Cells and Tissue
Overview of the Cellular Basis
Anatomy of a Generalized Cell
- A cell is composed primarily of four elements: carbon, hydrogen,
oxygen, and nitrogen plus many trace elements. Living matter is over
60 percent water. The major building material of the cell is protein.
- Cells vary in size from microscopic to over a meter in length.
Shape often reflects function. For example, muscle cells have a long
axis to allow shortening.
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- Cells have three major regions nucleus, cytoplasm, and plasma membrane.
- The nucleus, or control center, directs cell activity and
is necessary for reproduction. The nucleus contains genetic material
(DNA), which carries instructions for synthesis of proteins.
- The plasma membrane limits and encloses the cytoplasm and
acts as a selective barrier to the movement of substances into
and out of the cell. It is composed of a bi-lipid layer containing
proteins. The water-impermeable lipid portion forms the basic
membrane structure. The proteins (many of which are glycoproteins)
act as enzymes or carriers in membrane transport, form membrane
channels or pores, provide receptor sites for hormones and other
chemicals, or play a role in cellular recognition and interactions
during development and immune reactions (Figure
Specializations of the plasma membrane include microvilli
(which increase the absorptive area) and cell junctions (desmosomes,
tight junctions, and gap junctions).
- The cytoplasm is where most cellular activities occur. Its
fluid substance, the cytosol, contains inclusions, stored or inactive
materials in the cytoplasm (fat globules, water vacuoles, crystals,
and the like) and specialized bodies called organelles, each with
a specific function. For example, mitochondria are sites of ATP
synthesis, ribosomes are sites of protein synthesis, and the Golgi
apparatus packages substances for export from the cell. Lysosomes
carry out intracellular digestion, and peroxisomes disarm dangerous
chemicals in the cells. Cytoskeletal elements function in cellular
support and motion. The centrioles play a role in cell division
and form the bases of cilia and flagella (Figure
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- All cells exhibit irritability, digest foods, excrete wastes, and
are able to reproduce, grow, move, and metabolize.
- Transport of substances through the cell membrane:
- Passive transport processes include diffusion and filtration.
- Diffusion is the movement of a substance from an area of
its higher concentration to an area of its lower concentration.
It occurs because of kinetic energy of the molecules themselves.
The diffusion of dissolved solutes through the plasma membrane
is simple diffusion (Figure
3.3). The diffusion of water through the plasma membrane
is osmosis. Diffusion that requires a protein carrier is facilitated
- Filtration is the movement of substances through a membrane
from an area of high hydrostatic pressure to an area of lower
fluid pressure. In the body, the driving force of filtration
is blood pressure.
- Active transport processes use energy (ATP) provided by the
- In solute pumping, substances are moved across the membrane
against an electrical or a concentration gradient by proteins
called solute pumps. This accounts for the transport of amino
acids, some sugars, and most ions.
- The two types of ATP-activated bulk transport are exocytosis
and endocytosis. Exocytosis moves secretions and other substances
out of cells; a membrane-bound vesicle fuses with the plasma
membrane, ruptures, and ejects its contents to the cell exterior.
Endocytosis, in which particles are taken up by enclosure in
a plasma membrane sac, includes phagocytosis (uptake of solid
particles) and pinocytosis (uptake of fluids) (Figure
- Osmotic pressure, which reflects the solute concentration of a
solution, determines whether cells gain or lose water.
- Hypertonic solutions contain more solutes (and less water)
than do cells. In these solutions, cells lose water by osmosis and
- Hypotonic solutions contain fewer solutes (and more water)
than do the cells. In these solutions, cells swell and may rupture
(lysis) as water rushes in by osmosis.
- Isotonic solutions, which have the same solute-to-solvent ratio
as cells, cause no changes in cell size or shape (Figure
- Cell division has two phases, mitosis (nuclear division) and cytokinesis
(division of the cytoplasm).
- Mitosis begins after DNA has been replicated; it consists of
four stages: prophase, metaphase, anaphase, and telophase. The result
is two daughter nuclei, each identical to the mother nucleus.
- Cytokinesis usually begins during anaphase and progressively
pinches the cytoplasm in half.
- Mitotic cell division provides an increased number of cells
for growth and repair.
- Protein synthesis involves both DNA (the genes) and RNA.
- A gene is a segment of DNA that carries the instructions for
building one protein. The information is in the sequence of bases
in the nucleotide strands. Each three-base sequence (triplet) specifies
one amino acid in the protein.
- Messenger RNA carries the instructions for protein synthesis
from the DNA gene to the ribosomes. Transfer RNA transports amino
acids to the ribosomes. Ribosomal RNA forms part of the ribosomal
structure and helps coordinate the protein building process.
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- Epithelium is the covering, lining, and glandular tissue. Its functions
include protection, absorption, and secretion. Epithelia are named according
to arrangement (simple, stratified)(Figure
3.6) and cell shape (squamous, cuboidal, columnar) (Figure
- Connective tissue is the supportive, protective, and binding tissue.
It is characterized by the presence of a nonliving, extracellular matrix
produced and secreted by the cells; it varies in amount and consistency.
Fat, ligaments and tendons, bones, and cartilage are all connective
tissues or connective tissue structures.
- Nervous tissue is composed of cells called neurons, which are highly
specialized to receive and transmit nerve impulses and supporting cells.
Neurons are important in control of body processes. Nervous tissue is
located in nervous system structures - brain, spinal cord, and nerves
- Muscle tissue is specialized to contract, or shorten, which causes
movement. There are three types: skeletal (attached to the skeleton),
cardiac (forms the heart), and smooth (in the walls of hollow organs).
- Tissue repair (wound healing) may involve regeneration, fibrosis,
or both. In regeneration, the injured tissue is replaced by the same
type of cells. In fibrosis, the wound is repaired with scar tissue.
Epithelia and connective tissues regenerate well. Mature cardiac muscle
and nervous tissue are repaired by fibrosis.