2.1.5 Biological membranes Flashcards
What is the fluid mosaic model?
Theory; cell membrane structure with proteins embedded in a sea of phospholipids
What is a glycolipid?
Lipid or a phospholipid with a chain of carbohydrate molecules attached
What is a glycoprotein?
Protein with a chain of carbohydrate molecules attached.
What is a plasma membrane?
Synonym of a cell surface membrane
What is one function of cell membranes?
To allow diffusion of substances between the phospholipid barrier.
Where are cell membranes used in?
- Cell surface membrane between a cell and the cells exterior environment
- Surface membrane between the cell’s cytoplasm and the internal structure of an organelle
What is a cell membrane described as for diffusion? (feature)
Partially permeable
What are the functions of a cell surface membrane? (6)
- Separate cell’s components from external environment
- Regulating transport of materials in and out of the cell
- Contain enzymes involved with specific metabolic pathways (cell membrane is a site of chemical reactions)
- Contain antigens so that the immune system can recognise as “self” structure
- Release chemicals through exocytosis that signals to other cells
- Contain receptors for cell signalling of endocytosis; through hormones and drugs binding
What is NOT a function of the membranes in cells?
Supporting the cell is not a function of membranes
What is the main function of double membranes of organelles?
Compartmentalisation
What structure of the membrane of the mitochondria relates to increasing metabolic reactions internally?
- Double membrane with the inner membrane acting as highly folded cristae
- Therefore gives a large SA:V ratio for chemical reactions (aerobic respiration for example)
What is the main function of membranes of chloroplasts?
Inner membrane of the double membrane compartmentalises chlorophyll within the chloroplast
What is the function of membranes of epithelial cells?
Plasma membrane of epithelial cells lining the small intestine compartmentalises digestive enzymes
What are the functions of membranes in the rough endoplasmic reticulum?
- Compartmentalisation;
- Separates the proteins from the cell cytoplasm
- Holds ribosomes in place
What is the fluid mosaic model of the plasma membrane? State the components.
- Phospholipid bilayer
- Water filled channels
- Cholesterol
- Channel proteins
- Glycocalyx:
- Glycolipids
- Glycoproteins
- Protein receptor
What are the three forms of proteins on the phospholipid bilayer?
- Channel protein
- Carrier protein
- Proteinic enzymes, antigens or receptor sites
What are the function of channel proteins?
Allows ions with electrical charge surrounded by water to pass through
What is the function of carrier proteins?
Change their shape to carry specific molecules across the membrane.
What are the functions of proteinic enzymes, antigens or receptor sites that can be attached to a carrier protein?
Complementary cell signalling and synthesis of products
Function of cholesterol?
- Maintain mechanical stability
- Regulate the fluidity of the membrane
- Resist the effects of temperature change on the structure of the membrane
How are glycolipids and glycoproteins formed?
Formed from glycocalyx
How are glycocalyx’s formed?
Formed from carbohydrate molecules and attach to glycoproteins or glycolipids
What cells (and one structure) have adapted their cell surface membrane?
- Neurones
- White blood cells
- Root hair cells
- Inner membrane of mitochondria
What two ways is the cell-surface membrane of a neurone adapted?
- (a) More protein channels and carriers in the cell surface membrane
- (b) Protein channels cover the long axon and allows passage of ions
- (c) Passage of ions therefore give conduction of electrical impulses
- Myelin sheath contains several layers of cell surface membrane
How is the cell-surface membrane of a white blood cell adapted?
Cell surface membrane of white blood cells contains protein receptors allowing recognition of foreign antigens
How is the cell-surface membrane of a root hair cell adapted?
Multiple carrier proteins present to actively transport nitrate ions from the soil to cells
How is the inner membrane of a mitochondria adapted? (other than SA:V ratio)
think respiration!
Inner membrane contains;
- Multiple electron carriers
- Hydrogen ion channels
What is diffusion? What is this across in a membrane
Movement of molecules from an area of high concentration to an area of low concentration down the concentration gradient
*In cell membranes this is across a partially permeable membrane
What is facilitated diffusion?
Movement of molecules from an area of high concentration to an area of low concentration across a partially permeable membrane VIA assistance from protein channels or protein carriers.
Can substances pass through a membrane without the use of energy?
Yes, but only some substances and only through facilitated diffusion or simple diffusion
Why does facilitated diffusion not require energy?
- Facilitated diffusion is a passive process and therefore particles move down a concentration gradient
- Molecules already have enough of their kinetic energy therefore do not require more energy
What is a passive process?
A process that only uses kinetic energy and no energy from the hydrolysis of ATP.
Do all particles have kinetic energy?
Yes, though in different levels, all particles have kinetic energy.
How does simple diffusion work?
1) As the concentration increases in one area
2) Particles therefore move away towards an area of low concentration
3) They will move away until they become evenly dispersed
What is the summary of simple diffusion in cell membranes?
Particles move down a concentration gradient across a partially permeable membrane from an area of high concentration to an area of low concentration.
After even dispersion of particles, what is the term that is described?
equilibrium, no net diffusion
What molecules can undergo simple diffusion across a cell surface membrane?
- Carbon dioxide
- Oxygen
- Fat-soluble molecules
(If you said non-polar to water molecules that is correct)
Why can oxygen and carbon dioxide diffuse via simple diffusion across a partially permeable membrane down a concentration gradient from a high area of concentration to a low area of concentration via simple diffusion?
- They are small molecules
- Fat-soluble
- Can therefore be dissolved through the phospholipid bilayer through the hydrophobic fatty acid tails
Why can lipids diffuse across a partially permeable membrane, down a concentration gradient from an area of high concentration to a low concentration via simple diffusion?
As the fat soluble lipids can dissolve in the liquid bilayer through the hydrophobic fatty acid tails
Why can progesterone (produced from cholesterol) move across membranes?
- Progesterone is a lipid soluble and hydrophobic
- Progesterone dissolves through the phospholipid bilayer’ fatty acid tails which are also hydrophobic
How can water molecules be transported across a membrane?
Due to high level concentration of water outside of a cell;
- Significant direct diffusion occurs of water towards the cell surface membrane
- Specific water proteins known as aquaporins allows water molecules to diffuse through
How is a concentration gradient maintained as an example in the blood?
- Oxygen diffusing into the cell is used for aerobic respiration after diffusion into the mitochondria
- Carbon dioxide is take out by the blood so constant CO2 is being expelled from the cell via exocytosis
What are the five factors affecting the rate of simple diffusion?
- Temperature
- Diffusion distance
- Surface area
- Size of diffusing molecule
- Concentration gradient
How does temperature contribute to rate of simple diffusion?
Molecules that have more kinetic energy will move more and diffuse quicker
How does diffusion distance contribute to rate of simple diffusion?
A longer distance of molecules moving across the cell surface membrane means a slower diffusion rate
How does surface area contribute to the rate of simple diffusion?
Greater surface area means a greater simple diffusion rate
How does size of the diffusing molecule contribute to the rate of simple diffusion?
Smaller ions or molecules diffuse more rapidly than larger ions/molecules
How does the concentration gradient contribute to the rate of simple diffusion?
Steeper the gradient, the faster the molecules will diffuse to the other side.
Why can small molecules with electrical charge such as ions not diffuse across the phospholipid fatty acid tails in the membrane?
Insoluble in lipids and therefore the phospholipid bilayer as they cannot interact with hydrophobic tails.
How do small molecules with polarity (such as ions) diffuse across a membrane?
Diffuse through protein channels in the membrane.
What manages the permeability of membranes to water-soluble molecules? (such as ions)
Cholesterol molecules
What are channel proteins also known as? Why?
Water-filled channels; as ions that pass through the channels are water soluble and therefore hold water when they diffuse through.
What molecule cannot pass through the channel proteins?
Glucose molecules. Too big!
How do glucose molecules diffuse across the membrane?
Bind to a (transmembrane) carrier protein
How can cells control which type of molecule passes through?
Different cell types have specific transmembranes for each molecule which can be;
- Protein channels
- Carriers proteins
What are three main examples of how transmembrane protein channels are excessively used.
- Neurone plasma membranes
- Synapses (have neurone transmembrane carriers + some more :) )
- Cell surface membrane of epithelial cells
How does neurone plasma membranes excessively use transmembrane protein channels?
Have many channels specific to sodium or potassium ions (depends on the neurone)
How do synapses excessively use transmembrane protein channels additionally with neurone transmembranes?
Additionally
- Calcium ion channels
- Chloride ion channels
How does cell surface membranes of epithelial cells near the stomach excessively use transmembrane protein channels?
Have chloride ion transmembrane channels.
What is water potential?
Measure of the tendency of water molecules to diffuse from one region to another
In aqueous solution what is the solvent?
Water
What protein channels allow lots of water to diffuse through? (not channel proteins for ions)
Aquaporins
Do water molecules have kinetic energy?
Yes
What is osmosis?
Net diffusion of water molecules from an area of high concentration to an area of low concentration (across a partially permeable membrane if in cells)
What happens when solute molecules are added to water?
Water potential decreases.
What will happen if two aqueous solution containing water are separated by a partially permeable membrane?
Water molecules will diffuse from the high water potential to the low water potential area.
When the water potential is the same on both sides of a partially permeable membrane, what happens in terms of movement?
When the water potential is the same on both sides of a partially permeable membrane, what happens in terms of movement?
What units is water potential measured in?
kPa
What water potential does pure water have?
0 kPa
As solutes are added to pure water, what occurs to the water potential?
Water potential decreases becoming negative.
What is the usual water potential inside cells compared to the outside of pure water?
Water potential inside cells is more negative as they are filled with solutes.
If cells are placed in a solution of higher water potential what will occur?
Water will move into the cell down the water potential gradient from an area of a high concentration to a low concentration.
Net osmosis increases.
What happens if too much water enters a cell?
- Swelling of the cell for animals. Turgidity increases in plant cells
- Bursting of the animal cell as the plasma membrane breaks. Cytolysis.
In plant cells what prevents the cell from bursting from a high intake of water?
The rigid and strong cellulose cell wall
What is a swollen plant cell described as?
Turgid
What does turgidity of a plant cell do?
Presses against plant structure
What occurs when a cell is placed in a solution of lower water potential?
1) water will leave the cell via osmosis down a water potential gradient across the partially permeable membrane from a high area of concentration to a low concentration.
- Animal cells will crenate.
- Plant cells will plasmolyse becoming flaccid.
2) In Plant cells, the membrane will separate from the cellulose cell wall
3) In Plant cells, cells that are plasmolysed will be dehydrated and metabolic processes will cease
What is active transport?
Movement of substances against a concentration gradient (low concentration to a high concentration) across a cell membrane
What does active transport require?
- ATP
- Protein carriers
What is endocytosis?
Can be as passive process or an active process of bulk transport of molecules too large to pass through a cell membrane into a cell via
- a channel protein
- carrier proteins
What is exocytosis?
Passive facilitated diffusion or simple diffusion out of the cell or the active process of bulk transport of molecules loo large to pass through a cell membrane out of a cell via;
- channel proteins
- carrier proteins
How is the energy for bulk transport provided?
Hydrolysis of ATP
What are carrier proteins? What is their process?
- Proteins that binds to molecule(s) and change shape to shuttle the molecule(s) across the membrane.
- Have a site for hydrolysis of ATP to release energy to change shape
What is changing shape in a carrier protein also known as?
Conformation
What is bulk transport?
Transport or large molecules and particles in and out of the cell via use of hydrolysed ATP (energy)
Process of endocytosis?
Cell surface membrane surrounds and encloses a particle/vesicle
What are two types of endocytosis?
- phagocytosis
- pinocytosis
What is phagocytosis?
Intake of solid matter via endocytosis
What is Pinocytosis?
Intake of liquid matter via endocytosis
Why is ATP needed for endocytosis?
- To use molecular motor proteins and form the cell surface membrane around the vesicle/molecule
- Bring the vesicle/molecules into the cell
How do vesicles move into the cell?
Molecular motor proteins along cytoskeleton threads pulls the vesicles into the cell interior.
How is exocytosis performed in cells?
Vesicle containing the particles fuse with the cell surface membrane
Why is ATP needed in exocytosis of bulk transport?
To fuse the membranes together and to drag the vesicles using motor proteins
What factors affects the permeability of the cell membrane?
- Temperature
- Organic solvents
- pH
Effect of increasing temperature?
- More molecules have more kinetic energy and therefore diffuse faster across the plasma membrane
- This stops when the molecules are evenly dispersed in and out of the cell membrane
What occurs when temperature drops?
- Saturated fatty acids become compress
- Unsaturated fatty acids compress
What happens when unsaturated acids compress?
Their kink causes them to physically push other molecules away
- The kink therefore maintains fluidity.
What determines the membrane’s fluidity at cold temperatures?
Proportion of unsaturated and saturated fatty acids
What does cholesterol do as a buffer?
- Maintain mechanical stability
- Regulate the fluidity of the membrane
- Resist the effects of temperature change on the structure of the membrane
What does cholesterol do to the phospholipids from the effect of a lowered temperature?
Prevents phospholipid molecules from packing too closely together
What occurs when temperatures rise in terms of fluidity?
- Phospholipids acquire more kinetic energy and moves more
What does increased fluidity at an abnormal temperature result in?
- Permeability increases
- Increased membrane fluidity affects the infolding of the cell membrane during endocytosis
- Changes the ability of cells to release cell signalling chemicals through exocytosis
What does the presence of cholesterol do to oppose the increase in fluidity?
Reduces the increased fluidity to some extent.
Are proteins more or less stable than lipids?
Less stable
What occurs to proteins that are part of the phospholipid bilayer when too high of temperatures occur?
- Tertiary structure of the proteins begin to break and unfold becoming denatured (hydrogen bonds break)
- Cytoskeletal threads made of protein become denatured
What happens when both cytoskeletal threads and membrane embedded proteins become denatured?
- Holes will appear in the cell surface membrane
- Plasma membrane will become more permeable, increasing diffusion rate
- Membrane-embedded enzymes will also denature
- Rate of reaction will slow down/stop of specific metabolic pathways
What two things happens when a detergent (solvent) is added to phospholipid bilayer?
- Cell membrane will be damaged as the lipids will become dissolved.
- Micelle will form balls where hydrophilic heads face outwards and hydrophobic tails faces inwards
Two examples of solvents that could damage phospholipids?
- Acetone
- Ethanol
