Membranes and osmosis

1. Structure of Plasma membrane

* Two-layered structure:

lipid bilayer

*Made of lipid molecules

with protein molecules in

the lipid layer.

**PROTEINS aid in the movement of materials through the membrane

hydrophobictails

hydrophilicheads

hydrophilicheads

extracellular fluid(watery environment)

cytoplasm(watery environment)

phospholipid

bilayer

tails(hydrophobic)

head(hydrophilic)

Polar Head

Non-Polar Tails

* Bilayer can be considered a liquidliquid

FLUID MOSAIC MODEL: plasma membrane : made up of molecules that are free to flow among one another.

Kinds and arrangements of proteins + lipids vary from one membrane to another and give each type of membrane specific permeability properties.

Proteins

Membrane movement animation

Polar heads love water & dissolve.

Non-polar tails hide from water.

Carbohydrate cell markers

Fluid Mosaic Model of the cell

membrane

Outside of cell

Inside of cell (cytoplasm)

Lipid Bilayer

Proteins

Transport Protein Phospholipids

Carbohydratechains

Structure of the Cell Membrane

Go to Section:

Animations of membrane

structure

1. Maintaining a BALANCE in a CELL

Cells Maintain HOMEOSTASIS : internal balance

Selective permeability : allows

some materials to pass through

membrane while rejecting others.

Diffusion

Movement of molecules high concentration to lower concentration

Example: skunk, perfume, night after a dinner at Taco Bell!

1 A drop of dye is placed in water.

2 Dye molecules diffuse into the water; water molecules diffuse into the dye.

drop of dye

pure water

3 Both dye molecules and water molecules are evenly dispersed.

Diffusion animation

Osmosis

The diffusion of Water from areas of high concentration to areas of low concentration

Animation: How Osmosis Works

Transport across membranes

Passive transport of water by Osmosis• a. Isotonic

• b. Hypertonic

• c. Hypotonic

Solution concentration of water (solvent) outside of the cell is the same as concentration inside the cell (Animal cell normal / plant flaccid)

Isotonic Solution:

Hypotonic Solution: concentration of water (solvent)

outside the cell is higher than concentration inside the cell. Water rushes into the cell!

 Turgor pressure: Pressure that builds in a plant cell as a result of osmosis. Makes a plant cell firm. (Turgid) Plant cells are healthiest in a hypotonic environment.

 Lysed: pressure builds in animal cell (burst)

Hypertonic Solution: concentration of water (solvent) outside cell is lower than concentration inside the cell.

Water rushes out of the cell!

 Plasmolysis: loss of water from within a plant cell, causing cytoplasm to shrink, pulling inner plasma membrane away from cell wall. (wilting of plants)

 Crenation: (crenates: shrinks) animal cell loses water and it shrinks (collapses)

What type of solution are these cells in?

A CB

Hypertonic Isotonic Hypotonic

(a) isotonic solution (b) hypertonic solution (c) hypotonic solution

10 micrometers

equal movement of waterinto and out of cells

net water movement out of cells

net water movement into cells

Passive transport : movement of substances across plasma membranes without additional energy

Facilitated diffusion: diffusion of materials across a plasma membrane by transport (channel) proteins.

5. PASSIVE TRANSPORT: No energy required for this to happen

Active Transport : gradient from low to high. Energy is required.

Proteins throughout the membrane are “carriers” used for this purpose.

6. ACTIVE TRANSPORT: Requires energy in the form of ATP

Na+ ions: moved out of cell K+ ions: moved by the

same carrier into the cell

(This process is important in nerve and muscle function.)

Sodium-potassium pump Na out K in ACTIVE TRANSPORT = ATP

7. TRANSPORT of 7. TRANSPORT of LARGELARGE PARTICLESPARTICLES

 Endocytosis (Endo = In): cell surrounds and takes in material from environment. Material does not pass through the membrane; instead, it is engulfed and closed by a portion of

membrane and cytoplasm.  Pinocytosis: water

Phagocytosis: food/ minerals/ large particles

Exocytosis (Exo = Out): transport of materials out of cell across cell membrane.

Exocytosis

(extracellular fluid)

(cytoplasm)

food particle

particle enclosed in vesicle

phagocytosis

vesicle containing extracellular fluid

cell

pseudopod

pinocytosis

(a)

(b)

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Animated Review of Phagocytosis, Pinocytosis and Receptor Mediated Endocytosis.

Click on Active Transport, watch the brief overview of Active Transport and then click on Endocytosis in the bar below the animation