Levels of organization of living things
Living things are highly organized and structured, following a hierarchy on a scale from small to large.
The atom is the smallest and most fundamental unit of matter. It consists of a nucleus surrounded by electrons. Atoms form molecules.
A molecule is a chemical structure consisting of at least two atoms held together by a chemical bond. Many molecules that are biologically important are macromolecules, large molecules that are typically formed by combining smaller units called monomers. An example of a macromolecule is the deoxyribonucleic acid (DNA), which contains the instructions for the functioning of the organism that contains it.
All living things are made of cells; the cell itself is the smallest fundamental unit of structure and function in living organisms. A cell is the smallest unit of life. Most cells are so small that they cannot be viewed with the naked eye. Therefore, scientists must use microscopes to study cells.
The unified cell theory states that all organisms are composed of one or more cells, the cell is the basic unit of life, and new cells arise from existing cells. This requirement is why viruses are not considered living: they are not made of cells. To make new viruses, they have to invade and hijack a living cell; only then can they obtain the materials they need to reproduce.
Some cells contain aggregates of macromolecules surrounded by membranes; these are called organelles. Organelles are small structures that exist within cells and perform specialized functions.
Some organisms consist of a single cell and others are multicellular. Cells are classified as prokaryotic or eukaryotic. Prokaryotes are single-celled organisms that lack organelles surrounded by a membrane and do not have nuclei surrounded by nuclear membranes; in contrast, the cells of eukaryotes do have membrane-bound organelles and nuclei. Figure 2.1 illustrates the relative sizes of cells and cellular components.
In most multicellular organisms, cells combine to make tissues, which are groups of similar cells carrying out the same function.
Organs are collections of tissues grouped together based on a common function. Organs are present not only in animals but also in plants.
An organ system is a higher level of organization that consists of functionally related organs. For example vertebrate animals have many organ systems, such as the circulatory system that transports blood throughout the body and to and from the lungs; it includes organs such as the heart and blood vessels.
Organisms are individual living entities. For example, each tree in a forest is an organism. Single-cell prokaryotes and single-cell eukaryotes are also considered organisms and are typically referred to as microorganisms.
Figure 2.1: Relative sizes of cells and cellular components.
Cell structure and function
Prokaryotes are predominantly single-celled organisms of the domains Bacteria and Archaea. All prokaryotes have plasma membranes, cytoplasm, ribosomes, a cell wall, DNA, and lack membrane-bound organelles. Many also have polysaccharide capsules. Prokaryotic cells range in diameter from 0.1–5.0 μm.
Unlike prokaryotic cells, eukaryotic cells (Figure 2.2) have: 1) a membrane-bound nucleus; 2) numerous membrane-bound organelles such as the endoplasmic reticulum, Golgi apparatus, chloroplasts, mitochondria, and others; and 3) several rod-shaped chromosomes. Because a membrane surrounds eukaryotic cell’s nucleus (i.e. its DNA is surrounded by a membrane), it has a “true nucleus”. The word “organelle” means “little organ,” and organelles have specialized cellular functions. Eukaryotic cells tend to be 10 to 100 times the size of prokaryotic cells.
Animal cells (Figure 2.2 (a)) also have a centrosome and lysosomes. The centrosome has two bodies, the centrioles, with an unknown role in cell division. Lysosomes are the digestive organelles of animal cells.
Plant cells (Figure 2.2 (b)) have a cell wall, chloroplasts, and a central vacuole. The plant cell wall, whose primary component is cellulose, protects the cell, provides structural support, and gives shape to the cell. Photosynthesis takes place in chloroplasts. The central vacuole expands, enlarging the cell without the need to produce more cytoplasm.
Animal cells communicate through their extracellular matrices and are connected to each other by tight junctions, desmosomes, and gap junctions. Plant cells are connected and communicate with each other by plasmodesmata.
Figure 2.2: (a) a typical animal cell; (b) a typical plant cell; (c) a typical human cell.
The plasma membrane
Like prokaryotes, eukaryotic cells have a cell membrane or plasma membrane, which is a phospholipid bilayer embedded with proteins that separates the internal contents of the cell from its surrounding environment. The cell membrane has many functions, but the most basic one is to define the cell’s borders and keep the cell functional. The plasma membrane also controls the passage of organic molecules, ions, water, and oxygen into and out of the cell. Wastes (such as carbon dioxide and ammonia) leave the cell by passing through the plasma membrane as well.
The plasma membranes of cells that specialize in absorption fold into fingerlike projections that are called microvilli (singular microvillus). Such cells typically line the small intestine, the organ that absorbs nutrients from digested food. People with celiac disease have an immune response to gluten, which is a protein in wheat, barley, and rye. The immune response damages microvilli and, thus, afflicted individuals cannot absorb nutrients. This leads to malnutrition, cramping, and diarrhea. Patients suffering from celiac disease must follow a gluten-free diet.
The cytoplasm
The cytoplasm is the cell’s entire region between the plasma membrane and the nuclear envelope. It is comprised of organelles suspended in the gel-like cytosol, the cytoskeleton, and various chemicals. 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 in the cytoplasm. Glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, fatty acids, and derivatives of glycerol are also there. Ions of sodium, potassium, calcium, and many other elements also dissolve in the cytoplasm. Many metabolic reactions, including protein synthesis, take place in the cytoplasm.
The nucleus
Typically, the nucleus is the most prominent organelle in a cell (Figure 2.3). The nucleus (plural nuclei) houses the cell’s DNA and directs the synthesis of ribosomes and proteins.
Figure 2.3: The nucleus stores chromatin (DNA plus proteins) in a gel-like substance called nucleoplasm. The nucleolus is a condensed chromatin region where ribosome synthesis occurs. The boundary of the nucleus is called nuclear envelope. It consists of two phospholipid bilayers: an outer and an inner membrane. The nuclear membrane is continuous with the endoplasmic reticulum. Nuclear pores allow substances to enter and exit the nucleus.
The nuclear envelope
The nuclear envelope is a double-membrane structure that constitutes the nucleus’ outermost portion. Both the nuclear envelope’s inner and outer membranes are phospholipid bilayers. The nuclear envelope is punctuated with pores that control the passage of ions, molecules, and RNA between the nucleoplasm and cytoplasm. The nucleoplasm is the semi-solid fluid inside the nucleus, where the chromatin and the nucleolus are found.
Chromatin and chromosomes
To understand chromatin, it is helpful to first explore chromosomes, structures within the nucleus that are made up of DNA, the hereditary material. In prokaryotes, DNA is organized into a single circular chromosome. In eukaryotes, chromosomes are linear structures. Every eukaryotic species has a specific number of chromosomes in the nucleus of each cell. For example, in humans, the chromosome number is 46, while in fruit flies, it is 8.
Chromosomes are only visible and distinguishable from one another when the cell is getting ready to divide. When the cell is in the growth and maintenance phases of its life cycle, proteins attach to chromosomes, and they resemble an unwound, jumbled bunch of threads. These unwound protein-chromosome complexes are called chromatin. Chromatin describes the material that makes up the chromosomes both when condensed and decondensed.
The nucleolus
How does the nucleus direct the synthesis of ribosomes? Some chromosomes have sections of DNA that encode ribosomal RNA. A darkly staining area within the nucleus called the nucleolus (plural nucleoli) aggregates the ribosomal RNA with associated proteins to assemble the ribosomal subunits that are then transported out through the pores in the nuclear envelope to the cytoplasm.
Ribosomes
Ribosomes are the cellular structures responsible for protein synthesis. When viewed through an electron microscope, ribosomes appear either as clusters (polyribosomes) or single, tiny dots that float freely in the cytoplasm. They may be attached to the plasma membrane’s cytoplasmic side or the endoplasmic reticulum’s cytoplasmic side and the nuclear envelope’s outer membrane. Electron microscopy shows that ribosomes, which are large protein and RNA complexes, consist of two subunits: large and small (Figure 2.4).
Ribosomes receive their “orders” for protein synthesis from the nucleus, where the DNA transcribes into messenger RNA (mRNA). The mRNA travels to the ribosomes, which translate the code provided by the sequence of the nitrogenous bases in the mRNA into a specific order of amino acids in a protein. Amino acids are the building blocks of proteins. Because protein synthesis is an essential function of all cells (including enzymes, hormones, antibodies, pigments, structural components, and surface receptors), there are ribosomes in practically every cell. Ribosomes are particularly abundant in cells that synthesize large amounts of protein. For example, the pancreas is responsible for creating several digestive enzymes and the cells that produce these enzymes contain many ribosomes.
Figure 2.4: A large subunit (top) and a small subunit (bottom) comprise ribosomes. During protein synthesis, ribosomes assemble amino acids into proteins.