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Bacteria Cell Structure
Bacteria are prokaryotes, lacking well-defined nuclei and membrane-bound organelles, and with chromosomes composed of a single closed DNA circle. They come in many shapes and sizes, from minute spheres, cylinders and spiral threads, to flagellated rods, and filamentous chains. They are found practically everywhere on Earth and live in some of the most unusual and seemingly inhospitable places.
- Capsule – Some species of bacteria have a third protective covering, a capsule made up of polysaccharides (complex carbohydrates).
- Cell Envelope – The cell envelope is made up of two to three layers: the interior cytoplasmic membrane, the cell wall, and — in some species of bacteria — an outer capsule.
- Cell Wall – Each bacterium is enclosed by a rigid cell wall composed of peptidoglycan, a protein-sugar (polysaccharide) molecule. The wall gives the cell its shape and surrounds the cytoplasmic membrane, protecting it from the environment.
- Cytoplasm – The cytoplasm, or protoplasm, of bacterial cells is where the functions for cell growth, metabolism, and replication are carried out. It is a gel-like matrix composed of water, enzymes, nutrients, wastes, and gases and contains cell structures such as ribosomes, a chromosome, and plasmids. The cell envelope encases the cytoplasm and all its components. Unlike the eukaryotic (true) cells, bacteria do not have a membrane enclosed nucleus. The chromosome, a single, continuous strand of DNA, is localized, but not contained, in a region of the cell called the nucleoid. All the other cellular components are scattered throughout the cytoplasm.
- Cytoplasmic Membrane – A layer of phospholipids and proteins, called the cytoplasmic membrane, encloses the interior of the bacterium, regulating the flow of materials in and out of the cell.
- Flagella – Flagella (singular, flagellum) are hairlike structures that provide a means of locomotion for those bacteria that have them. They can be found at either or both ends of a bacterium or all over its surface. The flagella beat in a propeller-like motion to help the bacterium move toward nutrients; away from toxic chemicals; or, in the case of the photosynthetic cyanobacteria; toward the light.
- Nucleoid – The nucleoid is a region of cytoplasm where the chromosomal DNA is located. It is not a membrane bound nucleus, but simply an area of the cytoplasm where the strands of DNA are found. Most bacteria have a single, circular chromosome that is responsible for replication, although a few species do have two or more. Smaller circular auxiliary DNA strands, called plasmids, are also found in the cytoplasm.
- Pili – Many species of bacteria have pili (singular, pilus), small hairlike projections emerging from the outside cell surface. These outgrowths assist the bacteria in attaching to other cells and surfaces, such as teeth, intestines, and rocks. Without pili, many disease-causing bacteria lose their ability to infect because they’re unable to attach to host tissue. Specialized pili are used for conjugation, during which two bacteria exchange fragments of plasmid DNA.
- Ribosomes – Ribosomes are microscopic “factories” found in all cells, including bacteria. They translate the genetic code from the molecular language of nucleic acid to that of amino acids—the building blocks of proteins. Proteins are the molecules that perform all the functions of cells and living organisms.
Animal Cell Structure
Animal cells are typical of the eukaryotic cell, enclosed by a plasma membrane and containing a membrane-bound nucleus and organelles. Unlike the eukaryotic cells of plants and fungi, animal cells do not have a cell wall. This feature was lost in the distant past by the single-celled organisms that gave rise to the kingdom Animalia. Most cells, both animal and plant, range in size between 1 and 100 micrometers and are thus visible only with the aid of a microscope.
- Centrioles – Centrioles are self-replicating organelles made up of nine bundles of microtubules and are found only in animal cells. They appear to help in organizing cell division, but aren’t essential to the process.
- Cilia and Flagella – For single-celled eukaryotes, cilia and flagella are essential for the locomotion of individual organisms. In multicellular organisms, cilia function to move fluid or materials past an immobile cell as well as moving a cell or group of cells.
- Endoplasmic Reticulum – The endoplasmic reticulum is a network of sacs that manufactures, processes, and transports chemical compounds for use inside and outside of the cell. It is connected to the double-layered nuclear envelope, providing a pipeline between the nucleus and the cytoplasm.
- Endosomes and Endocytosis – Endosomes are membrane-bound vesicles, formed via a complex family of processes collectively known as endocytosis, and found in the cytoplasm of virtually every animal cell. The basic mechanism of endocytosis is the reverse of what occurs during exocytosis or cellular secretion. It involves the invagination (folding inward) of a cell’s plasma membrane to surround macromolecules or other matter diffusing through the extracellular fluid.
- Golgi Apparatus – The Golgi apparatus is the distribution and shipping department for the cell’s chemical products. It modifies proteins and fats built in the endoplasmic reticulum and prepares them for export to the outside of the cell.
- Intermediate Filaments – Intermediate filaments are a very broad class of fibrous proteins that play an important role as both structural and functional elements of the cytoskeleton. Ranging in size from 8 to 12 nanometers, intermediate filaments function as tension-bearing elements to help maintain cell shape and rigidity.
- Lysosomes – The main function of these microbodies is digestion. Lysosomes break down cellular waste products and debris from outside the cell into simple compounds, which are transferred to the cytoplasm as new cell-building materials.
- Microfilaments – Microfilaments are solid rods made of globular proteins called actin. These filaments are primarily structural in function and are an important component of the cytoskeleton.
- Microtubules – These straight, hollow cylinders are found throughout the cytoplasm of all eukaryotic cells (prokaryotes don’t have them) and carry out a variety of functions, ranging from transport to structural support.
- Mitochondria – Mitochondria are oblong shaped organelles that are found in the cytoplasm of every eukaryotic cell. In the animal cell, they are the main power generators, converting oxygen and nutrients into energy.
- Nucleus – The nucleus is a highly specialized organelle that serves as the information processing and administrative center of the cell. This organelle has two major functions: it stores the cell’s hereditary material, or DNA, and it coordinates the cell’s activities, which include growth, intermediary metabolism, protein synthesis, and reproduction (cell division).
- Peroxisomes – Microbodies are a diverse group of organelles that are found in the cytoplasm, roughly spherical and bound by a single membrane. There are several types of microbodies but peroxisomes are the most common.
- Plasma Membrane – All living cells have a plasma membrane that encloses their contents. In prokaryotes, the membrane is the inner layer of protection surrounded by a rigid cell wall. Eukaryotic animal cells have only the membrane to contain and protect their contents. These membranes also regulate the passage of molecules in and out of the cells.
- Ribosomes – All living cells contain ribosomes, tiny organelles composed of approximately 60 percent RNA and 40 percent protein. In eukaryotes, ribosomes are made of four strands of RNA. In prokaryotes, they consist of three strands of RNA.
Eucaryotic Cell Organelles
: The nucleus is the most obvious organelle in any eukaryotic cell. It is enclosed in a double membrane and communicates with the surrounding cytosol via numerous nuclear pores. Within the nucleus is the DNA responsible for providing the cell with its unique characteristics. The DNA is similar in every cell of the body, but depending on the specific cell type, some genes may be turned on or off – that’s why a liver cell is different from a muscle cell, and a muscle cell is different from a fat cell. When a cell is dividing, the nuclear chromatin (DNA and surrounding protein) condenses into chromosomes that are easily seen by microscopy.
: The prominent structure in the nucleus is the nucleolus. The nucleolus produces ribosomes, which move out of the nucleus and take positions on the rough endoplasmic reticulum where they are critical in protein synthesis.
: The cytosol is the “soup” within which all the other cell organelles reside and where most of the cellular metabolism occurs. Though mostly water, the cytosol is full of proteins that control cell metabolism including signal transduction pathways, glycolysis, intracellular receptors, and transcription factors.
: This is a collective term for the cytosol plus the organelles suspended within the cytosol.
: The centrosome, or MICROTUBULE ORGANIZING CENTER (MTOC), is an area in the cell where microtubles are produced. Plant and animal cell centrosomes play similar roles in cell division, and both include collections of microtubules, but the plant cell centrosome is simpler and does not have centrioles.
During animal cell division, the centrioles replicate (make new copies) and the centrosome divides. The result is two centrosomes, each with its own pair of centrioles. The two centrosomes move to opposite ends of the nucleus, and from each centrosome, microtubules grow into a “spindle” which is responsible for separating replicated chromosomes into the two daughter cells.
(animal cells only): Each centriole is a ring of nine groups of fused microtubules. There are three microtubules in each group. Microtubules (and centrioles) are part of the cytoskeleton. In the complete animal cell centrosome, the two centrioles are arranged such that one is perpendicular to the other.
: The Golgi apparatus is a membrane-bound structure with a single membrane. It is actually a stack of membrane-bound vesicles that are important in packaging macromolecules for transport elsewhere in the cell. The stack of larger vesicles is surrounded by numerous smaller vesicles containing those packaged macromolecules. The enzymatic or hormonal contents of lysosomes, peroxisomes and secretory vesicles are packaged in membrane-bound vesicles at the periphery of the Golgi apparatus.
: Lysosomes contain hydrolytic enzymes necessary for intracellular digestion. They are common in animal cells, but rare in plant cells. Hydrolytic enzymes of plant cells are more often found in the vacuole.
: Peroxisomes are membrane-bound packets of oxidative enzymes. In plant cells, peroxisomes play a variety of roles including converting fatty acids to sugar and assisting chloroplasts in photorespiration. In animal cells, peroxisomes protect the cell from its own production of toxic hydrogen peroxide. As an example, white blood cells produce hydrogen peroxide to kill bacteria. The oxidative enzymes in peroxisomes break down the hydrogen peroxide into water and oxygen.
: Cell secretions – e.g. hormones, neurotransmitters – are packaged in secretory vesicles at the Golgi apparatus. The secretory vesicles are then transported to the cell surface for release.
: Every cell is enclosed in a membrane, a double layer of phospholipids (lipid bilayer). The exposed heads of the bilayer are “hydrophilic” (water loving), meaning that they are compatible with water both within the cytosol and outside of the cell. However, the hidden tails of the phosopholipids are “hydrophobic” (water fearing), so the cell membrane acts as a protective barrier to the uncontrolled flow of water. The membrane is made more complex by the presence of numerous proteins that are crucial to cell activity. These proteins include receptors for odors, tastes and hormones, as well as pores responsible for the controlled entry and exit of ions like sodium (Na+) potassium (K+), calcium (Ca++) and chloride (Cl-).
: Mitochondria provide the energy a cell needs to move, divide, produce secretory products, contract – in short, they are the power centers of the cell. They are about the size of bacteria but may have different shapes depending on the cell type. Mitochondria are membrane-bound organelles, and like the nucleus have a double membrane. The outer membrane is fairly smooth. But the inner membrane is highly convoluted, forming folds (cristae) as seen in the cross-section, above. The cristae greatly increase the inner membrane’s surface area. It is on these cristae that food (sugar) is combined with oxygen to produce ATP – the primary energy source for the cell.
: A vacuole is a membrane-bound sac that plays roles in intracellular digestion and the release of cellular waste products. In animal cells, vacuoles are generally small. Vacuoles tend to be large in plant cells and play several roles: storing nutrients and waste products, helping increase cell size during growth, and even acting much like lysosomes of animal cells. The plant cell vacuole also regulates turgor pressure in the cell. Water collects in cell vacuoles, pressing outward against the cell wall and producing rigidity in the plant. Without sufficient water, turgor pressure drops and the plant wilts.
(plant cells only): Plant cells have a rigid, protective cell wall made up of polysaccharides. In higher plant cells, that polysaccharide is usually cellulose. The cell wall provides and maintains the shape of these cells and serves as a protective barrier. Fluid collects in the plant cell vacuole and pushes out against the cell wall. This turgor pressure is responsible for the crispness of fresh vegetables.
(plant cells only): Chloroplasts are specialized organelles found in all higher plant cells. These organelles contain the plant cell’s chlorophyll responsible for the plant’s green color. Chloroplasts have a double outer membrane. Within the stroma are other membrane structures – the thylakoids. Thylakoids appear in stacks called “grana” (singular = granum).
: Throughout the eukaryotic cell, especially those responsible for the production of hormones and other secretory products, is a vast network of membrane-bound vesicles and tubules called the endoplasmic reticulum, or ER for short. The ER is a continuation of the outer nuclear membrane and its varied functions suggest the complexity of the eukaryotic cell.
The smooth endoplasmic reticulum is so named because it appears smooth by electron microscopy. Smooth ER plays different functions depending on the specific cell type including lipid and steroid hormone synthesis, breakdown of lipid-soluble toxins in liver cells, and control of calcium release in muscle cell contraction.
: Rough endoplasmic reticulum appears “pebbled” by electron microscopy due to the presence of numerous ribosomes on its surface. Proteins synthesized on these ribosomes collect in the endoplasmic reticulum for transport throughout the cell.
: Ribosomes are packets of RNA and protein that play a crucial role in both prokaryotic and eukaryotic cells. They are the site of protein synthesis. Each ribosome comprises two parts, a large subunit and a small subunit. Messenger RNA from the cell nucleus is moved systematically along the ribosome where transfer RNA adds individual amino acid molecules to the lengthening protein chain.
: As its name implies, the cytoskeleton helps to maintain cell shape. But the primary importance of the cytoskeleton is in cell motility. The internal movement of cell organelles, as well as cell locomotion and muscle fiber contraction could not take place without the cytoskeleton. The cytoskeleton is an organized network of three primary protein filaments:
- microtubules
- actin filaments (microfilaments)
- intermediate fibers
Some Keywords…
eukaryote, nucleus, nucleolus, endoplasmic reticulum, centriole, Golgi, cytoskeleton, mitochondria, vacuole, cell
Cell Membranes
Cell Membranes
One universal feature of all cells is an outer limiting membrane called the plasma membrane.
In addition, all eukaryotic cells contain elaborate systems of internal membranes which set up various membrane-enclosed compartments within the cell.
Cell membranes are built from lipids and proteins.
The Plasma Membrane
The plasma membrane serves as the interface between the machinery in the interior of the cell and the extracellular fluid (ECF) that bathes all cells.
The lipids in the plasma membrane are chiefly phospholipids like phosphatidyl ethanolamine and cholesterol. Phospholipids are amphiphilic with the hydrocarbon tail of the molecule being hydrophobic; its polar head hydrophilic. As the plasma membrane faces watery solutions on both sides, its phospholipids accommodate this by forming a phospholipid bilayer with the hydrophobic tails facing each other.
Integral Membrane Proteins
Many of the proteins associated with the plasma membrane are tightly bound to it.
- Some are attached to lipids in the bilayer.
- In others – the transmembrane proteins – the polypeptide chain actually traverses the lipid bilayer. The figure shows a transmembrane protein that passes
- just once through the bilayer and another that passes through it 7 times. All G-protein-coupled receptors (e.g., receptors of peptide hormones, and odors each span the plasma membrane 7 times.
Peripheral Membrane Proteins
These are more loosely associated with the membrane. They are usually attached noncovalently to the protruding portions of integral membrane proteins.
Membrane proteins are often restricted in their movements.
A lipid bilayer is really a film of oil. Thus we might expect that structures immersed in it would be relatively free to float about. For some membrane proteins, this is the case. For others, however, their mobility is limited:
- Some of the proteins exposed at the interior face of the plasma membrane are tethered to cytoskeletal elements like actin microfilaments.
- Some proteins are the exterior face of the plasma membrane are anchored to components of the extracellular matrix like collagen.
- Integral membrane proteins cannot pass through the tight junctions found between some kinds of cells (e.g., epithelial cells).
Unit Membrane
Typical Structure – composed of protein and lipid (fat) molecules
Cell Membrane
Structure – same as unit membrane.
Function – acts as a boundary layer to contain the cytoplasm (fluid in cell)
– interlocking surfaces bind cells together
– selectively permeable to select chemicals that pass in and
out of cells
