CELL MEMBRANE & CELL TRANSPORT - Biology -...
Transcript of CELL MEMBRANE & CELL TRANSPORT - Biology -...
CELL MEMBRANE & CELL TRANSPORT
Homeostasis: Maintaining a Balance
Organisms must adjust to changes in their environment.
If not…DEATH! A formal definition is—
maintaining a stable internal condition despite what is going on externally.
What Maintains Homeostasis?
The PLASMA or CELL MEMBRANE maintains the proper concentrations of materials by controlling the passage of molecules in and out of the cell.
Therefore, the cell membrane’s function is to maintain HOMEOSTASIS through passive transport, active transport and cell communication!
Cell Membrane
X 53, 260
This electron micrograph of the cell membrane shows the appearance of the phospholipid bilayer using several staining processes. The magnification on this structure is 53,260 X
Characteristics of the Cell Membrane
The cell membrane is Selectively Permeable (or) Semi-permeable
It allows some things in/out and not others… • Oxygen, nitrogen,carbon dioxide, and
other small, nonpolar molecules can diffuse directly
• Water was once thought to move directly through, but it is now understood to travel through aquaporins (a type of transport protein).
• Ions, sugars, and larger molecules move through transport proteins or in vesicles.
Plasma/Cell Membrane-Structure
Phospholipid Bilayer with proteins embedded/ floating in it
Phospholipid Structure:
Polar Head (hydrophilic—”water loving”)
Nonpolar Tails—(hydrophobic—”water fearing”)
Phospholipid
Polar Head
Nonpolar Tails
Bilayer Arrangement Outside of cell
Inside cell
Proteins are embedded/floating in the lipid bilayer.
Protein in membrane
Cell Membrane Structure-Proteins
These proteins are needed
for the membrane to function
properly.
3 Types of Membrane Proteins
1. Transport - regulate what enters or leaves cell
2. Marker - identify the cell
3. Receptor - allow cells to communicate
Transport Proteins—Channel Proteins • Function as
“gates/passageways”
• Allow polar sugars, amino acids, and ions to cross the membrane.
• Special channel proteins:
– Gated ion channels—gates that open/close
– Carrier proteins—change shape to allow specific molecule to pass
– Aquaporins—allow water to diffuse through (called osmosis)
Marker Proteins • Cell’s “Name Tag”
• Protein sticks out of phospholipid layer
• Often has carbohydrates attached to outside end
• Functions in cell identification; identifies the cell to other cells and molecules
• Important in
– immunity—so various white blood cells in your body do not mistake your cells for foreign cells
- blood typing – so you can’t receive just anytype of blood
Receptor Proteins • Function as “messenger/receiver”
• Receive information from the
environment (extracellular fluid,
blood, interstitial fluid) and
transmit that info to the inside of
the cell
• Protein has specific shape/charge
to only allow certain molecules (like
hormones) to bond
• Triggers a response in cell
• Ex) epinephrine can bind to a
receptor protein and send a
message inside that says “break
down glycogen”
Solutions
• Solution: a mixture of a solute and a solvent – water and sugar (koolaid :-)
• Solute: the substance in a solution that is dissolved by the solvent – sugar
• Solvent: the substance in a solution that is dissolving the solute – water
Isotonic Solution • Concentration of
solute and solvent molecules on one side of the membrane is equal to/the same as the solution on the other side (often the cytoplasm in cell)
• Water moves in and out at equal rates, no net movement into or out of cell since
• Cell size would stay the same
Red blood cells in isotonic solution
X 1000
Note that all the cells appear normal.
Hypertonic Solution More solute molecules on one
side of the membrane than the
other
When a cell is in a hypertonic
solution, water moves out of the
cell shrinks (dehydrates)
Plants cells shrink (plasmolysis)
because cell membrane pulls
away from cell wall, so plant
wilts because water has left the
central vacuole. Animal cell
shrink (crenate). In both cases,
the cell may die.
Crenated red blood cells in hypertonic salt solution
X 1000
Notice that the cells have shrunk.
Hypotonic Solution
Fewer/less molecules on one side of the membrane than the other
When a cell is in a hypotonic solution, water moves into the cell
Cell will swell, and could burst (lyse)
Plant cells have vacuoles to collect extra water
Red blood cells in hypotonic solution
X 1000
Note that the pinkish cells have swollen (the little dip in the middle of a normal rbc is not visible and one side bows out).
CELL TRANSPORT
• Concentration gradient - a difference in the concentration of a particular substance across a space.
• Equilibrium is reached when the molecules are distributed evenly throughout a space.
Types of Transport
• Passive Transport—no energy required, molecules move from high to low concentration (down or with concentration gradient) – Diffusion – Osmosis – Facilitated Diffusion
• Active Transport—energy required, molecules move from low to high concentration (up or against the concentration gradient) – Pumps – Vesicles
Passive Transport
Diffusion, Osmosis, Facilitated Diffusion
Diffusion
is the process by which molecules spread from areas of high concentration, to areas of low concentration
Molecules are said to go “down” or “with” the concentration gradient.
Passive Transport-requires no energy
Osmosis
is the diffusion of water
molecules through a
semi-permeable
membrane requires no
energy
Ex. Water will move
toward the higher
concentration of solute
(and hence a lower
concentration of water).
Facilitated Diffusion (uses Transport Proteins, passive)
Moves substances (from high to low concentration) down the concentration gradient without using cell’s energy using channel/carrier proteins located in membrane
Active Transport
Requires energy from the cell because materials are being moved against the
concentration gradient
Why do cells need active transport?
• Cells must maintain a certain concentration of molecules (amino acids, sugars, etc.) inside their cytoplasm from the surrounding fluid
• Some of these must be moved against the concentration gradient (there are already more molecules inside the cell)
How does active transport work?
• Some types involve carrier proteins that function as “pumps”
• Other types use vesicles
• Energy is provided by ATP (more on ATP later)
Active Transport Using Pumps
Sodium/Potassium Pump
Na+ pumped out of a cell
K+ pumped into a cell
Important because it prevents cells from bursting by lowering the sodium inside causing less water to enter through osmosis.
Used by many cells, including nerve cells, to send a chemo-electric message.
Sodium-Potassium Pump
Sodium/Potassium pump: 3 Na+ out of cell (yellow
diamonds) 2 K + into cell
(red/purple squares) Here the energy of a
phosphate from ATP (shown in pink) is used to exchange sodium atoms for potassium atoms.
Proton (H+) Pump
• Forces protons out of a membrane enclosed space to create a proton gradient down which protons flow back in
• The cell uses one ATP to pump a proton out; that proton can be used to bring in large molecules with minimal energy output
Active Transport with Vesicles
Exocytosis & Endocytosis
Endocytosis
Substances are moved
into a cell by a vesicle that pinches off from the cell membrane
Requires energy (ATP)
Types of Endocytosis
Pinocytosis - when the nutrient particles are dissolved in a liquid; “cellular drinking”
Phagocytosis - when the nutrient particles are solids; “cellular eating”
Exocytosis Exocytosis- substances inside a vesicle are released
from a cell as the vesicle fuses with the cell membrane
Involves the cell expelling waste or the cell secreting cell products (ex. Hormones, insulin)
• Requires Energy (ATP)
Cell Transport Animations
• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__phagocytosis.html
• http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120068/bio03.swf::Sodium-Potassium%20Exchange%20Pump
• http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120068/bio02.swf::Endocytosis%20and%20Exocytosis