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chapter-5cellmembrane-new.pptx
Chapter-5
Cell membrane structure and function
Chapter at a glance
How Is the Structure of a Membrane Related to Its Function?
How Do Substances Move Across Membranes?
How Do Specialized Junctions Allow Cells to Connect and Communicate?
Functions of the plasma membrane:
It isolates the cell’s contents from the external environment
It regulates the exchange of essential substances
It allows communication between cells
It creates attachments within and between cells
It regulates biochemical reactions
Phospholipids are responsible for the isolating function of membranes
Proteins are responsible for selectively exchanging substances and communicating with the environment, controlling biochemical reactions, and forming attachments
Membrane structure
Membranes are “fluid mosaics” in which proteins move within layers of lipids
The “fluid mosaic” model of a membrane was proposed in 1972 by S. J. Singer and G. L. Nicolson
This model indicates that each membrane consists of a mosaic, or “patchwork,” of different proteins that constantly shift and flow within a viscous fluid formed by a double layer of phospholipids
The phospholipid bilayer is the fluid portion of the membrane
It consist of two very different parts:
A polar, hydrophilic head
Two nonpolar, hydrophobic tails
Plasma membranes face both exterior and interior watery environments
Water-soluble substances such as salts, amino acids, and sugars cannot easily cross phospholipid bilayers
However, very small molecules such as water, oxygen, and carbon dioxide as well as larger, lipid-soluble molecules can pass through this selective barrier
A variety of proteins form a mosaic within the membrane
Proteins are embedded within, or attached to, the phospholipid bilayer
Many proteins have attached carbohydrates (glycoproteins) on their outer membrane surface
Categories of membrane proteins
Receptor proteins
Recognition proteins
Enzymatic proteins
Attachment proteins
Transport proteins
Receptor and Recognition proteins
1. Receptor proteins trigger cellular responses upon binding of specific molecules, such as hormones, sent by other cells
2. Recognition proteins are glycoproteins that serve as identification tags on the surface of a cell
3. Enzymatic proteins are proteins that promote chemical reactions that synthesize or break apart biological molecules
4. Attachment proteins
anchor the
- cell membrane to the inner cytoskeleton,
-to proteins outside the cell,
- to other cells
Transport proteins: Regulate the movement of hydrophilic molecules through the membrane
There are two types of transport proteins
Channel proteins form channels whose central pores allow specific ions or water molecules to pass through the membrane
Carrier proteins have binding sites that can temporarily attach to specific molecules on one side of the membrane and then move them through the membrane to the other side
Understanding molecular movement
A fluid is a substance whose molecules can flow past one another and, therefore, have no defined shape
A solute is a substance that can be dissolved (dispersed as atoms, ions, or molecules) in a solvent
A solvent is a fluid capable of dissolving a solute
Gradient
The concentration of a substance defines the amount of solute in a given amount of solvent
A gradient is a physical difference in temperature, pressure, charge, or concentration of a particular substance in a fluid between two adjoining regions of space
Gradients cause molecules to move from one place to another
Gradients of concentration or pressure cause molecules or ions to move from one region to another in a manner that tends to equalize the difference
Cells use energy and cell membrane proteins to generate concentration gradients of various molecules and ions dissolved in their cytoplasm
Why gradients cause molecules to move from one place to another:
Molecules and ions in solution are in constant random motion
An increase in temperature increases the rate of this random motion
Random motion produces a net movement from regions of high concentration to regions of low concentration by a process called diffusion
Diffusion
Plasma membranes are selectively permeable because they only allow only certain ions or molecules to permeate
There are two types of movement across the plasma membrane
Passive transport is the diffusion of substances across cell membranes down concentration gradients
Energy-requiring transport is transport that requires the use of cellular energy
Passive transport
Passive transport includes :
simple diffusion,
facilitated diffusion,
osmosis
Simple diffusion: Substances move down their concentration gradients across a membrane. Examples include water, oxygen, carbon dioxide, and lipid-soluble molecules like alcohol and vitamins A, D, and E
Facilitated diffusion: Water soluble molecules like ions, amino acids, and sugars diffuse down their concentration gradients with the aid of channel and carrier transport proteins
Many cells have specialized water channel proteins called aquaporins.Their small size and positive charges attract the negative pole of water molecules making aquaporins selective for water molecules
Types of Diffusion Through the Plasma Membrane
Osmosis
Osmosis is the diffusion of water across selectively permeable membranes
Water diffuses from a region of high water concentration to one of low water concentration across a membrane
Dissolved substances reduce the concentration of free water molecules in a solution
Dissolved substances displace water molecules, lowering water concentration
Dissolved substances form hydrogen bonds with water molecules, reducing the number that are free to move across a water-permeable membrane
Types of solution
Isotonic solutions have equal concentrations of water and equal concentrations of dissolved substances
-No net water movement occurs across the membrane
A hypertonic solution is one with a greater solute concentration
Water moves across a membrane toward the hypertonic solution
A hypotonic solution has a lower solute concentration
Water moves across a membrane away from the hypotonic solution
The effects of osmosis are illustrated when red blood cells are placed in various solutions
When cells are placed into a hypertonic solution, they shrivel, owing to water loss
When cells are placed into a hypotonic solution, they swell, owing to water entry
Cells in isotonic solutions remain unaffected
Active transport/energy requiring transport
During active transport, membrane proteins use cellular energy to move molecules or ions across plasma membranes against their concentration gradients
Active transport proteins span the entire membrane
They often have a molecule binding site and an ATP binding site
When the high-energy third phosphate of bound ATP is released, some of its stored energy is donated to the protein to move molecules against gradients
Active transport proteins are often referred to as pumps
Active transport
recognition
site
Cells engulf particles or fluids by endocytosis. The engulfed particles are transported within the cell inside vesicles
There are three types of endocytosis
Pinocytosis (“cell drinking”) moves liquids into the cell
Receptor-mediated endocytosis moves specific molecules into the cell
Phagocytosis (“cell eating”) moves large particles into the cell
(extracellular fluid)
Receptor-Mediated Endocytosis
The plasma membrane extends pseudopods toward an
extracellular particle (for example, food). The ends of the
pseudopods fuse, encircling the particle. A vesicle called
a food vacuole is formed containing the engulfed particle.
Phagocytosis
Exocytosis
Exocytosis moves material out of the cell
--Cells use energy to dispose of undigested particles of waste or to secrete substances into the extracellular fluid by exocytosis
--Vesicles containing the material to be expelled move to the cell surface, where they fuse with the cell membrane, allowing their contents to diffuse into the outside fluid
Attachment Proteins
Desmosomes attach cells together
Desmosomes are found where cells need to adhere tightly together under the stresses of movement
Examples include the skin, intestine, and urinary bladder
Tight junctions make cell attachments leakproof Tight junctions are found where tubes and sacs must hold contents without leaking Examples include the skin and the urinary bladder
Gap junctions and plasmodesmata allow direct communication between cells
Cell-to-cell protein channels allowing for passage of hormones, nutrients, and ions in animal cells are gap junctions
Plant cells have holes in the walls of adjacent cells forming cytoplasmic connections called plasmodesmata
