Functions of Cell Membrane

1. Protect cell2.Control incoming and outgoing

substances3.Selectively permeable - allows some

molecules in, others are kept out

Phospholipid Bilayer

polarhead

nonpolartails

P –

hydrophobic molecules

hydrophilic molecules

cytosol

Phospholipid bilayer

Plasma membrane components

Membrane Permeability

Selective permeability The ability of a cell membrane to control

which substances and how much of them enter or leave the cell

Allows the cell to maintain a difference between its internal environment and extracellular fluid

Supplies the cell with nutrients, removes wastes, and maintains volume and pH

The Selectively Permeable Nature of Cell Membranes

Types of Membrane Proteins

Methods of Transport Across Membranes1. Diffusion -passive transport – no

energy expended2. Osmosis - Passive transport of

water across membrane3. Facilitated Diffusion - Use of

proteins to carry polar molecules or ions across

4. Active Transport- requires energy to transport molecules against a concentration gradient – energy is in the form of ATP

Concentration and Gradients

Concentration The number of molecules (or ions) of

substance per unit volume of fluid

Concentration gradient The difference in concentration between two

adjacent regions Molecules always move from a region of

higher concentration to one of lower concentration

Diffusion

Solute molecules moving from an area of high concentration to an area of low concentration

Random motion drives diffusion

Equilibrium is reached when there is an even distribution of solute molecules

(water)

Diffusion

Osmosis

Diffusion of water through a semi-permeable membrane Semi-permeable: permeable

to solvents (WATER), but not to large molecules

High [water] to low [water]

Dissolved molecules (i.e. glucose, starch) are called solutes

REMEMBER:

Water = solvent

Glucose, Starch = solutes

Osmosis

Effect of Water on Cells

Hypertonic Environment High [solute], low [water]

Hypotonic Environment High [water], low [solute]

Isotonic Environment [water] = [solute]

Isotonic

HypotonicHypertonic

Part 3 pg. 85

Osmosis in Living Cells

Cellulose in cell wall

Osmosis in Red Blood Cells

Isotonic

Hypotonic

Hypertonic

• Observe sheep RBCs via a wet mount of the sample

• Aliquot one drop the following solutions with a ½ drop of RBC to a slide

0.9% saline

10% NaCl

Distilled water

• Record observation in the table on page 85

Predictions?

Crenation

Effect of Water on RBC

Osmosis in Plant Cells

• Observe Elodea leaves via a wet mount of the sample

• Aliquot two drops the following solutions with a new Elodea leaf to a slide. Incubate for 10 minutes @ room temp.

10% NaCl

Distilled water

• Record observation in the table on page 85

Hypertonic

Hypotonic

Predictions?

Plasmolysis

Hydrostatic Pressure in Plants

a. Normal tomato on left – salt water gives effect (r) in 30min

b. Iris petal cells

c. Iris petal cells in wilted stage

Types of Transport

Transportation of Molecules

• Passive Transport

-Movement of molecules across a semi-permeable membrane

- no energy required

• Active Transport

-Movement of molecules across a semi-permeable membrane against a concentration gradient

- ENERGY – ATP

• Facilitated Diffusion

-Movement of molecules across a semi-permeable membrane with a protein

- no energy required

Active transport in two solutes across a membrane

Na+/K+ pump

Protein shape change

Figure 5.18

Transportprotein

1

FLUIDOUTSIDECELL

Firstsolute

First solute, inside cell, binds to protein

Phosphorylated transport protein

2 ATP transfers phosphate to protein

3 Protein releases solute outside cell

4 Second solute binds to protein

Second solute

5 Phosphate detaches from protein

6 Protein releases second solute into cell

exocytosis = vesicle fuses with the membrane and expels its contents

Exocytosis and endocytosis transport large molecules

Figure 5.19A

FLUID OUTSIDE CELL

CYTOPLASM

or the membrane may fold inward, trapping material from the outside (endocytosis)

Figure 5.19B

Receptor-mediated endocytosis

Cholesterol can accumulate in the blood if membranes lack cholesterol receptors

Figure 5.20

LDL PARTICLEPhospholipid outer layer

Protein

Cholesterol

Plasma membraneCYTOPLASM

Receptor protein

Vesicle

Membrane-Crossing Mechanisms