C4
Plants that use PEP carboxylase to fix CO₂ into a four carbon compound, which is then transferred to special cells, called bundle sheath cells, where the CO₂ is fixed by RuBisCo. C4 plants avoid photorespiration by keeping CO₂ concentrations high in the vicinity of RuBisCo.

Calvin-Benson cycle
(dark reactions, carbon-fixation reactions)
The reactions by which RuBisCo fixes carbon dioxide to a five carbon molecule (RUBP); the resulting six carbon molecule is quickly split and reduced, ultimately producing carbohydrate.

CAM Plants
Plants that use PEP carboxylase to fix CO₂ into a four carbon acid during the night. This acid is stored in the vacuole until daytime, when it is moved out of the vacuole and decarboxylated. The carbon dioxide is then fixed by RuBisCo in the Calvin-Benson cycle. CAM plants avoid photorespiration by building up a store of CO₂ during the night, and then using this store during the day to produce carbohydrates and maintain a relatively high CO₂ concentration in the vicinity of RuBisCo.

carbon fixation reactions
See Calvin-Benson cycle

catabolism
Catabolism encompasses the processes of cell metabolism in which the cell breaks down complex molecules to produce energy and reducing power

cell
The cell is the basic unit of life. All known cells (except a few specialized cell types) have certain basic components in common:

• cytoplasm (the living substance which makes up most of the cell volume)

• DNA, the genetic information that acts as a blueprint for the other components.

• Proteins, the machinery of the cell.

• Membranes, which separate the cell form its environment, work as a filter, and act as a communication relay with the outside

character
A particular phenotypic feature, such as hair, or skin, or leaf.

chloroplast
Chloroplasts are organelles found in plants and eukaryotic algae which conduct photosynthesis. These are one of the forms a plastid may take, and are generally considered to have originated as endosymbiotic cyanobacteria. In green plants they are surrounded by two lipid bilayer membranes, which correspond to the host cell and ancestral bacterium, though algal chloroplasts show notable variations from this pattern. The fluid within the chloroplast is called the stroma, corresponding to the cytoplasm of the bacterium, and contains tiny circular DNA and ribosomes. The genome is considerably reduced compared to that of free-living cyanobacteria, but the parts that are still present show clear similarities.

The structure and biochemistry of chloroplasts are also similar to those of cyanobacteria. Carbon dioxide is converted into carbohydrates via the Calvin cycle, which takes place in the stroma. Thylakoids, tiny membrane-enclose sacs, are present, and in most forms are arranged into stacks called grana. The thylakoid membranes are the site of photosynthesis, converting the energy of light into ATP and oxidizing water to oxygen. The photosynthetic proteins in the membrane bind chlorophyll, which is present with various accessory pigments. These give chloroplasts their colour, which may be green, golden, brown, or red.

chromosome
A chromosome is, minimally, a very long, continuous piece of DNA, which contains many genes, regulatory elements and other intervening nucleotide sequences. In the chromosomes of eukaryotes, the uncondensed DNA exists in a quasi-ordered structure inside the nucleus, where it wraps around histones (structural proteins, Fig. 1), and where this composite material is called chromatin. During mitosis (nuclear division), the chromosomes are condensed and called metaphasic chromosomes. This is the only natural context in which DNA is visible with an optical microscope. Prokaryotes do not possess histones or nuclei. In its relaxed state, the DNA can be accessed for transcription, regulation, and replication.

Autosomes are chromosomes that carry genes not related to sex determination; sex chromosomes carry genes related to the determinance of sexual characteristics.

Citric acid cycle
In biochemistry, the Citric Acid Cycle, also called Krebs cycle, Citrate Cycle, Tricarboxylic Acid Cycle or TCA, is a series of chemical reactions of central importance in all living cells that utilize oxygen as part of cellular respiration. In these aerobic organisms, the citric acid cycle is a metabolic pathway that forms part of the break down of carbohydrates, fats and proteins into carbon dioxide and water in order to generate energy.

The Krebs cycle is named after Sir Hans Adolf Krebs (1900-1981), who was awarded the 1953 Nobel Prize in Medicine for its discovery.

The citric acid cycle takes place within the mitochondria in eukaryotes, and within the cytoplasm in prokaryotes.

The citric acid cycle forms part of carbohydrate catabolism, protein catabolism and fat catabolism. All these three processes produce acetyl-CoA, a two-carbon acetyl group bound to coenzyme A. Acetyl-CoA is the main input to the citric acid cycle.

cleavage
An early embryonic stage of development during which the zygote divides to produce two, four, and then eight cells.

clone
Genetically identical cells or organisms. Cells may produce clones by fission (prokaryotes) or mitosis (eukaryotes. Some multicellular organisms produce clones by asexual reproduction or by vegetative propagation.

codominance
A phenomenon whereby both alleles of a gene in a diploid organism are expressed and active in the phenotype (ex. blood types of humans).

codon
A codon is a sequence of three bases in RNA.  Each codon specifies one amino acid. For example, the RNA sequence UUUAAACCC specifies three codons (UUU-AAA-CCC), which each specify one amino acid. This RNA sequence, then, encodes a protein sequence three amino acids in length (as we will see, it encodes Phenylalanine-Lysine-Proline). There are sixty-four possible codons.

Nearly all living things use the same genetic code. The standard version is given in the following tables, which show what amino acid each of the 4³ = 64 possible codons specify (Table 1), and what codons specify each of the 20 amino acids involved in translation.  For instance, GAU codes for the amino acid Asp (asparagine), and Cys (cysteine) is coded for by the codons UGU and UGC.  The bases in the table below are adenine, cytosine, guanine and uracil, which are used in the mRNA; in the DNA, thymine takes the place of uracil.

coenzymes
Small carbon molecules that bind to and are required for the activity of certain enzymes.

cofactors
Molecules essential for the activity of some enzymes; these can be inorganic ions or organic molecules.

competitive inhibitors
Inhibition of an enzyme where the substrate and inhibitor compete for the active site(s).

complementary base-pairing
The pairing of complementary nitrogenous bases between the two strands of the DNA molecule, between DNA and RNA, or between different segments of an RNA molecule. The pairings are as follows: adenine (A) pairs with either thymine (T) or uracil (U), while guanine pairs with cytosine.

condensation reaction
Also see dehydration synthesis
Common reaction of biosynthetic pathways, such as those that build macromolecules. During this reaction, two monomers are covalently linked, with the concurrent release of a molecule of water. The water forms from the combination of the hydroxyl (OH) group of one monomer and a hydrogen from the other monomer.

conjugation
Need Definition...

covalent bond
Covalent bonding is a form of chemical bonding characterized by the sharing of one or more pairs of electrons by two atoms to produce a mutual attraction. The atoms tend to share electrons, so as to fill their outer electron shells. Commonly covalent bond implies the sharing of just a single pair of electrons. The sharing of two pairs is called a double bond and three pairs is called a triple bond.  Covalent bonding tends to be stronger than other types of bonding.

crossover
Crossover is the process by which two chromosomes paired up during prophase I of meiosis exchange a distal portion of their DNA. Crossover occurs when two chromosomes, normally two homologous instances of the same chromosome, break and connect to each other's ends. If they break at the same locus, this merely results in an exchange of genes. This is the normal way in which crossover occurs. If they break at different loci, the result is a duplication of genes on one chromosome and a deletion on the other. If they break on opposite sides of the centromere, this results in one chromosome being lost during cell division.

Any pair of homologous chromosomes may be expected to cross over three or four times during meiosis. This aids evolution by increasing independent assortment, and reducing the genetic linkage between genes on the same chromosome.

cytokinesis
Cytokinesis refers to the division of a eukaryotic cell. Cytokinesis generally follows the replication of the cell's chromosomes, usually mitotically, but sometimes meiotically. Except for some special cases, the amount of cytoplasm in each daughter cell is the same. In animal cells, the cell membrane forms a cleavage furrow and pinches apart like a balloon. In plant cells, a cell plate forms, which becomes the new cell wall separating the daughters. Various patterns occur in other groups.

cytoplasm
The cytoplasm refers to everything bound by the plasma membrane except the nucleus. Perhaps a more useful distinction would be to talk about the cytosol. The cytosol is a watery matrix, bound by the plasma membrane, which contains the cell's organelles and other components. Small molecules, dissolved ions, and soluble macromolecules can also be found in the cytoplasm.