Download presentation
Presentation is loading. Please wait.
1
The Plasma Membrane SGN 7
2
The plasma membrane is a semipermeable barrier that allows for a managed internal environment (cellular homeostasis) separate from the external environment The plasma membrane encloses the cellular contents
3
The lipid based structure is produced by the EMS and fused into place, often with specialized imbedded proteins Independent organelle /endomembrane system membranes share many characteristics with PM or cell membrane
4
Membrane structure The membrane is composed primarily of lipids, but also significantly proteins and to a lesser degree carbs Components of the membrane are generally amphipathic molecules (for example phospholipids) or molecules with several different hydrophilic and hydrophobic regions (for example many types of embedded proteins)
5
The structure of the membrane is called a fluid mosaic phospholipid bilayer A bilayered arrangement of phospholipids with integral or peripheral proteins associated with it (“mosaic”)
6
A phospholipid is composed of two fatty acid tails (hydrophobic) and a third region that includes a charged phosphate group (hydrophilic), making the molecule amphipathic
7
In each layer the tails are oriented towards the middle of the membrane, creating a hydrophobic interior region and hydrophilic regions facing outward from the cell and inward to the cell’s aqueous interior The membrane is held together primarily by hydrophobic interactions between tail regions
8
Membranes are fluid and dynamic (fluid mosaic)
Phospholipids and some proteins float about within their layer Cholesterol in the membrane increases fluidity at low temps and decreases fluidity at high temps Unsaturated fatty acid tails keep lipids from packing together too tightly increase fluidity and permeability
9
Portions of membrane pinch off and reform to add new membrane or digest old membrane, and also in regard to secreting or taking in substances
10
But membranes are stabilized and organized due to…
Internal anchoring – membrane anchored to matrix of cytoskeleton within cell External anchoring – attachment to fibers of cell wall or in animals attachment to the extracellular matrix But membranes are stabilized and organized due to…
11
Membrane proteins serve many functions and are associated with the membrane in different ways
Two general classes of membrane protein Integral proteins Peripheral proteins
12
Integral proteins – primarily embedded within membrane, often extending beyond the membrane on the inside and/or outside of the cell Transport, anchoring, signal reception and transduction, etc. Receptors extending through the membrane Chloride pump – defective in individuals with Cystic fibrosis
13
Hydrophobic domains of protein are oriented towards the hydrophobic interior of the membrane, while hydrophilic domains are oriented towards outside or inside of cell, or towards lumen of channel
14
Peripheral proteins Typically on the internal side of the membrane and loosely associated with integral proteins or phospholipids Anchoring, internal transduction of signal, enzymes, etc.
15
Membrane carbohydrates
Found only on the outer surface of the membrane Typically oligosaccharides (10 or fewer monosaccharides) as glycolipids and glycoproteins Often serve as self markers , play a role in cell adhesion, and also as part of signal receptors
16
Traffic across the membrane
The tightly packed phospholipid bilayer, with a hydrophobic interior, allows passage of some but inhibits passage of most compounds, which can only pass with some assistance from the membrane
17
The smaller and more neutral a compound, the easier it can diffuse directly across the bilayer; increased size and charge or polarity inhibit direct diffusion Therefore the membrane is said to be selectively permeable, with mechanisms regulating whether compounds can pass or not, and at what rate Can diffuse freely through the bilayer Amino acid Cannot diffuse freely through the bilayer Glucose Carbon dioxide
18
Molecules and ions move across the membrane in three principle ways
Passive diffusion Facilitated diffusion Active transport
19
Passive diffusion Movement by diffusion through the phospholipids themselves Diffusion – random movement of substances leading to their movement from high concentration to low concentration Some compounds can diffuse through the membrane according to the concentration gradient on either side of the membrane (diffusion is movement down the concentration gradient) No metabolic energy required
20
Passive transport and gas exchange in animals
4. 3. 1. 2.
21
Involves movement of small, typically nonpolar, noncharged molecules (O2, CO2) and also moderately sized predominantly nonpolar molecules (testosterone, estradiol) H2O is so small and in such overwhelming concentration it is able to move by passive transport at a limited rate; movement of H2O determined by solute concentration on either side of the membrane
22
Facilitated diffusion
Movement by diffusion but through channel proteins Channel proteins are transport proteins that form pores/channels and are highly specific for substances that they transport Diffusion - No metabolic energy required
23
Can be open channels (ex. aquaporins) or gated channels (ex
Can be open channels (ex. aquaporins) or gated channels (ex. calcium channels) Gated channels – stimulus required to open or to induce operation Stimulus can be substance being transported or can be another compound or signal, such as voltage change in nerve cells Gated channel Open channel Aquaporins – H2O specific open channels
24
Active transport Movement of compounds against the concentration gradient (from low to high concentration) Energy allows protein pumps to move compound against concentration gradient, and maintain concentrations on either side of the membrane against concentration gradient
25
Electrogenic pumps move ions against electrochemical gradient (combines concentration and charge separation) Keeps inside of the cell more negative than outside by unequal distribution of anions and cations This “separation of charge/concentration” is a type of potential energy, which can be used in different ways
26
Proton pumps move H+ out of cell or across organelle membrane, creating electrochemical gradient
Proton pumps are instrumental in cellular respiration and photosynthesis In cellular respiration the energy of electrochemical gradients is converted into a molecular form of energy that is available to the cell, called ATP
27
Cotransporters move two compounds
Cotransproters can be antiport or symport In plants proton pumps are involved in cotransport with sucrose Cell establishes high H+ concentration outside of cell with proton pump A cotransport protein binds to H+ and sucrose on and diffusion of H+ into cell powers import of sucrose (combines active and facilitated transport) Allows cells to import sucrose against concentration gradient Absorption of glucose into cells from intestines starts with energy requiring antiport to create an electrochemical gradient; then uses facilitated diffusion and symport to import glucose
28
Sodium - potassium pump in animal nerve cells uses antiport
Three Na+ out for every 2 K+ in produces interior negative potential that is used to do cellular work, including transmiting nerve impulses
29
Mass or bulk transport across the membrane
Involves the movement of very large particles or even cells, as well as movement of smaller molecules in high concentration Requires energy to manipulate chunks of membrane
30
Exocytosis – loaded vesicles from the Golgi apparatus fuse with membranes and spill substances outside of cell (insulin, neurotransmitters)
31
Endocytosis – compounds bind to cell surface receptors or cell engulfs material; membrane then invaginates and pinches off internally, forming vesicle, which migrates to specific location within cell, for example, a lysosome, where material will be digested Phagocytosis – taking in large particles or cells (ex. microorganisms engulfed by white blood cells)) Pinocytosis – engulfing extracellular fluid, in which are dissolved desired solutes (ex. egg in ovary takes in nutrients secreted by surrounding cells)
32
Receptor mediated endocytosis (ex. cholesterol uptake)
33
Review of basic concepts of water and solute movement across membranes
Students should understand diffusion, osmosis and dynamic equilibrium
34
Students should understand conditions of tonicity: hypertonic, isotonic, hypotonic Students should understand the different responses of cells with cell walls versus cells without cell walls in solutions of different tonicity Refers to the solute concentration of the aqueous environment of the cell Indirectly tells us about water concentration “Hyper” – high solute/low water outside, so water flows out of the cell
Similar presentations
© 2025 SlidePlayer.com. Inc.
All rights reserved.