A&P Chapter 3: Cellular Form and Function

Cytology

Scientific study of cells

Cell theory generalizations

• All organisms are composed of cells and cell products

• Cell is the simplest structural and functional unit of life

• An organism’s structure and functions are due to activities of cells

• Cells come only from preexisting cells

Basic components of a cell

• Surrounded by a plasma (cell) membrane

• Cytoplasm within the cell

• Extracellular fluid (ECF) is located outside of cells

Plasma or cell membrane

Membrane that surrounds a cell, defines its boundaries, is made of proteins and lipids though composition can vary between regions of the cell

Cytoplasm

Gel within the cell, contained organelles, cytoskeleton, inclusions (stored or foreign particles), and clear gel called the cytosol or intracellular fluid

Extracellular fluid (ECF)

Fluid that includes any fluid outside of cells, including tissue (interstitial) fluid, blood plasma, lymph, and cerebrospinal fluid

Cytosol or Intracellular fluid

What is a plasma membrane made of?

Most of membranes are composed of lipids; phospholipids, cholesterol, and glycolipids

Characteristics of phospholipid cell membrane

75% of membrane lipids. Amphipathic molecules arranged in a bilayer. Hydrophilic phosphate heads face water on each side of membrane. Hydrophobic tails are directed toward the center, avoiding water. Drift laterally, keeping the membrane fluid

Characteristics of cholesterol cell membrane

20% of the membrane lipids. Holds phospholipids still and can stiffen membrane

Characteristics of glycolipid cell membrane

5% of the membrane lipids, phospholipids with short carbohydrate chains on extracellular face. Contribute to glyocalyx— carbohydrate coating on cell surface

How much do membrane proteins constitute on the cell membrane?

Constitute 2% of the molecules, but 50% of the weight of the membrane

Transmembrane proteins

Proteins that are able to pass completely through the membrane

Characteristics of transmembrane proteins

• Hydrophilic regions contact the water cytoplasm, extracellular fluid

• Hydrophobic regions pass through lipid of the membrane

• Most are glycoproteins

• Some drift in membrane, others anchored to cytoskeleton

Peripheral proteins

Proteins that adhere to one face of the cell membrane. Those on inner face are usually tethered to a transmembrane protein and the cytoskeleton

Functions of membrane proteins

Receptors, enzymes, channel proteins, carriers, cell-identity markers, cell-adhesion molecules

Channel proteins

Proteins on cell membrane that allow hydrophilic solutes and water to pass through membrane.

Types and characteristics of channel proteins

Leak channels- are always open

Gated channels- only open when triggered

Ligand-gated channels- respond to chemical messengers

Voltage-gated channels- respond to charge changes

Mechanically gated channels- respond to physical stress on cell

Carrier membrane proteins

Bind solutes and transfer them across membrane. This include pumps, carriers that consume ATP

Cell identity markers

glycoproteins acting as ID tags distinguishing the body’s own cells from foreign cells

Cell-adhesion molecules (CAMs)

mechanically link cell to another cell and to extracellular material

Microvilli

Extensions of the membrane that serve to increase surface area of the cell

Characteristics of microvilli

Provide 15 to 40 times more surface area to cells that have them. Best developed in cells specialized in absorption. On some absorptive cells they are very dense and appear as a fringe called the brush border. Some microvilli contain actin filaments that are tugged toward the center of cell to milk absorbed contents into cell

Glycocalyx

Carbohydrate layer covering plasma/cell membrane

Cilia

Hair-like processes, a little longer than microvilli

Characteristics of cilia

Multiple nonmotile cilia found on sensory cells of nose. Motile cilia less widespread, can be found in respiratory tract, uterine tubes, ventricles of brain, ducts of testes. 50 to 200 on each cell. Beat in waves sweeping materials across a surface in one direction. Power stroke followed by recovery stroke

Flagellum

Whiplike structure on cell surface. Much longer than cilium and stiffened by coarse fibers that support the tail. Movement is undulating with no power or recovery strokes. Only functional flagellum in humans is tail of sperm

Selectively permeable

Allow some things through the plasma membrane and organelle membranes, but preventing others from passing

Passive permeation mechanism with examples

Random molecular motion of particles provides necessary energy for things to pass through without requiring ATP. Ex. filtration, diffusion, osmosis

Active permeation mechanisms with examples

Active transport and vesicular transport that requires consumption of ATP

Carrier-mediated permeation mechanism

Uses a membrane protein to transport substances across membrane

Simple diWffusion

Net movement of particles from place of high concentration to plade of lower concentration

Where does simple diffusion occur, where does it go, and does it require energy

It occurs in air and water, does not require a membrane. Substances can diffuse through a membrane if membrane is permeable to substance, it flows down the gradient so no energy is required

Factors that affect diffusion rate through a membrane

Temperature (higher temp, higher motion), molecular weight (larger molecules move slower), steepness of concentration gradient (increased difference, increased rate), membrane surface area (increase area increase rate) membrane permeability (increase permeability, increase rate)

Osmosis

Net flow of water through a selectively permeable membrane. Water moves from an area of higher water (lower solute) concentration to an area of lower eater concentration

Aquaporins

Channel proteins in membrane specialized for water. enhances osmosis

Can water diffuse through phospholipid bilayers

How can cells speed up osmosis?

By installing more aquaporins

What is osmosis a crucial consideration in?

IV fluids

What do osmotic imbalances cause?

Diarrhea, constipation, edema

Hypotonic solution

Has a a lower concentration of nonpermeating solutes than intracellular fluid. Causes cell to absorb water, swell, and possibly burst (lyse)

Ex. distilled water

Hypertonic solution

Has a higher concentration on nonpermeating solutes than intracellular fluid, causes cell to lose water and shrivel (crenate)

Isotonic solution

Concentrations on nonpermeating solutes in ECF and ICF are the same, causes no change in cell volume

Ex. Normal saline (0.9% NaCl)

Carrier-mediated transport

Proteins (carriers) in cell membrane carry solutes into or our of cell (or organelle)

Specificity for protein carriers

• Solute (ligand) binds to receptor site on carrier protein

• Solute is released unchanged on other side of membrane

Three mechanisms of carrier-mediated transport

Facilitated diffusion, primary active transport, secondary active transport

Facilitated diffusion

Carrier moves solute down its concentration gradient

• Does not consume ATP

• Solute attaches to binding site, carrier changes conformation, then releases solute on other side of membrane

Primary active transport

Carrier moves solute through a membrane up its concentration gradient

• Carrier protein uses ATP for energy

Examples of primary active transports

• Calcium pump (uniport) uses ATP while expelling calcium from cell to where it is already more concentrated

• Sodium-potassium pump (antiport) uses ATP while expelling sodium and importing potassium into cell

Secondary active transport

Carrier moves solute through membrane, but only uses ATP indirectly

Example of secondary active transport

Sodium-glucose transporter (SGLT)

Sodium-glucose transporter (SGLT)

• Moves glucose into cell, up its concentration gradient, while also carrying sodium down its gradient

• Depends on primary transport performed by sodium-potassium pump

• Does not itself use ATP

• Works in kidney cells; prevents loss of glucose to urine

Sodium-potassium pump (Na+— K+ pump)

Important primary active transport

• Each pump cycle consumes one ATP and exchanges 3 Na+ for two K+

• Keeps K+ concentration higher and Na+ concentration lower within the cell than in ECF

Vesicular transport

Moves large particles, fluid droplets, or numerous molecules at once through the membrane in vesicles

Vesicles

Bubblelike enclosures of membrane

Endocytosis

Brings material into the cell

Exocytosis

Releases material from the cell

3 forms of endocytosis

Phagocytosis, pinocytosis, and receptor-mediated endocytosis

Phagocytosis

Engulfing and destroying large particles “cell eating”

How does phagocytosis work?

Pseudopods surround object, fuse to form internal phagosome, which merges with lysosome to form phagolysosome within which object is digested

Pinocytosis

Taking in droplets of ECF containing molecules useful in the cell; pinocytic vesicles in cytoplasm; “cell drinking”

Receptor-mediated endocytosis

Particles bind to specific receptors on plasma membrane

How does receptor-mediated endocytosis occurr?

A pit forms in membrane, cytosolic side covered in clathrin protein; forms clathrin-coated vesicle that is directed to destination within cell

Examples of exocytosis

• Release of insulin by endocrine cells

• Sperm cells release enzymes for penetrating egg

• Mammary gland cells release milk sugar

Also function to replace any plasma membrane lost by endocytosis

Cytosol

Clear, viscous, watery colloid within the cell

• Contains enzymes, other proteins, amino acids, ATP, electrolytes, dissolved gases, metabolic wastes

Cytoskeleton

Network of protein filaments and cylinders; composed of microfilaments, intermediate fibers, microtubules

Functions of the cytoskeleton

• Structural support, determines cell shape, organizes cell contents

• Directs movement of materials within cell and contributes to movements of the cell as a whole

Components of the cytoskeleton

Microfilaments, intermediate filaments, microtubules

Microfilaments (thin filaments)

• 6 nm thick; made of actin protein

• Form terminal web (membrane skeleton)

Intermediate filaments

• 8 to 10 nm thick; within skin cells; made of protein keratin

• Give cell shape, resist stress

Microtubules

• 25 nm thick; consists of protofilaments made of protein tubulin, radiate from centrosome; can come and go

• Maintain cell shape, hold organelles, act as railroad tracks for walking motor protein, make axonemes of cilia and flagella, mitotic spindle

Organelles

Internal structures of a cell that carry out specialized metabolic tasks

Nucleus

Typically largest organelle, contains the cell’s genetic material, most cells have 1 nucleus

Anuclear

A cell without a nucleus

Multinuclear

Cells with multiple nuclei

Nuclear envelope

double membrane around nucleus

Nuclear pores

perforations on the nuclear envelope, each formed by a ring of proteins called the nuclear pore complex

Function of nuclear pores

Regulate molecular traffic through envelope; hold two membrane layers together

Nuclear lamina

web of protein filaments that supports the nuclear envelope

Nucleoplasm

Material within nucleus; includes threadlike chromatin (DNA and proteins) and one or more nucleoli where ribosomes are produced

Endoplasmic reticulum (ER)

Network of interconnected membranous channels called cisterns

Rough endoplasmic reticulum

Parallel, flattened sacs covered with ribosome

Function of rough ER

• Continuous with outer membrane of nuclear envelope

• Produces phospholipids and proteins of nearly all cell membranes

• Synthesizes proteins that are packaged in other organelles or secreted from cell

Smooth endoplasmic reticulum

Tubular ER lacking ribosomes

Function of smooth ER

• Synthesizes steroids and other lipids

• Detoxifies alcohol and other drugs

• Calcium storage

Ribosomes

Small granules of protein and RNA responsible for protein synthesis; found in nucleoli, in cytosol, on outer surfaces of rough ER, and nuclear envelope

Function of ribosomes

Small granules of protein and RNA responsible for protein synthesis

Golgi complex

A system of membranous cisterns that synthesizes carbohydrates and modifies newly synthesized proteins

Function of the Golgi complex

• Synthesizes carbohydrates

• Receives newly synthesized proteins from rough ER

• Sorts proteins, splices some, adds carbohydrate moieties to some and packages them into membrane-bound Golgi vesicles

Golgi vesicles

• Some vesicles become lysosomes

• Some vesicles migrate to plasma membrane and fuse to it

• Some become secretory vesicles that store a protein product for later release

Secretory vesicles

Store a protein product for later release

Lysosomes

Package of enzymes bound by a membrane

Functions of lysosomes

• Intracellular hydrolytic digestion of proteins, nucleic acids, complex carbohydrates, phospholipids, and other substances

• Autophagy and autolysis

Autophagy

Digestion of cell’s surplus organelles

Autolysis

Digestion of a surplus cell by itself, “cell suicide”

Peroxisomes

Resemble lysosomes but contain different enzymes and are produced by ER

• Present in all cels; abundant in liver and kidney

Mitochondria

Organelles specialized for synthesizing ATP; powerhouses of the cell; energy is extracted from organic molecules and transferred to ATP

Characteristics of mitochondria

• Continually change shape from spheroidal to thread-like

• Surrounded by a double membrane

Cristae

Folds on inner membrane of the mitochondria

Matrix

Spaces between the folds on inner membrane of the mitochondria (cristae)

• Contain ribosomes, enzymes used for ATP synthesis, and small circular DNA molecules called mitochondrial DNA (mtDNA)