2.5 - Cell Membranes Flashcards
Two parts of a phospholipid
Hydrophilic phosphate head
Hydrophobic fatty acid tail
Why is the phosphate head hydrophilic?
A phosphate group is negatively charged
This means that it is polar
So can attract water molecules
What do many phospholipids form?
The phospholipid bilayer
How many layers are there in the phospholipid bilayer?
2 layers
What are individual phospholipids able to do?
What does this do to the membrane?
On their own layer they can move
This makes the membrane more fluid
Where can the phospholipids not move to?
When the hydrophobic tail is going to be in contact with water
Gives the membrane some stability
What feature does the membrane have?
It is selectively permeable
What does selectively permeable mean?
Why?
Only possible for small and non-polar molecules to move through the tails in the bilayer, such as oxygen and carbon dioxide
This allows the membrane to control what goes into and out of the cell
Keeps the membrane functioning properly
Structure of cholesterol
A steroid alcohol - lipid not made from glycerol or fatty acids
Consists of four carbon-based rings
Is a hydrophobic molecule
What does cholesterol being a non polar molecule allow it do?
Sit in the centre of hydrophobic part of the bilayer
They are small and hydrophobic, they can pass through the hydrophobic part of the cell membrane and any other membrane inside the cell
Function of cholesterol
Regulates the fluidity of the membrane, preventing it from becoming too fluid or stiff.
What is the fluid mosaic model?
Theory of cell membrane with proteins embedded in a sea of phospholipids
Glycolipid def
Lipid with a chain of carbohydrates attached
Glycoprotein def
Protein with a chain of carbohydrate molecules attached
Plasma membrane def
Cell surface membrane
What are cell membranes partially permeable to?
Some very small molecules simply diffuse through the cell membrane, in between its structural molecules.
Some substances dissolve in the lipid layer and pass through
Other substances pass through special protein channels or are
carried by carrier proteins.
What two parts of a cell do cell membranes function?
Surface of cells(plasma membrane)
Within cells
Roles of membrane on surface of cells
Separate cell components from external environment
Regulate transport of materials into and out of the cell
Contain enzymes involved in specific metabolic pathways
Has antigens so immune system won’t attack it
May release chemicals that signal to other cells
Contain receptors for chemical signal - site for cell communication or signalling
May be the site of chemical reactions
3 cells that have membranes within them
Mitochondria
Chloroplasts
Epithelial cells in small intestine
Membranes within mitochondria
Contain folded inner membranes called cristae - give larger SA for some reactions for aerobic respiration
Localise enough area needed for respiration to occur
Membranes within chloroplasts
Inner membranes called thylakoids membranes house chlorophyll
On the membranes some reactions of photosynthesis occur
Membranes within epithelial cells lining small intestine
Contain digestive enzymes
Catalyse final stages in breakdown of some sugars
What makes up the fluid mosaic model?
Mosaic - contain a phospholipid bilayer(double layer) with proteins floating in it
Fluid - lipid molecules can change places with each other, some proteins can move, giving it fluidity
What is the phospholipid bilayer made up of?
Two layers of phospholipid molecules
Hydrophilic (phosphate) heads - in contact with watery exterior(cytoplasm).
Hydrophobic (fatty acid) tails - in centre of membrane ways from water.
Channel protein function
Have pores and act as channels to allow ions to pass through the membrane
Carrier protein function
Able to change their shape to carry specific molecules across the membrane
Function of other proteins
May be attached to carrier proteins and function as:
Enzymes
Antigens
Receptors - for complementary-shaped signalling chemicals such as hormones
Function of cholesterol in fluid mosaic model
Helps regulate fluidity of the membrane
Helps maintain mechanical stability
Helps resist the effect of temperature changes on the structure of the membrane
What is the thickness of the cell membrane?
5 - 10 nanometers
Def of glycoalyx
Formed from carb chains attached to either proteins(glycoproteins) or lipids(glycolipids) outside of membrane
5 methods of movement across membranes
Simple diffusion Facilitated diffusion Osmosis Active Transport Co-transport
Function of peripheral proteins
Provide mechanical support
Connect to proteins to make glycoproteins and glycolipids
Cell recognition or as cell receptors
Function of integral proteins
Carrier proteins or channel proteins are involved in the transport of molecule across the membrane
Channel protein process
These form tubes that fill with water to enable water-soluble ions to diffuse
Process with carrier proteins
They bind to water-soluble ions and larger molecules, such as glucose and amino acids
They change shape to transport them to the other side of the membrane
Diffusion def
Passive movement of small, non-polar lipid soluble molecules such as CO2 and 02 from an area of high conc. to an area of low conc.
The molecules move directly through the phospholipid bilayer
Def of osmosis
The movement of water molecules from an area of high water pot. to an area of low water pot. through a partially permeable membrane
What is exocytosis?
Give an example
The process of moving materials from within a cell to the exterior of the cell
This process requires energy so is a type of active transport
Releasing a neurotransmitter for cellular communication
Effect of low temps(below 0) on structure and permeability of membrane
Phospholipids don’t have much energy, so can’t move very much, decreases fluidity
Phospholipids packed closely together so membrane is more rigid
Channel proteins and carrier proteins in membrane denature at very low temperatures - permeability increases
Ice crystals may form and pierce the membrane, making it highly permeable(then it thaws)
Effect of temps between 0 and 45C on membrane structure and permeability
Phospholipids can move around and aren’t packed as tightly together - the membrane is partially permeable
Increasing the temp. gives the molecules more K.E. - they move faster, so membrane becomes more ‘leaky’ and permeable
Membrane is more fluid
Effects of high temps(above 45C) on membrane structure and permeability
Phospholipid bilayer starts to melt(break down) and membrane becomes more permeable - and much more fluid
Channel proteins and carrier proteins in membrane denature
This means they can’t control what enters or leaves the cell - this increases permeability of the membrane
What is cell signalling?
Cell signalling is when cells communicate with one another by signals.
To detect these signals, cells must have proteins called ‘receptors’ on their cell surface.
In multicellular organisms, communication often mediated by hormones between cells
What are hormones?
Chemical messengers
Produced in specific tissues
Then they are released - have effects in other part of organism
What is a target cell?
Any cell that has a receptor for a specific hormone
Process of cell signalling
One cell releases a messenger molecule(e.g. hormone)
This molecule travels to another cell(e.g. in blood)
Messenger molecule detected by cell because it binds to a receptor on its cell membrane
What shape must receptor proteins be to bind to messenger molecules?
They must have a ‘complementary’ shape to messenger molecule
Role of glycoproteins
Cell adhesion - bind cells together in a tissue
Act as antigens on surface of cells - cells of immune system have receptors that detect glycoproteins and detect whether they are ‘self’ or ‘non self’
How do drugs work?
They bind to the receptors in cell membranes.
They trigger a response in cell or block receptor and prevent it from working e.g. antihistamines block releases of histamines when cells take damage, and causing inflammation.
What are passive processes?
Don’t require energy
Molecules move down their conc. gradient
E.g. diffusion, osmosis, facilitated diffusion
What are active processes?
Require energy in the form of ATP
Molecules move up or down their conc. gradient
E.g. active transport, bulk transport(vesicles need ATP)
Description of diffusion
Passive transport
Net movement of molecules from region of high conc. to low conc.
What molecules can pass through the membrane by diffusion?
Lipid-based molecules - e.g. steroid molecules
Small molecules - e.g. CO2 or 02 - small enough to pass between phospholipid molecules
What is facilitated diffusion?
The net movement of molecules and ions from a region of high conc. to a region of low conc.
Assisted by channel or carrier proteins(down conc. gradient)
What molecules can pass through phospholipid bilayer by facilitated diffusion?
- Charged particles/ions - channel proteins allow ions or charged particles through membrane, e.g. sodium ions.
The channel proteins form pores in membrane often shaped to allow only one type of ion through, and move down diffusion gradient - Larger molecules can pass through carrier proteins
E.g. glucose - carrier proteins are shaped so that a specific molecule can fit into them
Protein then changes shape to allow molecule to move either side and then moves down conc. gradient
How do proteins get pumped out of the cell(exocytosis)?
Molecule attaches to carrier protein, protein changes shape and this moves molecule across membrane releasing it to other side
Similar to facilitated diffusion, but ATP use to move substance against conc. gradient
Molecules can only move in one direction as during transport shape changes of carrier protein using ATP
So molecule cannot re-enter carrier protein as shapes no longer complementary
Process of bulk transport
It is possible because membranes can easily fuse, separate and ‘pinch off’
Energy needed in form of ATP - used to move the membranes around to form the vesicles that are needed and to move vesicles around cell.
Process of exocytosis(bulk transport)
- Vesicle pinch off from Golgi Apparatus
- Vesicle moves to plasma membrane
- Vesicle fuses with plasma membrane and contents of vesicle is released
Process of endocytosis
- A cell surrounds a substance with a section of its plasma membrane - engulfs it
- The membrane pinches off to form a vesicle inside cell containing the ingested substance
What is exocytosis?
Moving large quantities of material out of the cell(e.g. hormones)
What is endocytosis?
Moving large quantities of material into the cell(e.g. WBCs)
Osmosis in terms of water pot.
The movement of water molecules by diffusion from a region of high high water pot. to a region of low water pot across a partially permeable membrane
Water pot is a measure of the conc. of water molecules that are ‘free’ to diffuse
Effects of hypotonic solution on plant and animal cells
Animal - cell lyses(bursts)
Plant - cell stays turgid(normal)
Effect of isotonic solution on animal and plant cells
Animal - cell stays normal
Plant - cell becomes flaccid
Effect of hypertonic solution on animal and plant cells
Animal - cell becomes shrivelled, shrinks
Plant - cell becomes plasmolyzed
Effect of pure water moving into animal and plant cell by osmosis
Animal cell - becomes haemolysed(bursts open)
Plant cell - cell wall prevents bursting. Membrane pushed against cell wall and cell becomes turgid
Membranes are a fundamental part of the cell. They are found both at the surface of a cell and inside a cell.
(a) State three roles of membranes inside cells.
- make compartments within cell organelles - e.g. vesicles
- isolation of organelle contents from substances or metabolic pathways outside the cell, e.g. hydrolytic enzymes in lysosomes
- site of attachment for enzymes on organelle membranes, e.g. ribosomes and tRNA
- provide selective permeability
- creation of concentration gradients
