25-26 Cell Membrane

cell

structural unit of all living things

3 parts of cell

1. plasma mem

2. cytoplasm

3. nucleus

all cells composed of 

CHNO

trillions of cells

28-36 trillion

what is plasma mem

• defines extent of cell

• separates intracellular from extracellular

• function- selectively permeable barrier

fluid mosaic model plasma mem

• thin structure

• bilayer of lipids wt dispersed proteins
  → “sea of lipids wt floating iceburgs”

what do proteins do in plasma mem

form a constantly changing mosaic pattern 

lipid bilayer consists of

Phospholipids (75)

Cholesterol (20)

Glycolipids (5)

Phospholipids orient themselves in aqueous solutions such that

the polar heads face the interior and exterior of the cell with the tails forming center of membrane

bc of lipid content interior of mem is

hydrophobic

glycolipids (5%)

phospholipids wt attached sugar 

→ fatty acid tail nonpolar 

glycolipids are found ONLY

on the outer plasma mem surface

glycolipids

cholesterol (20%)

hydrocarbon rings btw phospholipid tails 

what does cholesterol do

stabilizes mem and fluidity of the mem

mem proteins

• ½ the mass

• responsible for most specialized mem functions

mem proteins are either 

integral (part in wt trails other out) or peripheral (either inside/outside mem) proteins

function of mem proteins

• formation of channel

• transporter proteins

• receptor proteins

• cell identity marker

• linker

• act as enzyme

linker

anchor proteins in cell mem or to other cells, allow cell movement, cell shape, and structure

glycocalyx

sticky carb/sugar coating on cell surface that is made up of glycolipids and glycoproteins

→ attached to mem proteins and lipids

• each cell hv diff pattern of sugars

• glycocalyx provides highly specific bio makers by which cells recognzie eachother

fluid nature of cell mem

dynamic- not stiff and constantly moving 

lipids in cell mem move

freely from side to side buy can NOT flip flop

proteins in cell mem move

some move freely

which protein movement restricted

peripheral proteins- b/c it is attatched to the intracellular cytoskeleton

microvilli

• special feature of mem 

• small fingerlike extensions of mem that project from free or exposed cell surfacem cire if actin filaments 

• SMALLEST 

function of microvilli

• increase surface area of plasma mem → SMALLEST

microvilli are absorptive cells in

intestines and kidney tubules (usally on top)

cilia

• extensions from surface of cell

• microtubules covered in plasma mem

• beat in coordinated wave-like motion

function of cilia 

move things over surface of sheet cells as they beat in coordinated fashion

flagellum

• only in human sperm cells

• microtubules covered in plasma mem

• propell entire cell

3 factors bind cells tg

1. glycoprotein of glycocalyx (sticky) act as an adhesive

2. wavy contours of mem of adjacent cells fit tg in a tounge and groove fashion (PUZZLE PIECES)

3. Special mem junctions

4. → tight junctions

5. → GAP junctions

6. → Desmosomes

Tight junctions

• dont want to leak out

• mem proteins hold tg/meet in middle

• EX: bladder stores urine tight junctions seal space btw cells

• keep inside lumen

connexon proteins

proteins in GAP junctions

GAP junctions

• allows ions flow from cell to cell

• → communication to sen message thru ions

• important in muscle (heart)

• → upper chamber of cell floods and contractcs at same time

• connexon proteins

desmosomes

• protein plaques

• → send out proteins (linker proteins) intracellular space to hold tg

• each like button: hold tg so don’t pull apart

• EX: skin (tension)

• → takes force over one and sitributes among all cells

• hemi-desmosome anchors to BM

hemi desmosome

½ desmosome