Order 1218450: A & P Chap 1-4

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3 Cellular form and function

Expected Learning Outcomes • Identify the three main parts of a cell, and list the general functions of each.

• Explain how cytoplasm and cytosol are different.

• Describe how lipids are distributed in a cell membrane, and explain their functions.

• Describe how carbohydrates are distributed in a cell membrane, and explain their functions.

• Describe how proteins are distributed in a cell membrane, and explain their functions.

• With respect to the following membrane transport processes – simple diffusion, facilitated diffusion, osmosis, active transport, exocytosis, endocytosis, phagocytosis, pinocytosis, & filtration: • State the type of material moving in each process. • Describe the mechanism by which movement of material occurs in each process. • Discuss the energy requirements and, if applicable, the sources of energy for each process. • Give examples of each process in the human body.

• Describe the effects of hypertonic, isotonic, and hypotonic conditions on cells.

• Demonstrate various cell transport processes and, given appropriate information, predict the outcomes of these demonstrations.

• Define the term organelle.

• For each different type of organelle associated with human cells: • Identify the organelle. • Describe the structure of the organelle. • Describe the function of the organelle

Introduction

• All organisms are composed of cells

• Cells are responsible for all structural and functional properties of a living organism

• Important for understanding • Workings of human body

• Mechanisms of disease

Development of the Cell Theory

• Cytology—scientific study of cells • Began when Robert Hooke coined the word cellulae to describe empty cell

walls of cork in 17th century

• Theodor Schwann concluded, about two centuries later, that all animals are made of cells

• Louis Pasteur demonstrated in 1859 that “cells arise only from other cells” • Refuted idea of spontaneous generation—living things arising from nonliving

matter

• Cell theory • All organisms 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 • Cells of all species exhibit biochemical similarities

Cell Shapes and Sizes

• About 200 types of cells in human body with varied shapes

• Squamous—thin, flat, scaly

• Cuboidal—squarish-looking

• Columnar—taller than wide

• Polygonal—irregularly angular shapes, multiple sides

• Stellate—star-like

• Spheroid to ovoid—round to oval

• Discoid—disc-shaped

• Fusiform—thick in middle, tapered toward the ends

• Fibrous—thread-like

• Note: A cell’s shape can appear different if viewed in a different type of section (longitudinal vs. cross section)

The Relationship Between Cell Surface Area and Volume

• Most cells about 10–15 micrometers (μm) in diameter • Egg cells (very large) 100 μm diameter

• Some nerve cells over 1 meter long

• Limit on cell size: an overly large cell cannot support itself, may rupture • For a given increase in diameter, volume

increases more than surface area • Volume proportional to cube of diameter

• Surface area proportional to square of diameter

Large cell

Diameter = 20 μm Surface area = 20 μm × 20 μm × 6 = 2,400 μm2

Volume = 20 μm × 20 μm × 20 μm = 8,000 μm3

Small cell

Diameter = 10 μm Surface area = 10 μm × 10 μm × 6 = 600 μm2

Volume = 10 μm × 10 μm × 10 μm = 1,000 μm3

Effect of cell growth:

Diameter (D) increased by a factor of 2 Surface area increased by a factor of 4 (= 𝐷2) Volume increased by a factor of 8 (= 𝐷3)

Basic Components of a Cell

• Plasma (cell) membrane • Surrounds cell, defines

boundaries • Made of proteins and lipids

• Cytoplasm • Organelles • Cytoskeleton • Inclusions • Cytosol (intracellular fluid,

ICF)

• Extracellular fluid (ECF) • Fluid outside of cells

includes tissue (interstitial) fluid

The Plasma Membrane

• Has intracellular and extracellular faces

• Functions • Defines cell boundaries • Governs interactions with other cells • Controls passage of materials in and out

of cell

• 98% of membrane molecules are lipids • 75% phospholipids

• Amphipatic 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 membrane fluid • 20% Cholesterol

• Can both holds phospholipids still and stiffen membrane or prevent close packing and make membrane more flexible

• 5% Glycolipids • Phospholipids with short carbohydrate

chains on extracellular face • Contributes to glycocalyx—carbohydrate

coating on cell surface

The Plasma Membrane

• 2% of the molecules but 50% of the weight of membrane are protein

• Integral proteins—penetrate membrane • Transmembrane proteins pass

completely through • Hydrophilic regions contact

cytoplasm, extracellular fluid • Hydrophobic regions pass

through lipid of the membrane • Some drift in membrane;

others are anchored to cytoskeleton

• Peripheral proteins • Adhere to one face of the

membrane (do not penetrate it)

• Usually tethered to the cytoskeleton

Some Functions of Membrane Proteins

The Glycocalyx

• Fuzzy coat external to plasma membrane • Carbohydrate moieties of

glycoproteins and glycolipids

• Unique in everyone but identical twins

• Functions • Physical Protection

• Self Identification (e.g. in immunity or transplant compatibly)

• Cell adhesion

Extensions of Cell Surface: Microvilli

• Extensions of membrane (1–2 μm) • Gives 15 to 40 times more

surface area

• Best developed in cells specialized in absorption

• On some absorptive cells they are very dense and appear as a fringe— “brush border”

Extensions of Cell Surface: Cilia • Cilia—hairlike processes 7–10 μm long

• Single, nonmotile primary cilium found on several cells such as • Balance cells in inner ear

• Motile cilia—respiratory tract, uterine tubes, ventricles of brain, ducts of testes • 50 to 200 on each cell • Beat in waves sweeping material across a surface in one direction • Power strokes followed by recovery strokes

Extensions of Cell Surface: Cilia

• Axoneme—core of motile cilium • Has 9 + 2 structure of

microtubules

• “Grows” from basal body

• Uses energy from ATP

Extensions of Cell Surface: Flagella and Pseudopod

• Tail of a sperm—only functional flagellum in humans

• Whip-like structure with axoneme identical to cilium’s • Much longer than cilium

• Movement is undulating, snake-like, corkscrew • No power stroke and recovery strokes

• Pseudopods—continually changing extensions of the cell that vary in shape and size • Can be used for cellular locomotion, capturing • foreign particles

Membrane Transport

• Plasma membrane is selectively permeable—allowing some things through, but preventing others from passing

• Passive mechanisms require no ATP • Random molecular motion of particles provides necessary energy

• Filtration, diffusion, osmosis

• Active mechanisms consume ATP • Active transport and vesicular transport

• Carrier-mediated mechanisms use a membrane protein to transport substances across membrane

Filtration

• Filtration—particles are driven through membrane by physical pressure

• Examples • Filtration of water and

small solutes through gaps in capillary walls • Allows delivery of water

and nutrients to tissues

• Allows removal of waste from capillaries in kidneys

Simple Diffusion

• Simple diffusion—net movement of particles from place of high concentration to place of lower concentration • Due to constant, spontaneous molecular motion • Molecules collide and bounce off each other

• Substances diffuse down their concentration gradient • Does not require a membrane • Substance can diffuse through a membrane if the membrane is permeable to

the substance

• Factors affecting diffusion rate through a membrane • Temperature:  temp.,  motion of particles • Molecular weight: larger molecules move slower • Steepness of concentrated gradient: difference,  rate • Membrane surface area:  area,  rate • Membrane permeability:  permeability,  rate

Osmosis

• Osmosis—net flow of water through a selectively permeable membrane • Water moves from the side where it

(water) is more concentrated to the side where it is less concentrated

• Solute particles that cannot pass through the membrane “draw” water from the other side

• Water can diffuse through phospholipid bilayers, but osmosis is enhanced by aquaporins—channel proteins in membrane specialized for water passage • Cells can speed osmosis by installing more

aquaporins

• Osmotic pressure—hydrostatic pressure required to stop osmosis • Increases as amount of nonpermeating

solute rises • Reverse osmosis—process of applying

mechanical pressure to override osmotic pressure • Allows purification of water

(a) Start

(b) 30 minutes later

Osmolarity and Tonicity • One osmole (osm) = 1 mole of dissolved

particles • Takes into account whether solute ionizes in

water • 1 M glucose is 1 osm/L • 1 M NaCl is 2 osm/L

• Osmolarity—number of osmoles per liter of solution • Body fluids contain a mix of many

chemicals, and osmolarity is the total osmotic concentration of all solutes

• Blood plasma, tissue fluid, and intracellular fluid are 300 milliosmoles per liter (mOsm/L)

• Tonicity—ability of a surrounding solution to affect fluid volume and pressure in a cell • Depends on concentration of

nonpermeating solutes

• Hypotonic solution

• Hypertonic solution

• Isotonic solution

(a) Start

(b) 30 minutes later

Effects of Tonicity on RBCs

• Hypotonic solution—causes cell to absorb water and swell • Has a lower concentration of nonpermeating solutes than intracellular fluid (ICF) • Distilled water is an extreme example

• Hypertonic solution—causes cell to lose water and shrivel (crenate) • Has a higher concentration of nonpermeating solutes than ICF

• Isotonic solution—causes no change in cell volume • Concentrations of nonpermeating solutes in bath and ICF are the same • Normal saline (0.9% NaCl) is an example

Carrier-Mediated Transport

• Transport proteins in membrane carry solutes into or out of cell (or organelle)

• Specificity • Transport proteins are specific for particular solutes • Solute (ligand) binds to receptor site on carrier protein • Solute is released unchanged on other side of membrane

• Three kinds of carriers 1. Uniport—carries one type of solute

• Example: Calcium pump

2. Symport—carries two or more solutes simultaneously in same direction (cotransport) • Example: sodium-glucose transporters

3. Antiport—Carries two or more solutes in opposite directions (countertransport) • Example: sodium-potasium pump removes Na+, brings in K+

• 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 on carrier, carrier changes conformation, then releases solute on other side of membrane

Primary active transport • Carrier moves solute through a

membrane up its concentration gradient

• The carrier protein uses ATP for energy

• Examples: • 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 • Each pump cycle consumes one ATP and

exchanges three Na+ for two K+

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

• Necessary because Na+ and K+ constantly leak through membrane

• Half of daily calories utilized for Na+−K+ pump

• Why? • Regulates solute concentration and thus

osmosis and thus cell volume • Maintains negatively charged resting

membrane potential • Maintains steep Na+ concentration gradient

allowing for secondary active transport

Secondary active transport

• Carrier moves solute through membrane but only uses ATP indirectly

• Example: sodium-glucose transporter (SGLT) (symport) • Moves glucose into cell while simultaneously

carrying sodium down its gradient

• Depends on the primary transport performed by Na+- K+pump

• Does not itself use ATP

• Prevents loss of glucose to urine

Vesicular Transport • Moves large particles, fluid

droplets, or numerous molecules at once through the membrane in vesicles—bubble-like enclosures of membrane

• Endocytosis—vesicular processes that bring material into cell • Pinocytosis—“cell drinking,”

taking in droplets of ECF containing molecules useful in the cell • Membrane caves in, then pinches off

pinocytic vesicle

• Phagocytosis - engulfing large particles using pseudopods creating phagosome

• Receptor-mediated endocytosis— particles bind to specific receptors on plasma membrane • Clathrin-coated vesicle

• Exocytosis—discharging material from the cell

Exocytosis and Transcytosis

• Exocytosis—discharging material from the cell

• Transcytosis • Transport of material across the cell by capturing it on one side and releasing it

on the other

• Receptor-mediated endocytosis moves it into the cell and exocytosis moves it out the other side

The Cell Interior

The Cytoskeleton • Network of protein filaments and

cylinders

• Composed of: microfilaments, intermediate fibers, microtubules

• Microfilaments • 6 nm thick • Made of actin protein • Forms terminal web under p.m.

• Intermediate filaments • 8–10 nm thick • Within skin cells, made of protein keratin • Give cell shape, resist stress

• Microtubules • 25 nm thick • Consist 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 proteins, make axonemes of cilia and flagella, form mitotic spindle

Organelles

• Internal structures of a cell, carry out specialized metabolic tasks

• Membranous organelles • Nucleus, mitochondria, lysosomes, peroxisomes,

endoplasmic reticulum, and Golgi complex

• Nonmembranous organelles • Ribosomes, centrosomes, centrioles, basal bodies

The Nucleus • Largest organelle (5 μm in

diameter) • Most cells have one nucleus • A few cell types are anuclear or

multinucleate

• Nuclear envelope—double membrane around nucleus • Perforated by nuclear pores formed

by rings of proteins • Regulate molecular traffic through

envelope • Hold the two membrane layers

together

• Nuclear envelope is supported by nuclear lamina • Web of protein filaments • Provides points of attachment for

chromatin • Helps regulate cell life cycle

• Nucleoplasm—material in nucleus • Chromatin (thread-like) composed of

DNA and protein • Nucleoli—masses where ribosomes

are produced

Endoplasmic Reticulum • System of channels (cisternae)

enclosed by membrane

• Rough endoplasmic reticulum— parallel, flattened sacs covered with ribosomes • Continuous with outer membrane of

nuclear envelope • Produces phospholipids and proteins of

the plasma membrane • Synthesizes proteins that are packaged

in other organelles or secreted from cell

• Smooth endoplasmic reticulum • Lack ribosomes • Cisternae more tubular and branching • Synthesizes steroids and other lipids • Detoxifies alcohol and other drugs • Calcium storage

• Rough and smooth ER are functionally different parts of the same network

Ribosomes

• Ribosomes—small granules of protein and RNA • Found in nucleoli, in

cytosol, and on outer surfaces of rough ER, and nuclear envelope

• They “read” coded genetic messages (messenger RNA) and assemble amino acids into proteins specified by the code

Golgi Complex

• System of cisternae that synthesizes carbohydrates and puts finishing touches on protein synthesis

• Receives newly synthesized proteins from rough ER

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

Lysosomes and Peroxisomes

• Lysosomes - Package of enzymes bound by a membrane

• Functions • Intracellular hydrolytic digestion of proteins, nucleic acids, complex

carbohydrates, phospholipids, and other substances • Autophagy—digestion of cell’s surplus organelles • Autolysis—“cell suicide”: digestion of a surplus cell by itself

• Peroxisomes - Resemble lysosomes but contain different enzymes and are produced by endoplasmic reticulum

• Function is to use molecular oxygen to oxidize organic molecules • Neutralize free radicals, detoxify alcohol, other drugs, and a variety of blood-

borne toxins • Break down fatty acids into acetyl groups for mitochondrial use in ATP synthesis • Reactions produce hydrogen peroxide H2O2 • Catalase breaks down excess peroxide to H2O and O2

• In all cells, but abundant in liver and kidney

Proteasomes

• Proteasomes—hollow, cylindrical organelle that disposes of surplus proteins

• Contain enzymes that break down tagged, targeted proteins into short peptides and amino acids

Mitochondria

• Specialized for synthesizing ATP

• Surrounded by a double membrane • Inner membrane has folds

called cristae

• Spaces between cristae called matrix • Matrix contains ribosomes,

enzymes used for ATP synthesis, small circular DNA molecule

• Mitochondrial DNA (mtDNA)

• “Powerhouses” of the cell • Energy is extracted from

organic molecules and transferred to ATP

Centrioles

• Short cylindrical assembly of microtubules arranged in nine groups of three microtubules each

• Two centrioles lie perpendicular to each other within the centrosome—small clear area in cell • Play important role in cell

division

• Form basal bodies of cilia and flagella • Each basal body is a centriole

that originated in centriolar organizing center and then migrated to the membrane

Inclusions

• Stored cellular products • Glycogen granules, pigments, and fat droplets