2 - membranes & cell organelles
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Quick Facts• Primary purpose is to maintain the internal environment of cells by regulating what goes in and what comes out.• Average width: 0.01μm• Primary components:
• Phospholipid bilayer• Protein molecules• Glycoproteins and glycolipids • Cholestrol: steroid that stabilizes the membrane
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Phospholipids
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Carbohydrate groups and ProteinsCarbohydrate
groups on glycoproteins and
glycolipids• Form antigen (label identifying cell as “self”)
Proteins• Facillitate the movement of substrate across the membrane
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7-9nm thick (nucleus & ER membranes = 5-7nm)
All biological membranes are the same : fluid-mosaic model§ phospholipid bilayer, proteins, carbohydrates and cholesterol
Membranes are fluid structures - individual lipid molecules & some proteins move about within the layers
Phospholipids§ Hydrophobic tails will face away from water to form a monolayer or micelle (if tails
are short)§ Therefore membrane is impermeable to water soluble (polar) molecules
Proteins carry out most other membrane functions,§ they are located throughout the membrane (hence the term mosaic)§ can cross both layers OR be confined to one§ Provide channels for water-soluble molecules & ions to pass
Cholesterol (between the phospholipid molecules)§ makes the membrane less fluid & more stable
Carbohydrates are usually on the outer surface & linked to protruding proteins
Membranes
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How does it regulate molecular transport?
Passive Transport• Diffusion• Osmosis• Facilitated diffusion (channel & carrier mediated)
Active Transport• Primary (uses chemical energy)• Secondary (uses electrochemical gradient)
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Passive transport: Diffusion•Relies on molecules being small enough to pass through the semi-permeable membrane …• or in the case of channel-mediated diffusion, can be through a protein channel•Movement of molecules governed by the concentration gradient (high to low)•Diffusion ceases when balance is achieved•Diffusion animation
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Passive transport: Osmosis• Occurs when molecules of a substrate are too large to pass through the semi-permeable membrane• Substrate molecules do not move, instead water crosses the membrane in order to achieve balance (low to high)• Diffusion ceases when balance is achieved• Osmosis animation
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Solute
Solvent(eg. Water)
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Passive transport: Facilitated diffusion•Molecules of substrate are unable to permeate the membrane but are instead transported through specialised protein channels.•Proteins change shape to facilitate movement•Molecules are transported along the concentration gradient (high to low)•Diffusion ceases when balance is achieved•Facilitated diffusion animation
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Facilitated diffusion
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Primary active transport•Active transport involves molecules moving against the concentration gradient•Molecules are transported along protein channels, proteins change shape to facilitate movement•Requires the input of energy in the form of ATP (Adenosine triphosphate)•Allows cell to retain ideal intracellular environment even when concentration of required ions in extracellular environment is low.•Primary active transport animation
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Secondary active transport•Same purpose as in primary active transport except …•ATP is not required to power the protein pump.•Instead, energy is gained by moving another substance in or out of the cell, but along the concentration gradient.•A bit like a wind-up toy, an input of energy will result in the output of energy,•Secondary active transport animation
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Bulk transport•Substrate can be moved in bulk by packaging it in a vesicle that either merges with the plasma membrane or breaks off from it.•Substrate exits the cell via EXOCYTOSIS•Substrate enters the cell via ENDOCYTOSIS, of which there are 3 types:
• Phagocytosis (taking in solids)• Pinocytotsis (taking in liquids)• Receptor mediated endocytosis
•Bulk transport animation
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Endocytosis
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Exocytosis
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• Isotonic: (iso - same) surrounding fluid and cells internal fluid are of equal concentration
• Hypotonic: (hypo - lower) surrounding solution has a lower concentration than the cells.Water will diffuse through the membrane into the cells
• Hypertonic: (hyper - higher) surrounding solution has a higher concentration than the cells
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Mitochondrion
Golgi apparatus
Centriole
smooth ER
Lysosome
Cytoplasm/Cytosol
Plasma membrane Cell Wall
Ribosome
rough Endoplasmic Reticulum (ER)
Nucleus
nuclear pore
nucleolus
nuclear membrane
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Eukaryote’s nuclear envelope is a double membrane
Prokaryotes lack a nuclear envelope DNA contains encoded instructions for every aspect of an organisms existence; structure, function and behaviour
DNA granular until mitosis, then DNA becomes organised into chromosomes
Nucleolus/nucleoli = aggregation of RNANuclear pores to allow movement between nucleus and cytoplasm
Control centreNucleus
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Produces chemical energy in the form of adenosine triphosphate (ATP) through the process of cellular respiration
Outer & inner membraneThe inner membrane contains many folds to provide a larger surface area for energy production
ATP produced by reactions on inner membrane
Only in eukaryotes
Energy supplying organelleMitochondrion
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Synthesize proteins through the addition of amino acids
Receive coded instructions via mRNAComposed of protein and rRNA (ribosomal RNA)
Free ribosomes in the cytosol make proteins for local use
Ribosomes connected to the ER make proteins for other parts of the cell or for export
Protein factoriesRibosomes
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Ribosomal products
Cell Type Protein Function Kept / exported
RBC Haemoglobin Carry O2 Kept
Pancreas Insulin Control glucose levels
Exported
Muscle Actin / Myocin
Movement Kept
Stomach Pepsin Digestion Exported
Liver Catalaze Speed up reaction
Both
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Network of intracellular membranesPrior to transport, packages substances in a transport vessicle
Production, processing, transport & storage of materials within a cell
Links with plasma membrane and other membranous organelles
Intracellular transportEndoplasmic Reticulum
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Stack of flat membrane sacsCollects identical proteins and condenses them in to a single export unit
Packages protein bundles in to excretory vessicles
Concentration and packagingGolgi Apparatus
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Organelles in action
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Large membrane-bound vessicles containing digestive enzymes
Used for breaking down unwanted structures or substances within the cell
Malfunction results in build up of toxic substances within the cell
Animation
Controlled destructionLysosome
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Peroxisomes § Prevent the dangerous build-up of hydrogen peroxide (a
byproduct of some cellular processes)
Endosomes § pass newly ingested material to lysosomes for digestion
Cilia & Flagella§ used for movement in many prokaryotic cells & some
eukaryotes§ covered by an extension of the cell membrane§ made up of 9 doublet microtubules & 2 single, central
microtubules
Other organelles
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Cytoplasm
A ribosome on the ER assembles around the mRNA, every 3 nucleotides codes a particular amino acid
The protein is packaged in a transport vessicle and transported through the cell along the ER
How it all comes togetherProtein production to excretion
An mRNA copy of a gene on the DNA is made. This copy then travels out in to the cytosol
Proteins are concentrated and then packaged in to a secretory vessicle.
Vessicle floats through the cytosol before merging with the plasma membrane
Nucleus
Ribosome
ER
Golgi Body
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SummaryOf structures and functions
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Membrane bound (called a tonoplast in plants), liquid filled spaces
food vacuoles (intracellular digestion)contractile vacuoles (water balance)plant cells typically have large ones filled with sap (turgidity & storage)
Present in plants, fungi, some protists, bacteria & animalsVacuole
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Double membrane, inner membrane contains many inner folds
Contains DNA, free ribosomes, starch grains and lipid droplets
Folded inner membrane forms thylakoids, stacks of these are called grana
The light-dependent reaction occurs in the chlorophyl containing grana
The light-independent reaction occurs in the fluid around the grana called the stroma
Only plants cells contain chloroplasts. Photosynthetic bacteria contain chlorophyll, but it is free-floating within the cell.
Sunlight trappersChloroplasts
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Only in plant, fungi & bacterial cellsPrimary cell wall made of non-living cellulose (chitin in fungi and bacteria)
Plants have secondary cell walls made of ligninThe walls of two cells are connected by a sticky pectin layer
Provides support, strength & prevents expansion
Freely allows water & dissolved substances through
Rigidity and supportCell Wall
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The cell skeletonMaintain the shape of the cellProvide a support structure for the
components of the cellMovement of materials within cellMovement of the cell itself - if required
Maintain the shape of the cellProvide a support structure for the
components of the cellMovement of materials within cellMovement of the cell itself - if required
Microtubules
• Hollow• Composed of tubulin
Microfilaments
• Solid• Thinner• More flexible
Intermediate filaments
• Very tough• Composed of proteins
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Occluding (tight) junctionsCell membranes “come
together”Function:
• hold cell together• help to maintain polarity
of cell• prevent passage of
molecules
NO movement of material between cells
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Communicating (gap) junctionsAligned protein channels in the membranes of adjoining cells
Permit the passage of: amino acids, sugars, salt ions, and other small molecules
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Anchoring junctions (Desmosomes)
Dense plaques of protein anchoring intermediate filaments that protrude in to the cytosol of neighbouring cells
Serve as a bridge connecting the actin cytoskeleton of neighbouring cells
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Connections between plant cells: plasmodesmata
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Signal to apoptose may come from inside or outside the cell
Cell death via apoptosis is a normal part of the cell cycle
Failure to apoptose will result in uncontrolled growth and reproduction.
Invasion in to healthy cells represents a malignant tumour
Cancer is the second highest cause of death in humans after heart disease
Animation of difference between apoptosis and necrosis
Programmed cell deathApoptosis
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Apoptosis The sequence of events