Cell
The basic structural and functional unit of a living organisms.
Plasma Membrane
Defines the extent of the cell and is the flexible outer boundary.
The lipid bilayer and proteins constantly changing in fluid mosaic.
Plays a dynamic role in cellular activity.
Separates intracellular fluid from extracellular fluid.
Contains thousands of substances (amino acids, sugars, fatty acids, vitamins, hormones, salts, waste products)
Allows cell to take in what it needs, when it needs it, and keep out what it does not
What are the 3 basic parts of a human cell?
Plasma membrane
Cytoplasm
Nuclues
Cytoplasm
Intracellular fluid
Nucleus
Control Center
Phospholipid (lipid bilayer)
75% of membrane lipids
Phosphate heads are polar and hydrophilic
Fatty acid tails are non-polar and hydrophobic
Glycolipids
5% of membrane lipids
Lipids with polar sugar groups on outer membrane surface
Fatty acid tails are non-polar
Cholesterol
20% of membrane lipids
Increases membrane stability
Hydroxyl group is polar
fused ring system is non-polar
Membrane Proteins
Allow communication with environment
1/2 of the mass of plasma membrane
Most specialized membrane functions
Some float freely, others are "tethered" to intracellular structures that make up the cytoskeleton and are restricted in their movement.
2 types - Integral and peripheral
Integral Proteins
Firmly inserted into the lipid bilayer
Some protrude from one membrane face only, but most are transmembrane that span the entire membrane and protrude on both sides
Have both hydrophobic and hydrophilic regions
Can interact with lipid tails and water
Function as transport proteins (channels and carriers), enzymes, or receptors
Peripheral Proteins
Loosely attached to integral proteins
Include filaments on intracellular surface for membrane support
Function as enzymes; motor proteins for shape change during cell division and muscle contraction; cell-to-cell connections
Six Functions of Membrane Proteins
- Transport
- Receptors for signal transduction
- Attachment to cytoskeleton and extracellular matrix
- Enzymatic activity
- Intercellular joining
- Cell-cell recognition
Lipid Rafts
~ 20% of outer membrane surface
Contain phospholipids, sphingolipids and cholesterol
More stable; less fluid that rest of membrane
May function as stable platforms for cell-signaling molecules, membrane invagination, and other functions
The Glycocalyx
-"Sugar covering" at cell surface
Lipids and proteins with attached carbohydrates (sugar groups)
- Every cell has different patterns of sugars
Specific biological markers for cell to cell recognition
Allows immune system to recognize "self" and "non-self"
Cancerous cells change it continuously
Cell Junctions - Tight Junctions
Adjacent integral proteins fuse to form impermeable junction encircling cell
Prevents fluids and most molecules from moving between cells
Tight junctions can be found between epithelial cells in the GI tract
Cell Junctions - Desmosomes
"Rivets" or "spot-welds" that anchor cells together at plaques (thickening on plasma membrane)
Linker proteins between cells contain plaques
Keratin filaments extend through cytosol to opposite plaque giving stability to cell
Reduces possibility of tearing
They are abundant in tissues subjected to great mechanical stress
Cell Junctions - Gap Junctions
Communication between adjacent cells
Transmembrane proteins form pores (connexons) that allow small molecules to pass cell to cell
For spread of ions, simple sugars, and other small molecules between cardiac or smooth muscle cells
Found in excitable tissues, such as the heart and smooth muscle to synchronize electrical activity and contraction
Passive Process Membrane Transport
Diffusion and filtration
No ATP required
Substances move down its concentration gradient
Diffusion
The tendency of molecules or ions to move from an area where they are in higher concentration to an area where they are in lower concentration.
Three forms -
simple diffusion
carrier and channel mediated facilitated diffusion
Osmossis
Simple Diffusion
Non-polar and lipid soluble (hydrophobic) substances diffuse directly through the lipid bilayer.
E.g. oxygen, carbon dioxide, fat-soluble vitamins
Facilitated Diffusion
Certain molecules and ions are transported passively even though they are unable to pass through the lipid bilayer.
They move through the membrane by binding to protein carriers in the membrane and is ferried across or by moving through water filled protein canals.
E.g. glucose, amino acids, and ions
Channel Mediated Facilitated Diffusion
Aqueous channels formed by transmembrane proteins
Selectively transport ions or water
Two Types -
Leakage Channels - always open
Gated Channels - Controlled by chemical or electrical signals
Osmosis (Passive Process)
Movement of solvent across selectively permeable membrane (water)
Water diffuses through plasma membranes though lipid bilayer/through specific water channels called aquaporins
Occurs when water concentration different on the two sides of a membrane
1. Osmolarity
2. Hydrostatic Pressure
3. Osmotic Pressure
1. Measure of total concentration of solute particles
2. The back pressure exerted by water against the membrane
3. The tendency of water to move into the cell by osmossis
Does osmosis cause the cell to shrink?
Yes, because change in cell volume disrupts cell function, especially in neurons.
Isotonic solutions
Have the same concentrations of non-penetrating solutes as those found in cells. Cells will retain their normal size and shape.
Hypertonic Solutions
Have a higher concentration of non-penetrating solutes as those found in cells. Cells lose water by osmosis and shrink. the solution contains a higher concentration of solutes that are present inside the cell.
Hypotonic Solutions
Solutions are more dilute (contain a lower concentration of non-penetrating solutes) than cells.
Cells take on water by osmosis until they become bloated and burst (lyse).
Lower concentration of solutes that are present inside cells.
Membrane Transport - Active Processes
Two types -
Active Transport
Vesicular Transport
Both require ATP to move solutes across a living plasma membrane because
-Solute is too large for chemicals
-Solute not lipid soluble
-Solute not able to move down concentration gradient
Active Transport-
Primary Active Transport
Requires carrier proteins (solute pumps)
Bind specifically and reversibly with substance
Moves against concentration gradient
Requires energy directly from ATP hydrolysis
Active Transport-
Secondary Active Transport
Requires carrier proteins (solute pumps)
Bind specifically and reversibly with substance
Moves against concentration gradient
Requires energy indirectly from ionic gradients created by primary active transport
Sodium - Potassium Pump
Located in all plasma membranes
Involved in primary and secondary active transport of nutrients and ions
Pumps against Na+ and K+ gradients to maintain high intracellular K+ concentration and high extracellular Na+ concentration
Maintains electrochemical gradients essential for functions of muscle and nerve tissues
It drives Na+ out of the cell against a steep concentration gradient and pumps K+ back into the cell.
It is crucial for cardiac, skeletal muscle, and neuron function.
Secondary Active Transport
Cotransport
Always transports more that one substance at a time
1 - Symport
2 - Antiport
1 - Substances transported in the same direction
2 - Substances transported in the opposite directions
Vesicular Transport
Transport of large particles, macromolecules, and fluids across membrane in membranous sacs called vesicles
Requires cellular energy (ATP)
Vesicular Transport Functions
1) Exocytosis
2) Endocytosis
3) Transcytosis
4) Vesicular Trafficking
1) transport out of cell
2) transport into cell (phagocytosis, pinocytosis, receptor-mediated endocytosis
3) Transcytosis - transport into, across, then out of the cell
4) transport from one area or organelle in cell one cell to another
Phagocytosis (Endocytosis)
Pseudopods engulf solids (often large or solid material, such as bacteria, cell debris, or inanimate particles) and bring them into the cell's interior.
These cells (phagocytes) are experts at ingesting and disposing of bacteria, other foreign substances, and dead tissue cells.
Pinocytosis (Endocytosis)
Fluid phase endocytosis (cell drinking)
Plasma membrane unfolds, bringing extracellular fluid and dissolved solutes inside of cell, then fuses with an endosome.
A routine activity in most cells
Most cells utilize to "sample" environment
Nutrient absorption in the small intestine
Receptor-mediated endocytosis
Allows specific endocytosis and transcytosis
cells use to concentrate materials in limited supply
Clathrin-coated pits provide main route for endocytosis and transcytosis
Uptake of enzymes, low-density lipoproteins, iron, insulin, and viruses, diphtheria, and cholera toxins
Exocytosis
Usually activated by cell surface signal or change in membrane voltage
Substances enclosed in secretory vesicle
v-snares - on vesicle
t-snares - on membrane and bind
Functions - hormone secretion, neurotransmitter release, mucus secretion, ejection of wastes
Generation of Resting Membrane Potential (RMP)
Produced by separation of oppositely charged particles (voltage) across me
membrane in all cells
Cells described as polarized
Typically ranges from -50 to -100 millivolts depending on cell type
Selective Diffusion Establishes RMP
In many cells Na+ affects RMP
Attracted into cell due to negative charge
Potassium's role in Resting Membrane Potential
The resting membrane potential is largely determined by K+ because at rest the membrane is much more permeable to K+ than Na+.
Sodium's role in Resting Membrane Potential
Sodium also contributes to a resting membrane potential because sodium is strongly attracted to the cell interior by its concentration gradient.
Ligands (1st messenger)
Binds to the receptor, changes shape and activates.
G Protein
A regulatory molecule that acts as a middleman or relay to activate (or inactivate) a membrane bound enzyme or ion channel.
Second Messengers
An intracellular chemical signal which connects plasma membrane events to the internal metabolic machinery of the cell.
Cyclic AMP and Ion Calcium
Typically activate protein kinase enzymes, which transfer phosphate group from ATP to other proteins.
Cytoplasm
"Cell forming material"
The cellular material between the plasma membrane and the nucleus.
Made up of 3 elements - the cytosol, organelles, and inclusions
Cytosol
The viscous, semi-transparent fluid in which the other cytoplasmic elements are suspended.
Organelles
The metabolic machinery of the cell. Each carries out a specific function for the cell. Some synthesize proteins, others package those proteins, etc.
Inclusions
Chemical substances that may or may not be present depending on the cell type. Such as stored nutrients, glycogen granules in the liver and muscle cells, lipid droplets in fat cells, melanin in skin and hair cells.
Mitochondria
Threadlike or lozenge-shaped membraneous organelles. They squirm, elongate, and change shape almost continuously. They are the power plants of the cells, providing most of its ATP supply.and ribosomes
Enclosed by 2 membranes.
The outer membrane is smooth and featureless and the inner membrane folds inward, forming shelf like cristae (crests) that protrude into the matrix, the gel-like substance.
They contain their own DNA, RNA, and ribosomes that can reproduce themselves.
Ribosomes
Small, dark staining granules composed of proteins and a variety of RNAs. Each has 2 globular subunits that fit together like the body and cap of an acorn. The are the site of protein synthesis.
Some float freely, some are attached to membranes.
Free Ribosomes
Float freely in the cytoplasm. They make soluble proteins that function in the cytosol, as well as those imported into mitochondria and some other organelles
Membrane Bound Ribosomes
Attached to membranes, forming a complex. They synthesize proteins destined either for incorporation into cell membranes or lysosomes, or for export from cell.
Endoplasmic Reticulum
An extensive system of interconnected tubes and parallel membranes enclosing fluid filled cavities. It coils and twists through the cytosol, with continuous outer nuclear membrane and accounts for about half of the cell's membrane.
Rough Endoplasmic Reticulum
The external surface is studded with ribosomes. Proteins assembled on these ribosomes thread their way into the fluid-filled interior of the ER cisterns. When complete, the newly made proteins are enclosed in vesicles for their journey to the Golgi apparatus where they undergo further processing.
Smooth Endoplasmic Reticulum
Consists of tubules arranged in a looping network. Its network does not play a role in protein synthesis.
Enzyme catalyze reactions involved with the following:
metabolize lipids
synthesize steroid-based hormones
absorb, synthesize, and transport fats
detoxify drugs
breakdown stored glycogen to form free glucose (in liver cells especially)
Skeletal and cardiac muscle cells have an elaborate amount that plays an important role in storing and releasing calcium ions during muscle contraction.
Golgi Apparatus
Consists of stacked and flattened membranous sacs, shaped like hollow dinner plates, associated with timey membranous vesicles.
Acts as the "traffic director" for cellular proteins
Major function is to modify, concentrate, and package the proteins and lipids made at the rough ER and destined for export from the cell. The "finishing touches"
The membranes are shaped like flattened rubber bands.
Peroxisomes
Membranous sacs containing powerful oxidases and catalases
Detoxify harmful or toxic substances
Catalysis and synthesis of fatty acids
Neutralize dangerous free radicals
-Oxidases convert to H2O (also toxic)
-Catalases convert H2O2 to water and oxygen
Lysosomes
Spherical membranous bags containing digestive enzymes
"Safe" site for intracellular digestion
Digest ingested bacteria, viruses, and toxins
Degrade nonfunctional organelles
Destroy cells in injured or non-useful tissue
Break down bone to release Ca2+
Endomembrane System
Overall function
-produce, degrade, store, and export biological molecules
-degrade potentially harmful substances
Includes ER, Golgi apparatus, secretory vesicles, lysosomes, nuclear and plasma membranes
Cytoskeleton
Elaborate series of rods throughout cytosol; proteins to other cell structures, non-membranous, gives it the shape of the cell
3 types
- Microfilaments
- Intermediate filaments
- Microtubules
Microfilaments
Thinnest of cytoskeletal elements
Dynamic stands of protein actin
Each cell unique arrangement of strands
Dense web attached to cytoplasmic side of plasma membrane-terminal web that gives strength, compression resistance
Involved in cell motility, change in shape, endocytosis and exocytosis
Intermediate Filaments
Tough, insoluble, ropelike protein fibers
Composed of tetramer fibrils
Resist pulling forces on cell; attach to desmosomes
(Neurofilaments in nerve cells; keratin filaments in epithelial cells)
Microtubules
Largest of cytoskeletal elements; dynamic hollow tubes; most radiate from centrosome
Composed of tetramer fibrils
Determine overall shape of cell and distribution of organelles
Mitochondria, lysosomes, secretory vesicles attach to microtubules, moved throughout cell by motor proteins