A cell is the smallest unit of a living thing. A living thing, whether made of one cell (like bacteria) or many cells (like a human), is called an organism. Thus, cells are the basic building blocks of all organisms.
Several cells of one kind that interconnect with each other and perform a shared function form tissues, several tissues combine to form an organ (your stomach, heart, or brain), and several organs make up an organ system (such as the digestive system, circulatory system, or nervous system). Several systems that function together form an organism (like a human being). Here, we will examine the structure and function of cells.
Several cells of one kind that interconnect with each other and perform a shared function form tissues, several tissues combine to form an organ (your stomach, heart, or brain), and several organs make up an organ system (such as the digestive system, circulatory system, or nervous system). Several systems that function together form an organism (like a human being). Here, we will examine the structure and function of cells.
Microscopy
Cells vary in size. With few exceptions, individual cells cannot be seen with the naked eye, so scientists use microscopes (micro- = “small”; -scope = “to look at”) to study them. A microscope is an instrument that magnifies an object. Most photographs of cells are taken with a microscope, and these images can also be called micrographs.
Cells vary in size. With few exceptions, individual cells cannot be seen with the naked eye, so scientists use microscopes (micro- = “small”; -scope = “to look at”) to study them. A microscope is an instrument that magnifies an object. Most photographs of cells are taken with a microscope, and these images can also be called micrographs.
Figure 1 . (a) Most light microscopes used in a college biology lab can magnify cells up to approximately 400 times and have a resolution of about200 nanometers. (b) Electron microscopes provide a much higher magnification, 100, 000x, and a have resolution of 50 picometers. (credit a:modification of work by "GcG"/wikimedia commons; credit b: modification of work by evan )
Light microscopes
Most student microscopes are classified as light microscopes. Visible light passes and is bent through the lens system to enable the user to see the specimen. Light microscopes are advantageous for viewing living organisms, but since individual cells are generally transparent, their components are not distinguishable unless they are colored with special stains. Staining, however, usually kills the cells.
Electron microscopes
In contrast to light microscopes, electron microscopes use a beam of electrons instead of a beam of light. Not only does this allow for higher magnification and, thus, more detail , it also provides higher resolving power. Living cells cannot be viewed with an electron microscope. As you might imagine, electron microscopes are significantly more bulky and expensive than light microscopes.
Cell size
At 0.1–5.0 µm in diameter, bacteria cells are significantly smaller than plant or animal cells, which have diameters ranging from 10–100 µm. As a cell becomes larger, it becomes more and more difficult for the cell to acquire sufficient materials to support the processes inside the cell.
Most student microscopes are classified as light microscopes. Visible light passes and is bent through the lens system to enable the user to see the specimen. Light microscopes are advantageous for viewing living organisms, but since individual cells are generally transparent, their components are not distinguishable unless they are colored with special stains. Staining, however, usually kills the cells.
Electron microscopes
In contrast to light microscopes, electron microscopes use a beam of electrons instead of a beam of light. Not only does this allow for higher magnification and, thus, more detail , it also provides higher resolving power. Living cells cannot be viewed with an electron microscope. As you might imagine, electron microscopes are significantly more bulky and expensive than light microscopes.
Cell size
At 0.1–5.0 µm in diameter, bacteria cells are significantly smaller than plant or animal cells, which have diameters ranging from 10–100 µm. As a cell becomes larger, it becomes more and more difficult for the cell to acquire sufficient materials to support the processes inside the cell.
Figure 2. This figure shows relative sizes of microbes on a logarithmic scale (recall that each unit of increase in a logarithmic scale represents a 10-fold increase in the quantity being measured).
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Organelles:
The cell membrane- All cells have a membrane made up of a phospholipid bilayer with embedded proteins that separates the internal contents of the cell from its surrounding environment. The cell membrane controls what goes into or out of the cell. Some substances are actively controlled by the cell (such as organic molecules, ions, etc.). Other materials are passed freely through the cell membrane. |
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The cytoplasm -
The cytoplasm is the gel-like fluid inside the cell. It holds the organelles and various chemicals and molecules. Even though the cytoplasm consists of 70 to 80 percent water, it has a semi-solid consistency, which comes from the proteins within it. However, proteins are not the only organic molecules found in the cytoplasm. Glucose and other simple sugars, amino acids, and fatty acids, are found there too. Ions of sodium, potassium, calcium, and many other elements are also dissolved in the cytoplasm. Many chemical reactions, including protein synthesis, take place in the cytoplasm.
The nucleus -
Typically, the nucleus is the most visible organelle in a cell . The nucleus (plural = nuclei) houses the cell’s DNA in the form of chromosomes and directs the synthesis of ribosomes and proteins.
Mitochondria -
Mitochondria (singular = mitochondrion) are often called the “powerhouses” or “energy factories” of a cell because they are responsible for cellular respiration.
The cell wall -
If you examine, the diagram of a plant cell, you will see the cell wall. The cell wall is a rigid covering that protects the cell, provides structural support, and gives shape to the cell. The major organic molecule in the plant cell wall is cellulose, a carbohydrate made up of glucose. Have you ever noticed that when you bite into a raw vegetable, like celery, it crunches? That’s because you are tearing the rigid cell walls of the celery cells with your teeth.
Chloroplasts -
Like the mitochondria, chloroplasts have their own DNA and ribosomes, but chloroplasts have an entirely different function. Chloroplasts are plant cell organelles that carry out photosynthesis. Photosynthesis is the series of reactions that use carbon dioxide, water, and light energy to make glucose and oxygen. This is a major difference between plants and animals; plants are able to make their own organic molecules, like sugars, while animals must ingest their food.
The central vacuole -
Vacuoles are essential components of plant cells. If you look at, you will see that plant cells each have a large central vacuole that occupies most of the area of the cell. Have you ever noticed that if you forget to water a plant for a few days, it wilts? That’s because as the water concentration in the soil becomes lower than the water concentration in the plant, water moves out of the central vacuoles and cytoplasm. As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the cell walls of plant cells results in the wilted appearance of the plant.
This text is adapted under a Creative Commons 4.0 license. You can find the original source here.
The cytoplasm is the gel-like fluid inside the cell. It holds the organelles and various chemicals and molecules. Even though the cytoplasm consists of 70 to 80 percent water, it has a semi-solid consistency, which comes from the proteins within it. However, proteins are not the only organic molecules found in the cytoplasm. Glucose and other simple sugars, amino acids, and fatty acids, are found there too. Ions of sodium, potassium, calcium, and many other elements are also dissolved in the cytoplasm. Many chemical reactions, including protein synthesis, take place in the cytoplasm.
The nucleus -
Typically, the nucleus is the most visible organelle in a cell . The nucleus (plural = nuclei) houses the cell’s DNA in the form of chromosomes and directs the synthesis of ribosomes and proteins.
Mitochondria -
Mitochondria (singular = mitochondrion) are often called the “powerhouses” or “energy factories” of a cell because they are responsible for cellular respiration.
The cell wall -
If you examine, the diagram of a plant cell, you will see the cell wall. The cell wall is a rigid covering that protects the cell, provides structural support, and gives shape to the cell. The major organic molecule in the plant cell wall is cellulose, a carbohydrate made up of glucose. Have you ever noticed that when you bite into a raw vegetable, like celery, it crunches? That’s because you are tearing the rigid cell walls of the celery cells with your teeth.
Chloroplasts -
Like the mitochondria, chloroplasts have their own DNA and ribosomes, but chloroplasts have an entirely different function. Chloroplasts are plant cell organelles that carry out photosynthesis. Photosynthesis is the series of reactions that use carbon dioxide, water, and light energy to make glucose and oxygen. This is a major difference between plants and animals; plants are able to make their own organic molecules, like sugars, while animals must ingest their food.
The central vacuole -
Vacuoles are essential components of plant cells. If you look at, you will see that plant cells each have a large central vacuole that occupies most of the area of the cell. Have you ever noticed that if you forget to water a plant for a few days, it wilts? That’s because as the water concentration in the soil becomes lower than the water concentration in the plant, water moves out of the central vacuoles and cytoplasm. As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the cell walls of plant cells results in the wilted appearance of the plant.
This text is adapted under a Creative Commons 4.0 license. You can find the original source here.
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Transport across the cell membrane
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Diffusion
Small molecules can pass through the cell membrane through a process called diffusion. Diffusion is the movement of molecules from an area where there is a higher concentration(larger amount) of the substance to an area where there is a lower concentration (lower amount) of the substance. The amount of a substance in relation to the total volume is the concentration. During diffusion, molecules are said to flow down their concentration gradient, flowing from an area of high concentration to an area of low concentration. Molecules flowing down a concentration gradient is a natural process and does not require energy. Diffusion can occur across a semipermeable membrane, such as the cell membrane, as long as a concentration gradient exists. Molecules will continue to flow in this manner until equilibrium is reached. At equilibrium, there is no longer an area of high concentration or low concentration, and molecules flow equally in both directions across the semipermeable membrane. At equilibrium, equal amounts of a molecule are entering and leaving a cell. |
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Osmosis
The diffusion of water across a membrane because of a difference in concentration is called osmosis. Let's explore three different situations and analyze the flow of water.
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Applications of Osmosis
How do marine animals keep their cells from shrinking? How do you keep your blood cells from bursting? Both of these questions have to do with the cell membrane and osmosis. Marine animals live in salt water, which is a hypertonic environment; there is more salt in the water than in their cells. To prevent losing too much water from their bodies, these animals intake large quantities of salt water and then secrete the excess salt. Red blood cells can be kept from bursting or shriveling if put in a solution that is isotonic to the blood cells. If the blood cells were put in pure water, the solution would be hypotonic to the blood cells, so water would enter the blood cells, and they would swell and burst
How do marine animals keep their cells from shrinking? How do you keep your blood cells from bursting? Both of these questions have to do with the cell membrane and osmosis. Marine animals live in salt water, which is a hypertonic environment; there is more salt in the water than in their cells. To prevent losing too much water from their bodies, these animals intake large quantities of salt water and then secrete the excess salt. Red blood cells can be kept from bursting or shriveling if put in a solution that is isotonic to the blood cells. If the blood cells were put in pure water, the solution would be hypotonic to the blood cells, so water would enter the blood cells, and they would swell and burst
This text is adapted under a Creative Commons 4.0 license. You can find the original source here.
Facilitated Diffusion
What happens if a substance needs assistance to move across or through the plasma membrane? Facilitated diffusion is the diffusion of solutes through transport proteins in the plasma membrane. Facilitated diffusion is a type of passive transport. Even though facilitated diffusion involves transport proteins, it is still passive transport because the solute is moving down the concentration gradient.
Small nonpolar molecules can easily diffuse across the cell membrane. However, due to the nature of the lipids that make up cell membranes, polar molecules (such as water) and ions cannot do so. Instead, they diffuse across the membrane through transport proteins. A transport protein completely spans the membrane, and allows certain molecules or ions to diffuse across the membrane. Channel proteins, gated channel proteins, and carrier proteins are three types of transport proteins that are involved in facilitated diffusion.
A channel protein, a type of transport protein, acts like a pore in the membrane that lets water molecules or small ions through quickly. Water channel proteins (aquaporins) allow water to diffuse across the membrane at a very fast rate. Ion channel proteins allow ions to diffuse across the membrane.
A carrier protein is a transport protein that is specific for an ion, molecule, or group of substances. Carrier proteins "carry" the ion or molecule across the membrane by changing shape after the binding of the ion or molecule. Carrier proteins are involved in passive and active transport.
What happens if a substance needs assistance to move across or through the plasma membrane? Facilitated diffusion is the diffusion of solutes through transport proteins in the plasma membrane. Facilitated diffusion is a type of passive transport. Even though facilitated diffusion involves transport proteins, it is still passive transport because the solute is moving down the concentration gradient.
Small nonpolar molecules can easily diffuse across the cell membrane. However, due to the nature of the lipids that make up cell membranes, polar molecules (such as water) and ions cannot do so. Instead, they diffuse across the membrane through transport proteins. A transport protein completely spans the membrane, and allows certain molecules or ions to diffuse across the membrane. Channel proteins, gated channel proteins, and carrier proteins are three types of transport proteins that are involved in facilitated diffusion.
A channel protein, a type of transport protein, acts like a pore in the membrane that lets water molecules or small ions through quickly. Water channel proteins (aquaporins) allow water to diffuse across the membrane at a very fast rate. Ion channel proteins allow ions to diffuse across the membrane.
A carrier protein is a transport protein that is specific for an ion, molecule, or group of substances. Carrier proteins "carry" the ion or molecule across the membrane by changing shape after the binding of the ion or molecule. Carrier proteins are involved in passive and active transport.
Active Transport
During active transport, molecules move from an area of low concentration to an area of high concentration. This is the opposite of diffusion, and these molecules are said to flow against their concentration gradient. Active transport is called "active" because this type of transport requires energy to move molecules.
As molecules are moving against their concentration gradients, active transport cannot occur without assistance. A carrier protein is always required in this process. Like facilitated diffusion, a protein in the membrane carries the molecules across the membrane, except this protein moves the molecules from a low concentration to a high concentration. These proteins are often called "pumps" because they use energy to pump the molecules across the membrane. There are many cells in your body that use pumps to move molecules. For example, your nerve cells (neurons) would not send messages to your brain unless you had protein pumps moving molecules by active transport.
This text is adapted under a Creative Commons 4.0 license. You can find the original source here.
During active transport, molecules move from an area of low concentration to an area of high concentration. This is the opposite of diffusion, and these molecules are said to flow against their concentration gradient. Active transport is called "active" because this type of transport requires energy to move molecules.
As molecules are moving against their concentration gradients, active transport cannot occur without assistance. A carrier protein is always required in this process. Like facilitated diffusion, a protein in the membrane carries the molecules across the membrane, except this protein moves the molecules from a low concentration to a high concentration. These proteins are often called "pumps" because they use energy to pump the molecules across the membrane. There are many cells in your body that use pumps to move molecules. For example, your nerve cells (neurons) would not send messages to your brain unless you had protein pumps moving molecules by active transport.
This text is adapted under a Creative Commons 4.0 license. You can find the original source here.
