3 Cells: The Living Units: Part A - PowerPoint Presentation

Slide1

3

Cells: The Living Units: Part A

Slide2

Figure 3.4f

Some glycoproteins (proteins bonded

to short chains of sugars) serve as

identification tags that are specifically recognized by other cells.

(f) Cell-cell recognition

Glycoprotein

Slide3

Membrane Junctions

Three types:

Tight junction

Desmosome

Gap junction

Slide4

Membrane Junctions: Tight Junctions

Prevent fluids and most molecules from moving between cells

Where might these be useful in the body?

Slide5

Figure 3.5a

Interlocking

junctional proteins

Intercellular

space

Plasma membranes

of adjacent cells

Microvilli

Intercellular

space

Basement membrane

(a) Tight

junctions:

Impermeable junctions prevent molecules

from passing through the intercellular space.

Slide6

Membrane Junctions: Desmosomes

“Rivets” or “spot-welds” that anchor cells together

Where might these be useful in the body?

Slide7

Figure 3.5b

Intercellular space

Plasma membranes

of adjacent cells

Microvilli

Intercellular

space

Plaque

Linker glycoproteins

(cadherins)

Intermediate

filament (keratin)

(b) Desmosomes:

Anchoring junctions bind adjacent cells together

and help form an internal tension-reducing network of fibers.

Basement membrane

Slide8

Membrane Junctions: Gap Junctions

Transmembrane proteins form pores that allow small molecules to pass from cell to cell

For spread of ions between cardiac or smooth muscle cells

Slide9

Figure 3.5c

Plasma membranes

of adjacent cells

Microvilli

Intercellular

space

Intercellular

space

Channel

between cells

(connexon)

(c) Gap junctions:

Communicating junctions allow ions and small mole-

cules to pass from one cell to the next for intercellular communication.

Basement membrane

Slide10

Membrane Transport

Plasma membranes are selectively permeable

Some molecules easily pass through the membrane; others do not

Slide11

Types of Membrane Transport

Passive processes

No cellular energy (ATP) required

Substance moves down its concentration gradient

Active processes

Energy (ATP) required

Occurs only in living cell membranes

Slide12

Passive Processes

What determines whether or not a substance can passively permeate a membrane?Lipid solubility of substanceChannels of appropriate sizeCarrier proteins

PLAY

Animation: Membrane Permeability

Slide13

Passive Processes

Simple diffusion

Carrier-mediated facilitated diffusion

Channel-mediated facilitated diffusion

Osmosis

Slide14

Passive Processes: Simple Diffusion

Nonpolar lipid-soluble (hydrophobic) substances diffuse directly through the phospholipid bilayer

PLAY

Animation: Diffusion

Slide15

Figure 3.7a

Extracellular fluid

Lipid-

soluble

solutes

Cytoplasm

(a) Simple diffusion

of fat-soluble molecules

directly through the phospholipid bilayer

Slide16

Passive Processes: Facilitated Diffusion

Certain lipophobic molecules (e.g., glucose, amino acids, and ions) use carrier proteins or channel proteins, both of which:

Exhibit specificity (selectivity)

Are saturable; rate is determined by number of carriers or channels

Can be regulated in terms of activity and quantity

Slide17

Facilitated Diffusion Using Carrier Proteins

Transmembrane integral proteins transport specific polar molecules (e.g., sugars and amino acids)

Binding of substrate causes shape change in carrier

Slide18

Figure 3.7b

Lipid-insoluble

solutes (such as

sugars or amino

acids)

(b) Carrier-mediated facilitated diffusion

via a protein

carrier specific for one chemical; binding of substrate

causes shape change in transport protein

Slide19

Facilitated Diffusion Using Channel Proteins

Aqueous channels formed by transmembrane proteins selectively transport ions or water

Two types:

Leakage channels

Always open

Gated channels

Controlled by chemical or electrical signals

Slide20

Figure 3.7c

Small lipid-

insoluble

solutes

(c) Channel-mediated facilitated diffusion

through a channel protein; mostly ions

selected on basis of size and charge

Slide21

Passive Processes: Osmosis

Movement of solvent (water) across a selectively permeable membrane

Water diffuses through plasma membranes:

Through the lipid bilayer

Through water channels called aquaporins (AQPs)

Slide22

Figure 3.7d

Water

molecules

Lipid

billayer

Aquaporin

(d) Osmosis

, diffusion of a solvent such as

water through a specific channel protein

(aquaporin) or through the lipid bilayer

Slide23

Passive Processes: Osmosis

Water concentration is determined by solute concentration because solute particles displace water molecules

Osmolarity: The measure of total concentration of solute particles

When solutions of different osmolarity are separated by a membrane, osmosis occurs until equilibrium is reached

Slide24

Figure 3.8a

(a)

Membrane permeable to both solutes and water

Solute and water molecules move down their concentration gradients

in opposite directions. Fluid volume remains the same in both compartments.

Left

compartment:

Solution withlower osmolarity

Rightcompartment:Solution with greater osmolarity

Membrane

H2O

Solute

Solutemolecules(sugar)

Both solutions have the

same osmolarity: volume

unchanged

Slide25

Figure 3.8b

(b)

Membrane permeable to water, impermeable to solutes

Both solutions have identical

osmolarity, but volume of the

solution on the right is greater

because only water is free to move

Solute molecules are prevented from moving but water moves by osmosis.Volume increases in the compartment with the higher osmolarity.

Leftcompartment

Rightcompartment

Membrane

Solutemolecules(sugar)

H

2

O

Slide26

Importance of Osmosis

When osmosis occurs, water enters or leaves a cellChange in cell volume disrupts cell function

PLAY

Animation: Osmosis

Slide27

Tonicity

Tonicity: The ability of a solution to cause a cell to shrink or swell

Isotonic: A solution with the same solute concentration as that of the cytosol

Hypertonic: A solution having greater solute concentration than that of the cytosol

Hypotonic: A solution having lesser solute concentration than that of the cytosol

Slide28

Figure 3.9

Cells retain their normal size and

shape in isotonic solutions (samesolute/water concentration as insidecells; water moves in and out).

Cells lose water by osmosis and shrink in a hypertonic solution (contains a higher concentration of solutes than are present inside the cells).

(a) Isotonic solutions

(b) Hypertonic solutions

(c) Hypotonic solutions

Cells take on water by osmosis until

they become bloated and burst (lyse)

in a hypotonic solution (contains a

lower concentration of solutes than

are present in cells).

Slide29

Summary of Passive Processes

Also see Table 3.1

Process

Energy Source

Example

Simple diffusion

Kinetic energy

Movement of O

2

through phospholipid bilayer

Facilitated diffusion

Kinetic energy

Movement of glucose into cells

Osmosis

Kinetic energy

Movement of H

2

O through phospholipid bilayer or AQPs