Chapter 3- the plasma membrane Flashcards
Loss of homeostasis in cells often leads to
Disease
What are the 3 basic parts of the cell?
- Plasma membrane
- Cytoplasm- intracellular fluid containing organelles
- Nucleus
Which cells do not have a nucleus?
Red blood cells
basic functions of different cell types in the body (8)
- Connecting body parts (fibroblast of connective tissue)
- Form linings (epithelial cells)
- Gas transport (erythrocyte)
- Movement (smooth, skeletal, and cardiac muscle)
- Storage (adipose cells)
- Immune defenses (leukocytes)
- Control cells (nerve cells)
- Reproduction (sperm and eggs)
Fluid mosaic model
The plasma membranes consist of a phospholipid bilayer with proteins randomly dispersed in it. The mosaic comes from the proteins embedded in the layer. Some are anchored, some float within it.
Why is the membrane important?
separates the intracellular fluid from the extracellular fluid
Selectively permeable definition
The membrane allows substances in or out, or blocks them from entering or leaving
Which 3 macromolecules make up the chemical composition of cells?
Lipids, proteins, and carbohydrates
Importance of lipids in the cell (2)
- Fatty acid tails- hydrophobic portions that face the inside of the membrane. Tails help prevent crossing of water soluble molecules and create a boundary.
- The combination of hydrophobic and hydrophilic regions leads to ability of cells to reseal when damaged or torn
Which macromolecules constitute most of the cell specialized membrane functions?
Proteins
What are the 3 main types of membrane proteins?
Integral, peripheral, and motor proteins
Integral proteins
These proteins are embedded in the plasma membrane. They transport proteins in and out of cells, and are used for enzymes, receptors, and cell-cell recognition (important for immune function)
Peripheral proteins
loosely attached to integral protein. Are not found in the lipid bilayer. They are used for enzymes, motor proteins, cell-cell attachment.
Motor proteins
plasma membrane protein that helps the cell to change shape, like when cells divide
Functions of membrane proteins (6)
- Transport proteins in and out of cells.
- Receptor proteins
- Enzymes
- Cell-cell recognition
- Attachment proteins
- Intercellular junctions
Transport proteins importance and function
Transport proteins in and out of cells. Some proteins form channels through which a particular solute can be selectively moved. Other proteins actively pump substances across the membrane surface by using ATP.
Receptor proteins importance and function
relay messages to the cell interior when bound to/exposed to certain chemical messengers (hormones, etc). Some receptor proteins have receptor sites that are specific to a chemical messenger.
What happens when a protein is bound to a chemical messenger?
It changes shape
Enzymes
Proteins that catalyze chemical reactions. Some enzymes act alone, others act as a team to catalyze sequential steps.
Cell-cell recognition proteins importance
helps with recognizing “self” from “non-self”, important for immune function
Attachment proteins
helps hold some membrane proteins in place, maintains cell shape. Can be located inside of the cell or outside, depending on function
Intercellular junctions
link cells together, used for cell movement/migration in tissues
Which aspects of cells determine their function in the body?
Size, shape, content of cell
Principle of complementarity of structure and function
the activities of cells are dictated by their shapes, and by the types and relative numbers of the subcellular structures they contain. For example, the flat, tile-like epithelial cells lining the inside of your cheek fit closely together to form a barrier that protects underlying tissues from bacterial invasion
Cells only arise from other cells by
Mitosis/meiosis
Extracellular materials
substances contributing to body mass that are found outside the cells
3 main classes of extracellular materials
- Extracellular fluid
- Cellular secretions
- Extracellular matrix
Extracellular fluid
includes interstitial fluid, blood plasma, and cerebrospinal fluid. ECF dissolves and transports substances in the body
Interstitial fluid
the fluid in tissues that bathes all of our cells and contains thousands of substances. The cell extracts from this fluid the substances it needs at the time
Cellular secretions
include substances that aid in digestion (intestinal and gastric fluids) and some that act as lubricants (saliva, mucus, and serous fluids).
Extracellular matrix
cells secrete a jellylike substance composed of proteins and polysaccharides that assembles in the extracellular space. The molecules act like glue to hold the cells together. Most abundant extracellular material
Where is the extracellular matrix most abundant
Connective tissue
Cholesterol
Cholesterol provides structural support to stiffen the membrane- increases membrane stability. A type of lipid
Glycolipids
Chain of sugars attached to a lipid
Glycoproteins
Chain of sugars attached to a protein
Glycocalyx
The combination of glycolipids and glycoproteins on the extracellular surface. Each individual’s is unique, and each cell type will have a slightly different arrangement. This is important for immune system function, as it helps the immune system to recognize foreign cells. It acts as a specific biological marker
What is the purpose of cell junctions?
Cell junctions permanently or temporarily attach a cell to one or more other cells. Most cells in the body form a cell junction.
Types of cell junctions (4)
- Tight junction
- Desmosomes
- Cadherins
- Gap junctions
Tight junction
integral proteins of adjacent cells fuse together. This junction is impermeable- nothing passes through it. Tight junctions between epithelial cells of the stomach prevent gastric juice from “leaking out”, which could be damaging
Desmosomes
anchoring junctions from one cell to another that prevents separation. Like Velcro. Bind cells together to form sheets, resist shearing forces when pulled/stretched
Cadherins
protein filaments extend from cell surface and link to filaments of other cells. Inside the cell, a plaque holds the cadherins in place. Keratin filaments hold plaque in place to prevent movement/shifting
Gap junctions
Communication junctions. Hollow cylinders formed by proteins connect adjacent cells. Different proteins= selective passage of molecules/substances through channels, like ions, simple sugars, small molecules, etc. Importance- cells need to know what’s happening in cells around them.
What happens to the glycocalyx of a cancerous cell?
It changes almost continuously to keep ahead of immune system recognition mechanisms and avoid destruction
Where are gap junctions present?
Present in electrically excitable tissues, like the heart and smooth muscle where ion passage from cell to cell helps synchronize their electrical activity and contraction
Passive membrane transport
movement of molecules across the membrane down their concentration gradient (diffusion) with no ATP required. High to low concentration, downhill movement
What is the driving force of diffusion?
Kinetic energy of molecules. In areas of high molecule concentration- molecules collide and bounce off one another frequently. More collisions= dispersal of molecules
Diffusion speed is determined by (3)
- Concentration- greater concentration difference between 2 areas leads to faster diffusion
- Molecular size- smaller molecules diffuse faster. Large molecules don’t have hard collisions with other molecules
- Temperature- higher temperatures increases kinetic energy and results in faster diffusion rates
Types of diffusion (3)
- Simple diffusion
- Facilitated diffusion
- Osmosis
Simple diffusion
diffusion of substance directly through the lipid bilayer. Most molecules are small in size and nonpolar. Ex- most gases, steroid hormones, fatty acids.
Facilitated diffusion
diffusion of molecules through the membrane by use of a protein (transmembrane integral protein)
Types of facilitated diffusion (2)
- Carrier mediated
2. Channel mediated
Carrier mediated facilitated diffusion
transmembrane proteins used to carry large molecules through the membrane. Proteins change shape while moving substance, will usually only transfer a specific substance. Limits- the cell can only move substances as fast as proteins become available to them
Channel mediated facilitated diffusion
transmembrane proteins form water filled channels through which molecules can pass. Selective- size of channels determines what can/can’t pass through. Proteins can form leaky or gated channels
Leaky channels
Channel is open all the time, allowing for a small amount of substance to pass continuously
Gated channels
Have doors on one or both sides- Doors are closed until they come into contact with a specific substance. Usually controlled by chemical signals.
Osmosis
diffusion of water through a selectively permeable membrane. Movement of water across a semipermeable membrane from a less concentrated solution to a more concentrated one until concentration is equal on both sides of the membrane, size of solutes don’t matter. One solute particle will displace one water molecule. Can occur without proteins or with the use of aquaporins
In a closed container, diffusion will eventually produce
In a closed container, diffusion eventually produces a uniform mixture of molecules. The system reaches equilibrium and there is no net movement of molecules
Which factors determine whether a given substance can cross the plasma membrane? (2)
- Lipid solubility- the more lipid soluble, the more readily it will diffuse across
- Size- the smaller the molecule, the more readily it will diffuse across
Molecules that are not sufficiently small or lipid soluble can still diffuse across the membrane if they are assisted by a carrier molecule.
Osmolarity
total concentration of all solute particles in a solution. A solution with a high osmolarity will have a greater number of solute particles than a solution with low osmolarity
One solute molecule displaces
One water molecule
Water moves by osmosis until
Hydrostatic pressure is equal to osmotic pressure and equilibrium is established. When they are equal, no net movement of water is observed.
The higher the number of non diffusible solutes in a cell, the higher the
osmotic pressure and the greater the hydrostatic pressure must be to resist further net water entry (pressure can’t get too high in an animal cell)
Hydrostatic pressure
the pressure of water on the inner cell wall
Osmotic pressure
tendency of water to move into a cell by osmosis
When equal volumes of aqueous solutions of different osmolarity are separated by a membrane that is permeable to all molecules in the system
net diffusion of both solute and water occurs, each moving down its own concentration gradient.
Tonicity
ability of a solution to change the shape of cells by altering the cells by altering the cells internal water volume
Water follows
solutes- any change in solute concentration on either side of a membrane will also cause a change in water concentration.
Isotonic solutions
have the same concentration of nonpenetrating solutes as those found inside the cell. No net loss or gain of water observed. Cell size/shape remains the same. Ex- .9% NaCl solutions (basis of IV therapy), extracellular fluid
Hypertonic solutions
have a higher concentration of solutes than inside the cell. Water moves out of the cell. Cell can shrivel up or crenate. Ex- 10% NaCl solutions
Hypotonic solutions
have a lower concentration of solutes than inside the cell. Water moves into the cell. Cell will swell up until they burst (lyse). Ex: distilled water (has no solute), anything below .9%
Active membrane transport
movement of molecules across the plasma membrane that requires energy input (use of ATP). Active transport also requires transport proteins
Why use energy for transport?
Molecules may be too big, too charged, insoluble in lipid membrane, or moving against their concentration gradient
Types of active transport
- Primary active transport
2. Secondary active transport
Primary active transport
energy required to do work comes directly from ATP hydrolysis by pumps (transport proteins). Hydrolysis of ATP leads to transfer of phosphate group from ATP to the transport protein. Phosphorylation of protein pump leads to a change in protein shape- allows protein to move molecule across the membrane. Ex: sodium potassium pump
Sodium potassium pump
Uses sodium potassium ATPase enzyme. Pumps Na and K against their gradients in opposite directions across the membrane. Effect- K+ concentration is higher inside the cell than outside, Na concentration is higher outside the cell than inside. The pump operates continuously- Na and K leak slowly but continuously through leakage channels in the plasma membrane
Importance of the sodium potassium pump
Importance- ATPase pumps maintain electrochemical gradient necessary for function of muscle and nervous tissue
Electrochemical gradient
Ions diffuse according to electrochemical gradients- recognizes the effect of both electrical (charged) and concentration of ions (chemical) forces
Secondary active transport
indirectly uses energy stored in concentration gradients of ions created by primary active transport
Cotransport protein couples movement of a solute down its concentration gradient (the driving molecule) to movement of a second molecule against its concentration gradient (the driven molecule). Example- moving Na out of cell creates concentration gradient. The cotransport protein pumps Na+ back into the cell, and carries another molecule with it
Symporter
movement of two transported substances in the same direction
Antiporter
movement of two transported substances in the opposite direction
Uniporter
Movement of one substance
Vesicular transport
A type of active transport, requires the use of ATP. movement of fluids with large particles and macromolecules inside membranous sacs called vesicles. This is how your body cells get rid of the stuff they’re making
Functions of vesicular transport (4)
- Endocytosis
- Exocytosis
- Transcytosis
- Vesicular trafficking
Endocytosis
Movement into the cell. Begins with the infolding of the plasma membrane. There are 3 types
Exocytosis
Movement out of the cell, vesicular transport used to expel substances from cell interior into ECF. Substance to be ejected is surrounded by a secretory vesicle. Secretory vesicle travels to plasma membrane, fuses with it, and dumps contents out of the cell
Transcytosis
movement of substances into, across, then out of the cell. Common in the endothelial cells lining blood vessels- provides a quick means to get substances from the blood to the interstitial fluid.
Vesicular trafficking
movement of a substance from one area of the cell to another
Types of endocytosis (3)
- Phagocytosis
- Pinocytosis
- Receptor-mediated endocytosis
Phagocytosis
cell eating- cell engulfs large and/or solid material (clump of bacteria, cell debris). Pseudopods form around particle, forming an endocytic vesicle called a phagosome. Pseudopod formation involves receptors, formation is specific. The phagosome usually fuses with lysosome, where contents are digested. Ex- macrophages (immune system) engulf bacteria or dead cells- dead cells must be disposed of or they will trigger an inflammatory response
Pinocytosis
cell drinking- cell brings in small volume of extracellular fluid containing small particles. This droplet enters the cell and fuses with a sorting vesicle called an endosome. No receptor use- endocytosis is not a specific process. This process is ongoing. Important in cells that absorb nutrients, like the cells that line the intestines
Receptor-mediated endocytosis
allows specific endocytosis and transcytosis to occur. Extracellular substances bind specific receptor proteins. Substances can be specifically concentrated in vesicles and brought into the cell
Which substances are transported during receptor mediated endocytosis?
These substances include insulin and some other hormones, enzymes, low density lipoproteins, and iron. Flu viruses, diphtheria, and cholera toxins hijack this route to enter cells
Fate of contents in vesicles during receptor mediated endocytosis
Substrate can be distributed through the cell. Vesicle can fuse with lysozyme for digestion of concentrated substance
How does stimulation begin for exocytosis
Stimulation begins by binding of hormone to membrane receptor or a change in membrane voltage
In exocytosis, how does the vesicle dump the contents out of the cell?
V-snare transmembrane proteins on vesicles must recognize certain plasma membrane proteins (t-snares) and bind with them. The binding rearranges the lipid monolayers
V snare proteins
Transmembrane proteins on vesicles, important for exocytosis
t-snare proteins
Plasma membrane proteins, important for exocytosis
Exocytosis functions (4)
- Hormone secretion
- Neurotransmitter release
- Mucus secretion
- Waste removal
Voltage
electrical potential energy resulting from separation of oppositely charged particles in cells (ions)
Membrane potential
A voltage generated across the plasma membrane by the selective permeability of the membrane
Resting membrane potential
voltage difference across cell membrane when cell is at rest- the charge separation only exists at the membrane
Resting membrane potential range
-50 to -90 mV, average of -70 mV
Electrically polarized
All cells are electrically polarized- the inside of the cell is negatively charged while the outside of the cell is positively charged
How is resting potential created? (5)
- Diffusion causes ionic imbalances to polarize the membrane
- Potassium ions (K+) have a pivotal role here. Remember K+ concentration is higher inside the cell than outside
- K+ leaks out of the cell, but large/negatively charged proteins stay inside the cell- inside of the cell is negatively charged
- As inside of the cell becomes more negatively charged, K+ is attracted back inside
- When the K+ concentration gradient is balanced by electrical gradient (equal movement of K+ into and out of the cell)- resting membrane potential is -90 mV
How is the resting membrane potential maintained?
Active transport maintains electrochemical gradients to keep the cell in a steady state. Remember- Na+-K+ pump continuously pumps Na+ out of K+ into cell
Cells can respond to both
extracellular chemicals (hormones, neurotransmitters) and to other surrounding cells. These interactions are used to maintain homeostatic balance in the body
Molecules used by cells to interact with their environment (2)
- Cell adhesion molecules (CAMs)
2. Plasma membrane receptors
Cell adhesion molecules
are found on almost every cell in the body. They play key roles in embryonic development and wound repair (where cell mobility is important)
Types of glycoproteins found on cell surface (2)
- Cadherins
2. Integrins
Cadherins and integrins function (4)
- Cells can adhere themselves to other cells or to molecules in the extracellular space
- Used as “arms” to allow cell migration
- Immune response- can be used to signal white blood cells to infected/injured area of the body
- Transmit information outside of cell to inside- causing migration, cellular division, specialization
Plasma membrane receptors
Integral proteins (mostly glycoproteins) that serve as binding sites at the membrane surface
Functions of plasma membrane receptors (2)
- Contact signaling
2. Chemical signaling
Contact signaling- plasma membrane receptors
cellular recognition by physical contact between cells
Importance- normal cellular development and immunity rely on contact signaling
Ligand
A chemical messenger that binds a specific receptor and initiates a response
Ligand examples
Ligands include most neurotransmitters, hormones, and paracrines
Paracrines
chemicals that act locally and are rapidly destroyed
Chemical signaling- plasma membrane receptors
Overall process: ligand binds to receptor- receptor structure changes- cell proteins are altered.
Why can one ligand have different effects on 2 different types of cells?
The specific response is linked to the cell’s internal machinery (its structure and function), not the ligand itself. For example, acetylcholine stimulates skeletal muscle to contract but inhibits cardiac muscle
G protein
a regulatory molecule that acts as a middle-man to activate or inactivate a membrane bound enzyme or ion channel.
G protein coupled receptors
This generates one or more intracellular signals (second messengers), which connect plasma membrane events to the internal metabolic machinery of the cell.
Certain enzymes will at as second messengers and will transfer phosphate groups from ATP to other proteins, activating a series of enzymes that bring about the desired cellular activity
The amplification effect of this chain of effects is significant- like a video going viral
G protein coupled receptors importance
The G protein signaling system is a key signaling pathway involved in neurotransmission, smell, vision, and hormone action.
