4 (plasma membrane and organelles) and 5 (endomembrane system and bulk transport processes) Flashcards
Cell inside and the cell outside
The cell’s inside is alive and the cell’s outside is dead (it is a dead extracellular environment)
What does a membrane provide?
A membrane provides special conditions within the cell as only certain conditions are compatible with life.
What must a cell do?
manufacture cellular materials (molecules such as lipids, carbohydrates, glycoproteins etc.)
Obtain raw materials - these raw materials are all in the non-living space, for us that is the extracellular fluid whereas amoeba have to find their raw materials in the environment that they live in
Remove waste - maybe breakdown of parts of molecules, chemical waste etc which would damage the cell if it stayed there
Generate required energy
Control all of the above/be organised
Therefore the cell’s conditions must allow these things to happen
Why do we need organelles?
In general, a cell must complete many different processes which require different conditions and therefore need seperate compartments which are known as organelles
Organelles
Provide special conditions for specific processes
Keeps incompatible parts apart
Allow specific substances to be concentrated
Form concentration gradients (high concentration inside the organelle perhaps and low concentration on the outside of the organelle and the cell would use this concentration difference to make things happen)
Package substances for transport or export (in vesicles)
Each organelle membrane has to…
Allow for different conditions inside each organelle
Allow specific substances to be concentrated in one area
Key organelles in eukaryotic cells
Endoplasmic reticulum (ER)
Nucleus
Mitochondria
Golgi apparatus
Organelles unique to animal cell
Lyososomes
Centrosomes
Organelles unique to plant cells
Central vacuole
Chloroplast
Cell wall
What are cells bounded by?
Plasma membrane
Many cellular organelles are also bounded by…
Membranes so that each organelle can provide its own special conditions as their internal environment is different to the cells. They also have to control what is going in and out of that compartment and therefore they must have their own membranes.
All cellular membrane are composed of a …
Phospholipid bilayer
Phospholipid bilayer
The phospholipid bilayer consists of two layers of phospholipids, with a hydrophobic, or water-hating, interior and a hydrophilic, or water-loving, exterior. The hydrophilic (polar) head group and hydrophobic tails (fatty acid chains) are depicted in the single phospholipid molecule.
What is allowed and what is prevented from diffusion through the phospholipid bilayer?
Can diffuse through the membrane (passively) - hydrophobic, uncharged molecules
Restricted by membrane - Hydrophilic, charged molecules (hydrophilic needs facilitated diffusion to create a channel)
Phospholipid
Phospholipids are a class of lipids that are a major component of all cell membranes. They can form lipid bilayers because of their amphiphilic characteristic. The structure of the phospholipid molecule generally consists of two hydrophobic fatty acid “tails” and a hydrophilic “head” consisting of a phosphate group.
Hydrophobic part of phospholipid
Hydrophobic - Having no affinity for water; tending to coalesce (combine) and form droplets in water.
The fatty acid chains are the uncharged, nonpolar tails, which are hydrophobic. Since the tails are hydrophobic, they face the inside, away from the water and meet in the inner region of the membrane.
The composition of fatty acids affects membrane fluidity.
Why does the hydrophobic component face inwards?
These part of the phospholipid bilayer does not like water therefore they form the bilayer structure and face into each other in order to avoid the aqueous environment that nearly makes up everything else. (all blood and extracellular fluids are water based)
Hydrophilic part of the phospholipid
Hydrophilic - Having an affinity for water (easily dissolvable).
The phosphate group is the negatively-charged polar head, which is hydrophilic.Since the heads are hydrophilic, they face outward and are attracted to the intracellular and extracellular fluid.
Cholesterol’s role in the phospholipid bilayer
Cholesterol interacts with the fatty acid tails of phospholipids to moderate the properties of the membrane: Cholesterol functions to immobilise the outer surface of the membrane, reducing fluidity. It makes the membrane less permeable to very small water-soluble molecules that would otherwise freely cross.
Cholesterol stabilises membrane fluidity
Unsaturated phospholipids
Unsaturated tails have double bonds and, as a result, have crooked, kinked tails. As you can see above, saturated fatty acids tails are arranged in a way that maximizes interactions between the tails. These interactions decrease bilayer fluidity.
The unsaturated tails mean that packing is prevents/bends mean that they can’t pack together as closely because saturated tails can pack together closely.
SA:V ratio
SA:V ratio is the limiting factor for cell size. The larger a cell gets the smaller the SA:V ration, so there is not sufficient surface area to allow the required from occurring.
Why are animal cells between 10-100 micrometers in diameter?
A small cells has a greater surface area to volume ratio than a larger cell
This dimension gives them the most surface area to volume ration therefore they can communicate effectively and optimally with their environment
Plasma membrane
A semi-permeable barrier meaning that somethings can cross and other things are prevented from crossing
Controls movement of substances in and out of the cell
This interaction with the environment limits the maximum size of a cell
A small cells has a greater surface area to volume ratio than a larger cell
Passive transport
Passive transport involves the movement of particles with a concentration gradient and does not require any energy from the cell e.g. osmosis, diffusion and facilitated diffusion (requires a channel or carrier)
Diffusion
Definition - Diffusion is the movement of a substance from an area of high concentration to an area of low concentration.
Membranes are permeable to lipid soluble (hydrophobic) molecules as steroid hormones and gases (for example testosterone and oxygen)
Molecule is moving along the concentration gradient (from high to low concentration). Since the molecules move down their concentration gradient there is no energy required for this process.
In contrasts, the membrane restricts movement of water soluble and charged molecules such as glucose, ions and water and therefore these kinds of molecules are not transported via diffusion but by other ways instead
Concentration gradient
A concentration gradient occurs when a solute is more concentrated in one area than another.
Facilitated diffusion
Movement of hydrophilic molecules (e.g. glucose, water and ions) requires membrane proteins called channels and carriers. These aid the movement of specific substances down their concentration gradient
No energy is required but some channels open or close in response to signals therefore they can regulate what comes through
Carriers undergo a shape change to help guide the molecule
Channel vs carrier
A channel is a reasonably unchanged structure, acts like a tunnel where molecules can move down. A carrier operates the same way but there needs to be some sort of molecular shift in the structure of that molecule when something travels through it but no energy is required to make that happen
Each channel protein is specific to an ion. This is the only way ions can travel through the membrane. They are trans membrane proteins. Carrier proteins- these are proteins which allow larger or polar molecules through the membrane.
Osmosis
Osmosis is the movement of a solvent across a semipermeable membrane toward a higher concentration of solute (lower concentration of solvent)
A type of facilitated diffusion
Movement from a high water (low solute) concentration to a low water (high solute) concentration
Cells osmoregulate to prevent swelling or shrinking under varying conditions. Not enough water, the cell with shrink or if there is too much water, the cell could burst. In both of these cases, the cells can die
Movement of water across a cell membrane requires …
Aquaporins - water proteins
Active transport
The movement of molecules across a cell membrane into a region of higher concentration, assisted by enzymes and requiring energy (ATP).
Requires transport proteins, which are carriers (take things in and out) that use energy. The energy is usually supplied by ATP
Move specific substance AGAINST their concentration gradient
Active transport allows a cell to have an internal concentration of a substance that is different from its surroundings. For example, it may be higher inside the cell than it is outside of the cell.
Co-transport
A type of secondary active transport across a biological membrane in which a transport protein couples the movement of an ion (usually Na+ or H+) down its electrochemical gradient to the movement of another ion or molecule against a concentration or electrochemical gradient.
Indirect active transport (needs energy)
One substance pumped across the membrane and its concentration gradient is use to power the movement of a second substance against its concentration gradient
Co-transport example
An example is the Na+/glucose cotransporter, which couples the movement of Na+ into the cell down its electrochemical gradient to the movement of glucose into the cell against its concentration gradient.
Defects in transport proteins are responsible for many diverse diseases including…
albinism
Wilson’s disease
Cystic fibrosis
Roles of membrane proteins
Transporters are membrane proteins but membrane proteins have more roles, often specific to a cell type (could be said to give a cell membrane its character)
signal transduction
Cell recognition
Intercellular joining
Linking cytoskeleton and extracellular matrix
Roles of membrane proteins - signal transduction
these membrane proteins’ job is to detect signals (normally have receptors that react to a signalling molecule)
It is the process of taking a message and relaying it to the cell. It is relaying messages from the body (or environment) into the cell
Roles of membrane proteins - cell recognition
Often involves glycoproteins (proteins with added sugars). Glycoprotein recognition tags on cells.
Allows cells to interact with each other close up
Roles of membrane proteins -intracellular joining
Some proteins form long-lasting connections between cells (tissues for example)
Various cellular junctions
Roles of membrane proteins -linking cytoskeleton and extracellular matrix
Allows a cell to physically connect with protein structures outside the cell (extracellular matrix - proteins around the cell)
Helps with cell shape and stabilises proteins
Glycoproteins vs proteoglycans
Proteoglycans - Proteoglycans are proteins with extensive sugar additions.
Glycoproteins - A protein with one or more covalently attached carbohydrates.
Endomembrane system
System of membranes within the cell. It is a system because they work together to come to the same end point. The collection of membranes inside and surrounding a eukaryotic cell that performs metabolic functions and regulates protein traffic.
Includes: nuclear envelope Endoplasmic reticulum (ER) (rough and smooth) Golgi apparatus Vesicles Lysosome Vacuoles Plasma membrane
Think of these as a factory line with the start being at the top and the finish being at the plasma membrane
This membrane system is interconnected by direct physical contact or transfer by vesicles
Endoplasmic reticulum
An extensive membranous network in eukaryotic cells composed of ribosome-studded (rough) and ribosome-free (smooth) regions.
How do the sER and rER look on an electron micrograph?
The parallel lines on an electron micrograph are the membranes of the ER
On the rER, there are dark dotted structures on the membrane known as ribosomes
The sER appears more tubular and lacks ribosomes
Functions of the sER
metabolism of carbohydrates (various forms of sugars)
Lipid synthesis for membranes (and other places too) (makes them and ultimately arranges them to be transported to where they are needed in the cell)
Detoxificiation of drugs and poisons
Storage of calcium ions (used as a signal in the cell)
Extensive sER cells are active in these processes. The amount of sER can be increased or decreased in order to meet demand. Cells are not fixed and not all cells are the same, it depends on their function.
How does the sER work in pain relief?
When you take a drug (painkiller etc.), the sER has enzymes which it is going to use in order to break down a particular kind of drug
One of the functions of the sER is that it detoxifies drugs and poisons therefore when a drug is given not all of the drug taken gets through into the particular area it is needed due to some of the drug being detoxified
When you increased the drug concentration, the sER can’t detoxify all of the drug that has been taken therefore some gets through and allows for pain relief. In response to this, more sER is formed which creates more enzymes in order to decrease the amount of drugs getting through
The amount of this organelle can change depending on what it needs to do
Functions of the rER
Rough appearance due to ribosomes. The ribosomes are involved in the making of proteins that are going to be secreted or proteins that are going to end up sitting in the membrane
Involved in protein synthesis
Secreted and membrane bound proteins (involved in transport) enter the lumen (interior) of the rough ER and are processed by the rough ER and the rest of the endomembrane system for release from the cell or retention on the cell membrane (there are two options)
Note that the synthesis of cytoplasmic proteins occur on free ribosomes (ribosomes that are not located on the rER. These ribosomes are in the cellular fluid and not on the membrane)
Ribosomes
A complex of ribosomal RNA and protein molecules that functions as a site of protein synthesis in the cytoplasm.
Vesicle
A membranous sac in the cytoplasm of eukaryotic cells.
Proteins being made on the rER are then released in a vesicle (transport vesicle as it is transporting proteins). Vesicles are tiny membrane bound spheres with contents that head towards the Golgi.
Golgi complex
An organelle in eukaryotic cells consisting of stacks of flat membranous sacs that modify, store and route products of the endoplasmic reticulum and synthesize some products, notably non-cellulose carbohydrates.
Series of membrane sacs (flattened spheres) and associated vesicles
Receives, modifies, sorts and ships proteins arriving from the rER
Has polarity - cis and trans face
Cis and trans face of Golgi complex
Vesicles from ER arrive at the cis face. Processed vesicles leave at the trans face (think of it as C comes before T). The Golgi only operates correctly if cis is receiving and trans is releasing.
Functions of the Golgi
glycosylation
Sorting proteins
Directing vesicle traffic
Golgi complex function - glycosylation
Addition (or modification) of carbohydrates to proteins (putting sugars on proteins effectively) (some proteins arrive with carbohydrates already in place that the ER has put in but they may need some biochemistry on them to change the types of sugars or they might need to start anew)
Important for secreted or cell surface proteins (help interact with the water environment that surrounds them)
Golgi also produce many polysaccharides which may also be secreted from the cell
Golgi complex function - sorting proteins
Adds molecular markers to direct proteins to the correct vesicles before “budding” from the trans face
For example - a special phosphorylated sugar (mannose 6-phosphate) identifies proteins that will become lysosomal enzymes ( proteins that are going to the lysosome are enzymes that are going to break things down, you must take care where they go as they can destroy biological molecules which is what the cell is made of, important that they are tagged with the sugar in order for identification for location
Golgi complex function - directing vesicle traffic
Adds molecular ‘tags’ to vesicles (not on the proteins) leaving the trans face to direct them to the correct targets
Such tags are often short proteins exposed on the vesicle surface
Acts as docking sites when they reach their target
Some tags direct vesicles to the lysosome, others direct to secretory pathways
Imported for release and surface expression
Exocytosis
The cellular secretion of biological molecules by the fusion of vesicles containing them with the plasma membrane.
Transports material (in most cases glycoproteins) out of the cell or delivers it to cell surface
Vesicle fuses with the membrane and what was the interior of the vesicle has now fused and the interior is now continuous with the extracellular space therefore the contents of the vesicle can now leak out
Regulated exocytosis
Releases hormones and neurotransmitters
Used to release signals to other cells in order to communicate with other cells
Must be heavily regulated as these have to be released at precise timings
Constitutive exocytosis
releases extracellular matrix proteins
Proteins are needed in the ECM and this process is not regulated
Endocytosis
Cellular uptake of biological molecules and particulate matter via formation of vesicles from the plasma membrane. (brings molecules into the cell
Phagocytosis
A type of endocytosis in which large particulate substances or small organisms are taken up “cellular eating” by a cell. Carried out by some protists and certain immune cells in animals.
Form a phagocytic vacuole (NOT vesicle) which is ‘digested’ by the lysosomes (membrane encloses food or other particle and beings it into the cell)
In humans this occurs in macrophages
Pinocytosis
A type of endocytosis in which the cell ingests “cellular drinking” extracellular fluids and its dissolved solutes.
Uptake of extracellular fluid containing various solutes such as proteins and sugars which the cell will want to use
Uptake vesicle is formed with the aid of a coat protein (which protein coats the vesicle)
The uptake is non-selective (it isn’t picking anything in particular, it just weeps stuff into the vesicle)
Receptor-mediated endocytosis
A process by which cells absorb various solutes by the inward budding of the plasma membrane (invagination).
Specialised form of pinocytosis
This process is selective and only brings in what the cell wants
Allows the cell to take up bulk quantities (relatively large amounts) of specific substance which may be presents at only low concentrations in the extracellular fluid
Receptor proteins are used to selectively capture the required solute
Lysosomes
A membrane-enclosed sac of hydrolytic enzymes found in the cytoplasm of animal cells and some protists.
Phagocytic vacuoles fuse with lysosomes which are membrane bound organelles made by the rER and Golgi body containing hydrolytic (water is part of the way in which they carry out the breakdown of the substance) enzymes
The interior of a lysosome is acidic which is required for the enzymes to be active. This protects the rest of the cell from damage should any of these enzymes leak out
They degrade proteins, lipids, carbohydrates and nucleic acids and release breakdown products into the cell (release monomers after breakdown which can often be used again
Lysosome function
Lysosomes digest and recycle unwanted cellular materials
This process is called autophagy and is important for cell health
Lysosomal digestion is also important in apoptosis (programmed cell death) in which whole cells ‘intentionally’ die (you want them to be removed after they have died as you do not want them taking up space, leaking out substances instead you want them to be digested away so that they can be reused again)
Defects in lysosomal enzymes can result in lysosomal storage diseases
Lysosomal storage disease
Tay-Sachs
Vacuoles
A membrane-enclosed vesicle whose specialised function varies in different kinds of cells.
Large vesicles dervied from the rER and Golgi
Part of the endomembrane system
Food vacuoles are involved in phagocytosis
Vacuoles are important in plants as they can perform lysosome-like functions. The large central vacuole in plants absorbs water allowing plant cells to grow without a large increase in cytoplasm
