B2.1 - Membrane + Membrane Transport

  
1. Define the term Amphipathic and give an example of an amphipathic molecule 

-An amphipathic molecule contains both hydrophilic and hydrophobic properties
-An example of an amphipathic molecule is a phospholipid, which contains a hydrophobic, phosphate head and a hydrophobic, lipid tail.  

2. Describe the role of lipids in forming membrane structures 

• Phospholipid molecules organise themselves, so that hydrophobic tails are shielded from water.  

• Hydrophobic tails point inwards, whereas hydrophilic heads point outward 

• This results in the formation of phospholipid bilayers, which creates the structure of the membrane 

• Glycolipids and the glycocalyx aid in cell-to-cell adhesion and recognition  

• Membrane structural integrity  

• Lipids ensure membrane permeability 

• Lipids (cholesterol) control membrane fluidity

3. How do lipid bilayers act as barriers?

• Separate intra- and extracellular environments  

• Control movement of hydrophilic molecules  

• Semi-permeable  

• Aid in creation of transmembrane proteins channels  - selective permeability is possible

4. What are the factors that can affect rate of diffusion across membranes? 

• The concentration gradient  

• Temperature 

• Higher the temperate, faster the rate of diffusion 

• Surface area 

• Greater surface area, faster the rate of diffusion 

• Substrate concentration gradient  

• Presence of moisture

5. Distinguish between integral and peripheral proteins. 

Integral:

Embedded in one or both of the lipid layers of the membrane

Hydrophobic on at least part of the surface

Transmembrane proteins which extend across the membrane

Hydrophilic parts project through regions of the phosphate head on each side

Extraction results in disruption of the membrane

Peripheral:

Attached to a surface of the bilayer

Hydrophilic on the surface

Non-transmembrane protein

No regions are projected to the phosphate head

Easily extracted from the membrane

6. What is the role of Aquaporins? 

• Aquaporins make up the protein channels in osmosis 

• Increases water permeability across membrane

• They are transmembrane integral proteins with a pore through which water molecules can pass in either direction.  

• The properties of the pore prevent other particles (e.g., protons) from passing  

• Aquaporins are integral proteins which are bidirectional and permit rapid movement of water in and out the cell, through the formation of hydrophilic channels that span the membrane.  

7. How are water molecules transported across membranes? 

Water molecules are transported by the process of osmosis, the movement of an area from high to low water potential. It uses protein channel, aquaporins, to transport molecules through a pore which is bidirectional.   

8. Why does the transport of water molecules require protein channels? 

Because it is polar  

9. What is the role of channel proteins on membrane surfaces? 

• Helps molecules be moved across and against concentration gradient 

• Selective transport of ions which can normally not pass through  

• Help transport substances AGAINST concentration gradient

10. Define facilitated diffusion. (MENTION: concentration gradient, ATP, protein channel, example)  

Facilitated diffusion is the movement from high to low concentration. It requires channel proteins to facilitate the diffusion, which allow molecules to move across the cell membrane. There is no ATP needed due to the channel proteins. An example is made up by voltage-gated channels, e.g., the sodium-gated channel.  

11. What is the difference between pump proteins and channel proteins? 

Pump proteins:

Use energy (ATP)

Active transport

Move particles in one direction only

Move particles against concentration gradient

Channel proteins:

Do not use energy (ATP)

Passive transport

Move molecules in either direction

Move particles through down the concentration gradient

12. What is the difference between semi permeable membrane and selectively permeable membrane? 

Semi permeable membrane	Selectively permeable membrane
Some substances are able to pass through	More control over the membrane
Simple diffusion	Facilitated diffusion, active transport

 

Describe the structure and function of glycoproteins.

• Glycoproteins are proteins attached to a carbohydrate. The attachments aid in the process of cell-to-cell recognition and cell adhesion 

• Conjugated proteins with carbohydrates 

• Component of the plasma membrane, with protein part embedded in the membrane and carbohydrate part projecting outwards 

• Cell signalling – act as receptors  

14. Describe the structure and function of glycolipids. 

• Glycolipids are lipids attached to a carbohydrate. They help with cell-to-cell recognition and cell adhesion 

• Molecules consisting of carbohydrates which are linked to lipids  

• Carbohydrate part usually consists of a single monosaccharide/short chain, whilst the lipid part contains one or two hydrocarbon chains, which fit into the core of the membrane  

• In plasma membrane of all eukaryotic cells, attached carbohydrate projecting outwards into the extracellular environment of the cell  

• Cell signalling – act as receptors 

15. Compare and contrast glycoproteins and glycolipids. 

Glycoproteins	Glycolipids
Aid with cell-to-cell recognition	Aid with cell-to-cell recognition
Aid in cell adhesion	Aid in cell adhesion
Contains carbohydrates	Contains carbohydrates
Contains proteins	Does not contain proteins
Does not contain lipids	Contains lipids
Carbohydrate is extracellular	Carbohydrate is extracellular

16. What is a glycocalyx? 

• A glycocalyx is a structure in the cell membrane which holds tissue together 

• The glycocalyx is composed of glycolipids and glycoproteins. It binds cells together and prevents tissue from falling apart 

17.Draw the fluid mosaic model

Check:
- don't leave space between phosphate heads, lipid tails pointed inwards, two-layers (BILAYER), cholesterol molecules between the lipid tails, peripheral protein, glycoprotein, glycolipid
-write hydrophilic, hydrophobic = amphipathic TOGETHER.  

18. What are the different types of membrane proteins 

• Integral proteins – embedded in the phospholipid bilayer – mostly hydrophobic  

• Peripheral proteins – on outside, attached to surface (proteins or lipids) on one side or the other  

• Transmembrane proteins – openings on the in- and outside – stretch across from one side of the membrane to the other