Biology & Human Physiology

Biomolecules and enzyme action

Biomolecules fall into four main classes: carbohydrates, proteins, lipids and nucleic acids. Carbohydrates are sugars and their polymers, with a general formula close to (CH_2O)_n, while proteins are polymers of amino acids linked by peptide bonds. Lipids are a broad group that is insoluble in water but soluble in organic solvents, and nucleic acids are polymers of nucleotides. Cholesterol is a sterol, a steroid lipid built on a four-ring skeleton. It is a structural component of animal cell membranes and a precursor of steroid hormones and bile salts.

Starch and glycogen are both glucose polymers used for energy storage, but they are found in different organisms. Starch is the storage polysaccharide of plant cells, whereas glycogen is the storage polysaccharide of animal cells, chiefly in the liver and skeletal muscles.

Protein synthesis depends on adaptor molecules. Transfer RNA (tRNA) carries a specific amino acid and has an anticodon that pairs with the matching mRNA codon. Aminoacyl-tRNA synthetases attach the correct amino acid to each tRNA. mRNA carries the genetic code, snRNA takes part in RNA processing such as splicing, and miRNA regulates gene expression, so none of these carries amino acids.

Enzymes are biological catalysts, and their rate depends on several factors at once. The rate of an enzyme-catalysed reaction is controlled by substrate concentration, temperature and pH. The rate rises with substrate concentration until the enzyme is saturated. Each enzyme has an optimum temperature and pH, because deviation alters the three-dimensional structure of the active site.

Allosteric enzymes are an important exception to simple Michaelis-Menten behaviour. They are usually oligomeric, with several subunits, and each subunit has a regulatory site in addition to its active site. An effector molecule binds the regulatory site and changes the enzyme's activity. Because of cooperative binding, the velocity versus substrate concentration curve is sigmoidal rather than hyperbolic.

Cell organelles and endomembrane system

The membrane-bound organelles of the endomembrane system work together to make, modify and transport proteins and lipids. The rough endoplasmic reticulum (RER) carries ribosomes on its surface and synthesises proteins, including the hydrolytic enzymes that are later packaged into lysosomes. Golgi bodies modify, sort and package these proteins but do not make them. Lysosomes are therefore filled with enzymes that were synthesised on the RER.

The smooth endoplasmic reticulum (SER) lacks ribosomes and is involved in lipid synthesis and detoxification. In vertebrates it is especially abundant in liver cells (hepatocytes), where its cytochrome P450 enzymes metabolise drugs, alcohol and other toxic compounds into more water-soluble forms for excretion.

Mitochondria are semi-autonomous organelles. Besides the nucleus, they possess their own small circular DNA, their own ribosomes and the machinery to synthesise some of their own proteins. This feature supports the endosymbiotic theory of their origin.

Do not confuse organelles with chromatin units. Nucleosomes are units of chromatin inside the nucleus and are not separate membrane-bound organelles.

Human blood and circulation

Blood consists of plasma together with cellular components. Platelets are small cell fragments that adhere to damaged blood vessel walls and take part in clot formation. They release clotting factors that help convert fibrinogen into fibrin, so a sharp fall in the platelet count raises the risk of bleeding. Oxygen is carried mainly by haemoglobin in red blood cells, not by platelets.

Lymph is formed when plasma and small molecules leak from blood capillaries into the tissue spaces. Red blood cells are too large to pass through capillary walls, so they remain in the blood. Lymph still contains white blood cells, mainly lymphocytes. The key difference between blood and lymph is therefore the absence of red blood cells in lymph.

The heart muscle (myocardium) needs its own blood supply, because the chambers cannot take oxygen from the blood they pump. The coronary arteries arise from the base of the aorta and supply oxygenated blood to the myocardium. Other arteries serve other organs: the pulmonary artery carries deoxygenated blood to the lungs, the renal artery supplies the kidneys and the hepatic artery supplies the liver.

Plant meristem cells

Meristematic cells are small, actively dividing cells found in the growing regions of a plant. Their typical features are dense cytoplasm, a thin primary cell wall, a relatively large nucleus and no vacuoles or only very small ones. Large central vacuoles and thick walls are features of mature cells, such as parenchyma, rather than of meristems.

The apical meristem, located at the tips of roots and shoots, is responsible for primary growth in length. Its continuous cell division produces new cells that elongate, so roots and shoots grow longer. Collenchyma gives mechanical support, and the cortex stores food and provides support. Neither of these tissues drives elongation of the organ.

Heart structure in vertebrates

The number of heart chambers differs among vertebrates and reflects how completely oxygenated and deoxygenated blood are separated. Fishes have a two-chambered heart. Amphibians and most reptiles have three chambers, which allows partial mixing of blood. Among reptiles, the crocodile has a four-chambered heart with two atria and two completely separated ventricles. Snakes, turtles and lizards such as chameleons have three-chambered hearts.

Double circulation means that blood passes through the heart twice in one complete cycle, once through the pulmonary circuit and once through the systemic circuit. Mammals show complete double circulation because their heart has four chambers with completely separated atria and ventricles, so oxygenated and deoxygenated blood do not mix. Fishes have a single circulation, amphibians mix blood only partially, and invertebrates generally lack this arrangement.

Phloem and plant vascular tissues

The vascular tissues of plants are xylem and phloem. Phloem translocates organic food, chiefly sucrose and other sugars made in the leaves by photosynthesis, to storage organs and growing parts of the plant. Water and dissolved minerals are carried upward from the roots mainly by xylem. Oxygen moves by diffusion through stomata and intercellular air spaces rather than through vascular tissue.

A complex tissue is made of more than one cell type working together as a unit. Xylem is a complex tissue that contains tracheids, vessels, xylem parenchyma and xylem fibres. Parenchyma, sclerenchyma and collenchyma are simple tissues, each made of one basic cell type.