Cell Structure. The Cell ESSENTIAL to the study of biology Simplest form of life Every organism’s...
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Transcript of Cell Structure. The Cell ESSENTIAL to the study of biology Simplest form of life Every organism’s...
Cell Structure
The Cell
ESSENTIAL to the study of biology
Simplest form of life Every organism’s basic
unit of structure and function
Named by Robert Hooke in 1665 after observing cork cells (cell walls) under microscope.
Prokaryotic vs. Eukaryotic Cells
Prokaryote “before” “nucleus”/ NO
NUCLEUS/few organelles Bacteria DNA is concentrated in
nucleoid (non membrane-bound)
Eukaryote “true” “nucleus” / many
membranous organelles Protists, plants, fungi,
animals Nucleus with nuclear
membrane holds DNA
Why so small?
Metabolism requires that cells stay small As a cell grows, its
volume grows proportionately more than its surface area
Cells need a high surface area to volume ratio to exchange materials with their environment through plasma membrane.
Compartmental Organization of Cells
Compartments (ORGANELLES) provide different local environments (pH, etc.)Incompatible but equally important
processes can occur next to each other in different “rooms”
Cytoskeleton & Related Organelles
Cytoskeleton Maintains shape of cell Responsible for
movement of cell and movement of organelles within cell
Made of three types of protein fibers:
Microtubules, microfilaments, & intermediate filaments
Cytoskeleton & Related Organelles
Microtubules Hollow tubes Made up of A- and B-
tubulin Responsible for:
• Cell motility• cilia/flagella
• Chromosome movements (mitosis)
• centrioles
• Movement of organelles
• Maintenance of cell shape
Cytoskeleton & Related Organelles
Components of Cytoskeleton:Microtubules – 25 nm diameterIntermediate Filaments – 8 – 12 nm
diameterMicrofilaments – 7 nm diameter
Cytoskeleton & Related Organelles
Intermediate Filaments Made up of fibrous
proteins Made up of keratin Responsible for:
• Structural support• Maintenance of cell
shape• Anchors nucleus &
certain organelles
Cytoskeleton & Related Organelles
MicrofilamentsMade up of 2 intertwined strands of
actinResponsible for:
• Muscle contraction• Cytoplasmic streaming• Cell motility (pseudopodia)• Cell division (cleavage furrow)• Maintenance of/changes in cell shape
Centrioles Only found in animal
cells Visible only during cell
division 9+0 arrangement of
microtubules May give rise to cilia &
flagella May be involved in
formation of spindle fibers in animal cells, but not plants!
Flagella and Cilia
Structures for cell motility
Flagella (long & few in #)
Cilia (short & numerous)
9 + 2 internal structure
Basal body has 9+0
structure like centrioles
dynein
microtubule
Figure 4.25Page 73
Cellular Organelles
Nucleus: “control center” of the cell Surrounded by a nuclear
envelope Contains DNA Nucleolus: site of ribosome
synthesis
Cellular Organelles
Ribosomes Site of protein assembly Free and bound ribosomes
• Free: float through cytoplasm (make proteins for use inside that cell)
• Bound: attached to Rough ER (make proteins to be transported out of the cell)
Cellular Organelles
Endoplasmic Reticulum:Made up of membranous tubules and
cisternae (sacs)Smooth ER: NO ribosomes attached
• Synthesis and transport of lipids• Controls glucose glycogen conversion in
liver & muscles• Detoxification of drugs and other poisons• Sarcoplasmic reticulum (muscle ER) stores
calcium needed in muscle contraction.Rough ER: ribosomes attached
• Synthesis & transport of proteins
Endomembrane System
Smooth and Rough ER
Endomembrane System
Golgi Apparatus: Products of the
Endoplasmic Reticulum are modified and stored here
Modifies & packages proteins
Endomembrane System
Lysosomes: Used by cells to
digest macromolecules
Sac of hydrolytic enzymes
Apoptosis: • Programmed cell
death Usually found only
in animal cells
Endomembrane System Vacuoles:
Food vacuoles (storage)
Contractile vacuoles (pump extra water out of cells in freshwater protists)
Central vacuole (plant cells)
• Stores organic compounds, inorganic ions (K+, Cl-), and water
• Surrounded by tonoplast
Endomembrane System
Peroxisomes:Contain enzymes that transfer
hydrogen from various substances to oxygen, producing H2O2 as a byproduct
Various functions:• Break fatty acids down into smaller
molecules for cellular respiration• Detoxify alcohol in liver
Energy-related organelles Mitochondria
Site of cellular respiration (Energy from the breakdown of organic molecules is used to phosphorylate ADP to produce ATP)
“powerhouse of the cell”
More metabolic activity = more mitochondria
Energy-related organelles
Mitochondrial Structure: Outer membrane Inner membrane:
• Cristae = large surface area makes more efficient at producing energy
Intermembrane space
Mitochondrial matrix
Energy-related organelles
Chloroplasts: Found in plants and
eukaryotic algae Site of photosynthesis Contain the green
pigment chlorophyll
Energy-related organelles Chloroplast
Structure Thylakoids
• Grana = stacks of thylakoids
• (Light Dependent Phase)
Stroma• Fluid outside the
thylakoids• (Calvin Cycle)
Cellular Organelles
Cell Wall Found only in plant cells Protects the cell Maintains
cell shape Prevents excessive uptake
of water Holds plant up against
gravity Primary Cell Wall-thin;
cellulose Secondary Cell Wall-
thicker; found in woody plants
Cellular Organelles
Extracellular Matrix: Found in animal cells Made up of
glycoproteins (collagen) & proteoglycans
• Proteins + carbohydrates
Provides support and anchorage for cells
Differs from one cell type to another
Intercellular Junctions
Neighboring cells are connected to one another
Plant cells: Plasmodesmata:
• Channels in the cell wall through which strands of cytoplasm pass through and connect the living contents of adjacent cells
Intercellular Junctions (Animal Cells)
Tight junctions-membrane proteins interlock
Desmosomes, (anchoring junction)-intermediate filaments “sew” membranes together
Gap junctions- channels align allowing materials to flow between cells
Intercellular Junctions
Tight junctions: Membranes of
neighboring cells are fused
Form a continuous “belt” around a cell
Example: junction between epidermis of the skin
Intercellular Junctions Desmosomes
Anchoring junctions
Act as rivets Muscle cells
are held together by desmosomes.
What happens when a muscle is torn?
Intercellular Junctions
Gap junctions Communicating
junctions Cytoplasmic
channels between adjacent cells
Salts, sugars, AAs, etc. can pass through
Membrane Structure & Function
Functions of the Cell Membrane
Isolates the cytoplasm from the external environment
Regulates the flow of materials into and out of the cell
Communicates with other cells
The Fluid Mosaic Model Currently accepted model of
the cell membrane Proposed by Singer and
Nicolson in 1972 Phospholipid bilayer
Hydrophilic “head” – exposed to the outside
Hydrophobic “tail” – hides inside
Membrane proteins are randomly dispersed in phospholipid bilayer
Fluid Mosaic Model
Fluidity of the Membrane The lipids and
proteins can drift throughout the membrane
Membrane is NOT stiff/rigid
Cholesterol makes the membrane stronger by limiting the movement of phospholipids
Membrane as a Mosaic Lipid bilayer has
membrane proteins embedded in it
Integral proteins Go through the
membrane (both sides)
Peripheral proteins attached to the
surface of the membrane
Selective Permeability of the Cell Membrane
The cell membrane can “choose” what enters and exits a cell
“Gatekeeper of the Cell”
Passive Transport
Def’n: Diffusion of a substance that does
NOT require the input of energy by the cell
3 types of passive transport:DiffusionOsmosisFacilitated diffusion
Diffusion
Movement of molecules from high to low concentration until equilibrium is reached.
Passive Transport= no energy required
What substances may diffuse across membrane? Nonpolar (non-charged) molecules; small polar molecules
Diffusion Each substance
diffuses down its OWN concentration gradient and is unaffected by concentration gradients of other substances
Diffusion
Does all movement stop once equilibrium is reached?? NO!! Equal rates in all
directions
Osmosis Def’n:
The passive transport of water across a selectively permeable membrane
Hyper-, hypo-, iso- tonic• RELATIVE TERMS!!
• Always referring to solute concentration
Water moves from areas of lower concentration of solutes (hypotonic) to areas of higher solute concentration (hypertonic)
Osmosis in Plant and Animal Cells Animal Cells:
Cell crenate/shrinks Cytolysis
• Occurs when a cell is in a hypotonic solution
• Water goes from solution into cell
Plant Cells: Turgid- vacuole
swells Flaccid-vacuole
shrinks (plasmolysis)
Facilitated Diffusion
Def’n:The diffusion of large molecules
across the cell membrane using transport proteins
Glucose; ions Does NOT require an input of energy
Solute is still moving down its concentration gradient
Facilitated Diffusion
Transport proteins are specific for their solutes
Transport proteins can become saturated
Some are gated channels: Chemical or electrical
stimulus causes them to open
Example
Which direction will sucrose move?
Which direction will glucose move?
Which direction will fructose move?
Pop Quiz – Passive Transport1. What is passive transport across
the cell membrane?
2. Describe 3 types of passive transport discussed yesterday.
3. If a blood cell is placed in a hypotonic solution what would be the result?
4. What type of solution is preferred in plant cells.
Active Transport
Def’n:The pumping of solutes against their
gradientsRequires an input of energy by the cellUsed so cells can “stockpile” extra
supplies (storage of any substance in a very high concentration; i.e. Iodine stored in thyroid; glycogen stored in liver
Sodium/Potassium Pump
Electrogenic Pumps
Voltage across membranes = stored energy that can be used for cellular work
Sodium-Potassium Pump:3 Na+ OUT of the cell for every 2 K+
pumped inNet transfer of one positive charge from
cytoplasm to extracellular fluid Very important for transferring signals
between nerve cells
Sodium-Potassium Pump
Cotransport
Substance that has been pumped across a membrane can do work as it “leaks” back by diffusion
Another substance “hitches a ride”
Endocytosis & Exocytosis
Def’n:The movement of large molecules
(polysaccharides, proteins, etc.) across the membrane using vesicles
Endocytosis = cell takes in macromolecules
Exocytosis = cell secretes macromolecules
Endocytosis Cell takes in macromolecules
by forming vesicles made from the plasma membrane
Phagocytosis = “cell eating” Large molecules
Pinocytosis = “cell drinking” Small molecules & liquids
Receptor-mediated endocytosis = seeks out specific molecules
Exocytosis
The cell secretes macromolecules by the fusion of vesicles with the plasma membrane
Used to release hormones, chemical signals, etc.
Pop Quiz 2
1. What is Active Transport across the cell membrane?
2. What does it mean when molecules are moving up the concentration gradient?
3. Describe (in detail) how the sodium potassium pump works in nerve cells.
4. What is the difference between phagocytosis and pinocytosis?