1 osmosis in animal cells diffusion and facilitated diffusion membranes and phospholipids osmosis...

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Osmosis in animal cells

Diffusion and facilitated diffusion

Membranes and phospholipids

Osmosis and water potential

Macro exchange surfaces - root hairs

Macro exchange surfaces -alveoli

Active transport

Review

Bulk transport

Roles of cell membrane parts

The fluid mosaic model

Osmosis in plant cells

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Membrane structure

Membranes and phospholipidsAlthough very thin, membranes regulate very precisely indeed the substances that enter and leave the cell.

Membranes also have receptors to enable hormones to influence certain cells

PhospholipidsWhen mixed with water, phospholipid molecules spontaneously assemble to form membrane-like structures.

Their polar heads point outwards towards the surrounding charged water molecules, and their non-polar tails point inwards.

CHECK OUT BIOLOGICAL

MOLECULES - PHOSPHLIPIDS

Under certain conditions they form bilayers, the basis of cell membranes

Here phospholipid molecules have

formed a spherical ‘micelle’

Here phospholipid molecules have formed a bilayer

DETAILED VIEW OF BILAYER

PHOSPHOLIPID MOLECULE

HYDROPHOBIC TAIL

HYDROPHILIC HEAD

These face inwards forming a non-polar hydrophobic interior

These face the aqueous (water-containing) medium around the membrane.

The molecules move around by diffusion (driven by kinetic energy).

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Membrane structure

Features of the fluid mosaic model

High power TEM, cell membrane x 100000

The double line seen at very high power is thought to be

the 2 phospholipid layers. The bilayer is about 7 nm wide.

Membranes also contain proteins and the model of membranes accepted at present is called the ‘fluid

mosaic’ model.

The phospholipid bilayer is the fluid part because phospholipid

molecules can move aroundThe protein molecules form a

mosaic pattern set in the phospholipid bilayer

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Membrane structure

Features of the fluid mosaic model

3 INTRINSIC PROTEINS

Carbohydrate (polysaccharide ) part of a glycoprotein, or gylcolipid

EXTRINSIC PROTEIN

PHOSPHOLIPID LAYER

Some phospholipids tails are unsaturated. The more unsaturated

they are, the more fluid the membrane, because bent tails fit together more

loosely.

Most protein molecules are mobile, moving

around freely. Others are fixed like islands to

structures in the membrane and do not

move

Some proteins are embedded in the

outer layer (extrinsic) and

some in the inner layer (intrinsic).

Hydrophobic protein areas are anchored in the

hydrophobic inner part of the

membrane.

Recheck unsaturated phospholipid structure

inside

outside

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Membrane structure

Roles of the components of cell membranesPhospholipids: Because their tails

are non-polar, water soluble molecules such as ions, cannot

pass through them.

Cholesterol molecules: These too have hydrophilic

heads and hydrophobic tails. They fit neatly

between phospholipid molecules and help

maintain the fluidity and stability of the membrane.

Cholesterol molecules, being hydrophobic, help prevent ions or polar molecules from

passing through. This is especially important in myelin sheaths around nerves, where ion

leakage would slow down impulse transmission

Transport proteins provide

hydrophilic channels for the passage of ions

and polar molecules

Membrane enzymes are sometimes present. e.g. small intestine cell

membranes have enzymes which hydrolyse disaccharides.

Glycolipids and glycoproteins These lipids

and proteins have carbohydrate chains which

jut out in from the membrane

They stabilise the membrane and

also act as receptor

molecules for hormones and

neurotransmitters

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Transport across the plasma membrane


Diffusion is the net movement of molecules down a

concentrated gradient.

Factors affecting the rate of diffusion across membranes include:

The steepness of the concentration gradient. The

greater the difference in concentration, the faster the rate.

Temperature: Molecules have more kinetic energy at

high temperatures and diffuse faster

Surface area: the greater the surface

area, the more molecules or ions that can cross it.

The type of molecule or ion: Large molecules diffuse more slowly than small ones. Non-polar molecules

diffuse faster

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All these can only cross the membrane through hydrophilic channels created by protein molecules. Diffusion through these channels is called facilitated diffusion, because it is ‘made easy’, or ‘made possible.

PHOSPHOLIPID BILAYER



Oxygen is uncharged and non-polar. It passes across the phospholipid bilayer quickly

PERMEABLE

Carbon dioxide is polar but small enough to pass through rapidly

PERMEABLE

Water molecules, despite being polar, can diffuse across rapidly because they are so small

PERMEABLE

Amino acids glucose

nucleotidesIMPERMEABLE

H+, Na+, K+, Mg+, Ca2+, Cl-, HCO3

-

IMPERMEABLE

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CHECK OUT CYSTIC

FIBROSIS

Plasma membranes contain many different types of protein channel, each type allowing only 1 kind of molecule or ion to diffuse through it.

The rate of facilitated diffusion depends on how many appropriate channels there are, and whether they are open.

In cystic fibrosis a protein channel in lung and gut epithelial cells which normally allows sodium chloride to move out of the cells is faulty. As a result chloride ions cannot move out.

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Terms you need to know include solute, solvent and solution.e.g. in sugar solution the sugar is the solute and the water is the solvent.

Before osmosis starts

Osmosis finished

There is a net movement of water

molecules from A to B until an

equilibrium is reached where

solution A has the same water

concentration as B

High solute concentration

A B

A B

The solute molecules (red) are too large to pass through the

selectively permeable membrane.

Cell membranes are semi-

permeable. They only allow certain molecules (small)

through.

Water potential is the tendency of water to move from 1 place to another

The symbol for water potential is Ψ

Water always moves from a region of high water potential to low water potential.

Pure water has the highest water potential. The effect of solute molecules is to lower

water potential

By convention the water potential of pure water is zero. Increasing solute

concentrations produce increasingly negative values for water potential.

Osmosis – water potential and solute potential

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Osmosis in animal cells – red blood cells

Red cell bursts Red cell remains normal

Red cell shrinks

In (hypotonic) pure water or dilute solution

In a (isotonic) solution with the same concentration as

the red cell

In a (hypertonic) more concentrated solution

Low concentration of solute molecules, high concentration of water molecules

High concentration of solute molecules, low concentration of

water molecules

Movement of water into or out of red blood cells by osmosis in solutions of different concentration

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vacuole vacuole vacuole

cell undergoing plasmolysis

normal turgid

In a hypertonic solution

In an isotonic solution

In a hypotonic solution

Pressure potential is particularly important in plant cells.

They have a strong and rigid cell wall and if water enters the plant cell protoplast by osmosis and increases the protoplast volume, the confining cell wall causes a pressure build-up.

This is the pressure potential, and it increases the water potential of the cell inside until it equals the external water potential. At this point water entry stops. The cell is now described as turgid

The cell wall prevents the cell from bursting, as would happen with an animal cell under these conditions

- turgidity

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vacuole

vacuole vacuole


normal turgid

In a hypertonic solution

In an isotonic solution

In a hypotonic solution

For plant cells the water potential is thus a combination of solute potential and pressure potential, as follows:

e.g. concentrated sucrose

Water leaves the cell by osmosis. As it does so the

protoplast gradually shrinks until there is no pressure on the cell wall

At this point the pressure potential is zero and the

water potential is equal to the solute potential

As the protoplast continues to shrink it pulls away from the cell wall. This process is called

plasmolysis and the cell is said to be plasmolysed (protoplast not touching

the cell wall).

The point at which the pressure potential has just reached zero and plasmolysis is about to occur is referred to as incipient plasmolysis.

Ψ ΨS ΨP+=

ΨS Solute potential

ΨP Pressure potential

- plasmolysis

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Osmosis in plant cellsOsmotic changes in plant

cells can be easily observed using a light microscope

Rhubarb epidermal strips or the swollen storage leaves of

onion bulbs contain a red pigment which highlights the protoplasts in sharp contrast

to the cell walls.

Light micrograph of plasmolysed red onion cells

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Active transportActive transport is the pumping of ions across membranes against a diffusion

gradient, using energy from ATP.

Like facilitated transport it is achieved by special transport proteins but in active transport ATP is required to change the 3D shape of the protein

and therefore move the ‘bound’ ion or molecule across.

Most cells have active transport pumps, Check out some examples of

active transport:

Glucose reabsorption in the kidney

Nerve cell resting potential

Ion uptake by plant root hairs

ATPADP

high concentration

low concentration

Membrane protein pump

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Bulk transport

Adherence

Ingestion

Formation of phagosome

Lysosome

Phagolysosome

FusionDestruction of

microbe

Release of microbial

debris

Diffusion, osmosis and active transport refer to the movement of individual particles

across membranes

Mechanisms also exist for the bulk transport of materials in and out of

cells (endo- and exocytosis).

Phagocytosis or ‘cell eating’. The bulk uptake of solid materials. Cells which do this are phagocytes, e.g.

some white blood cellsPinocytosis or ‘cell drinking’. The

bulk uptake of liquid.

- endocytosis

Stages in phagocytosis of a bacterium by a white blood cell

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Bulk transportExocytosis is the reverse of

endocytosis

- exocytosis

It happens, for example, in the secretion of digestive enzymes from

the pancreas

Secretory vesicles from the Golgi body carry the enzymes to the cell surface and release them to the outside of the

cell

Plant cells use exocytosis to get their cell wall building materials to the outside of the plasma membrane

Secretory vesicle containing secretory product, e.g. enzyme

Golgi apparatus

EM of pancreatic acinar cell secreting

protein

Diagram of Golgi apparatus secretion

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WHAT YOU SHOULD KNOW AT THE END OF

THIS UNIT

describe and explain the fluid mosaic model of membrane structure, including an outline of the roles of phospholipids, cholesterol, glycolipids,

proteins and glycoproteins;

outline the roles of membranes within cells and at the surface of cells;

describe and explain the processes of diffusion, osmosis, active transport, facilitated diffusion, endocytosis and exocytosis

(terminology described in the IOB’s publication Biological Nomenclature should be used; no calculations involving water potential will be set);

*investigate the effects on plant cells of immersion in solutions of different water potential;

use the knowledge gained in this section in new situations or to solve related

problems.

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inside

outside

PHOSPHOLIPID MOLECULE

HYDROPHOBIC TAIL

HYDROPHILIC HEAD

1.

2.

Carbohydrate (polysaccharide ) part of a glycoprotein, or gylcolipid

channel protein

extrinsic protein

LABEL THE PARTS SHOWN

(3)

(3)

Name ____________________

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Cholesterol molecules:

Transport proteins

Glycolipids and glycoproteins

They stabilise the membrane and also act as receptor molecules for hormones and neurotransmitters

3.

help maintain the fluidity and stability of the membrane. help prevent ions or polar molecules from passing through

provide hydrophilic channels for the passage of ions and polar molecules

Oxygen PERMEABLE

Carbon dioxide PERMEABLE

Water PERMEABLE

Amino acids IMPERMEABLE

H+

IMPERMEABLEglucose

Na+ IMPERMEABLE

IMPERMEABLE

4.

GIVE THE FUNCTIONS OF THESE MEMBRANE COMPONENTS

STATE WHETHER THE MEMBRANE IS PERMEABLE OR IMPERMEABLE TO EACH OF THESE SUBSTANCES

(3)

(6)

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In cystic fibrosis a _______________ in lung and gut epithelial cells which normally allows ______________ to move out of the cells is faulty.

Water potential is the ___________________________________________:

The effect of solvent molecules is to ___________ water potential

5.

6.

channel protein

sodium chloride ions

Fill in the gaps

tendency of water to move from 1 place to another

decrease the

(2)

(2)

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vacuole vacuole vacuole


normal turgid

In a _____________ solution In an_________________ solution

In a _______________ solution

Ψ ΨS ΨP+=

ΨS Solute potential

ΨP Pressure potential

7.

8. Write an equation showing the relationship between solute potential, pressure potential and water potential. Use the correct symbols

Fill in the missing words

(3)

(4)

hypertonic isotonic hypotonic

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vacuole

This cell is in concentrated sucrose solution

The point at which the pressure potential has just reached zero and plasmolysis is about to occur is referred to as _________________.

Active transport is the:

Glucose reabsorption in the kidney

Nerve cell resting potential

Ion uptake by plant root hairs

9.

10.

11.

pumping of ions across membranes against a diffusion gradient, using

energy from ATP.

Give 2 examples of active transport:

incipient plasmolysis

(1)

(3)

(2)

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bulk transport out of a cell

Secretory vesicle containing secretory product, e.g. enzyme

Golgi apparatus

EM of pancreatic acinar cell secreting

protein

12.

13.

Define these termsPhagocytosis

Pinocytosis

Exocytosis

Endocytosis bulk transport into a cell

membrane transport of solids

membrane transport of liquids

Label the parts

(4)

(2)

Total /38

1 osmosis in animal cells diffusion and facilitated diffusion membranes and phospholipids osmosis...

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phospholipid molecules

membrane structurefeatures

protein molecules

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cell membrane x

water soluble molecules

net movement of molecules