Biol 2430 Anatomy and Physiology

Lect #2Muse5/5/10

THE CELL

The cell is the smallest structural and functional unit of the body. Most

of the chemical reactions that sustain life occur inside cells.

An Introduction to Cells• Sex cells (germ cells)

– Reproductive cells

– Male sperm

– Female oocyte (a cell that develops into an egg)

• Somatic cells (soma = body)

– All body cells except sex cells

An Introduction to Cells• A cell is surrounded by a watery medium known

as the extracellular fluid (interstitial fluid)

– Plasma membrane (cell membrane) separates

cytoplasm from the extracellular fluid

– Cytoplasm

• Cytosol = liquid

• Intracellular structures collectively known as organelles

Chapter 3Cells

• vary in size• possess distinctive shapes• measured in micrometers

Cell functions are similar in all cells

• Cells maintain a selective barrier called the plasma membrane between their cytoplasm and the extracellular environment. All substances that enter or leave the cell must pass across the barrier.

• Cells contain hereditary material carrying encoded instructions for the synthesis of most of the cellular components. This hereditary material is duplicated prior to cell reproduction so that each new cell carries a full set of instructions.

• Cells carry out metabolic activities, which are catalyzed chemical reactions that result in the synthesis and breakdown of organic molecules.

Components of a cell• A generalized body cell has four principal

divisions: – the plasma membrane– the cytoplasm– cytoplasmic organelles– the nucleus.

A Composite Cell

• hypothetical cell• major parts

• nucleus• cytoplasm• cell membrane

Cell Membrane• The proteins are divided into two categories: integral and peripheral.

– The integral proteins form the majority of membrane proteins. They penetrate and are embedded in the bilayer, bound to the non polar tail regions.

• The transmembrane proteins span the bilayer completely and may form channels (pores) for transport of substances across the membrane.

• Integral proteins also may lie partly submerged in one side or the other. They have several functions.

– Some integral proteins serve as cell surface enzymes.– Integral proteins bound to carbohydrates may form receptor

sites for chemical messages from other cells, such as endo crine glands.

– Some also function as markers, or antigens, which identify cell types.

• The peripheral proteins are loosely bound to the membrane surface and can be easily removed from it. Their functions are not as well known as those of integral proteins. They may be involved in structural support and changes in membrane shape during cell division or cell movement.

Cell Membrane• outer limit of cell• controls what moves in and out of cell• selectively permeable•phospholipid bilayer

• water-soluble “heads” form surfaces• water-insoluble “tails” form interior• permeable to lipid-soluble substances

• cholesterol stabilizes the membrane• proteins

• receptors• pores, channels, carriers• enzymes• CAMS (Cellular Adhesion Molecules)• self-markers

Cell Membrane

The Plasma Membrane

Intercellular JunctionsTight junctions

• close space between cells• located among cells that form linings

Desmosomes• form “spot welds” between cells• located among outer skin cells

Gap junctions• tubular channels between cells• located in cardiac muscle cells

Cell Adhesion Molecules

• guide cells on the move

• selectin – allows white blood cells to “anchor”

• integrin – guides white blood cells through capillary walls

• important for growth of embryonic tissue

• important for growth of nerve cells

Ribosomes • Structure

– Ribosomes are small granules composed of ribosomal RNA and almost 80 different proteins.

– They occur as individual granules or in clusters called polyribosomes.

– They may be free in the cytoplasm (free ribosomes) or attached to the membranes of the endoplasmic reticulum.

• Function– Ribosomes are the site of protein synthesis.– Free ribosomes are involved in the synthesis of proteins for the

cell’s own use; for example, in the renewal of enzymes and membranes.

– Attached ribosomes are the site of synthesis of proteins that are secretory products to be released from the cell.

Golgi Apparatus• Function• The Golgi apparatus is the site of accumulation, concentration,

packaging, and chemical modification of the secretory products synthesized on the rough ER. – The transport vesicles pinch off from the ER and carry the secretions to

the Golgi apparatus, where the secretions fuse with its cisternae.– The large condensing vacuoles concentrate the secretion and package

them to become secretory granules.– Secretory granules, which are large, densely packed, membrane-

bounded structures, unload their contents via exocytosis upon nervous or hormonal stimulation.

– The Golgi apparatus also chemically modifies the molecules synthesized in the ER for incorporation into the plasma membrane. It adds fatty acid residues to certain proteins to convert them to lipoproteins, and it synthesizes and attaches carbohydrate side chains to proteins to form glvcoproteins.

– The Golgi apparatus processes proteins that function intracellularly, such as the lysosome enzymes.

The Nucleus is the largest organelle

• It is present in all cells of the body except mature red blood cells, which lost their nuclei as they developed.

• Generally, each cell has a single nucleus, but some giant cells, such as megakaryocytes of bone marrow, osteoclasts of bone, and skeletal muscle cells, may have several nuclei.

Nucleus• Structure

– The nuclear envelope consists of a double membrane separated by the perinuclear space.

• The inner membrane is smooth. The outer membrane often contains ribosomes and is continuous with the surrounding ER.

• The inner and outer membranes fuse at irregular intervals around the nucleus to form nuclear pores, which allow for exchange of materials between the nucleus and the cytoplasm.

– Chromatin appears as irregular clumps or granules material dispersed throughout the nucleus.

• Chromatin is composed of coiled strands of DNA bound to basic proteins called histones, varying amounts of RNA, and other nonhistone proteins and enzyme systems.

• In a dividing cell, the chromatin is condensed and coiled into discrete units, the chromosomes. Human cells contain 23 pairs of chromosomes.

– The nucleoplasm is the matrix that surrounds the chromatin. It is composed of proteins, metabolites, and ions.

– The nucleolus is a spherical structure composed of RNA and protein. The size of the nucleolus and the number present vary in different cell types. It is missing in cells that do not synthesize protein, such as spermatozoa. It is the site of ribosome production

The cytoplasm contains a complex network of structural components

• Microfilaments– Structure

• Microfilaments are solid thread-like cylinders made of protein and found in a variety of sites within the cell.

– Function• Microfilaments are responsible for contractility of cells,

which is a property of all cells but is especially well developed in muscle cells.

• Contractility is responsible for cell locomotion and movements associated with phagocytosis, pinocytosis, and cell division.

Structural Components• Microtubules

– Structure• Microtubules are hollow tubes present everywhere

in the cytoplasm in all cells.• They are composed of protein tubulin molecules.

– Function• Microtubules contribute to the cytoskeleton, or

supporting elements, of the cell.• They also are involved in cell division, cell

movements, and the transport of materials from one area of the cell to another.

Structural Components• Centrioles

– Structure• In a nondividing cell, two centrioles are located near the

nucleus and Golgi apparatus in a specialized region called the centrosome.

– Function• Centrioles function in cell division and also as the site of the

formation of cilia and flagella.• Centrioles are self-replicating and divide prior to cell division.

Following replication, each original centriole and its duplicate migrate to opposite nuclear poles where they induce the forma tion of the spindle apparatus during cell division.

Structural Components

• Cilia and flagella– Structure

• Both cilia and flagella are motile processes that extend out from the cell surface.

• They are composed of longitudinal microtubules, which are arranged as two single tubules surrounded by a ring of nine regu larly spaced double tubules.

– Function• Both cilia and flagella function in movement.• Cilia are able to move fluid or a layer of mucus over the

surface of the cells on which they occur, while the flagellum of the sperm cell propels the cell.

Cytoplasmic OrganellesEndoplasmic Reticulum

• connected, membrane-bound sacs, canals, and vesicles• transport system• rough ER

• studded with ribosomes• protein synthesis

• smooth ER• lipid synthesis

•added to proteins arriving from rough ER

• break down of drugsRibosomes

• free floating or connected to ER• provide structural support

Cytoplasmic OrganellesGolgi apparatus•stack of flattened, membranous sacs•modifies, packages and delivers proteins

Mitochondria•membranous sacs with inner partitions•generate energy

Vesicles•membranous sacs•store substances

Cytoplasmic OrganellesLysosomes

• enzyme-containing sacs• digest worn out cell parts or unwanted substances

Peroxisomes• enzyme-containing sacs• break down organic molecules

Centrosome• two rod-like centrioles• used to produce cilia and flagella• distributes chromosomes during cell division

Cytoplasmic Organelles

Cilia• short hair-like projections• propel substances on cell surface

Flagellum• long tail-like projection• provides motility to sperm

Microfilaments and microtubules• thin rods and tubules• support cytoplasm• allows for movement of organelles

Cytoplasmic Organelles

Inclusions

• temporary nutrients and pigments

Cell Nucleus

• control center of cell

• nuclear envelope• porous double membrane• separates nucleoplasm from cytoplasm

• nucleolus• dense collection of RNA and proteins• site of ribosome production

• chromatin• fibers of DNA and proteins• stores information for synthesis of proteins

Movements Into and Out of the Cell

Passive (Physical) Processes• require no cellular energy• simple diffusion•facilitated diffusion• osmosis• filtration

Active (Physiological) Processes• require cellular energy• active transport• endocytosis• exocytosis• transcytosis

Diffusion • The random movement of particles (molecules or ions)

under the influence of their own thermal energy, from an area of their higher concentration to an area of their lower concentration, or “downhill.”

• Diffusion of molecules or ions may take place in a liquid, gas, or solid or through nonliving or living mem branes that are permeable to them.– Diffusion in a liquid is the movement of solute and solvent

particles in all directions through a solution, or in both directions through a permeable membrane.

– Net diffusion is the movement of particles from an area of their own high concentration to an area of lower concentration; that is, along their own concentration gradients. Net diffusion means more particles are diffusing in one direction than in the other.

The rate of net diffusion of particles in a solution is increased by the following

factors:• A higher concentration gradient because

there are more particles• A low molecular weight because large

particles are not as easily moved by colliding with each other

• An increase in temperature because higher temperature increases random particle movement.

Simple Diffusion

• movement of substances from regions of higher concentration to regions of lower concentration• oxygen, carbon dioxide and lipid-soluble substances

Osmosis

• movement of water through a selectively permeable membrane from regions of higher concentration to regions of lower concentration• water moves toward a higher concentration of solutes

OsmosisOsmotic Pressure – ability of osmosis to generate enough pressure to move a volume of water

Osmotic pressure increases as the concentration of nonpermeable solutes increases

• hypertonic – higher osmotic pressure• hypotonic – lower osmotic pressure• isotonic – same osmotic pressure

Facilitated Diffusion• In facilitated diffusion, the carrier substance combines with the solute

molecules to form a solute-carrier complex, which is soluble in the lipid-bilayer, and thus transports the solute across the membrane.

• Once on the other side, the solute is released. The carrier breaks away from the complex, returns to the exterior of the membrane, and repeats the process.– The carriers exhibit specificity; i.e. they are highly selective in

distinguishing between closely related molecules.– Facilitated diffusion can be inhibited by competitive and

noncompetitive inhibitor molecules, which closely resemble the solute molecules.

– The rate of passage of a solute through facilitated diffusion depends on:

• its concentration difference on both sides of the membrane• the number of carrier molecules available• how rapidly the solute-carrier complex formation takes place.

Facilitated Diffusion

• diffusion across a membrane with the help of a channel or carrier molecule• glucose and amino acids

Filtration

• smaller molecules are forced through porous membranes• hydrostatic pressure important in the body• molecules leaving blood capillaries

Active Transport• carrier molecules transport substances across a membrane from regions of lower concentration to regions of higher concentration• sugars, amino acids, sodium ions, potassium ions, etc.

Endocytosis (endo = inner) • Means taking into the cell. • It includes phagocytosis and pinocytosis.

– Phagocytosis (phago = to eat) is the engulfing of large solid sub stances by foldings of the plasma membrane to form a phagocytic vesicle.

• The phagocytic vesicle fuses with a lysosome and the lysosomal enzymes destroy the contents.

• Specialized phagocytic cells in the body remove disintegrating cells, foreign matter, and bacteria.

– Pinocytosis (pino to drink) is the engulfing of small drops of extracellular fluid, which may contain dissolved nutrients, and incorporating them into the cell.

• Receptor-mediated endocytosis refers to the binding of receptor molecules on the cell surface with specific substances known as ligands. The receptor-ligand complex then undergoes endocytosis for transport into the cell.

Endocytosis• cell engulfs a substance by forming a vesicle around the substance• three types

• pinocytosis – substance is mostly water• phagocytosis – substance is a solid• receptor-mediated endocytosis – requires the substance to bind to a membrane-bound receptor

Endocytosis

Exocytosis• reverse of endocytosis• substances in a vesicle fuse with cell membrane• contents released outside the cell• release of neurotransmitters from nerve cells

Transcytosis• endocytosis followed by exocytosis• transports a substance rapidly through a cell• HIV crossing a cell layer

Stages of replication • The two strands of DNA are unwound and separated

(unzipped) by un winding enzymes, which cause the weak hydrogen bonds between the paired bases to break.

• The enzyme DNA polymerase, using the four kinds of complementary nucleotides freely present in the nucleus, matches and attaches the nucleotides to the exposed bases on each unzipped, single stranded DNA.

• Two complete DNA double helices are formed, each identical in nucleotide sequence to the original DNA helix that served as the templates. Thus, the genetic information is copied exactly.

• Such replication is termed semiconservative because it conserves each strand of the original DNA double helix while each also receives a newly synthesized matching partner strand.

DNA Replication

• hydrogen bonds break between bases• double strands unwind and pull apart• new nucleotides pair with exposed bases• controlled by DNA polymerase

The cell cycle and mitosis

• The cell cycle, in cells that are capable of dividing, refers to the events in a cell’s life span in the period between the time it was formed by cell division to the beginning of the next cell division.

• The greatest portion of the cycle (about 90%) is devoted to growth and synthesis, called interphase, with a smaller portion devoted to nuclear and cell division, or mitosis.

The Cell Cycle

• series of changes a cell undergoes from the time it forms until the time it divides• stages

• interphase• mitosis• cytoplasmic division

The Cell Cycle

Interphase

• very active period• cell grows• cell maintains routine functions• cell replicates genetic material to prepare for nuclear division• cell synthesizes new organelles to prepare for cytoplasmic division• phases

• G phases – cell grows and synthesizes structures other than DNA• S phase – cell replicates DNA

Mitosis• produces two daughter cells from an original somatic cell• nucleus divides – karyokinesis• cytoplasm divides – cytokinesis • stages

• prophase – chromosomes form; nuclear envelope disappears• metaphase – chromosomes align midway between centrioles• anaphase – chromosomes separate and move to centrioles• telophase – chromatin forms; nuclear envelope forms

Mitosis

Control of Cell Division• cell division capacities vary greatly among cell types

• skin and blood cells divide often and continually• neuron cells divide a specific number of times then cease

• chromosome tips (telomeres) that shorten with each mitosis provide a mitotic clock

• cells divide to provide a more favorable surface area to volume relationship• growth factors and hormones stimulate cell division

• hormones stimulate mitosis of smooth muscle cells in uterus• epidermal growth factor stimulates growth of new skin

• tumors are the consequence of a loss of cell cycle control

• contact (density dependent) inhibition

Stem and Progenitor CellsStem cell

• can divide to form two new stem cells• self-renewal

• can divide to form a stem cell and a progenitor cell• totipotent – can give rise to every cell type• pluripotent – can give rise to a restricted number of cell types

Progenitor cell • committed cell• can divide to become any of a restricted number of cells • pluripotent

Stem and Progenitor Cells

The telomere paradox