Module 3: Cells and Membranes Flashcards
Membranes are…
continuous, unbroken sheets, enclosing compartments
- structure allows fusion without losing continuity
Eukaryotic cells internal membranes define organelles…
(5)
- Nuclear envelope
- Mitochondrial membranes
- Chloroplast membranes
- Lysosomal membrane
- Endoplasmic membrane
Membrane Functions
- Compartmentalization (compartments have different contents and activities)
- Provide a selectively permeable barrier (prevent unrestricted exchange of molecules)
- Transporting solutes (exchange of molecules across membrane)
- Scaffold for biochemical activities
- Responding to external signals (signal transduction)
- Intercellular interaction (mediates interaction between cell and neighbors)
- Energy transduction (conversion of one form of energy into another)
Cell membrane is made of the following components…
Lipids
Proteins
Carbohydrates
- composed of two layers of lipids and the major type, phospholipids
Phospholipid arrangement
- can arrange themselves spontaneously
- arrangement depends on size of the polar head groups
- if head is large and bulky with 1 hydrophobic tail that is buried, spheres called MICELLES
- if contain smaller heads and have two hydrophobic tails, form BILAYER with a closed structure and inner space
- if added to water form enclosed bilayers called LIPOSOMES
- membranes are self healing and spontaneously rearrange
Membranes are dynamic because…
they are continually moving, forming and reforming, said to be fluid
Lipids can move…
laterally within the membrane
Following features of lipids affect fluidity…
- length of fatty acid tail, longer = less fluid
- presence of carbon-carbon double bonds, fewer = less fluid
- cholesterol, inserts into lipid bilayer of animal cell membranes, can increase or decrease fluidity depending on temperature
Lipids are found in the membrane in…
lipid rafts
- move in the plane of membrane but don’t flip flop
Several functions of membrane proteins…
- Transporters that move ions and molecules
- Receptors that allow cells to receive signals
- Enzymes that catalyze chemical reactions
- Anchors that attach to other proteins
Two types of Membrane proteins
- Integral: permanently associated with the membrane and can span entire lipid bilayer (transmembrane)
- Peripheral: temporarily associated with lipid bilayer or with integral proteins via non covalent bonds, can be in or outside the cell
Membrane Carbohydrates
Attached from outside of cell
- covalently linked together
– Protein (glycoprotein)
– Lipid (glycolipid)
The membrane is a fluid…
mosaic model
- core of membrane is lipid bilayer
- its a fluid structure that allows molecules to move laterally within the membrane
- contains mixture of different molecules
- dynamic structure, components are mobile and can engage in different interactions
Plasma membrane is…
selectively permeable
Which molecules can move across the bilayer?
- some molecules are more permeable than others, gases and nonpolar molecules can move across the bilayer
- macromolecules are too large to move across
- ## small uncharged polar molecules can move across the bilayer
Movement of substance into the cells…
influx
Movement of substance out of cells…
efflux
when one movement of substance exceeds the other…
net flux
Diffusion
- spontaneous process
- movement of molecules form high conc to low conc
Passive Transport
- occurs across a membrane via diffusion
– no energy required - driven by concentration gradient across the membrane
What are the 2 mechanisms for passive transport?
- Simple diffusion
- small molecules and simple molecules, go directly across membrane - Facilitated diffusion
- need help of a protein to get across
- molecules move down the concentration gradient through a protein transport
What are the two types of membrane transports?
- Channel protein
- changes shape around the molecule and releases what attached to it to the other side
- selective
- some exist in either open or closed conformation
- can change shape form open and usable to closed - Carrier protein
- specific molecules bind to transporter and moves down concentration gradient
- molecule bonds on one side, triggers conformational change, molecule exposed to other surface of membrane, diffuses down membrane
Osmosis
- diffusion of water from high [H2O] to low [H2O]
Why is osmotic pressure important?
- Need to consider tonicity of the solution a cell is surrounded by
- if equal, isotonic
Hypertonic solutions
- if a solution is hypertonic relative to cell (higher [solute] than cell), cell will shrink
- animal cells will shrivel and die (crenation)
- plant cells will pull away from wall (plasmolysis)
Hypotonic solutions
- if a solution is hypotonic relative to a cell (lower [solute] than cell) cell will swell
- animal cells will swell and burst
- if its a red blood cell, hemolysis
- plant cells will push against surrounding wall, i.e., turgor pressure
Active transport
- movement of substances against concentration gradient
- requires energy
- use protein carriers (active transporters)
- substance binds and moves against [gradient]
- active transporters change conformation upon binding to substance, movement is one direction against gradient
- two types: primary and secondary
Primary active transport
Directly uses energy to move substances against gradient
Electrochemical Gradients
- energy stored in the [gradient] that is established through primary active transport can produce electrochemical gradient
- its a gradient that has both an electrical (charge) component and a chemical component
- can drive movement of molecules through secondary active transport
Secondary active transport
- cells use transport proteins to move ions across membrane
– this drives movement of other substances against their [gradient] - ex: cells may actively pump protons across membrane using ATP, the pump creates a proton [gradient], also called chemical gradient
- because protons charged, also electrical gradient
- movement of protons down their [gradient] drives the movement of other molecules against their [gradient] (co-transport)
- uses the energy stored in these gradients to move other substances against their own gradients.
Two types of co-transport
- Antiport
- two solutes move in opposite direction - Symport
- two solutes move in same direction
What do you find in a typical Animal Cell?
- Endoplasmic reticulum
- Golgi apparatus
- Lysosomes
- Mitochondria
- Cytoskeleton
What other things do you find in a Plant Cell
- Chloroplast
- Vacuole
- Cell wall
Cytoplasm
The entire contents of the cell minus the nucleus
Cytosol
Region outside the organelles but inside the plasma membrane
What is surrounded by membranes?
Many internal organelles inside the eukaryotic cell
- either physically connected by membrane “bridges” or transiently by vesicles that form by budding off an organelle
Endomembrane system (EMS)
Interconnected organelles within the cell
- allows specific functions to take place within defined spaces of the cell
- Includes:
– Plasma membrane & Nuclear envelope
– ER & Golgi apparatus
– Lysosomes
– Vesicles that move between the above
Exocytosis
- The process where a vesicle fuses with the cell membrane
- vesicle that’s budded off from the EMS can fuse with the plasma membrane and deliver its contents into the extracellular space
Endocytosis
- Process where material from outside the cell is brought into a vesicle that can fuse with organelles
- all variations of this have:
– cell membrane invaginates, forms a pocket around the target material
– Pocket pinches off inside the cell, results in the material surrounded by a newly created intracellular vesicle formed from the cell membrane
Phagocytosis
- delivery to a lysosome via a phagosome
- phagosome fuses with primary lysosome to form secondary lysosome, activates lysosomal hydrolytic enzymes
- contents digested in a secondary lysosome
What does Phagocytosis provide?
Nutrition - products move into cytoplasm
Defense - kill microorganisms
Done by professional phagocytes in animals - macrophages and neutrophils
Lysosomes contains lysozyme - enzyme that degrades bacterial cell walls
ACIDIC PH IN LYSOSOME KILLS SOME MICROORGANSIMS
Oxidases on inner surface of phagosome produce H2O2 - kills some bacteria
Pinocytosis
- means ‘‘cell drinking’’
- extracellular fluids taken up w/o recognition by plasma membrane surface
- results in smaller vesicles and the vesicle does not need to merge with a lysosome
- any molecules present in enclose fluid enters the cell
Receptor-mediated endocytosis
- cell membrane receptors proteins have specific binding to certain substances
- coated pit is the site where receptors on the extracellular side are concentrated, indented surface
- Clathrin is attached to the cytoplasmic side, dynamic changes bring the vesicle into the cell
- Material delivered to a network of tubules & vesicles, ENDOSOMES
- early endosomes are located near peripheral region of cell
- late endosomes are interior part of the cell
– receive material from early endosomes and Golgi apparatus
What stores the cells DNA?
Nucleus
The nuclear membrane is perforated by protein openings called…
Nuclear pores
- allow molecules to move into and out of the nucleus
- the pores are essential for the nucleus to communicate with the rest of the cell
The outer membrane of the nuclear envelope is physically continuous with the…
Endoplasmic reticulum
- large organelle in most eukaryotic cells
- produces many of the lipids and proteins used inside and outside the cell and is a network of interconnected tubules and flattened sacs, interior space is called the lumen
- highly folded nature allows for large membrane surface area to fit inside cell
- can produce a lot in a small amount of time b/c a lot of SA
RER
- has ribosomes associated with the membrane ad exposed to the cytosol
– ribosomes are sites of PROTEIN SYNTHESIS
– amino acids are assembled into polypeptides - ribosomes may also be free in the cytosol
SER
- lacks ribosomes
- parts of the SER bud off to produce vesicles that are free to move in the cytosol
– the SER is the site of FATTY ACID and PHOSPHOLIPID SYNTHESIS - there is lot of SER in cells that are specialized for lipid production
The outer membrane of the nuclear envelope is physically continuous with …
the ER
Golgi apparatus
- not physically attached to ER but is next stop for vesicles that bud from ER
- Has three functions
1. Further modify proteins and lipids produced in ER
2. Sort proteins and lipids as they move to final destination
3. Add carbohydrates to protiens and lipids
Golgi A. consists of flattened sacs called…
Cisternae
- these are surorunded by small vesicles which transport proteins form ER to Golgi A. and between gogli A. and cell membrane /other organelles
Enzymes within golgi A chemically modify…
protiens and lipids
- each step perfomed in different region
- different sets of enzymes catalyze specific reactions
Lysosomes
SPecialized vesicles dervied from golgi A
- degrade damadged or uneeded macromolecules such as protiens, nucleic acids, lipids, and complex carbohydrates
- gogli A can package these macromolecules into vesilces that fuse with lysosomes
Enzymes found inside a lysosome are…
synthesized in the RER, sorted in the Golgi A then packaged into lysosomes
Lysosomes contain specialized proteins in their…
membranes
- includes a proton pump, keeps internal pH of lysosome around 5
Why is it important to keep the contents of the lysosome separate from the rest of the cell?
B/c the optimal pH of the cell is 7, proteins and organelles within the cytosol would degrade if exposed to a pH of 5 inside lysosomes
Which two membranous organelles are not part of the EMS?
Mitochondria and chloroplast
- both specialized in harnessing energy for the cell
- Semi-autonomous
– grow and multiply independently and contain own circular genomes
Mitochondria
- harness energy from chemical compounds such as sugars and convert it into ATP (ADP means used)
- rod shaped, present in almost all eukaryotic cells
- Have 4 parts:
1. outer membrane - integral proteins, from large, nonselective membrane channels, porins
2. intermembrane space - aqueous compartment between outer and inner membrane
3. Inner membrane - highly folded (convoluted)
- electron transport chain and ATP synthase
4. Matrix - citric acid cycle, ribosomes, DNA
Chloroplasts
- where photosynthesis takes place
- captures suns energy to synthesize simple sugars for use in the plant cell
- surrounded by double mebrane and have internal membrane bound compartment
– thylakoid (pigment that captures light) - thylakoid menbrane contains light collecting molecules called pigments
– chlorophyl is most common, gives green colour in plants
Overall chloroplast structure
Thylakoids- flattened membrane sacs within chloroplasts
Space inside thylakoid - lumen
orderly stacks of thylakoids - grana
space surrounding thylakoid - stroma
Origin of Mitochondria and Chloroplasts
chloroplasts resemble cyanobacteria in organization and biochemistry
- originated as endosymbiotic relationship btwn cyanobacterial cell that has been engulfed by a eukaryotic cell
Mitochondria resemble free living bacteria in organization and biochemistry
- like chloroplasts, mitochondria originated as endosymbiotic bacteria, in this case, proteobacteria
