NOTES FOR BIOLOGY 1001
SECTION 005
Spring 2005
DR. STEVEN POMARICO
CHAPTER 5
A CLOSER LOOK AT CELL MEMBRANES
>>>The cell (or plasma) membrane is usually the outermost barrier of the cell.
The plasma membrane contains the cell but still allows the cell to interact with its environment.
The plasma membrane functions:
1. Isolate the cell environment
2. Regulate exchange between inside and outside
3. Communicate with other cells
4. Identify the cell type
---Plasma membrane is the membrane at the boundary of every cell that acts as a selective barrier, thereby regulating the cell’s chemical environment.
1. Membranes are made of lipid
2. Phospholipids are molecules which can form membranes naturally
3. Cell membranes are phospholipid bilayers
Evidence: The phospholipid content of red blood cells is just enough to cover the cell with two layers.
The red blood cell has no internal membranes.
4. Biological membranes contain proteins
Evidence: Membranes isolated from red blood cells contain proteins in addition to lipids.
Singer and Nicolson: FLUID MOSAIC MODEL (See fig. 5.3)
-proteins embedded in the lipid bilayer
-Hydrophilic zones = polar head groups + hydrophilic portion of proteins
-Hydrophobic zone = 2 x hydrophobic tails + hydrophobic portion of proteins
---Fluid for membranes means there is lateral two-dimensional movement of the lipids and some proteins in the plane of the membrane. However, transverse (flip-flop) across the membrane is rare.
---Mosaic aspect of membranes means that there are proteins interspersed within the lipid bilayer.
Some functions of membrane proteins
-transport proteins
-channel proteins - pores
-carrier proteins - selective revolving doors
-receptors
-cell-to-cell recognition
The membrane proteins have two types of spacial arrangements.
1. integral proteins
Some functions of integral membrane proteins (See fig 5.5)
-transport proteins
-channel proteins - pores
-carrier proteins - selective revolving doors
-enzymes
-receptors
-cell-to-cell communication
-adhesion
-cell recognition
2. peripheral proteins
Membranes have a bifacial orientation (i.e. sidedness)
-different lipid composition
-different proteins or protein orientation
-carbohydrate (always outside)
>>>>>Membranes are built to be selectively permeable.
---Selective permeability
Two factors which influence permeability
-solubility characteristics of the substance crossing the membrane
-presence of transport proteins
>>>TRANSPORT ACROSS MEMBRANES
Transport is movement.
Movement of molecules requires two factors
1. Movement must be through a fluid - liquid or gas
2. There must be a concentration gradient
-difference in the amount of molecules from one place to another
Molecules in a fluid always move. If there is a concentration gradient then the movement has a specific direction.
>>>>>Passive transport: Movement down a concentration gradient
---Concentration gradient is a concentration change over a distance in a particular direction.
---Diffusion (See fig. 5.8) is the net movement of a substance down a concentration gradient.
-movement is from greater concentration to lower concentration
-the greater the concentration difference, the faster the diffusion
-net movement continues until the system reaches equilibrium
-usually occurs over short distances
Much of the movement of substances across membranes occurs by diffusion and therefore is a form of passive transport.
---Passive transport is diffusion of a substance across a biological membrane.
-Does not require the cell to expend energy
-Depends on the permeability of the membrane
>>>>>Membranes are built to be differentially permeable.
---Differential permeability is the property of biological membranes which allows some substances to cross more easily than others.
Two factors which influence permeability (see fig 6-5)
-solubility characteristics of the substance crossing the membrane -facilitated diffusion by a carrier protein or a channel protein
>>>>>>Specific proteins facilitate diffusion of selected solutes
---Facilitated diffusion (see fig.6-5) is diffusion across a membrane with the help of a protein.
Three types of facilitated diffusion
-Bind-and-release (see fig. 6-5c)
-Selective channel
A pore the size of the solute
-Gated channel
Like a selective channel only with a door
>>>ENERGY-REQUIRING TRANSPORT ACROSS MEMBRANES
Cells need to move molecules against a concentration gradient (from low concentration to high concentration)
Doing this requires energy because it’s fighting diffusion.
(Like pumping water uphill)
>>>Active transport of solutes against a concentration gradient. (See fig 6-9)
---Active transport is an energy-requiring process during which a transport protein pumps a molecule across a membrane against its concentration gradient.
-Energy requiring
-Uses energy from ATP
-Used to maintain ion gradients (e.g., Ca2+ ions)
>>>>>>Osmosis is the passive transport of water
---Osmosis (see fig 6-6) is the diffusion of water across a differentially permeable membrane.
-Influenced by factors which govern diffusion
-Some solute molecules reduce the number of water molecules which can diffuse by forming a hydration shell of bound water
---Osmotic pressure (see fig. 6-7) is a measure of the tendency of a solution to take up water when separated from pure water by a differentially permeable membrane.
The principles of osmosis:
1. Osmosis is the diffusion of water across a differentially permeable membrane.
2. Water moves from high concentration to low concentration
3. Dissolved substances decrease the amount of free water molecules
>>>>>Cellular survival depends on balancing water movement
Cells without cell walls are not tolerant to excessive water movements.
Three water balancing scenarios (see fig 6-8):
1. Isotonic environment
No net movement of water, and the cell volume remains stable.
2.Hypertonic environment
Water moves out of the cell and the cell shrivels
3. Hypotonic environment
Water moves into the cell, the cell swells and eventually bursts
Note these are relative terms comparing one side of the membrane with the other.
---Isotonic is an equal concentration of solute compared to the inside of a cell.
---Hypertonic is a greater concentration of solute than the inside of a cell.
---Hypotonic is a lower concentration of solute than the inside of a cell.
Cells with walls
Cells with cell walls are more tolerant to excessive water movements, but still exhibit cellular changes.
Three water balancing scenarios
1. Isotonic environment
No net movement of water, and the cell becomes flaccid or limp.
2. Hypertonic environment
Water moves out of the cell and the cell shrivels
3. Hypotonic environment
Water moves into the cell. The cell swells and becomes turgid.
-This is the ideal state for most plants.
-provides mechanical support
-plants maintain this state by being hypertonic compared to their environment.
>>>>>Endocytosis, the transport of large molecules
---Endocytosis is the cellular uptake of large molecules (a.k.a. macromolecules) and particulate substances (food) by the localized pinching off of a region of the plasma membrane to form a vesicle.
Three types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis (see fig. 6-10).
---Phagocytosis (cell eating) is the endocytosis of solid particles. This may involve the formation of pseudopodia
---Pinocytosis (cell drinking) is the endocytosis of fluid droplets.
---Receptor-mediated endocytosis (see fig 6-10) is the process of importing specific marcomolecules into the cell by the inward budding of vesicles formed from coated pits, occurs in response to the binding of specific nutrients to receptors on the cell’s surface.
>>>>>Exocytosis moves material out of the cell.
---Exocytosis is the cellular secretion of large molecules (a.k.a. macromolecules) by the fusion of vesicles with the plasma membrane.