Cell. Structure

Сell, in biology, the basic membrane-bound unit that contains the fundamental molecules of life and of which all living things are composed. A single cell is often a complete organism in itself, such as a bacterium or yeast. Other cells acquire specialized functions as they mature. These cells cooperate with other specialized cells and become the building blocks of large multicellular organisms, such as animals and humans. Although cells are much larger than atoms, they are still very small. The smallest known cells are a group of tiny bacteria called mycoplasmas; some of these single-celled organisms are spheres about 0.3 micrometre in diameter, with a total mass of 10⁻¹⁴ gram—equal to that of 8,000,000,000 hydrogen atoms. Cells of humans typically have a mass 400,000 times larger than the mass of a single mycoplasma bacterium, but even human cells are only about 20 micrometres across. It would require a sheet of about 10,000 human cells to cover the head of a pin, and each human organism is composed of more than 75,000,000,000,000cells. Different parts of a cell carry out different activities so that the cell can function normally. 1. Nucleus

• Controls all the cell activities

• Contains  DNA, which carries the 

information for producing new cells 2. Cytoplasm 

• The place where many cell activities happen 3. Cell membrane 

• Controls what goes into and out of a cell 4. Cell wall 

• Gives shape and support to a cell

1. 2. 3. 4. 5. 6.

membrane-bound yeast nucleus cytoplasm cell wall to give shape

оточений оболонкою дріжджі ядро цитоплазма стінка клітини надавати форму

Cell cycle

Most cells both eukaryotic and prokaryotic divide through the Mitosis processes which is asexual. An example of this might be eukaryotic human skin cells and or prokaryotic bacteria; they replicate themselves exactly but not sexually. Eukaryotic cells are different from the prokaryotic cells however because they contain much more DNA and their reproduction process is more complicated. Eukaryotic cells also reproduce sexually through Meiosis in more developed (higher on the evolutionary scale) plants and animals. During, the sexual type of cell division called Meiosis special cells called gametes (eggs and sperm) come together to create a finished product called a zygote (fetus).

When cells reach maturity, they must either stop growing and remain as they are, or divide and reproduce themselves. Some cells like the nerve and muscle cells in the human body don’t divide but stay the same as they reach maturity, this makes them rarer and more finite. Other cells divide when they reach maturity through the Mitosis or Meiosis types of cell division. The cells that do divide do so in what is called the cell cycle, the cycle of a cell life from one division to the next.

A cell cycle from one division to the next is also called a generation as it is with human beings from one set of parents to the children.

The main differences between Meiosis and Mitosis is that one is a form of replication without sexual contact and the other is a blending of chromosomes through the sexual process. When a cell divides by Mitosis, it makes an exact copy of its DNA and transfers this to a daughter cell, sort of like cloning. There are individual steps during the process of Mitosis called interphase, prophase, metaphase, anaphase and telophase. Being one of the most important components of a cell, the plasma membrane, or cell membrane plays a huge role in regulating how things get in and out of cells. The plasma membrane's unique structure enables it to let only certain molecules pass through the cell. This ability is called selective permeability. Most of the plasma membrane in most cells is made up of a phospholipid bilayer with proteins either embedded into it or sticking out from either side of the membrane. The heads of the phospholipids absorb water from the extracellular environment while the tails of the phospholipids repels it. This system helps the membrane regulate the flow of water in and out of the cell, and is also responsible for letting in hydrophobic or water repelling molecules into and out of the cell as well.

Molecules may enter the cell via two main hinds of transport: active and passive. Passive transport is the movement of molecules through the cell membrane that requires no energy. The molecules passing through the membrane move from the area of the higher concentration to the area with a lower concentration. In other words, the molecules move along the gradient and therefore do not need energy to help them move through the membrane.

Diffusion is another word for passive transport and is categorized by two ways: facilitated and simple. In simple diffusion, the molecules are either hydrophilic or small enough to pass through the phospholipid bilayer without any assistance. In facilitated diffusion, proteins embedded in the cell membrane, provide a doorway for particular solutes to go through. Even though the proteins move in order to make openings for solutes, this process does not use any energy. Water is crucial for the survival of cells. Water is able to diffuse through the membrane by a method of passive transport called osmosis. Osmoregulation is the control of the water balance in cells. The amount of water in and out of cells has a great effect on the concentration gradient inside and outside cells. When there is more water inside the cell and more solutes outside the cell, the cell is hypotonic to the gradient fluid. In other words, the cell is in a hypertonic solution. As a result of this, more water molecules leave the cell.

	cell division	поділ клітин
	to reach maturity	достигати, дозрівати
	daughter cell	дочірна клітина
	finite	обмежений
	ability	здатність
	sticky	липкий, неприємний
	to diffuse	розповсюджувати, поширювати
	to facilitate	полегшувати
	to solute	розчиняти
	particular	особливий
	osmosis	осмос (однобічна дифузія, спонтанний перехід)
	gradient	градієнт (міра зростання або спадання в просторі якоїсь фізичної величини)
	to absorb water	вбирати, всмоктувати воду