cellular organisms that do not have a “true” nucleus. A nucleus is the control center of a cell. A nucleus contains the genetic
material packed into chromosomes, and it is associated with other organelles that function in the production of amino acids and proteins based on
what the genetic material dictates. Prokaryotes have some genetic material, but it is not as well organized as it is in eukaryotes. Still, prokaryotes
are able to reproduce. Examples of these organisms include bacteria and blue-green algae.
STRUCTURES OF ANIMAL CELLS
You have organs and are made up of cells. Your organ systems perform certain functions in you, the entire organism. Cells have organelles that
perform certain functions in the cell. Although it takes millions and millions of cells to create you, each cell functions on its own and metabolizes
individually.
Holding it all together: The plasma membrane
n
The fluid inside a cell (intracellular fluid) is called plasma or cytoplasm (cyto- means cell). The membrane holding the fluid in the cell is called a
plasma membrane, also called the cell membrane.
The cells themselves are floating in a type of fluid, called a matrix. The matrix is insoluble — substances do not dissolve in its fluid. The matrix just
supports the cells. The fluid that squeezes in between each and every cell is called extracellular fluid because it is outside of the cell.
The job of the plasma membrane is to separate the chemical reactions occurring inside the cell from the chemicals that are floating in the
extracellular fluid. If the plasma membrane didn’t separate the inside and the outside of the cell, waste products excreted from the inside of the
cell to the outside could flow back inside.
Structures in a typical animal cell.
Controlling the show: The nucleus
Every cell of every living thing has a nucleus, and every nucleus in every living thing contains genetic material. The genetic material directs the
production of proteins that make the entire organism function; the nucleus makes the entire cell function.
In the nucleus of cells that are not currently dividing, clumps of thread-like genetic material called chromatin appear. Right before a cell divides, the
chromatin bunches up into chromosomes, which contain DNA (deoxyribonucleic acid).
DNA has two strands, each of which has sequences of nitrogenous bases that form the genetic code. The genetic code, which is derived from the
nucleotide bases in the genes on strands of DNA, is “interpreted,” and then a ribonucleic acid (RNA) molecule called messenger RNA (mRNA)
is produced from the DNA template. The mRNA uses the information from the genetic code for certain amino acids — the building blocks of protein
the cell. The amino acids are then taken by transfer RNA (tRNA) to an organelle called a ribosome, where the final proteins are made.
Proteins either contribute to the structure of the cell, or they contribute to the function of the cell, meaning that they are used as enzymes in
metabolic processes. Either way, it is the genetic material housed in the nucleus that ultimately controls the structure and function of each and
every cell.
Each nucleus has a round mass inside it called a nucleolus. The nucleolus produces the third type of RNA molecule — ribosomal RNA (rRNA). This
type of RNA helps to make ribosomes, which get transferred from the nucleus to the cytoplasm to help in making proteins.
Surrounding each nucleus is a double layer formed from proteins and lipids that separates the nucleus from the cytoplasm. This two-layered
structure is called the nuclear envelope or nuclear membrane.
The factory of the cell: The endoplasmic reticulum
The endoplasmic reticulum (ER) is a series of canals that connects the nucleus to the cytoplasm of the cell. The part of the ER that is dotted with
ribosomes is called rough ER; the part of the ER that has no ribosomes is called smooth ER.
Ribosomes on the rough ER serve as the place for the synthesis of proteins that are directed by the genes to be put together in the ER. (Other
proteins are put together on ribosomes attached to other organelles or floating free in the cytoplasm.) The smooth ER contains transport vesicles
that shuttle cellular products from cytoplasm to organelle, from organelle to organelle, or from organelle to plasma membrane. In addition to
protein synthesis, the ER is involved in the metabolism of lipids (fats).
The main function of ER is to make and transport proteins. The ER is essentially the “womb” for new protein chains. Protein synthesis, or
production, begins in the nucleus, with the mRNA molecule carrying the genetic information as to what amino acids (proteins) should be produced.
The tRNA molecules bring the amino acids from the cytoplasm to the ribosomes, which are produced by rRNA. At the ribosomes, the amino acids
are joined together to form a protein, and the protein is stored in the ER until it can be moved to the Golgi apparatus.
Preparing for distribution: The Golgi apparatus
In biology, as well as other sciences, structures usually are named for the person who found them. In this case, the Italian scientist Camillo Golgi
finds fame. The Golgi apparatus is very close to the ER; in the figure above, it looks like a maze with water droplets splashing off of it. The “water
droplets” are transport vesicles bringing material from the ER to the Golgi apparatus.
Inside the Golgi apparatus, products produced by the cell, such as hormones or enzymes, are packaged for export to other organelles or to the
outside of the cell. The Golgi apparatus surrounds the product to be secreted with a sac called a vesicle. The vesicle finds its way to the plasma
membrane, where certain proteins allow a channel to be produced so that the products inside the vesicle can be secreted to the outside of the cell.
Once outside the cell, the products can enter the bloodstream and be transported through the body to where they are needed.
Lysosomes really clean up
Lysosomes are special vesicles formed by the Golgi apparatus to “clean up” the cell. They are the garbage men (or sanitation engineers) of the cell.
Lysosomes contain digestive enzymes, which are used to break down products that may be harmful to the cell and “spit” them back out into the
extracellular fluid. Lysosomes also remove dead organelles by surrounding the dead organelle, breaking down the proteins of the dead organelle,
and releasing them to reconstruct a new organelle. Because the lysosome acts upon its own cell, the process is called autodigestion.
Peroxisomes break down hydrogen peroxide
Peroxisomes are little sacs of enzymes produced by smooth ER to help protect the cell from toxic products. You know how hydrogen peroxide is
helpful when you use it to clean out a wound because it kills bacteria? Well, too much hydrogen peroxide inside you could kill you. Hydrogen
peroxide is normally produced in some metabolic reactions, so it is inside you. However, hydrogen peroxide becomes harmful to the cells of the
body if too much accumulates, so the key is to keep breaking it down to keep it from accumulating.
The powerhouses of the cell: The mitochondria
The ER supplies the products, the Golgi apparatus distributes the products, and the mitochondria supply the energy for all of those processes to
take place.
When you get a bill for electricity, the amount of electricity your household used in the past month is measured in kilowatt hours. Inside an
organism, the amount of energy a cell uses is measured in molecules of adenosine triphosphate (ATP). The mitochondria produce the ATP, and to
do it, mitochondria use products of glucose metabolism as fuel.
The Fluid-Mosaic Model of the Cell Plasma Membrane
The fluid-mosaic model describes the plasma membrane of animal cells. The plasma membrane that surrounds these cells has two layers (a bilayer)
of phospholipids (fats with phosphorous attached), which at body temperature are like vegetable oil (fluid). And the structure of the plasma
membrane supports the old saying, “Oil and water don’t mix.”
Each phospholipid molecule has a head that is attracted to water (hydrophilic: hydro = water; philic = loving) and a tail that repels water
(hydrophobic: hydro = water; phobic = fearing). Both layers of the plasma membrane have the hydrophilic heads pointing toward the outside; the
hydrophobic tails form the inside of the bilayer.
Because cells reside in a watery solution (extracellular fluid), and they contain a watery solution inside of them (cytoplasm), the plasma membrane
forms a circle around each cell so that the water-loving heads are in contact with the fluid, and the water-fearing tails are protected on the inside.
The fluid-mosaic model of plasma membranes.
Proteins and substances such as cholesterol become embedded in the bilayer, giving the membrane the look of a mosaic. Because the plasma
membrane has the consistency of vegetable oil at body temperature, the proteins and other substances are able to move across it. That’s why the
plasma membrane is described using the fluid-mosaic model.
The molecules that are embedded in the plasma membrane also serve a purpose. For example, the cholesterol that is stuck in there makes the
membrane more stable and prevents it from solidifying when your body temperature is low. (It keeps you from literally freezing when you’re
“freezing.”) Carbohydrate chains attach to the outer surface of the plasma membrane on each cell. These carbohydrates are specific to every
person, and they supply characteristics such as your blood type.
How Cell Substances Transport through the Plasma Membrane
The plasma membrane surrounding animal cells is where the exchange of substances inside and outside of cells takes place. Some
substances need to move from the extracellular fluid outside cells to the inside of the cell, and some substances need to move from the
inside of the cell to the extracellular fluid.
Some of the proteins that are stuck in the plasma membrane help to form openings (channels) in the membrane. Through these
channels, some substances such as hormones or ions are allowed to pass through. They either are “recognized” by a receptor (a
protein molecule) within the cell membrane, or they attach to a carrier molecule, which is allowed through the channels. Because the
plasma membrane is choosy about what substances can pass through it, it is said to be selectively permeabl e.
Permeability describes the ease with which substances can pass through a border, such as a cell membrane. Permeable means that
most substances can easily pass through the membrane. Impermeable means that substances cannot pass through the membrane.
Selectively permeable or semipermeable means that only certain substances are able to pass through the membrane.
Transporting substances across the plasma membrane can require that the cell use some of its energy. If energy is used, the transport
is called active. If molecules can pass through the plasma membrane without using energy, the molecules are using passive transport.
Helping the molecules across: Active transport
Sometimes, the molecules are just too big to easily flow across the plasma membranes or dissolve in the water so that they can be
filtered through the membrane. In these cases, the cells must put out a little energy to help get molecules in or out of the cell.
Embedded in the plasma membrane are protein molecules, some of which form channels through which other molecules can pass.
Some proteins act as carriers — that is, they are “paid” in energy to let a molecule attach to itself and then transport that molecule
inside the cell.
Passive transport of molecules
A membrane can allow molecules to be passively transported through it in three ways: diffusion, osmosis, and filtration.
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Diffusion: Sometimes organisms need to move molecules from an area where they are highly concentrated to an area where
the molecules are less concentrated. This transport is much more easily done than moving molecules from a low concentration to
a high concentration. To go from a high concentration to a low concentration, in essence the molecules need to only “spread”
themselves, or diffuse, across the membrane separating the areas of concentration.
In the human body, this action occurs in the lungs. You breathe in air, and oxygen gets into the tiniest air sacs of the lungs, the
alveoli. Surrounding the tiniest air sacs of the lungs are the tiniest blood vessels — capillaries. The capillaries in the lungs, called
pulmonary capillaries, contain the lowest concentration of oxygen in the body, because by the time the blood gets to the tiniest
vessels, most of the oxygen has been used up by other organs and tissues.
So, the tiniest air sacs of the lungs have a higher concentration of oxygen than do the capillaries. That means that the oxygen
from the alveoli of the lungs can spread across the membrane between the air sac and the capillary, getting into the bloodstream.
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Osmosis: This term is used when talking about water molecules diffusing across a membrane. Basically, the diffusion of water
(osmosis) works as described in the preceding bullet. However, with osmosis, the concentration of substances in the water is
taken into consideration. If a solution is isotonic, that means the concentrations of the substances (solutes) and water (solvent) on
both sides of the membrane are equal. If one solution is hypotonic, there is a lower concentration of substances (and more water)
in it when compared to another solution. If a solution is hypertonic, there is a higher concentration of substances in it (and less
water) when compared to another solution.
For example, the blood in your body contains a certain amount of salt. The normal concentration is isotonic. If suddenly there is
too high a concentration of salt, the blood becomes hypertonic (too many salt molecules). This excess of salt forces water out of
the blood cells in an attempt to even things out. But the effect this action has is actually that of shrinking the blood cells.
This shrinking of cells is called crenation (not cremation). If too much fluid is in the bloodstream, the blood cells have too few
molecules of salt in comparison, making them hypotonic. Then, the blood cells take in water in an attempt to normalize the blood
and make it isotonic. However, if the blood cells need to take in too much water to bring everything back into balance, they can
swell until they burst. This bursting of cells is called hemolysis (hemo = blood; lysis = break apart).