AP BIOLOGYLIVING ENVIRONMENT REVIEW

Living Environment Review Topic 1: Introduction

A. Life

Life is defined by the properties and processes characteristic of living things. Individual living things are called organisms. Organisms are composed of basic living units known as cells. In multicellular (as opposed to unicellular) organisms, cells come together to form tissues, which come together to form organs. Organs working together are referred to as organ systems (for example, the digestive system or the circulatory system)

B. Life Functions

Life functions are those processes carried out by living things that are necessary to maintain life and maintain homeostasis.

Nutrition is the process by which organisms obtain and process food (organic nutrients) to supply the energy and building materials necessary for growth and maintenance. Autotrophs can synthesize their own food, while heterotrophs depend on the consumption of other organisms for food.

Transport is the process by which substances are absorbed, circulated, distributed and released by living things. Passive transport is the movement of solutes (as in diffusion) or water (as in osmosis) across a membrane from an area of high concentration to an area of low concentration, without requiring an expenditure of energy by the cell. Active transport is the movement of substances from an area of low concentration to an area of high concentration, requiring an expenditure of energy by the cell.

Cellular Respiration is the processes by which energy stored in food is converted into a form that can be directly used by cells (ATP)

Excretion is the process by which the waste products of metabolism are removed from living things.

Water tends to move from high water concentration/low solute concentration to low water concentration/high solute concentration

Synthesis is the process by which small, simple molecules are chemically combined to produce larger, more complex molecules.

Regulation is the processes by which living things coordinate and control their life functions, usually by responding to stimuli (changes within and around them).

Growth involves the increase in cell number and/or cell size.

Reproduction is the process by which new cells arise from preexisting cells. When cells in multicellular organisms divide to produce new cells, the result may be the growth of the organism or the repair of damaged tissues. Cell division may also result in the production of new organisms. Genetic information is transmitted from parent cell to daughter cells during this process.

C. Metabolism

When all life functions are considered together, the chemical and biological activities are known collectively as metabolism. Metabolic processes underlay all the life functions described above. Metabolic processes are essentially chemical in nature. As such, they use chemical reactants, result in chemical products and wastes, and are affected by various physical and chemical conditions that exist in the cell.

D. Homeostasis

Homeostasis refers to the normal condition of balance and dynamic equilibrium that exists within cells and organisms. Changes in the cell’s physical or chemical environment can upset this equilibrium or stability leading to disease or death.

E. Structure and Function

In biology, function is determined by structure. This is true whether it’s about organs, tissues, cells or molecules. A good example are receptor proteins found on the surface of a cell. Receptor proteins receive specific chemical messages (hormones, neurotransmitters) from other cells. Those messages are received by the receptors because they have corresponding structures/shapes. We will see this relationship between structure and function again and again (see: enzymes).

Living Environment Review Topic 2: Biochemistry

A. Biochemistry

The cell is composed of the same elements found in the abiotic (non-living) environment. The most abundant elements found in living things are carbon (C), hydrogen (H), oxygen (O), and nitrogen (N).

Organisms are made up of and depend on inorganic and organic compounds. A compound is a substance made up of two or more different elements bonded together. If the bond is the result of the transfer of electrons from one atom to another, the atoms are converted into charged ions and the resulting bond and compound are called ionic. If the bond is the result of electrons being shared between atoms, the resulting bonds are called covalent and the resulting compound a molecule.

B. Organic Compounds

Organic compounds contain both carbon and hydrogen. They are usually synthesized from subunits (building block) through dehydration synthesis or polymerization, and can be digested (broken down) through hydrolysis.

Carbohydrates consist of the elements C, H, and O, and usually include a 2:1 ratio of hydrogen to oxygen atoms. The basic unit of the carbohydrate is the simple sugar or monosaccharide, the names of which typically end in “-ose”. Glucose (C6H12O6) is an example. Monosaccharides can be chemically combined to make more complex carbohydrates like disaccharides or polysaccharides. Carbohydrates are used primarily for fuel (simple sugars), energy storage (polysaccharides like starch in plants and glycogen in humans), and structural support (cellulose in plant cell walls, chitin in animal exoskeletons).

Lipids consist of the elements C, H, and O, but with higher hydrogen to oxygen ratios. Lipids include fats, steroids, waxes and oils. Lipids are used primarily for long-term energy storage, insulation, cushioning, cellular communication and as major components of cellular membranes.

Proteins consist of the elements C, H, O, N and sometimes sulfur (S). A protein is made of subunits called amino acids. Each type of amino acid has a unique side chain that determines that amino acid’s chemical properties. Amino acids that have been chemically combined form large chains called polypeptides. Most

proteins are large and complex and are made up of one or more polypeptide chains. There is an extremely large number of different kinds of proteins, distinguished from one another by differences in the number, kind or sequence of amino acids present. Proteins have a wide variety of functions (cell communication, structural support, transport) but their most important function is to act as enzymes for biochemical reactions.

Nucleic Acids (DNA, RNA) consist of the elements C, H, O, N, and P. A nucleic acid is composed of subunits called nucleotides. Nucleic acids are used primarily for storing and transmitting genetic information.

C. Inorganic Compounds

Water makes up between 60% and 98% of a living organism and is essential for transport and the chemical activities of the cell.

Salts are ionic compounds consisting of metallic and nonmetallic elements. Salts help living organisms maintain osmotic (water) balance and supply ions necessary for cellular chemical reactions

Inorganic acids and bases are important for maintaining proper hydrogen ion concentration (pH/acidity/alkalinity).

D. Enzymes

Enzymes are proteins that catalyze (speed up) chemical reactions by lowering the amount of energy needed to start the reaction (the reaction’s activation energy). Enzymes are sometimes referred to as organic catalysts.

Each chemical reaction that occurs in living systems requires a specific enzyme. The reactants of an enzyme-catalyzed reaction are called substrates. Substrates bind to an area of the enzyme called the active site during the reaction. The specificity of an enzyme’s function depends on the complementary shapes of the substrate and active site. Enzymes are not used up by the reactions they catalyze and can go on to catalyze the same reaction again and again.

Enzymes are usually named for their substrates, and the name typically ends in “-ase”.

Enzymes function best under specific environmental conditions. Changes in temperature or pH can alter the shape of the active site (denaturation) and slow the rate of reaction. The rate of reaction will also be influenced by the relative concentration of enzyme and substrate.

Living Environment Review Topic 3: Cells

A. Cell Theory

Cells are the basic unit of structure and function of living things. All living things are made of cells. Viruses are considered non-living because they are not composed of cells despite sharing many other characteristics with living organisms.

All cells come from preexisting cells through the process of cell division.

Cells are composed of subcellular structures known as organelles that carry out life functions and help maintain homeostasis. Prokaryotic cells (bacteria, archaea) are small, primitive and lack membrane-enclosed organelles like the nucleus. Eukaryotic cells (animals, plants, fungi, protists) are larger and have many membrane-enclosed organelles like the nucleus.

B. Organelles Found in All Cells

The Cell/Plasma Membrane is the outer membrane of the cell and regulates the transport of materials into and out of the cell. Like other membranes found in the cell, it is composed of a selectively permeable phospholipid bilayer and embedded proteins. The proteins might act as enzymes, receptors involved in receiving chemical signals from other cells, or transport proteins necessary to move large, polar or charged particles across the membrane (carrier proteins and channels facilitate diffusion; pumps carry out active transport).

Cytoplasm is the jelly-like environment surrounded by the cell membrane in which the organelles of the cell are suspended and within which diffusion and biochemical processes occur.

Ribosomes are the sites of protein synthesis, where amino acids are put together according to instructions originally encoded for by the DNA of the cell (the cell’s genetic information).

C. Organelles Found in Eukaryotic Cells

The Nucleus, surrounded by the nuclear membrane, is where DNA is stored in the form of chromosomes. As the carrier of the cell’s genetic information, it is said that the nucleus directs the cell’s activities. The nucleolus, within the nucleus, is involved in synthesizing ribosomes.

The Endoplasmic Reticulum (ER) is a series of interconnected channels associated with storage, synthesis, and transport of substances (mainly proteins) within the cell. There is both rough and smooth endoplasmic reticulum. Rough endoplasmic reticulum is studded with ribosomes.

The Mitochondria are the sites of cellular respiration.

The Golgi Apparatus is a series of small, membrane-bound sacs that synthesize, package, and secrete cellular products.

Vacuoles/Vesicles are small membrane-bound compartments used for storage and transport within cells. Plant cells have very large central vacuoles.

Chloroplasts (plants) are the sites of photosynthesis.

Cell walls (plants, fungi, bacteria) are nonliving structures that surround, protect and support cells. In plants, cell walls are composed of cellulose.

Lysosomes (more common in animals) contain digestive enzymes and help with digestion of large molecules within the cell.

Centrioles (animals) function during cell division.

Living Environment Review Topic 4: Energy

A. Photosynthesis

Photosynthesis is the process by which light energy is converted into the chemical energy of organic molecules like glucose. This process can be summarized by the formula below.

Carbon Dioxide (CO2) + Water (H2O) + Light Energy Glucose (C6H12O6) + Water (H2O) + Oxygen (O2)

Photosynthesis occurs in two stages. The light-dependent reaction uses the green pigment chlorophyll to capture light energy in the thylakoid membranes of the chloroplast. The light-independent reaction (or Calvin cycle) produces sugar in the stroma of the chloroplast by fixing carbon dioxide from the atmosphere.

Both stages proceed only when light is present. Photosynthesis works best in red and blue light and worst in green.

B. Cellular Respiration

Cellular respiration is the process by which the chemical energy of organic molecules, such as glucose, is released and converted to a usable form – stored in molecules of ATP.

If the energy transfer involves the use of oxygen, the process is aerobic respiration and occurs in the mitochondria. This process can be summarized by the formula below.

Glucose (C6H12O6) + Water (H2O) + Oxygen (O2) Carbon Dioxide (CO2) + Water (H2O) + ATP Energy

If oxygen is not used, the process is the less efficient anaerobic respiration.

C. ATP

ATP

(adenosine triphosphate) stores the cell’s usable energy. When ATP is hydrolyzed, usable energy is released and ADP (adenosine diphosphate) is produced. Cellular respiration converts ADP back into ATP, storing usable energy and “recharging” the ATP molecule.

Living Environment Review Topic 5: Genetics

A. DNA

Nucleic acids are composed of subunits called nucleotides. A DNA (deoxyribonucleic acid) nucleotide has three parts: a phosphate group, a deoxyribose sugar, and a nitrogenous base of which there are four types: adenine (A), thymine (T), cytosine (C), or guanine (G).

The DNA molecule consists of two complementary chains of nucleotides. The DNA molecule has a “twisted ladder” structure referred

to as a double helix. The sides of the ladder are composed of alternating phosphate and deoxyribose molecules (sugar-phosphate backbone). Each rung of the ladder is composed of a complementary base pair held together by weak hydrogen bonds. A is complementary to T and C is complementary to G. The sequence of bases in a DNA molecule is where genetic information is encoded. This genetic code determines the sequence of amino acids in the cell’s proteins.

B. DNA Replication

The exact duplication of the genetic material is accomplished through DNA replication. The process is summarized to the right. Copying errors result in changes to the DNA sequence known as mutations.

C. Genes

Genes are segments of DNA that control the traits of an organism by directing protein synthesis. Genes are found at specific locations (loci) on chromosomes in the cells of different organisms. Sexually reproducing organisms have chromosomes that come in pairs (homologous pairs) and therefore carry two copies of every gene (one from the mother, one from the father). Sexually reproducing individuals pass one copy of every gene on to their offspring.

Different versions of genes called alleles, with slightly different base sequences, code for different phenotypes or observable characteristics (for example, different alleles for the eye color gene exist that result in different eye colors). An organism’s genotype refers to its combination of alleles. A homozygous genotype consists of two copies of the same allele. A heterozygous genotype consists of two different alleles. Some alleles are dominant over other recessive alleles and will always determine the phenotype when present in a heterozygous genotype. Sometimes the relationship between different alleles in a heterozygous genotype is more complicated.

D. RNA

The genetic control of cellular activities involves RNA (ribonucleic acid) as well as DNA. There are three major different differences between the structure of DNA and RNA molecules. In RNA, ribose is substituted for deoxyribose, Uracil (U) is substituted for thymine (T), and there is a single chain of nucleotides, not a double.

There are three types of RNA. Messenger RNA (mRNA) carries the DNA message from the DNA in the nucleus to the ribosomes in the cytoplasm. Transfer RNA (tRNA) carries amino acids in the cytoplasm to the ribosomes. Ribosomal RNA (rRNA) makes up the ribosomes of the cell.

E. Protein Synthesis

DNA serves as a template for the synthesis of complementary mRNA molecules. mRNA carries the specific code determined by the base sequence of a gene on the original DNA molecule to ribosomes in the cytoplasm. tRNA molecules pick up and transfer specific amino acid molecules to appropriate three-letter mRNA sequences known as codons at the ribosome. Specific polypeptides are thusly formed by joining amino acids together in an order originally specified by the sequences of bases in a gene on a DNA molecule.

F. Gene Expression

Not all genes are continuously expressed (“turned on”). For example, all of an organism’s body cells contain a complete, identical set of genes. But different types of cells differ in which of those genes are expressed and when. Whether or not genes are expressed can be influenced by environmental factors like light or temperature.

Living Environment Review Topic 6: Evolution

The types of species found on earth today evolved from earlier, distinctly different forms. Most species that have existed are extinct. All living things share common ancestry and so all living things are related. Some types of living things are more closely related to each other than others, having evolved from a common ancestor more recently. These evolutionary relationships can be discovered by comparing DNA sequences and visualized using a phylogenetic tree.

Evolution refers to the change in the genetic composition of a population from one generation to the next. For example, an allele becoming more or less common over time. Individuals don’t evolve, populations do.

Natural selection causes populations to evolve so that they are better adapted (better suited) to their environments. In any population, variation or genetic differences exist. Competition over limited resources means that some individuals will survive and reproduce at higher rates than others. Genetic variations that allow for greater reproductive success (adaptations that increase fitness) will be more likely to be passed on, and so become more common in a population over time. This is evolution by natural selection.

Living Environment Review Topic 7: Ecology

Ecology is the study of the interdependency between living things, other living things, and the non-living environment.

Interacting individuals belonging to the same species are referred to as a population. Interacting populations are referred to as a community. A community and the non-living factors with which it interacts is referred to as an ecosystem.

In any ecosystem, producers (autotrophs) capture energy from the environment. Consumers (heterotrophs) pass the energy along consumer to consumer. Energy transfer is inefficient and energy is lost as heat along the way. Because of this, energy must be continuously added to an ecosystem.

Decomposers help recycle matter by breaking down dead organisms and returning nutrients to the environment.

Biodiversity (a measure of the number of different types of living things in an ecosystem) is important for maintaining stability. Human activity disrupts biodiversity and destabilizes natural ecosystems.