2.1.5 - Biological Membranes Flashcards
What are the roles of a cell surface membrane?
- acts as a partially permeable barrier (between cell and environment)
- controls what enters/leaves the cell
- cell communication (cell signalling)
- allow recognition with other cells
- some chemical reactions
- sites of chemical reactions
What are the roles of a membrane within a cell?
- comparementisation (divides cell and keeps cell organelles separate)
- partially permeable barrier (between organelles and cytoplasm and within organelles)
- can form vesicles
- control what enters/leaves organelles
- sites of chemical reactions
What is cell signalling?
communication between cells
-one cell releases a molecule, which attaches to another cell’s receptor so that it is detected by the cell
What is the fluid mosaic model?
a model which shows the arrangement of molecules (eg. phospholipids, proteins, etc) in a cell membrane
What is the role of phospholipids within the fluid mosaic model of a cell membrane?
- acts as a barrier to large polar (dissolved) substances -water soluble substances can’t dissolve through as hydrophobic centre to bilayer don’t let them through
- fat soluble and small molecules can pass straight through
Describe the structure of phospholipids in the fluid mosaic model of a cell membrane
- form a bilayer
- hydrophilic phosphate heads on outside
- hydrophobic fatty acid chains on inside
- about 7nm thick
What is the role of cholesterol within the fluid mosaic model of a cell membrane?
- makes membrane less fluid and more rigid
- reduces permeability to charged particles
- interacts with phospholipids (hydrophobic end interacts with heads + hydrophilic ends interact with tails -pulls them together for stability)
What is the role of glycolipids within the fluid mosaic model of a cell membrane?
- act as antigens (cell markers)
- stabilise membrane by forming hydrogen bonds with surrounding water molecules
What is the role of glycoproteins within the fluid mosaic model of a cell membrane?
- stabilise membrane by forming hydrogen bonds with surrounding water molecules
- act as antigens (cell markers)
- act as receptors (for cell signalling)
What is the role of proteins within the fluid mosaic model of a cell membrane?
-control what enters/leaves cells
different types: channel proteins, carrier proteins, etc
What factors affect membrane structure and
permeability?
- temperature
- solvent
Why is it important that cell membranes are fluid?
- diffusion of substances across membrane
- fusing of membranes
- movement and shape change of cells
How does an increase of temperature affect the permeability of a membrane?
- increases permeability
- increased kinetic energy of phospholipids
- creates gaps between bilayer (disrupts it)
- molecules can pass through gaps
What is the structure and permeability of a membrane like below 0°C?
- phospholipids don’t have much energy so can’t move much (are packed closely together) so membrane is rigid
- channel and carrier proteins in membrane deform so permeability increases
What is the structure and permeability of a membrane like around 0-45°C?
- phospholipids can move and aren’t so packed together
- membrane is partially permeable
- as temp increases, permeability increases (more energy to move so more fluid)
What is the structure and permeability of a membrane like above 45°C?
- phospholipid bilayer begins to melt
- membrane becomes more permeable
- water inside cells puts pressure on membrane
- channel and carrier proteins denature
How do solvents affect the permeability of a membrane?
- solvents increase a membrane’s permeability
- solvents dissolve lipids in membrane so membrane looses it structure
How do molecules move across membranes?
- diffusion
- facilitated diffusion
- active transport
- endocytosis
- exocytosis
What is diffusion?
- net movement of particles down a concentration gradient (from a region of high concentration to a region of low concentration)
- passive transport
What is simple diffusion across a membrane?
small, non-polar molecules in high concentration on one side can easily diffuse directly across the phospholipid bilayer
What is facilitated diffusion across a membrane?
diffusion of larger or polar molecules/ions across a membrane through protein channels or carrier proteins
How do molecules move across a membrane through carrier proteins?
- large molecule attaches to a carrier protein in membrane
- protein changes shape
- molecule is released on opposite side of membrane
How do molecules move across a membrane through channel proteins?
- channel proteins have pores which charged particles diffuse through
- different channel proteins facilitate the diffusion of different charged particles
What is active transport across a membrane?
movement against a concentration gradient (from low to high concentration) using carrier proteins and ATP
How do molecules move across a membrane via active transport?
- molecule/ion binds to receptors in carrier protein’s channel
- on inside of cell, ATP binds to carrier protein and is hydrolysed into ADP and a phosphate
- binding of phosphate to protein causes protein to change shape (opening up way into inside of cell)
- molecule is released into inside of cell
- phosphate molecule is released and rejoins ADP to make ATP
- carrier protein goes back to original shape
What is endocytosis?
the bulk transport of material into a cell
What is exocytosis?
the bulk transport of materials out of a cell
What is osmosis?
movement of water across a partially-permeable membrane down a water potential gradient (from a more dilute solution to a more concentrated one)
What is an isotonic solution?
solution with the same water potential as the cell
What is a hypotonic solution?
solution with higher water potential than cell
What is a hypertonic solution?
solution with lower water potential than cell
What happens to an animal cell in an isotonic solution?
- water potential is equal inside and outside the cell
- water enters and leaves cell at equal rate
- equilibrium
- cell doesn’t change
What happens to an animal cell in a hypotonic solution?
- higher water potential outside cell
- water enters cell
- cell swells and bursts
- lysis
What happens to an animal cell in a hypertonic solution?
- lower water potential outside cell
- water leaves cell
- cell shrinks
- crenation
What happens to a plant cell in an isotonic solution?
- water potential is equal inside and outside the cell
- water enters and leaves cell at equal rate
- equilibrium
- cell doesn’t change
What happens to a plant cell in a hypotonic solution?
- higher water potential outside cell
- water enters cell
- cell swells and becomes turgid (increased turgor pressure against cell walls)
What happens to a plant cell in a hypertonic solution?
- lower water potential outside cell
- water leaves cell
- cell becomes flaccid
- cytoplasm pulls away from cell wall (plasmolysis)
- contents of cell shrink
