Ch4 cell membranes and transport

fluid mosaic model definitions

Explains how biologicsl moleculws are arranged to form cell membrwnes

Explains passive and active mobement between cells and surrounding

Fluid mosaic 4 features

- separate internal and external environment

- controls exchange of materials across surfaces

- selectively permeable

- phospholipid bilayer

Why is the model called 'fluid'

Phospholipids and proteins can move about by diffusion

Phospholipid molecules move sideways in their layers

Some protein molecules move, others remain fixed

'Mosaic' model

the scattering of the different proteins within the phospholipid

Phospholipid polarity

Hydrophilic heads and hydrophobic tails

Phospholipid 2 structures

Ball like micelles

Sheet like bilayers

Phospholipid single layers

Phospholipids spread over one surface of water

Hydrophilic heads in water

Hydrophobic tails out

Micelles

Formed when phospholipids are shaken with water

Stable structure, in a ring with heads out, shielding tails in which point together

Phospholipid sheets

Bilayer , two layers of phospholipid molecules

Protein arrangement in membrane

- some float within their layer, some are fixed

- some are embedded in upper or lower layers, which's some span entire membrane

Why do protein stay in the membrane

- protein shave hydrophobic and hydrophilic regions

Hydrophobic amino acids are next to the hydrophobic fatty acid tails and are repelled by surrounding water environment on either side of membrane

Glycocalyx

Glycolipids and glycoproteins

Many proteins and lipids have short branching carb chains attached to form these

Fluidity affecting factors of membrane

- more unsaturated fatty acid tails = more fluid

- longer tail = less fluid

- higher temp= more fluid

How do phospholipids act as a barrier

- Tails are non polar and face in , so it is difficult for polar substances to pass through membrane

- barrier to most soluble substances

- prevents leakage of things e.g. sugars, amino acids , proteins

Cholesterol

A small, lipid related molecule with a hydrophilic head and hydrophobic tail

Cholesterol structure and location

-hydrophilic heads and hydrophobic tails

Fits between phospholipids with their heads at the surface

How does cholesterol provide mechanical stability

Strengthens membranes by getting in between phospholipids and reducing fluidity

Prevents cells bursting open and breaking quickly

cholesterol and myelin sheath

Hydrophobic regions of cholesterol prevents ions/polar molecules from passing through membrane

- myelin sheet surrounds nerve cells and is made of many layers of cell surface membrane

- prevents leakage of ions which would slow down nerve Impulses

Low temperatures and cholesterol

At low temperatures phospholipid tails pack close

Cholesterol prevents this from happening too much, prevented too much rigidity

Maintaining correct fluidity allows cells to survive colder temperatures

High temperatures and cholesterol

Cholesterol stops in crease in fluidity by interacting with phospholipids

Glycoproteins and glycolipids as receptor molecules + examples

- carb chains help them to act as receptor molecules

- receptor molecules bind with substances at cell surface membrane

E.g. signalling receptors

signalling receptors

Coordinate activities of the cell

Recognises messenger molecules like hormones and neurotransmitters

When messenger molecules bind to signalling receptor, a series of chemical reactions start inside the cell

Signalling receptor examples

Glucagon receptor in liver cells. Only cells with glucagon receptors are affected by glucagon

Glycoproteins and Glycolipids cell to cell recognition

Some Glycolipids and glycoproteins act as cell markers/antigens to allow cells to recognise each other

- carbohydrates chains bind to complementary sites in other cells

- important for immune responses

Transport proteins

Many proteins can act as these

They provide hydrophilic channels/passageways for ions and polar molecules to pass through membrane

Each transport protein is specific to a particular ion/molecule

Two types channel or carrier

Enzymes in membrane

Enzyme membrane proteins found in cell surface membrane

Proteins and cytoskeleton

Some proteins attached to a system of protein filaments = cytoskeleton

Maintains and decides cell shape and involved in shape changes when cells move

Cell signalling

The molecular mechanisms by which cells detect and respond to external stimuli including communication between cells

Two types of cell signals

electrical and chemical

1st stage of first part of pathway

A stimulus causes cells to secrete a specific chemical called a ligand e.g. glucagon

2nd stage of first path of pathway

Ligand is transported to target cells

Signalling molecules usually are small for easy transport

3rd stage of first path of pathway

Ligand binds to cell surface receptors on target cells. Receptors are protein molecules on cell surface membrane. Cell surface receptor is a specific shape and recognises ligand

What does ligand do to receptor

Ligand changes receptor shape. Receptor spans the membrane so message is passes to inside of cell. This allows it to interact with next component of signalling pathway so message gets transmitted

Ligand

A biological molecule which binds specifically to another molecule such as cell surface membrane receptor during cell signalling

Transduction

Occurs during cell signalling and is the process of converting a signal from one method of transmission to another

G protein

Causes release of second messenger

Acts as a switch to bring about release of second messenger

Second messenger

A small molecule which diffuses though the cell relaying the message

Process of 2nd messenger

- stimulation of one receptor results in many second messenger molecules made in response causing amplification of original signal

- 2nd messenger activates an enzyme which activates further enzymes increasing amplification in each stage

- enzymes required to change cell metabolism are produced

Signalling cascade

Events triggered by the G protein

3 other ways a receptor can alter the activity of a cell

1. Opening ion channel resulting in change in membrane potential

2. Acting directly as a membrane bound enzyme

3. Acting as an intracellular receptor when the initial signal passes straight through cell surface membrane

Why can some signalling molecules like steroids pass directly across cell surface membrane

Hydrophilic signal molecules cant cross but hydrophobic can diffuse directly

Steroids are fatloving/lipophilic and hydrophobic so can cross. Membrane is made of phospholipids so lipophillic things can go straight through

Cell to cell contact

Another mechanism of signalling

E.g. when lymphocytes detect foreign antigens on other cells

3 methods of substance movement across membranes

Diffusion

Facilitated diffusion

Osmosis

Active transport

Bulk transport- endo and exycytosis

Simple diffusion

The net movement of molecules or ions from high to low concentration down a concentration gradient, as a result of the random movement of particles

How does molecule size affect diffusion

Bigger molecule = slower

How does polarity of molecules affect diffusion

Non polar = easier diffusion

Non polar substances are soluble in non polar phospholipid tails compared to polar substances

Facilitated diffusion

Diffusion of molecules from high to low concentration through specific hydrophilic membrane protein molecule channels

What do proteins do in facilitated diffusion

Provides hydrophilic areas that allow molecules and ions to pass through membrane which would otherwise be less permeable to it

Why is facilitated diffusion needed

Larger molecules e.g. glucose and amino acids and na and cl ions cannot diffuse through bilayer. They need proteins to help. Channel or carrier proteins involved

Channel protein

Membrane protein of fixed shape. Water filled pore through which selected hydrophilic ions or molecules can pass by facilitating diffusion or active transport

Channel protein structure

- water filled pores that allow charged substances to diffuse across membrane

- gated part on inside surface opens and closes like a pore for ion exchange

- some channels need single or several proteins

- some gated channel proteins need atp

Carrier protein

A membrane protein which changes shape to allow the passage into to out of cell of specific ions or molecules by facilitating diffusion or active transport

Carrier protein structure

- can flip between 2 shapes so binding site is alternating

Open to one side then the other allows molecule or ion to cross membrane

- some change shape spontaneously

- some require energy and are involved in active transport

How is rate of diffusion in channel and carrier proteins affected

- affected by how many channel or carrier proteins molecules there are in the membrane

- whether or not channel proteins are open

Osmosis

The net diffusion of water molecules from high to low water potential through a partially permeable membrane

Water potential

A measure of the tendency of water to move form one place to another

Water potential affecting factors

1. Concentration of solution

2. How much pressure is applied to it

- higher pressure on one side increase wp because applying pressure increases tendency of water to move form

Water potential unit

kPa

Water potential of substances value

Water = 0 kPa

Other substances are all lower than 0 kPa

Water has highest

Hypotonic

High wp

Hypertonic

Low wp

isotonic

when the concentration of two solutions is the same

Cytolysis

Animal cell bursts in too much water

Crenation

Animal cell shrinks and shrivels

Why do plant cells reach water equilibrium fast

Cell wall is so inelastic, volume rarely changes so pressure increases fast so very little water needed to achieve equilibrium

Protoplast

Contents of cell excluding cell wall

Plasmolysis

Loss of water from plant or prokaryote cell to the point where protoplast shrinks away form cell wall

Incipient plasmolysis

Point at which plasmolysis is about to occur when a plant cell or prokaryote is losing water. Protoplast is exerting no pressure on cell wall

Active transport

Th movement of molecules/ions through transport proteins across a cell membrane against their concentration gradient using atp for energy

Active transport process

Some ions are more concentrated inside cell that outside

Carrier proteins called pumps are specific to a particular molecule or ion

Requires energy unlike facilitated diffusion as its path is against conc gradient

Energy from atp is used to make carrier protein change shape, transferring molecules/ions across membrane in this process

Sodium - potassium pump

A membrane protein/s that moves sodium ions out of a cell and potassium ions into a cell using ATP

Sodium potassium pump process

- found in all cell surface membranes

- pumps 3 sodium ions out and 2 potassium into cell for each atp molecule used

- net result is inside of cell becoming more negative than outside

- a potential difference is created along the membrane; needed for nervous communication

- pumps has receptor site for atp. Receptor site acts as an atp enzyme and hydrolyses it into adp and phosphate to release energy

Active transport examples

- reabsorption in kidneys;certain useful molecules and ions have to be reabsorbed into blood after filtration into tubules

- absorption of digestive products in gut

- in plants used to load sugar from photosynthesising cells of leaves into phloem for transport

- in plants used to load inorganic ions from soil into root hairs

Bulk transport

Endocytosis and exocytosis

Sometimes cells need to transport materials across membrane in a much larger scale than possible by other mechanisms

Endocytosis

The bulk movement of liquids or solids ( pinocytosis or phagocytosis ) into a cell by the unfolding of the cell surface membrane to form vesicles containing the substance; active process requiring atp

Endocytosis process

Cell surface membrane engulfs material to form a small sac/vesicle/vacuole

Two forms ; phago or Pino cytosis

Phagocytosis

Cell eating

Bulk uptake of solid material

Cells specialising in this are called phagocytes

Vacuoles are called phagocytic vacuoles

Pinocytosis

Cell drinking bulk uptake of liquid

Vacuoles or vesicles formed are often extremely small in which the process is called micro pinocytosis

phagocyte

A type of cell that ingests and destroys pathogens or damaged body cells by the process of phagocytosis

Exocytosis

The bulk movement to liquids or solids out of a cell by fusion of vesicles contains the substance with the cell surface membrane; active process requiring atp

Exocytosis process

- materials are removed from cells. Vesicles dues to plasma membrane and expel contents

- secretory vesicles from Golgi body carry enzymes to cell surface membrane and release their contents

exocytosis examples

plant cells use exocytosis to get their cell wall building materials to the outside to cell surface membrane

Secretion of digestive enzymes from cells of pancreas

Extrensic protein

Protein that covers half of membrane

Intrinsic protein

Proteins that span the whole of the phospholipid bilayer

How to compare rates of diffusion

Ratio of surface area : volume

Higher number = faster because more surface area of very unit volume

Membrane permeable substances

O2 co2 and h2o

Impermeable substances

Ions e.g. ca2+ k+

Glucose and amino acids

Which methods of transport can go both ways across a cell surface membrane

Active transport, facilitated diffusion, dffiusion and osmosis

Which part of a glycoprotein is hydrophilic

Protein part and the carbohydrate chain

Which part of a glycolipid is hydrophilic

Lipid tail is hydrophobic

Carbohydrate head/sugar head is hydrophilic