Membrane Flashcards
What is a cell membrane?
- Double layer of lipids and proteins that surrounds a cell
Functions of the cell membrane:
- Separates cytoplasm from the external environment
- Controls the movement of substances in and out of cells and organelles
- Involved in a variety of cellular processes: cell adhesion
What is the phospholipid bilayer and its function?
- Thin Polar membrane made of two layers of phospholipid molecules
- Held together by strong hydrophobic interactions
Function: - Act as a barrier that keeps ions and other large molecules where they are needed and prevents them from diffusing into areas where they should not be
- Selectively allows some small molecules (non-polar) to pass through into and out of the cell
What three factors affect fluidity?
- Temperature
- Presence of Cholesterol
- Phospholipids (length and saturation)
Describe how temperature affects fluidity of phospholipid bilayer?
- Lipids acquire thermal energy when heated
- Energetic lipids move around more arranging and rearranging randomly making the membrane more fluid
Tm of a membrane:
melting temperature
- temperature across which the membrane transitions from a crystal-like to a fluid-like organisation
- Increased temperature –> increased fluidity
Describe how temperature affects cholesterol?
- Role is bi-directional
- At high temperatures: Holds membrane together using both its hydrophobic and hydrophilic ends raising Tm and keeping it from melting
- At low temperatures: Fill in between phospholipids disrupting their intermolecular interactions preventing membrane from freezing
Due to Cholesterol: Increased temperature = ____ and decreased temperature = ____
decreased fluidity; increased fluidity
Describe how phospholipid length affects fluidity:
- Longer phospholipids have a higher melting point
- Increased length = Increased strength of interaction and decreased fluidity
Describe how phospholipid saturation affects fluidity:
- Saturated phospholipids have no double bonds and have straight unkinked tails
- Unsaturated phospholipids have double bonds and have kinked tails
Unsaturated phospholipids
- Reduce ability of the phospholipids to pack tightly
- Reduces the strength of hydrophobic interactions
Increased saturation of phospholipids
= Increased strength of interaction/packing ability and decreased fluidity
What are membrane proteins?
Proteins which interact with or apart of biological membranes
What are the functions of membrane proteins? (6)
- Transport
- Enzymatic activity
- Signal transduction
- Cell-cell recognition
- Inter-cellular joining
- Attachment to cytoskeleton and extracellular matrix
What are integral proteins and the four types?
- Type of membrane protein that is permanently attached to the biological membrane.
1. Transporter
2. Receptor
3. Enzyme
4. Anchor
Transporter protein function:
involved in the movement of ions
Receptor protein function:
- bind to external molecules and perform signal transduction
- Extracellular signal is converted to intercellular signal
Enzyme protein function:
- enzymatic activity
- Potential for the binding of an extracellular ligand to cause enzymatic activity on intracellular side
Anchor protein function:
- physically link intracellular structures with extracellular structures
What are lipid anchored proteins?
- Proteins covalently attached to lipids embedded within the cell membrane either side of the cell membrane
- Lipid serves to anchor protein to the cell membrane
What are the functions of lipid anchored proteins?
- Lipidation is the covalent attachment of lipids to proteins
- Allows for interaction of proteins with cellular membranes and protein domains.
- Lipidation can “hide” an enzyme away from
its substrate to inactivate it and activate it by substrate presentation
What are peripheral proteins and their purpose?
- Interact with the surface of the cell membranes
- messengers
- support
What are the functions of peripheral proteins?
- Support: direct and maintain the cytoskeleton and ECM
- Communication: messengers to pass information from the ECM to organelles within cytoplasm
- Interact with the surface of the membrane and can attach and detach
What are membrane carbohydrates and what are their types?
- Carbohydrates found on the outside surface of cells
- Glycoprotiens if linked to extracellular proteins
- Glycolipids (Phospholipid molecules )
- Some are part of proteoglycans and insert their amino acid chain directly among the lipid fatty acids.
Function of Membrane Carbohydrates:
- Cell adhesion
- Cell recognition
- Structural role as physical barrier
Describe glycoproteins:
- Highly specific sites for recognition and high-affinity binding by other proteins
- Do not have serial repeats so they are rich in information
Describe glycolipids:
- consist of a carbohydrate that is linked to lipid molecule by a covalent bond
What are proteoglycans?
- Compound consisting of a protein bonded to glycosaminoglycan groups - Occur in connective tissue
What is simple diffusion?
- Net passive movement of molecules or particles from regions of higher to lower concentration across a concentration gradient
- Spontaneous process requiring no input of energy; passive diffusion
What is facilitated diffusion?
- Where substances cross the cell with help of proteins such as channel proteins and carrier proteins
What proteins are involved in facilitated diffusion?
- Channel proteins: less selective and usually mildly discriminate between cargo based on size and charge
- Carrier: more selective
What is osmosis?
Movement of water from a region of higher water potential to region of low water potential.
What are the three types of osmotic solutions?
- Hypotonic solution: high water potential = water will move into cell
- Hypertonic solution: low water potential = water will move out of cell
- Isotonic: equal water potential in and out of cell
What is the charge of the extracellular space and cytosol side of the cell membrane?
- The extracellular space is positively charged
- The cytosol is negatively charged
What is active transport?
- When a substance must move into or out of the cell against its concentration gradient so the cell must use energy to move the substance
What is an example of active transport?
Na+/K+ transmembrane protein pump:
- Concentration of sodium is greater in blood then in the cell and concentration of potassium ion is greater inside the cell
- A low sodium concentration inside the cell is maintained by the sodium-potassium pump
- Purpose: to transport sodium and potassium ions across the cell in the ratio of 3:2
- In each pump 3 sodium ions move out for every 2 potassium ions brought in which creates an electrochemical potential
What are the two types of bulk transport and are they active or passive?
Active: transports large molecules or large amounts
1. Endocytosis: into cell
2. Exocytosis: out of cell
What is endocytosis?
- A cellular process in which large molecule and substances are brought into the cell
- the material is internalised and surrounded by an area of cell membrane which then buds off inside the cell to form a vesicle containing the ingested material
What are the three types of endocytosis?
- Pinocytosis (Cell drinking)
- Phagocytosis (Cell eating)
- Receptor-mediated
What is pinocytosis?
- Main function to absorb extracellular fluids
- Importance: uptake of nutrients along with the removal of waste products and signal transduction
What are the steps in pinocytosis?
- Molecules in extracellular fluid bind to cell membrane
- Triggers cell membrane to create a fold around the fluid containing the molecules to be ingested
- Cell membrane folds back on itself and creates a pouch
- Pouch is then pinched off at the cell membrane and can migrate into the cytosol of the cell
What is phagocytosis?
- In multicellular organisms phagocytosis is a defensive reaction against infection and invasion of the body by foreign substances (antigens).
What are the steps in phagocytosis?
- Plasma membrane entraps the food molecule
- A vacuole forms within the cell to contain the food particle
- Lysosomes fuse with the food vacuole
- Enzymes of the lysosomes digest the food particle
What is receptor mediated endocytosis and how does it occur?
- cells uptake molecules by binding them to specific receptor proteins on their surface, which then triggers the cell membrane to fold inward, forming a vesicle containing the bound molecules
What is exocytosis?
- a natural process of transporting molecules from within a cell to the outside space
How does exocytosis occur?
- Vesicles containing the fluid enclosed by a lipid bilayer fuse with the plasma membrane to release their contents outside the cell
What are the three main pathways in exocytosis?
- Constitutive Secretory Pathway
- Regulated Secretory Pathway
- Lysosome Secretory Pathway
What is metabolism?
- a set of chemical reactions carried out to maintain the living state of the cells in an organism
What are the two main types of metabolic pathways?
- Anabolic: pathways synthesise molecules and require energy.
- Catabolic: break down molecules and produce energy
Why do enzymes assist in metabolic pathways?
Enzymes help facilitate in metabolic chemical reactions since almost all metabolic reactions take place non-spontaneously
Describe in terms of oxidation and reduction whether you gain or lose hydrogen oxygen and electrons:
Oxidation:
- Lose electrons
- Lose hydrogen
- Gain oxygen
Reduction:
- Gain electrons
- Gain hydrogen
- Lose oxygen
What is the first law of thermodynamics?
- conservation of energy
- energy cannot be created nor destroyed but may change from one form to another
- The energy in a closed system remains constant
What is an example of thermodynamics in biological interactions?
- Photosynthesis, where chemical energy is stored in the form of glucose –> Cellular respiration production of ATP.
What is the second law of thermodynamics?
- As energy is transferred or transformed, more and more of it is wasted
- wasted energy goes toward entropy - There is a natural tendency of any isolated system to degenerate into a more disordered state.
What is entropy?
Entropy (S): is a measure of disorder of a system
Describe order in terms of entropy in organisms:
Organisms maintain order (low entropy) by constantly consuming energy and releasing heat, thereby increasing the overall entropy of the environment
What is Gibbs Free Energy and why is it a useful measurement?
- a measure of the amount of usable energy in that system
- The change in Gibbs free energy (ΔG) during a reaction provides useful information about the reaction’s energetics and spontaneity
What are Exergonic and Endergonic reactions and what type of metabolic pathways are they?
- Exergonic reactions are spontaneous (∆𝐺<0) = Catabolic
- Endergonic reactions are NOT spontaneous (∆𝐺>0) = Anabolic
What are enzymes and what do they do?
- Biological catalyst produced by a cell responsible for reducing the activation energy within intracellular or extracellular biochemical reactions
- Enzymes helps break down substrate by speeding up the process and lowering energy required from organism
Describe the two models of enzyme function:
- Lock and Key: Substrate same as activation site
- Induced Fit Model: Active site morphs to match substrate
What are two ways enzymes assist in metabolism?
- Lower activation energy required
- Speed up reactions
What factors affect enzyme activity? (5)
- Substrate concentration
- Enzyme concentration
- Temperature
- pH
- Presence of inhibitors
What is meant by the presence of inhibitors in enzymes and what are the three types?
- A substance that slows down or stops the normal catalytic function of an enzyme by binding to the enzyme
1. Reversible competitive inhibition
2. Reversible non-competitive inhibition
3. Irreversible inhibition
What is competitive and non-competitive inhibition?
- Competitive: Not the same shape but can still bind with the active site and inhibit substrate from binding
- Non-competitve: Binds to different area of enzyme and changes the shape of the enzyme so substrate can’t bind
What is irreversible inhibition?
- Inactivates an enzyme by binding to its active site with a strong covalent bond
- Permanently deactivates the enzyme
- Cannot be reversed
- Irreversible inhibitors do not resemble substrates eg. CO
- Addition of excess substrate doesn’t reverse this process
What are allosteric enzymes and their characteristics?
- Group of regulatory enzymes whose catalytic activities are controlled by noncovalent binding to other molecules call activators or inhibitor
- Molecules involved in allosteric regulation can either increase (stimulate) or decrease (inhibit) enzyme activity.
What are the characteristics of allosteric enzymes?
- Multi-subunit and possess a catalytic and regulatory site
- Regulated by the binding to its regulatory site
- Sigmoid growth curve
Feedback Inhibition
Where the product from any particular step in a metabolic pathway can inhibit enzyme function
Feed-Forward Activation
A product from any step can tell an enzyme to speed up
