Anatomy and Physiology – Cell Structure and Functions

Anatomy and Physiology – Cell Structure and Functions

CELL FUNCTION

• The structural and functional characteristics of different types of cells are
• determined by the nature of the proteins present. Cells of various types have
• different functions because cell structure and function are closely related. It
• is apparent that a cell that is very thin is not well suited for a protective
• function. Bone cells do not have an appropriate structure for nerve impulse
• conduction. Just as there are many cell types, there are varied cell functions.
• The generalized cell functions include movement of substances across the
• cell membrane, cell division to make new cells, and protein synthesis.
• Movement of substances across the cell membrane
• The survival of the cell depends on maintaining the difference between
• extracellular and intracellular material. Mechanisms of movement across the
• cell membrane include simple diffusion, osmosis, filtration, active
• transport, endocytosis, andexocytosis.
• Simple diffusion is the movement of particles (solutes) from a region of
• higher solute concentration to a region of lower solute concentration.
• Osmosis is the diffusion of solvent or water molecules through a selectively
• permeable membrane. Filtration utilizes pressure to push substances through
• a membrane. Active transport moves substances against a concentration
• gradient from a region of lower concentration to a region of higher
• concentration. It requires a carrier molecule and uses energy. Endocytosis
• refers to the formation of vesicles to transfer particles and droplets from
• outside to inside the cell. Secretory vesicles are moved from the inside to the
• outside of the cell by exocytosis.
• Cell division
• Cell division is the process by which new cells are formed for growth,
• repair, and replacement in the body. This process includes division of the
• nuclear material and division of the cytoplasm. All cells in the body (somatic
• cells), except those that give rise to the eggs and sperm (gametes), reproduce
• by mitosis. Egg and sperm cells are produced by a special type of nuclear
• division called meiosis in which the number of chromosomes is halved.
• Division of the cytoplasm is called cytokinesis.
• Somatic cells reproduce by mitosis, which results in two cells identical to the
• one parent cell. Interphase is the period between successive cell divisions. It
• is the longest part of the cell cycle. The successive stages of mitosis are
• prophase, metaphase, anaphase, and telophase. Cytokinesis, division of the
• cytoplasm, occurs during telophase.

Meiosis is a special type of cell division that occurs in the production of the gametes, or eggs and sperm. These cells have only 23 chromosomes, onehalf the number found in somatic cells, so that when fertilization takes place the resulting cell will again have 46 chromosomes, 23 from the egg and 23 from the sperm.

• DNA replication and protein synthesis
• Proteins that are synthesized in the cytoplasm function as structural
• materials, enzymes that regulate chemical reactions, hormones, and other
• vital substances. DNA in the nucleus directs protein synthesis in the
• cytoplasm. A gene is the portion of a DNA molecule that controls the
• synthesis of one specific protein molecule. Messenger RNA carries the
• genetic information from the DNA in the nucleus to the sites of protein
• synthesis in the cytoplasm.
• There are many different types, sizes, and shapes of cells in the body. For
• descriptive purposes, the concept of a "generalized cell" is introduced. It
• includes features from all cell types. A cell consists of three parts: the cell
• membrane, the nucleus, and, between the two, the cytoplasm. Within the
• cytoplasm lie intricate arrangements of fine fibers and hundreds or even
• thousands of miniscule but distinct structures called organelles.
• Cell membrane
• Every cell in the body is enclosed by a cell (Plasma) membrane. The cell
• membrane separates the material outside the cell, extracellular, from the
• material inside the cell, intracellular. It maintains the integrity of a cell and
• controls passage of materials into and out of the cell. All materials within a
• cell must have access to the cell membrane (the cell's boundary) for the
• needed exchange.
• The cell membrane is a double layer of phospholipid molecules. Proteins in
• the cell membrane provide structural support, form channels for passage of
• materials, act as receptor sites, function as carrier molecules, and provide
• identification markers.
• Nucleus and Nucleolus
• The nucleus, formed by a nuclear membrane around a fluid nucleoplasm, is
• the control center of the cell. Threads of chromatin in the nucleus contain
• deoxyribonucleic acid (DNA), the genetic material of the cell. The nucleolus
• is a dense region of ribonucleic acid (RNA) in the nucleus and is the site of
• ribosome formation. The nucleus determines how the cell will function, as
• well as the basic structure of that cell.
• Cytoplasm
• The cytoplasm is the gel-like fluid inside the cell. It is the medium for
• chemical reaction. It provides a platform upon which other organelles can
• operate within the cell. All of the functions for cell expansion, growth and
• replication are carried out in the cytoplasm of a cell. Within the cytoplasm,
• materials move by diffusion, a physical process that can work only for short
• distances.
• Cytoplasmic organelles
• Cytoplasmic organelles are "little organs" that are suspended in the
• cytoplasm of the cell. Each type of organelle has a definite structure and a
• specific role in the function of the cell. Examples of cytoplasmic organelles
• are mitochondrion, ribosomes,endoplasmic reticulum, golgi apparatus,
• and lysosomes.
• Organelles
• Organelles are parts of the cell which are adapted and/or specialized for
• carrying out one or more vital functions, analogous to the organs of the
• human body (such as the heart, lung, and kidney, with each organ
• performing a different function). Both eukaryotic and prokaryotic cells have
• organelles, but prokaryotic organelles are generally simpler and are not
• membrane-bound.
• There are several types of organelles in a cell. Some (such as
• the nucleus and golgi apparatus) are typically solitary, while others (such
• as mitochondria, chloroplasts,peroxisomes and lysosomes) can be numerous
• (hundreds to thousands). The cytosol is the gelatinous fluid that fills the cell
• and surrounds the organelles.
• Eukaryotic
• Cell nucleus: A cell's information center, the cell nucleus is the most
• conspicuous organelle found in a eukaryotic cell. It houses the
• cell's chromosomes, and is the place where almost all DNA replication
• and RNA synthesis (transcription) occur. The nucleus is spherical and
• separated from the cytoplasm by a double membrane called the nuclear
• envelope. The nuclear envelope isolates and protects a cell's DNA from
• various molecules that could accidentally damage its structure or
• interfere with its processing. During processing, DNA is transcribed, or
• copied into a special RNA, called messenger RNA (mRNA). This
• mRNA is then transported out of the nucleus, where it is translated into a
• specific protein molecule. The nucleolus is a specialized region within
• the nucleus where ribosome subunits are assembled. In prokaryotes,
• DNA processing takes place in the cytoplasm.
• Mitochondria and Chloroplasts: generate energy for the
• cell. Mitochondria are self-replicating organelles that occur in various
• numbers, shapes, and sizes in the cytoplasm of all eukaryotic
• cells. Respiration occurs in the cell mitochondria, which generate the
• cell's energy by oxidative phosphorylation, using oxygen to release
• energy stored in cellular nutrients (typically pertaining to glucose) to
• generate ATP. Mitochondria multiply by binary fission, like prokaryotes.
• Chloroplasts can only be found in plants and algae, and they capture the
• sun's energy to make carbohydrates through photosynthesis.
• Endoplasmic reticulum: The endoplasmic reticulum (ER) is a transport
• network for molecules targeted for certain modifications and specific
• destinations, as compared to molecules that float freely in the cytoplasm.
• The ER has two forms: the rough ER, which has ribosomes on its surface
• that secrete proteins into the ER, and the smooth ER, which lacks
• ribosomes. The smooth ER plays a role in calcium sequestration and
• release.
• Golgi apparatus: The primary function of the Golgi apparatus is to
• process and package the macromolecules such as proteins andlipids that
• are synthesized by the cell.
• Lysosomes and Peroxisomes: Lysosomes contain digestive
• enzymes (acid hydrolases). They digest excess or worn-outorganelles,
• food particles, and engulfed viruses or bacteria. Peroxisomes have
• enzymes that rid the cell of toxic peroxides. The cell could not house
• these destructive enzymes if they were not contained in a membranebound system.
• Centrosome: the cytoskeleton organiser: The centrosome produces
• the microtubules of a cell – a key component of thecytoskeleton. It
• directs the transport through the ER and the Golgi apparatus.
• Centrosomes are composed of two centrioles, which separate during cell
• division and help in the formation of the mitotic spindle. A single
• centrosome is present in the animal cells. They are also found in some
• fungi and algae cells.
• Vacuoles: Vacuoles sequester waste products and in plant cells store
• water. They are often described as liquid filled space and are surrounded
• by a membrane. Some cells, most notably Amoeba, have contractile
• vacuoles, which can pump water out of the cell if there is too much
• water. The vacuoles of plant cells and fungal cells are usually larger than
• those of animal cells.
• Eukaryotic and prokaryotic
• Ribosomes: The ribosome is a large complex
• of RNA and protein molecules. They each consist of two subunits, and
• act as an assembly line where RNA from the nucleus is used to
• synthesise proteins from amino acids. Ribosomes can be found either
• floating freely or bound to a membrane (the rough endoplasmatic

reticulum in eukaryotes, or the cell membrane in prokaryotes).