Life Science: GED Test Prep

The Cell

Today we know that a cell is the building block of life. Every living organism is composed of one or more cells. All cells come from other cells. Cells are alive. If blood cells, for example, are removed from the body, given the right conditions, they can continue to live independently of the body. They are made up of organized parts, perform chemical reactions, obtain energy from their surroundings, respond to their environments, change over time, reproduce, and share an evolutionary history.

All cells contain a membrane, cytoplasm, and genetic material. More complex cells also contain cell organelles. Here is a description of cell components and the functions they serve. Also, refer to the figures.

• The cell wall is made of cellulose, which surrounds, protects, and supports plant cells. Animal cells do not have a cell wall.
• The plasma membrane is the outer membrane of the cell. It carefully regulates the transport of materials in and out of the cell and defines the cell's boundaries. Membranes have selective permeability—meaning that they allow the passage of certain molecules, but not others. A membrane is like a border crossing. Molecules need the molecular equivalent of a valid passport and a visa to get through.
• The nucleus is a spherical structure, often found near the center of a cell. It is surrounded by a nuclear membrane and it contains genetic information inscribed along one or more molecules of DNA. The DNA acts as a library of information and a set of instructions for making new cells and cell components. In order to reproduce, every cell must be able to copy its genes to future generations. This is done by exact duplication of the DNA.
• Cytoplasm is a fluid found within the cell membrane, but outside of the nucleus.
• Ribosomes are the sites of protein synthesis. They are essential in cell maintenance and cell reproduction.
• Mitochondria are the powerhouses of the cell. They are the site of cellular respiration (breakdown of chemical bonds to obtain energy) and production of ATP (a molecule that provides energy for many essential processes in all organisms). Cells that use a lot of energy, such as the cells of a human heart, have a large number of mitochondria. Mitochondria are unusual because unlike other cell organelles, they contain their own DNA and make some of their own proteins.
• The endoplastic reticulum is a series of interconnecting membranes associated with the storage, synthesis, and transport of proteins and other materials within the cell.
• The Golgi complex is a series of small sacs that synthesizes, packages, and secretes cellular products to the plasma membrane. Its function is directing the transport of material within the cell and exporting material out of the cell.
• Lysosomes contain enzymes that help with intracellular digestion. Lysosomes have a large presence in cells that actively engage in phagocytosis—the process by which cells consume large particles of food. White blood cells that often engulf and digest bacteria and cellular debris are abundant in lysosomes.
• Vacuoles are found mainly in plants. They participate in digestion and the maintenance of water balance in the cell.
• Centrioles are cylindrical structures found in the cytoplasm of animal cells. They participate in cell division.
• Chloroplasts exist in the cells of plant leaves and in algae. They contain the green pigment chlorophyll and are the site of photosynthesis—the process of using sunlight to make high energy sugar molecules. Ultimately, the food supply of most organisms depends on photosynthesis carried out by plants in the chloroplasts.
• The nucleolus is located inside the nucleus. It is involved in the synthesis of ribosomes, which manufacture proteins.

In a multicellular organism, individual cells specialize in different tasks. For example, red blood cells carry oxygen, white blood cells fight pathogens, and cells in plant leaves collect the energy from sunlight. This cellular organization enables an organism to lose and replace individual cells, and outlive the cells that it is composed of. For example, you can lose dead skin cells and give blood and still go on living. This differentiation or division of labor in multicellular organisms is accomplished by expression of different genes.

Molecular Basis of Heredity

What an organism looks like and how it functions is largely determined by its genetic material. The basic principles of heredity were developed by Gregor Mendel,who experimented with pea plants in the nineteenth century. He mathematically analyzed the inherited traits (such as color and size) of a large number of plants over many generations. The units of heredity are genes carried on chromosomes. Genetics can explain why children look like their parents, and why they are, at the same time, not identical to the parents.

Phenotype and Genotype

The collection of physical and behavioral characteristics of an organism is called a phenotype. For example, your eye color, foot size, and ear shape are components of your phenotype. The genetic makeup of a cell or organism is called the genotype. The genotype is like a cookbook for protein synthesis and use. Phenotype (what an organism looks like or how it acts) is determined by the genotype (its genes) and its environment. By environment we don't mean the Earth, but the environment surrounding the cell. For example, hormones in the mother's body can influence the gene expression.

Reproduction

Asexual reproduction on the cellular level is called mitosis. It requires only one parent cell, which, after exactly multiplying its genetic material, splits in two. The resulting cells are genetically identical to each other and are clones of the original cell before it split.

Sexual reproduction requires two parents. Most cells in an organism that reproduces sexually have two copies of each chromosome, called homologous pairs—one from each parent. These cells reproduce through mitosis. Gamete cells (sperm and egg cells) are exceptions. They carry only one copy of each chromosome, so that there are only half as many chromosomes as in the other cells. For example, human cells normally contain 46 chromosomes, but human sperm and egg cells have 23 chromosomes.At fertilization, male and female gametes (sperm and egg) come together to form a zygote, and the number of chromosomes is restored by this union. The genetic information of a zygote is a mixture of genetic information from both parents. Gamete cells are manufactured through a process called meiosis whereby a cell multiplies its genetic material once, but divides twice, producing four new cells, each of which contains half the number of chromosomes that were present in the original cell before division. In humans, gametes are produced in testes and ovaries. Meiosis causes genetic diversity within a species by generating combinations of genes that are different from those present in the parents.

Alleles

Alleles are alternative versions of the same gene. An organism with two copies of the same allele is homozygous, and one with two different alleles is heterozygous. For example, a human with one gene for blue eyes and one gene for brown eyes is heterozygous, while a human with two genes for blue eyes or two genes for brown eyes is homozygous. Which of the two genes is expressed is determined by the dominance of the gene.

An allele is dominant if it alone determines the phenotype of a heterozygote. In other words, if a plant has a gene for making yellow flowers and a gene for making red flowers, the color of the flower will be determined by the dominant gene. So if the gene for red flowers is dominant, a plant that has both the gene for red and the gene for yellow will look red. The gene for yellow flowers in this case is called recessive, as it doesn't contribute to the phenotype (appearance) of a heterozygote (a plant containing two different alleles). The only way this plant would make yellow flowers is if it had two recessive genes—two genes both coding for yellow flowers.

For some genes, dominance is only partial and two different alleles can be expressed. In the case of partial dominance, a plant that has a gene that codes for red flowers and a gene that codes for white flowers would produce pink flowers.

A Punnett square can be used to represent the possible phenotypes that offspring of parents with known genotypes could have. Take the example with the yellow and red flower. Let's label the gene for the dominant red gene as R and the gene for yellow flowers as r. Cross a plant with yellow flowers (genotype must be rr) with a plant with red flowers and genotype Rr. What possible genotypes and phenotypes can the offspring have? In a Punnett square, the genes of one parent are listed on one side of the square and the genes of the other parent on the other side of the square. They are then combined in the offspring as illustrated here:

The possible genotypes of the offspring are listed inside the square. Their genotype will be either Rr or rr, causing them to be either red or yellow, respectively.

Sex Determination

In many organisms, one of the sexes can have a pair of unmatched chromosomes. In humans, the male has an X chromosome and a much smaller Y chromosome, while the female has two X chromosomes. The combination XX (female) or XY (male) determines the sex of humans. In birds, the males have a matched pair of sex chromosomes (WW), while females have an unmatched pair (WZ). In humans, the sex chromosome supplied by the male determines the sex of the offspring. In birds, the sex chromosome supplied by the female determines the sex.

Plants, as well as many animals, lack sex chromosomes. The sex in these organisms is determined by other factors, such as plant hormones or temperature.

Identical twins result when a fertilized egg splits in two. Identical twins have identical chromosomes and can be either two girls or two boys. Two children of different sex born at the same time can't possibly be identical twins. Such twins are fraternal. Fraternal twins can also be of the same sex. They are genetically not any more alike than siblings born at different times. Fraternal twins result when two different eggs are fertilized by two different sperm cells.

When meiosis goes wrong, the usual number of chromosomes can be altered. An example of this is Down syndrome, a genetic disease caused by an extra chromosome.

Changes in DNA (mutations) occur randomly and spontaneously at low rates. Mutations occur more frequently when DNA is exposed to mutagens, including ultraviolet light, X-rays, and certain chemicals. Most mutations are either harmful to or don't affect the organism. In rare cases, however, a mutation can be beneficial to an organism and can help it survive or reproduce. Ultimately, genetic diversity depends on mutations, as mutations are the only source of completely new genetic material. Only mutations in germ cells can create the variation that changes an organism's offspring.

Biological Evolution

Mutations cause change over time. The result of a series of such changes is evolution, or as Darwin put it, "descent with modification." The great diversity on our planet is the result of more than 3.5 billion years of evolution. The theory of evolution argues that all species on Earth originated from common ancestors.

Evidence for Evolution

Several factors have led scientists to accept the theory of evolution. The main factors are described here.

• Fossil record. One of the most convincing forms of evidence is the fossil record. Fossils are the remains of past life. Fossils are often located in sedimentary rocks, which form during compression of settling mud, debris, and sand. The order of layers of sedimentary rock is consistent with the proposed sequence in which life on Earth evolved. The simplest organisms are located at the bottom layer, while top layers contain increasingly complex and modern organisms, a pattern that suggests evolution. The process of carbon dating has been used to confirm how old the fossils are and that fossils found in the lower layers of sedimentary rock are indeed older than the ones found in the higher layers. This helps scientists to chart evolutionary history based on time. And new fossils are turning up all the time; for example, the fossil called Tiktaalik, which was found in 2006, is believed to mark the transition from fish to land animals.
• Biogeography. Another form of evidence comes from the fact that species tend to resemble neighboring species in different habitats more than they resemble species that are in similar habitats but far away.
• Comparative anatomy. Comparative anatomy provides us with another line of evidence. It refers to the fact that the limb bones of different species, for example, are similar. Species that closely resemble one another are considered to be more closely related than species that do not resemble one another. For example, a horse and a donkey are considered to be more closely related than are a horse and a frog. Biological classifications (kingdom, phylum, class, order, family, genus, and species) are based on how organisms are related. Organisms are classified into a hierarchy of groups and subgroups based on similarities that reflect their evolutionary relationships. The same underlying anatomical structures of groups of bones, nerves, muscles, and organs are found in all animals, even when the function of these underlying structures differ.
• Embryology. Embryology provides another form of evidence for evolution. Embryos go through the developmental stages of their ancestors to some degree. The early embryos of fish, amphibians, reptiles, birds, and mammals all have common features, such as tails.
• Comparative molecular biology. Comparative molecular biology confirms the lines of descent suggested by comparative anatomy and fossil record. The relatedness of two different species can be found by comparing their DNA.

Darwin also proposed that evolution occurs gradually, through mutations and natural selection. He argued that some genes or combinations of genes give an individual a survival or reproductive advantage, increasing the chance that these useful combinations of genes will make it to future generations. Whether a given trait is advantageous depends on the environment of the organism. We can witness the changes in populations of living organisms: antibiotic-resistant bacteria, super lice that are resistant to chemical treatments, and the increased frequency of dark-colored moths versus the lightly colored variety (Biston betularia) after the industrial revolution in Britain. These are all examples of evidence of natural selection.

Natural selection is only one of several mechanisms by which gene frequency in a population changes. Other factors include mating patterns and breeding between populations.

Interdependence of Organisms

The species in communities interact in many ways. They compete for space and resources, they can be related as predator and prey, or as host and parasite.

Plants and other photosynthetic organisms harness and convert solar energy and supply the rest of the food chain. Herbivores (plant eaters) obtain energy directly from plants. Carnivores are meat eaters and obtain energy by eating other animals. Omnivores eat both meat and plants. Decomposers feed on dead organisms. The flow of energy can then be represented as follows:

Sun → Photosynthetic organisms → Herbivores → Carnivores or Omnivores → Decomposers

The food chain is not the only example of the interdependence of organisms. Species often have to compete for food and space, so that the increase in population of one can cause the decrease in population of the other.

Organisms also may have a symbiotic relationship (live in close association), which could be classified as parasitism, mutualism, or commensalism. In a parasitic relationship, one organism benefits at the expense of the other. Commensalism is symbiosis in which one organism benefits and the other is neither harmed nor rewarded. In mutualism, both organisms benefit.

Under ideal conditions, with ample food and space and no predators, all living organisms have the capacity to reproduce infinitely. However, resources are limited, limiting the population of a species.

Humans probably come closest to being a species with seemingly infinite reproductive capacity. Our population keeps increasing. Our only danger seems to come from viruses and bacteria, which at this point we more or less have under control. When we need more food, we grow more, and when we need more space, we clear some by killing off other biomes. By doing this, humans modify ecosystems and destroy habitats through direct harvesting, pollution, atmospheric changes, and other factors. This attitude is threatening current global stability, and has the potential to cause irreparable damage.

Behavior of Organisms

Even the most primitive unicellular organisms can act to maintain homeostasis. More complex organisms have nervous systems. The simplest organism found to have learning capability is a worm, suggesting a more complex nervous system. The function of the nervous system is the collection and interpretation of sensory signals, messages from the center of the nervous system (brain in humans) to other parts of the body. The nervous system is made of nerve cells, or neurons, which conduct signals, in the form of electrical impulses. Nerve cells communicate by secreting excitatory or inhibitory molecules called neurotransmitters. Many legal and illegal drugs act on the brain by disrupting the secretion or absorption of neurotransmitters, or by initiating a response by activating the receptors that neurotransmitters would normally bind to. Behavior can also be affected by hormonal control. Hormones are produced in one part of the body and are transported by the circulatory system to another part of the body where they ultimately act.

Many animals have sense organs that enable them to detect light, sound, and specific chemicals. These organs provide the animals with information about the outside world. Animals engage in innate and learned social behavior. These behaviors include hunting or searching for food, nesting, migrating, playing, taking care of their young, fighting for mates, and fighting for territory.

Plants also respond to stimuli. They turn toward the Sun and let their roots run deeper when they need water.