Cell Membranes and Transport (4) Flashcards
Cell membrane properties
Separates the inside of a cell from the outside environment, making it possible for the inside to be different
About 7nm thick
Allows cell-communication and cell identification
Is a partially permeable membrane that controls the movement of molecules in and out of cells
Consists of a phospholipid bilayer with embedded membrane proteins
What is the cell membrane
A double layer of phospholipids. Forming a structure called phospholipid bilayer
What do the hydrophobic and hydrophilic tails do in the cell membrane
Allow phospholipids to form the bilayer structure of cell surface membranes
Hydrophilic heads are attracted to polar water molecules and point outwards towards the aqueous environment outside and inside the cell - Cytoplasm or Tissue Fluid
Hydrophobic tails are repelled by by polar water molecules and point inwards forming the inner core of the membrane, making it impermeable to hydrophilic substances
What is a monolayer
If phospholipids are spread over the surface of water, they form a single layer called a monolayer
- The polar, hydrophilic heads are in the water
- The non-polar, hydrophobics tails are projecting out of the water
What are micelles
If phospholipids are mixed with water, they form stable ball-like, single layered structures called micelles
Polar hydrophilic heads face outwards into the water
Nonpolar hydrophobic tails point inwards as they are repelled by the water molecules
What is a phospholipid bilayer
Phospholipid bilayer is the basic structure of all cell membranes
If phospholipids are mixed with water, they can also form two-layered structures called bilayers.
The nonpolar hydrophobic tails are protected inside by the polar hydrophilic heads
What shapes can bilayers be
Spherical
Sheets
What is the role of phospholipids in cell membrane
- The phospholipid bilayer acts as a barrier
- The fatty acid tails are nonpolar and hydrophobic, forming the main part of cell membranes and makes it difficult for polar molecules and ions and freely move through membranes.
- This means that useful water-soluble molecules such as sugars, amino acids and ions and small proteins cannot leak out of the cell, and unwanted water-soluble molecules cannot enter the cell - Phospholipid can also act as signaling molecules.
- Some phospholipids have been chemically modified to act as receptor molecules and play roles to cell-to-cell communication and cell identification
- For example, glycolipids are phospholipids with carbohydrate chains attached to them.
What does the cell membrane contain
Phospholipids
Other types of lipid molecules
Different kinds of proteins
What is the fluid mosaic model
The molecular structure of cell membranes
It is called the fluid since the individual lipid and protein molecules can move around freely by diffusion within the bilayer
- The lipids move mainly within their own layers
- Some proteins move within only one layer, some move within both layers and others are fixed in position by structures inside or outside the cell
- The movements of the lipid molecules are rapid while the larger protein molecules move more slowly
It is called mosaic because of the pattern the scattered protein molecules form when the cell membrane is viewed from above
How is the fluidity of a cell membrane determined
- The degree of saturation of the phospholipid fatty acid tails
- The more unsaturated they are, the more fluid the membrane
- The bends in the tails that the C=C bonds create result in more space between the phospholipids and this weakens the hydrophobic interactions that keep them together - The length of the phospholipid fatty acid tails
- The longer they are, the less fluid the membrane
- More hydrophobic interactions that keep the fatty acids together - The amount of cholesterol molecules present
- The more cholesterol, the less fluid the membrane at normal 37 temperatures, cholesterol molecules prevent/reduce the movements of the phospholipids
- At low temperatures, cholesterol maintains fluidity by preventing tight packing of phospholipids. - Temperature
- The lower the temperature, the less fluid the membrane
- Some organisms can vary the balance between saturated and unsaturated fatty acids and the amount of cholesterol in their membranes as the ambient temperature changes to maintain membrane functions even at very low temperatures.
What is cholesterol
An essential component of biological molecules in both animals and plant
Where are cholesterol molecules found and What is the role of cholesterol
Cholesterol molecules fit neatly between the phospholipid molecules with their polar -OH groups located at the membrane surface and their nonpolar C rings pointing towards the core of the membrane
This helps maintaining the structural integrity and fluidity of cell membranes by providing mechanical strength
Cholesterol at low temperature
- Increases fluidity and flexibility of the cell membrane, preventing it from becoming too rigid since the presence of cholesterol molecules prevent close packing of the phospholipid tails
Cholesterol at high temperatures
- The hydrophobic interactions between the phospholipid tails and the cholesterol molecules help to stabilize the cell membrane, preventing it from becoming too fluid and result in the cell breaking up
Where is cholesterol found
Cell surface membranes in animal cells have equal parts cholesterol and phospholipid
Cholesterol is much less common in plant cell membranes
Cell membranes in prokaryotes do not contain any cholesterol, instead they have other compounds serving the same function
What are are intrinsic (integral) proteins
Proteins that are embedded within the cell membrane
Can be found in the inner layer, in the outer layer or, most commonly, spanning the whole membrane. Intrinsic proteins spanning the whole membrane are called transmembrane proteins
Have hydrophobic and hydrophilic regions
- The hydrophobic regions consist of hydrophobic amino acids and they attach the intrinsic proteins to the phospholipid fatty acid tails
- The hydrophilic regions consist of hydrophilic amino acids and they face either towards the aqueous environment inside or outside the cell
- In transmembrane proteins, the hydrophobic regions cross the membrane and they are often made up of one or several alpha-helical chains
- Some transmembrane proteins are hydrophilic pores and their insides are lined with hydrophilic amino acids.
Most of the intrinsic proteins float in the phospholipid layers, although some are fixed to structures inside or outside the cell and do not move
What is the fluid mosaic model (znotes)
Individual phospholipids and some protein molecules move around by diffusion - Fluid
Mosaic - pattern produced by scattered protein molecules when the surface of the membrane is viewed from above
What is the phospholipid bilayer (znotes)
In aqueous environments, the hydrophobic region (fatty acid tails) of the phospholipid molecules face inwards and away from the water, whereas, the hydrophilic region (phosphate heads) face outwards and towards the water
This causes the phospholipid molecules to arrange into a bilayer
This provides the basic structure of membranes
It is selectively permeable
It acts as a barrier to most water soluble substances
What is the fluidity of the phospholipid bilayer (znotes)
- The more unsaturated the tails, the more fluid the membrane as unsaturated fatty acid tails are bent because of the kinks due to double bonds and therefore, fit together more loosely
- The longer the tail the less fluid the membrane
- The higher the temperature, the more the kinetic energy of the phospholipid molecules and the higher the fluidity
What does cholesterol do at different temperatures (znotes)
Regulates the fluidity of membranes
Helps with mechanical stability
Its hydrophobic region prevents molecules from passing through the membrane in the myelin sheath
Low temps - Cholesterol increases the membranes fluidity, preventing it from becoming too rigid
High temps - stabilize the cell when the membrane could otherwise become too fluid
What are the components of the phospholipid bilayer and their functions
Glycolipids and Glycoproteins
Carbohydrate chains
Proteins
Channel proteins
Carrier proteins
Cell surface receptors
Cell surface Antigens
What are the functions of glycoproteins and glycolipids in the phospholipid bilayer
Glycolipids and glycoproteins - Carbohydrates chains that are attached to membrane protein (glycoprotein) and phospholipids (glycolipid) project out into the watery fluids surrounding the cell, where they form hydrogen bonds to stabilize the membrane structure
What is the function of carbohydrate chains
Carbohydrate chains act as receptors
1. Signaling receptors - the receptors recognize messenger molecules like hormones and neurotransmitters. When the messenger molecule binds to the receptor, chemical reactions are triggered inside the cell
2. Endocytosis - this group of receptors binds to molecules to be engulfed by the cell surface membrane
3. Cell adhesion - binding cells to other cells in tissues and organs. Some glycolipids and glycoproteins act as antigens, allowing cell-cell recognition
What is the function of proteins in the phospholipid bilayer
Transport proteins provide hydrophilic channels for ions and polar molecules. Enzymes catalyze the hydrolysis of molecules. Cytoskeletons made of protein filaments help maintain the shape of the cell
What is the function of channel proteins in the phospholipid bilayer
Water-filled pores that allow charged substances usually ions to diffuse through the membrane.
They have a fixed shape and can be gated to control ion exchange
This does not use ATP and is in facilitated diffusion
