Plasma membrane (Chapter 1)

Page 1

The fluid mosaic model

The plasma membrane is comprised of a double layer of phospholipids molecules represented by a head and two tails. The head is hydrophilic phosphate while the tail is a hydrophobic fatty acid

This means that the head can dissolve in water whereas the tails are repelled from the water solutions and are forced to face inwards towards each other forming a phospholipid bilayer

Specialised protein molecules are embedded in the bilayer in various patterns forming mosaics, As a result of the work of SJ Singer and Garth Nicolson in the 1970s the structure of the plasma membrane can be understood by using a fluid mosaic model

Page 2

Sterols increase flexibility

Phospholipids alone do not provide the flexibility required in membranes, another type of lipid called cholesterol is interspersed among the phospholipid molecules. Cholesterol interferes with the interactions between the lipid tails making the membrane more flexible ( in plants it is phytosterol not cholesterol that increases membrane flexibility)

At low temperatures, the phospholipid molecules in the membrane cluster together more closely as they don't have much energy to move around and the fluidity in reduced, when cholesterol is inserted between some of the phospholipid molecules, fluidity increases due to the greater distance between the molecules and their increased freedom of movement

At high temperatures, there is more space between the phospholipid molecules because of their greater energy levels fluidity is increased, when cholesterol is inserted it reduces fluidity

cholesterol therefore regulates plasma membrane fluidity and acts as a buffer against fluctuations in temperature

Page 3

Membrane proteins

Associated with each membrane is a set of membrane proteins that enable the membrane to carry out distinctive functions, surface proteins allow cell-cell interactions and communication and the exchange of substances with the external environment. Proteins on the external plasma membrane surface can be involved in signalling and communication between cells and helps to keep a cell anchored n its place. Proteins that span the membrane (transmembrane) can regulate the movement of substances across the membrane

Receptor proteins

Receptor proteins have receptor sites on their surface that detect molecules such as hormones, Each receptor is specific for a single molecule or a small number of molecules with related structures to which the receptor binds.

Molecules that bind to receptors are called ligands which includes proteins, lipids, ions and carbohydrates

Molecules bind with a lock and key fit. once they have bound their ligand, receptor proteins can be activated (or inactivated) which can lead to enzyme activation, ion channel opening or the avalibility of a new ligand binding site

Some membrane receptor proteins carry a carbohydrate molecule, giving them their name of glycoprotein

Recognition proteins

Recognition proteins called MHC proteins act as markers that identify the cell as a normal body cell belonging to the individual

Eg. The immune system can recognise recognition proteins on self cells and leave them be, but identify an invader by the non-self molecule it has and mount an immune response against it

Adhesion proteins

Adhesion proteins link cells together to maintain the three dimensional structure and the normal functioning of tissues

Transport proteins

Water soluble and ions cant pass through the Lipid bilayers, so transport proteins allow movement of specific stances across a membrane

Page 4

Recall, Recap & Revision

The plasma membrane forms a barrier between the internal and external environment of cells. organelles in eukaryotic cells are surrounded by membranes

The plasma membrane is composed of a phospholipid bilayer. Hydrophobic tails are stuck in the middle of hydrophilic heads

The structure of the plasma membrane explained by a fluid mosaic model

Cholesterol (animals) and phytosterol ( plants and bacteria) increase membrane flexibility

Embedded in the plasma membrane are a variety of membrane proteins that enable the membrane to carry out its distinctive activities

Membrane proteins allow cells to function appropriately, respond to chemical messages and recognise each other. They also link cells together and allow substances to move through membranes

Recap 1.1

List three functions of plasma membranes

List three features of plasma membranes

Recount why plasma membranes can be described by a fluid mosaic model '

Describe how cholesterol makes membranes more flexible

Recap 1.2

Name four types of membrane proteins

State the functions of the different types of membrane proteins

Revision

https://www.visionlearning.com/en/library/Biology/2/Membranes-I/198/quiz

https://learn.genetics.utah.edu/content/cells/membranes/

Page 5

Movement through membranes

The internal environment of a cell is regarded to be all the material contained within its plasma membrane. Extracelluar fluid bathes the outside of the plasma membrane, providing the liquid medium through which nutrients are supplied waste is removed. The plasma membrane is described as selectively or differentially permeable because it allows some substances through and not others

The plasma membrane's permeability properties ensure that essential molecules such as glucose, amino acids and lipids enter the cell readily, Gasses and small hydrophobic molecules diffuse directly across the phospholipid bilayer. Many molecules are unable to move directly through (size, shape and polarity matter)

Polarity

Water molecules are highly polar; the oxygen atom (one pole) of the water molecule has a negative charge and the hydrogen atoms (other pole) are positively charged, this results in the oxygen being attracted to the hydrogen atoms of nearby water molecules and forming hydrogen bonds with it

A hydrogen bond is a weak intermolecular chemical bond between a hydrogen atom on one molecule and a second

More substances dissolve water than any other liquid, this is due to the polarity of water molecules; it means that water molecules can interact with other charged particles such as polar molecules or ions

Polar molecules can form hydrogen bonds with water molecules ( which are also polar) and so dissolve - they are called hydrophilic ( water loving). Non-polar substances will not dissolve in water because they cannot form hydrogen bonds with water molecules they are hydrophobic ( water fearing)

Lipids are non-polar, or hydrophobic substances. They do not dissolve in water but they do dissolve in each other

For substances to form a solution, the rule is that "like dissolves in like"

Polar solvent (e.g water) + polar solute = solution

Non-polar solvent + non-polar solute = solution

Movement of hydrophilic and hydrophobic substances across the membrane

The plasma membrane is widely considered to be thought of as a sandwich that surrounds the cell. The bread of the sandwich is made up of the hydrophilic heads. The non-polar hydrophobic tails of the lipid bilayer are like butter spread thickly between the pieces of the bread. The butter does not mix with the water, which is a polar substance. Therefore substances that dissolve in water cannot pass through the non-polar 'butter' the membrane, However molecules that are non-polar can pass through the membrane freely, as long as they are not too big

Small polar molecules of substances such as water, ethanol and other small molecules pass through the plasma membrane, although they do so quite slowly, simple diffusion of substances occurs when substances move from a region where they are in higher concentration a region where they are lower in concentration

Channel proteins

Channel proteins forms narrow passageways in which small ions can diffuse through rapidly across their concentration gradient, from a ion concentration to a lower ion concentration.

Only ions of a specific size and shape can pass through a particular channel protein, many types of channel proteins are also receptors that open only up to a specific ligand

Facilitated diffusion

Carrier proteins bind to specific molecules on one side of the membrane, after binding their ligand the protein changes shape and releases substance on the other side

( Movement through carrier proteins is another example of facilitated diffusion)

Active transport

Carrier proteins use ATP ( Adenosine triphosphate) to power the movements of specific ions or small chemicals concentration gradient, ATP-powered pumps work in only one direction and therefore effectively are one-way valves

As the input of energy is required to move molecules or ions through a membrane against a concentration gradient this movement is called active transport

Type of movement: Movement through the PM: Example:

simple diffusion Through phospholipid molecules Water, ethanol

facilitated diffusion Through channel and carrier proteins Ions, glucose

active transport Through carrier proteins Ions

Membrane diversity

Nucleus: Coordinates activities of the cell

Cytoplasm: Material outside the nucleus that includes the cytosol and other organelles

ER: Helps distribute materials throughout the cell

Golgi Apparatus: Systems of the membrane that package and store substances before their release

Nucleolus: Involved in the manufacture of proteins within the cell

Vacuoles: Temporary storage sacks

Cytosol: Fluid surrounding the organelles

Mitochondrion: Site of cellular respiration

Importing and exporting across membranes

At times large particles or even whole cells have to be moved into a cell across its plasma membrane, Sometimes the opposite is applied and large particles and whole cells have to be moved out of a cell

The large size these particles makes movement through the membrane by diffusion or active transport impossible. Vesicles transport such molecules across the membrane, outwards in the case of exocytosis and inwards in the case of endocytosis

During exocytosis, a small membrane-bound vesicle moves through the cytoplasm the plasma membrane. The vesicle fuses with the plasma membrane and then releases its contents to the exterior of the cell, During endocytosis, the plasma membrane is drawn inwards and engulfs particles which then stores or transports the material within the cytoplasm

Endocytosis and exocytosis are both examples of bulk transport (active processes)

Page 6

Recall, recap & revision

The phospholipid bilayer and embedded membrane proteins control the movement of substances into and out of organelles and between the cell and its external environment

The plasma membrane is differentially permeable- it allows some substances through and not others

The physical and chemical nature of a substance determines the way in which it will move across the plasma membrane-either freely or with the assistance required

Polarity allows polar ( hydrophilic) substances to dissolve in water ( polar) but not in non-polar substances. Non-polar (hydrophobic) substances can dissolve in other non-polar substances

Transport proteins facilitate the movement of substances across membranes

Energy is not directly required for the movement of substances through channel proteins and carrier proteins. Energy is required for the movement of substances through ATP-powered pumps

The specific set of transport proteins determines which ions or molecules can cross a membrane

Exocytosis and endocytosis move very large particles across a plasma membrane

In exocytosis a cytoplasmic vesicle fuses with the plasma membrane, releasing the vesicle contents outside the cell

In endocytosis a small patch of the plasma membrane sinks inward and seals back on itself forming vesicles inside the cytoplasm

Recap 1.3

Distinguish between intracellular and extracellular

Explain why the plasma membrane needs to be able to allow some substances to pass through it

In terms of polarity, explain the expression 'like dissolves in like' Name two types of substances that can diffuse through lipid bilayers and two types of substances that cannot move directly through

Name two types of substances that can diffuse through lipid bilayers and two types of substances that cannot move directly through

Recap 1.4

Create a table naming the three types of membrane transport proteins. In the table describe their structure, the type of movement, and if energy is required for movement

In what way can the permeability of a plasma membrane be likened to a sieve? In what way is this analogy insufficient to explain membrane permeability?

Explain how specific membrane transport proteins maintain the diversity of the internal environment of different cell organelles

Recap 1.5

outline one similarity and one difference between endocytosis and exocytosis

Distinguish between endocytosis, phagocytosis and pinocytosis

Revision

https://quizizz.com/join/game/U2FsdGVkX1%252BqXyHAM3LH7h4cfYal6bB1Cndt5s2iCpKOjzSk%252BqAdP%252F%252B%252B1JV8DuJp?gameType=solo

https://quizizz.com/admin/quiz/583e090731a203003b0597d0/passive-and-active-transport

http://www.phschool.com/science/biology_place/biocoach/biomembrane1/quiz.html

Page 7

Organelles in action

Within the cell, various organelles work in collaboration to move substances from one part of the cell to another and prepare it for export from the cell. Proteins are the workplaces of the cell they are important in structures and in moving molecules across the plasma membrane

To understand the role played by various cell compartments we will follow the journey of protein molecules in the secretory pathway

The nucleus

The nucleus is one of the most prominent structures in the eukaryotic cell. It is the information centre of the cell and controls all activities of the cell because it controls the production of proteins, The code that directs all activities of the cell is found in the nucleoid acid, DNA, in the chromosomes

The nucleus is contained within a double membrane called the nuclear envelope, The outer membrane continuous with the endoplasmic reticulum (ER), another membrane organelle that helps distribute materials throughout the cells. The nuclear envelope contains numerous openings called molecular pores. These pores are channels through which water-soluble molecules can move between the nucleus and the cytoplasm

Structures known as nucleoli are present in the nucleus. These are responsible for the synthesis of ribosomal RNA (rRNA) and the assembly of ribosomes from rRNA and proteins. Messenger RNA (mRNA) molecules carry the coded instructions for assembling proteins, which catalysed by ribosomes

Ribosomes synthesis proteins

Ribosomes are extremely small organelles that exist free-floating in the cytosol of cells or attached to membranes the endoplasmic reticulum. They are abundant in cells, reflecting the work that they do in producing a constant supply of different kinds of proteins

Ribosomes consist of two subunits that are composed of the nucleic acid RNA and various proteins, In eukaryote cells, ribosomal RNA (rRNA) molecules are synthesised in the nucleolus. They pass through the nuclear pores into the cytosol. They begin the process of synthesizing polypeptides from amino acids according to the instructions given by mRNA copied from DNA within the nucleus

Cells with high rates of protein synthesis have prominent nucleoli and many ribosomes

Endoplasmic reticulum

The endoplasmic reticulum (ER) is an interconnecting system of thin membranes sheets dividing the cytoplasm into compartments and channels. The membrane of the ER is able to pinch off into small vesicles and deliver proteins to all parts within the cell ( The ER is an intracellular transport system)

Most of the ER in cells is studded with ribosomes and thus is known as rough endoplasmic reticulum (rough ER). Proteins produced by ribosomes attached to the rough ER can move about the cell in vesicles that bud off from the ER. Proteins produced in the rough ER can also be exported, transported to the extracellular side of the plasma membrane, or secreted into other cells. ( The ER is also an intercellular transport system)

Only proteins that are destined to remain embedded in an organelle membrane or the plasma membrane, or be exported from the cell, are produced by ribosomes on the rough ER. Cytoplasmic proteins are synthesised by free ribosomes in the cytosol

Some parts of the ER has no ribosomes attached and this area is known as the smooth ER, its main role is transport cells it also detoxifies drugs

Golgi apparatus

The Golgi apparatus/body puts on the final touches to enzymes and its packaged and stored before being secreted from the cell, The Golgi apparatus consists of a system of membranes, parts of the Golgi apparatus membrane are able to pinch off into small vesicles. These vesicles can move to the plasma membrane and discharge their contents to the outside of the cell

The secretory pathway

Protein-building instructions (mRNA) move from the nucleus into the cytoplasm

The protein chains are manufactured on ribosomes embedded in the membrane of the rough ER

The protein forms and folds inside the ER lumen. Some additional modifications o the protein may also take place

lipids are manufactured in the membrane of the very smooth ER

Vesicles that bud from the ER membrane transport unfinished proteins and lipids to a Golgi apparatus

Proteins and lipids take on their final form inside the Golgi apparatus

Vesicles budding from the Golgi membrane transport finished products to the plasma membrane where they are released by exocytosis

Page 8

Recall, recap & revision

In the secretory pathway, cellular products move through a cellular transport system,are packaged and are then released from the cell through the plasma membrane

Recap 1.6

Explain the role of the nucleus in the secretory pathway

Describe the structure and function of ribosomes

Describe what the endoplasmic reticulum and Golgi apparatus have in common and describe how they differ functionally

Distinguish between rough ER and smooth ER

Revision

https://www.purposegames.com/game/3c217b49

https://www.biologycorner.com/quiz/qz_cell.html

https://www.quia.com/quiz/717688.html

Page 9

Concept summary

How do cells maintain life?

Phospholipid bilayer

This flexible structure allows the cell to grow, expand, contract, break, and reassemble

The two-dimensional fluid is made up of a double layer of phospholipid molecules

The fluid mosaic model

The phospholipid had is dissolved in water, whereas the hydrophobic tails are forced to face inwards towards each other

Individual phospholipid molecules are highly mobile within the membrane

Specialised protein molecules are embedded in the bilayer: transmembrane proteins, receptors/ligands, glycoproteins, proteins for recognition, adhesion and transport

Cholesterol (animals) and phytosterol (plants) increase membrane flexibility

Polarity: like dissolves in like

Hydrophillic solutions: Polar solvent + Polar solute = solution

Hydrophobic solutions: Non-polar solvent + non-polar solute = solution

Movement of substances through membranes

The plasma membrane is differentially peremable, size, charge and polarity affect permeability

Transport across membranes

Simple diffusion: occurs through phospholipid molecules, such as water and ethanol

Facilitated diffusion: Occurs through channel proteins and carrier proteins such as ions and glucose

Active transport: Occurs through carrier proteins, such as ions; requires ATP

Bulk transport

Exoyosis

Endocytosis- includes phagocytosis ( solids) and pinocytosis ( liquids)

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Chapter glossary

Active transport: The process whereby cells actively transport substances across a membrane against a concentration gradient (from a low concentration gradient to a higher concentration of the substance) consumes energy

Adenosine triphosphate (ATP): A high-energy compound composed of adenine and ribose with a chain of three phosphate groups attached; it releases energy for cellular actions when its last phosphate group is removed and it is converted to ADP

Adhesion proteins: Proteins on the surface of cells are involved binding with other cells or to an extracellular matrix in a process called cell adhesion

Amino acids: A nitrogen-containing compound that is the building block of proteins

Aquaporin: A type of membrane protein that facilitates e transport of water

ATP-powered pump: A type of membrane protein that uses the energy from ATP to move molecules across the membrane ain't a concentration gradient

Bulk transport: The transport of large quantities of materials into or out of the cytoplasm all at the one time

Carrier protein: A protein within a membrane that assists other molecules to cross the membrane in facilitated and active transport

Channel protein: A protein that forms a channel within a membrane to allow the passage of substances across the membrane

Cholesterol: A type of lipid found in animal cell membranes that stabilises membrane fluidity

Chromosomes: A thread-like structure made of nucleic acids and proteins that encode genetic information

Cristae: The folding of the inner membranes into the matrix of a mitochondrion, thus increasing the total surface area of the inner membrane

Cytoplasm: The material outside the nucleus that includes the cytosol and other organelles

Cytoskeleton: A network of filaments within a eukaryotic cell that provide structural support, anchorage, shape, motility and the capacity to move and arrange organelles within the cell

Cytosol: The fluid part of the cytoplasm surrounding the organelles

Differentially permeable: Describes a membrane that allows some substances but not others to pass across it

Diffusion: The passive movement of molecules from a high to a low concentration of those substances

Endocytosis: The movement of solids or liquids into a cell from the environment via vesicle information

Endoplasmic reticulum (ER): An organelle in eukaryotic cells consisting of an interconnecting system of thin membrane sheets

Exocytosis: The movement of solids or liquids from a cell to the environment via vesicle fusion with the plasma membrane

Extracellular fluid: All fluid outside the cells in multicellular organisms

Facilitated diffusion: A form of diffusion that requires a substance to be attached to a specific carrier molecule to move across the membrane

Fluid mosaic model: A model that explains the structure and function of the plasma membrane

Glycoprotein: A protein with an attached carbohydrate group

Golgi apparatus: A collection of membranes that package and store substances into vesicles in preparation for their release from the cell

Hydrogen bond: A weak chemical bond between a hydrogen atom on one molecule and a second, more electronegative element, usually an oxygen-nitrogen atom, on another molecule

Hydrophilic: Describes substances such as polar molecules d ionic compounds that dissolve readily in water

Hydrophobic: Describes substances such as non-polar molecules that are insoluble in water

Intercellular: Occurring between cells

Intracellular: Occurring within a cell or cells

Ion: Atom or molecules with an overall positive or negative charge

Ligand: A molecule that binds to a receptor

Messenger RNA (mRNA): RNA copied from DNA that conveys the instructions needed for polypeptide synthesis from the nucleus to the cytoplasm

Non-self: A molecule that is not recognised by the immune system as being part of the organism itself

Nuclear envelope: The double membrane that surrounds the nucleus in eukaryotic cells and separates DNA from the cytosol

Nuclear pore: An opening in the nuclear envelope

Nucleolus: A granular structure within the nucleus where ribosomal RNA is transcribed and ribosome subunits are assembled

nucleus: A membrane-bound compartment in eukaryotic cells that contains the chromosomal DNA

Phagocytosis: The bulk transport of solids into a cell inside a vesicle

Phospholipid bilayer: The two layers of phospholipids that form a plasma membrane

Phytosterol: A type of lipid found in plant cell membranes

Pinocytosis: The bulk transport of lipids into a cell inside vesicles

Plasma membrane: The insoluble boundary of the living cell that maintains the contents of the cell and regulates the movement of substances into and out of the cell. All cells have a plasma membrane

Polarity: A term that refers to a molecule having distinct regions opposite charge

Polypeptide: A liner polymer built from amino acid monomers

Receptor: A molecule on the surface or interior of the cell that binds specifically to a substance (ligand) to detect or receive a stimulus

Recognition proteins: A protein that acts as a marker on membranes

Ribosomal RNA (rRNA): An RNA strand that serves as a structural component of a ribosome

Ribosome: A small structure comprising RNA and proteins where amino acids are joined to form polypeptides

Rough endoplasmic reticulum (rough ER): ER with ribosomes attached

Secretory pathway: The movement or proteins produced ribosomes attached to the ER, through a series of compartments and vesicles, via the Golgi apparatus, to secretory vesicles for export from the cell by exocytosis

Smooth endoplasmic reticulum (smooth ER): ER with no ribosomes

Transmembrane protein: A protein with one or more regions that span the membrane

Transport protein: A protein that carries molecules across membranes

Vesicles: A small, membrane-bound sac in the cytoplasm that transports, stores or digests substances

Chapter review questions

Remembering

Name the molecules responsible for each os the following features of the plasma membrane

a. Ability to change shape

b. Flexibility

c. Ability to reseal a puncture

2. Explain why a plasma membrane described as a 'lipid bilayer'

3. Name the four types of membrane proteins and construct a table that summarise their functions.

4. Define 'hydrophobic' and 'hydrophilic'

5. Name two methods of membrane transport that do not require protein channels or carriers

Understanding

6a. Describe the part of the plasma membrane describes as fluid. Which part is described as a mosaic?

6b. Explain why these terms are used

7. Plasma membranes are selectively permeable. what does this mean?

8. Explain why certain white blood cells are known as phagocytes.

9. Explain the role of the Golgi apparatus in the transport of materials out of the cell. Describe the kinds of materials it packages.

Applying

10. Write a few sentences to explain the process by which carbon dioxide might move from the external environment into a cell

11. By means of an annotated diagram, relate the structure of the plasma membrane to its function

12. Predict the function of a cell that contains more rough ER than smooth ER

13. Explain the concept of 'like dissolves in like' and the significance of this to molecules in the cell

14 The effect of drinking a glass of wine can be felt fairly quickly

a. Predict whether alcohol (ethanol) is water-or lipid-soluble. suggest how it moves through the plasma membrane

b. Caffeine is a polar molecule. How would the speed of its effects compare with that of ethanol?

15. Radioactively labelled amino acids were supplied to a pancreatic cell that produces digestive enzymes to be released into the digestive system

a. In which organelles of the cell would the amino acids subsequently be detected? List them in order.

b. In what form would they appear in these organelles? you might like to present your answers as a diagram

Analysing

16. when viewed with an electron microscope, small sac-like vesicles are seen to form inside a cell by budding rom the plasma membrane, then detach rom the plasma membrane

a. Name these vesicles

b. Describe the function of these structures

c. Name the process described

d. An observer described the process as pinocytosis. what would you say to argue for or against this observation

17. Explain what a hydrophobic substance does not readily dissolve in water

18. Compare and contrast the structure and role of membrane transport proteins

19. Discuss reasons why dfferent types of cell organelles have different types of transport proteins in their membranes