Biological membranes

What are the functions of membranes within cells?

Compartmentalisation: seperate contents of organelles from the cytoplasm

Site of attachment of enzymes/ribosomes

Creation of concentration gradients

Provide selective permeability within organelles

Form vesicles to transport substances.

What is the function of membranes at the surface of cells?

Cell signalling/recognition as self or non self

Cell communication

Provide selective permeability to cells.

What is the fluid mosaic model?

Phospholipids are free to move within the bilayer, they are fluid.

Proteins embedded in the bilayer vary in shape,size and position and are randomly arranged between phospholipids like a mosaic.

What is the structure and function of phospholipids?

Form bilayer with hydrophobic tails pointing inwards away from aqueous environment,

hydrophilic heads pointing outwards interacting with the aqueous environment, forming a hydrophobic core.

Function: provide a barrier to large,polar molecules and ions. Only allow small non-polar, fat-soluble molecules to diffuse through the bilayer eg oxygen,carbon dioxide.

Explain why large and polar molecules cannot pass through the phospholipid bilayer

Large- phospholipid bilayer acts as a barrier

Polar- not soluble in the phospholipid bilayer

Why can fat soluble molecules pass through to phospholipid bilayer

Not repelled by the phospholipid bilayer

What are intrinsic proteins.

Transmembrane proteins

embedded through both layers of the membrane.

Have amino acids with hydrophobic R-groups which interact with the hydrophobic core to keep them in place Examples:channel and carrier proteins and glycoproteins.

What is the function of channel proteins?

Channel proteins provide a hydrophillic channel

allow specific small polar molecules or ions to cross the membrane by passive movement

hydrophilic pathways provided by these proteins can allow water molecules or small ions to diffuse very quickly from one side of the membrane to the other.

What is the function of carrier proteins?

Can change shape to reveal hydrophillic interiors but needs energy ATP.

Allows movement of large polar molecules and ions across the membrane by both passive and active transport.

What are the structure and functions of glycoproteins?

Sticking out of the cell surface membrane with an attached carbohydrate chain. functions include:

cell adhesion which is holding cells together in a tissue.

acting as antigens

recognition of cells as self/non self

receptor site for chemical signals/hormones

receptor site on transport proteins

attach to water molecules to stabilise membrane

cell signalling

What is cell signalling?

Communication between cells by a trigger molecule being released by one cell which attaches and causes a change in another cell.

How does the cell surface membrane contribute to cell signalling?

1.trigger molecule released by a cell by exocytosis

2.glycoproteins act as receptors which are specific so that the shape of the receptors and trigger molecule are complementary.

3.trigger molecule attaches to receptor which causes a change inside the cell.

Give an example of cell signalling?

Drugs act by binding to receptors eg B Blockers are used to reduce the response of the heart to stress.

What is the structure and function of glycolipids?

Lipid usually a phospholipid which an attached carbohydrate chain, act as antigens

recognised by cells as self/non self.

What is the function of extrinsic proteins?

Present in one side of the bilayer, hydrophillic r-groups on external surface which interact with hydrophillic heads of the phospholipids.

What is the structure and function of cholesterol?

Fit between phospholipids they have a hydrophilic end which interact with the hydrophilic heads and a hydrophobic head which interacts with the hydrophobic tails pulling them together making them non-rigid.

function:

stabilizes the membrane structure by regulating the fluidity of the membrane

prevents membranes becoming too solid by preventing phospholipids from grouping too closely together and crystallising

How does temperature affect membranes?

Increasing the temperature means phospholipids

1.have more kinetic energy, increasing their movement

2.increases number of gaps between phospholipids

3.bilayer becomes more fluid membrane begins to lose its structure

4.increased peremability of membrane,easier for particles to diffuse across

5. proteins denature and can no longer control peremability making it easier again for particles to diffuse across.

What happens in the beetroot practical?

As temperature increases, plasma membrane becomes damaged, membrane becomes more permeable and more pigment leaks out by diffusion, transmission decreases as the pigment absorbs all the light.

What are the control variables?

Same beetroot used

All samples have the same size/surface area

In the waterbaths at the same time.

Why does a low pH cause the pigment to leak out the cell

Low pH denatures membrane proteins by changing the tertiary structure, therefore membrane peremability increases

Why does water not affect the plasma membrane?

Because in the phospholipid bilayer the hydrophobic tails face inwards away from the water, the hydrophilic heads face outwards interacting with the water helping to keep the bilayer intact.

What do organic solvents such as alcohols do to plasma membranes, give an example.

Dissolve the membrane disrupting cells by making them more fluid and more permeable.

Example: alcohol in wipes dissolve the membranes of bacteria and kill them.

What happens when it is a pure/strong alcohol?

They are very toxic, dissolve membranes and destroy cells in the body.

What is simple diffusion?

The net movement of particles from a reigon of high concentration to a reigon of low concentration. Down a concentration gradient.

What are the factors affecting the rate of diffusion?

Diffusion distance, temperature, concentration gradient, surface area and number of channel proteins.

What happens as the diffusion distance increases?

The rate of diffusion slows down as more collisions have taken place, this is why cells are generally microscopic.

What happens as the temperature increases?

Rate of diffusion increases as particles have more kinetic energy and move at higher speeds.

What happens as the concentration gradient increases?

Greater the difference in concentration between the two reigons the rate of diffusion increases because the overall movement is larger.

What can diffuse across the phospholipid bilayer?

Hydrophobic molecules they are partially permeable as the hydrophobic core repels positive or negative charge therefore meaning only small,non polar fat-soluble molecules can diffuse through down a concentration gradient.

What is facillitated diffusion?

Diffusion across a membrane through channel or carrier proteins that allows hydrophillic substances to cross the membrane without interacting with the hydrophobic core of the phospholipid bilayer.

How do carrier proteins work in facillitated diffusion?

Have specific binding sites which specific molecules attach to which changes the tertiary structure of the protein and allows the chemical across the membrane where it is released.

How do channel proteins work in facillitated diffusion?

Proteins with a central pore,

lined with hydrophillic amino acids that hydrophillic substances can pass through one side of the membrane to the other via facillitated diffusion.

Selective so only certain chemicals can pass through certain protein channels.

How do channel proteins require no metabolic energy

Particles have their own kinetic energy so can move down a concentraiton gradient

What is active transport?

The movement of molecules from a reigon of low concentration to a reigon of high concentration which requires metabolic energy in the form of ATP and uses carrier proteins.

Outline the general process of active transport.

Molecule to be transported binds to the receptor site on the carrier protein (outside of cell low conc)

Molecule of ATP binds to carrier protein inside of cell where it is hydrolysed to ADP and phosphate.

Binding of phosphate molecule causes carrier protein to change shape.

Molecule transported to other side of membrane and released.

Phosphate molecule is released from carrier protein and recombined with ADP in respiration to form ATP.

Carrier protein returns to original shape.

What is bulk transport?

Another form of active transport.

Large molecules such as enzymes, hormones and whole cells like bacteria are too large to move through the channel or carrier proteins,

so they are moved in and out of the cell by bulk transport.

What is endocytosis?

Particles are transported into cells.

What is the process of endocytosis?

Cell membrane folds inwards to form a cavity around the particles, invagination.

Membrane enfolds the particle until it fuses and forms a vesicle.

Vesicle pinches off and moves into the cell for further processing.

What is exocytosis?

Particles are transported out of the cell eg. secretion of extracellular enzymes.

What is the process of exocytosis?

Vesicles formed by the golgi apparatus move towards and fuse with the cell surface membrane

Contents of the vesicle are released to the outside of the cell.

What is osmosis?

The diffusion of water down a water potential gradient (high w.p to a more negative w.p) through a partially permeable membrane.

Explain importance of osmosis in plant support

-Water enters vaccuole

-Pressure against cell wall creates turgor pressure

-Turgid cells support plants

What is water potential?

The pressure exerted by water molecules as they collide with a membrane, unit Pa or kPa.

What is the water potential of pure water?

0Pa which is the highest possible water potential as pure water is the most concentrated, the more concentrated the solution the more negative the water potential.

What is hydrostatic pressure?

If the solution is in a closed system eg a cell, and water moves in the hydrostatic pressure increases.

What happens in an animal cell when the external solution has a higher water potential than the cytoplasm, cell solution?

Water moves into the cell by osmosis, hydrostatic pressure increases, the thin cell-surface membrane cannot stretch much and cant withstand the increased hydrostatic pressure so the cell swells and bursts which is cytolysis.

What happens in an animal cell when the external solution has a lower water potential than the cytoplasm, cell solution?

Water moves out of the cell by diffusion and there is a reduction in the volume of the cell, the cell surface membrane puckers and the cell becomes shrunken and shrivilled, crenation.

What happens in an plant cell when the external solution has a higher water potential than the cytoplasm, cell solution?

Water enters the cell by osmosis, turgor pressure increases and resists further entry of water, cell swells and becomes turgid and the cell surface membrane pushes against the cell wall.

What happens in an plant cell when the external solution has a lower water potential than the cytoplasm, cell solution?

Water leaves the cell by osmosis, reduction in the volume of the cytoplasm, cell-surface membrane pulls away from cell wall and cell becomes plasmolysed and cell contents shrink.

What is turgor pressure?

the water pressure inside the vacuole of the plant cell in response to the force of water within the cell

What do animal cells have in place?

Control mechanisms so cells are continously surrounded by aqueous solutions with equal water potential, eg plasma in blood. unlike plants which are often surrounded by pure water.

Osmosis practical

- Cut up a potato into identical cylinders

- Get some beakers with different sugar solutions in them, one should be pure water and another should be a very concentrated sugar solution and some in between.

- Measure the mass of the cylinders and leave each one in each beaker for a day

- Take them out, dry them and weigh them again

- If water has been drawn in by osmosis they will have increased in mass

- If water has be drawn out they will decrease in mass

- Dependant variable is the chip mass

- Independent variable is the concentration

Why is it neccessary to calculate the percentage change in mass

Different starting masses of plant pieces

Allows comparison between plant pieces of different mass

Control variables in practical

-discs same sa:v ratio

-same variety/part of plant

-no skin on plant

-removing excess water before reweighing/drying

-same volume of sucrose solution

-same temp

0% mass change

No change in mass occurs when water potential of sucrose solution= water potential of plant tissue

-Plot graph of sucrose solution against percentage change in mass

-find which sucrose solution gives no change in mass of the plant tissue to find the water potential of the plant tissue

Why is the graph represented as a line graph

Both variables are continuous

-concentration on x axis as it is an independent variable

-change in mass on y axis as it is a dependent variable

Outline the process of the beetroot practical

1.Cut beetroot into 6 identical pieces

2.Place each cube into a test tube with equal volumes of distilled water

3.Place in waterbaths at different temperatures

4.Filter each solution out into a cuvette and measure the absorbance.

Plot a graph of temperature against absorbance, as high temps make the membrane more peremeable, the higher the temperautre the greater the absorbance as the more pigment leaks out

Measuring the rate of diffusion

Make agar jelly mixed with phenolpthalein

Place in hydrochloric acid and measure the time taken for them to turn colourless

Change the factor and repeat, eg conc of Hcl or SA of agar jelly to investigate the changing factors