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Chapters 6 & 7 (Concept 6.7 (Cell walls of plants
Extracellular…
Chapters 6 & 7
Concept 6.7
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Cell walls of plants
- Extracellular structure distinguishes plant from animal cells
- Prokaryotes, Fungi and some eukaryotes have cell walls
- made of cellulose fibers
Primary cell walls have multiple layers
- Primary- thin & flexible
- Middle Lamella - thin layer between primary walls of adjacent cells
- secondary - between plasma membrane & primary cell wall
Extracellular Matrix (ECM) of Animal Cells
- Animals lack cell walls
-Covered by an ECM
- made up of glycoproteins
- bind to receptor proteins in plasma membrane call integrins
Can regulate cells behavior by communicating through integrins
- Can influence the activity of genes in the nucleus
- mechanical signaling may occur through cytoskeletal changes the\at trigger chemical signals in the cell
Cell Junctions
- Neighboring cells interact & communicate through direct physical contact
Plasmodesmata in Plant cells
- Channels that perforate plant cell walls
- water & small solutes, sometimes RNA, can pass from cell to cell
Tight Junctions - Membranes of neighboring cells are pressed together, preventing leakage
Desmosomes - Anchoring junctions, fasten cells together into strong sheets
Gap junctions- Communication junctions, provide cytoplasmic channels between adjacent cells
Concept 6.4
Endomembrane system includes;
- Nuclear envelope
- Endoplasmic reticulum
- Golgi aparatus
- Lysomes
- Vesicles & vacuoles
- Plasma Membrane
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Golgi Apparatus - Shipping and receiving center
- Receives transport vesicles from ER and sends them off to other places
Products of the ER are modified & stored & sent to other destinations.
- Extensive in cells specialized for secretion
Consists of a group of associated flattened membrane sacs, cisternae
1 side of Golgi stack is Cis face
- on the same side & located near ER
The other side is the Trans face
- on opposite side, gives rise to vesicles that pinch off & travel to other sites
Removes some sugar monomers & substitutes others. Membranes phosopholipids may be altered in the Golgi
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Lysosomes; Digestive Compartments - Membranous sac of hydrolytic enzymes used to digest macro molecules
Work best in acidic environments found in lysosomes.
- Excessive leaking from a large # can destroy a cell by self-digestion
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Hydrolytic enzymes and lysosomal membrane are made by rough ER, transferred to Golgi for further processing
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Concept 7.3
Diffusion - molecules spread out evenly
each molecule moves randomly but diffusion of a population may be directional
- Concentration Gradient - The region along which the density of a chemical increases or decreases
- Passive Transport - The cell does not have to expend energy
Effects of Osmosis on Water Balance
- Osmosis- Diffusion of water across a selectively permeable membrane
- Diffuses from lower solute concentration (hypotonic) to higher concentration (hypertonic)
- Water will move back & forth until the concentration are equal on both sides
- The balance of water between the cell & its environment are crucial to organisms
Water Balance of Cells Without Cell Walls
- Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water
- Depends on its concentration of solutes that cannot cross the membrane
Isotonic Solution - Concentration of the solute is the same as inside the cell, no net water movement
- Hypertonic Solution- Solute concentration is greater than inside the cell; cell loses water
- Hypotonic Solution- Solute concentration is less than inside the cell; cell gains water
- Cells withou cell walls will shrivel in a hypertonic solution, and lyse (burst) in a hypotonic solution
Hypertonic & hypotonic environments create osmotic problems for organisms without rigid walls
- Bacteria & Archaea that live in hypersaline environments have cellular mehanisms to balance interal & external solute concentrations
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Water Balance of Cells with Cell Walls
- Cell walls help maintain water balance
- A plant cell in a hypotonic solution swells until the wall can't hold anymore. (TURGID) firm
- If a plant cell is isotonic, there is no movement into the cell; (FLACCID) limp
- In hypertonic plant cell loses water, membrane pulls way from cell wall, plant wilts, a potentially lethal effect (PLASMOLYSIS)
Facilitated Diffusion
- Transport proteins speed up the passive movement across the plasma membrane
- Channel proteins provide corridors that allow a specific molecule or ion to cross
- Aquaporins facilitate the diffusion of water
-Ion channels facilitate the transport of ions
- Some ion channels called gated channels open or close in response to a
stimulus
EX> Nerve cells
Carrier Proteins undergo a small change in shape that translocates the solute-binding site across the membrane
- This change in shape can be triggered by the binding & release of transported molecule
Concept 7.1
Lipids & proteins - Staple ingredients of the membrane, most abundant lipids are phospholipids,
- Amphipathis -molecule has both a hydrophilic (loves water) and hydrophobic(hates water) region
- Phosopholipid bilayer is the boundary between 2 aqueous compartments
- Hydrophobic tails are sheltered inside the membrane
- Hydrophilic heads are exposed to water on either side of bilayer
Fluid Mosaic model - membrane is mosaic of protein molecules bobbing in bilayer
- Continually being refined as new discoveries are made about membrane structure
Proteins are not randomly distributed in the membrane
- Groups of proteins are often associated in long-lasting specialized patches
The fluidity of membranes
- Held together mainly by weak hydrophobic interactions
- most of the lipids and some proteins can move sideways within the membrane
- very rarely a lipid may flip-flop switching from one bilayer to another
The sideways movement of phosopholipids is rapid
Proteins are larger so they move slower, but some drift
Cooler temperatures cause membranes to become solid
- The temp at which the membrane solidifies depends on the type of lipids
- Rich in unsaturated fatty acids are more fluid than saturated fatty acids
- Membranes must be fluid to work properly
At warm temps cholesterol restrains movement of phospholipids
- at cool temps it maintains fluidity by preventing tight packing
- Plants use related steroid lipids to buffer membrane fluidity
Evolution of Differences in Membrane Lipid Composition
- Variations of cell membranes appear to be evolutionary adaptations
- Fish that live in extreme cold have membranes that remain fluid
- Bacteria & Archaea that live in high heat have more solid membranes
-Ability to change the lipid composition in response to temp changes has evolved in organisms
- Overall natural selection has favored organisms whose mix of membrane lipid ensure an appropriate level of membrane fluidity
Membrane Proteins & Their Functions
- A membrane is a collage of different proteins, clustered in groups, embedded in fluid matrix
- Phosopholipds form the main fabric of the membrane
- Proteins determine membranes functions
Different types of cells contain different sets of membrane proteins
- Various membranes of the cell have a unique collection of proteins
- Integral Proteins- penetrate the hydrophobic interior of the bilayer
- Majority are transmembrane proteins span the membrane
- consist of stretches of non polar amino acids coiled into a helices
- Peripheral Proteins loosely bound to the surface of membrane
Cell surface-surface membranes carry out several functions
-Transport
- Enzyme activity
- Signal transduction
- Attaching a cell to another cell
A single membrane protein may carry out multiple functions
Cell-surface proteins are important in the medical field
- Protein CD4 on the surface of immune cells helps HIV infect these cells
- HIV can't enter a cell lacking CCR5
This info has been key to developing a treatment
The Role of Membrane Carbohydrates in Cell-Cell Recognition
- Recognize each other by binding to molecules containing carbohydrates on plasma membrane
Membrane Carbohydrates are short branched chains
- some covalently bonded to lipids forming glycolipids
- Most are covalently bonded to proteins forming glycoproteins
Synthesis & Sidedness of Membranes
- Distinct inside & outside faces
- May differ in lipid composition
- each protein has directional orientation in the membrane
Concept 7.4
- Facilitated diffusion is still passive because the solute moves down gradient, requires no energy
-Some transport proteins can move solutes against gradient
The Need for Energy in Active Transport
- Active transport requires energy
- Usually in the form of ATP
- To move against concentration gradient
-All proteins involved in active transport are carrier proteins
How Ion Pumps Maintain Membrane Potential
- Membrane Potential id the voltage across a mambrane
- Voltage is created by differences in the distribution of positive & negative Ions across the membrane
- The cytoplasmic side is negative in charge
Electrochemical Gradient - 2 combined forces, drive the diffusion of ions across a membrane
- Chemical force (ions gradient)
- Electrical force (effect of membrane potential on ions movement)
Electrogenic pump - Transport protein that generates voltage
- Sodium-potassium pump - Major electronic pump of animal cells
- Proton pump - Plants Fungi & Bacteria, transports H+ out of cell
- Electrogenic pumps help store energy that can be used for cellular work
Cotransport - Occurs when active transport of a solute indirectly drives transport of other substances
Concept 6.1
Microscopes
Electron Microscopes (EM) - Use electron beams, reveals sub cellular structures
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Light Microscopes (LM) - Uses light, maginifies image up to 1,000 X
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Concept 6.3
The Nucleus
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Nuclear Lamina - net-like protein filaments, maintains shape
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Concept 6.8
- Cells rely on the integration of structures & organelles in order to function
- None of the cells components work alone
The large cell is a macrophage it helps defend against infections by ingesting bacteria into phagocytic vesicles
Cellular functions arise from cellular order:
-All the processes require energy, which mitochondria supply in ATP
- The cell is a living unit greater than the sum of all its parts
Concept 7.2
- A cell must exchange materials w/ its surroundings
- Plasma membranes are selectively permeable
The permeability of the Lipid Bilayer
- Nonpolar molecules are hydrophobic
- Tey can dissolve in the lipid bilayer& cross easily w/out help of membrane proteins
- Hydrophilic molecules do not cross membrane easily
- Proteins built into membrane play key roles in transport
Transport Proteins
- Allow passage of hydrophilic substances, is specific for the substance it moves
- Chanel Proteins have a hydrophilic channel that some ions can use as a tunnel through the membrane
- Aquaporins greatly facilitate the passage of water moleculs
- Carrier proteins - bind to molecules & change shape to help cross
Concept 7.5
Small molecules & water enter or leave the cell through lipid bilayer or transport proteins
- Large molecules cross membrane in bulk
- Bulk transport requires energy
Exocytosis
- Transport vesicles migrate to the membrane, fuse with it, & release contents outside the cell
- Many secretory cells use exocytosis to export their products
Endocytosis
- THe cell takes in macromolecules by forming vesicles from membrane
- Reverse of of Exocytosis
- Phagocytosis (cellular Eating)
- Pinocytosis (cellular drinking)
- Receptor-mediated endocytosis
Phagocytosis - a cell engulfs a particle in a vacuole, the fuses with a lysosome to digest the particle
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Receptor-mediated endocytosis- binding of specific solutes to receptors triggers vesicle formation
- Receptor proteins, receptors, & other molecules are transported in vesicles
- Emptied receptors are recycled to the plasma membrane
Human cells use these to take in cholesterol, which is carried in particles called low-density lipoproteins (LDLs)
- Disease familial hypercholesterolemia have missing or defective LDL receptor proteins
Concept 6.6
Cytoskeleton - network of fibers extending through the cytoplasm
-Organizes the structure & activities of the cells
- Composed of Microtubules, Microfilaments, & Intermediate filaments
Helps support the cell & maintain shape
- Interacts w/ Motor Proteins to move the cell
- Vesicles travel along the cytoskeleton
Microtubules
- thickest, hollow rods
- made of dimers of tubulin
- Shape the cell
-guide movement
- seperate chromosomes during cell division
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In animal cells, the centrosome has a pair of centrioles, with 9 triplets arranged in a ring
Control Flagella & Cilia, projections from cells, uses to move cell
- Basal body anchors them to cell
-Motor protein, dynein, bending movement
Microfilaments
- Actin filaments, thinnest component
- Solid rods
- help support cell shape
form a cortex inside plasma membrane
- bundles make up core of microvilli of intestinal cells
Function in cellular motility contain myosin
-Crawl along surface by extending pseudopodia
- Cytoplasmic streaming is a circular flow of cytoplasm w/in cells, by actin-myosin interactions
Intermediate Filaments
- Larger than Micofilaments but smaller than microtunules
- Permanent cell structures
- Support shape and fix organelles in place (nucular lamina)
Life at the Edge
- The plasma membrane is the edge of life
- Separates living cell from from its surroundings
- Controls inbound and out bound traffic
- Exhibits selective permeability, allowing some substances to cross more easily