Human Anatomy and Physiology: The Cell, Transport and Protein Synthesis
The cell is the building blocks of our bodies. They are incredibly complex structures consisting of many parts. They have many important processes as well such as Transport and Protein Synthesis that are key to running the cell.
Generalized Cells
Generalized cells are cells that have the basic parts that are common amongst all cells. Generally, all cells are made up of three main regions: the nucleus, the cytoplasm and a plasma membrane. The nucleus is found towards the center of the cell. It is surrounded by the cytoplasm, which is then surrounded by the plasma membrane. Each of these regions contain their own unique structures/organelles.
Generalized Cells — Organelles and Structures
Nucleus
The control center of the cell is known as the nucleus. The nucleus contains the genetic material that is known as deoxyribonucleic acid (DNA). DNA is like a blueprint that has all the instructions needed for building protiends. DNA is also necessary for cell reproduction. While typically oval or spherical, the nucleus usually changes shape depending in the shape of the cell.
Nucleus — Nuclear Envelope
The nucleues has a double membrane barrier. This membrane is called to nuclear envelope, or nuclear membrane. Between the two membranes is a fluid filled space. At numerous points, the layers fuse and nuclear pores form in the fused regions. Like othe rcellular membranes, the nuclear membrane allows some but not all substances to pass through it. The nuclear membrane encloses a fluid called nucleoplasm.
Nucleus — Nucleoli
The nucleus contains at least one small round bodies called nucleoli. Nucleoli are where ribosomes are created. These ribosomes then will eventually migrate to the cytoplasm so they can act as sites of protein synthesis.
Nucleus — Chromatin
When the cell is not dividing, the DNA is combining with protein and a network forms chromatin which is then scattered throughout the nucleus. When the cell is divining, the chromatin coils and condenses to form rodlike bodies caled chromosomes.
Plasma Membrane
The plasma membrane is a transparent barrier that contains the cell contents and separates it from the surrounding environment. The plasma membrane is made of a phospholipid bilayer in which the two layer of lipids (fats) are arranged tail to tail in which the protien molecules float. The protiens for a constantly cchangning pattern withing the membrane. A majority of the lipids in these layers are phospholipids, hence then name. The polar phospholipid heads are hydrophilic (“water loving”, will easily dissolve or mix with water), and are attracted to water which is the primary component of fluid both in and out of the cell so they lie on both the inner and outer surfaces of the membrane. The tails are hydrophobic (“water hating”) and avoid water so they line up on the centre of the membrane. Because of the properties, the phospholipids are self orientating so when torn the membrane can quickly reseal itself.
The proteins found in the bilayer are important to the specialized functions of the membrane. Some of the proteins are enzymes while some protrude from the cell to act as receptors for chemical messengers such as hormones. Others may act as binding sites to anchor the cell to a certain place or to certain structures.
However, most of the proteins in the membrane are involved in transport. For example, some of these proteins can cluster together and form protein channels. These channels allow water and water soluble molecules or ions to pass through. Some protiens also act as carriers. This means they bind to a substance and move it through the membrane. Branching sugar proteins called glycoproteins are found attached to most proteins on the extracellular space).
Plasma Membrane — Membrane Junctions
While some cell types are “loose” and not interconnected, many types are knit together tightly. These types of cells are bound together in three possible ways: glycoproteins acting as an adhesive, membranes of adjacent cells fitting together in a “tongue and groove” fashion, and special membrane junctions being formed. These junctions are the most important of the three. There are three types of junctions:
Tight Junctions:
These are impermeable junctions that use leakproof sheets to bind cells together and prevent substances from passing thourgh the space between cells. The plasma membrane fuses together to form this sheet.
Desmosomes:
Desmomes are anchoring judgment that are found along the sides of adjacent cells. These junctions protect the cell from mechanical stress such as being pulled apart. Structurally they are like buttons of adjacent plasma membranes. Thick protein filaments extend between the adjacent plasma membranes forming a system of strong wire.
Gap Junctions:
This type of junction is primarily found in the heart. In these types of junctions, the neighbouring cells are connected by hollow cylinders made of proteins. These proteins are called connexons. The cylidnders go across the entire width of the neighbouring membranes. Chemical molecules are able to pass through these connexon channels between cells.
Cytoplasm
The cytoplasm is the cellular material that lies outside of the nucleus but is still in the plasma membrane. Most cellular activities occur here. The cytoplasm consists of three major elements. These are the cytosol, organelles and inclusions. The cytosol is the fluid in the cytoplams that all the other elements reside in. The cytosol contains nutrients and other dissolved substances. The organelles are like the machinery of the cell. Each organelle has a specialized role within the cell that is key to keep the cell running. Inclusions are certain chemical substances the could be present in the cell depending on the cell type. This can include various things such as pigments for skin cells or secretory products.
Cytoplasm — Mitochondria
The mitochondria is a organelle that is typically depicted to be like a sausage. However, the mitochondria actually squirms and lengthens and changes constantly. The mitochondrial wall consists of two plasma membranes. The outer one is smooth while the inner on is full of protrusions called cristae.
Reactions in which oxygen is used to break down foods are caused by enzymes dissolved in the mitochondria’s fluid as well as enzymes the are part of the cristae membranes. When these foods are broken down, energy is released in the form of heat and some of the energy is captured and used to make ATP (Adenosine Triphosphate) molecules. ATP molecules are kind of like the energy currency of the body. ATP provides the energy required for all cellular work. Since the mitochondria is the primary source of ATP, it is often called the “powerhouse” of the cell.
Cytoplasm — Ribosomes
Ribosomes are small dark bodies that consist of proteins and a form of RNA called ribosomal RNA. Ribosomes are the sites of protiend synthesis in the cell. Ribosomes can be found either floating around in the cell or attatched to the rough ER.
Cytoplasm — Endoplasmic Reticulum
The endoplasmic reticulum (ER) is a system of tube like structures that flow throughout the cytoplasm. The ER makes up approximately half of the cells membranes. One of its purposes is to have a circulatory system type function. It acts as a channel for the transport of various substances (mainly proteins) to different parts fo the cell. There are two types of endoplasmic reticulum.
Rough endoplasmic reticulum (rough ER) is called this because it is studded with ribosomes. As most of the building blocks of cellular membranes are built here, the rough ER is often thought of as the cell’s membrane factory. The proteins made here are transported through the tubules of the rough ER, in which they get folded into functional three-dimensional shapes that are then transported to other parts of the cell in transport vesicles.
The smooth ER plays no role in protein synthesis, but is responsible for lipid metabolism (breakdown and synthesis of cholesterols and fats), as well as the detoxification of drugs and pesticides. Its for this reason that the liver cells are full of smooth ER.
Cytoplasm — Golgi Apparatus
The golgi apparatus looks much like a stack of flattened sacks along with many small vesicles. It is found close to the nucleus and it acts as a traffic man for cellular proteins. Its main function is modifying and packaging proteins in certain ways depending on where they are to be transported.
As proteins that are prepped for transport accumulate, the sacs of the golgi apparatus start to swell. The swollen ends are then pinched off and form secretory vesicles. These vesicles now travel to the plasma membrane. Once the vesicles reach the plasma membrane, they fuse with it, the membrane ruptures and the proteins are ejected out of the cell.
Cytoplasm — Lysosomes
Lysosomes are membraneous bags that appear in varying sizes. These bags contain powerful digestive enzymes. These enzymes are powerful enough to even digest worn out or non-usable cells as well as any foreign substance that enters the cell, lysosomes function as the cell’s demolition sites.
Cytoplasm — Peroxisomes
Peroxisomes are membraneous sacs containing powerful enzymes called oxidase. These enzymes use molecular oxygen to detoxify numerous harmful substances such as alcohol and formaldehyde. However, the main function is to disarm dangerous free radicals.
Free radicals are very reactive chemicals with unpaired electrons that have the ability to scramble the structure of proteins and nucleic acids. These free radicals are normal byproducts of cellular metabolism but if they accumulate they can have extremely horrible consequenses for the cell. Peroxisomes convert free radicals into hydrogen peroxide (H2O2). An enzyme called catalase converts excess hydrogen peroxide into water.
Cytoplasm — Cytoskeleton
The cytoskeleton is an elaborate network of protein structures. The cytoskeleton acts like the cells bones and muscles by determining cell shape, supporting other cell organelles and provides the machinery for cellular transport.
The cytoskeleton is made up of a variety of pieces which are: microtubules, intermediate filaments and microfilaments. Intermediate filaments are key to the cell as they provide internal wires to resist pulling forces on the cell. Microfilaments (ex. actin and myosin) are mainly used for cell mobility and in producing changes in cell shape. Microtubules are important as they determine the overall shape of a cell and the distribution of organelles. Microtubules are very important in cell division.
Transport in Cells
Intracellular fluid (nucleoplasm and cytosol) is a solution that contains certain amounts of substances such as gases, salts and nutrients that are all dissolved in water. Interstitial fluid is the fluid is the fluid outside of the cell and this fluid is full of stuff nutrients like amino acids and sugars, regulatory substances like hormones, as well as waste products. Cells have to extract the things they need from this fluid in order to remain healthy. The measure of the amount of nutrients and other substances in the solution or in the cell is called the solute concentration.This is done through the plasma membrane. The plamsa membrane is selectively permeable. This means that it only allows certain substances to pass through it meaning it can allow the nutrients to enter the cell while keeping any undesirable substances out.This also works the other way with the membrane keeping the important proteins within the cell while letting the waste products out.
Substances can pass through through the plasma membrane in two main ways: passively or actively. Passive transport is when the substances are transported across the membrane without using any of the cell’s energy. In active transport, the cell provides the energy (ATP) that drives the process.
Passive Transport
The main form of passive transport is diffusion. Diffusion is the process in which molecules will move to a region of low concentration from a region of high concentration. The key concept driving this process is kinetic energy.
All molecules contain kinetic energy. This energy causes the molecules to move about randomly at very high speeds and they often collide and change direction. This causes the molecules to generally travel to areas of lower concentration as there are fewer molecules to collide with.
Molecules will be able diffuse through the membrane if any of the following statements are true:
- The molecule is small enough to pass through pores in the membrane
- The molecules are lipid soluble
- The molecules are assisted by a membrane carrier
There are four main forms of diffusion.
Osmosis:
Osmosis is the diffusion of water across a selectively permeable membrane like the plasma membrane. Water is highly polar, meaning is it repelled by the lipid core of the plasma membrane. However, there are special pores known as aquaporins that are created by certain protiens in the membrane. These pores allow water to pass through easily. There are some dangers when it come to water in cells.
There are some dangers relating to the movement of water in cells. The ability of an extracellular solution to make water move into or out of a cell by osmosis is known as its tonicity. The terms hyerptonic, hypotonic, and isotonic are used to describe whether a solution will cause water to move into or out of a cell. A solution is hypertonic to the cell if the solute concentration of the solution is higher than the solute concentration inside the cell. When a cell is placed in a hypertonic solution, the water rushes out to the area of low density, causing the cell to lose volume and shrink. In a hypotonic solution, the solute concentration in the solution is lower than that of the cell. If the solutes are not able to cross the membrane, the solution is hypotonic to the cell and causes a rush of water into the cell, causing the cell to swell and potentially burst. Finally, an isotonic solution is when the solute concentration is the same both out and in the cell. This means there is no flow of water and the cell remain stable.
Simple Diffusion:
Simple diffusion is the unassisted diffusion of solutes through the plasma membrane (or any similar selectively permeable membrane). These molecules are small enough to pass through the pores that are found over the surface of the membrane.
The other two types are forms of facilitated diffusion. Facilitated diffusion is when a passage is provided for certain necessary substances that are both lipid insoluble and too large to pass through membrane pores. In facillitated diffusion, a protien membrane channel is used or a protien carrier molecule is neded to act as a transport vehicle. Carrier-mediated facilitated diffusion is when a protein carrier for a specific type of molecule binds to the molecule. Channel-mediated facilitated diffusion occurs through a channel protein and typically involves ions that are selected based on their size or charge.
Active Membrane Transport
Active membrane transport is when a cell uses ATP to help with moving substances across the membrane. Substances moved actively are typically unable to pass in the desired direction through diffusion. This can be for a number of reason. For example, the molecule could be too large, or it may have to move against the concentration gradient or they may not be lipid soluble. There are two forms of active membrane transport: active transport (also known as solute pumping) and vesicular transport.
Active Transport:
Active transport is similar to the facilitated diffusion processes mentioned above as both need protein carriers to combine with the substances to be transported. However, while facilitated diffusion moves utilizing kinetic energy, active transport uses ATP to energize the protein carriers (called solute pumps). Amino acids, some sugars and many ions are transported by solute pumps. As these typically are moving against the concentration gradient, they need ATP to power them through. One example of such a pump is the sodium-potassium pump.
The sodium-potassium pump carries sodium ions out of the cell and potassium ions into the cell. This pump is incredibly important to the transmission of impulses in nerve cells. There is usually more cells outside of the cell than in so they would remain in the cell unless forced out. Similarly, there are more potassium ions inside the cells that there are outside and any potassium ions that managed to leak out have to be actively pumped back in.
Vesicular Transport:
Vesicular transport is for substances that cannot get through the membrane by and of the mentioned methods. Using ATP, vesicular transport moves substances in and out of cells without actually moving them through the membrane. There are two types of vesicular transport: exocytosis and endocytosis.
Exocytosis is the movement of substances out of the cell. This process is the key to how cells are able to actively secrete hormones mucus and other cell products as well as how they are able eject cellular wastes. The product that is to be released is first pacaged into a membranous sac known as a vesicle. The vesicle then moves towards the plasma membrane and fuses with it. The sac then ruptures, causing the contents to spill out of the cell. The fusing on the vesicle to the plasma membrane involves transmembrane proteins on the vesicles. These transmembrane proteins recognize specific plasma membrane proteins adn bind with them. This binding causes the membranes to fuse.
Endocytosis includes the processes that use ATP and engulf extracellular substances by enclosing them in a small vesicle. After the vesicle is formed, it detaches from the plasma membrane and moves into the cytoplasm. From there, it typically fuses with a lysosome and the contents of the vesicle are then digested by the enzymes in the lysosome.
If the engulfed substance is large, the cell separates them from the environment by utilizing flowing cytoplasmic extensions called pseudopods (comes from latin; “fake feet”). The entire process is known as phagocytosis (literally meaning “cell eating”).
Another form of endocytosis that involves engulfing extracellular fluid instead of molecules or substances is pinocytosis. Pinocytosis is a process that starts by the plasma membrane indenting. This indent forms a small pit . The edges of the pseudopods then fuse around the droplet of extracellular fluid containing dissolved proteins or fats.
The main method for taking specific smaller molecules is receptor-mediated endocytosis. In this, receptor proteins on the plasma membrane bind only with certain substances. Once the molecules have attached to the receptor proteins, they are engulfed in a vesicle and brought into the cell.
Protein Synthesis
Proteins are key parts of a cell’s life. Proteins are the cell’s main building blocks. Some proteins become enzymes that are catalysts for every chemical reactions in the cells. Believe it or not, DNA is responsible for the structure of these enzymes that are needed in every chemical reaction the cell has.
DNA is made so that information is encoded in the sequence of bases that run along each side of the DNA molecule. There are 4 different bases that are used many times in each DNA molecule. These are adenine, cytosine, guanine and thymine (A, C, G, T). These bases are made so that in the two strands, adenine and thymine are always paired together while cytosine and guanine are always paired together. Each sequence of three bases represent a certain amino acid (the building blocks for proteins that are assembled during protien synthesis). For example, the sequence CCT calls for the protein glycine. Different variations of these four bases allow the cell to make all the different types of proteins it needs. It has been estimated that a single gene has between 300 and 3000 base pairs in sequence.
However, DNA by itself is pretty useless as its just a set of information and the ribosomes that are known as protein factories are found in the cytoplasm while DNA never leaves the nucleus. This means the cell needs a method of decoding DNA and then a way to transport that information to the ribosomes where proteins can then be built using it. This decoder and transporter is called ribonucleic acid or RNA. RNA is another type of nucleic acid like DNA but it is by no means the same thing. RNA differs from DNA in a number of ways but some key differences are that it is single stranded as opposed to DNA’s double helix, its sugar is ribose rahter than deoxyribose, and instead of the base called thymine (T), RNA has uracil (U).
There are three types of RNA used in protein synthesis. Transfer RNA (tRNA), ribosomal RNA (rRNA) which helps for the ribosomes and messenger RNA (mRNA) which are long single nucleotide strands that resemble half of a DNA molecule.
Protein synthesis consists of two major steps: transcription and translation. Transcription is when mRNA that is complementary to the desired DNA strand is made at the DNA gene and translation is when the information in the mRNA molecule is decoded and used to assemble proteins at the ribosomes.
Transcription:
Transcription is the transfer of information from the DNA’s base onto a piece of complementary mRNA sequence. The only nucleic acids involved in transcription are DNA and mRNA. On the DNA sequence, each set of three bases corresponding to an amino acid is called a triplet and the respective sets on the mRNA are called codons. Since the mRNA is reflecting the DNA strand, it would be the exact opposite of the DNA. For example, if the DNA strand was AAT-TCG-CTG, the mRNA strand would be UUA-AGC-AGC.
Translation:
During translation, the base sequence found on is converted into an amino acid sequence. Translation occurs in the cytoplasm and uses all three mentioned types of RNA. After the mRNA has attached to the ribosome, tRNA then starts its work. The job of tRNA is to transport amino acids to the ribosome where the enzymes in the ribosome binds together the amino acid in the sequence specified by the mRNA. The tRNA also is able to recognize what the mRNA codons are referencing. They do this through their own three base sequence called an anticodon that is on their head. This anticodon can bind to the complimentary codon. Once the first tRNA has placed itself in the correct position on the mRNA, the ribosome moves the mRNA along and brings on the next codon in position so its ready for the next tRNA. As amino acids are brought to their correct positions along the mRNA, they are joined by enzymes. As an amino acid bonds to the chain, its tRNA is released and moves away to go and collect another amino acid. Then the last codon is read, the protein is released.
