Cell Membrane & Endomembrane System Flashcards
Functions of the Cell Membrane
Transport processes
Protective barrier/boundary
Cell-cell communication
Cellular signalling
Signal conduction
Cell to cell recognition
Cell shape
Protective barrier/boundary
- Membranes define boundaries of cells and the boundaries of compartments (organelles).
Cell-cell communication
- Specific plasma membrane gap junction proteins (desmosomes, tight junctions) of adjacent cells lines up and form pipelines between the two cells.
Transport processes
- Membranes control what information is received from other cells or external environment.
- Membranes control the import and export of signals from other cells or the external environment.
- Membranes are flexible and have a capacity for movement and expansion.
Cellular signalling
- Membrane proteins serve as receptors that recognise and bind specific molecules in the extracellular environment. This triggers a series of molecular events in the cell which can lead to a cellular response e.g. hormones.
Signal conduction
- Membranes have electrical properties which can serve as a mechanism of signal conduction when a cell receives an electrical, chemical or mechanical stimulus, e.g. Neurons and muscle cells.
Cell to cell recognition
- Some glycoproteins in the plasma membrane act as identification tags that are specifically recognised by membrane proteins to other cells.
Cell shape
- Cytoskeletal may be noncovalently bound to membrane proteins, a function that helps maintain cell shape and stabilises the location of certain membrane proteins.
Fluid-Mosaic Model- FLUID
individual phospholipids and proteins can move side- to-side within the layer, like it’s a liquid.
Fluid-Mosaic Model- MOSAIC
the pattern produced by the scattered protein molecules when the membrane is viewed from above.
Lipid bilayer
2 layers of phospholipids.
Hydrophilic
molecules do not pass through easily.
Hydrophobic
molecules pass through easily.
The semi-permeable membrane
- Substances that are soluble in lipids pass through easily.
- Small molecules and larger hydrophobic molecules move through the membrane easily, e.g. O2, CO2, H2O.
- Ions, hydrophilic molecules larger than water, and large molecules such as proteins do not move through the membrane on their own.
Structural components- LIPIDS
- Phospholipids (75%)
- Cholesterol (20%)
- Glycolipids (5%)
Structural components- PROTEINS
- Integral (involved in cell transport)
- Peripheral
Phospholipids
- Individual units that make up the membrane.
- Phospholipid has a head and a tail:
* Head = hydrophilic phosphate
group.
* Tail = 2 hydrophobic strings of
carbon and hydrogen atoms. - Watery fluids are found both inside a cell (intracellular fluid) and outside a cell (extracellular fluid).
- The hydrophobic tails of membrane phospholipids are organised in a manner that keeps them away from water.
Phospholipids are ‘amphiphatic’- Exhibit both hydrophilic and hydrophobic properties.
CELL HOMEOSTASIS
The membrane maintains the internal equilibrium of the cell by controlling entry to and exit from the cell.
Movement of phospholipids
- Phospholipids can move sideways/laterally (approx 10 to the power of 7 times per second)
- Phospholipids can flip flop across membrane (approx once a month)
Cholesterol
- Cholesterol is an amphiphatic molecule with a polar head which attaches to the polar part of phospholipids and its function is to stiffen the plasma membrane.
- Cholesterol molecules are made up of four rings of hydrogen and carbon atoms.
- They strengthen the membrane by preventing some small molecules from crossing it.
- Cholesterol molecules also prevent the phospholipid tails from coming into contact and solidifying.
- This ensures that the cell membrane stays fluid and flexible.
Glycolipids
- Glycolipids are molecules which have a fatty acid tail and a carbohydrate head (chains of sugars) and their function is in cell to cell recognition and communication.
- Short carbohydrate chain covalently attached and this is exposed on the outer surface of the cell.
- They provide stability for the cell and help cells join to other cells to form tissues.
- Glycolipids play in humans is their contribution to blood type.
- There are four main blood types: A, B, AB and O, and this variation stems from the different glycolipids present on the surface of red blood cells (erythrocytes).
Integral proteins
- These proteins have one or more hydrophobic regions with an affinity for the hydrophobic interior of the lipid bilayer.
- These proteins are embedded in the membrane and cannot be easily removed.
- Generally transport proteins:
* Channel proteins
* Carrier proteins
Peripheral proteins
- These proteins lack hydrophobic sequences and therefore do not penetrate the lipid bilayer.
- Instead they associate with the membrane surfaces and through weak electrostatic forces.
- They bind either to the polar heads of the membrane lipids or the hydrophilic portions of the integral proteins that extend out of the membrane.
2 Classes of transport proteins
- Carrier Proteins: bind solute on one side of membrane, deliver it to other side by conformational change in protein (turnstile).
- Channel Proteins (Ion Channels): form hydrophilic pores in membrane through which solutes (mainly ions) can diffuse (trapdoor).
Cell membrane transport
Transport across the lipid bilayer of the cell membrane can be divided into two distinct types depending on the energy requirement of the process:
(a) Passive transport
(b) Active transport
Passive Transport
DOES NOT require energy. It occurs
because of the tendency for dissolved molecules to move or diffuse from higher to lower concentrations.
* Passive transport can be divided into two types:
* Simple diffusion
* Facilitated diffusion
Simple Diffusion
whereby molecules move from a region of higher concentration to a region of lower concentration.
Facilitated Diffusion
whereby molecules depend on specialised membrane proteins to aid the passage of ions or polar molecules across the hydrophobic interior of the membrane.
Active Transport
DOES require the input of energy.
* Active transport moves solutes against their concentration gradient which require work.
* Therefore this type of transport requires special membrane proteins that are often referred to as pumps. Because of the energy input, active transport can result in the generation of a concentration gradient across the membrane.
* Examples: Sodium-potassium pump which exchange sodium
(Na+) out of the cell for potassium (K+) inside the cell.
Active Coupled transport
ATP-powered pump that transports a specific solute can indirectly drive the active transport of several other solutes across the membrane.
Two types of co-transport
- Symport coupled transport- molecules move in the same direction across the membrane.
E.g. Glucose and sodium from outside to inside the cell. - Antiport coupled transport– molecules move in opposite directions across the membrane.
E.g. Sodium potassium pump to pump sodium outside the cell and pump potassium into the cell.
ENDOMEMBRANE SYSTEM
The organelles that are “connected” (tangibly or via transport) together by phospholipid membranes.
Endomembrane System- Function
- Membranes provide compartmentalisation, which allows different places in the cell to have different environments.
- With this arrangement, the cell can “build” in some areas, “break down” in others, and reserve other areas for storage or other functions.
- Nuclear membranes (envelope), the endoplasmic reticulum, the Golgi apparatus, lysosomes, vesicles, endosomes and the cell membrane.
Rough Endoplasmic Reticulum
synthesizes proteins and packages them in vesicles commonly go to Golgi apparatus
Smooth Endoplasmic Reticulum
synthesizes lipids and also performs various other functions
Transport Vesicles
shuttle proteins and lipids to various locations such as the Golgi apparatus
Golgi Apparatus
modifies proteins and lipids from the ER; sorts them and packages them in vesicles
Lysosome
contains digestive enzymes that breakdown worn-out cell parts or substances entering the cell at the plasma membrane
Secretory Vesicles
fuse with the plasma membrane as secretion occurs
Incoming Vesicles
brings substances into the cell that are digested when vesicle fuses with a lysosome.
