2.4 Cell Membrane And Transport

Cell Membrane Structure and Function

Cell Membrane• Every cell is encircled by a membrane and most

cells contain an extensive intracellular membrane system.

• Membranes fence off the cell's interior from its surroundings.

• Membranes let in water, certain ions and substrates and they excrete waste substances.

• Without a membrane the cell contents would diffuse into the surroundings, information containing molecules would be lost and many metabolic pathways would cease to work.

The cell would die!

Membrane StructureThe cell is highly organized with many

functional units or organelles inside. Most of these units are limited by one or more membranes.

To perform the functions of an organelle, the membrane is specialized in that it contains specific proteins and lipid components that enable it to perform its unique roles. 

In essence membranes are essential for the integrity and function of the cell.

The Cell Membrane

Cell membranes

What is their structure?

We don’t know currently

There are a number of hypotheses and we will consider the one which is

currently accepted........

Structure - The Cell Membrane

The fluid mosaic model of membrane structure

• The membrane is a mosaic (mixture) of different protein molecules floating in a bilayer (double layer) of phospholipids

• Each phospholipid has a hydrophilic (water-loving) head & hydrophobic (water-hating) tails

• Because of this feature of phospholipids, the lipid bilayers assemble themselves spontaneously

The fluid mosaic model

Lipids arranged in bi-layer with proteins embedded or associated with them.

Phospholipid Molecule Model

glycerol

fatty acids (hydrophobic) – hate water

phosphate (hydrophilic) – like water

Draw This

Membrane Lipidsform a Bi-layer

Outside layer

Inside Layer

The fluid mosaic model

• This proposes that the cell membrane is made up of 2 main layers – lipids and proteins.

• The lipids form themselves into a bi-layer with the water seeking ends (hydrophilic) facing out & the water hating ends (hydrophobic) facing in.

• The proteins are embedded in this layer but can move around or flip over.

• Special carrier molecules take in important elements, like ions, at the cell membrane, using energy supplied by the cell and use the proteins that are embedded in the lipid layer.

Membrane Function

• Sometimes the elements bind to the proteins, which flip over, thus transporting the element into the cell.

• Some proteins form a ‘pore’ through which the element can pass from the outside to the inside of the cell membrane.

• The movement of the phospholipid and protein components through the plasma membrane permits the membrane to change shape—this is known as fluidity.

• This flexibility is crucial to many different types of cells.

The cell membrance is a complex 3d circular structure

The cell membrane is a complex 3 d structureThe cell membrane is a complex 3 d structure.

Structure

• Fluid-like composition…like soap bubbles

• Composed of:– Lipids in a bi-layer – what is this?– Proteins embedded in lipid layer (called

trans-membrane proteins)– Proteins floating within the lipid sea (called

integral proteins)– Proteins associated outside the lipid bi-

layer (peripheral proteins).

The fluid mosaic model

Composition of the cell membrane

Phospholipids

Peripheral Proteins

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Integral Proteins

Trans – membrane proteins

CellMembrane

Phospholipids

• Make up 75% of cell surface membrane• Amphipathic – hydrophilic head (water-

loving and polar) and hydrophobic tail (water-hating and non-polar)

• Phospholipid bilayer forms spontaneously in aqueous environment eg. extra-cellular fluid and cytosol

• Selectively permeable – small molecules and lipid soluble molecules pass through easily. Hydrophilic substances cannot diffuse and rely on membrane pore/channels to pass

Cholesterol

• A steroid which makes up 20% of animal membranes (but rarely found in plants) (in humans cholesterol is present in almost same proportion as phospholipids)

• Also amphipathic – fit between phospholipids• Important because it makes cell membrane more rigid

and prevents membrane being too fluid & breaking up (37degrees = relatively high temp) = mechanical stability

• Hydrophobic regions prevent ions & polar molecules passing through – especially important in neurons

Glycolipids

• 5% of membrane lipids have short carbohydrate chains attached

• Occur on external surface of membrane

Proteins

• Variety of functions:– Transport proteins for ions and polar

molecules– Enzymes: eg ATPase in mitochondrial

membrane, chloroplast membrane, intestinal wall cells (for hydrolysis of disaccharides)

Glycoproteins

• Most proteins in plasma membrane have short carbohydrate molecules attached

• H-bond with water to help stabilise membrane structure

• May act as receptor for hormones, neurotransmitters

• Antigens = glycoproteins that help cells recognise each other. Each cell has it’s own antigen

Proteins - Glycoproteins• transmembrane proteins span the entire

membrane & are usually glycoproteins:

• Four main functions– Act as channels: Na+/K+ Pump to maintain ion

concentrations either side of the membrane– Transporters: some proteins identify & attach to

specific substances eg. nutrients, neurotransmitters

– Receptors: recognise & bind to target molecules such as hormones

– Enzymes eg. ATPase

Functions of membranes within cells (organelle membranes)

• Intracellular membranes have a structure very similar to that of cell surface membranes

• However, proportions of molecular components differs considerably eg. chloroplast membrane contains very little carbohydrate

• Almost all cellular process involves intracellular membranes:

Transport across the cell membrane

• Passive transport does NOT require energy– Diffusion – small uncharged molecules– Osmosis - water– Facilitated diffusion - glucose

• Active transport REQUIRES ENERGY– Ion pumps– Endocytosis– Exocytosis

Active and Passive Transport Summary

What does the membrane do?

• allows for different conditions

between inside and outside of cell

• subdivides cell into compartments

with different internal conditions

• allows release of substances from

cell via vesicle fusion with outer

membrane:

Membrane Permeability

• Biological membranes are physical barriers, but which allow small uncharged molecules to pass…

• They are described as semi-permeable

Because;• Lipid soluble molecules and small molecules

pass through• Big molecules and charged ones do NOT

pass through

Membrane Semi-permeability

1) lipid soluble solutes go through faster

1) smaller molecules go faster

1) uncharged & weakly charged go

faster

2) Channels or pores may also exist in

membrane to allow transport of

larger molecules 12

Its about concentration

The concentration of the solution, with respect to other solutions is

important

Isotonic --- when both solutions have the same concentration of dissolved substances

Hypertonic --- a solution with a higher concentration of dissolved substances

Hypotonic --- a solution with a lower concentration of dissolved substances

Animal cells

Plant cells

Two types of transport

Passive and Active

Passive Transport

• Involves concentration gradients ONLY.

• NO CELL ENERGY is used—this is why it is called “passive”

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Passive Transport3 types

• Diffusion- simple movement from regions of high concentration to low concentration.

• Osmosis- specifically the diffusion of water across a semi-permeable membrane.

• Facilitated diffusion - protein transporters which assist in diffusion.

KNOW T

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Diffusion

Smell Particles & Air Particles

Solution

Smell Particles diffused evenly into the Air Particles

Diffusion is the passive movement of particles from a high concentration of particles to a

lower concentration until they are spread out evenly

Diffusion

• Movement generated by random motion of particles.

• Movement always from region of high concentration to regions of low concentration.

• Increased water pressure is caused by water moving to decrease a concentration gradient or concentration difference between two areas.

Solutions

• A solution is made up of two parts: the solute and the solvent.

• The solute dissolves in the solvent

• Before a gas can diffuse across a membrane it must dissolve into a liquid.

Diffusion & Gas Exchange

• Animal cells use oxygen, so oxygen is less concentrated inside the cell than outside. This causes oxygen to diffuse into the cell

• Carbon dioxide is produced in an animal cell, so it is more concentrated inside than outside – so it diffuses out of the cell

Diffusion for Photosynthesis

• Diffusion of gases also happens in leaves:

• For photosynthesis to happen Carbon Dioxide has to get inside the leaves. It diffuses in through stomata

• Water vapour and oxygen diffuse out of the leaf at the same time

Water + Carbon Dioxide → Oxygen and Glucose (& a little water)

Diffusion & Photosynthesis

CO2 O2

Rate of Diffusion

• The rate of diffusion depends on:

• Size of the particles: smaller = faster

• Temperature (eg. kinetic energy): hotter = faster

• The concentration gradient: the higher it is = faster the rate.

• State of the particles: gas > liquid > solid

• Distance - thickness of the exchange surface: thinner = faster

• Surface area available: larger = faster

Facilitated Diffusion

• Transport proteins carry specific molecules across the cell membrane

• Movement is along a concentration gradient (i.e. From higher to lower)

• Each type of transport protein will carry only one type of molecule.

• This is how glucose is moved.

Facilitated Diffusion

GlucoseTransport protein

Concentrationgradient

Transport Proteins• Move solutes faster across

membrane

• Highly specific to specific

solutes

• Can be inhibited by drugs

• Also involved in ACTIVE

transport

Glucose

Cell membrane

Transport protein

Glucose binds to thetransport protein

The transport protein turns over and releases glucoseonto the inside of the cell, along the concentration gradient

Concentrationgradient

Types of Protein Transporters: Ion Channels

• Work by facilitated diffusion No E!

• Deal with small molecules... ions

• Open pores are “gated”- Can change shape.– How? Do a diagram to show how you think

this might work.

• Important in cell communication

Carrier molecule

Cell membrane

Transport protein

The carrier molecule binds to the transport protein, whichopens the pore allowing it to move through the cell membrane.The pore closes once the carrier is inside the cell.

Concentrationgradient

It is possible to stop the action of transport protein with drugswhich will block the pore.

Osmosis

• Osmosis is a special type of diffusion.

• Osmosis is the diffusion of water.

Osmosis is the movement of water molecules from a high

concentration of water to a low concentration of water through

a partially-permeable membrane

Osmosis

Osmosis

Osmosis Experiment

1. Fill the partially permeable membrane with strong sugar solution

2. Tie it to a capillary tube and stand it in a weak sugar solution

3. Use your ideas about osmosis to explain why the liquid rises in the tube

Glass tube Level of sugar solution

Weak sugar solution

Visking Tubing with Strong Sugar solution

Osmosis and Cells - Notes

• Cells are surrounded by a cell membrane that separates the contents of the cell from the outside environment. The cell membrane has tiny holes in it. This allows small molecules to pass through, but not large ones. The cell membrane is partially-permeable.

• Osmosis occurs when two solutions are separated by a partially permeable membrane

Class Activity: Correctly Label

Weak Solution

Strong Solution

Solute Molecule

Water Molecule

Partially Permeable Membrane

Which way will the water flow???

Osmosis

Osmosis and Animal Cells

haemolysis

Hypotonic solution

Hypertonic solution

Osmosis & Plant Cells

Turgid Cell Plasmolysed Cell

Hypotonic solution Hypertonic

solution

Plasmolysis in Elodea

Plasmolysis in Elodea: http://www.mrphome.net/mrp/Membrane_Transport.html

Active Transport• Cell Energy is used to move substances

across the cell membrane

• The substances are moved against the concentration gradient i.e. from where there is less to where there is more.

Transport proteins

• Substances are moved molecule by molecule.

• It is similar to facilitated diffusion except that cell energy (ATP) is used in the process.

ATP = Adenosine Triphosphate

Salt ion

Cell membrane

Transport protein

Ion binds to thetransport protein

The transport protein turns over and releases the iononto the inside of the cell, against the concentration gradient

Concentrationgradient

Energy is used

Moving many large molecules at once—Endocytosis

• Endocytosis • Transports macromolecules and large particles into

the cell.• Part of the membrane engulfs the particle and folds

inward to “bud off.”• The cell membrane envelopes the material • If material is liquid the process is called

pinocytosis• If material is solid the process is called

phagocytosis

Pseudopodia extend to engulf food

A food vacuole is formed

Pinocytosis works the same, but with no food, only liquid

How Endocytosis works

Moving many large molecules at once—Exocytosis

• Material is packaged inside the cell and the package fuses with the cell membrane while the material goes out of the cell.

How exocytosis works

Vacuole containing particlesis moved closeto the cell membrane

Fuses with the cellmembrane to expelthe particles

•Animation;•YouTube - Endocytosis & Exocytosis

Exocytosisthe finer detail

Bulk Transport

Ion Channels

• Work fast: No conformational changes needed

• Not simple pores in membrane:– specific to different ions (Na, K, Ca...)– gates control opening– Toxins, drugs may affect channels

• saxitoxin, tetrodotoxin• cystic fibrosis

Sodium-Potassium Pump

Na-K Pump Model: Part I

• 3 Na+ bind to inner region of protein

• Na+ binding triggers phosphorylation of protein. ATP ADP + Pi

• Phosphorylation causes conformation change and Na+ binding site faces outside

• 3 Na+ released to outside

Na-K Active Pump: Part II

• 2 K+ ions on outside are able to bind• K+ binding causes de-phosphorylation

and new conformation change• 2K+ ions exposed to inside and released

Cyclic process uses ATP energy to drive Na & K ion transport against conc. Gradient

Root Hair Cells and Active Transport

• Root hair cells take in minerals using active transport.

• If the concentration of minerals is higher in the root hair than in the soil, then the cell needs to use energy to actively transport the minerals into the cell against the concentration gradient.

Active Transport and Villi• When there’s lots of nutrients in the gut

they diffuse naturally into the blood. But...

• Sometimes there are less nutrients in the gut than in the blood, so the villi cells of our gut use active transport to absorb these nutrients against their concentration gradient.

Villi and Glucose Uptake

Passive vs. Active Transport