- Cells are the smallest functional units of the body.
- They are grouped together to form tissues, each of which has a specialised function.
e.g. blood, muscle, bone.
- Different tissues are grouped together to form organs.
e.g. heart, stomach, brain.
- Organs are grouped together to form systems.
e.g. the digestive system is responsible for taking in, digesting and absorbing food and involves a number of organs, including the
stomach and intestines.
- The human body develops from a single cell called the zygote, which results from the fusion of the ovum (female egg cell) and the spermatozoon (male germ cell).
Cell multiplication follows and, as the fetus grows, cells with different structural and functional specializations develop
STRUCTURE OF THE CELL
- A cell consists of a plasma membrane inside which there are a number of organelles floating in a watery fluid called cytosol
- Organelles are small structures with highly specialized functions, many of which are contained within a membrane.
- They include the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, microfilaments, and microtubules.
The cytosol (intracellular fluid) is the fluid portion of the cytoplasm that surrounds organelles and constitutes about 55% of total cell volume. Although it varies in composition and consistency from one part of a cell to another, cytosol is 75–90% water plus various dissolved and suspended components. Among these are different types of ions, glucose, amino acids, fatty acids, proteins, lipids, ATP, and waste products. Also present in some cells are various organic molecules that aggregate into masses for storage.
The cytosol is the site of many chemical reactions required for a cell’s existence.
For example, enzymes in cytosol catalyze glycolysis.
organelles are specialized structures within the cell that have characteristic shapes; they perform specific functions in cellular growth, maintenance, and reproduction.
The cytoskeleton is a network of protein filaments that extends throughout the cytosol. Three types of filamentous proteins contribute to the cytoskeleton’s structure, as well as the structure of other organelles. In the order of their increasing diameter, these structures are microfilaments, intermediate filaments, and microtubules.
These are the thinnest elements of the cytoskeleton. They are composed of the protein actin, Microfilaments have two general functions:
They help generate movement and provide mechanical support. With respect to movement, microfilaments are involved in muscle contraction, cell division, and cell locomotion.
Microfilaments provide much of the mechanical support that is responsible for the basic strength and shapes of cells.
Microfilaments also provide mechanical support for cell extensions called microvilli.
These filaments are thicker than microfilaments but thinner than microtubules. Several different proteins can compose intermediate filaments, which are exceptionally strong. They are found in parts of cells subject to mechanical stress, help stabilize the position of organelles such as the nucleus, and help attach cells to one another.
These are the largest of the cytoskeletal components and are long, unbranched hollow tubes composed mainly of the protein tubulin. The assembly of microtubules begins in an organelle called the centrosome. The microtubules grow outward from the centrosome toward the periphery of the cell. Microtubules help determine cell shape. They also function in the movement of organelles such as secretory vesicles, of chromosomes during cell division, and of specialized cell projections, such as cilia and flagella.
The centrosome, located near the nucleus, consists of two components: a pair of centrioles and pericentriolar material. The two centrioles are cylindrical structures, each composed of nine clusters of three microtubules (triplets) arranged in a circular pattern. Surrounding the centrioles is pericentriolar material which contains hundreds of ring-shaped complexes composed of the protein tubulin. These tubulin complexes are the organizing centers for growth of the mitotic spindle, which plays a critical role in cell division.
- Cilia (singular is cilium) are numerous, short, hairlike projections that extend from the surface of the cell.
- Each cilium contains a core of 20 microtubules surrounded by plasma membrane. The microtubules are arranged such that one pair in the center is surrounded by nine clusters of two fused microtubules (doublets). Each cilium is anchored to a basal body just below the surface of the plasma membrane.
- A cilium displays an oarlike pattern of beating.
- Many cells of the respiratory tract, for example, have hundreds of cilia that help sweep foreign particles trapped in mucus away from the lungs.
- The movement of cilia is also paralyzed by nicotine in cigarette smoke. For this reason, smokers cough often to remove foreign particles from their airways. Cells that line the uterine (fallopian) tubes also have cilia.
- Flagella (singular is flagellum) are similar in structure to cilia but are typically much longer.
- Flagella usually move an entire cell.
- A flagellum generates forward motion along its axis by rapidly wiggling in a wave-like pattern.
- The only example of a flagellum in the human body is a sperm cell’s tail, which propels the sperm toward the oocyte in the uterine tube
- Ribosomes are the sites of protein synthesis. The name of these tiny organelles reflects their high content of one type of ribonucleic acid, ribosomal RNA (rRNA),
- Structurally, a ribosome consists of two subunits, one about half the size of the other. The large and small subunits are made separately in the nucleolus.
- Once produced, the large and small subunits exit the nucleus separately, then come together in the cytoplasm.
- Some ribosomes are attached to the outer surface of the nuclear membrane and to an extensively folded membrane called the endoplasmic reticulum.
- These ribosomes synthesize proteins destined for specific organelles, for insertion in the plasma membrane, or for export from the cell. Other ribosomes are “free” or unattached to other cytoplasmic structures. Free ribosomes synthesize proteins used in the cytosol.
- Ribosomes are also located within mitochondria, where they synthesize mitochondrial proteins.
The endoplasmic reticulum or ER is a network of membranes in the form of flattened sacs or tubules. The ER extends from the nuclear envelope (membrane around the nucleus), to which it is connected, throughout the cytoplasm. The ER is so extensive that it constitutes more than half of the membranous surfaces within the cytoplasm of most cells.
Cells contain two distinct forms of ER, which differ in structure and function.
- Rough ER
- Smooth ER
- Rough ER is continuous with the nuclear membrane and usually is folded into a series of flattened sacs.
- The outer surface of rough ER is studded with ribosomes, the sites of protein synthesis. Proteins synthesized by ribosomes attached to rough ER enter spaces within the ER for processing and sorting.
- In some cases, enzymes attach the proteins to carbohydrates to form glycoproteins. In other cases, enzymes attach the proteins to phospholipids, also synthesized by rough ER.
- These molecules may be incorporated into the membranes of organelles, inserted into the plasma membrane, or secreted via exocytosis.
- Thus rough ER produces secretory proteins, membrane proteins, and many organellar proteins.
- Smooth ER extends from the rough ER to form a network of membrane tubules.
- Unlike rough ER, smooth ER does not have ribosomes on the outer surfaces of its membrane.
- However, smooth ER contains unique enzymes that make it functionally more diverse than rough ER.
- Because it lacks ribosomes, smooth ER does not synthesize proteins, but it does synthesize fatty acids and steroids, such as estrogens and testosterone.
- In liver cells, enzymes of the smooth ER help release glucose into the bloodstream and inactivate or detoxify lipid-soluble drugs or potentially harmful substances, such as alcohol, pesticides, and carcinogens (cancer-causing agents).
- In liver, kidney, and intestinal cells a smooth ER enzyme removes the phosphate group from glucose-6-phosphate, which allows the “free” glucose to enter the bloodstream.
- In muscle cells, the calcium ions (Ca2) that trigger contraction are released from the sarcoplasmic reticulum, a form of smooth ER.
- Most of the proteins synthesized by ribosomes attached to rough ER are ultimately transported to other regions of the cell.
- The first step in the transport pathway is through an organelle called the Golgi complex.
- It consists of 3 to 20 cisternae, small, flattened membranous sacs with bulging edges.
- The cisternae are often curved, giving the Golgi complex a cuplike shape.
- The cisternae at the opposite ends of a Golgi complex differ from each other in size, shape, and enzymatic activity.
- The convex entry or cis face is a cisterna that faces the rough ER.
- The concave exit or trans face is a cisterna that faces the plasma membrane.
- Sacs between the entry and exit faces are called medial cisternae.
- Transport vesicles from the ER merge to form the entry face. From the entry face, the cisternae are thought to mature, in turn becoming medial and then exit cisternae.
- Lysosomes are membrane-enclosed vesicles that form from the Golgi complex.
- Lysosomes inside contain as many as 60 kinds of powerful digestive and hydrolytic enzymes can break down a wide variety of molecules
- The lysosomal membrane also includes transporters that move the final products of digestion, such as glucose, fatty acids, and amino acids, into the cytosol.
- Lysosomal enzymes also help recycle worn-out cell structures.
- A lysosome can engulf another organelle, digest it, and return the digested components to the cytosol for reuse.
- In this way, old organelles are continually replaced. The process by which entire worn-out organelles are digested is called autophagy
- Lysosomal enzymes may also destroy the entire cell that contains them, a process is known as autolysis
- Autolysis occurs in some pathological conditions and also is responsible for the tissue deterioration that occurs immediately after death.
- Another group of organelles similar in structure to lysosomes, but smaller, are the Peroxisomes.
- Peroxisomes, also called microbodies, contain several oxidases, enzymes that can oxidize (remove hydrogen atoms from) various organic substances.
- For instance, amino acids and fatty acids are oxidized in peroxisomes as part of normal metabolism.
- In addition, enzymes in peroxisomes oxidize toxic substances, such as alcohol. Thus, peroxisomes are very abundant in the liver, where detoxification of alcohol and other damaging substances occurs.
- A by-product of the oxidation reactions is hydrogen peroxide (H2O2), a potentially toxic compound.
- However, peroxisomes also contain the enzyme catalase, which decomposes H2O2. Because production and degradation of H2O2 occur within the same organelle, peroxisomes protect other parts of the cell from the toxic effects of H2O2.
- As lysosomes degrade proteins delivered to them in vesicles. Cytosolic proteins also require disposal at certain times in the life of a cell.
- Continuous destruction of unneeded, damaged, or faulty proteins is the function of tiny barrel-shaped structures consisting of four stacked rings of proteins around a central core called
- For example, proteins that are part of metabolic pathways are degraded after they have accomplished their function.
- A typical body cell contains many thousands of proteasomes, in both the cytosol and the nucleus.
- Proteasomes were so named because they contain myriad proteases, enzymes that cut proteins into small peptides.
- Once the enzymes of a proteasome have chopped up a protein into smaller chunks, other enzymes then break down the peptides into amino acids, which can be recycled into new proteins.
- Some diseases could result from failure of proteasomes to degrade abnormal proteins. For example, clumps of misfolded proteins accumulate in brain cells of people with Parkinson’s disease and Alzheimer’s disease. Discovering why the proteasomes fail to clear these abnormal proteins is a goal of ongoing research.
- Because they generate most of the ATP through aerobic (oxygen requiring)respiration, mitochondria are referred to as the “powerhouses” of the cell.
- A cell may have as few as a hundred or as many as several thousand mitochondria,
depending on the activity of the cell.
- Active cells, such as those found in the muscles, liver, and kidneys, which use ATP at a high rate, have a large number of mitochondria. For example, liver cells have about 1700 mitochondria.
- A mitochondrion consists of an outer mitochondrial membrane and an inner mitochondrial membrane with a small fluid-filled space between them chondrial membrane contains a series of folds called cristae.
- The central fluid-filled cavity of a mitochondrion, enclosed by the inner mitochondrial membrane, is the matrix.
- The enzymes that catalyze these reactions are located on the cristae and in the matrix of the mitochondria.