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 Structure
The 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 Lipids form 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 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 DRAW THIS Integral Proteins Trans – membrane proteins Cell Membrane

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
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
1 2

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”
KNOW THIS

Passive Transport 3 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 THESE

Diffusion Diffusion is the passive movement of particles from a high concentration of particles to a lower concentration until they are spread out evenly Smell Particles & Air Particles Solution Smell Particles diffused evenly into the Air Particles

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 CO 2 O 2

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 Glucose Transport protein Concentration gradient

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 the transport protein The transport protein turns over and releases glucose onto the inside of the cell, along the concentration gradient Concentration gradient

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, which opens the pore allowing it to move through the cell membrane. The pore closes once the carrier is inside the cell. Concentration gradient It is possible to stop the action of transport protein with drugs which 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 Experiment
Fill the partially permeable membrane with strong sugar solution
Tie it to a capillary tube and stand it in a weak sugar solution
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 the transport protein The transport protein turns over and releases the ion onto the inside of the cell, against the concentration gradient Concentration gradient 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 particles is moved close to the cell membrane Fuses with the cell membrane to expel the particles
Animation;
YouTube - Endocytosis & Exocytosis

Exocytosis the 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

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2.4 Cell Membrane And Transport

by Toni Foley on May 10, 2011

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2.4 Cell Membrane And Transport Presentation Transcript