The specific structure of a biological mechanism is closely related to its purpose. This specialization of parts facilitates the smooth interactions of life on three levels:

• Among molecules, cells, and other internal components of an organism

(a substrate's shape is specific to the active site of its enzyme)

• Between two organisms

(a hummingbird's long, thin beak is perfectly designed to get nectar from flowers)

• Between an organism and its external environment

(an elephant's long, muscular trunk facilitates drinking and food-getting)

Molecules and Cells

• Plasma Membrane

The plasma membrane is composed of phospholipids, proteins, and cholesterol. The phosphate component is hydrophilic, while the fatty acid tails of the lipid portion are hydrophobic. Because the membrane is a bilayer, the fatty acid tails face in towards each other and the phosphates face out. This construction allows for the insertion of protein carriers that transport molecules into and out of the cell.

Ion pumps, a specific type of protein carrier, are specifically shaped to carry the ions that the cell needs.

• DNA

DNA consists of two long strands of phosphates and deoxyriboses with nitrogenous bases attached to the sugars. Simply, DNA is composed of a sequence of nucleotides. However, if DNA were left in this elementary state, the amount needed to code for an entire organism would not fit into the nucleus. DNA must be twisted into a helical state and then folded around histones and condensed further into looped domains. The form resulting from this folding is a chromosome.

• Chloroplasts and Mitochondria

The increased surface area created by the shape of the mitochondria, mitochondrial membrane, and the thylakoids found in chloroplasts, allows for an optimum rate of cellular respiration or photosynthesis.

• Cellulose

The fibrous structure of cellulose provides the necessary rigidity for the cell wall of a plant cell.

• Electron Transport System

The electron transport system consists of a sequence of electron carriers lined up one after the other. This construction allows for the movement of electrons through each cytochrome, thereby releasing the energy needed to make ATP.

• Restriction Enzyme Recognition

Each enzyme has a specific nucleotide sequence which is the restriction site, or cut site.

• Cell Differentiation

Cell differentiation allows cells to specialize to correspond to their specific function. In this way every cell can carry out a specific function correctly by using this unique division of labor.

Heredity and Evolution

• Adaptations

As a result of natural selection, all adaptations are the best design for their function.

One example of an important adaptation is the fact that birds have hollow bones to facilitate flight.

• Gamete Structure

The structure of the complementary gametes, the egg and sperm, is such that their fusion is possible. The egg is large and immobile with more cytoplasm than the small flagellated sperm cell. This fusion is vital to the creation of a new zygote. If the structure of the gametes did not serve their function, sexual reproduction would not be possible.

• tRNA

The tRNA molecules are folded so that they have a sequence of three nucleotides (anticodon) in the clover-leaf structure. The opposite end of the molecule is designed to carry the amino acid that corresponds to the the anticodon.

• Chromosomal Puffs

As DNA is ready to be transcribed, the two strands of DNA separate at a specific site. The separation remains localized so that the entire chromosome does not have to be unraveled.

• Viruses

Viruses consist of a nucleic acid core and a protein coat. This simplistic structure means that the virus's only function is to manipulate the functions of the host cell by using the host cell DNA and proteins to replicate its own. The virus only has what it requires to infect a cell and directs the cell to do everything else - reproduction and protein synthesis.

Organisms and Populations

• Vertebrate Systems

**This is not an exhaustive list of examples of form fitting function.

The Integumentary System

1. Amphibians have a permeable skin to allow for gas exchange, while reptiles have a protective leathery coating that prevents dehydration.
2. Protective layers such as skin, feathers, mucus, scales, and hair not only provide physical protection, but insulate the organism as well.

The Nervous System

1. The central nervous system is a combination of the brain and spinal cord and serves as the central communcation center. All incoming and outgoing impulses are routed through the CNS. Because the brain and spinal cord are in the central part of the body, it is easier to coordinate incoming and outgoing messages between the CNS and PNS (peripheral nervous system).
2. There are three different types of neurons - sensory neurons, interneurons, and motor neurons. The length of the dendrites and axons vary according to the function of the neuron. For example, a sensory neuron has long and skinny dendrites that extend to the extremities in order to collect information from the external environment. In contrast, the motor neurons have short dendrites, because they only need to receive information from a nearby interneuron. However, the axon of a motor neuron is longer than that of a sensory neuron, because the motor neuron delivers the impulses to effectors far away from the interneuron that it receives the message from.

The Muscular System

1. Particular shapes and arrangements of muscles can be related to the types of movement of the organism. (muscles are antagonistic to one another)
2. Sliding-filament hypothesis

The Endocrine System

1. Ductless glands secrete hormones into the circulatory system and therefore do not require ducts.
2. Cell surface receptors allow water-soluble hormones to diffuse through the plasma membrane.
3. Cytoplasmic receptors bind with steroid hormones and travel to nucleus to affect a change.

The Reproductive System

MALE

1. Penis = Internal fertilization
2. Sperm

-Lack of nucleic and cytoplasmic fluid

-Flagella for propulsion against the pull of gravity

-Penetrates the egg and contributes chromosomes

3. Scrotum

-Protects the sperm

-Keeps testes away from body and at a lower temperature

FEMALE

1. = Internal fertilization
2. Egg

-Large

-Immobile

-Nutrient-rich

3. Ovary

-Produces and houses eggs

-Releases egg during ovulation

4. Cilia

-Moves egg through Fallopian tube

-Draws egg into the uterus

5. Uterus

-Nurtures and protects embryo

6. Nipples

-Secrete milk produced by mammary glands

-Formed to fit mouth of offspring

7. Labia majora and Labia minor = protection
8. Clitoris = stimulation

The Respiratory System

1. Nasal hairs catch and filter incoming air.
2. Mucus maintains a moist respiratory surface and catches extraneous particles.
3. The alveoli provide a massive amount of surface area in the lungs for diffusion.
4. Gill filaments are thin and permeable to facilitate gas exchange.
5. The circulatory system is in close proximity to the respiratory system.

The Circulatory System

1. Blood vessels are specifically designed to be an optimal size for their specific function.

ex: Capillaries are only a single layer of cells thick to allow for easy exchange between cells.

2. Arterial walls are thick to compensate for the pressure within the arteries.
3. Valves in the heart and veins prevent backflow, thus keeping blood flowing in one direction.
4. The four-chambered heart (in alligators, crocodiles, birds, and mammals) is designed so that oxygen-rich and oxygen-deficient blood never mix.

The Immune System

1. Each antibody is formed to react with a specific antigen.

The Excretory System

1. The nephron is made up of a single layer of cells so osmosis and active transport can occur easily between the blood and the kidney.

The Digestive System

1. The structure of an organism's system is adapted to the particular feeding habits of the animal; for example, a wolf has sharp canine teeth to pierce and hold onto its prey
2. The individual parts of the digestive system are specialized to suit their functions; for example, the esophagous has thick, longitudinal, and circular muscles for pushing food down into the stomach.
3. Various sorts of enzymes are produced which accomodate various kinds of foods; enzymes are specific in terms of the reactions that they catalyze.
4. The intestines are long and have a large surface area (provided by the villi) to make absorption as efficient as possible.

The Skeletal System

1. The composition of the bone is hard and resilient. These structures provide support and protection for the internal organs.
2. Joints allow the organism to move.

• Xylem and Phloem

Since xylem transports water against gravity, its walls are thicker than those of phloem. The explanation for the transport of water is known as the cohesion-tension model.

The tracheids and vessel elements of xylem are non-living, and at their junctions, the cell walls have deteriorated, so they form a long tube. Because the cells are not alive, the xylem cells cannot perform active transport.

The walls of the phloem are thinner since there is no tension or pressure imposed upon them as it is in the xylem.

The sieve-tube elements and companion cells actively transport the products of photosynthesis and other organic molecules. The explanation for the movement of organic materials through a plant is the pressure-flow hypothesis.

• Stomates and Guard Cells

Stomates are the "holes" through which water transpires and gases flow (CO₂ flows in and O₂ flows out). Guard cells regulate the opening and closing of these holes. Because of their thicker inner cell wall, the hole will open when the cells are turgid.

• Roots

Not only does the branching network of roots anchor the plant, it also extends far enough into the soil to reach underground reservoirs of water. Root hairs provide extra surface area for the absorption of water and minerals. The root cap at the tip of a root allows the root to push through tightly packed soil without damaging the meristematic cells.

• Flowers

Flowers are colorful, fragrant and nectar-rich to attract pollinators (i.e. insects and birds). Some flowers even have special patterns on the petals to guide the pollinators to the pollen. Flowers often complement the structures of their pollinators.

Self-pollinating flowers have the anther higher than the pistil so that pollen grains can fall directly onto the pistil to facilitate fertilization.

• Seeds

Seeds consist of the embryo, food for the embryo, and a hard coat which serves as protection for the embryo.

• Fruits

Fruits are designed to facilitate dispersal of seeds. (e.g. apples taste good so deer will eat them)

• Polymorphic Parasites

Some parasites exist in more than one form, depending on their life cycle and host. According to which host they are in, they change form and function. One example of this polymorphism is malaria; it exists as a trophozoite, merozoite, and sporozoite depending on whether it is in its human or mosquito host. (Don't worry about the details of the life cycle, just be aware that malaria is an example of a polymorphic parasite.)