CHAPTER 6

Energy for living processes

Energy is required for an organism to carry out the basic living processes such as movements, digestion, reproduction, response, secretion, etc. The energy must be supplied constantly to the cells so that chemical reactions at the cellular level can be carried out. This in turn allows the basic living processes to be carried out by the organism.

Energy is available in organic food molecules in the form of carbohydrates. Respiration is required where food molecules are broken down and energy is produced. The energy is subsequently supplied to the cells so that the cells can carry out various processes at the cellular level. Processes at the cellular level include active transport, synthesis of protein, cell division, formation of gamete, nerve transmission and so on.

The main substrate for energy production

The main substrate for respiration is glucose. For humans and animals, glucose is derived from the digestion of carbohydrates. In plants, glucose is derived from photosynthesis.

Two types of respiration

Respiration can be defined as a metabolic process that involves the breaking down of organic nutrients, such as glucose, into simpler products to produce energy for the cells. There are two types of respiration, external respiration and internal respiration.

External respiration is the breaking down of organic nutrients through gaseous exchanges between body tissues and the environment. It involves breathing or inhaling of air containing oxygen from the atmosphere into the lungs followed by exhaling of air containing carbon dioxide back to the atmosphere.

Internal respiration, on the other hand, is the breaking down of nutrients in the cells through cellular respiration. It involves a series of chemical reactions in the cells. No oxygen is used in this respiration. As such, the respiration is categorised into two groups as follows

i.Aerobic respiration	– the respiration that uses oxygen
ii.Anaerobic respiration	– the respiration in the absence of oxygen

Aerobic respiration

Aerobic respiration is the breaking down of glucose in the presence of oxygen to release chemical energy. Oxygen is required to oxidise the glucose and the products of the oxidation are arbon dioxide, water and energy. Glucose is completely oxidised in order to release all its chemical energy.

Anaerobic respiration

Anaerobic respiration is a type of cell respiration which occurs in the absence of oxygen to release energy. Glucose is broken down in the absence of oxygen to release chemical energy. However, the glucose is not completely broken down and not all the energy in the glucose is released. Some of the energy in the glucose is stored as the by-product from the anaerobic respiration. Only small amounts of energy are released. Anaerobic respiration occurs in the cytoplasm.

Anaerobic respiration in human muscles

• Anaerobic respiration occurs in human muscles during vigorous exercise or vigorous activities, such as when a person is running. Oxygen is needed at a higher rate and is transported quickly to the muscles for rapid cell respiration. This enables the release of sufficient energy for the vigorous muscle activities to take place.

Anaerobic respiration in yeast

Yeast is a microorganism that undertakes anaerobic respiration. Such respiration is called fermentation. During fermentation, yeast secretes the enzyme called zymase. The zymase hydrolyses glucose in the absence of oxygen to form ethanol, carbon dioxide and energy.

The enzyme zymase secreted by the yeast speeds up the fermentation process. Only a small amount of energy is released in the fermentation process. A large amount of energy is still stored in the ethanol as chemical energy. This is because the glucose is not completely broken down in anaerobic respiration.

Comparison between aerobic respiration and anaerobic respiration

Different respiratory structures in organisms

Breathing is the mechanical process of gas exchanges. It involves the taking in of oxygen into the lungs (inhalation) and the removal of carbon dioxide from the lungs (exhalation). All organisms need respiratory structures for the gaseous exchanges. Their structures must be well adapted so as to maximise the rate of these exchanges. 

The gaseous exchanges occur via diffusion and take place on the surfaces of the respiratory structures. The respiratory surfaces must be adapted for maximum gaseous exchanges and this is done by increasing the total surface area. The larger the surface area of the respiratory surfaces, the higher will be the rate of diffusion for gaseous exchanges. 

Several characteristics should be present in respiratory surfaces for them to achieve the optimum rate for gaseous exchanges.

Respiratory structures and the breathing mechanisms in organisms

Protozoa (unicellular organisms)

Protozoa are unicellular microorganisms such as the Amoeba and paramecium. They are very small in size and have large total surface area to volume ratios. The gaseous exchanges are achieved by simple diffusion and they occur rapidly and efficiently across the thin plasma membrane

Oxygen from the atmosphere diffuses into the cells down the partial pressure gradient while carbon dioxide diffuses out of the cells through their permeable membranes via the same mechanism. The respiratory structure of the protozoa is thus a very simple one.

Respiratory structures and the breathing mechanism in humans

The human respiratory system consists of a complex respiratory structure. It is made up of the nasal cavity, pharynx, tracheae, bronchi, bronchioles and lungs. The respiratory process involvea air entering through the nostrils and subsequently into the pharynx, tracheae, bronchi, bronchioles, finally ending in air sacs called alveoli. There are as many as 700 million air sacs surrounded by a capillary network for the purpose of gaseous exchanges.

Human respiratory system

The trachea and bronchi are strengthened by cartilage rings (C-shaped). This is to prevent the respiratory tube from collapsing during breathing. At the same, the cartilage rings keep the tube open and allowed the passage of air.

The epithelium lining of the trachea and bronchi is moist. This is to trap dust and microorganisms present in the inhaled air. The trapped particles are moved by the cilia on the epithelial lining to the pharynx and then to the stomach for removal. The trachea branches into two bronchi each of which goes into a lung. Each of the bronchi branches into bronchioles and subsequently, branch and re-branch into finer tubes, finally ending in alveoli. Oxygen enters the alveoli and diffuses through the epithelium lining and the capillary walls into the blood.

The Breathing Mechanism in Humans during Inhalation and Exhalation

The breathing mechanism involves air entering the lungs during inhalation and air moving out of the lungs during exhalation. The lungs do not have muscles but breathing is made possible by the action of a set of antagonistic intercostals muscles and the action of the diaphragm muscles

The Exchange of Oxygen and Carbon Dioxide at the Alveoli

The exchange of oxygen and carbon dioxide at the alveoli is made possible because of the partial pressure of these two gases existing between the alveoli and the blood capillaries. This partial pressure exists because each component of gas in a mixture of gases exerts its own pressure. Partial pressure can defined as the fraction of the total pressure exerted by the gas.

In the atmosphere, there is 21% oxygen, so the partial pressure of oxygen Po in the atmosphere is 21% x 760 mmHg (total atmospheric pressure) = 159.6 mmHg. In the same way, there is 0.03% of CO₂ so that the partial pressure of CO₂ Pco in the atmosphere is 0.03% x 760 mmHg = 0.23 mmHg.

Inhalation

During inhalation, the partial pressure of oxygen P_o in the inhaled air in the alveoli is higher (105mmHg) compared to the partial pressure of oxygen P_o in the blood capillaries of the lungs (95mmHg). The higher partial pressure of oxygen in the alveoli forces the oxygen to be dissolved in the layer of moisture on the walls of the alveoli : it subsequently diffuses out into the blood capillaries.

 

Gaseous exchange across the alveolus surface and surrounding blood capillaries

Exhalation

Carbon dioxide, which is produced by the body cells, is brought by the blood capillaries to the alveoli. Hence, the partial pressure of carbon dioxide P_co in the blood capillaries entering the alveoli is higher (45 mmHg) than the partial pressure of carbon dioxide P_co in the alveoli (40mmHg). The difference in the partial pressure forces the carbon dioxide in the blood capillaries to diffuse into the alveoli and it is expelled during exhalation. 

Transport of Respiratory Gases

uring gaseous exchanges, oxygen diffuses into the blood capillaries. Once it is in the blood capillaries, the oxygen has to be transported to the body cells where it is needed. To be transported, oxygen dissolves in the plasma, diffuses in the red blood cells and combines with a respiratory pigment called haemoglobin to form oxyhaemaglobin.

The transport of oxygen is achieved via the blood circulatory system. Oxygenated blood flows from the lungs to the whole body in the form of oxyhaemoglobin. Oxyhaemoglobin is not stable. When cells lack oxygen, the oxyhaemoglobin breaks down and releases the oxygen to diffuse into the cells.

Transport of carbon dioxide

The respiration at the body cells releases carbon dioxide. The carbon dioxide diffuses into the blood capillaries due to the differences in partial pressure. In the capillaries, the blood dissolves in the plasma and enters the red blood cells. A large part of the carbon dioxide is converted into bicarbonate ions. However, some carbon dioxide combines with haemoglobin to form carbaminohaemoglobin in the red blood cells.

The carbon dioxide is then transported to the alveoli in the form of carbaminohaemoglobin and bicarbonate ions in the blood plasma. At the alveoli, carbon dioxide is released from its compound, diffuses into the alveoli and is subsequently expelled during exhalation.

Gaseous Exchanges at the Body Cells

The gaseous exchanges at the body cells are also achieved by means of the gradient of the gas partial pressure. The partial pressure in the blood capillaries is higher than the partial pressure of oxygen in the body cells. As a result, oxygen is forced out of the blood capillaries into the body cells.

In addition, the low partial pressure in oxygen in the body cells results in the low affinity of haemoglobin to oxygen in the cells. This condition makes it more favourable for the oxygen to be released and diffuse into the body cells.

At the same time, the body cells contain a high concentration of carbon dioxide due to respiration at the cells. Hence, the partial pressure of carbon dioxide in the cells is higher than the partial pressure of carbon dioxide in the blood capillaries surrounding them. The carbon dioxide is forced to diffuse out of the body cells into the blood capillaries.

Rate of respiration and vigorous exercise

When a person is doing vigorous activities such as running, more energy is needed by the body to undertake the physical movements. This in turn increases the metabolic rate; the cells require more glucose and oxygen as more energy is given out in cellular respiration. This results in an increase in the breathing rate and heart beat. The increased breathing rate ensures that more oxygen is inhaled and the increase of the heart beats ensures that more blood is being circulated around the body with more oxygen being transported to the respiring body cells. At the same time, the excess carbon dioxide is expelled to the lungs.

The regulation of oxygen and carbon dioxide contents in the body

In humans, the control centre that regulates the basic rhythm of breathing is located in the medulla oblongata in the brain. The centre regulates the rhythm of breathing by controlling the strength and frequency of contraction and relaxation of the intercostal muscles and diaphragm muscles.

Breathing centre in the medulla oblongata

The control centre consists of a special group of cells called central chemoreceptors. The central chemoreceptors in the centre are triggered when there are changes in the oxygen and carbon dioxide concentrations in the body.

During vigorous activities, the rate of respiration in the body cells increases and this in turn increases the concentration of carbon dioxide in the blood. The high concentration of carbon dioxide in the blood lowers the pH value of the blood. The drop in the pH value is detected by the central and peripheral chemoreceptors. We shall look at how both chemoreceptors regulate the rhythm of breathing.

Central chemoreceptors

The drop in the pH value of the blood and tissue fluid bathing the brain stimulates the central chemoreceptors to emit nerve impulses to the respiratory centre. The respiratory centre sends nerve impulses to the intercostal muscles and the muscles of the diaphragm. The intercostal muscles and the diaphragm muscles contract rapidly, causing the rate of breathing and heart beats to increase.

The increases in the rate of breathing and the heart beats enable more oxygen to be supplied all over the body and more carbon dioxide to be produced. This will continue until the level of the pH returns to normal.

Peripheral chemoceptors

Besides the central chemoreceptors, periphery chemoreceptors also respond to changes of oxygen in the body. A decrease in the concentration of oxygen to a very low value, such as at high altitudes where the atmospheric oxygen value is low, will stimulate the periphery receptors to emit nerve impulses. The nerve impulses are then sent to the breathing centre at the medulla oblongata.

The breathing centre responds by sending nerve impulses to the respiratory muscles. Action of the muscles becomes more rapid and will increase the rate of breathing and ventilation. Nerve impulses are also sent to the heart causing the rate of the heart beats to increase. This causes more oxygen and glucose to be carried faster to the muscles for rapid cell respiration and releases energy for vigorous muscle activities. The increased rate of blood circulation also helps to remove carbon dioxide formed during cell respiration more rapidly.

CHAPTER 5

Mitosis

Mitosis is the process of nuclear division within the cell which results in the production of two daughter cells from a single parent cell. The daughter cells are identical to one another and to the original parent cell. Mitosis occurs in all body cells (or somatic cells) except in the reproductive cells (gametes).

A somatic cell contains two complete sets of chromosomes. One set is derived from the female parent and the other from the male parent. Each set has 23 chromosomes, giving a total of 46 chromosomes in a single somatic cell.

A somatic cell with two sets of chromosomes is called a diploid cell (2n). Haploid cells ( n ) are cells with single sets of unpaired chromosomes. All reproductive cells (gametes) are haploid. If the parent cell is haploid ( n ), the mitosis producing the daughter cells will also be haploid. If the parent cell is diploid (2n), the mitosis producing the daughter cells will also be diploid.

The Significance of Mitosis

Mitosis is important in growth, cell replacement, asexual reproduction, reproduction in plants and ensuring genetical identity between parents and daughters.

Growth

Mitosis increases the number of cells in organisms and this is the basis of growth and development in multi-cellular organisms.

Cell replacement

Mitosis is important to replace dead or damaged cells from injuries with new ones. When damaged tissues are repaired, the new cells must be exact copies of the cells being replaced so as to retain the normal function of these cells.

Asexual reproduction

New organisms are formed when the division of cells by mitosis takes place in unicellular organisms. Each new cell grows and becomes an entire organism. The binary fission of an amoeba and the budding of yeast cells are examples of mitosis in unicellular organisms.

Vegetative reproduction

This is the process of asexual reproduction in plants which also involves mitosis. Tulips and onions are reproduced by bulbs, which are short stems under the ground. New bulbs sprout from the old ones and each new bulb gives rise to a new leafy plant. Potatoes are reproduced by tubers, which are enlarged parts of the short stems located underground. The "eyes" become tiny buds. Each bud becomes a shoot, which penetrates the soil and grows upwards. The buds also form roots.

Genetical identity

Mitosis ensures that all new cells that are formed in an organism carry the same genetic information, therefore sharing the same characteristics as the parent cell.

The Cell Cycle

A cell undergoes a sequence of events from the time it is formed until it divides completely into two. This sequence of activities exhibited by cells is called the cell cycle. Two main phases in the cell cycle are the interphase and the mitotic phase (M phase). The mitotic phase is the phase where the division of cell takes place through the processes of mitosis and cytokinesis.

The interphase (G1, S and G2 subphases)

The interphase is not part of mitosis but it is part of the cell cycle and accounts for 90% of the whole cycle. Before mitosis begins, three sub-phases take place in the interphase. These are the G1 (synthesis of new organelles), S (replication of chromosomes), and G2 phases (synthesis of proteins necessary for mitosis). All these sub-phases are necessary preparatory stages for a successful mitotic division.

In the interphase, the cell is engaged in metabolic activity and the chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleolus may be visible. The cell may contain a pair of centrioles (or microtubule organising centres in plants) both of which are the organisational sites for microtubules. During this phase, the cell enlarges and cellular organelles double in number, the DNA replicates, and protein synthesis occurs. The chromosomes are not visible and the DNA appears as uncoiled chromatin.

Just before mitosis begins, two chromomoses, each consisting of two chromatids, appear as thread-like structures. The nucleus is also large and prominent.

Mitosis (M phase)

Mitosis is divided into four phases, namely, the prophase, metaphase, anaphase and telophase. Although separation of the phases makes mitosis easier to understand, it is important to note that mitosis is a continuous process, without pause between phases.

Cytokinesis

The process of cytokinesis is different in animal and plant cells. We shall look at each of them separately.

Cytokinesis in animal cells

Cytokinesis is the process where the cytoplasm of a cell is physically divided to form two daughter cells. This occurs after the mitotic division of the nucleus. Cytokinesis in the animal cell begins shortly after the separation of the sister chromatids in the anaphase of mitosis. The process begins when a ring of actin and myosin filaments constricts the plasma membrane at the equator. Eventually, the cell breaks at the constricted region and the parent cell is divided into two cells.

Cytokinesis in plant cells

Cytokinesis in plant cells is different from that in animal cells. Unlike the animal cells, plant cells construct a cell plate in the middle of the cell. The cell plate begins to enlarge and finally comes into contact with the existing plasma membrane. At the end of the process, a new cell wall is formed on each side of the cell plate and two daughter cells are produced.

Meiosis

Meiosis is the cell division that takes place in the reproductive organs. In human beings and animals, meiosis occurs in the testes in males (to produce sperms) and the ovaries in females (to produce ovules). Meiosis in plants occurs in the anthers of flowers (to produce male gametes in the pollen) and in the ovaries of flowers (to produce egg cells in the ovules).

This cell division can be divided into two stages, namely, Meiosis I and Meiosis II. In meiosis I (first meitotic division), the homologous chromosomes in a diploid cell divide, producing four (haploid) daughter cells. It is this step in meiosis that generates genetic diversity. Meiosis I produces four daughter cells called gametes. Each daughter cell will have half of the chromosome numbers from the parent cell. The haploid cells form one set of chromosomes from each pair of homologous chromosomes. Gametes that are produced are haploid ( n ) whereas the parent cell is diploid (2n). Gametes have different genetic material from the parent cell.

The Significance of Meiosis

Allowing trait inheritance in offspring for the continuity of life

The transmission of traits from one generation to the next generation occurs during meiosis. Genes, also known as units of inheritance, are the inherited characteristics that are passed from parents to their offsprings through the genes in the sperms and ova. Genes present themselves in pairs. One is inherited from the father and the other is from the mother. During fertilisation, when the nucleus of a sperm fuses with the nucleus of an ovum, genes from both parents will be present in the nucleus of the zygote. The genes will determine the traits that will be inherited by the offsprings.

Maintaining the diploid chromosomal number from generation to generation

The maintenance of the diploid number of chromosomes in each generation is important. Meiosis produces gametes that are haploid and when fertilisation happens, a haploid sperm fuses with a haploid ovum to form a diploid zygote (one set of haploid chromosomes each from the paternal and maternal sides). The diploid chromosomal number is maintained in each generation. Thus, meiosis ensures genetic continuity from generation to generation.

Production of haploid gametes in sexual reproduction

In meiosis, the number of chromosomes is reduced by one half to produce gametes (sperm and ova). This ensures that the haploid generation receives a mixed set of genes. During sexual reproduction, the haploid gametes fuse together to form diploid offsprings and therefore restores the diploid conditions in each generation.

Producing genetic variation among offspring

Most importantly, recombination and the independent assortment of homologous chromosomes allow for a greater diversity of genotypes in the population. Meiosis results in unique combinations of maternal and paternal chromosomes during metaphase I. Crossing over during meiosis results in genetic exchange between the members of each pair of homologous chromosomes. This produces genetic variation in gametes that result in genetic and phenotypic variation in a population of offsprings.

The Process of Meiosis

Meiosis is a two-part nuclear division in which the number of chromosomes is halved. Meiosis I reduces the number of chromosomes and Meiosis II divides the double stranded chromosomes to form single stranded chromosomes. Meiosis creates daughter cells each of which receives half the number of chromosomes of the parent cell.

A human cell contains 46 chromosomes; therefore after the process of meiosis, the four daughter cells have 23 chromosomes each. Sex cells or gametes are produced in animals during meiosis. During sexual reproduction, the male and female gametes unite and create a new being called a zygote. The zygote receives two sets of 23 chromosomes from the gametes. These add to become the necessary 46 chromosomes and a diploid or 2n cell.

CHAPTER 4

Elements In The Cell

The basic unit of matter is the element. It cannot be broken down to a simpler form by chemical reactions. The elements that are essential to living things include calcium (Ca), phosphorus (P), potassium (K), sulphur (S), chlorine (Cl), sodium (Ng), magnesium (Mg) and iron (Fe). The most common elements in living organisms are hydrogen (H), carbon (C), oxygen (O) and nitrogen (N). 

Chemical Compounds In The cell

The chemical compounds of the cells consist of two major groups; organic compounds and inorganic compounds. Organic compounds are compounds that contain the element called carbon. Examples of organic compounds are carbohydrates, lipids, proteins and vitamins. These compounds are synthesised by the cell.
Inorganic compounds are compounds that do not contain carbon atoms. Examples of inorganic compounds are mineral salts and water. These are the compounds that are not synthesised by the cell. These compounds are obtained from the external environment. 

The Importance of Water in A Cell

About 70% of the human body consists of water. It is an inorganic compound and consists of hydrogen (H) and oxygen (O) atoms. The water molecules are polar molecules due to the uneven distribution of electrons in the molecules where the hydrogen end is more positive and the oxygen end is more negative.

Introduction

Carbohydrates are made up of carbon, hydrogen and oxygen atoms and are commonly known as sugars and starches. Carbohydrates contain a functional group called a carbonyl. The carbonyl group can exist in two forms, as an aldehyde or a ketone. Table below shows the location of the carbonyl group in the carbon chain.

A monosaccharide is a monomer. Monomers are small basic molecules which can be joined together to form a polymer. So, monomers are basic building blocks of carbohydrates. In other words, a monosaccharide is the simplest carbohydrate which cannot be broken down into other simpler forms of carbohydrates.

A monosaccharide is soluble in water and has a characteristic sweet taste. The molecular formula consists of six-carbon atoms and the common monosaccharide is C₆H₁₂O₆

Disaccharides

Disaccharides are known as "complex" or "double" sugars. Like monosaccharides, they have the sweet-taste characteristic. Examples of disaccharides are maltose (malt sugar), sucrose (cane sugar) and lactose (milk & sugar)

Disaccharides are formed when two monomers (monosaccharides) are joined together by the condensation reaction (or the dehydration synthesis). A molecule of water is removed during this process

The molecular formula of disaccharides is C₁₂H₂₂O₁₁. A disaccharide can be broken down to two monosaccharides by a chemical reaction called hydrolysis (or digestion). The process involves the addition of water and requires the presence of specific enzymes.

Polysaccharides

Coriolus versicolor if one of polysaccharide source

Polysaccharides are polymers of monosaccharides or disaccharides formed by the condensation of glucose monomers. The polymers are long-chained molecules (macromolecules) formed by linking thousands of monosaccharide molecules. 

The general formula of polysaccharides is (C₆H₁₀O₅)_n, where n is the number of monomers. Polysaccharides are insoluble in water and do not taste sweet. 

Reducing Sugars and Non-Reducing Sugars

All monosaccharides have reducing properties. Similarly, some disaccharides maltose and lactose ave reducing properties. They are known as reducing sugars. However, sucrose is a non-reducing sugar. There are two common test reagents used to test for reducing sugars:

i.

Benedict's reagent (alkaline solution of CuSO4)

ii.

Fehling's reagent (alkaline solution of CuSO4) Reducing sugars reduce Cu2+ (a blue solution) to Cu+ (a reddish brown precipitate) in both Benedict's and Fehling's reagents. So, the reddish brown precipitate indicates the presence of a reducing sugar. But a non-reducing sugar (sucrose) cannot reduce Benedict's reagent to the reddish brown precipitate because the carbonyl groups of the glucose and fructose units are locked up in the glycosidic bond Introduction Proteins are complex molecules. Examples of proteins are enzymes, antibodies and hormones. Proteins are important for growth and metabolism in the body. Proteins are also responsible for attacking and destroying invading pathogens. The basic building units (monomers) of proteins are amino acids. Proteins or the polypeptide chains are formed when amino acids join each other in a particular sequence. Polypeptides have many monomers ranging from a few to hundreds and thousands Amino acids contain an amino group (-NH2) which is basic and a carboxyl group (-COOH) which is acidic. The general formula for amino acids is shown in figure below. Amino acids are not soluble in organic solvents but are soluble in water. Types of Amino Acids There are two types of amino acids, essential amino acids and non-essential amino acids. Essential Amino Acids Amino acids can be divided into two groups, essential amino acids and non-essential amino acids. Essential amino acids are amino acids that cannot be synthesised by the human body but are needed by the body. An example of an essential amino acid is leucine. Amino acids can only be obtained from the food we eat. There are altogether nine essential amino acids. Non-essential Amino Acids Non-essential amino acids are amino acids that can be synthesised by the human body. They are derived from other amino acids. There are altogether 11 non-essential amino acids. Table below shows the essential amino acids and non-essential amino acids. The Formation and Breakdown of Dipeptides When joined together, two amino acid molecules form a dipeptide. The process of this combination is called the condensation reaction. In the reaction, a water molecule is removed. The resulting bond attaching the two amino acids is called the peptide bond. The hydrolysis reaction is the process of breaking down the proteins with the addition of a water molecule. This process involves the presence of specific enzymes. The Formation ant Breakdown of Polypeptides When three or more amino acids are linked together by peptide bonds with the removal of water a polypeptide is formed. A polypeptide chain can be hydrolysed to form individual amino acids with the addition of water molecules. The formation and breakdown of polypeptides A protein molecule is made of one or more polypeptides linked together by various chemical bonds. The process of breaking up large and complex protein molecules is via the hydrolysis reaction which involves the addition of a water molecule. This process also involves the presence of specific enzymes. The peptides bond can also be broken by hydrolysis with heat dilute acids or enzymes. The loss of the three-dimensional structures of a protein molecule is known as denaturation. Changes in temperature, pH and salt concentration can also cause denaturation.  Various Structure of proteins The protein structure is classified into four levels: i. the primary structure ii. the secondary structure iii. the tertiary structure iv the quaternary structure The primary structure is the linear sequence of amino acids in a protein molecule. The secondary structure is formed when the peptide chain (a chain of amino acids) becomes folded or twisted forming a helix or pleated sheet. The tertiary structure is formed when the polypeptide helix (secondary-structured protein chain) is bent and twisted into a compact structure. Examples of proteins with tertiary structures are enzymes, hormones and antibodies. The quaternary structure is formed when different polypeptide chains (tertiary-structure protein molecules) combine with associated non-protein groups to form a large complex protein molecule. Lipids Lipids are organic compounds containing carbon, hydrogen and oxygen. Some lipids also contain other elements such as nitrogen and phosphorus. Lipids are non-polar hydrophobic compounds that are insoluble in water but soluble in organic solvents (lipophilic) such as ether and benzene. Lipids are important as a source of energy, as storage of long-term energy and they form the main component in the structure of cell membranes. In terms of size, lipids are medium-sized molecules compared to the macromolecules of polysaccharides, proteins and nucleic acids.  Types of Lipids  There are four main types of lipids. They are: i. fats and oils ii. waxes iii. phospholipids iv. steroids Fats and Oils At room temperature, fats are solids while oils are liquids. They are almost similar chemically. To form fats and oils, two different kinds of organic molecules are needed – glycerol and fatty acids. Glycerol is a three-carbon alcohol with a three hydroxyl group (-OH). Glycerol is a colourless, odourless, sweet-tasting and syrupy liquid. A fatty acid is an organic acid. Its molecular structure consists of a long hydrocarbon tail with a carboxyl group (-COOH) at one end. Different fatty acids have different hydrocarbon tails. Glycerol plays the role of a "holder" molecule which bonds with fatty acids at its hydroxyl group to form a lipid molecule. The properties of a lipid depend on the types of its fatty acids. The glycerol in any lipid is always the same (it is the fatty acids that change, giving the many different types of lipids). Formation and Breakdown of Triglyceride Each molecule triglyceride is formed by the condensation reaction between a glycerol and three fatty acids. The bonds formed are called the ester bonds. Three molecules of water are removed in the condensation reaction The breakdown of triglycerides involves the hydrolysis reaction. The hydrolysis occurs in the presence of enzymes and the triglycerides are broken down into glycerol and fatty acids. Saturated and Unsaturated Fatty Acids Fats often contain only saturated fatty acids but oils usually contain unsaturated fatty acids. Introduction Enzymes are protein molecules which act as biological catalysts to speed up a biochemical reaction in the cell. However, they themselves remain unchanged at the end of the reaction. The substance acted on by the enzyme is called the substrate and the products of the reaction are called the products. Metabolism The biomedical reaction that occurs in a cell of a living organism is called metabolism. As is already known, metabolism requires the action of specific enzymes to take place and it involves the transformation of one type of substance into another. Metabolism consists of anabolism and catabolism. i. Anabolism the synthesis of a complex molecule from simple molecules and the energy-consuming (endergonic) process ii. Catabolism the breaking down of a complex molecule into simple molecules and energy-releasing (exergonic) process. Examples of catabolism are digestion and decomposition. The activation energy is the amount of energy required to activate a reaction. The reaction can take place faster because enzymes can lower the activation energy.  The activation energy is the amount of energy required to activate a reaction. The reaction can take place faster because enzymes can lower the activation energy.  Carbohydrates, proteins, lipids and enzymes are essential for the survival of the cells. The deficiency of any one of the chemical components will affect the function of the cells.

CHAPTER 3

Introduction

Life processes are being carried out by living cells at all times to enable them to stay alive. Examples of these processes are absorbing water and nutrients (minerals, ions, glucose and amino acids from the surroundings), excreting waste products (urea and uric acid that are not needed by the cells) and exchanging respiratory gases (oxygen and carbon dioxide) during respiration

Exchanging respiratory gases during respiration happen in alveolus

The cells absorb certain substances to enable them to carry out biochemical reactions. Examples of the biochemical reactions they undergo are respiration and photosynthesis. For example, animal and plant cells need oxygen for respiration while plant cells also need water, carbon dioxide and light to carry out photosynthesis.

Cells also need to excrete waste products produced within their internal environment. Examples of substances that have to be excreted by plant cells are excess oxygen and waste products such as carbon dioxide. Waste products that are formed during biochemical reactions within the cells must be excreted because they are poisonous and can cause harm to the cells.

Necessity for Movement of Substances Across the Cell Membrane

The movement of substances in and out of cells is necessary as this helps the cells to obtain substances they require such as the oxygen (animal cell), carbon dioxide (plant cell), glucose and other nutrients.

The movement also allows the excretion of wastes; this enables the cells to maintain a suitable pH level and ionic concentration for them to perform their enzyme activities. The movement of substances also allows the exchange of gases and the transfer of substances between cells.

The movement of substances in and out of cells is conducted through the cell membrane known as the plasma membrane. It is basically a boundary that separates the internal environment of the cells from the external environment that surrounds them

Figure below shows the movement of substances in and out of the cells through the plasma membrane. The plasma membrane regulates the exchange of substances between the content of a cell and its external environment.

The Structure of The Plasma Membrane

How does the structure of the plasma membrane
that allows substances to move in and out of a cell, look like? The structure of this membrane can be explained by a membrane model called the fluid mosaic model
proposed by Singer and Nicholson in 1972.

In this model, the plasma membrane consists of a phospholipid bilayer
, in which carrier and pore proteins are embedded. This bilayer surrounds the internal environment of the cell and thus separates the cytoplasmic fluid of the cell from the external environment.

Movement of Substances Across the Plasma Membrane

The movement of substances across the plasma membrane can be divided in two parts – passive transport and active transport.

Passive transport

Passive transport is the movement of substances across the plasma membrane from a region of high concentration
to a region of lower concentration
. In passive transport, no energy is required
by a cell to move the substances through the cell membrane. The following are examples of passive transport:

i.	simple diffusion
ii.	facilitated diffusion
iii.	osmosis

Active Transport

Active transport involves the movement of ions and molecules against the concentration gradient. So this transport is like the pumping process of molecules or ions across the plasma membrane from a region of low concentration to a region of high concentration. The mechanism is similar to facilitated diffusion where carrier proteins are used except that the movement of molecules is now against the concentration gradient.

Since it requires pumping and taking the ions or molecules across the cell membrane against the concentration gradient, energy is required by the cell. The cell must use its own internal or metabolic energy to transport the ions or molecules across its membrane. The energy is provided by ATP.

The mechanism of active transport for sodium ions from the inside of the cell (lower concentration) to the outside of the cell (high concentation) across the plasma membrane 

Process of Passive Transport and Active Transport

Simple Diffusion

An example of simple diffusion is the gaseous exchange in the alveoli and blood capillaries during respiration

i.	Oxygen in the alveoli diffuses across the alveolar and capillary walls into the blood capillaries of the lungs. This is because the concentration of oxygen in the alveoli is higher than that in the blood capillaries.
ii.	Carbon dioxide diffuses across the capillary and alveolar walls from the blood capillaries into the alveoli. This is because the concentration of carbon dioxide in the blood capillaries is higher than that in the alveoli.

Other processes of simple diffusion are the gaseous exchange through the stomata of leaves during photosynthesis, the gaseous exchange in unicellular organisms and the evaporation of water through the stomata from cell leaves during transpiration.

The Effect of Hypotonic, Hypertonic, and Isotonic Solutions on Plant Cells 

A Plant Cell in A Hypotonic Solution

The external solution has a lesser concentration of the solute than that in the cell sap. So, the external solution has a greater concentration of water molecules compared to the concentration of water molecules in the cell sap.

The water molecules move into the cell by osmosis and push the cell contents outwards, against the cellulose cell wall. The pressure outward onto the cell wall is called the turgor pressure. This pressure makes the cell very turgid, rigid and firm. This turgidity provides the plant with its mechanical support, enabling it to stay upright.

When the cell is turgid, water molecules will diffuse into and out of the cell at the same rate (dynamic equilibrium).

So, the effect of the hypotonic solution is to increase the size and volume of the cell, making it rigid and turgid.

A Plant Cell in A Hypertonic Solution

The external solution has a greater concentration than that of the solute in the cell sap. So, the external solution has a lesser concentration of water molecules than the concentration of the water molecules in the cell sap.

The water molecules diffuse out of the central vacuole and cytoplasm of the cell via osmosis. The vacuole shrinks and becomes smaller. The cytoplasm also shrinks, becoming smaller in size and is pulled away from the cell wall. The cell loses water and becomes flaccid, causing the plant to soften and wilt.

The process whereby the cell loses water is called plasmolysis. So, plasmolysis is the loss of water from the cell by osmosis. This becomes evident when the cell contents pull away from the rigid cell wall as the water moves out and the plant becomes soft and less turgid. If the plasmolysis continues, the cell will die and in turn, the plant itself will also die

A Plant Cell in An Isotonic Solution

Water, solute and other substances are stored mainly in the large central vacuole. In the plant cell, the plasma membrane is enveloped by a wall made of cellulose. The external solution has the same concentration of water molecules as that of the water molecules in the cell sap. So, water diffuses into and out of the cell at the same rates.

The original size of the cell is not changed because the net movement of water across the plasma membrane is zero. Its volume and size remain the same. Hence, the isotonic solution does not affect the cell size and shape.

An Animal Cell In A Hypotonic Solution

The external solution has a lesser concentration of solute than that in the cell sap. So, the external solution has a greater concentration of water molecules compared to the concentration of water molecules in the cell sap.

The water molecules move into the cell by osmosis, inflating and swelling the cell and finally, rupturing it. This is because the animal cell has a very thin wall and this wall is unable to withstand the strong osmotic pressure developing within the cell. The bursting of the cell by the diffusion of water into the cell is called haemolysis.

An Animal Cell In A Hypertonic Solution

When a red blood cell is placed in a hypertonic solution, the external solution has a greater concentration of solute that that in the cell sap. So, the external solution has a lesser concentration of water molecules than the concentration of water molecules in the cell sap.

Water diffuses outside the cell by osmosis. The cell loses water to the external environment, and this causes the cell to shrivel and the plasma membrane to crinkle. This situation is called a crenation. If the cell continues to lose its water contents, the cell will probably die.

An Animal Cell In An Isotonic Solution

When an animal cell is immersed in an isotonic solution, the original size of the cell is not changed because the net movement of water across the plasma membrane is zero. This is because the external solution has the same concentration of water molecules as that in the cell sap. So, water diffuses into and out of the cell at the same rates. The cell retains its size and the shape remains in the form of a biconcave disc

The Effect and Applications of Osmosis in Everyday Life

We shall now look at the applications of osmosis in everyday life. Two aspects of the application will be considered, namely, the wilting of plants and the preservation of food.

Wilting in Plants

The excessive use of chemical fertilisers usually causes the wilting in plants. The soil water becomes more concentrated and hypertonic compared to the cell sap of plant roots. This is because fertilisers, such as potassium nitrate which are added to the soil, dissolve in soil water. In this situation, water diffuses from the cell sap into the soil by osmosis and results in the cell being plasmolysed. A plant becomes wilted because the flaccid cells cannot provide support anymore.

In addition, the wilting in plants may also be caused by a shortage of water. When the soil dries up, the soil water becomes more concentrated and hypertonic. So, plants lose water by osmosis and the cells become flaccid.

Wilting refers to the loss of rigidity in non-woody and herbaceous plants. This situation occurs as a result of the diminishing water content in the cells and the turgid pressure in the non-lignified plant cells falls towards zero. Within a short period, the cytoplasm of a plant cell is damaged by plasmolysis. A wilted plant will eventually die if the period of plasmolysis is prolonged.

When water is made available, the cells will promptly recover and the plant gains its turgidity.

Food Preservation

Food preservation is the process of treating and handling food; the concept of osmosis and diffusion is applied during this process. Food preservation is the process conducted to stop or greatly slow down the spoilage of food, preventing food-borne diseases while maintaining the nutritional value, density, texture and flavour of the food.

Natural preservatives such as salt, sugar and vinegar can be used to preserve different types of food such as fruits, vegetables and fish. When these preservatives are added to the food, the surrounding solution becomes hypertonic compared to the contents of the food. Water leaves the food by osmosis and preservatives enter the cell sap.

Preservation prevents the growth of bacteria, fungi and other micro-organisms which can spoil the food. In this way, the food will have a longer shelf life.