ocr Biology - 2.5 Plasma Membranes

What is compartmentalisation?

The formation of separate membrane-bound areas in a cell

Why is compartmentalisation in cells important?

*Metabolism involves many different and often incompatible reactions

*Reactions contained in separate parts of the cell allows the specific conditions required for cellular reactions, such as chemical gradients to be maintained.

*Protects vital cell components

What is the structure of plasma membranes?

Composed of a phospholipid bilayer, proteins and carbohydrates (usually attached to proteins/lipids)

What is the fluid mosaic model?

Phospholipid molecules form a continuous bilayer, which are continuously moving ('fluid'), with proteins of different shapes and sizes embedded (tiles in a 'mosaic'), and with cholesterol present.

What is a phospholipid?

A molecule consisting of glycerol, two fatty acids and a phosphate-linked head group

What is the role of phospholipids in plasma membranes?

Form a barrier to dissolved substances.

Hydrophilic heads interact with water but the tails form a hydrophobic core, preventing water-soluble substances (e.g. ions) passing through it.

What is the role of cholesterol in plasma membranes?

Give the membrane stability.

The molecules fit between the phospholipids, binding to the hydrophobic tails, causing them to pack more closely together. This makes the membrane less fluid and more rigid.

What is the role of proteins in plasma membranes?

Control what enters and leaves the cell

Channel proteins allow small or charged particles through. Carrier proteins transport molecules and ions across the cell membrane by active transport and facilitated diffusion.

What is the role of glycoproteins and glycolipids in plasma membranes?

Act as receptors for messenger molecules

✤They stabilise the membrane by forming hydrogen bonds with surrounding water molecules.

✤Sites for the binding of drugs, hormones, antibodies

✤Antigens- CSM involved in the immune response

What is the difference between intrinsic and extrinsic proteins?

Intrinsic- embedded through both layers of the membrane, they have amino acids with hydrophobic R-groups on their external surfaces, which interact with the hydrophobic core of the membrane, keeping them in place.

Extrinsic- present in one side of the bilayer, they have hydrophilic R-groups on their external surfaces, interacting with the polar heads of the phospholipids or with intrinsic proteins.

Give examples of intrinsic proteins

-Channel proteins

-Carrier proteins

-Glycoproteins

give an example of a membrane bound protein and its function

ATP-synthase

-catalyses the production of ATP during oxidative phosphorylation on the inner mitochondrial membrane

What are channel proteins?

Provide a hydrophilic channel that allows the passive movement of polar molecules and ions down a concentration gradient through membranes.

How are channel proteins held in position?

By interactions between the hydrophobic core of the membrane and the hydrophobic R-groups on the outside of the proteins.

What are carrier proteins?

An intrinsic protein involved in both passive transport (down CG) and active transport (against CG) into cells. Often involving the shape of the protein changing.

What are glycoproteins? What is their role?

Intrinsic proteins embedded in the plasma membranr with carbohydrate chains of varying lengths and shapes attached.

Involved in:

✤cell adhesion in tissues

✤receptors for cell signalling and the messenger will bind to the specific glycoprotein receptor site

glycolipids- 'antigens' or cell markers- help in the regulation of the immune response to identify cells as 'self'

What are glycolipids? What is their role?

Lipids with carbohydrate chains attached.

Called 'antigens' or cell markers- help in the regulation of the immune response to identify cells as 'self'

what is the function of cholesterol in cell membranes?

Makes the phospholipids pack more closely together, which makes the membrane less fluid and more rigid.

-helps to maintain the shape of animal cells

What do cells communicate with each other using? How does this work?

Cells communicate with each other using messenger molecules:

1. one cell releases a messenger molecule (e.g. a hormone)

2. this molecule travels (e.g. in the blood) to another cell.

3. the messenger molecule is detected by the cell because it binds to a receptor on a cell membrane

how do cell receptors work?

these receptors bind a target molecule

the glycomolecule undergoes a conformational change and initiates a chain of reactions known as a 'CASCADE' that can lead to a cell-level response to the binding of certain signalling molecules

What is a target cell?

A cell that responds to a particular messenger molecule

what factors influence the permeability of cell membranes?

✤Temperature

✤Solvents

How does temperature affect membrane permeability?

✤Temperature increased> phospholipids will gain kinetic energy and will move even more> membrane more fluid and begins to lose its structure

✤Continues to increase> cell breaks down completely

✤Carrier and channel proteins denature> cannot control what enters/leaves cell> increased permeability

How does solvent concentration affect the permeability of cell membranes, how can solvent concentration be controlled?

✤Less-polar molecules such as alcohols, or non-polar molecules such as benzene, can easily pass through the membrane by simple diffusion.

✤Their presence dissolves the membrane, disrupting cells.

✤Membrane becomes more fluid and more permeable

Describe the permeability of the plasma membrane

It is selectively permeable.

Small, uncharged polar molecules and hydrophobic molecules are Free to Pass. (E.g. H2O, CO2, O2)

Large polar molecules (glucose) and ions Cannot Pass

unless by a carrier or channel protein

what is the practical to investigate cell membrane permeability?

beetroot PAG

1. 5 small pieces of beetroot of equal size cut using a cork borer.

2. pieces throughly washed in running water

3. placed in 100ml of distilled water in a water bath

4. temp of water bath increased in 10* intervals

5. Samples of the water containing the beetroot taken 5 mins after each temp is reached.

6. Absorbance of each sample measured using a colorimeter with a blue filter: ensure to calibrate first

7. Experiment done 3 times, each time with fresh beetroot pieces and a mean calculated for each temperature.

8. plot a graph of results

The higher the permeability of the membrane, the more pigment is released, so the higher the absorbance of the liquid.

why do beetroot samples need to be rinsed first?

To remove excess pigment that has leaked through physically broken cell membranes

what is the conclusion of the practical investigating cell membrane permeability?

the darker the solution, the more pigment that the cell membrane has released.

this is reflected in a higher absorbance reading

what absorbance is the colorimeter calibrated to?

520 nm

What filter is used in the colorimeter in the practical investating beetroot membrane permability at different temperatures?

Blue filter

What are the results of the beetroot membrane permeability practical?

positive correlation: as the temperature of the water bath increased, the membrane became more permable as the solution was darker and more concentrated so more betalain pigment escaped the beetroot cell

higher light absorbance shown on colorimeter at higher temperatures

what is the pigment which diffuses out of the beetroot cell at high temperatures?

betalain pigment

(gives the red colour)

explain what is meant by the term simple protein

a protein without a prosthetic group/non-protein component.

What is diffusion?

Diffusion is the net movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration.

When will diffusion continue until?

A concentration equilibrium between the two areas- a balance or no difference in concentrations- the particles do not stop moving but the movements are equal in both directions.

What is a concentration gradient?

The path from an area of higher concentration to an area of lower concentration.

In diffusion, particles move ____________ a concentration gradient

Down

Diffusion is a _____________ process. What does this mean?

passive- no energy is needed for it to happen

Why does diffusion happen?

Particles in a liquid or gas have kinetic energy and are moving randomly, such that an unequal distribution of particles will eventually become an equal distribution.

What is the rate of diffusion over long distances?

Particles move at high speeds and are constantly colliding, which slows down their overall movement.

This means that over short distances diffusion is fast, but as diffusion distance increases, the rate of diffusion slows down because more collisions have taken place.

_____________, _______________ molecules such as _____________ and _______________ are able to __________________ easily through spaces between ______________________

small, non-polar molecules such as oxygen and carbon dioxide are able to diffuse easily through spaces between phospholipids.

________________ is also ___________ enough to fit between ___________________, so it's able to _______________ across plasma membranes even though it's _____________. This is called _____________

Water is also small enough to fit between phospholipids, so it's able to diffuse across plasma membranes even though it's polar. This is called osmosis.

The __________________________ of the membrane ______________ substances with a ________________ or ______________ ____________ (________) so they cannot easily pass through.

The hydrophobic interior of the membrane repels substances with a positive or negative charge (ions) so they cannot easily pass through.

What are the factors affecting the rate of diffusion?

❤Temperature- the higher it is, the higher the rate of diffusion as particles have more kinetic energy and move at higher speeds.

❤Concentration difference- the higher it is, the higher the rate of diffusion as the overall movement from the higher to lower concentration will be larger.

❤Surface Area- the larger the surface area, the higher the rate of diffusion

❤Thickness of membrane- the thinner the exchange surface, the higher the rate of diffusion.

What are the two ways rate of diffusion can be calculated?

Distance travelled/time

Volume filled/time

Outline an investigation into diffusion in model cells.

1. Make up some agar jelly with phenolphtalein and dilute sodium hydroxide> jelly will be a pink colour

2. Fill a beaker with some dilute hydrochloric acid. Using a scalpel, cut out a few cubes from the jelly and put them in the beaker of acid.

3. If you leave the cubes for a while, they'll eventually turn colourless as the acid diffuses into the agar jelly and neutralises the sodium hydroxide.

What is facilitated diffusion?

Diffusion across a membrane through protein channels.

What type of molecules does facilitated diffusion allow the transport of into or out of a cell membrane?

❤larger molecules - amino acids, glucose

❤ions

❤polar molecules

Facilitated diffusion moves large molecules, ions or polar molecules ______________ a concentration gradient and is a _____________ process.

down