Movement in and out of the cell membrane

Fluid compartments in our bodies are separated by membranes

Composition of body fluids

Greater number of osmotically active particles

Distribution of Total body fluids

Some organisms have a CELL WALL

• Plants (cellulose)• Algae (polysaccharide)• Fungi (chitin)• Prokaryotes(peptidoglycan)

Why have a cell wall?1. Mechanical support2. Protection3. Cell-cell communication4. Maintenance of structure (turgor)5. Prevent water loss

Membranes

2007-2008

Where are membranes located?

• Plasma membrane (double membrane)

• Membrane-bound organelles (single or double)

• Secret Universe• Introduction to cell

membrane

Why do we need plasma membranes made

Our cells inhabit an aqueous environment• …but they must let ‘stuff’ (nutrients, ions,

molecules,waste products) in and out• …..and they need to communicate with each

other…How is this achieved?....

Functions of the Cell membrane

1. Protective barrier2. Cell-Cell signalling3. Transport of nutrients, products and waste products4. Localisation of function within organelles5. Semi-permeable: controls entry and exit of

substances6. Self-sealing!7. Flexible, mobile fluid mosaicCell membrane function

Let’s meet the components of the cell membrane

• Phospholipid bilayer• Cholesterol• Carbohydrates

(glycoproteins)• Proteins

Phospholipid bilayer

polarhydrophilicheads

nonpolarhydrophobictails

polarhydrophilicheads

Membrane Proteins

• Proteins determine a membrane’s specific functions• Cell membrane &

organelle membranes each have unique collections of proteins

• Channels, pumps, receptors

NH2

H+

COOH

Cytoplasm

Retinalchromophore

Nonpolar(hydrophobic)a-helices in thecell membrane H+

Porin monomer

b-pleated sheets

Bacterialoutermembrane

proton pump channel in photosynthetic bacteria

water channel in bacteria

function through conformational change (shape change)

Examples

Membrane glycoproteins (carbohydrates)

‘Chemical identification cards’Play a key role in cell-cell recognition• ability of a cell to distinguish

one cell from another– Antigens

• basis for rejection of foreign cells by immune system

Let’s Review…

Let's build a membrane from scratch…And now…let’s make a membrane!

Movement across the Cell Membrane

How do things get into and out of our cells?

PASSIVE MECHANISMSThese don’t require energy

Simple diffusionFacilitated diffusion

Osmosis

ACTIVE MECHANISMSThese require energy (usually ATP) to transport substances (often against their concentration gradient)

Endocytosis/ exocytosisProtein pumps(cotransport)

Active Mechanisms: Endocytosis and Exocytosis

• A simple one...• Animation• Animation 2

Endocytosis/ exocytosis are

import/export of materials by

infolding/outfolding of the cell membrane

How do things get into and out of our cells?

PASSIVE MECHANISMSThese don’t require energy

Simple diffusionFacilitated diffusion

Osmosis

ACTIVE MECHANISMSThese require energy (usually ATP) to transport substances (often against their concentration gradient)

Endocytosis/ exocytosisProtein pumps(cotransport)

Simple diffusion…in the real world

• Gas exchange in the lungs• Gas exchange in plants

Simple Diffusion2nd Law of Thermodynamics governs biological systems: the universe tends towards disorder (entropy)

Diffusion: movement of small, soluble particles from high low concentration

DiffusionMovement is from HIGH to LOW concentration• “passive transport”• no energy needed

diffusion osmosis

movement of water

Factors affecting diffusion

1. Temperature2. Surface area for diffusion

3. Distance for diffusion4. (size of particle)

5. (charged or uncharged)

Diffusion in the real world

Diffusion animation 2• Gas exchange at the alveoli — oxygen from air to blood,

carbon dioxide from blood to air. • Gas exchange for photosynthesis — carbon dioxide from air

to leaf, oxygen from leaf to air. • Gas exchange for respiration — oxygen from blood to tissue

cells, carbon dioxide in opposite direction. • Transfer of neurotransmitter substance at a nerve synapse. • Osmosis — diffusion of water through a semipermeable

membrane.

Facilitated DiffusionFacilitated diffusion is diffusion of specific molecules through protein channels in the cell membrane– no energy is required

open channel = fast transport

facilitated = with help

high

low

Channels for facilitated diffusionMembrane becomes semi-permeable with protein channels : specific channels allow specific material across cell membrane

inside cell

outside cell

sugaraaH2O

saltNH3

Which substances are transported by facilitated diffusion?

• Glucose• Urea• Amino acids• Animation

Factors affecting the rate of facilitated diffusion are the same as for simple diffusion

1. Temperature2. Surface area for diffusion

3. Distance for diffusion

Osmosis

A special case: facilitated diffusion of water molecules

across a semi-permeable membrane

Osmosis is facilitated diffusion of water

Facilitated diffusion of water from

high concentration of water to

low concentration of water across a semi

-permeable membrane

Examples of Osmosis in Biology

• Absorption of water by plant roots. • Re-absorption of water in the kidney. • Re-absorption of tissue fluid into blood

capillaries. • Absorption of water in the GI tract

Water passes through special water pores - Aquaporins

Aquaporins

• Structure, function and dynamics of aquaporins

Peter AgreJohn Hopkins

Roderick MacKinnonRockefeller

Water moves across a membrane from

the hypotonic solution to the

hypertonic solution

• Animation of osmosis 1• Animation of osmosis 2:

why water balance matters

Comparing ‘water concentration’ of different solutions

Direction of osmosis is determined by comparing total solute concentrations on either side of the membrane:

• Hypertonic - more solute, less water

• Hypotonic - less solute, more water

• Isotonic - equal solute, equal water

hypotonic hypertonic

water

net movement of water

freshwater balanced saltwater

Water potential?• It can be confusing to talk about the concentration of

water molecules, since ‘concentration’ is usually reserved for a solute (e.g. glucose,

• Instead we use the term water potential• High water potential = lots of water (high water

concentration/ low solute concentration• LOW water potential = little water (low water

concentration/high solute concentration)

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

Low water potential (relatively few water molecules compared to solute molecules)

High water potential (relatively many water molecules compared to solute molecules)

How do things get into and out of our cells?

PASSIVE MECHANISMSThese don’t require energy

Simple diffusionFacilitated diffusion

Osmosis

ACTIVE MECHANISMSThese require energy (usually ATP) to transport substances (often against their concentration gradient)

Protein pumpsEndocytosis

(cotransport)

Active transport 1: Protein pumps

• Active transport uses energy (ATP) to transport substances AGAINST a concentration gradient into/out of the cell

• The energy is used to change the shape of the ‘protein pump’ and thus import/export specific molecule

• Animation• protein pumps in plants

ATP

low

high

Active transport in the real world

• Re-absorption of glucose, amino acids and salts in the kidney.

• Sodium/potassium pump in cell membranes (especially nerve cells)

• Uptake of nutrients (e.g. nitrate ions)in plant root hair cells

Transport summarysimplediffusion

facilitateddiffusion

activetransport

ATP