Principles of Biology

Viruses exhibit some of the characteristics of life but lack others.

For example, some microorganisms live in boiling hot springs, whereas some moths have tongues the exact length of the flower from which they feed.

In this infrared image the polar bear‘s body heat hardly registers; only the uninsulated eyes and mouth show temperatures significantly warmer than the environment.

Viruses are often not considered living because they are not made of cells, nor are they capable of reproducing on their own.

If the individuals change to the point where they can no longer interbreed, a speciation event has occurred, and species diversity has increased.

Forensic scientists also analyze biological evidence left at crime scenes, such as insect parts or pollen grains.

Science also requires considerable imagination

There are, however, areas of knowledge and human experience that the methods of science cannot be applied to.

The boundary between these two forms of study is often blurred because most scientific endeavors combine both approaches.

A hypothesis can be shown to be false or eliminated, but it can never be proven true.

Scientific reasoning is more complex than the scientific method alone suggests.

The immediate goal of basic science is knowledge for knowledge’s sake, though this does not mean that in the end, it may not result in an application.

Pseudoscience is claims or beliefs that are represented as scientific fact but cannot be evaluated using the scientific method.

A scientific theory is the foundation of scientific knowledge.

We cannot break gold atoms down into anything smaller while still retaining the properties of gold.

Most of an atom's volume, greater than 99 percent, is empty space.

When an organism dies, it is no longer ingesting 14C, so the ratio will decline.

The PET camera shows the medical team, which of the patient’s tissues are taking up the most glucose.

This is known as the octet rule, which states, except for the innermost shell, that atoms are more stable energetically when they have eight electrons in their valence shell, the outermost electron shell.

They do not circle the nucleus like the earth orbits the sun, but we find them in electron orbitals.

In reversible reactions, reactants turn into products, but when the product's concentration goes beyond a certain threshold, some of these products convert back into reactants.

The opposite electrical charges of the resulting sodium cation and chloride anion result in the formation of an ionic bond.

The shared electrons spend more time near the oxygen nucleus than they spend near the hydrogen nuclei.

If it were not for hydrogen bonding, water would be a gas rather than a liquid at room temperature.

If strong bonds were used to hold the two strands together, instead of the weaker hydrogen bonds, the cell would need to invest a lot more energy into reproduction order to first break these bonds.

If this did not happen, plants and animals living in the water would freeze into a block of ice and would not be able to move around freely, making life in cold temperatures difficult, if not impossible.

Before it overflows, the water forms a dome-like shape above the rim of the glass.

As the temperature rises, the hydrogen bonds between water continually break and reform.

Because an atom of hydrogen has just one proton and one electron, a positively charged hydrogen ion is simply a proton.

Without this buffer system, the pH in our bodies would fluctuate too much, and we would fail to survive.

When they analyzed the contents of the flask a week later, they found amino acids.

The three-dimensional shape or conformation of a molecule is critical to determining its function.

Structural differences lead to differences in chemical properties which will cause the isomers to function differently.

This type of reaction is called a dehydration synthesis, a condensation reaction, which means “to put together while losing water".

When a monomer forms a covalent bond with another monomer as a result of a water molecule being released, this reaction is called dehydration synthesis.

Because of these structural differences, each molecule has different chemical properties; for example, the sugar fructose is sweeter than the sugar glucose.

Tightly packed chains of glucose give cellulose its rigidity and high tensile strength, which is very important to plant cells.

All lipids share one major characteristic: they are all hydrophobic (or at least have a hydrophobic region, as in phospholipids).

In mammals, fats are stored in specialized cells called adipocytes, where globules of fat occupy most of the space in the cell.

The phospholipid bilayer consists of two adjacent layers of phospholipids arranged tail to tail.

Each cell in a living system may contain thousands of different proteins, each with a unique function.

Its shape is important because it allows hemoglobin to attach and release oxygen molecules easily.

The term protein is not usually used until the polypeptide chain(s) have folded into their distinct three-dimensional shape and can carry out their unique function(s).

When the liquid egg white is placed in a hot pan, the heat denatures the protein.

The loss of one amino acid changes the primary structure of the protein.

In eukaryotic cells, DNA never leaves the nucleus, so a specific type of RNA, messenger RNA (mRNA), helps to relay information from the nucleus to other parts of the cell.

This is the Central Dogma of Life, which holds true for many organisms.

Close your eyes and picture a brick wall. What is the basic building block of that wall? Most would answer, it is a single brick.

Viruses are inactive outside of a host organism. Therefore, they do not grow and develop, nor can they reproduce on their own.

Cytotechnologists (cyto - = “cell”) are professionals who study cells. They are trained to determine which cellular changes are normal and which are abnormal.

Like bacteria, archaea are found in nearly every habitat on earth, even extreme environments that are very cold, very hot, very basic, or very acidic.

When there is insufficient surface area to support a cell’s increasing volume, a cell will either divide or die.

In eukaryotic cells, membrane-bound organelles allow different functions to be compartmentalized in different areas of the cell.

They can disassemble and reform quickly, which enables a cell to change its shape and move.

Cilia lining the cells of the respiratory tract move particulate matter toward the throat where it is then trapped in mucus.

The word “organelle” means “little organ” and organelles have specialized cellular functions just as your body's organs have specialized functions.

The rough endoplasmic reticulum (RER) is so named because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewed through an electron microscope.

A lysosome is like a wrecking crew that takes down old and unsound buildings in a neighborhood.

Mitochondria are oval-shaped, double-membrane organelles that have their own ribosomes and DNA.

The endomembrane system of eukaryotic cells includes the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, vesicles, as well as the plasma membrane.

Because the name 'protist' serves as a catchall term for eukaryotic organisms that are not animal, plant, or fungi, it is not surprising that very few characteristics are common to all protists.

Scientists hypothesize that host cells and bacteria formed a mutually beneficial endosymbiotic relationship when the host cells ingested aerobic bacteria and cyanobacteria but did not destroy them.

As the central vacuole shrinks, it leaves the cell wall unsupported and appears to wilt.

Desmosomes occur in patches on the membranes of cells... They keep cells together in a sheet-like formation in organs and tissues that stretch, like the skin, heart, and muscles.

The central vacuole expands, enlarging the cell without the need to produce more cytoplasm.

Fluid refers to the fact that materials making up the membrane move and are not rigid.

The lipid tails are nonpolar and are hydrophobic. The hydrophobic lipid tails meet in the inner region of the membrane and exclude the watery intracellular and extracellular fluid.

The glycocalyces found in a person’s body are a result of that person’s genetic makeup. They help identify cells as belonging to the same individual.

Plasma membranes enclose the borders of cells, but rather than being a static bag, they are dynamic and constantly in flux.

The perfume is at its highest concentration in the bottle and is at its lowest at the edges of the room.

Each separate substance in an environment has its own concentration gradient, independent of the concentration gradients of other materials in that same environment.

In this situation, water will move down its concentration gradient and enter the cell. This may cause an animal cell to burst or lyse.

Water moved into the cells from an area of low solutes in the blood to an area of high solutes in the cytoplasm resulting in the cells bursting.

Water moves across a semipermeable membrane in osmosis because there is a difference in solute concentration between the inside of the cell and the outside of the cell.

The word pinocytosis means “cell drinking” and was named at a time when the assumption was that the cell was purposefully taking in extracellular fluid.

People with this condition have life-threatening levels of cholesterol because their cells cannot remove the lipid from their blood.

Adenosine triphosphate (ATP) is the primary energy currency of all living cells.

Both anabolism and catabolism occur simultaneously and continuously within cells.

To better understand entropy, think of a student’s bedroom. If no energy or work were put into it, the room would quickly become messy.

No energy transfer is completely efficient, and all transfers trend toward disorder.

Think: exergonic means energy is exiting the system.

Phosphate groups are negatively charged and therefore repel one another when they are arranged in series, as is the case with ADP and ATP.

On their own, most of the chemical reactions in a cell happen too slowly for life to be maintained.

The induced-fit model expands on the lock-and-key model by describing a more dynamic binding between enzyme and substrate.

In non-competitive inhibition, an inhibitor molecule binds to the enzyme in a location other than the active site, often called an allosteric site.

Cells have evolved in such a way that they can use the products of their chemical reactions for feedback inhibition of enzyme activity.

Like an electrical plant, plants and animals also must take in energy from the environment and convert it into a form that their cells can use.

The transfer of electrons between atoms allows the cell to transfer and use energy in small increments rather than a single, destructive burst.

Phosphate groups are negatively charged and as a result, repel one another.

Scientific evidence suggests that atmospheric oxygen levels were very low, if not nonexistent, when life first evolved on the planet.

Glycolysis is their sole source of ATP, and if this pathway is interrupted, these cells will die.

Unlike glycolysis, the citric acid cycle is a closed loop.

Electron transport is a series of chemical redox reactions that resemble a bucket brigade or a ball rolling down a staircase.

ATP synthase, an integral protein and an enzyme, acts as a tiny generator which allows hydrogen ions to easily diffuse across the inner membrane.

Mitochondrial disorders can arise from mutations in nuclear or mitochondrial DNA, and they result in the production of less energy than is normal in body cells.

Lactic acid build-up causes muscle stiffness and fatigue.

Obligate anaerobes live and grow in the absence of oxygen. Oxygen is a poison to these microorganisms and kills them upon exposure.

Triglycerides store about twice as much energy as carbohydrates and are made of glycerol and three fatty acids.

These organisms are called photoautotrophs, literally “self-feeders using light,” because they use light to generate their own food.

When a wolf eats a deer, it obtains energy that was initially produced by plants.

After the energy is released, the “empty” energy carriers, NADP+ and ADP + Pi, return to the light-dependent reactions to obtain more energy.

It is easy to think of light as just something that allows living organisms to see, but light is a form of energy.

Chlorophyll a reflects the colors in the middle of the visible spectrum: green and yellow light. This explains why most plants appear to be green in color.

Photosystem II was named for the order of its discovery rather than for the order in which it functions.

When water is split, oxygen (O 2) and hydrogen ions (H+) are formed and accumulate in the thylakoid space.

This process is called carbon fixation because CO2 is “fixed” from its inorganic form into the organic form of 3-PGA.

The process of photorespiration wastes the energy carriers that were produced in the light-dependent reaction and does not lead to the production of glucose.

Prokaryotic photosynthetic autotrophic organisms have infoldings of the plasma membrane for chlorophyll attachment and photosynthesis.

If the DNA from all 46 chromosomes in a human cell were laid out end-to-end, it would measure approximately two meters!

Chromosomes consist of double-stranded DNA molecules wrapped around proteins.

Neurons found in the brain and spinal cord lack centrioles and are therefore amitotic, meaning they do not divide.

The mitotic spindles are broken down into amino acid monomers that will be used to assemble the cytoskeleton for each daughter cell.

The cleavage furrow deepens as the actin ring contracts, and eventually, the membrane and cell are cleaved into two separate identical daughter cells.

Because the separation of the sister chromatids during anaphase is an irreversible step, the cycle will not proceed until each pair of sister chromatids is firmly anchored to spindle fibers.

Even minor mistakes allow subsequent mistakes to occur more readily.

The actin ring contracts inward, pulling the plasma membrane toward the center of the cell until the cell is pinched in two.

As a result, some bacteria like E. coli can divide in as little as twenty minutes.

Bacterial cytokinesis is directed by a ring composed of a protein called FtsZ.

In asexual organisms, if the environment changes and an individual is negatively impacted, then all individuals would be negatively impacted due in part to the lack of genetic variation.

“Sex chromosome” is misleading. Many non-sex determining genes are found on the X chromosome and autosomes do contain genes involved in sex determination.

If this did not occur, the number of chromosomes would double with every future round of fertilization.

This has resulted in XX males who are phenotypically male, even though they have 2 X chromosomes.

Humans have 23 chromosome pairs, which results in over eight million (223) possibilities.

The four daughter cells produced at the end of meiosis are genetically unique because of processes like crossing over and independent assortment.

The alignment should occur differently in almost every meiosis.

Chromosomal disorders are characteristically noticeable and often fatal.

In contrast, polyploidy is very common in plants, and polyploid plants tend to be larger and more robust than the euploids of their species.

Notably, specific translocations have occurred with several cancers and with schizophrenia.

Mendel selected a simple biological system, the common garden pea plant, and conducted methodical, quantitative analyses using large sample sizes.

The blending hypothesis of inheritance stated that when two individuals made an offspring, their original parental traits were lost because their traits blended together when the offspring was formed.

Mendel’s decision to use traits that show discontinuous variation allowed him to see experimentally that offspring were not a result of “blending.”

Instead, Mendel’s results demonstrated that the violet flower trait was retained and the white flower trait had disappeared entirely in the F1 generation.

The fact that Mendel confirmed his work with statistical analysis made it relatively easy for others to repeat his experiments and verify his results.

The white-flowered allele is therefore considered recessive. An allele is considered recessive if it is masked (does not appear) in the F 1 offspring.

It is the phenotype that is observed in a heterozygous individual that determines which trait is dominant and which trait is recessive.

Which chromosome, paternal or maternal, will align on what side of the metaphase plate is unknown and leads to several different possible genetic arrangements.

For example, if neither parent has the disorder, but their child does, then both parents must be heterozygous for the gene to be passed down.

Observable traits are referred to as dominant, and non-expressed traits are described as recessive.

If Mendel had chosen an experimental system that exhibited these genetic complexities, it’s possible that he would not have understood what his results meant.

Pleiotropy is a pattern of inheritance where one gene controls two or more different characteristics.

Each gene that an individual inherits has a small additive effect on the overall phenotype, a concept known as polygenic inheritance.

Long before scientists visualized chromosomes under a microscope, the father of modern genetics, Gregor Mendel, began studying heredity in 1843.

Each chromosome contains hundreds or thousands of genes, organized linearly on chromosomes like beads on a string.

Interestingly, the Himalayan phenotype is the result of an allele that produces a temperature-sensitive gene product that only produces pigment in the cooler extremities of the rabbit’s body.

Sturtevant correctly presumed that genes are arranged in serial order on chromosomes and that recombination between homologs can occur anywhere on a chromosome with equal likelihood.

The carrier may never know they have a defective allele unless they have a child with the condition, or they have their genome sequenced.

With X-linked recessive conditions, males either have the condition or they do not; they cannot be carriers.

Given the intricate orchestration of gene expression, cell migration, and cell differentiation during prenatal development, it is amazing that the vast majority of newborns are healthy and free of major birth defects.

Wilkins shared Franklin’s X-ray crystallography data with Watson and Crick without her permission.

Rosalind Franklin died of ovarian cancer in 1958 at the age of 37. As a result, she was not awarded the Nobel Prize for her contribution in the discovery of the DNA structure because it is not given posthumously.

The two strands of DNA are anti-parallel in nature; that is, one strand will have the 3' carbon of the sugar in the “upward” position, whereas the other strand will have the 3' carbon in the “downward” position.

The bonding causes the two strands to spiral around each other in a shape called a double helix.

The semi-conservative method suggests that each of the two “old” parental DNA strands acts as templates.

As the DNA double-helix opens, Y-shaped structures called replication forks are formed.

This essentially means that telomere shortening is associated with aging.

Individuals with flaws in their nucleotide excision repair genes show extreme sensitivity to sunlight and often develop skin cancers early in life.

Somatic cell mutations can be extremely dangerous to the individual organism, but are not passed on to their offspring, therefore they are not heritable.

The ends of eukaryotic chromosomes pose a problem, as DNA polymerase is unable to extend them without a primer.

The central dogma states that genes specify the sequences of RNAs, which in turn specify the sequences of proteins.

The specific sequence of a promoter is very important because it determines whether the corresponding gene is transcribed all of the time, some of the time, or hardly at all.

For example, eukaryotic mRNAs last for several hours, whereas the typical prokaryotic mRNA lasts no more than five seconds.

Humans, for example, have just over 20,000 genes, yet the human body produces over 80,000 different proteins.

Therefore, tRNAs are the molecules that actually “translate” the language of RNA into the language of proteins.

This is powerful evidence that all existing life on earth shares a common origin.

The ribosome shifts one codon at a time, catalyzing each process that occurs in the three sites.

The genetic code is degenerate because 64 triplet codons in mRNA specify only 20 amino acids and three stop codons.

Cells would have to be enormous if every gene were expressed in every cell all the time.

If a gene is to remain turned off, or silenced, the histone proteins and DNA have different modifications that signal a closed chromosomal configuration.

The Ukrainian-born American geneticist Theodosius Dobzhansky famously wrote that “nothing makes sense in biology except in the light of evolution.”

One might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends.

The capacity for reproduction in all organisms exceeds the availability of resources to support their numbers.

The Grants observed evolutionary events in which the beak shape of the medium ground finch changed from one generation to the next.

The wings of bats and insects, however, evolved from very different structures and as a result are quite different.

Highly beneficial alleles may, over a very few generations, become “fixed.” If an allele becomes “fixed,” it means that every individual of the population will carry that allele.

Hardy, who was not even a biologist, pointed out that if there are no factors that affect an allele frequency, those frequencies will remain constant from one generation to the next.

The Hardy-Weinberg equilibrium states that a population’s allele and genotype frequencies are inherently stable unless some kind of evolutionary force is acting on the population.

There is a delicate balance between having enough enhancements to attract a mate but not so much that it results in being predated.

In order for a disaster to be categorized as a bottleneck effect and genetic drift, it must be one that kills for reasons unrelated to the organism’s traits, such as a hurricane or lava flow.

The original Dutch colonists that settled in South Africa were only a small sample of the total Dutch population; however, just by chance, those that arrived had a higher-than-normal proportion of these rare faulty alleles.

The resulting fossil record tells the story of the past and shows the evolution of form over millions of years.

For example, some snakes have pelvic bones despite having no legs.

Great ape embryos, including humans, have a tail structure during their development that is lost by the time of birth.

These duplications allow one copy to be modified by mutation, selection, and drift, while the second copy continues to produce a functional protein.

When critics of evolution say evolution is “just a theory,” they are implying that there is little evidence supporting it and that it is still in the process of being rigorously tested.

Evolution has no goal of making faster, bigger, more complex, or even smarter species.

Evolution is sometimes mistakenly interpreted to mean that individuals evolve, when in fact only populations can evolve as their gene frequencies change over time.

Central dogma: The flow of genetic information in cells from DNA to mRNA to protein

Chromosomal Theory of Inheritance: a theory proposing that chromosomes are the genes' vehicles and that their behavior during meiosis is the physical basis of the inheritance patterns that Mendel observed

crossing over: (also, recombination) the exchange of genetic material between homologous chromosomes resulting in chromosomes that incorporate genes from both parents

entropy: the measure of randomness or disorder within a system

founder effect: a magnification of genetic drift in a small population that migrates away from a large parent population carrying with it an unrepresentative set of alleles

hydrophilic: describes a substance that dissolves in water; water -loving

law of dominance: in a heterozygote, one trait will conceal the presence of another trait for the same characteristic

natural selection: the greater relative survival and reproduction of individuals in a population that have favorable heritable traits, leading to evolutionary change

Water also moves from an area of low solutes to an area of high solutes until equilibrium is met.

pseudoscience: claims or beliefs that are portrayed as scientific fact but cannot be evaluated using the scientific method

the region of the endoplasmic reticulum that is studded with ribosomes and engages in protein modification

vestigial structure: a physical structure present in an organism but that has no apparent function and appears to be from a functional structure in a distant ancestor

zygote: a fertilized egg produced when a sperm and egg fuse

Figure1.17b National Cancer Institute Wikimedia Commons Creative Commons CC0 1.0 Universal Public Domain

Figure 2.15 Miansari66 Wikimedia Commons Creative Commons CC0 1.0 Universal Public Domain https://commons.wikimedia.org/wiki/File:Salt_,_Lake_salt.JPG

Figure 3.1 “photo”: modification of work by NASA; credit “illustration”: modification of work by Courtney Harrington.

Figure 3.15 Yash Deshpande Wikimedia Commons Creative Commons Attribution 4.0 International https://commons.wikimedia.org/wiki/File:Tarota_(Casia_toral)_plant_with_raindrops.jpg

Figure 4.11 Sukulya Wikimedia Commons Creative Commons Attribution 4.0 International https://commons.wikimedia.org/wiki/File:Microbiologist.jpg

Micrographs provided by the Regents of University of Michigan Medical School © 2012 Anatomy and Physiology OpenStax Creative Commons Attribution License v4.0

Modification of work by Mariana Ruiz Villareal Concepts of Biology OpenStax Creative Commons Attribution License v4.0

Figure 5.19 Modified by Elizabeth O’Grady original work by NASA (cyano) Wikimedia Commons Creative Commons CC0 1.0 Universal Public Domain

Figure 5.26 "dam": modification of work by "Pascal"/Flickr; "waterfall": modification of work by Frank Gualtieri Concepts of Biology OpenStax Creative Commons Attribution License v4.0

Modification of work by Mariana Ruiz Villareal Biology 2E OpenStax Creative Commons Attribution License v4.0

Figure 6.27 Modification of work by NASA/Jeff Schmaltz, MODIS Land Rapid Response Team at NASA GSFC, Visible Earth Catalog of NASA images.

Figure 7.13 Modification of work by NASA Biology 2E OpenStax Creative Commons Attribution License v4.0

Figure 8.11 "diagrams": modification of work by Mariana Ruiz Villarreal; credit "mitosis micrographs": modification of work by Roy van Heesbeen; credit "cytokinesis micrograph": modification of work by the Wadsworth Center, NY State Department of Health.

Modified by Elizabeth O’Grady original work by Guy Leonard Wikimedia Commons Creative Commons Attribution-Share Alike 3.0 Unported.

Figure 10.2 MRC Laboratory of Molecular Biology Wikimedia Commons Creative Commons Attribution - Share Alike 4.0 International https://commons.wikimedia.org/wiki/File:Rosalind_Franklin .jpg

Figure 10.4 Zephyris Wikimedia Commons Creative Commons Attribution - Share Alike 3.0 https://en.wikipedia.org/wiki/Obsolete_models_of_DNA_structure#/media/File:A -DNA,_B-DNA_and_Z -DNA.png

Figure 11.1 “wolf”: modification of work by Gary Kramer, USFWS; credit “coral”: modification of work by William Harrigan, NOAA.

Figure 11.16 Romanes copy of Ernst Heaeckel Wikimedia Commons Creative Commons CC0 1.0 Universal Public Domain

Modified by Elizabeth O'Grady Original work by Clark et al.

Viruses exhibit some of the characteristics of life but lack others.

In this infrared image the polar bear‘s body heat hardly registers; only the uninsulated eyes and mouth show temperatures significantly warmer than the environment.

Viruses are often not considered living because they are not made of cells, nor are they capable of reproducing on their own.

There are, however, areas of knowledge and human experience that the methods of science cannot be applied to.

Scientific reasoning is more complex than the scientific method alone suggests.

Most of an atom's volume, greater than 99 percent, is empty space.

The PET camera shows the medical team, which of the patient’s tissues are taking up the most glucose.

This is known as the octet rule, which states, except for the innermost shell, that atoms are more stable energetically when they have eight electrons in their valence shell, the outermost electron shell.

They do not circle the nucleus like the earth orbits the sun, but we find them in electron orbitals.

If this did not happen, plants and animals living in the water would freeze into a block of ice and would not be able to move around freely, making life in cold temperatures difficult, if not impossible.

Before it overflows, the water forms a dome-like shape above the rim of the glass.

As the temperature rises, the hydrogen bonds between water continually break and reform.

When they analyzed the contents of the flask a week later, they found amino acids.

The three-dimensional shape or conformation of a molecule is critical to determining its function.

Structural differences lead to differences in chemical properties which will cause the isomers to function differently.

This type of reaction is called a dehydration synthesis, a condensation reaction, which means “to put together while losing water".

The phospholipid bilayer consists of two adjacent layers of phospholipids arranged tail to tail.

Each cell in a living system may contain thousands of different proteins, each with a unique function.

The term protein is not usually used until the polypeptide chain(s) have folded into their distinct three-dimensional shape and can carry out their unique function(s).

The loss of one amino acid changes the primary structure of the protein.

In eukaryotic cells, DNA never leaves the nucleus, so a specific type of RNA, messenger RNA (mRNA), helps to relay information from the nucleus to other parts of the cell.

Close your eyes and picture a brick wall. What is the basic building block of that wall? Most would answer, it is a single brick.

Viruses are inactive outside of a host organism. Therefore, they do not grow and develop, nor can they reproduce on their own.

When there is insufficient surface area to support a cell’s increasing volume, a cell will either divide or die.

Cilia lining the cells of the respiratory tract move particulate matter toward the throat where it is then trapped in mucus.

The word “organelle” means “little organ” and organelles have specialized cellular functions just as your body's organs have specialized functions.

The endomembrane system of eukaryotic cells includes the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, vesicles, as well as the plasma membrane.

Because the name 'protist' serves as a catchall term for eukaryotic organisms that are not animal, plant, or fungi, it is not surprising that very few characteristics are common to all protists.

Scientists hypothesize that host cells and bacteria formed a mutually beneficial endosymbiotic relationship when the host cells ingested aerobic bacteria and cyanobacteria but did not destroy them.

As the central vacuole shrinks, it leaves the cell wall unsupported and appears to wilt.

The lipid tails are nonpolar and are hydrophobic. The hydrophobic lipid tails meet in the inner region of the membrane and exclude the watery intracellular and extracellular fluid.

The glycocalyces found in a person’s body are a result of that person’s genetic makeup. They help identify cells as belonging to the same individual.

Plasma membranes enclose the borders of cells, but rather than being a static bag, they are dynamic and constantly in flux.

Each separate substance in an environment has its own concentration gradient, independent of the concentration gradients of other materials in that same environment.

In this situation, water will move down its concentration gradient and enter the cell. This may cause an animal cell to burst or lyse.

Water moved into the cells from an area of low solutes in the blood to an area of high solutes in the cytoplasm resulting in the cells bursting.

People with this condition have life-threatening levels of cholesterol because their cells cannot remove the lipid from their blood.

Adenosine triphosphate (ATP) is the primary energy currency of all living cells.

To better understand entropy, think of a student’s bedroom. If no energy or work were put into it, the room would quickly become messy.

Phosphate groups are negatively charged and therefore repel one another when they are arranged in series, as is the case with ADP and ATP.

ATP synthase, an integral protein and an enzyme, acts as a tiny generator which allows hydrogen ions to easily diffuse across the inner membrane.

Lactic acid build-up causes muscle stiffness and fatigue.

Triglycerides store about twice as much energy as carbohydrates and are made of glycerol and three fatty acids.

These organisms are called photoautotrophs, literally “self-feeders using light,” because they use light to generate their own food.

Photosystem II was named for the order of its discovery rather than for the order in which it functions.

The process of photorespiration wastes the energy carriers that were produced in the light-dependent reaction and does not lead to the production of glucose.

If the DNA from all 46 chromosomes in a human cell were laid out end-to-end, it would measure approximately two meters!

Even minor mistakes allow subsequent mistakes to occur more readily.

Humans have 23 chromosome pairs, which results in over eight million (223) possibilities.

Instead, Mendel’s results demonstrated that the violet flower trait was retained and the white flower trait had disappeared entirely in the F1 generation.

Each chromosome contains hundreds or thousands of genes, organized linearly on chromosomes like beads on a string.

Wilkins shared Franklin’s X-ray crystallography data with Watson and Crick without her permission.

This essentially means that telomere shortening is associated with aging.

Humans, for example, have just over 20,000 genes, yet the human body produces over 80,000 different proteins.

This is powerful evidence that all existing life on earth shares a common origin.

The Grants observed evolutionary events in which the beak shape of the medium ground finch changed from one generation to the next.

Evolution has no goal of making faster, bigger, more complex, or even smarter species.