A&P Lesson 2 - Cellular Form and Function

Robert Hooke

• made improvements to compound microscope

• first to see and name cells

• published first comprehensive book of microscopy

Robert Hooke Cell Theory

1. cells are the building blocks of all plants and animals

2. all cells come from the division of pre-existing cells

3. cells are the smallest units that perform all vital physiological functions

4. each cell maintains homeostasis at the cellular level

How big are most human cells?

10 to 15 um in diameter

How big are egg cells?

100 um diameter

how big are nerve cells

some are over 1 m long

can large cells support themselves?

no - risk for rupture

what happens when a cells increase in diameter?

the volume increases more than the surface area

Formula for volume proportional to cube of the cell’s radius

V = (4/3)(π r³)

Formula for surface area proportional to square of the cell’s radius

SA = (4)(π r²)

What are the 2 main classes of cells in the human body?

somatic cells and sex cells

Somatic Cells

All cells in the body except the ones created by meiosis (sex cells)

• includes neurons, osteocytes, muscle cells, epithelial cells, and more

Sex Cells

aka germ cells; the reproductive cells in the body

• male sperm cells

• female oocyte (egg)

Representative Cell

What are the major components of a cell?

• plasma/cell membrane

• cytoplasm

• extracellular fluid (ECF)

Plasma/Cell membrane

• defines cell boundaries (separates the outside from inside)

• made of proteins and lipids

Cytoplasm

• located inside the cell

• region between the plasma membrane and nucleus

• contains organelles, cytoskeleton, inclusions, and cytosol/intracellular fluid

Extracellular Fluid (ECF)

• located outside of cell

• includes any fluid outside the cells including tissue (interstitial) fluid, blood plasma, lymph, and cerebrospinal fluid

Cytosol vs Cytoplasm

Cytoplasm- everything withing the cell membrane; contains organelles cytosol and other substances

Cytosol- the fluid portion of the cytoplasm

Functions of the Plasma membrane

1. physical barrier

2. regulates entry/exit of materials into and out of the cell

3. responds to changes in the extracellular fluid

4. provides support to the cell

What is the plasma membranes composition?

~98% lipids (75% phospholipids, 20% cholesterol, 5% glycolipids)

Phospholipids

• 75% of membrane lipids

• amphipathic molecules arranged in a bilayer

• hydrophilic phosphate heads and hydrophobic tails

• drift laterally keeping membrane fluid

Cholesterol

• 20% of the membrane lipids

• holds phospholipids still and can stiffen or loosen the membrane

Glycolipids

• 5% of the membrane lipids

• phospholipids with short carbohydrate chains on extracellular face

• contribute to the glycocalyx

Glycocalyx

carbohydrate component on glycoproteins and glycolipids located external to plasma membrane

Functions of the glycocalyx

• protection from physical and chemical injury

• immunity to infection

• defense against cancer

• transplant compatibility

• cell adhesion

• fertilization

• embryonic development

Transmembrane proteins

• pass completely through membrane

• most are glycoproteins

• hydrophilic regions contact the cytoplasm on the inside and extracellular fluid on the outside

• hydrophobic regions pass through lipid region of the membrane

• some drift in membrane and some are anchored to cytoskeleton

Peripheral proteins

• adhere to one face of the membrane

• those on inner face usually tethered to the transmembrane protein and cytoskeleton

Membrane proteins

• constitute 2% of the molecules but 50% of the weight of the membrane

What are the functions of membrane proteins?

• receptors

• enzymes

• channel proteins

• carriers

• cell-identity markers

• cell-adhesion molecules (CAMs)

Receptors

bind to chemical signals to trigger internal changes; may cause production of a second messenger

Enzymes

catalyze reactions including digestion of molecules and production of second messengers

Channel proteins

allow hydrophilic solutes and water to pass through membrane

• leak channels and gated channels

leak channels

channel proteins that are always open

gates (gated channels)

channel proteins that open only when triggered

• ligand-gated channels

• voltage-gated channels

• mechanically gated channels

ligand-gated channels

gated channels that respond to chemical messengers

voltage-gated channels

gated channels that respond to charge changes

mechanically-gated channels

respond to physical stress on a cell

Carriers

bind solutes and transfer them across membrane

• pumps

pumps

carriers that consume ATP to move substances across the membrane AGAINST its concentration gradient

cell-identity markers

glycoproteins acting as identification tags

cell-adhesion molecules (CAMs)

mechanically link cell to another cell and to extracellular material

Simple diffusion

net movement of particles from place of high concentration to place of lower concentration without the need for energy input

• substances diffuse down their concentration gradient

• due to constant, spontaneous molecular motion

What factors affect diffusion rate through a membrane?

• temperature - ↑ temp. causes ↑ motion of particles

• molecular weight - smaller molecules move faster

• steepness of concentration gradient - ↑ gradient causes ↑ rate

• membrane surface area - ↑ surface area causes ↑ rate

• membrane permeability - ↑ permeability causes ↑ rate

osmosis

net flow of water through a selectively permeable membrane

• enhanced by aquaporins

aquaporins

channel proteins in the membrane that are specialized for water passage

how does water move in osmosis?

from an area of higher water (lower solute) concentration to an area of lower water (higher solute) concentration

What do osmotic imbalances cause?

diarrhea, constipation, edema

osmotic pressure

hydrostatic pressure required to stop osmosis

hydrostatic pressure

fluid pressure on the membrane

reverse osmosis

process of applying mechanical pressure to override osmotic pressure

• allows purification of water

osmolarity

osmotic concentration of all solutes; quantity of non-permeating solutes per liter of solution

blood plasma, tissue fluid, and intracellular fluid are ___ milliosmoles per liter (mOsm/L)?

300 mOsm/L

tonicity

ability of a surrounding solution to affect fluid volume and pressure in a cell

• if a solute is more concentrated on one side of the membrane it causes osmosis of water towards the more concentrated side

Hypotonic solution

causes cell to absorb water, swell and possibly burst (lyse)

• lower concentration of nonpermeating solutes than intracellular fluid inside the cell

• ex. distilled water

Hypertonic solution

causes cell to lose water and shrivel (crenate)

• has higher concentration of nonpermeating solutes than intracellular fluid

Isotonic solution

causes no change in cell volume

• same concentration of nonpermeating solutes in extra and intracellular fluid

• ex. saline

carrier-mediated transport

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

• carriers exhibit specificity for their particular solutes

• carriers also exhibit saturation

What are the 3 kinds of carrier proteins?

• uniport

• symport

• antiport

uniport

carrier that moves on type of solute

• ex. calcium pump only moves calcium

symport

carrier that moves 2 or more solutes simultaneously in the same direction (cotransport)

• ex. sodium-glucose transporters move both sodium and glucose at the same time and in the same direction

antiport

carrier that moves 2 or more solutes in opposite directions (countertransport)

• ex. sodium-potassium pump moves Na+ out of the cell and K+ into the cell

What are the 3 mechanisms of carrier-mediated transport?

1. facilitated diffusion

2. primary active transport

3. secondary active transport

Facilitated diffusion

carrier moves solute down its concentration gradient

• doesn’t 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 to its concentration gradient

• the carrier protein uses ATP for energy

• ex. calcium pump uses ATP while expelling calcium from cell to where it is already more concentrated

Secondary active transport

carrier moves solute through membrane, but only uses ATP indirectly

• ex. sodium-glucose transporter (SGLT)

Sodium-potassium pump (Na+/K+ pump)

• each pump cycle consumes one ATP and exchanges 3 NA+ for 2K+

• Keeps K+ concentration higher and Na+ concentration lower within the cell compared to the extracellular fluid

What are the functions of the sodium-potassium pump?

• maintains the NA+ gradient for use in secondary active transport

• Regulates solute concentration and thus osmosis and cell volume

• maintains negatively charged resting membrane potential

• produces heat

Vesicular transport

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

Vesicles

bubble-like enclosures of membrane

Endocytosis

brings material into cell; exocytosis releases material from cell

What are the 3 forms of endocytosis?

1. phagocytosis

2. pinocytosis

3. receptor-mediated endocytosis

Phagocytosis

engulfing and destroying large particles; cell eating

• pseudopods surround object, fuse to form internal phagosome, which merges with lysosome to form phagolysosome within which the 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

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

Transcytosis

transport of material across the cell by capturing it on one side and releasing it on the other

Exocytosis

discharge material from cell; essentially the reverse of endocytosis

• also functions to replace any plasma membrane lost by internalization during endocytosis