cell membranes

What is the function of the cell surface membrane?

It separates the internal cell environment from the external environment.

Name three structures surrounded by intracellular membranes.

Nucleus, mitochondria and endoplasmic reticulum.

What else do membranes do besides separating compartments?

They control exchange of substances and act as interfaces for communication.

What does partially permeable mean?

The membrane allows some substances to cross but restricts others.

What transport processes can occur across membranes?

Diffusion and active transport.

What proteins are involved in cell communication?

Receptor proteins.

What can receptor proteins bind?

Substances such as hormones and antigens.

What is the basic structure of the cell membrane?

A phospholipid bilayer with additional components.

What is a phospholipid?

A lipid consisting of glycerol, a phosphate group and two fatty acid tails.

How many fatty acid tails does a phospholipid have?

Two.

What forms the phosphate head?

A phosphate group.

What forms the lipid tail?

Two fatty acid tails.

What is the phosphate head's polarity?

Polar.

What is the fatty acid tail's polarity?

Non-polar.

What does hydrophilic mean?

Attracted to or able to interact with water.

Why is the phosphate head hydrophilic?

It is polar and can interact with polar water molecules.

What does hydrophobic mean?

Repelled by or unable to interact with water.

Why are fatty acid tails hydrophobic?

They are non-polar and cannot interact with polar molecules.

What does hydro mean?

Water.

What does philic mean?

Loving.

What does phobic mean?

Hating.

What happens when phospholipids are spread over water?

They form a monolayer.

How are phospholipids arranged in a monolayer?

Hydrophilic heads face the water and hydrophobic tails point away.

What is a phospholipid monolayer?

A single layer of phospholipids.

What is a phospholipid bilayer?

A two-layered sheet of phospholipids.

What is the basic structure of a cell membrane?

A phospholipid bilayer.

Where are the hydrophilic heads positioned in a bilayer?

Facing the aqueous environments on either side.

Where are the hydrophobic tails positioned?

Facing inwards away from water.

What non-lipid components are found in the phospholipid bilayer?

Proteins and cholesterol, plus glycolipids and glycoproteins at the surface.

What are the two main categories of membrane proteins?

Intrinsic and extrinsic proteins.

What is another name for an intrinsic protein?

Integral protein.

What is another name for an extrinsic protein?

Peripheral protein.

Where are intrinsic proteins found?

Embedded within the membrane.

What determines the arrangement of an intrinsic protein?

Its hydrophilic and hydrophobic regions.

Where are extrinsic proteins found?

On the outer or inner surface of the membrane.

What are membrane proteins involved in?

Transport and communication.

Where is cholesterol found?

Between phospholipids.

What is the main role of cholesterol?

Regulating membrane fluidity.

What does cholesterol do at low temperatures?

Increases membrane fluidity and prevents the membrane becoming too rigid.

How does cholesterol prevent excessive rigidity at low temperatures?

It prevents phospholipid tails packing too closely.

What does cholesterol do at high temperatures?

Prevents the membrane becoming too fluid.

How does cholesterol stabilise membranes at high temperatures?

It interacts with phospholipid tails and causes them to pack more closely.

What does cholesterol bind to?

Hydrophobic phospholipid tails.

How does cholesterol affect membrane mechanical strength?

It increases mechanical strength and stability.

What could happen without cholesterol?

Membranes could break down and cells could burst.

What is a glycoprotein?

A protein with a carbohydrate attached.

What is a glycolipid?

A lipid with a carbohydrate attached.

Where are glycoproteins and glycolipids found?

On the cell surface.

What is one function of glycoproteins and glycolipids?

Cell-to-cell communication.

What can glycoproteins and glycolipids bind?

Substances such as hormones.

What other role can glycoproteins and glycolipids have?

They can act as cell markers or antigens.

What do cell markers allow?

Cell-to-cell recognition.

What are ABO blood group antigens?

Glycolipids and glycoproteins with slightly different carbohydrate chains.

What is the fluid mosaic model?

A model describing membrane structure as a fluid phospholipid bilayer containing a mosaic of components.

Why is the model called a mosaic?

The different components create a scattered pattern.

Why is the model called fluid?

Many phospholipids and proteins can move within the bilayer.

How do phospholipids mainly move?

Sideways within their own layer.

Can membrane proteins move?

Many can move within the bilayer, although some are fixed.

What gives the membrane its mosaic appearance?

The distribution of proteins and other components within the phospholipid bilayer.

What determines the position of membrane proteins?

The structure of the proteins.

What type of molecules can pass between phospholipids?

Small non-polar molecules.

Why can small non-polar molecules cross the bilayer?

They can fit between phospholipids and interact with hydrophobic tails.

What type of molecules need transport proteins to cross?

Large polar molecules.

What proteins allow large polar molecules to cross?

Channel and carrier proteins.

Who proposed the fluid mosaic model?

Singer and Nicolson.

When was the fluid mosaic model first proposed?

1972.

Why can scientific models of membranes change?

New discoveries and technological advances can provide evidence that previous models cannot explain.

What should a scientific model do?

Represent the structure or process using the available evidence.

What happens if new evidence does not fit a model?

The model may be altered or replaced.

What did the Gorter and Grendel model propose?

That membrane phospholipids formed a bilayer.

What evidence supported the Gorter and Grendel model?

The extracted phospholipids from red blood cell membranes had twice the area of the plasma membrane if arranged as a monolayer.

What was a problem with the Gorter and Grendel model?

It did not explain membrane proteins or movement of lipid-insoluble molecules.

What did the Davson and Danielli model propose?

Proteins were arranged in layers above and below the phospholipid bilayer.

What evidence supported the Davson and Danielli model?

Membranes controlled movement of substances and electron micrographs showed two dark lines with a lighter band between.

What evidence contradicted the Davson and Danielli model?

Freeze-etched electron micrographs showed globular structures scattered throughout the membrane.

What did improved protein analysis show?

Membrane proteins were globular, varied in size and had hydrophobic regions.

What did Singer and Nicolson propose?

A fluid membrane containing both peripheral and integral globular proteins.

What evidence supported the fluid mosaic model?

Freeze-etched micrographs showed proteins extending into the membrane centre.

What biochemical evidence supported the fluid mosaic model?

Membrane proteins were shown to be free to move within the bilayer.

What is important to understand about membrane models?

They are interpretations of data and can change when new evidence becomes available.

What should you call the membrane surrounding the cell?

Cell surface membrane or plasma membrane.

What factors can affect membrane permeability?

Temperature and pH.

What organism/tissue can be used to investigate membrane permeability?

Beetroot tissue.

What pigment is found in beetroot cells?

A dark purple-red pigment called betalain.

What happens when beetroot membrane permeability increases?

More pigment leaks out of the cells.

How can the amount of beetroot pigment in solution be measured?

Using a colorimeter.

What does a colorimeter measure?

How much light is absorbed by or transmitted through a coloured liquid.

What does greater absorbance indicate?

A darker coloured solution and therefore more pigment.

What does lower transmission indicate?

A darker coloured solution and therefore more pigment.

Why is a colour filter used in a colorimeter?

To select the correct wavelength for measuring the specific pigment.

What pigment is measured in the beetroot practical?

Betalain.

What must a colorimeter be before measurements?

Zeroed/calibrated.

What is used to calibrate the colorimeter?

Distilled water.

What is a cuvette?

A small container holding liquid for measurement in a colorimeter.

What is the independent variable when investigating temperature?

Temperature.

What is the dependent variable?

The amount of pigment released, measured by absorbance.

Why must beetroot pieces be the same size?

To ensure equal surface area and volume.