Biology and Cells Notes
Biology & Cells
Topics of Study (SL and HL)
A2.2 Cell structure [SL/HL]
B2.2 Organelles and compartmentalization [SL/HL]
B2.3 Cell specialisation [SL/HL]
B2.1 Membranes and membrane transport [SL/HL]
HL only:
A2.1 Origins of Cells [HL only]
A2.3 Viruses [HL only]
A2.2 Cell Structure [SL/HL]
Guiding questions:
What are the features common to all cells and the features that differ?
How is microscopy used to investigate cell structure?
Linking questions:
What explains the use of certain molecular building blocks in all living cells?
What are the features of a compelling theory?
B2.2 Organelles and Compartmentalization [SL/HL]
Guiding questions:
How are organelles in cells adapted to their functions?
What are the advantages of compartmentalization in cells?
Linking questions:
What are examples of structure–function correlations at each level of biological organization?
What separation techniques are used by biologists?
B2.3 Cell Specialisation [SL/HL]
Guiding questions:
What are the roles of stem cells in multicellular organisms?
How are differentiated cells adapted to their specialized functions?
Linking questions:
What are the advantages of small size and large size in biological systems?
How do cells become differentiated?
B2.1 Membranes and Membrane Transport [SL/HL]
Guiding questions:
What plausible hypothesis could account for the origin of life?
What intermediate stages could there have been between non-living matter and the first living cells?
Linking questions:
For what reasons is heredity an essential feature of living things?
What is needed for structures to be able to evolve by natural selection?
A2.2.1—Cells as the Basic Structural Unit of All Living Organisms
Cell theory states that all living organisms are made of cells.
It is usually also accepted that:
all cells arise from other cells.
cells are the smallest discrete unit of life.
A2.2.2—Microscopy Skills
Microscopy:
The term “cell” was first used by Robert Hooke (1665) after he looked at cork tissue using a light microscope.
Magnification: How large the image appears.
Resolution: the shortest distance between two points on a specimen that can still be distinguished by the observer as separate objects.
Electron microscope (developed in the1950’s) --- uses a beam of electrons with a wavelength much shorter than the wavelength of visible light --- so can resolve to 0.2 nm (x 1000 better than light microscope).
Microscopy Calculations:
Magnification = \frac{Size \ of \ Image}{Size \ of \ object}
A2.2.3—Developments in Microscopy
Electron Microscopy (EM)
EM can achieve high resolutions down to 0.2nm.
EM can be used to image a wide range of samples, but the preparation of samples kills the cells
EM can either be transmission electron microscopy (TEM) (through a sample) and scanning electron microscopy (SEM) (imaging the surface of a sample).
Freeze Fracture
Freeze fracture reveals the internal surfaces of cells and organelles, and by freezing samples rapidly, freeze fracture can preserve biological structures without introducing artifacts that can happen in EM or staining.
Freeze fracture can be combined with other techniques, such as immunolabeling, to provide additional information about specific components within the sample.
Cryogenic Electron Microscopy (Cryo-EM)
Cryo-EM allows for the determination of high-resolution structures of biological macromolecules, including proteins and protein complexes.
Cryo-EM samples are frozen very quickly, preserving structure and so avoiding the need for staining or fixation of the sample.
Cryo-EM can handle large biological complexes that are difficult to crystallize for X-ray crystallography.
Fluorescent Stains and Immunofluorescence in Light Microscopy
Fluorescent Stains
Fluorescent stains are chemical compounds that bind to specific structures or molecules within a cell and emit light when exposed to specific wavelengths of light.
Fluorescent staining of brain cells:
Immunofluorescence
Immunofluorescence uses antibodies labeled with fluorescent dyes to detect specific proteins or antigens within cells or tissues. This generates better images of the distribution of proteins in a cell that can help in the, understanding of cellular processes, and diagnosis of diseases.
Immunofluorescent staining of bone marrow:
A2.2.4—Structures Common to Cells in All Living Organisms
Typical cells have DNA as genetic material and a cytoplasm composed mainly of water, which is enclosed by a plasma membrane composed of lipids.
A2.2.5—Prokaryote Cell Structure
Key structural features you should know in prokaryotes:
Prokaryotic cell wall
Plasma membrane
Cytoplasm
70S Ribosome
Nucleoid with loop of naked DNA (forming a single chromosome)
You should be able to identify these in either Bacillus or Staphylococcus prokaryotic cells.
Group Task: Make a 2D model of a prokaryotic cell. Each individual part of the cell must be made out of paper or some other material. (Just drawing the cell is not allowed!). Include annotations:
Size range in micrometres/nanometres for each structure.
A description of the structure and function of each part - what is its purpose and does it rely on other internal structures?
Finally - for your own notes:
A. Draw a fully labeled diagram of a prokaryotic cell.
B. Do this for a Gram-positive eubacteria such as Bacillus or Staphylococcus.
C. Also find and print an electron micrograph of a prokayotic cell (unlabeled) - and then label as many structures as you can on the electron micrograph.
A2.2.6—Eukaryote Cell Structure
Key structural features you should know in eukaryotes:
Cell wall
80S Ribosome
Nucleus
Mitochondria
Cytoplasm
Plasma membrane
Lysosome
Rough/Smooth endoplasmic reticulum
Secretory vesicle
Sap vacuole
Golgi apparatus
Microvilli
Chloroplast
Cytoskeleton/Microtubules/Microfilaments
Group Task: Make a 2D model of a eukaryotic cell. Each individual part of the cell must be made out of paper or some other material. (Just drawing the cell is not allowed!). Include annotations:
Size range in micrometres/nanometres for each structure.
A description of the structure and function of each part - what is its purpose and does it rely on other internal structures?
Finally - for your own notes:
A. Draw fully labeled diagrams of a plant and an animal eukaryotic cell. Do this for a human pancreatic cell and a leaf palisade cell.
B. Also find and print an electron micrograph of a eukaryotic cell (unlabeled) - and then label as many structures as you can on the electron micrograph.
A2.2.7, A2.2.8, A2.2.9
A2.2.7—Processes of Life in Unicellular Organisms
Use the resources available to make a summary of the processes of life and how they act in the unicellular organism, Paramecium.
The summary can be in several forms, slide/canva/quizlet etc.
A2.2.8—Differences in Eukaryotic Cell Structure Between Animals, Fungi and Plants
Include presence and composition of cell walls, differences in size and function of vacuoles, presence of chloroplasts and other plastids, and presence of centrioles, cilia and flagella.
A2.2.9—Atypical Cell Structure in Eukaryotes
Use numbers of nuclei to illustrate one type of atypical cell structure in aseptate fungal hyphae, skeletal muscle, red blood cells and phloem sieve tube elements.
A2.2.10, A2.2.11
A2.2.10—Cell Types and Cell Structures Viewed in Light and Electron Micrographs
Application of skills: Students should be able to identify cells in light and electron micrographs as prokaryote, plant or animal.
In electron micrographs, students should be able to identify these structures: nucleoid region, prokaryotic cell wall, nucleus, mitochondrion, chloroplast, sap vacuole, Golgi apparatus, rough and smooth endoplasmic reticulum, chromosomes, ribosomes, cell wall, plasma membrane and Microvilli.
A2.2.11—Drawing and Annotation Based on Electron Micrographs
Application of skills: Students should be able to draw and annotate diagrams of organelles (nucleus, mitochondria, chloroplasts, sap vacuole, Golgi apparatus, rough and smooth endoplasmic reticulum and chromosomes) as well as other cell structures (cell wall, plasma membrane, secretory vesicles and microvilli) shown in electron micrographs.
Students are required to include the functions in their annotations.
A2.2.12—Origin of Eukaryotic Cells by Endosymbiosis
The origin of eukaryotic cells can be explained by the endosymbiotic theory.
Evidence includes the presence in mitochondria and chloroplasts of 70S ribosomes, naked circular DNA and the ability to replicate.
A2.2.13—Cell Differentiation as the Process for Developing Specialized Tissues in Multicellular Organisms
Differentiation occurs when cells express specific genes associated with a particular cell type. gene expression is often triggered by changes in the environment.
Once cells differentiate, they only express genes responsible for producing proteins characteristic of that specific cell type.
Differentiated cells are important in multicellular organisms as they perform specialized functions within the body.
differentiated cells often lose the ability to undergo cell division and make new copies of themselves. So multicellular organisms maintain unspecialized cells known as stem cells.
Stem cells serve the purpose of making more cells as needed and retain the capacity for self-renewal.
A2.2.14—Evolution of Multicellularity
Development of cells, tissues, and organs in multicellular organisms facilitates coordination and communication. Many fungi and eukaryotic algae and all plants and animals are multicellular.
Advantages of multicellularity:
Enables growth to larger body sizes.
Allows for cell specialization, leading to groups of cells with specific functions.
Both of these may improve the chances of adaptation in diverse environments.
Evolution of multicellularity likely occurred in stages:
Single-celled organisms clumped together over time.
These clumps formed specialized cells, especially reproductive cells.
Groups of specialized cells started to fold, giving rise to tissues.
Over time, tissues became more complex, forming organs.
Multicellularity has evolved repeatedly.
Volvox is a good example of how multicellularity may have evolved in many organisms
A2.1 Origins of Cells [HL only]
A2.1.1—Conditions on Early Earth and the Pre-Biotic Formation of Carbon Compounds
Conditions on early Earth were ideal for the pre-biotic formation of carbon compounds due to several factors:
Lack of Free Oxygen and Ozone: Without free oxygen, there was no ozone layer to block ultraviolet (UV) radiation. This allowed more UV light to reach the surface, providing energy for chemical reactions that helped form organic molecules.
High Concentrations of Carbon Dioxide and Methane: The atmosphere was rich in carbon dioxide (CO2) and methane (CH4), which trapped heat, leading to higher global temperatures. These conditions promoted chemical activity and the formation of carbon-based compounds.
Ultraviolet Light Penetration: UV radiation acted as a key energy source, breaking down simple molecules and facilitating the recombination of atoms into more complex organic compounds like amino acids and nucleotides.
These factors increased the potential for spontaneous formation of carbon compounds through chemical processes that we cannot see happening at the present time on earth.
A2.1.2—Cells as the Smallest Units of Self-Sustaining Life
Differences Between Living and Non-Living Things:
Living Things:
Composed of cells
Can grow and reproduce
Maintain internal balance (homeostasis)
Metabolize (consume energy and produce waste)
Respond to environmental changes
Non-Living Things:
Lack cellular structure
Cannot grow or reproduce
Do not maintain internal balance
No metabolism or energy use
Do not respond to stimuli without external force
Why Viruses Are Considered Non-Living:
Lack cellular structure
Cannot reproduce independently (require a host)
No metabolism or energy production
Inert and inactive outside a host cell
Do not grow or develop on their own
A2.1.3—Challenge of Explaining the Spontaneous Origin of Cells
Cells as Complex Structures: Cells are highly complex structures that can only be produced by the division of pre-existing cells.
Necessary Requirements for First Cells:
Catalysis: The presence of catalysts e.g. enzymes was essential to speed up the chemical reactions necessary for life.
Self-Replication: Molecules capable of replicating themselves (e.g., RNA) were needed to pass on genetic information.
Self-Assembly: The ability of molecules to organize themselves into functional structures was crucial for forming early cell components.
Compartmentalization: The emergence of membrane-bound compartments allowed for the isolation of biochemical processes, key to cell function.
Challenge to Spontaneous Cell Formation: These complex requirements make the spontaneous formation of a fully functional cell highly unlikely.
A2.1.3—Challenge of Explaining the Spontaneous Origin of Cells
Preparation of Sterile Broth: Louis Pasteur prepared a nutrient-rich broth in a flask and subjected it to heat to ensure all existing microorganisms were killed. This step aimed to create a sterile environment within the flask.
Swan Neck Flask Design: The flask was specially designed with a swan neck shape for its opening. This unique design allowed air to enter the flask but prevented dust and microorganisms from directly reaching the sterile broth. The curved neck acted as a barrier to potential contaminants.
Exposure to Air: Pasteur left the flask exposed to the open air for an extended period. During this time, air could freely enter the flask, but any particles or microorganisms present in the air were trapped in the swan neck.
Observation of Broth: Despite prolonged exposure to the air, Pasteur observed that the nutrient broth in the main body of the flask remained free from microbial growth. This observation provided evidence against the concept of spontaneous generation, as it demonstrated that life did not spontaneously arise from non-living matter under these conditions.
A2.1.4—Evidence for the Origin of Carbon Compounds
The Miller and Urey experiment (1953) tested whether early Earth conditions could produce organic molecules.
They simulated the atmosphere using gases like methane, ammonia, hydrogen, and water vapor, with no oxygen, and applied electric sparks to mimic lightning.
After a week, the experiment produced amino acids, key building blocks of life. This demonstrated that under early Earth-like conditions, simple chemicals could naturally form complex organic molecules, supporting the idea that life’s precursors could arise from non-living matter.
A2.1.5—Spontaneous Formation of Vesicles by Coalescence of Fatty Acids Into Spherical Bilayers
Phospholipids are molecules that are amphipathic:
Amphipathic phospholipids have hydrophilic and hydrophobic properties.
The phosphate ends are attracted to water in cellular fluid and so hydrophobic ends of the molecule are forced towards each other, away from the water forming a micelle structure.
This gave a simple compartment for reactions to occur in.
A2.1.6—RNA as a Presumed First Genetic Material
RNA world hypothesis
The RNA World Hypothesis suggests that early life forms may have relied on RNA as genetic material and as an enzyme, before the formation of DNA and proteins.
Evidence:
RNA Replication: RNA can replicate itself, which means that early life forms could have used RNA to store genetic information AND replicate it, allowing early cells to divide and make more cells.
Catalytic Activity: RNA can function as a catalyst in biochemical reactions, in a similar way to protein enzymes. This function is seen in ribozymes, which are RNA molecules that speed up reactions and so act like an enzyme. In cells now, ribozymes are essential components of the ribosome, where they help in the formation of peptide bonds during protein synthesis.
These characteristics support the idea that RNA could have been the central molecule in early life forms, capable of both storing genetic information and performing essential biochemical reactions.
A2.1.7—Evidence for a Last Universal Common Ancestor
All life on Earth is believed to have evolved from LUCA around 4.2 billion years ago.
The evidence for a Last Universal Common Ancestor (LUCA) includes:
Universal Genetic Code: All known life forms use the same genetic code to translate DNA into proteins, indicating that this code originated with LUCA and was passed down to all descendants.
Shared Genes Across Organisms: Fundamental genes and biochemical processes are conserved across all living organisms, suggesting they were present in LUCA and inherited by subsequent life forms.
Shared characteristics among species: such as the vertebrate forelimb bone structure, point to a common ancestry.
Amino acids forming protein molecules: are universally shared among organisms.
Extinction of Competing Life Forms: Other early forms of life may have existed but became extinct due to competition with LUCA’s descendants, which were likely more successful or better adapted.
These factors support the idea that all current life forms share a common ancestry in LUCA.
A2.1.8—Approaches Used to Estimate Dates of the First Living Cells and the Last Universal Common Ancestor
To estimate the dates of the first living cells and the Last Universal Common Ancestor (LUCA), scientists use several methods:
Fossil Record: The oldest microfossils, about 3.5 billion years old, provide evidence of early life but not the very first cells.
Radiometric Dating: This technique dates the oldest rocks, suggesting life began at least 3.5 billion years ago.
Molecular Clock: By measuring genetic mutation rates and comparing them with known fossil dates, scientists estimate the timing of LUCA.
Phylogenetic Analysis: Comparing genetic data across species helps estimate when LUCA and early life forms existed.
Comparative Genomics: Studying conserved genes and pathways across all organisms infers the timing of LUCA’s traits.
These methods reflect the vast evolutionary timescale, with LUCA appearing around 4.2 billion years ago.
A2.1.9—Evidence for the Evolution of the Last Universal Common Ancestor in the Vicinity of Hydrothermal Vents
Evidence for LUCA's evolution near hydrothermal vents includes:
Fossilized Evidence: Microfossils found in ancient hydrothermal vent precipitates, dating around 3.5 billion years, suggest that early life thrived in these environments.
Conserved Sequences: Genomic analysis shows that many conserved genes and biochemical pathways present in all life forms are similar to those in modern extremophiles, indicating that LUCA may have adapted to hydrothermal vent conditions.
Chemical Composition: Rich in compounds like hydrogen and methane, hydrothermal vents provide potential building blocks for early biological molecules.
Energy Source: High-energy compounds, such as hydrogen sulfide, could have fueled early life processes.
These findings support the idea that LUCA likely evolved in the unique conditions of hydrothermal vents.
A2.3 Viruses
A2.3.1—Structural Features Common to Viruses
Genetic Material: Either DNA (double-stranded or single-stranded) or RNA (double-stranded or single-stranded).
Capsid: A protein coat that surrounds and protects the viral genetic material. It is made up of protein subunits called capsomeres, giving the virus its characteristic shape.
small, fixed size Envelope (in some viruses):An outer covering derived from the host cell membrane, surrounding the capsid. The envelope contains viral proteins and glycoproteins and plays a role in the virus's ability to enter host cells.
Relatively few features are shared by all viruses: ; nucleic acid (DNA or RNA) as genetic material; a capsid made of protein; no cytoplasm; and few or no enzymes.
Diversity of viruses
Bacteriophage lambda
Coronaviruses
HIV
A2.3.2-Diversity of Structure in Viruses
Helical, Polyhedral, Spherical, Complex are the shapes of viruses.
A2.3.3—Lytic Cycle of a Virus and A.2.3.4—Lysogenic Cycle of a Virus
In bacteriophage lambda, the virus can follow two different life cycles: the lytic cycle and the lysogenic cycle.
Lytic cycle: The phage injects its DNA into the host (typically E. coli), hijacks the cell's machinery to replicate its DNA, produce proteins, and assemble new phage particles. The host cell eventually bursts (lyses), releasing new phages to infect other cells.
Lysogenic cycle: Instead of immediately replicating, the phage DNA integrates into the host's genome as a prophage. It remains dormant, replicating along with the host cell's DNA during normal cell division. Under stress or certain conditions, the prophage can reactivate and enter the lytic cycle, causing the host cell to lyse and release new phages.
The key difference is that in the lysogenic cycle, the viral DNA remains dormant in the host for an extended period, while in the lytic cycle, the virus rapidly replicates and destroys the host.
A2.3.3—Lytic Cycle of a Virus
Phage attaches to the bacterial cell.
Phage injects its genetic material into the host.
Phage inserts its genetic material into the host genome.
The host cell synthesizes viral proteins.
Viral components assemble to form complete virus particles.
Bacterial cell lyses, releasing new virus particles to infect other cells.
A.2.3.4—Lysogenic Cycle of a Virus
Attachment and Entry:
Phage attaches and injects its DNA into the host bacterial cell.
Integration (Lysogeny):
Viral DNA integrates into the bacterial chromosome, becoming a prophage.
Replication with the Host:
Prophage replicates along with the host DNA during bacterial cell division.
Maintenance of Lysogeny:
Lysogen remains dormant, passing the integrated viral genes to daughter cells.
Induction (Transition to Lytic Cycle):
Under certain conditions, the lysogenic cycle may transition to the lytic cycle.
Replication, Assembly, and Lysis:
Phage DNA replicates, new virions are assembled, and the bacterial cell is lysed, releasing virions.
A2.3.5—Evidence for Several Origins of Viruses From Other Organisms
Viruses have features that are similar to those found in living organisms. This suggests that viruses may have developed from primitive cells.
Evidence:
Diversity of Viruses - Viruses have a wide range of forms, which suggests they may have evolved from multiple organisms rather than having one single common ancestor.
Obligate Parasitism and Convergent Evolution - Convergent evolution can be seen where unrelated viruses develop similar characteristics due to similar environmental pressures. An example of this would be when viruses demonstrate extreme obligate parasitism because they rely completely on host cells for replication.
Shared Genetic Code - Viruses have the same genetic code as living organisms. Many viral genetic sequences are similar to those of their hosts. This suggests that viruses may have evolved alongside or even from early organisms.
A2.3.6—Rapid Evolution in Viruses
Rapid evolution in viruses allows them to quickly adapt to host defenses and antiviral treatments.
Key factors include:
High Mutation Rates: RNA viruses, like influenza and HIV, have higher mutation rates due to less accurate replication processes.
Reassortment and Recombination: Viruses can exchange genetic material, especially segmented viruses like influenza, leading to new strains.
Selective Pressure: Immune responses and antiviral drugs create an environment where only certain viral variants survive.
Short Generation Times: Rapid replication enables multiple generations in a short period, facilitating quick evolutionary changes.
Examples
Influenza Viruses
Antigenic Drift: Small mutations lead to variations that can evade immunity, necessitating annual vaccine updates.
Antigenic Shift: Genetic reassortment can create novel strains, leading to pandemics (e.g., H1N1 in 2009).
HIV
High Mutation Rate: HIV’s reverse transcriptase generates diverse variants, complicating treatment.
Latency: HIV can remain dormant in immune cells, in a lysogenic cycle.
Treatment
Vaccine Development: Influenza vaccines must be updated regularly to match circulating strains.
Antiviral Resistance: HIV treatment can be made more difficult by emerging drug-resistant strains.
B2.2 Organelles and Compartmentalization [SL/HL]
B2.2.1—Organelles as Discrete Subunits of Cells That Are Adapted to Perform Specific Functions
B2.2.2—Advantage of the Separation of the Nucleus and Cytoplasm Into Separate Compartments
B2.2.3—Advantages of Compartmentalization in the Cytoplasm of Cells
Prokaryotic vs Eukaryotic Cells
Prokaryotic cells: relatively small and exchange materials directly with the external environment. No membrane bound internal structures.
Key structural features you need to know about eukaryotes: Cell wall 80S Ribosome Nucleus Mitochondria Chloroplast Plasma membrane Cytoplasm
Eukaryotic cells: have a “true” nucleus that is membrane bound. Some internal structures are membrane bound to achieve compartmentalization.
Key structural features you need to know about prokaryotes: Cell wall 70S Ribosome Nucleoid with naked DNA plasmids Cytoplasm Plasma membrane Pili and Flagella
Why compartmentalize in eukaryotic cells?
Internal membranes allow discrete processes to proceed in the cytoplasm. This gives the cell greater control over the number and complexity of processes that can be carried out.
non-membrane bound structures
membrane-bound and non-membrane bound structures
“organelle” - originally defined a membrane bound structure in the cell - now commonly used for all internal structures with a function: BSCB (british Society for Cell Biology): A ribosome is a cell organelle. It functions as a micro-machine for making proteins.
Organelles are discrete structures within cells that have adapted to perform a specific function.
Examples are: nuclei, vesicles, ribosomes and the plasma membrane.
Mitochondria and Chloroplasts are also organelles.
BUT - the cell wall, cytoskeleton and cytoplasm are not considered organelles
B2.2.2—Advantage of the Separation of the Nucleus and Cytoplasm Into Separate Compartments
Eukaryotes - compartments
In eukaryotic cells some parts of the protein making process (protein synthesis) happen first in the nucleus and then the synthesis of proteins is completed in the cytoplasm.
the compartmentalization of the nucleus and cytoplasm allows for modification of some parts of protein synthesis.
Prokaryotes - no compartments
In prokaryotic cells there is no nucleus, so all stages of protein synthesis happen in the cytoplasm. This means that unlike in eukaryotes, protein synthesis in prokaryotes cannot be modified.
So, in eukaryotes, compartmentalization allows for separation of processes which gives more control and opportunities to modify key processes.
B2.2.3—Advantages of Compartmentalization in the Cytoplasm of Cells
Compartmentalisation advantages:
Allows concentration of metabolites and enzymes to be controlled in one part of the cell e.g.
Lysosomes (a compartment formed from the golgi membrane) carry enzymes for cell component breakdown.
Phagocytic vacuoles as part of endocytosis can be the site of bacterial breakdown in the immune system.
Allows separation of incompatible biochemical processes.
B2.2.4—Adaptations of the Mitochondrion for Production of ATP by Aerobic Cell Respiration
Adaptations: a double membrane with a small volume of intermembrane space, large surface area of cristae and compartmentalization of enzymes and substrates of the Krebs cycle in the matrix.
B2.2.5—Adaptations of the Chloroplast for Photosynthesis
Adaptations: the large surface area of thylakoid membranes with photosystems, small volumes of fluid inside thylakoids, and compartmentalization of enzymes and substrates of the Calvin cycle in the stroma.
B2.2.6—Functional Benefits of the Double Membrane of the Nucleus
Pores in the nuclear membrane are needed for the transport of substances out of the nucleus and the nuclear membrane forms vesicles during cell division.
B2.2.7—Structure and Function of Free Ribosomes and of the Rough Endoplasmic Reticulum
free ribosomes of synthesise proteins for use within the cell membrane-bound ribosomes on the rough endoplasmic reticulum synthesise proteins in vesicles for secretion.
Some vesicles are used for transport of proteins within the cell (Lysosomes).
B2.2.8—Structure and Function of the Golgi Apparatus
The golgi receives proteins in vesicles from the rough endoplasmic reticulum.
The golgi processes the proteins, folding them into their final shape.
Then the proteins are carried in vesicles either out of the cell (secretion) or used inside the cell as lysosomes.
B2.2.9—Structure and Function of Vesicles in Cells
Vesicles have the same structure as the plasma membrane and are involved in transport within the cell and secretion.
B2.3 Cell Specialization
B2.3.1—Production of unspecialized cells following fertilization and their development into specialized cells by differentiation
Cell differentiation gives organisms the ability to perform complex functions e.g. nerves, muscle connective tissue and blood cells all combine to make a pump – the heart.
Chemical signaling controls the type of cells made in an embryo and into adulthood by regulating which genes are expressed from all the genes in the DNA.
These gene regulating chemicals are called morphogens.
The morphogens will form a concentration gradient, higher where they are made, lower further away. This is important as the concentration determines the type of cell that will form.
B2.3.2—Properties of Stem Cells
Stem cells possess two fundamental characteristics:
Self-renewal: They can continuously undergo division and replication.
Potency: They possess the ability to transform into distinct cell types.
As a cell undergoes differentiation to become specialized, it loses its capacity to give rise to other cell types, which often results in limited stem cell resources.
B2.3.3—Location and Function of Stem Cell Niches in Adult Humans
Stem cell niches are special places in the body where a group of adult stem cells is kept ready for future growth and transformation.
In the human body, you can find stem cell niches in places like the bone marrow, hair follicles, heart, intestines, and brain.
B2.3.4—Differences Between Totipotent, Pluripotent and Multipotent Stem Cells
There are three main types of stem cells in human development:
Totipotent - Can become any cell type and even create new organisms.
Pluripotent - Can form many different cell types.
Multipotent - Can only turn into a specific group of related cell types.
Totipotent and pluripotent stem cells, like those in embryos, are called embryonic stem cells. Multipotent stem cells, such as those in the bone marrow, are known as adult stem cells.
B2.3.5—Cell Size as an Aspect of Specialization
As a cell gets larger both the volume and surface area increase. However, the surface area increases at a slower rate compared to the volume. This means that a small cell will have a larger surface area:volume ratio than a larger cell.
A greater volume of the cell means more processes/reactions can go on in the cell,
A greater surface area means more exchange of substrates and wastes.
Cells need to efficiently cope with 3 factors:
a. Metabolic rate
b. Excretion of wastes
c. Heat exchange
Small cells can do this as they have a high surface area: volume ratio.
Large cells are not as efficient as they have a low surface area: volume ratio. This limits cell size.
Type of cell Size range (micrometres)
Sperm cell 60
Ovum cell 120 micrometers
White blood cell 12-15 micrometers
Red blood cell 6.2-8.2 micrometers
neuron 3-18 micrometers
Striated muscle cell 10-100 micrometers
B2.3.6—Surface Area-to-Volume Ratios and Constraints on Cell Size
If the 3 green cubes represent single cells then the SA:V ratio reduces as the cell gets larger.
But, if the large cube is multicellular and made up of smaller 1cm side length cells then the SA of the large block =
6cm2 x 27 = 162cm2
The volume is the same = 27cm3
So the SA:Vol ratio = 162:27 = 6:1
This means the multicellular retains a high SA:Vol ratio
B2.3.7—Adaptations to Increase Surface Area-to-Volume Ratios of Cells
Cells can adapt their shape to increase their surface area