THE CELL
Historical Background
- Robert Hooke—credited with the naming of the cell after looking at cork.
- Leeuwenhoek—credited with studying and describing the first living cells.
- Schleiden—stated all plants were made from cells.
- Schwann—stated all animals were made from cells.
Together with Virchow, they devised the cell theory.
- All cells come from other cells
- The cell is the smallest unit of life
- All living things are made from cells
- Watson and Crick-the structure of DNA.
Plasma membrane/Cell membrane is the outer limit of the cell that encloses different cellular organelles (Fig. 1.1).
- Is a selectively permeable membrane (only allows certain substances to pass back and forth as they please).
- Ions (Na+, K+) are not able to pass through the membrane by themselves; they must be helped.
- Have proteins (called integral proteins) embedded inside the plasma membrane that will act as transport mechanisms for these larger molecules.
- Is a lipid bilayer (a double membrane made from phospholipids and proteins).
- A phospholipid has a hydrophilic (water loving) end made from phosphorous and a hydrophobic (water hating) end made from 2 lipid (fat) molecules.
- Contains integral proteins and peripheral proteins (part of the inner and outer surface of the membrane are used as “markers”).
- Each cell in our body has a peripheral protein (a marker) that is unique to us…this is how our body recognizes which cells are self, and which cells are foreign.
- Many cells have sugar groups attached to them that are called glycocalyx. This sugar is sticky and helps the cells stay attached to each other.
- Each type of cell in our body uses a different type of glycocalyx.
- This is how our body recognizes cells apart (which is a stomach cell, brain cell, etc).
- Cells lining the absorptive surface possess microscopic finger like projections called microvilli that increase surface area.
Membrane Junctions
- Tight protein molecules adhere the cells together like cement, and there is no way to break this junction without tearing cell.
- Dermatomes use proteins like threads to sew the cell together, which can be undone.
- Gap plasma membrane of two or more cells fuse to form a bridge, and materials can pass back and forth between cells.
Movement through the Membrane
Passive Transport
- Movement of a molecule without any type of work being done by cell.
- Cell will create a natural current (called the concentration gradient) that moves things in, out, and around.
Diffusion: Movement of molecules from areas of high concentration to areas of low concentration, through a permeable membrane.
Osmosis: Is the movement of water through a semi-permeable membrane, from a solution of low concentration to one of higher concentration.
- Isotonic the percentages in and out of the cell are equal, so net flow is zero.
- Hypertonic there is more dissolved stuff outside the cell than inside, so water leaves the cell and it shrinks.
- Hypotonic there is less dissolved stuff outside the cell than inside, so water rushes into the cells and it swells.
This movement in and out of cells creates a pressure gradient that can be measured and this is what our bodies use to filter (blood, lymph).
Active Transport
- The cell must use energy in order to move things around, in or out.
- Goes against the concentration gradient
- Exocytosis—getting stuff out of the cell
- Endocytosis—getting stuff into the cell (pinocytosis—brings in small stuff and liquids while phagocytosis brings in large particles).
Facilitated Diffusion
It is a carrier-mediated process. This mechanism does not require energy but the rate of transport is more rapid than diffusion process. It is dependent on concentration gradient.
Parts of Cells
Cytoplasm is the jelly-like gel that fills the cell and holds the organelles in place.
Mitochondria is the “powerhouse of the cell”.
- Have a double membrane and their own DNA (were once a bacteria).
- Site of cellular respiration, which breaks down sugars to form ATP (cellular energy).
- Inner membrane is highly folded to form cristae that increase the surface area to make more ATP.
Ribosome is the site of protein synthesis.
- Ribo means proteins.
- Free ribosomes float in the cytoplasm and make proteins for the cell's own use.
- Attached ribosomes make proteins to be shipped out of the cell.
Endoplasmic reticulum (ER) is the subway system. It has a system of canals and channels through the cytoplasm.
Smooth ER produces lipids and carbohydrates.
Golgi apparatus is the packaging house of the cell (the Post office) is found near the nucleus.
- Anything that is to be sent out of the cell is sent to the GB to be packaged.
- Makes packages called vesicles.
Lysosomes are the suicide sacs
- Structures containing digestive enzymes that break down old, decaying cell parts.
- Will split open in order to release the enzymes.
Peroxisomes
- When cells break down food, they naturally make hydrogen peroxide (H2O2) which is toxic to the cells.
- Peroxisomes break down H2O2 into water and oxygen for the cells use.
Cytoskeleton is an elaborate system of protein rods that run through the cytoplasm.
- Some rods are used to give the cell its shape and structure (like our bones).
- Some rods are used to hold the organelles in place.
- Some rods help move organelles around.
Centrosome is used in cell division of animal cells.
- Make protein fibers that are used in mitosis and meiosis to move structures around.
Nucleus is the brain of the cell
- Has a protective double membrane around it called the nuclear envelope/membrane contains the cell's DNA.
- Contains a smaller organelle called the nucleolus (this was once a bacteria for it has its own DNA).
Nucleolus contains the DNA that tells the cell how to make ribosomes for protein synthesis.
CELL DIVISION
Mitosis
Mitosis is the term for cell division that produces two daughter cells identical to the parent cell. In humans, each of these cells will have 46 chromosomes.
The five stages of mitosis are:
- Interphase: Interphase is the stage where the cell carries on it's normal processes. DNA replicates during this stage.
- Prophase: During prophase all the fun begins. First the chromatin begins to coil up and condense. Once that happens they are referred to as chromatids. When two chromatids join, the pair is called a chromosome. The chromosome is held together by the centromere. Chromosomes first become visible during prophase. The nuclear membrane, as well as the nucleoli, disappears, and the centrioles move to opposite ends of the cell while the mitotic spindle forms between them.
- Metaphase: During metaphase, the chromosomes all meet up at the middle of the cell (Fig. 1.2). The centromeres of the chromosome align with the spindle fiber.
- Anaphase: The chromosome splits at the centromere, and each chromatid pulls apart and moves toward opposite poles of the cell as the spindle fibers shorten.
- Telophase: The most obvious landmark of telophase is the formation of the cleavage furrow which will divide the cytoplasm, and hence the cell, in two. The chromosomes once again become chromatin (long, unwound threads of DNA), and the nuclear membrane reforms. The final result of mitosis has been the formation of two identical daughter cells, each contain 46 chromosomes.
Meiosis
Meiosis is the type of cell division that produces sex cells. Spermatogonia and oogonia are primitive sex cells and have 46 chromosomes (the same number as other cells in our bodies), but in order for them to become mature gametes (sperm and eggs) they must reduce this number to 23. To do this, meiosis has more stages:
- Interphase I: The same events take place in this stage as those that take place in the interphase stage of mitosis. DNA replicates.
- Prophase I: Just like in mitosis, the chromatin begins to coil up and condense, the nuclear membrane disappears and the centrioles begin their migration while the spindle fiber forms between them. The difference between this stage and what happens in mitosis is that crossing over occurs. This is where pieces of chromosomes exchange with pieces of other chromosomes.
- Metaphase I: During metaphase, the chromosomes all meet up at the middle of the cell. The centromeres of the chromosome align with the spindle fibers.
- Anaphase I: During anaphase of mitosis, the centromere split and each chromatid moves to opposite pole. This does not happen here. The centromere does not split, and instead whole chromosomes undergo this migration.
- Telophase I: Same as in mitosis in that the cleavage furrow forms and divides the cell into two daughter cells. Each of these cells contains 23 chromosomes.
- Interphase II: Ha! Fooled you…there is no interphase II. The DNA has already been replicated.
- Prophase II: Same as during prophase of mitosis. Each of the cells has 23 chromosomes (remember that each chromosome is actually 2 chromatids).
- Metaphase II: Chromosomes meet at the middle.
- Anaphase II: This time the centromere does split and one chromatid of the chromosome goes to one pole and the other to the opposite pole.
- Telophase II: Same as in mitosis except that each cell has 23 chromosomes.
TISSUES
Tissue is a group of cells with similar structure and function. The cells differ in appearance according the particular types of tissue to which they belong and the specialized functions they perform. There are four groups of tissues:
- Connective tissue
- Epithelial tissue
- Muscle tissue
- Nerve tissue
Connective Tissue
Connective tissue is the most widespread and abundant tissue in the human body. Many different types of cells are found in connective tissues, and connective tissue comes in many varieties. One feature of connective tissue is the presence of a matrix, which is composed of the ground substance and the fibers.
Fibers Found in Connective Tissue
Collagen: Collagen fibers are white in appearance and very strong, and are the most abundant fibers.9
Elastic: The elastic fibers, as you might guess are able to stretch and so they provide flexibility. With branches they appear yellow.
Reticular: The thinnest fibers. Also branched. The fibers are made of special cells called fibroblasts.
Loose Connective Tissue (Fig. 1.3)
- Areolar: Found in the superficial fascia and the lamina propria of mucous membranes. Areolar is the most widespread type of connective tissue. Its matrix is semi-fluid and it contains all three types of fibers. There is much space between all of the fibers and cells.
- Adipose: Adipose tissue has lot of fat cells, which are packed in tightly. The nucleus of each fat cell is pushed to the side of the cell. Found in subcutaneous tissue and serves several functions like protection and insulation.
- Reticular: Has lots of reticular cells accompanied by reticular fibers. This type of connective tissue forms the framework for different organs.
Dense Connective Tissue
Dense fibrous connective tissue can be further classified into regular or irregular. Dense regular has many collagen fibers arranged in parallel rows. Dense irregular contains many collagen fibers as well, but the arrangement has no regular pattern.10
Elastic: Contains elastic fibers, which permit stretching.
Types of Cartilage
Special Features of Cartilage: Unlike other forms of connective tissue, cartilage has a poor blood supply. Chondrocytes are the cells found in cavities called lacunae. The lacunae give the appearance of bubbles under the microscope.
Elastic Cartilage
Provides support with flexibility. Contains elastic fibers.
Fibrocartilage
Have collagen fibers mostly. Compose the intervertebral discs where it offers support and cushioning.
Hyaline Cartilage
Hyaline cartilage (Fig. 1.4) is found at articular surfaces. You can't really see the fibers here, and the matrix appears glassy.
Other Types of Connective Tissue (Fig. 1.5)
Bone is a type of connective tissue in which the matrix is very hard. The bone cells, called osteocytes, secrete the matrix. Many collagen fibers are embeded in the matrix.11
Blood is a type of connective tissue with a fluid matrix.
Mesenchyme is found in the embryo. Gives rise to all other types of connective tissue.
Mucous is found in the umbilical cord.
Epithelial Cells
Epithelial tissue always has a free surface. Cells are close together and attached to basement membrane.
Epithelial tissue types are named by their shape and number of layers.
Layer Designations
- Simple: One layer of cells
- Stratified: More than one layer of cells
- Pseudostratified: One layer of cells that appears to be many layers.
Cell Shapes
- Squamous: Flat cells
- Cuboidal: Cube shaped cells
- Columnar: Column shaped cells, tall and thin.
Types of Epithelial Tissue
- Simple squamous: Found in the alveoli of the lungs, lines blood vessels. One layer of flat cells permits filtration and diffusion.
- Stratified squamous: Two types: keratinized and non-keratinized. Keratinized found in dry areas like the skin and non-keratinized stratified squamous is found in wet areas like the esophagus and vagina. The many layers of cells offer protection of these areas.
- Simple cuboidal: One layer of cube shaped cells. Simple cuboidal lines the kidney tubules. Its function is secretion and absorption.
- Stratified cuboidal: Many layers of cube shaped cells. Stratified cuboidal lines the mammary glands and salivary glands. Its function is to offer protection of the areas it lines.
- Simple columnar: Two types: ciliated and non-ciliated. Non-ciliated can be found in the gastrointestinal tract. You will also find goblet cells here (which secrete mucus). Ciliated simple columnar lines the fallopian tubes, and the cilia beat to move the oocyte through the tube.
- Stratified columnar: Like other stratified epithelial types, its function is to protect and also secrete.
- Uncommon epithelial type.
- Pseudostratified columnar: Lines the upper respiratory tract where it functions to produce and move mucus. (Has cilia and goblet cells).
- Transitional: Found in the lining of the urinary bladder. The cells change shape from a rounder appearance to a flatter one as the bladder fills, so their function is to permit distention of the bladder.
Glandular Epithelium
- Endocrine glands: Release their products (hormones) directly into the blood. Examples of endocrine glands are the thyroid, pituitary, and ovary.
- Exocrine glands: Release product (sweat, oil, ear wax, milk, etc) into a duct.
Three Types of Multicellular Exocrine Glands
- Holocrine: The whole cell and contents are secreted into duct. Example: Sebaceous glands.
- Apocrine: Part of the cell pinches off and is secreted into the duct. Example: Mammary glands.
- Merocrine: Only the products are secreted into the duct. Most exocrine glands are this type.
Types of Muscle
There are three types of muscles (Fig. 1.6), namely the skeletal muscle, cardiac muscle and smooth muscle.
Skeletal Muscle
Skeletal muscle may be called voluntary muscle (because it can be manipulated by conscious effort) or striated muscle (because it appears to be striped). The muscle cell, also called a fiber, contains many nuclei. Each muscle fiber is made up of bundles of smaller fibers called myofibrils. Each myofibril is made up of smaller fibers still called myofilaments. Some of these myofilaments are thick (made of myosin) and some are thinner (made of actin, tropomyosin and troponin). Myofilaments each have several sarcomeres, which are the contractile units of the muscle. Cell structures have different names than you'll find in other types of cells, and the muscle cells even have structures that can't be found in other types of cells.
Muscle Cell Structures
- Sarcolemma is the cell membrane.
- Sarcoplasm is the cytoplasm.
- Sarcomere is the contractile unit of the muscle. The sarcomere (Fig. 1.7) extends from one Z line of a myofibril to the next Z line of that myofibril.
- Z lines (or Z disc) are found in the middle of each I band.
- I band is thinner, and lighter colored striations that alternate with A bands.
- A band is thicker, darker color striations that alternate with I bands.
- Myofilaments make up myofibrils. Consist of myosin (thicker), actin, tropomyosin, and troponin (all thinner).
- Myofibrils make up muscle cells (fibers).
- Connective tissue coverings of the fibers:
- Endomysium: Around an individual muscle fiber.
- Perimysium: Around a fascicle (bundle of fibers).
- Epimysium: Around many fascicles.
Skeletal muscles produce movement by contracting (Fig. 1.8) which pulls the insertion bone towards the origination bone.
Smooth Muscle
Because smooth muscle has no A or I bands, it does not have the striated appearance of skeletal muscle. Smooth muscle is involuntary, meaning that no thought or conscious effort is needed to cause muscle contraction. There are two types of smooth muscle:
- Single unit: Located in the viscera: Gastrointestinal tract, uterus, bladder and small arteries.
- Multi unit: Located in large arteries, respiratory tract and iris.
Cardiac Muscle
Cardiac muscle is located, as you might guess, only in the heart. It has a striated appearance and is involuntarily controlled. Cardiac muscle also has a feature that is foreign to the other muscle types: intercalated discs.
Muscle Contraction
The physiology of muscle contraction is very involved and can be very confusing. Basically, this happens:
- The energy for muscle contraction comes from the breaking of two bonds in ATP.
- Acetylcholine released by a neuron into the neuromuscular junction binds to receptors on the motor-end plate of the muscle cell. The nerve impulse conducts over sarcolemma and triggers the release of calcium ions from sacs in the sarcoplasmic reticulum into the sarcoplasm.
- Calcium ions in the sarcoplasm combine with troponin molecules, which allows myosin to interact with actin. The thin myofilaments are pulled toward the center of the sarcomere, which causes the sarcomere and the myofibrils to shorten and contract. Relaxation of the muscle occurs when calcium and troponin separates, preventing myosin-actin interaction.
- All or None Law: Muscle cells either contract with all possible force, or they do not contract at all. Remember that this applies to individual muscle fibers, not entire muscles.
- Skeletal muscle is voluntary and only contracts when stimulated. Cardiac and smooth muscle are involuntary.
- Muscles move from the point of insertion toward the origination site.
BONE STRUCTURE
Bone (Fig. 1.9) is a type of connective tissue where the matrix is hard and calcified. The matrix contains many collagen fibers. The term for these layers of the matrix is lamellae.
- Each of these white spots in the graphic is an osteocyte within a lacuna.
- In the center of each osteon is a passageway called the Haversian canal (Fig. 1.10). Within this canal are the blood vessels that supply nutrients to the osteocytes by way of the canaliculi. Radiating out from the Haversian canal, like spokes on a wheel, are the canaliculi.Osteocytes are not the only types of cells found in bone:
- Osteoblasts are the cells that lay down new bone.
- Osteoclasts are the cells that destroy old bone so that growth can occur. Osteoclasts also begin to destroy bone when the body is deficient of calcium, as well as when bone is not used.
Other Terms to Know
Spongy bone: Spongy bone has no Haversian systems. Instead the arrangement is like a web (Trabeculae).
Diaphysis: The shaft of a long bone.
Epiphysis: The end or head of a long bone.
Periosteum: A covering over the surface of the bone. Made of dense connective tissue.
Articular cartilage: Hyaline cartilage, which covers the epiphyseal surface where a bone form a joint with another bone. Epiphyseal surfaces, which are not part of an articulating surface, are covered by the periosteum.
The markings on bones (such as tuberosities, lines, crests, and spines) serve as sites of attachment for muscles and/or ligaments.
Openings in bones (such as foramen, fossa, fissures, etc.) serve as passageways for blood vessels and/or nerves.
Projections on bones (like heads and condyles) take part in forming joints.
Types of Bones
Long bones: Long bones are made primarily of compact bone, except at the ends (epiphysis) which has only a thin layer of compact bone covering a great deal of spongy bone. Examples of long bones include the humerus and femur. They are bones that are longer than they are wide.
Short bones: These are bones shaped like the carpals and tarsals, and contain mostly spongy bone.
Flat bones: Examples of flat bones are those of the skull. Contain spongy bone in between surrounding layers of compact bone.
Irregular bones: Examples are the vertebrae.
Sesamoid bones: An example is the patella. Sesamoid bones are enclosed in tendons.
Nervous System
There are two distinct types of cells found in nervous system.
- Neurons or nerve cells with their processes called neuritis that convey motor or sensory impulses (Fig. 1.11).
- Neuroglia, ependyma and Schwann cells are the types of cells that are non-excitable with numerous functions including mechanical support for the neurons.
The size of neurons varies considerably in size, some being the largest cells in the whole body. The nervous tissue containing the cell bodies of neurons is some times called grey matter. Aggregations of nerve cell bodies are known, as nuclei or ganglia and they are dark in appearance.
A neuron consists of three main parts:
- Cell body: The largest part contains the nucleus and much of the cytoplasm (area between the nucleus and the cell membrane), most of the metabolic activity of the cell, including the generation of ATP (Adenosine triphosphate compound that stores energy) and synthesis of protein.
- Dendrites: Short branch extensions spreading out from the cell body. Dendrites receive stimulus (Action potentials) and carry impulses from the environment or from other neurons and carry them toward the cell body.
- Axon: A long fiber that carries impulses away from the cell body. Each neuron has only one axon. The axon ends in a series of small swellings called axon terminals.
Neurons may have dozens or even hundreds of dendrites but usually only one axon.
The axons of most neurons are covered with a lipid layer known as the myelin sheath.
The myelin sheath both insulates and speeds up transmission of action potentials through the axon.
In the peripheral nervous system, myelin is produced by Schwann cells, which surround the axon.
Gaps (nodes) in the myelin sheath along the length of the axon are known as the nodes of Ranvier's.
Integumentary System
The skin (Fig. 1.12) is the body's largest organ, and serves many functions: Regulation of body temperature, protection from environment, excretion of some wastes and absorption of some chemicals and vitamins, and sensations.
- Epidermis has five layers and is made up of stratified squamous epithelial tissue.
- Stratum corneum is the outermost layer of the epidermis. The cells here are dead.
- Stratum granulosum is found in thicker skin, the stratum lucidum will be found in between stratum corneum and stratum granulosum. Cells die in the stratum granulosum.
- Stratum spinosum is the layer in which cells appear spiny under the microscope.
- Stratum basale is the germinating layer where mitosis takes place that forms new skin cells. As new cells are formed, the older cells are pushed out layer by layer until they die and are eventually sloughed off. Cells containing pigment are found here.
- Dermis is thicker than the epidermis and consists of connective tissue (dense irregular fibrous). The dermis has two layers—the papillary layer and the reticular layer.
- Papillae form ridges on the skin for increased friction. This is what forms fingerprints.
- Arrector pilli muscle is attached to hairs. It is smooth muscle. When the muscle contracts, the hair “stands up.” (Goose-bumps).
- Sebaceous gland produces sebum (oil).
- Hair bulb is the root of the hair.
- Meissner's corpuscle sense pain and temperature. Touch receptors.
- Sweat gland secretes sweat.
- Subcutaneous tissue (Superficial fascia) will have areolar connective tissue and adipose tissue.
