Cell Membrane and Transport Across Membranes

Cell Membrane

The selective permeable structure located in the outer part of the cell that absorbs or excludes molecules according to their properties is called the cell membrane. The cell membrane is alive and flexible. The main ingredients in its structure are protein (65%), phospholipid (33%) and a small amount of carbohydrates (2%).

• Glycoproteins, which are located in the structure of the cell membrane, are responsible for the recognition of the cells, the selection of the substances to be taken into the cell and the response to some hormones.
• The cell membrane forms outward extensions called microvilli 0.6-0.8 microns outward to increase the absorption surface. Especially in the intestinal cells where digestive products are absorbed, the number of microvilli is very high. Also, flagella, pseudopods are formed in the cell membrane.
• The cell membrane has the properties to move and pull each other.
• The cell membrane is not in a straight line. It can be very indented depending on the cell type. (Like microvillus)

Microvillus

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• The cell surface undergoes various changes (mocrovillus, mesosome, pseudopod) to facilitate substance exchange. Phagocytosis and pinocytosis events may occur as a result of these morphological changes.
• If the dice are torn, it can repair itself to a certain point. The cell membranes are not adherent to each other and there is intercellular fluid between them.

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Mesosome

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Pseudopod

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Fluid Mosaic Model 

• The cell membrane model adopted today is the fluid mosaic membrane model developed by Singer and G. Nicholson in 1972. According to this model, there are two rows of lipid layers in the structure of the membrane. The lipid layer is in constant motion and flowing.
• Protein molecules are placed irregularly (mosaic) between the lipid layer.
• Carbohydrate molecules bind to fat molecules to form glycolipids and protein molecules to form glycoproteins. Some glycoproteins in the membrane of the red blood cells determine the blood group. It allows neighboring cells to recognize each other. This feature is important in tissue transplants.
• This layer formed by glycoproteins and glycolipids is called the glycocalyx layer. These molecules act as receptors in the cell. Disruption of the glycocalyx layer causes uncontrolled division (cancer) of the cells.
• The glycocalyx layer allows the cell to recognize foreign substances and respond to hormones.
• There are holes called pore in the cell membrane. The pores give the cell selective permeability. Small pieces can pass through the pores, while large pieces cannot.

Fluid Mosaic Model

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Functions of the Cell Membrane

1. It surrounds the cytoplasm organelles of the cell.
2. It protects and shapes the cell.
3. It regulates the exchange of substances between the organelles and the cytoplasm, between the cell and its outer environment.
4. It organizes the electric balance.
5. It regulates osmotic balance.
6. Recognizes foreign proteins thanks to glycoproteins. Therefore, it has the feature of recognizing neighboring and foreign cells.
7. It has secretory function. Transmits alert.
8. Helps photosynthesis and respiration in prokaryotic cells.
9. It provides cellulose synthesis in plant cells.

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Transition of Molecules through the Cell Membrane;

• Large molecules (starch, glycogen, protein) cannot pass through the cell membrane. Small molecules (O2, CO2, H2O, glucose, amino acid) pass easily.
• Fat-soluble compounds (vitamins A-D-E-K) pass easily through the cell membrane.
• The degreasing agents (alcohol, acetone, ether, chloroform) can easily pass through the cell membrane as they cause disruption in the cell membrane.
• Neutral molecules can pass through the cell membrane more easily than ions.

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Cell Wall

• The cytoplasm in the plant cell is covered with a lifeless cover called the cell wall. The main structure of this structure is cellulose. It gives the cell a specific shape and provides support to the plant.
• In addition, there is a cytoplasm membrane around the cytoplasm. Young plant cells, sperm and egg cells of algae and fungi are without walls.
• As the cell division ends, a middle lamellar occurs. Cellulose accumulates on both sides of the intermediate lamellae, and the secondary wall forms by accumulating substances such as lignin and fertilizer on it. The task of the secondary wall is to make the cell more resistant.
• Since there is no exchange of substances from the primary and secondary walls, the intermediate lamellae remains in some places without thickening, these are called passages. Items are exchanged through these passages.

Cell Wall

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Transport across Cell Membrane

The continuation of the cells' lives depends on the fact that they take the substances they need from their environment and give their metabolic residues to this environment. If the substance to be passed through the cell is of a size that can pass through the membrane, it can pass in two ways; passive and active transport.

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Passive Transport

Passive transport is the phenomenon that the substances pass directly through the cell membrane, thanks to the density difference without wasting energy. All living and inanimate cells can do passive transport. Passive transport is divided into two as diffusion and osmosis.

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1. Diffusion:

Movement of molecules from very dense to less dense media is called diffusion. Energy and enzyme are not used. This phenomenon is only seen in molten materials and gases. The diffusion rate in solid state is almost negligible. Some factors affect the diffusion rate, these are;

• As the density difference between the two media increases, the diffusion rate increases.
• Temperature and motion accelerate diffusion.
• The diffusion rate increases as the number of pores and the width of the pore increases in the cell membrane.
• As the molecular size of the diffusing substance increases, diffusion slows down.
• The pressure difference accelerates diffusion.

Diffusion

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2. Osmosis:

Osmosis is the transition of solvents from a low-dense medium to a very dense medium, through a selectively permeable membrane, without wasting energy.

Osmosis

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If the dissolved substance concentration of the liquid in the cell is more than the cell, such liquids are called hypertonic compared to the cell, if less, it is called hypotonic, and if equal, isotonic.

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Plasmolysis: 

It is the shrinkage of the plant cell by losing water when it is placed in a more dense (hypertonic) environment than itself. If this shrinkage persists for a long time, the plant cell will die.

Plasmolysis

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Deplasmolysis: 

When the plasmolized cell is placed in a less dense (hypotonic) environment than itself, it swells by taking water. For this reason, the phenomenon of plasmolysis normally does not cause cell death, but if it remains in this state, the cell will eventually die.
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Hemolysis

If the animal cell waits for a long time in a hypotonic environment, the turgor pressure applied to the cell membrane increases with the effect of water taken into the cell, and the cell cannot withstand the increasing pressure after a while and the cell bursts. This event is called hemolysis. Since the elastic pressure of the cell wall outside the cell membrane in plant cells compensates for osmotic pressure, there is no burst.

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Active Transport

• Active transport is a form of transport that opposes diffusion. In active transport, a substance is transported from the low density side to the high density side under the control of the cell membrane. The energy required for active transport is provided from ATP.
• Although active transport stops in a cell, passive transport may continue. In this case, while the cell is alive, the density difference of some substances between the outer and the inner of the cell may disappear. As a result, death becomes inevitable for the cell.
• Active transport is an essential event in providing ions and necessary anion-cation concentrations that are specific for cell functions in the cell.

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Endocytosis

It is the method of absorbing molecules that are too large to pass through the cell membrane by diffusion or active transport. If the substance taken into the cell with endocytosis is solid, it is called phagocytosis and if it is liquid, it is called pinocytosis.

Endocytosis

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Phagocytosis

In this case, the particle to be ingested comes onto the plasma membrane. The false feet formed by the cell are turned over the particle, resulting in a vacuole with a foreign particle inside.

Phagocytosis occurs in a large number of cellular animals and some multicellular cells. In a cell, the nutrition of the cell is done in this way and a food vacuole is created with the membrane that turns the part inside. But in multicellular phagocytosis is a defensive event. For example, it is in this way that white blood cells destroy microbes in our body.

Pinocytosis

It is the intake of liquids. In pinocytosis, the circumference of the fluid to be taken with active extensions of the membrane, such as fringe, is wrapped and a vacuole occurs in the cell. The phenomenon of pinocytosis has been well demonstrated in the amoeba.

Mikropinositoz

The ions coming to the cell surface are taken into a structure surrounded by the membrane by the collapse of the membrane.

Exocytosis

It is the mode of transport that allows large molecules inside the cell to be thrown out of the cell. Non-digestible waste of molecules inside the cell is brought to the cell membrane in vacuole and excreted by the merger of the vacuole membrane and the cell membrane. Vacuole membrane is opened from the joint and discharges waste materials.

Exocytosis

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The Original Author: Mrs. Şerife (Erden) SARICA

Cell and Cell Structure

Cell

The cell is the smallest unit of a living thing that can display structural and functional features. The presence of the cell began to be recognized by the discovery of the microscope. Robert Hooke identified the cell in 1665 in the section he received from the cork.

With the development of the microscope, information about the cell has developed and Cell Theory has emerged. According to cell theory:

• The basic structure and task unit of living things are cells.
• All living things are made up of one or more cells.
• Cells are independent, but can also join the division of labor.
• New cells are formed by dividing existing cells. With cell division, hereditary substances in the parent cells are transferred to the offspring.
• All metabolic events take place in the cell.

Animal Cell

Plant Cell

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Types of Cells

Cells are divided into two as prokaryotes and eukaryotes according to their structure.

1. Prokaryotic Cell: Bacteria, cyanobacteria and arcs have a prokaryotic cell structure. All prokaryotic organisms are one-celled. Bacteria and arches can only be made from prokaryotic living things. In prokaryotic cells;

• They have no nucleus.
• Organelles surrounded by membrane are not found, only the membrane-free ribosome is found.
• It has cell wall, cell membrane, cytoplasm, ribosome and nucleic acids (DNA, RNA).
• Its genetic material is distributed throughout the cytoplasm.

Prokaryotic Cell

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2. Eukaryotic Cell: Cells with nuclear membranes and membrane organelles. Plants, animals, fungi and animals in the protozoans have a eukaryotic cell structure.

• They have nucleus.
• It has organelle with and without membranes.
• It can be found in single-celled and multi-celled organisms.

Eukaryotic Cell

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Shape of Cells

The shape of the cells varies according to the work it does and where it is located. Because there is a close relationship between its structure and function. For example, the immobile egg cell is generally spherical, while the mobile sperm cell is long and flagellated. The white blood cells change their shape as they make the amoeba move. Tissue cells can be cubic, cylindrical, prismatic or flat under pressure.

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Size of cells

The size of the cell varies. The egg cell, which is the largest cell of human, is 200 microns in diameter. In addition, some of the nerve cells can be 1 to 1.5 meters tall with their axons. Red blood cells are 7.5 microns in diameter. Cell size is not related to body size. For example, the kidney and liver cells of elephant and mouse are the same size. The size of the organism is related to the excess number of cells.

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Number and Color of Cells

The number of cells in an adult varies with body size. A person has an average of 1013-1014 cells, excluding blood cells. In all vertebrate animals and humans, the number of cells of some organs (such as the central nervous system, retina, corti organ) is fixed. It does not reproduce later, only its volumes increase.

The cells are mostly colorless. However, some cells may appear in yellow, brown, black and other colors, depending on the pigment type in their cytoplasm.

The consistency of the cell also varies according to the cell type. This consistency is determined by the water and colloid substances it carries. For example, although the brain cells are very soft, some cells of the skin that are exposed to external effects are hard.

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Chemical Structure of the Cell

Inorganic and organic substances are found in the structure of a living cell.

Inorganic Substances:

Water: It plays an important role in the cell's vital activities. There are different amounts of water in the cells depending on the type of living being, the task of the cell, whether it is old or young.

Elements and Inorganic Compounds:

C, H, O, N, K, Ca, Mg, Fe, S and P are the basic elements of the cytoplasm. In the absence of these elements, the cell cannot fully function. For example, hemoglobin in Fe deficiency and chlorophyll cannot occur in Mg deficiency. Inorganic substances in living cells are acids, bases and salts. they are ionized in cell water. Inorganic substances adjust the cell's pH balance and osmotic pressure.

Organic Matters:

Organic substances that make up the cell; carbohydrates, lipids, proteins, nucleic acids and enzymes.

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Cell Structure

Cells are basically examined in three parts.

1. Cell membrane
2. Cytoplasm and cell organelles
3. Nucleus

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Cell Structure

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The Original Author: Mrs. Şerife (Erden) SARICA