Biology

Cell division

New cells are needed for an organism, or part of an organism, to grow. They are also needed to replace cells that bceome worn out or brocken and are used to repair damaged tissue. However, the new cells must have the same genetic information as the originals so they can do the same job.

Growth and differentiation

At the moment of cinception, a potential new human is just one cell. By the time you are an adult, scienctists have estimated that your body will contain around 37.2 trillion cells – although estimates vary from 15 – 100 trillion! Almost all of these cells are the result of mitosis. The growth that takes place is amazing. Growth is a permanent increase in size as a result of cell divison or cell enlargement.

The cells of your body, or any complex multicellular organism, are not all the same. They are not all the same at the original cell either. This is because, as cells divide, grow and develop, they also begin to differentiate. 

The cell cycle

The length of the cycle varies considerably. It can take less than 24 hours, or it can take several years, depending on the cells invloved and the stage of life of the organism. The cell cycle is short as a baby develops before it is born, when new cells are being made all the time. It remains fairly rapid during childhood, but the cell cycle slows down oce puberty is over and the body is sdult. However, even in adults, there are regisons where there is ontined growth or a regular replacement of cells. They include hair follicles, the skin, the blood, and the lining of the digenstive system.

The cell cycle in normal, healthy cells follows a regular pattern:

-       Stage 1: this is the longest stage in the cell cycle. The cells grow bigger, increase their mass, and carry out normal cell activities. Most importantly they replicate their DNA to form two copies of each chromosome ready for cell division. They also oncrease the number of sub – cellular structures such as mitochondria, ribosomes and chlorplasts ready for the cells to divide.

-       Stage 2: Mitisis: in this process one set of chromosomes is pulled to each end of the dividing cell and the nucleus dividers.

-       Stage 3: this is the stage during which the cytoplasm and the cell membranes also divide to form two identical daughter cells.

In some parts of an animal or plant, mitotic cell division carries on rapidly all the time. For example, you constantly loose cells from the skin’s surface and make new cells to replace them. In fact, about 300 million of your body cells die every minute, so cell division by mitosis is very important. In a child, mitotic divisoins produce produce new cells faster than the old ones die.  As an adult, cell death and mitosis keep more or less in balance. When you get very old, mitosis slows down and you show the typical signs of ageing.

Differentiation in animal cells

In the early development of an animal and plat embryos, the cells are unspecialised. Each one of them also known as the stem cell can become any type of cell that is needed.

In animals, many types of cells become specialised very early in life. By the time a human baby is born, most of its cells are specialised to carry out a particular job, such as nerve cells, skin cells, or muscle cells. They have differentiated. Some of their genes have been switched on and others have been switched off. As a result, different types of specialised cells have different sub – cellular structuress to carry out specific functions. Most specialised cells can divide to produce more muscle cells, for example. Some differentiated cells, such as red blood cells and skin cells, cannot divide at all and so adult stem cells replace dead or damaged cells. Nerve cells do not divide once they have differentiated and they are not replaced by stem cells. As a result, when nerve cells are damaged they are not usually replaced.

In a mature animal, little or no growth takes place. Cell division is almost entireley restricted to repair and replacement of damaged cells, and each differentiated cell type divides only to make more of the same cell.

Differentiation in plant cells

In contrast to animal cells, most plant cells are able to differentiate all throught their lives. Undifferentiated cells are formed to at active reigions of the stems and roots, known as the meristems. In these areas, mitosis takes place almost continoisly. The cells then elongate and grow before they finally differentiate.

Plants keep growing all through their lives at these ‘growing points’ . the plant cells produced do not diffremtiate until they are in their final position in the plant. Even then, the differentiation is not perminant. You can move a plance cell from one part of the plant to the another. There it can redifferentiate and become a completely different type of cell. You cannot do that with animal cells – once a muscle cell, always muscle cell.

Organisation and the digestive system

Tissues and organs

As you have seen, cells are the basic building blocks of all living organisms.

Unicellular and simple multicellular organisms carry out all the exchanges they need across their cell membranes.

Large multicellular organisms may contain billions of cells and they have to overcome the problems linked to their size.

They have evolved different ways of exchanging materials.

During the development of a multicellular organisms go beyond specialised cells.

Similar specialised cells are often found grouped together to form a tissue.

Tissues

A tissue is a group of cells with similar structure and function working together.

For example, muscular tissue can contract to bring about movement.

Glandular tissue contains secretory cells that can produce and release substances such as enzymes and hormones.

Epithelial tissue covers the outside of your body as well as your internal organs.

Organs

Organs are collections of tissues.

Each organ contains several tissues, all working together to perform a specific function.

For example, the stomach is an organ involved in the digestion of the food. It contains:

-       - Muscular tissue, to churn the food and digestive juices of the stomach together.

-      -  Glandular tissue, to produce the digestive juices that break down food

-       - Epithelial tissue, which covers the inside and outside of the organ.

The pancreas is an organ that has two important function.

It makes hormones to control blood sugar, as well as some of the enzymes that digests food.

It contains two very different types of tissue, which produce these different secretions.

The human digestive system

The digestive system

Your digestive system is between 6 and 9m long – 9 million times or 6 orders of magnitude longer than an average human cell! The digestive system of humans and other mammal’s exchanges substances with the environment. The food you take in and eat is made up of large insoluble molecules. Your body cannot absorb these molecules. They need to be broken down or digested to form smaller, soluble molecules that can then be absorbed and used by your cells. This process of digestion takes place in your digestive system, one of the major organ systems of the body.

Digestive Systems

The digestive system is a muscular tube that squeezes your food through it.

It starts at one end with your mouth, and finishes at the other end with your anus. The digestive system contains many different organs. These are glands such as the pancreases and salivary glands. These glands make and release digestive juices containing enzymes to break down your food.

The chemistry of food

Carbohydrates, lipids, and proteins are the main compounds that make up the structure of a cell. They are vital components in the balanced diet of any organism that cannot make its own food. Carbohydrates, lipids, and proteins are all large molecules that are often make up by smaller molecules joined together as part of the cell metabolism.

Carbohydrates

Carbohydrates provide us with the fuel that makes all of the other reaction of life possible. They contain the chemical elements carbon, hydrogen and oxygen.

All carbohydrates are made up of units of sugars.

-       Some carbohydrates contain only one sugar unit. The best known is glucose. Other carbohydrates are made up of two sugar units joined together, for example sucrose, the compound we call ‘sugar’ in everyday life. These small carbohydrate units are referred to as simple sugars.

-       Complex carbohydrates such as starch and cellulose are made up of long chains of simple sugar units bonded together.

Carbohydrate – rich foods include bread, potatoes, rice and pasta. Most of the carbohydrates you eat will be broken down to glucose used in cellular respiration to provide energy for metabolic reactions in your cells. The carbohydrates cellulose is an important support material in plants.

Lipids

Lipids are fats (solids) and oils (liquids). They are most efficient energy store in your body and an important source of energy in your diet. Combined with other molecules, lipids are very important in your cell membranes, as hormones, and in your nervous system. Like carbohydrates, lipids are made up of carbon, hydrogen, and oxygen. All lipids are insoluble water.

Lipids are made up of three molecules of fatty acids joined to a molecule of glycerol. The glycerol is always the same, but the fatty acids vary. Lipid – rich food includes all the oils, such as olive oil and corn oil, as well as butter, margarine, cheese and cream. The different combination of fatty acids affected whether the lipid will be a liquid oil or a solid fat.

Proteins

Proteins are used for building up the cells and tissues of your body, as well as the basis of all your enzymes. Between 15 and 16% of your body mass is protein. Protein is found in tissues ranging from your hair and nails to the muscles that move you around and the enzymes that control your body chemistry. Proteins are made up of the elements carbon, hydrogen, oxygen, and nitrogen. Protein – rich foods include meat, fish, pulses and cheese.

A protein molecule is made up of long chains of small units called amino acides. There are around 20 different amino acids, and they are joined together into long chains by special bonds. Different arrangements of the various amino acids give you different proteins.

Catalysts and enzymes

In everyday life, you control the rate of chemical reactions all the time.

You increase the temperature of your oven to speed up chemical reactions when we cook, and you cool food down in the fridge to slow down the reaction that cause food to go off.

Sometimes people use special chemicals known as catalysts to speed up reactions. A catalyst speeds up a chemical reaction, but it is not used up in the reaction. You can use a catalysit over and over again.

Factors affecting enzyme action

A contanier of milk left at the back of the fridge for a week or two will be disguisting. The milk will go off as enzymes in bacteria break down the protein structure. Leave the milk in the sun for a day the same thing happens – but much faster.

Temperature affects the rate at which chemical reactions take place, even when they are controlled by biological catalyst.

Biological reactions are affected by the same factors as any other chemical reactions. These factors includeconcentration, temperature, and surface area.

However, in living organisms, an increase in temperature only increases the rate of reaction up to a certain point.

The effect of temperature on enzyme action

The reactions that take place in the cells happen at relatively low temperatures. As with other reactions, the rate of enzyme – controlled reacctions increases as the temperature increases.

However, for most organisms this is only true up to temperatures of about 40°C. after this, the protien structure of the enzyme is affected by the high temperature. The long amino acid chains begin to unravel, and as a result, the shape of the active site changes. The substrate will no longer fit in the active site. The enzyme is said to have been denatured. It can no longer acts a cataylst, so the rate of the reaction drops drematacally. Most human enzymes work best at 37°C, which is human body temperature.

Without enzymes, none of the reactions in your body would happen fast enough to keep you alive. This is why it is dangerous if your temperature goes too high when you are ill. Once your body temperature reaches about 41°C, your enzymes start to be denatured which would result in death.

Effect of pH on enzyme action

The shape of the active site of an enzyme comes from the forces between the fifferent parts of the protien molecule. These forces hole the folded chains in place. A change in pH addects these forces. That’s why it changes the shape of the moleule. As a result, the specific shape of the active site is lost, so the enzyme no longer acts as a catalyst. Different enzymes work best at different pH levels. A change in pH can stop them working completley.

The information in the cells

Each of your cells has a nucleus that contains chromosomes. Chromosomes carry the genes that contain the instructions for making both new cells and all the tissues and organs needed to make an entire new you!

A gene is a small packet of information that controlls a characteristic, or part of a charicteristic, of your body. It is a section of DNA, the unique moclecule that makes up your chromosomes.

Most of your characteristics are the result of many different genes rather than a single gene. The genes are grouped together on chromosomes. A chromosome may carry several hundred or even thousands of genes.

You have 46 chromosomes in the nucleus of your body cells. They are arranged in 23 pairs. In each pari of chromosomes, one chromeosome is inherited from the father and one from the mother.

The cell cycle

The length of the cycle varies considerably. It can take less than 24 hours, or it can take several years, depending on the cells invloved and the stage of life of the organism. The cell cycle is short as a baby develops before it is born, when new cells are being made all the time. It remains fairly rapid during childhood, but the cell cycle slows down oce puberty is over and the body is sdult. However, even in adults, there are regisons where there is ontined growth or a regular replacement of cells. They include hair follicles, the skin, the blood, and the lining of the digenstive system.