Water Potential Review - CHISD · Passive transport is the movement of a substance across a...

Water Potential Review

Which way will the water flow?

Ψs = -7

Ψs = -4

Chapter 7 Review

Membrane Structure and Function

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Selective Permeability

The plasma

membrane exhibits

selective

permeability

It allows some

substances to

cross it more

easily than others

Cellular membranes are fluid mosaics of lipids and

proteins

Phospholipids (fluid part)

Are the most abundant lipid in the plasma membrane

Are amphipathic, containing both hydrophobic and

hydrophilic regions

Proteins (mosaic part) can be

Transmembrane (integral)

Peripheral

Fluid Mosaic Model


Lipids

Phospholipid structure

Consists of a hydrophilic “head” and hydrophobic

“tails”

CH2

O

P O O

O

CH2 CH CH2

O O

C O C O

Phosphate

Glycerol

Fatty acids

Hydrophilic

head

Hydrophobic

tails

–

CH2 Choline + N(CH3)3


Lipids

The structure of phospholipids

Results in a bilayer arrangement found in cell

membranes

Hydrophilic

head

WATER

WATER

Hydrophobic

tail


Lipids

Saturated fatty acids

Have the maximum number of hydrogen atoms

possible

Have no double bonds

Are typically solid at room temperature

Stearic acid


Unsaturated fatty acids

Have one or more double bonds

Are typically liquid at room temperature

cis double bond

causes bending

Oleic acid

Lipids


How fluid?

The type of hydrocarbon tails in phospholipids

Affects the fluidity of the plasma membrane

Fluid Viscous

Unsaturated hydrocarbon tails with kinks

Saturated hydro-

Carbon tails

Membrane fluidity


How fluid?

The steroid cholesterol

Has different effects on membrane fluidity at

different temperatures

Cholesterol within the animal cell membrane

Cholesterol

Thinking Question

Would you expect an amoeba that lives in a

cold climate to have a higher or lower

amount of saturated hydrocarbon chains in

its phospholipids compared to a species that

lives in a warm climate?

Thinking Question

Would you expect an amoeba that lives in a

cold climate to have a higher or lower

amount of saturated hydrocarbon chains in

its phospholipids compared to a species that

lives in a warm climate?

Hint: saturated hydrocarbons are MORE fluid

at room temperature.

Thinking Question

Would you expect an amoeba that lives in a cold climate

to have a higher or lower amount of saturated

hydrocarbon chains in its phospholipids compared to a

species that lives in a warm climate?

Fluid Viscous

Unsaturated hydrocarbon tails with kinks

Saturated hydro-

Carbon tails

Membrane fluidity


Fluid-Mosaic Model

Membrane proteins (mosaic part) are

dispersed and individually inserted into the

phospholipid bilayer

Phospholipid

bilayer

Hydrophilic region

of protein

Hydrophobic region of protein

Membrane Carbohydrates

Cell-cell recognition

Is a cell’s ability to distinguish one type of neighboring cell from another

Short carbohydrates bound to lipids (glycolipids) or proteins (glycoproteins) cover the outer surface of cells

These carbohydrates mediate cell-cell recognition

Crossing the Membrane

Cells need to control the exchange of material with their environment

Hydrophobic molecules (non-polar)

Are lipid soluble and can pass through the membrane rapidly

Hydrophilic molecules (polar, ionic)

Do not cross the membrane rapidly

Need help of transport proteins

Passive vs. Active Transport

Passive transport is the movement of a substance across a membrane with no energy investment

Involves the process of simple diffusion

Or facilitated diffusion

Active transport requires an energy input


Passive Transport

Simple Diffusion

Is the tendency for molecules to move from

areas of high concentration to areas of low

concentration

Small, hydrophobic molecules can move

across the membrane this way Molecules of dye Membrane (cross section)

Net diffusion Net diffusion Equilibrium

(a)

Facilitated Diffusion

In facilitated diffusion

Transport proteins speed the movement of

molecules across the plasma membrane

Does NOT require energy input (no ATP)


Facilitated Diffusion

Transport proteins

Provide corridors that allow a specific molecule

or ion to cross the membrane

EXTRACELLULAR

FLUID

Channel protein Solute

CYTOPLASM

(a)


Special Case of Facilitated Diffusion Osmosis

Is the diffusion of water across a selectively permeable

(semipermeable) membrane

Lower

concentration

of solute = Higher

concentration of

water

Higher

concentration

of solute = Lower

concentration of

water

Water moves from an area of higher

water concentration to an area

of lower water concentration

s

Tonicity

Tonicity Is the ability of a solution to cause a cell to gain or lose water

If a solution is isotonic The concentration of solutes is the same as it is inside the cell

There will be no net movement of water

If a solution is hypertonic The concentration of solutes is greater than it is inside the cell

The cell will lose water

If a solution is hypotonic The concentration of solutes is less than it is inside the cell

The cell will gain water

Tonicity Diagram

5% NaCl 5% NaCl 10% NaCl 20% NaCl

Isotonic

Hypotonic

Hypertonic

Hypotonic

Hypertonic

Water


Osmosis in Cells

Water balance in cells without walls

Such as animal or protist

Hypotonic solution Isotonic solution Hypertonic solution

H2O H2O H2O H2O

Lysed Normal Shriveled/Crenation


Osmosis in Cells

Water balance in cells with walls

Such as plant cells

H2O H2O H2O H2O

Turgid (normal) Flaccid Plasmolyzed

Hypotonic solution Isotonic solution Hypertonic solution

Thinking Question

Imagine you are in the hospital recovering

from an operation and require intravenous

fluids. Why should you be concerned about

the concentration (tonicity) of the solution

in the IV bags?

Active Transport

Active transport

Moves substances against their concentration

gradient from low to high

Requires energy, usually in the form of ATP


Review: Passive and Active Transport Compared Passive transport. Substances diffuse spontaneously

down their concentration gradients, crossing a

membrane with no expenditure of energy by the cell.

The rate of diffusion can be greatly increased by transport

proteins in the membrane.

Active transport. Some transport proteins

act as pumps, moving substances across a

membrane against their concentration

gradients. Energy for this work is usually

supplied by ATP.

Diffusion. Hydrophobic

molecules and (at a slow

rate) very small uncharged

polar molecules can diffuse

through the lipid bilayer.

Facilitated diffusion. Many

hydrophilic substances diffuse

through membranes with the

assistance of transport proteins,

either channel or carrier proteins.

ATP

Movement of Macromolecules

Transport of large molecules across the plasma membrane occurs by exocytosis and endocytosis

In exocytosis

Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents

In endocytosis

The cell takes in macromolecules by forming new vesicles from the plasma membrane


EXTRACELLULAR

FLUID Pseudopodium

CYTOPLASM

“Food” or

other particle

Food

vacuole

1 µm

Pseudopodium

of amoeba

Bacterium

Food vacuole

PINOCYTOSIS

0.5 µm Plasma

membrane

Vesicle

Endocytosis

Three types of endocytosis

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 0.25 µm

RECEPTOR-MEDIATED ENDOCYTOSIS

Receptor

Ligand

Coat protein

Coated

pit

Coated

vesicle

Plasma

membrane

Coat

protein

Key Points of Chapter 7

Cellular membranes are fluid mosaics of lipids and proteins

Membrane structure results in selective permeability

Passive transport is diffusion of a substance across a membrane with no energy investment

Active transport uses energy to move solutes against their gradients

Bulk transport across the plasma membrane occurs by exocytosis and endocytosis

Water Potential Review - CHISD · Passive transport is the movement of a substance across a...

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