Cell Membrane Flashcards
Describe phospholipid bilayer and outline its functions.
-Each phospholipid is amphipathic
-In an aqueous environment, they arrange to form a bilayer where the phosphate heads interact with the aqueous medium and the hydrocarbon tails form a hydrophobic core in the interior of the bilayer
Functions:
Major component of cell membranes, it:
1. regulates movement of substances in and out of the cell by acting as a barrier to ions which are charged and polar, and large molecules
2. acts as a buondary between intracellular and extracellular aqueous environment and
3. allows compartmentalisation within a cell
Describe cholestrol and outline its function.
-Found in between phospholipid molecules in membranes of eukaryotes.
-Has a characteristic 4 ring structure
-slightly amphipatic, hydrophobic 4 ring structure + OH group
-Hydroxyl group of cholestrol aligns with charged phosphate heads of phospholipids, rest of it interacts with the hydrophobic core of the membrane
Function:
Regulates membrane fluidity:
-membrane is prevented from being overly-fluid at warmer temperatures as cholestrol restricts phospholipid movement through its interactions with the phospholipids
-Membrane is prevented from being overly firm at lower temperatures, prevents the close packing of phospholipids, prevents solidification
Cholestrol helps to anchor proteins in the membrane
Outline the functions of membranes at the surface of cells and membranes within the cell.
- Membrane allows compartmentalisation which allow
-unique environments to be formed for highly specialised activities
-spatial separation of biochemical processes and thus their sequential operation within a cell
-accumulation ions to high concentrations - Membranes act as a surface for chemical reactions to occur in a sequential manner
-membranes may have functionally-related proteins grouped together so that sequential biochemical processes can occur - Membranes increases surface area for chemical reaction
e.g. inner mitochondrial membrane highly folded, hold more e- transport chains and ATP synthase - Membranes surface topography enable communication between a cell and its surroundings
-the unique combination of proteins/glycoproteins/glycolipids on surface of different cells enable
(a) cell-cell recognition and adhesion so that tissue formation is possible
(b) ligands to recognise specific receptors so that signal transduction can occur
Explain simple diffusion and factors affecting rate.
-Movement of non-polar molecules and small polar water molecules directly through the membrane, down its concentration gradient, without assistance of any transport proteins.
Factors:
- Molecular size: smaller the particle, faster (less drag)
- Solubility in lipid bilayer
-since the membrane is essentially non-polar, non-polar substances move through the membrane faster compared to polar substances - Concentration gradient: steeper gradient, faster rate
- Kinetic energy of molecules:
-higher temperature: higher KE, higher rate of diffusion
-higher KE of phospholipids, membrane becomes highly fluid -> formation of more transient pores - Surface area of cell membrane: higher SA, higher rate
- Distance: higher distance, slower rate
Explain how facilitated diffusion of polar or charged particles occurs
using channel and carrier proteins.
Facilitated diffusion: similar to diffusion BUT transport proteins are needed as ions or polar molecules cannot diffuse through hydrophobic core of plasma membrane
Channel protein:
-transmembrane protein
-provides a hydrophilic pore through which only a particular ion or polar/charged molecule can diffuse readily through from one side of the membrane to the other
-e.g. water channel proteins (aquaporins)
-some channel proteins may be gated. Gated channels can close to prevent access to the hydrophilic pore but will open with arrival of a chemical or electrical stimulus
Carrier protein: usually a transmembrane protein
exists in two alternate conformations:
-the hydrophilic interior of the carrier protein contains a binding site for its solute and is exposed to one side of the membrane
-a conformation change in the protein occurs when the solute binds to its binding site
-solute is now exposed to the other side of the membrane where it is released
Define osmosis.
-The movement of water molecules from a region of higher water potential to a region of lower water potential through a selectively permeable membrane.
-Can be simple diffusion whereby water molecules diffuse directly across the membrane through the transient pores or facilitated diffusion through the aquaporin proteins.
Define active transport.
Active transport is the transport of polar molecules or ions across a membrane against a concentration gradient with the expenditure of ATP.
-Involved specialised carrier proteins called pumps.
-Movement of substances is in one direction only.
Using suitable examples, describe how active transport of polar or
charged particles occur.
- 3 cytoplasmic Na+ binds*** to sodium-potassium pump
- Binding of Na+ stimulates phosphorylation by ATP
- Phosphorylation causes conformational change in protein expelling Na+ outside
- 2 extracellular K+ binds the protein and trigger release of phosphate group
- Protein returns to original conformation releasing K+ inside the cell
- Na+ sites become receptive again
Explain why ATP is needed for bulk transport.
ATP used in bulk transport:
-energy from ATP is used to extend the membranes around the materials to form the vesicles
-to move the vesicles within the cell
-ATP required for invagination or extension of cell membrane
-rearrangement of cytoskeleton
Explain the difference between active transport and bulk transport.
Bulk transport is an active process as it requires ATP. However it is not active transport as it does not transport molecules across a membrane through a transmembrane carrier protein.
Active transport: up/against concentration gradient
Bulk transport: both down and up a concentration gradient
Describe the process of endocytosis, i.e. phagocytosis, pinocytosis
and receptor-mediated endocytosis.
Endocytosis: Infolding or extension of cell surface membrane to form a vesicle or vacuole, allowing cell to acquire macromolecules and particulate matter respectively.
Phagocytosis:
1. filaments in the cytoskeleton are rearranged to help form pseudopodia
2. pseudopodia are the outward extensions of the membrane which wraps around and engulf the particle
3. the ends of the pseudopodia fuse and a vesicle/vacuole containing the solid matter is pinched off and moves into the cytoplasm
Pinocytosis: A small area of the plasma membrane invaginated and liquids are taken into the cell via vesicles (e.g. a human egg cell takes up nutrients from surrounding follicles )
Describe the process of receptor-mediated endocytosis.
-Protein receptors are embedded in membranes that are exposed to the extracellular fluid
-Extracellular substances that bind to the receptors are called ligands
-Invagination of the membrane occurs to form vesicles containing the ligand-receptor complexes which are then transported within the cell/organelle
-Receptor proteins are usually clustered in regions of the membrane called coated pits
Describe the process of exocytosis.
-Secretion of macromolecules (e.g. waste products) by fusion of vesicles with the plasma membrane
-Membrane of the vesicle fuses with the cell surface membrane to release contents of the vesicle to the extracellular environment
Describe what happens in Prophase.
- Chromatin condenses to form chromoseomes. Each chromosome comprises 2 sister chromatids joined at the centromere.
- Centrioles move to opposite poles and start to organise spindle fibres.
- Nucleolus disappears and nuclear envelope disintegrates into vesicles.
Describe what happens in metaphase.
- Chromosomes align at the metaphase plate.
- Each chromosome is attached to 2 kinetochore
microtubules (1 from each pole) at the centromere.
