聆听与学习科学/细胞
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All Living organisms are made up of small units, called cells. Cells are the building blocks, of a living organism. Whether the organism is a bacteria, insect, fish, plant, tree, bird, animal, or a human being, the unit of the organism, is still a cell. Organisms which have only one cell, are called unicellular organisms. Bacteria, archaea, protozoa, amoeba are examples, of unicellular organisms. Most other living organisms, insect, fish, plant, tree, bird, animal, or a human being, are multi cellular. The cell size of a human being, or an elephant, is about the same. Larger organisms, have more number of cells. A Human being, has about 100 trillion cells. The basic components and functionality of a cell, of any living organism, is the same. So study of cells, is like studying life itself.
Though the basic components, are the same in any cell, in larger organisms, cells specialise in functions. For example, in our body, Blood cells transport oxygen and nutrition. Brain Neurones, which are cells in the brain, are involved in the thinking process. Cells, in the eye, help deduct light signals. Cells, in the tongue, help deduct taste. Cells, in the nose, help deduct odours. Nerve Neurones, which are cells in the nerves, transmit signals to and from the brain. The sense of touch, of say, heat or cold, is transmitted to the brain, by nerve cells.
Cells are, the structural or functional unit, of an organism. Many cells make, a tissue. For example, bone tissue. Many tissues make an organ. For example, a finger bone. Many organs make an organ system. For example, skeletal system. An organism consists of many organ systems. For example, the brain, the heart, the lung, the kidney, the liver, are organs in our body. The blood system, the nerve system, the respiration system, the digestive system, are examples of organ systems in our body. All the organ systems work together, in a inter related way, in the organism.
Robert Hooke, was an English scientist. He designed the microscope, which can enlarge the cell image. He examined a piece or dried cork, under a microscope. He saw honeycomb like structures, with many units. He called each of them, as a cell. The meaning of the word cell, is a small room. The branch of science, which deals with the study of cells, is called cytology. Robert Hooke is considered, as the father of cytology.
Cell theory was first developed by German scientists, Schleiden and Schwann in 1839. Some basic concepts of cells, are outlined below.
All living organisms, comprises of a basic living unit, called a cell. All the cells, originate from, a single main cell. Cells, divide and create, more cells. This is how, living organisms grow. Whether it is a plant, animal, or human being, this concept of growth, from a single cell remains the same. In higher level organisms, Cells specialise, in different functionalities. For example, Blood cells, nerve cells, muscle cells, kidney cells etc., are specialised cells.
Cells are very small in size, they are usually, measured in microns. A micron, is one millionth of a metre. It is also, one thousandth of a mille metre. A Human cell is typically, 10 to 20 microns in size. With the naked eye, we cannot see anything, smaller than 100 microns. We use an instrument called, a light or compound microscope, to magnify the image of the cell. In a simple sense, a microscope acts like, a magnifying glass. A 10 X magnifying glass, will magnify an image, ten times. A 40 X magnifying glass, will magnify an image, forty times. A light microscope in a laboratory, is a device with many lenses, and mechanical components. It is a more advanced and sophisticated instrument, than the one used by Robert Hooke. Different microscopes have different, magnification capacity. A microscope which has, a magnifying capacity of 2000 X, magnifies an object 2000 times. For example a cell of size 10 microns, will be seen as 20000 microns, or 20 millimetres. This is good enough, to see the cell, and some of its components. But, it is not good enough to see, all the internal components of a cell, in detail.
An electron microscope, uses a beam of electrons, to get an image of an object. It has, a magnifying capacity, of 200 thousand X. With this, we are now able to study, the components of a cell, in molecular detail.
Though a cell is very small, a lot of activity takes place in it. It is much more complex, and fascinating, than a modern factory. Many of the things which happen within the cell, are organic chemical reactions. The process of chemical digestion, and its related reactions, are called metabolism. Cell Metabolism is a total, of all the chemical reactions, in the cell. Two key chemical reactions, are glycolysis and photosynthesis. The breakdown of sugars, and release of energy, is called glycolysis. The process of using suns energy to build sugars, is called photosynthesis.
There is a lot of common chemistry, which happen in all cells. It is surprising and interesting, to know that, carbon, hydrogen, oxygen and nitrogen, constitute most of the cell. These four elements, account for 99%, of a body’s atoms. Hydrogen and oxygen is present, as H 2 O or water. Water is the best and most, abundant solvent in the body. Most other substances, are dissolved in water. Water contributes to 60%, of our body weight. It is also involved, in most of the chemical reactions.
Carbohydrates are compounds of Carbon, Hydrogen and Oxygen. Sugar is a carbohydrate. Sugars provide living beings, with energy. It also is a substance, in the structure, of the cell. The simplest of sugars, is called glucose. It has the formula, C 6, H 12, O 6. This is the sugar, created by photosynthesis, with the suns energy. This is the sugar, that is broken down in glycolysis, which releases energy.
Amino acids are biologically important organic compound. Amino acids mainly have, a carboxyl group, C O O, and an N H 2 group. Amino acids are used widely in cells, to build proteins.
Proteins are basically, a long chain of amino acids. In Proteins, these long chains of amino acids, are twisted around like a knot. Proteins are important of an organism. They participate in virtually every process, within cells.
Chemical reactions sometimes, require a catalyst to make, the reaction happen. Enzymes are proteins, which help complex reactions, that happen in cells.
Lipids are fat cells, used by the body. Their main function, is to store energy. They also act as structural components, of cell membranes.
The cell, is the basic unit, of an organism. An organism has many cells. Is an organism, just a collection of many cells? No, it is much more than that. An organism is a system of organs, which work in a coordinated way. A human body, is an example, of an organism. Like any other organism, it comprises of cells. In a simple sense , a cell is like a plastic bag, containing a fluid, with a central nucleus or head quarters. The cell cannot exist, by itself. It requires a mechanism, to supply substances to it, and take away substances from it. The blood system, plays this role. Blood flows in thin tubes, called capillaries, which reaches all the hundred trillion cells, in the body. Blood carries, oxygen and nutrition, to the cells. Blood takes away, carbon dioxide and waste, from the cell. The lung breathes in oxygen, and gives it to the blood. The lung takes out the carbon dioxide, from the blood, which we breathe out. The digestive system creates nutrients, and gives it to the cells, via blood. The kidneys filter out the waste, from the blood, for disposal. This way all the cells, in the body are inter related, and contribute to the functionality, of the whole organism. There are many such functional systems, in the human body. For example, the nerve system, the respiratory system, the digestive system, etc, all contribute to the working, of the organism.
A cell has 3 prominent components. Cell Membrane. Nucleus. D N A. Cytoplasm.
The cell membrane, is also called, as the plasma membrane. The cell membrane is the skin, which holds all the other components, of the cell inside it. It also acts as a gateway, to the other parts of the organism. It protects the contents of the cell, from the environment, outside the cell. The cell, however has a need to interact, with its environment. It needs oxygen and nutrition, from the environment. It needs to give out, carbon dioxide and waste, to the environment. The cells act as a selective gateway, for this process. The cell membrane is said to be, selectively permeable. The process by which it allows, substances to move in and out, is called diffusion and osmosis. Diffusion is a process by which, a substance dissolved in water, spreads out to be uniformly present. Osmosis is a process by which solvent molecules, move through a semi permeable membrane, to a region of higher concentration. These processes selectively allows, oxygen and nutrition molecules, to enter the membrane. It allows carbon dioxide and waste material, to exit the cell through the membrane. The cell membrane allows the cell, to operate as an independent unit, at the same time selectively interact, with the organism.
The cell nucleus, is like the brain of the cell. It is the head quarters, from which instructions are issued, to the rest of the cell. The nucleus has its own, skin or membrane. It is like a tiny bag, inside the cell. The nucleus contains chromosomes. For example, the human nucleus contains, 23 pairs of chromosomes. Each Chromosome, is made up of thousands of genes. These genes are called, the D N A. The genes of the D N A, is what makes us, or any other organism. The D N A, is an organic compound called, di nucleic acid. Genes are possibly, the most fascinating part, of an organism. They contain all the knowledge and information required, to build the organism. This is how, a living organism grows from a single cell, to the size of any organism, human being or elephant. When a cell divides, the chromosomes inside the nucleus, is replicated in the new cell. It is a remarkable fact, that all the hundred trillion cells in our body, have exactly the same set, of 23 pairs of chromosomes. The genes in the chromosomes, acts like a blueprint to produce, other substances inside the cell. Genes have encoded information, for this. The chromosome is a long chain of genes, a pair of which is twisted, in a double helix form. This is like two snakes, coiled around each other. Gene expression is the process by which, the information in gene is decoded, to form other molecules, like a protein molecules. The D N A works along, with another molecule, called R N A. The R N A, is the selective copy, of part of the D N A. R N A, stands for RiboNucleic acid. We know that the cells in our body, could specialise in different functions. Though all of them have the same chromosomes, different genes can express themselves in different cells. This is how they build different organs, muscle, bone, kidney, liver, eyes, brain etc. That is, the genes are selectively expressed, in each type of functional cell. That is the fascinating aspect of genes, which are contained in the packet, smaller than a pin head.
The gel like material within the cell, is called the cytoplasm. Cytoplasm has fatty acid, sugar, and amino acid molecules, dissolved in it. These are required for the working, of the cell. The enzymes in it break down, larger molecules for use, by the organelles. For example it breaks down Glucose, to be used by the mitochondria. Many tiny organs are present, inside the cell, in the cytoplasm. They are called organelles. Some of the prominent organelles, contained in the cytoplasm are,
Cytoskeleton. Mitochondria. Centrioles. Endoplasmic Reticulum. Ribosomes. Golgi complex. Vacuoles. Lysosomes.
Cytoskeleton, is a cellular scaffolding or skeleton, that criss crosses the cytoplasm. It gives a structure to the cell. It holds the organelles in place. It also acts like a high way, for movement of molecules, within the cell.
Mitochondria, is like the power plant of the cell. It is the source of energy, for the cell. It uses organic compounds, derived from nutrients, to produce A T P. A T P stands for, adenosine triphosphate. The mitochondria is the place, where oxygen is combined, with food molecules. This way the food is digested, and energy released. There might be more than one, mitochondria in a cell. This depends, upon the amount of energy needed, from the cell. For example a muscle cells, which supplies a lot of energy, contains thousands of mitochondria. Nerve cells have only a few.
Centrioles are rod like structures, made of short microtubules. Centrioles play a important role, in cellular division.
This has two important functions. Transporting molecules such as proteins, from one place in the cell, to another place in the cell. Helping in the Synthesis, of proteins and lipids.
Ribosomes, are small organelles, where proteins are synthesised. Ribosomes interacts with, R N A molecules, to produce chains of amino acids. Proteins, are long chains, of amino acids. Ribosomes, are protein factories, of the cell.
Golgi complex modifies, sorts, and packages, different substances in the cell. It gathers simple molecules and combines them, to make complex molecules. It acts like a post office. It packages and labels items, and sends them to specific parts, of a cell.
Vacuoles, are organelles that have, secretory, excretory and storage functions.
They are little digestion machines. When a cell absorbs some food, they release enzymes. These powerful enzymes, break down or digest food, in the cell. The lysosomes break down, harmful cell products, and waste materials. They also digest invading bacteria. Sometimes, lysosomes breakdown the parent cell itself, when the parent cell is ready to die.
There is a simple and important difference, between animals and plants. Plants, produce their own food. Animals, cannot produce their own food. They depend on plants and other animals, for their food. All the components we have discussed, are common to both, animal and plant cells. Plant cell has some special organelles, to help produce their own food, and some other specialised functionality. Three organs are prominently different, in plant cells.
Cell Wall. Vacuole. Plastid.
A cell wall is a rigid layer, outside the cell membrane. It surrounds the membrane. It contains, cellulose and proteins. The cell wall provides, structural support and protection. Plants do not have bones. Plants need other ways, to become structurally stable. The cells walls in a plant provide, shape and structural support. Cell walls have tiny pores, to allow water and nutrients to flow, in and out of the cell.
Plant cells have a central vacuole, which is much larger than an animal cell. It can occupy 30 to 90%, of the cell volume. It is a storage place, for the liquid called, sap. Cell sap is basically water, in which some salts, and other substances are dissolved. When a plant droops, it means the vacuoles, have lost its water. It also stores, other substances like the pigments, which give flowers their colour.
Plastids, are found only in plant cells.
The types of plastids are
Leucoplasts. Chromoplasts. Chloroplasts.
They occur in regions, not exposed to sunlight, like in roots. They are mainly involved, in storage of food.
They contain pigments, which are green, yellow, red and orange in colour.
Chloroplasts are the centre, for production of food. Chloroplasts have chlorophyll, which is green in colour. This is what gives plants, their green colour. Chloroplasts are involved, in photosynthesis. They harvest light energy from the sun, and manufacture organic food molecules. Sugars and starches are examples, of the food that they produce. They use water and carbon dioxide, and release oxygen. Plants are a source of food for all animals, including human beings. Chloroplasts, play a central role, in producing this food.