This is a woman who knows what she wants, she doesn't let other sway her opinion, and she doesn't give second chances to anyone who has betrayed her, she knows that someone who hurts you once will hurt you forever. She ignores those who aren't worth her time, leaves those who don't deserve her, and goes to the one who really loves her and shows her affection.
3.2.1 Asexual Reproduction As previously mentioned, in asexual reproduction one individual produces offspring that are genetically identical to itself. These offspring are produced by mitosis. There are many invertebrates, such as sea stars and sea anemones, that produce asexually. We will now take a look at the common forms of asexual reproductions. (a) Budding In this form of asexual reproduction, an offspring grows out of the body of the parent (see Figure 3.1 for an example of budding in process). According to Figure 3.1, process (1) of Hydra budding, a bud begins to form on the tubular body of an adult Hydra. Process (2) shows the bud developing a mouth and tentacles. Next, the bud detaches itself from the parent in process (3). In the final process (4), the new Hydra is fully developed and will find its own location for attachment. (b)Gemmulation (Internal Buds) In this form of asexual reproduction, a parent releases a specialised mass of cells that can develop into offspring. Some freshwater sponges exhibit this type of reproduction. One in particular is Spongilla (see Figure 3.2). (c) Fission The parent organism grows in size and divides into two or more organisms. Binary fission implies the splitting of parent organism into two new organisms, whereas multiple fissions imply a division into more than two daughter organisms. Figure 3.3 shows this method of reproduction in process. (d) Fragmentation In this type of reproduction, the body of the parent breaks into distinct pieces, each of which can produce an offspring. An example of this type would be the starfish. Look at how the starfish undergoes fragmentation in Figure 3.4 below. (e) Regeneration In regeneration, if a piece of a parent is detached, it can grow and develop into a completely new individual. An example of this would be echinoderms (see Figure 3.5) (f)Parthenogenesis This type of reproduction involves the development of an egg that has not been fertilised into an individual. Animals like most kinds of wasps, bees and ants that have no sex chromosomes reproduce by this process. Some reptiles and fish are also capable of reproducing in this manner. In some organisms, parthenogenesis occurs under specific conditions. For example, when aphids get enough food to eat in the spring season, they resort to asexual reproduction; this is because it is a quicker means of producing offspring (see Figure 3.6). However, these creatures also undergo sexual reproduction.
Plant hormones are also known as plant growth substances. Unlike animal hormones, plant hormones are not manufactured in special organs. Instead, they are made by cells in many different parts of the plant. They regulate many aspects of plant growth and development, from seed formation and germination to the maturity and death of the plant. There are five major types of plant hormones: auxins, gibberellins, cytokinins, abscisic acid and ethene. Table 2.3 below provides a brief explanation of their respective functions. Auxins Synthesised in the apical meristems to promote primary growth by increasing the rate of cell elongation. Gibberellins Formed in young leaves to stimulate the growth of shoots and leaves. Cytokinins Synthesised in the roots and transported to other parts of the plant. When combined with auxins, it stimulates cell division. Abscisic acid Synthesised in chloroplast to promote dormancy in some seeds and stimulates the closing of stomata. Ethene/Ethylene Released from ripening fruit, nodes of stems, ageing leaves and flowers. Involved in seed dormancy, fruit ripening and leaf abscission.
1.3.3 Amino Acid and Peptide Amino acid is the basic unit that forms protein. All amino acids have the same basic structure but differ only at the side group (-R group). Look at Figure 1.13 to have a clear picture of the structure of amino acid. There are 20 types of amino acids. Amino acids are classified into four groups according to their side group. The four groups are: (a) Amino acid without polarised R group Example: glysine (Gly) (b) Amino acid with polarised R group Example: serine (Ser) (c)Amino acid with acidic R group (negatively charged) Example: aspartic acid (Asp) (d)Amino acid with basic R group (positively charged) Example: lysine (Lys) When one amino acid is combined with another amino acid, condensation will occur. As a result, a peptide bond will be formed between the amino acids. The new molecule is called dipeptide (see Figure 1.14). A dipeptide might form tripeptide with another amino acid. When this continues, a polypeptide might form. Protein is the combination of polypeptides. You might ask the number of polypeptides that can be formed here. Twenty amino acids can form unlimited types of polypeptides. Protein is a complex macromolecule. It contains thousands of atoms in its structure. One molecule of protein is made from C, H, O and N. Rarely, it also consists of S and P. Protein is made from amino acids. Three-dimensional protein structures are organised into four levels (see Figure 1.15), namely: (a)Primary Polypeptide chain that is composed of amino acid linear sequences; (b)Secondary Folding and coiling of polypeptide chain; (c)Tertiary Folding of -helix (shaped like telephone wire) polypeptide to form packed globular protein molecules; and (d) Quartenary The arrangement of more than one polypeptide chain to form a protein molecule. We can use structure or composition to classify proteins. At a high temperature (40ÀC), protein denaturalisation might happen. The structures of our body are made from proteins. These also act as hormones and enzymes.
(a) Water is bipolar One molecule of water consists of two hydrogen atoms and one oxygen atom (see Figure 1.6a). The structure of water forms an angle of 104.5À (see Figure 1.6b). As a result, weak positively charged hydrogen atoms and a weak negatively charged oxygen atom are present in a water molecule. The result is a biopolarised molecule. (b) Water molecules are networked through hydrogen bonds Water molecules bond when hydrogen meets oxygen. The molecules come together through a weak hydrogen bond. Water molecules appear collectively. This is the reason why water is a stable matter. (c) Water is in liquid state at room temperature Water has a higher boiling and melting point in comparison with other matters that have the same relative molecular mass. It means more energy is needed to break down the hydrogen bonds in the water. Thus, water can appear in liquid state at room temperature. (d) Water is the universal solvent Bipolarity in water makes water a good solvent with most charged solutes. When water meets charged solutes (for example, Na+ Cl), an electrostatic reaction between the molecules will happen. Non-charged solutes like oil will not form any reaction with water. (e) Water has low viscosity Water can flow easily. This beneficial feature allows water to enter and leave cells efficiently. (f) Water has a high surface tension (high adhesion) Due to this reason, water molecules stick together. It explains why you could observe rain water in the form of droplets. For plants, high adhesion helps through the capillary action. Thus, absorbed water can be distributed well in plants via transpiration. (g) Water has a high specific heat capacity High specific heat capacity means more energy is needed to increase the temperature (1ÀC) of 1kg of water. As a result, fluid temperature is very stable in all the cells of our body. (h) Water density is maximum at 4ÀC Have you ever wondered why an ice cube floats in a glass of water? Why does it not sink? The answer is very simple ice has a lower density compared to water. To undertand, you have to recall the molecular structure of water. This feature helps aquatic organisms to survive during winter because only the upper layer of the lake is frozen (0ÀC) while the temperature inside the lake would be slightly warmer at 4ÀC.
A molecule is formed when two or more atoms join together chemically. A compound is a molecule that contains at least two different elements. All compounds are molecules but not all molecules are compounds. So are hydrogen (H2), oxygen (O2), nitrogen (N2), water (H2O), carbon dioxide (CO2) and methane (CH4) molecules or compounds? In this section, we are going to explore carbon dioxide and water as examples of small molecules. They may be small but they are very important to us. 1.2.1 Carbon Dioxide. Carbon dioxide (CO2) is one of the simplest and commonest molecules in the universe. It has only three atoms one carbon and two oxygen atoms. It is easy for carbon atoms to combine with oxygen atoms because the outer shell (valence shell) of a carbon atom has only four electrons in it, leaving room for four more before it is filled up. In the same way, the outer shell of an oxygen atom has only six electrons in it, leaving room for two more to make eight. When two oxygen atoms share their electrons with one carbon atom, all three of the atoms can fill up their shells the carbon atom has four electrons of its own, plus four more that it shares with the oxygen atoms, and each oxygen atom has six electrons of its own, plus two more that it shares with the carbon atom. We call this a covalent bond. Plants make their cells mostly out of carbon. The way plants get carbon is by breathing in carbon dioxide and breaking off the oxygen, which they then breathe out again. So the carbon in carbon dioxide is what all plants are made of, and the oxygen becomes the oxygen we breathe. When a plant dies, decays or burns, the carbon in it returns to the air, where it mixes with oxygen to become carbon dioxide again. In the last hundred years or so, carbon dioxide emission has become a big problem for everyone on Earth. We have been burning hydrocarbons as gasoline for cars, heating oil for houses and coal for factories that a lot of carbon has been released into the air, where it makes a lot more carbon dioxide than usual. Carbon dioxide is good for plants but it acts like a warm blanket around the Earth, trapping heat on Earth instead of releasing the heat into space. This is the main cause of global warming. (a) Where is it found? (i) Carbon dioxide is found in the atmosphere. About 0.03% of the air is carbon dioxide; and (ii) It is found in lakes, ponds, streams and oceans. (b) Where does it come from? (i) It is produced by almost all living organisms, both plants and animals. Plants release carbon dioxide mostly at night; (ii) It is released into the air every time we exhale; (iii) Even organisms without lungs or gills, such as insects, plants and bacteria, release carbon dioxide into the environment; and (iv) All aquatic organisms release carbon dioxide into the water. This gas either bubbles to the surface or dissolves in the water. Most of the carbon dioxide found in the water is produced by the decomposition of dead organisms, mostly bacteria. (c) Carbon dioxide and plants a nest relationship (i) Most of the plant material in an aquatic environment is made up of algae; (ii) During daylight, all plants use carbon dioxide and release oxygen. This process, which requires light, is called photosynthesis; (iii) At night, the opposite happens. Plants use oxygen and give off carbon dioxide. This process is called respiration; and (iv) All dead plants use a lot of oxygen and give off a lot of carbon dioxide as they rot and decay. (d) Carbon dioxide and animals another exciting relationship! (i) All animals use oxygen and give off carbon dioxide; and (ii) Dead animals continue to use oxygen and give off carbon dioxide as they rot and decay. 1.2.2 Water Did you know that 90% of cellular contents is made of water? Thus, without water, there would be no life! Water, along with carbohydrates and fat, are important sources for life Water is a stable medium for most of the biochemical reactions in living things. In addition, it acts as the intercell and intracell transporter for most dissolved nutrients. Heat is also transported through water. From the evolutionary point of view, life started from water. In fact, most organisms live in the aquatic system. Water has several unique physical and chemical characteristics (see Figure 1.5). Its molecules are small, polarised and form hydrogen bonds with other molecules. You will be amazed by the special features of water and understand why it is so important to living things.
Oxygen is a colourless, odourless and tasteless gaseous chemical element which appears in great abundance on Earth, trapped by the atmosphere. Many people are familiar with oxygen, because it is a vital component of the respiration process; without oxygen, most organisms will die within minutes. The atomic number of oxygen is eight, and it is identified by an O symbol on the periodic table of elements. It is a key catalyst in many chemical reactions. Oxidation is one such reaction, and it occurs when oxygen mixes with other elements and compounds. Oxygen also plays a role in combustion. By mass, oxygen is the most abundant element in the human body. If you think about it, this makes sense, since most of the body consists of water or H2O. Oxygen accounts for 61-65% of the mass of the human body. Even though there are many more atoms of hydrogen in your body than oxygen, each oxygen atom is 16 times more massive than a hydrogen atom. Oxygen has many uses, as shown below: (a) We use oxygen for respiration and it is an ongoing process which will only stop when we die. We inhale oxygen and exhale carbon dioxide. This process is the same for other living things such as animals, plants and bacteria. (b) When plants and animals die and decompose, oxygen is used. (c) When you burn a fire, it uses oxygen. (d) When metals are rusting, oxygen is used. Most available oxygen comes from photosynthesis by plant on land and phytoplankton on the ocean's surface. Some oxygen is made in the atmosphere, when sunlight breaks down water. Most oxygen is stored in the oxide minerals of the Earth's crust and mantle, called the lithosphere, but is bound to rocks and unvailable for use. Phytoplanton floating on the surface of the ocean.
The cells of animals, plants and microorganisms have a similar chemical composition. A cell contains several thousand substances that are involved in a variety of chemical reactions. Some elements are present in cells in relatively large quantities while others are in small quantities. The four elements that are present in large quantities are oxygen, carbon, nitrogen and hydrogen (98%). Sulphur, phosphorus, chlorine, potassium, magnesium, sodium, calcium and iron together comprise 1.9%. All other elements are present in the cell in small amounts (less than 0.01%). The key factor in the reactivity of the atoms of various chemical elements is the number of bonds they can form with other atoms. Table 1.1 shows the five elements, together with examples of the structures of simple compounds involving the elements in which the bonds are represented by lines linking the atoms together. Carbon has a special place in the chemistry of life because with its four bonds, it can link with other carbon atoms to form chains, loops and networks providing the structural basis for complex compounds that may contain many thousands of carbon atoms. Cells are also made up of compounds. What is the difference between an element and a compound? The difference between an element and a compound is that an element is a substance made of the same type of atoms, whereas a compound is made of different elements in definite proportions. Iron, copper, hydrogen and oxygen are examples of elements. Examples of compounds include water (H2O) and salt (Sodium Chloride NaCl). There are two types of compounds organic and inorganic. Organic compoundsare extracted from living organisms. These substances whose molecules contain one or more carbon atoms covalent bonded with another element or radical (including hydrogen, nitrogen, oxygen, halogens as well as phosphorus, silicon and sulphur). A few exceptions are carbon monoxide, carbon dioxide, carbonates, cyanides, cyanates, carbides and thyocyanates, which are considered inorganic. Examples of organic compounds are carbohydrates, lipids, proteins and nucleic acids. Inorganic compoundsare extracted from non-living things. They are any compound not containing carbon atoms. Inorganic compounds have salt forming capacity while organic compounds do not form salts. Next, we are going to explore the important elements and their uses. Here, we will discuss carbon dioxide and water, and move on to organic compounds in detail. We will look at the elements that are present and their uses in the cells. Elements are listed in order of decreasing abundance, with the most common element (by mass) listed first. Approximately 96% of body weight consists of only four elements oxygen, carbon, hydrogen and nitrogen. Calcium, phosphorus, magnesium, sodium, potassium and sulphur are macronutrients or elements which the body needs in a significant amount.