Fluid Mosaic Model of the plasma membrane PDF

Title Fluid Mosaic Model of the plasma membrane
Course An Introduction to Physiology
Institution University of Leicester
Pages 15
File Size 1.2 MB
File Type PDF
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Summary

Fluid Mosaic Model of the plasma membrane...


Description

Fluid Mosaic Model of the plasma menbrane The fluid-mosaic model describes the plasma membrane that surrounds animal cell. The membrane has 2 layers of phospholipids (fats with phosphorous attached), which at body temperature are like vegetable oil (fluid).

1. Fluid Mosaic Model  Because cells reside in a watery solution (extracellular fluid), and they contain a watery solution inside of them (cytoplasm), both layers of phospholipids (1) have the hydrophilic heads (2) facing outwards into the water and the hydrophobic tails (3) facing inwards, avoiding contact with water. •

Cholesterol molecules are among the phospholipids.



Protein molecules (4) float in the phospholipid bilayer.



Many of the phospholipids and proteins have short chains of carbohydrates (5) attached to them, on the outer surface of the membrane. They are known as glycolipids (6) and glycoproteins (7). There are also other types of glycolipid with no phosphate groups.

The fluid mosaic model of membrane structure. This is called the fluid mosaic model of membrane structure: • 'fluid' because the membran is fluid (molecules within the membrane can move around within their own layers) • 'mosaic' because the protein molecules are mosaiclly aranged • 'model' because no-one has ever seen a membrane looking like the diagram - the molecules are too small to see even with the most powerful microscope.

Component Phospholipids

The role of the components of cell membrane Roles 

Have a hydrophobic head and a fatty acid tail --> form a



bilayer separating the cell from the outside. They are fluid --> components can move around freely.



Permeable to small and/or non-polar molecules



Impermeable to large molecules and ions --> prevent these substances from passing thorough.

Cholesterol

 

Maintains the fluidity of the membrane Increases the stability of the membrane: sits between fatty acid tails --> making the barrier more complete, preventing molecules like water and ions from passing through the membrane. Without cholesterol the membrane would easily split apart.

Proteins and glycoproteins (Protein molecules + carbohydrates)



Channel proteins allow the movement of some substances, such as the large molecule sugar, into and out of the cell as they can‘t travel directly through the cell surface membrane. The channels can be opened or closed to control the substances‘ movement.

Carrier proteins actively move substances across the cell surface membrane, using energy from ATP.



Cell surface receptors are glycoproteins responsible for the binding of an extracellular signalling molecule (hormones and cell surface antigens) and transduction of its messages into one or more intracellular signalling molecules, which changes the cell‘s  behaviour.  Help to interact with other cells. 

Help to recognise cells: glycoproteins are specific for cells from a particular individual or a particular tissue

Glycolipids (Phosopholipid molecules + carbohydrates)

 Cell signalling  Cell surface antigens  Cell adhesion (adhese to neighbouring cells to form tissues).

2. Roles of cell surface membranes • • • • • •

Structural, keeping the cell contents together. Separate cell components from the outside environment  Allows cells to communicate with each other by cell signalling. Allows recognition of other external substances. Allows mobility in some organisms, e.g. amoeba. Selectively permeable barrier. Regulating the transport of materials into or out of cells  The site of various chemical reactions.

# 20. Passive and active transport across cell membranes

Substances can enter or leave a cell in 2 ways: 1) Passive a) Simple Diffusion b) Facilitated Diffusion c) Osmosis (water only) 2) Active a) Molecules b) Particles

I. Passive transport across cell membranes 1. Diffusion  Molecules and ions move freely in gases and liquids, each type of these particles tends to spread out evenly within the space available. This is diffusion. •

Diffusion is:

+ the net movement of molecules + from a region of its higher concentration to a region of its lower concentration. + down a concentration gradient + no energy is used. Source: northlandcollege.edu



Some molecules and ions are able to pass through cell membranes --> The membrane is permeable. Some substances cannot pass through cell membranes --> The membrane is partially permeable.



Example: O2 is at a higher concentration outside a cell (inside the cell it is being used in respiration). Oxygen molecules are small and do not carry an electrical charge --> they can pass freely through the

phospholipid bilayer -->O2 diffuses from outside to the inside of the cell, down its concentration gradient.

2. Facilitated diffusion •

Facilitated diffusion is

+ the movement of specific molecules + down a concentration gradient + with the aid of special channel or carrier protein. + no energy is used. •

Ions or electrically charged molecules are not able to diffuse through the phospholipid bilayer because they are repelled from the hydrophobic tails. Large molecules are also unable to move through the phospholipid bilayer freely.



However, the cell membrane contains special channel protein that provide hydrophilic passageways for these special ions and molecules. Diffusion through these channel proteins is called facilitated diffusion. Like 'ordinary' diffusion, it is entirely passive.



Each carrier protein has its own shape and only allows one molecule (or one group of closely related molecules) to pass through. Selection is by size; shape; charge.

Glucose and aminoacids are transporteg by facilitated diffusion.

3. Osmosis •

Osmosis is

+ the diffusion of water molecules + from a region of their higher concentration (dilute solution) to a region of their lower concentration (concentrated solution) + across a partially permeable membrane + down a water potential gradient. + no energy is used. •

The solute molecules are too large to get through the membrane. Water molecules carry tiny electrical charges but they ar small --> can move freely through the phospholipid bilayer of most cell membranes --> diffuse across cell membranes.

In the picture: - The concentration of sugar molecules is higher on the concentrated solution (L) and lower on the diluted one (R). - The concentration of water molecules is higher on the (R) and lower on the (L).

It is confusing to talk about the 'concentration of water', so we can say that a diluted solution (R) has a hige water potential and a concentrated solution (L) has a low water potentia.

There is a water potential gradient between the 2 sides. The water molecules diffuse down this gradient, from a high water potential (R) to a low water potential (L). Water potential (ψ) is measured in pressure units, kilopascals (kPa): - Pure water ψ = O kPa. - Solutions ψ = negative: e.g a dilute sucrose solution ψ = -250 kPa a concentrated sucrose solution ψ = -4000 kPa. - Water moves by osmosis down a water potential gradient, from a high ψ (- 250 kPA) to a low ψ (- 4000 kPA).

Summary of passive transport

II. Active transport across cell membranes Sometimes substances are required to be move against the concentration gradient, or faster than they would by passive transport. In these cases, active processes are used, which require energy. There are many

occasions when cells need to take in substances which are only present in small quantities around them. E.g. root hair cells in plants take in nitrate ions from the soil. Their concentration are often higher inside the root hair cell than in the soil, so the diffusion gradient is from the root hair à the soil. Despite this, the root hair cells still can take nitrate ions in, by active transport.

The active transport is done using carrier (transporter) proteins in the cell membrane. These use energy from the breakdown of ATP to move the ions into the cell. The carrier proteins are ATPases. Each carrier protein is specific to just one type of ion or molecule. Cells contain many different carrier proteins in their membranes.

Source: McGraw- Hill companies. III. Endocytosis and exocytosis (bulk transport) Macromolecules are too large to move with membrane proteins and must be transported across membranes in vesicles. Exocytosis • • •

the transport of macromolecules out of a cell in a vesicle the object is surrounded by a membrane inside the cell to form a vesicle  the vesicle is moved to the cell membrane. the membrane of the vesicle fuses with the cell membrane, expelling its contents outside the cell.

Endocytosis • • •



the transport of macromolecules into a cell in a vesicle the cell puts out extensions around the object to be engulfed the membrane fuses together around the object, forming a vesicle.

there are 2 types of endocytosis: phagocytosis (cell eating) and pinocytosis (cell drinking).

Video: Cell membrane animation https://www.youtube.com/watch?v=vh5dhjXzbXc...


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