Biology: Topic 1: Cell Biology Flashcards
(159 cards)
1
Q
What are living organisms composed of?
A
Composed of cells.
2
Q
Outline cell theory (3 points).
A
1) Cells are the basic unit of life.
2) New cells are formed from pre-existing cells.
3) Cell and cell products make up all structures in living cells.
3
Q
What are the 7 functions of life?
A
Metabolism Respiration Sensitivity Homeostasis Growth Reproduction Excretion Nutrition
4
Q
What are common features of cells? 4 points.
A
- Surrounded by a membrane.
- Contain genetic material.
- Chemical reactions occurring (catalyzed by enzymes)
- Have their own energy release system for power.
5
Q
Why can’t the cell theory be proven true?
A
It would require us to examine every single cell which is impossible.
6
Q
How can cell theory be proven false?
A
If someone were to find a discovery that violates the existing cell theory.
7
Q
What 3 cells fit the cell theory even though they disapprove cell theory?
A
- Skeletal Muscle.
- Giant Algae.
- Multi Nucleated fungi cell.
8
Q
What are unicellular cells?
A
- Also known as single-celled organisms.
- Organisms that consists of one cell only.
9
Q
Definition of metabolism?
A
The chemical reactions that occur in a cell, including cell respiration to release energy.
10
Q
Definition of Hydrolysis?
A
Break down of Water
11
Q
How do you calculate magnification?
A
M = I/A
12
Q
Are cell walls found in animal or plant cells?
A
Plant cells only.
13
Q
How many functions of life do unicellular cells carry.
A
All 7; Metabolism, respiration, sensitivity, homeostasis, growth, reproduction, excretion, nutrition.
14
Q
What are functions of life?
A
They are things that all organisms must do to stay alive.
15
Q
What are chemical reactions in the cell known as?
A
Metabolism.
16
Q
What are the rate of chemical reactions in the cell proportional to?
A
The volume.
17
Q
What happens if the ratio of the cell is too small?
A
1) Substances will not enter the cell as quickly as they are required and waste products will gather together (Accumulate) because they are produced more rapidly.
2) Cells may overheat because the metabolism produces heat faster than it is lost over the cell’s surface.
18
Q
What is cell differentiation?
A
The process by which a cell becomes specialized in order to perform a specific function.
19
Q
How does cell differentiation work?
A
Differentiation is the process during development whereby newly formed cells become more specialised and distinct from one another as they mature.
20
Q
What are 3 uses of stem cells?
A
- Embryonic stem cells.
- Cord blood stem cells.
- Adults stem cells.
21
Q
What are embryonic stem cells used for?
A
… embryonic stem cells can be guided into becoming any type of cell that can be used to repair and regenerate diseased of damaged tissues in people.
22
Q
How are stem cells used to treat Stargardt’s syndrome?
A
Stem cells replace the photo receptors.
23
Q
How are stem cells used to treat leukemia?
A
Stem cells help stimulate new bone marrow growth and restore the immune system.
24
Q
What is a cell?
A
- Smallest unit of life.
- The cell is the basic structural, functional, and biological unit of all known living organisms.
25
Are prokaryotic cells unicellular or multicellular?
Unicellular.
26
What do human check cells consist of? (Structure)
1. plasma membrane
2. cytoplasm
3. nucleus
4. mitochondria
27
What do moss leaf cells consist of? (Structure)
1. cell wall
2. plasma membrane
3. cytoplasm
4. chloroplasts
5. sap in vacuole
6. nucleus
7. vacuole membrane
28
Why does skeletal muscle not fit the cell theory?
1. made up of muscle fibres which are enclosed in a membrane (fits)
2. but they are much larger than most cells - 300mm+ (doesn't fit)
3. and they contain hundreds of nuclei (doesn't fit)
29
Why does giant algae not fit the cell theory?
1. can grow to 100mm but are not multicellular (doesn't fit) since they only contain one nucleus, rather than hundreds of nuclei
30
Why does aseptate fungi not fit the cell theory?
1. consists of thread like structures called hyphae
2. hyphae not divided up into subunits by plasma membrane
3. but consist of long undivided sections which contain many nuclei (doesn't fit)
31
How many cells in a unicellular organism?
1. One
32
Give two examples of unicellular organisms.
1. Paramecium
| 2. Chlamydomonas
33
What happens to a cells surface area to volume ratio as it grows larger?
1. The SA:V decreases
34
What does the surface area of a cell determine?
1. The rate at which materials enter/leave the cell
35
What does the volume of a cell determine?
1. the rate at which materials are used or produced
36
What happens if a cell becomes too large?
1. it may not be able to take in essential materials or excrete waste substances quickly enough
37
Why are large organisms multicellular?
Understand why. 1. SA:V decreases as a cell increases 2. if a cell becomes too large, it may not be able to take in essential materials or excrete waste substances quickly enough 3. the cell can no longer survive at this size, so it divides 4. large organisms therefore consist of many cells (multicellular)
38
Name two advantages of being a multicellular organism.
1. cells can take in/excrete essential materials/ waste substances fast enough 2. multicellular allows division of labour, which means different groups of cells can become specialised for different processes
39
Through what process does a cell become specialised?
1. the process of differentiation
40
What do emergent properties arise from?
1. the interaction of the component parts of a complex structure
41
Define genome.
1. an organisms entire set of genes
42
In multicellular organisms, 100% of the genome is in each cell. Why is this significant?
1. each cell has the instructions to develop into any type of cell
43
Define stem cells. What are their two key qualities?
1. cells that have the capacity to divide and differentiate along different pathways - Self renewal: They can continuously divide and replicate - Potency: They have the capacity to differentiate into specialised cell types
44
Where are adult stem cells found?
1. most human tissues including: skin, liver and bone marrow
45
Give two examples of diseases that can be treated with stem cells.
1. Stargadt's muscular dystrophy (embryonic stem cells) 2. Leukemia (adult stem cells)
46
What is Stargadt's muscular dystrophy?
1. a genetic disease causing loss of vision
47
What sort of disease is leukemia? What happens when a person has leukemia?
1. cancer 2. abnormally large numbers of white blood cells are produced in the bone marrow
48
How is leukemia treated?
1. adult stem cells used 2. large needle inserted into a large bone, usually the pelvis and fluid is removed from the bone marrow 3. stem cells are extracted from this fluid and are stored by freezing them. They are adult stem cells so only have potential to produce blood cells 4. high dose of chemotherapy drugs to kill all cancer cells in bone marrow. Bone marrow loses its ability to produce blood cells. 5. Stem cell then returned to patient's body. The cells re-establish themselves in the bone marrow, multiply and start to produce red and white blood cells. 6. In many cases this procedure cures leukemia completely.
49
How is Stargadt's muscular dystrophy being treated using stem cells?
researchers have developed methods for making.... 1. embryonic stem cells develop into retina cells (remember that embryonic stem cells can differentiate along any pathway)
50
Define resolution.
1. is the ability of the microscope to show two close objects separately in the image
51
What does resolution depend on?
1. the wavelength of the rays used to form the image 2. the shorter the wavelength, the higher the resolution
52
Cells are divided into two types. Based on what? What are the two types?
1. based on structure 2. prokaryotic and eukaryotic
53
Cells are divided into two types based on their structure. Compare the two structures. (How are they different)
1. eukaryotic cells are divided up by membranes into separate compartments such as the nucleus and mitochondria 2. whereas prokaryotic cells are not compartmentalized.
54
Do mitochondria have a nucleus?
1. NO 2. they have a nucleoid region - it is not bound by a membrane therefore it is not a nucleus
55
What can surface area to volume ratio help to explain?
1. why cells divide 2. many other phenomena kind of a useless card but could be something to consider if they ask you to explain "why cells divide" or 'some other phenomena'.
56
Name the process prokaryotic cells undergo to divide.
1. binary fission (meaning splitting in two)
57
What can you observe in a eukaryotic cell using a light microscope? How about an electron microscope?
1. light: cytoplasm enclosed in plasma membrane 2. electron: ultrastructure is visible
58
Name all the organelles in a eukaryotic cell.
Organelle with single membrane: 1. rough endoplasmic reticulum 2. smooth endoplasmic reticulum 3. lysosome 4. Golgi apparatus; vesicles and vacuoles Organelle with double membrane: 5. nucleus 6. mitochondrion 7. chloroplast
59
What is the role of the lysosome?
1. site of hydrolysis / digestion / breakdown of macromolecules
60
What is the Golgi apparatus?
1. an assembly of vesicles and folded membranes involved in the sorting, storing and modification of secretory products
61
Give similarities of prokaryotic and eukaryotic cells.
1. both have a cell membrane 2. both contain ribosomes 3. both have DNA and cytoplasm
62
Give differences between prokaryotic and eukaryotic cells.
1. size - eukaryotic bigger
2. ribosomes - eukaryotic bigger
3. reproduction - eukaryotic can reproduce sexually as well as asexually
4. membrane-bound organelles - eukaryotes have them
5. DNA (4 things, found in first chapter of genetics)
63
State 2 models of membrane structure? Which one superseded the other (and is therefore the one we use today)?
1. Davson-Danielli Model
| 2. Singer-Nicolson Model Singer-Nicolson superseded Davson-Danielli and is therefore the one we use today.
64
Outline the membrane structure in the Davson-Danielli Model.
- bilayer of phospholipids in centre of membrane - protein on either side (like phospholipid sandwich)
65
Give three examples of evidence that did not fit with the Davson-Danielli Model.
1. freeze-fracture electron micrographs 2. analysis of membrane protein 3. fusion of cells with membrane proteins tagged with different coloured flourescent markers
66
What does the fluid mosaic model show?
cell membrane structure (of animal cell because it includes cholesterol, more on that later)
67
In the fluid mosaic model diagram, how are phospholipids shown?
phospholipids: - oval (phosphate head) - two parallel lines attached (fatty acid tails)
68
What is the biological term for 'not attracted to water'? What is the biological term for 'attracted to water'?
- hydrophobic - hydrophilic
69
What is the term that describes a molecule which has one part attracted to water, and one part not attracted to water?
amphipathic
70
What part of a phospholipid is attracted to water?
Hydrophilic Head
71
What part of a phospholipid is not attracted to water?
Hydrophobic Tail
72
What is a structure of a phospholipid?
- Consist of a polar head (hydrophilic) composed of a glycerol and a phosphate molecule
- Consist of two non-polar tails (hydrophobic) composed of fatty acid (hydrocarbon) chains
- Because phospholipids contain both hydrophilic (water-loving) and lipophilic (fat-loving) regions, they are classed as amphipathic
73
How do prokaryotes divide? and what is it used for?
1) By binary fission.
| 2) asexual reproduction
74
How does binary fission work in prokaryotes?
1) The singular circular chromosome is replicated and two copies of chromosomes move to opposite ends of the cell.
2) Division of the cytoplasm of the cell quickly follows.
3) Each of the daughter cells contains one copy of the chromosome so they are genetically identical.
75
What are the 4 components of the plasma membrane?
1 - Phospholipids
2 - Cholesterol
3 - Proteins
4 - Carbohydrates
76
What is the cell membrane?
It is a barrier that separates a cell from its surrounding environment.
77
What is the fluid mosaic model?
1) The fluid mosaic model describes the structure of a cell membrane.
78
Cholesterol is a type of .....?
....Lipid but is not a fat or oil.
79
What group of substances do lipids belong to?
Steroids.
80
Where are is cholesterol positioned on membranes?
Positioned between phospholipids.
81
What is the importance of cholesterol in the cell membrane?
1) One role of cholesterol is to help give the cell membrane extra support.
2) Cholesterol helps to immobilize some of the lipid molecules around them.
82
What are the 7 modes of transport across the membrane?
1) Diffusion,
2) Osmosis
3) Active Transport
4) Facilitated Diffusion
5) Endocytosis
6) Exocytosis
7) Movement of vesicles within the cell
83
What is the cell cycle?
```
Interphase
Prophase
Metaphase
Anaphase
Telaphase
E.C
```
84
What are adult stem cells used for?
Used to treat dozens of diseases and conditions.
85
What happens during interphase?
An active period in the life of a cell when many metabolic reactions occur, including protein synthesis, DNA replication, and an increase in # of mitochondria and chloroplasts.
86
What happens during prophase?
Chromatin supercoils to become chromosomes. Proteins called histones help w supercoiling. Nuclear mem. breaks down.
87
What happens during metaphase?
Microtubules grow and attach to centromeres of each double stranded chromosome. Chromosomes line in at equator (middle) of the cell.
88
What happens during anaphase?
Each centromere divides. Sister chromatids separate. Spindles pull single stranded chromosomes towards the two poles of the cell.
89
What happens during telaphase?
Chromosomes pulled into tight group near MTOC (microtubule organizing centre). Nuclear membrane reforms. Chromosomes uncoil to form chromatin. Nucleolus forms. UNSEPARATED cell.
90
What is cytokinesis?
Occurs after mitosis. Different in plant and animal cells. PHYSICAL CELL DIVISION.
91
What is the movement of vesicles within the cell?
Transport vesicles can move molecules between locations inside the cell, e.g., proteins from the rough endoplasmic reticulum to the Golgi apparatus.
92
What is mitosis?
The division of the nucleus into two genetically identical daughter nuclei. Growth, embryonic development, tissue repair and asexual reproduction involve mitosis.
93
What happens during prophase?
- Chromosomes become shorter and more condensed in the process called supercoiling.
- The nuclear envelope begins to break down and disintegrate.
- Microtubules that form the mitotic spindle begin to develop from the centrosomes in the cell.
- Centrosomes move towards the poles as the spindle grows and lengthens.
94
What happens during metaphase?
- The spindle fibers are attached to the centromeres of the chromosomes.
- Chromosomes move towards the equator of the cell and line up along the metaphase plate.
- The other ends of the microtubules of the spindle are attached to poles of the cell.
95
What happens during anaphase?
- The pairs of sister chromatids are pulled apart by the spindle fibers towards the poles.
- The chromatids are now considered chromosomes.
- The chromosomes move to the poles as a result of the shortening of the microtubule.
- After anaphase the cell now has two genetically identical nuclei at each end of the cell.
96
What happens during telophase?
- Nuclear membranes now begin to form around each set of chromosomes.
- Chromosomes begin to uncoil to form chromatin again.
- The spindle fibers break down and nucleoli reform in each nucleus.
- The cell elongates and gets ready for cytokineis.
97
How does cytokinesis happen in animal cells?
Ring of contractile
| Proteins pull inwards to form cleavage furrow. Squeezes centre of cell until two cell break apart from each other.
98
How are tumours created?
Cell division that happens too quickly can cause tumours.
99
What is meiosis?
It is the process of reduction division in which the number of chromosomes per cell is cut in half throughout the separation of homologous chromosomes in a diploid cell.
100
What is the difference between anaerobic and aerobic?
One requires oxygen and the other doesn't.
101
How many daughter cells are produced that are haploid?
Four
102
What is a diploid number and what is a haploid number?
A diploid number is a term used to a cell that contains both sets of homologous chromosomes. A haploid cell that contains only a single set of chromosomes.
103
What are Gametes?
Gametes are egg and sperm.
104
What are Alleles?
One of the numbers of the different forms of a genes.
105
What are Chromatids?
A chromatid is one of the two identical sister parts of a duplicated chromosome.
106
What is Homologous?
A term used to refer to chromosomes that each have a corresponding chromosome from the opposite sex parent.W
107
Difference between electron and light microscopes.
Electron microscope: Uses beams of electrons
Light microscope: Rays of visible light to form highly magnified images of tiny areas materials or biological specimens.
108
Difference between electron and light microscopes.
Electron microscope: Uses beams of electrons
Light microscope: Rays of visible light to form highly magnified images of tiny areas materials or biological specimens.
109
What is the Davson-Danielli model? 4 main features.
1) Protein-lipid sandwich
2) Lipid bilayer composed of phospholipids (hydrophobic tails inside, hydrophilic heads outside)
3) Proteins coat outer surface (PROVED WRONG)
4) Proteins do not permeate lipid bilayer (PROVED WRONG)
110
How did they falsify the davson-danielli model?
Using freeze fracture
111
What is hypertonic solution?
Is a solution with a higher osmolarity (higher solute concentration) then the other solution. If cells are placed into a hypertonic solution, water will leave the cell causing the cytoplasm’s volume to shrink and thereby forming indentations in the cell membrane.
112
What is hypotonic solution?
Is a solution with a lower osmolarity (lower solute concentration) then the other solution. If cells are placed in a hypotonic solution, the water will rush into the cell causing them to swell and possibly burst.
113
When does cytokinesis occur?
After mitosis and is different in plant and animal cells.
114
What is the interphase?
Interphase is an active period in the life of the cell in which many metabolic reactions occur, including protein synthesis, DNA replication and production or mitochondria and/or chloroplasts.
115
What are the three levels of magnification on a typical high school microscope?
- x40 (low power)
- x100 (medium power)
- x400 (high power)
116
What is the formula to calculate magnification?
size of image/actual size of specimen
117
Limitations on cell size (5)
- surface area to volume ratio is important in the limitation of cell size
- large numbers of chemical reactions take place in the cytoplasm of the cells (metabolism)
- the rate of the reactions (metabolic rate) is proportional to the volume of the cell
- substances used for the reactions must be absorbed by the cell and the waste products must be removed; the rate at which substances cross this membrane depends on its surface area
- surface area:volume ratio is also important for heat production and loss
118
Outline the ways in which Paramecium demonstrates the functions of life (6)
- Paramecia are surrounded by small hairs called cilia which allow it to move (responsiveness)
- Paramecia engulf food via a specialised membranous feeding groove called a cytostome (nutrition)
- Food particles are enclosed within small vacuoles that contain enzymes for digestion (metabolism)
- Solid wastes are removed via an anal pore, while liquid wastes are pumped out via contractile vacoules (excretion)
- Essential gases enter (e.g. O2) and exit (e.g. CO2) the cell via diffusion (homeostasis)
- Paramecia divide asexually (fission) although horizontal gene transfer can occur via conjugation (reproduction)
119
Outline the ways in which Scenedesmus demonstrates the functions of life (4)
- Scenedesmus exchange gases and other essential materials via diffusion (nutrition / excretion)
- Chlorophyll pigments allow organic molecules to be produced via photosynthesis (metabolism)
- Daughter cells form as non-motile autospores via the internal asexual division of the parent cell (reproduction)
- Scenedesmus may exist as unicells or form colonies for protection (responsiveness)
120
Emergent properties (4)
- multicellular organisms have properties that emerge from the interaction of their cellular components
- multicellular organisms can be regarded as cooperative groups
- the characteristics of the whole organism, including the fact that it is alive, are known as emergent properties
- emergent properties arise from the interaction of the component parts of a complex structure
121
Cell differentiation (5)
- specialized tissues can develop by cell differentiation in multicellular organisms
- different cells perform different functions
- often a group of cells specialize in the same way to perform the same function (tissue)
- the development of cells in different ways to carry out specific functions is called differentiation
- involves the expression of some genes and not others in a cell's genome (cells have all genes needed to specialize in every possible way but only expresses certain ones)
122
Stem cell (3)
- can divide again and again to produce copious quantities of new cells; good for the growth of tissues or the replacement of cells that have been lost/damaged
- not fully differentiated; can differentiate in different ways to produce different cell types
- essential in embryonic development for the above reasons
123
What is an example of a non-therapeutic use for embroynic stem cells
- produce large quantities of striated muscle fibres/meat for human consumption
124
Resolution
Making the separate parts of an object distinguishable by eye
125
Electron microscopes (3)
- much higher resolution than light microscopes
- reveal the ultrastructure of cells
- needed to see viruses with diameter of 0.1 micro-metres
126
Prokaryotes (6)
- simple cell structure without compartments
- no nucleus; has nucleoid instead which contains DNA
- DNA not associated with proteins
- cell wall
- do not have crytoplasmic organelles apart from ribosomes
- ribosomes are 70S (smaller than those in eukaryotoes)
127
How do prokaryotes divide? (5)
- binary fission
- used for asexual reproduction
- circular chromosome is replicated and the two copies of the chromosome move to opposite ends of the cell
- division of the cytoplasm
- each of the daughter cells contains one copy of the chromosome so they are genetically identical
128
Eukaryotes (5)
- compartmentalized cell structure, meaning that the cell is divided up by single or double membranes into compartments
- has nucleus
- DNA associated with histone proteins
- organelles in the cytoplasm ('compartments')
- ribosomes are 80S (bigger than those in prokaryotes)
129
Hydrophilic
- substances that are attracted to water
- all substances that dissolve in water are hydrophilic, including polar molecules such as glucose and chloride ions
- substances that water adheres to (ie. cellulose) are also hydrophilic
130
Hydrophobic
- substances that are insoluble in water, but dissolve in other solvents
- molecules that are non-polar are hydrophobic
- all lipids are hydrophobic
131
Amphipathic
Substances that are partially hydrophilic and partially hydrophobic (ie. phospholipid)
132
7 ways of transportation across the membrane
- Active Transport
- Endocytosis
- Exocytosis
- Movement of vesicles
- Movement of vesicles
- Diffusion
- Facilitated Diffusion
- Osmosis
133
4 types of stem cells and functions
Totipotent – Can form any cell type, as well as extra-embryonic (placental) tissue (e.g. zygote)
Pluripotent – Can form any cell type (e.g. embryonic stem cells)
Multipotent – Can differentiate into a number of closely related cell types (e.g. haematopoeitic adult stem cells)
Unipotent – Can not differentiate, but are capable of self renewal (e.g. progenitor cells, muscle stem cells)
134
What kingdom do prokaryotes belong to and what two domains?
Monera
Archaebacteria
Eubacteria
135
Binary Fission and process
Form of asexual reproduction used by prokaryotic cells
- The circular DNA is copied in response to a replication signal
- The two DNA loops attach to the membrane
- The membrane elongates and pinches off (cytokinesis), forming two cells
136
Prokaryotes feature (9)
```
Cytoplasm
Nucleoid
Plasmids
Ribosomes
Cell membrane
Cell wall
Slime capsule
Flagella
Pili
```
137
What four kingdoms can eukaryotes belong too
Protista
Fungi
Plantae
Animalia
138
Function and Structure of Lysosome
Structure: Membranous sacs filled with hydrolytic enzymes
Function: Breakdown / hydrolysis of macromolecules (presence in plant cells is subject to debate)
139
Function and Structure of Cell Wall
Structure: External outer covering made of cellulose (not an organelle per se, but a vital structure)
Function: Provides support and mechanical strength; prevents excess water uptake
140
Function and Structure ofVacuole (large and central)
Structure: Fluid-filled internal cavity surrounded by a membrane (tonoplast)
Function: Maintains hydrostatic pressure (animal cells may have small, temporary vacuoles)
141
Function and Structure of chloroplast
Structure: Double membrane structure with internal stacks of membranous discs (thylakoids)
Function: Site of photosynthesis – manufactured organic molecules are stored in various plastids
142
Function and Structure of centrosome
Structure: Microtubule organising centre (contains paired centrioles in animal cells but not plant cells)
Function: Radiating microtubules form spindle fibres and contribute to cell division (mitosis / meiosis)
143
Function and Structure of mitochondrion
Structure: Double membrane structure, inner membrane highly folded into internal cristae
Function: Site of aerobic respiration (ATP production)
144
Function and Structure of Golgi apparatus
Structure: An assembly of vesicles and folded membranes located near the cell membrane
Function: Involved in the sorting, storing, modification and export of secretory products
145
Function and Structure of Endoplasmic Reticulum
Structure: A membrane network that may be bare (smooth ER) or studded with ribosomes (rough ER)
Function: Transports materials between organelles (smooth ER = lipids ; rough ER = proteins)
146
Function and Structure of Nucleus
Structure: Double membrane structure with pores; contains an inner region called a nucleolus
Function: Stores genetic material (DNA) as chromatin; nucleolus is site of ribosome assembly
147
Function and Structure of plasma membrane
Structure: Phospholipid bilayer embedded with proteins (not an organelle per se, but a vital structure)
Function: Semi-permeable and selective barrier surrounding the cell
148
Function and Structure of cytoskeleton
Structure: A filamentous scaffolding within the cytoplasm (fluid portion of the cytoplasm is the cytosol)
Function: Provides internal structure and mediates intracellular transport (less developed in prokaryotes)
149
Function and Structure of Ribosomes
Structure: Two subunits made of RNA and protein; larger in eukaryotes (80S) than prokaryotes (70S)
Function: Site of polypeptide synthesis (this process is called translation)
150
Structure of phospholipids
- Consist of a polar head (hydrophilic) composed of a glycerol and a phosphate molecule
- Consist of two non-polar tails (hydrophobic) composed of fatty acid (hydrocarbon) chains
- Because phospholipids contain both hydrophilic (water-loving) and lipophilic (fat-loving) regions, they are classed as amphipathic
151
Phospholipid Arrangement in Membranes:
- Phospholipids spontaneously arrange into a bilayer
- The hydrophobic tail regions face inwards and are shielded from the surrounding polar fluids, while the two hydrophilic head regions associate with the cytosolic and extracellular fluids respectively
152
Properties of the Phospholipid Bilayer
- The bilayer is held together by weak hydrophobic interactions between the tails
- Hydrophilic / hydrophobic layers restrict the passage of many substances
- Individual phospholipids can move within the bilayer, allowing for membrane fluidity and flexibility
- This fluidity allows for the spontaneous breaking and reforming of membranes (endocytosis / exocytosis)
153
2 types of proteins in phospholipid bilayers
- Integral proteins are permanently attached to the membrane and are typically transmembrane (they span across the bilayer)
- Peripheral proteins are temporarily attached by non-covalent interactions and associate with one surface of the membrane
154
Functions of membrane proteins
- Junctions – Serve to connect and join two cells together
- Enzymes – Fixing to membranes localises metabolic pathways
- Transport – Responsible for facilitated diffusion and active transport
- Recognition – May function as markers for cellular identification
- Anchorage – Attachment points for cytoskeleton and extracellular matrix
- Transduction – Function as receptors for peptide hormones
155
Function of cholesterol in phospholipid bilayer
- Cholesterol functions to immobilise the outer surface of the membrane, reducing fluidity
- It makes the membrane less permeable to very small water-soluble molecules that would otherwise freely cross
- It functions to separate phospholipid tails and so prevent crystallisation of the membrane
- It helps secure peripheral proteins by forming high density lipid rafts capable of anchoring the protein
156
Components of a plasma membrane
Phospholipids – Form a bilayer with phosphate heads facing outwards and fatty acid tails facing inwards
Cholesterol – Found in animal cell membranes and functions to improve stability and reduce fluidity
Proteins – May be either integral (transmembrane) or peripheral and serve a variety of roles
157
Fluid-Mosaic Model
Cell membranes are represented according to a fluid-mosaic model, due to the fact that they are:
- Fluid – the phospholipid bilayer is viscous and individual phospholipids can move position
- Mosaic – the phospholipid bilayer is embedded with proteins, resulting in a mosaic of components
158
2 models of membrane structure
- Davson-Danielli - proteins form distinct layers (sandwich)
| - Singer-nicolson - Proteins embedded with bilayer (fluid-mosaic)
159
Basic Structure of phospholipids
- A polar organic molecule (e.g. choline, serine)
- A phosphate group
- A glycerol molecule (replaced by sphingosine in sphingomyelin)
- Two fatty acid tails (may be saturated or unsaturated)
