Chapter 5 Biological Membranes Flashcards
Define Compartmentalisation
The formation of separate membrane bound areas in a cell
What is the difference between the cell surface membrane and plasma membrane?
Plasma Membranes are only around organelles
Cell Surface Membranes are around the whole cell
Both made of a phospholipid bilayer
What makes up the plasma membrane? Why is it called the fluid mosaic model?
Phospholipid bilayer with hydrophilic phosphate heads positioned outwards, and hydrophobic fatty acid tails positonied inwards
Proteins scattered through the bilayer
Both the phospholipids and the proteins can move
How and why do phospholipids arrange themselves when in water?
Phospholipids contain a hydrophilic region, the charged phosphate head, and hydrophobic regions, the fatty acid tails.
When in water, the charged phosphate heads are positioned in the solvent, dissolving and forming hydrogen bonds with water
However, the fatty acid tails are positioned inwards, shielded from the aqueous environment of the cell, as they are hydrophobic and repelled by water
Therefore, a phospholipid bilayer forms
What is the structure and function of cholesterol?
4 hydrophobic carbon rings and a chain, & a hydrophilic hydroxyl group
Positioned between phospholipids to prevent them grouping together too much and crystallising, preventing disruption of membrane fluidity
Hydrophilic heads and hydroxyl interact, & hydrophobic fatty acid tails and 4 carbon rings interact, pulling the membrane together and adding stability
What are intrinsic proteins and how are they held in place in the membrane?
Proteins embedded in the membrane which are positioned through both layers of the phospholipid bilayer
They are held in place by the interactions of the hydrophobic R groups on the surface of the proteins and the hydrophobic membrane core (fatty acid tails)
Name 2 types of membrane protein involved with the transport of substances, and how they do this.
Channel Proteins- provide a hydrophilic channel to allow charged objects to pass through. Held like regular intrinsic proteins
Carrier Proteins- Involved in passive and active transport, including changing shape to allow molecules to enter the cell
What are glycoproteins and their function?
Intrinsic proteins containing a carbohydrates chain
Play a role in cell adhesion and cell signalling, acting as receptors
E.g acetylcholine on nerve cells prevent impulse transmission
What are glycolipids and their function?
Lipids attached with a carbohydrate chain
Acts as antigens/ cell markers to be recognised by immune system
What are extrinsic proteins?
Present on only one side of the phospholipid bilayer
Held in place by hydrophilic R groups on surface of proteins interacting with hydrophilic phosphate heads
Why is it important for proteins to be in the right position?
They can be the site of chemical reactions, with these reactions only occurring if the orientation of the protein is correct
How does temperature affect cell membranes?
When temperature increases, phospholipids have more kinetic energy, move more
Increases fluidity of membranes, increases permeability, cell membrane breaks down
At high temperatures, proteins denature so permeability affected
How do solvents affect membranes? E.g alcohols
Solvents at high concentrations destroy cells by dissolving cell membranes
At lower concentrations, toxins enter cell membrane and disrupt membrane
Define diffusion
The passive net movement of particles from an area of high concentration to low concentration, caused by the random movement of particles
How does diffusion distance affect rate?
At high speeds, particles collide, slowing down overall movement
Short distances diffusion fast as fewer collision
Longer distances, more collision occur
How does temperature and concentration difference affect diffusion?
Temperature - Higher KE, move more and faster, higher rate of diffusion
Concentration Difference- Greater difference= Greater rate, as overall movement larger
What is simple diffusion?
Diffusion in the absence of a barrier or membrane
What substances can pass straight through the cell surface membrane? E.g And why?
Small and uncharged
Cannot pass through the phospholipid bilayer, or repelled by it
E.g O2, Co2
What factors affect diffusion across membranes
Surface Area= Increased, increased points of diffusion
Thickness= Diffusion distance
(Viscosity, size)
What is facilitated diffusion? What extra factor affects it?
Diffusion across a membrane through protein Channels
Affected by number of channel proteins
Define Active Transport
The movement of molecules from an area of low concentration to high concentration requiring energy and carrier proteins
What is the process of active transport?
- Desired molecule binds to carrier protein
- ATP binds to molecule. It is hydrolysed to ADP and P
- Binding of phosphate causes the protein to change shape ( in tertiary structure), allowing the desired molecule to be released into the cell
- Phosphate is released, forming ATP when combining with P, returning the protein to its original shape
What is the process of endocytosis? What are the three types?
Bulk transport of material into the cell
- Material comes into contract with cell surface membrane. Invagination (pinching inwards of cell membrane) occurs
- The membrane folds around the material, until it eventually fuses, forming a vesicle
- The vesicle pinches off, moving into the cytoplasm on the micro tubules using motor proteins and energy
Pinocytosis=Liquids
Phagocytosis=Solids
Receptor Mediated= Certain molecules only
What is the process of exoctyosis?
The bulk movement of materials from inside the cell to outside
1. Vesicles, most likely from Golgi, move to the cell surface membrane via the transport function of the cytoskeleton, along microtubules with motor proteins and ATP
2. The vesicle fuses with the cell surface membrane (effectively rebuilding and rejuvenating the membrane)
3. The material is released out of the cell
What parts of endocytosis and exocytosis require energy?
Movement of vesicles
Fusion of cell membranes
Changing shape of the cell
Define Osmosis
The passive net movement of water molecules from an area of high water potential to low water potential across a semi permeable membrane
Define Water Potential
The pressure exerted by water molecules as they collide with a container, measure in kPa, Symbol = psi
Pure water=0, higher solute concentration means negative values e.g -20kPa
Why does osmosis occur?
Only free water molecules can move
With side of lower water potential, fewer free water molecules so less movement, on high water potential more free water molecules available to move so more movement high to low
Define hydrostatic pressure
Increased pressure caused by the movement of water by osmosis
Define Hypo, Hyper and Iso
Hypo= less than
Iso= Same as
Hyper= More than
But when referring to water potential, think about solute concentration
What happens to an animal cell in hypo, and hyper tonic solutions?
Hypotonic- Water moves into the cell, higher hydrostatic pressure, cell membrane burst, =Cytolysis
Hypertonic- Water moves out of the cell, puckering = crenation
What happens to plants cells in hypo, iso and hypertonic solutions?
Hypo- water moves in, increased hydrostatic pressure pushes membrane against cell walls, =turgid
Isotonic= flaccid
Hypertonic- water moves out, cell surface membrane pulls away from cell wall= Plasmolysis
What are the uses of plasma membranes?
Formation of vesicles
Separates organelles from cytoplasm, compartmentalisation
Site of chemical reactions
What is the purpose of the phospholipid bilayer in the cell surface membrane?
Provides a large barrier which prevents polar or large compounds entering the cell (unless a channel/carrier protein is present)
How does the cell and its membrane help with cell signalling?
Cell releases chemicals via exocytosis
Glyoproteins/glycolipids act as receptors
Receptors specific to certain types of molecules only, complimentary shapes
Triggers cascade of events in the cell in response, therefore being signalled
Why does facilitated diffusion through glut transporters not require metabolic energy?
Relies on the kinetic energy of the particles as they diffuse down their concentration gradient through the channel proteins, rather than relying on ATP to cause conformational changes
What are the roles of membranes?
Compartmentalisation, so organelles can provide a different distinct environment to the cytosol of the cell
Site of chemical reactions e.g oxidative phosphorylation
Provide attachment site for enzymes
Formation of vesicles, transport within the cell
Cell surface membrane
How does increasing concentration affect the rate of active transport and facilitated diffusion?
Increasing concentration initially increases the rate of both, up to a point, limiting factor
All proteins being used/ channels so rate cannot increase further
What do phospholipids look like in detergent?
Vesicle shaped
