Bio G11 A2 & B2 Unit Test Prep PDF

**A2.1 Origins of Cells** Guiding Question: 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? **The Atmosphere of Prebiotic Earth:** Oparin and Haldane suggested that the prebiotic...

**A2.1 Origins of Cells** Guiding Question: 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? **The Atmosphere of Prebiotic Earth:** Oparin and Haldane suggested that the prebiotic Earth's atmosphere was significantly different than today. Evidence suggests that Earth's prebiotic atmosphere was primarily composed of nitrogen, carbon dioxide and water vapor with smaller amounts of methane and hydrogen. There was no oxygen gas (O~2~) or ozone (O~3~) present in the atmosphere, as oxygen gas was first released into the atmosphere by photosynthesis. The lack of an ozone layer resulted in high levels of ultraviolet light entering the atmosphere. The high concentrations of carbon dioxide resulted in higher temperatures due to the greenhouse effect. Oparin and Haldane proposed that the energy available from ultraviolet light and higher temperatures (as well as lightning and volcanic activity) on the prebiotic Earth allowed the spontaneous generation of organic compounds. Over time the organic molecules became more complex, eventually leading to self-replicating structures and to the first cells. Cell theory: 1. Cells are the basic unit of life. 2. All organisms are composed of at least one cell. 3. All cells come from pre-existing cells. Living organism has... -------------------------------------------------- Have at least one cell Are capable of carrying out metabolism Maintain homeostasis Respond to stimuli Are capable of reproduction Grow and develop Contain genetic information (in the form of DNA) **Viruses are not Alive:** Living organisms should possess **[all]** of the characteristics of life. Viruses do have genetic material (DNA or RNA); however, they do **not** possess most of the characteristics of life. - Viruses do **not** have cells which carry out metabolism and homeostasis. Viruses **cannot** respond to stimuli, and they do not grow. - Viruses are **not** capable of reproducing themselves. They are replicated by host cells. Origin of Cells: Cells are highly complex structures that can currently only be produced by division of pre-existing cells. The following were required for the development of the first cells: - Catalysis - Self-replication of molecules - Self-assembly - The emergence of compartmentalization **Miller Urey Experiment:** Miller and Urey carried out an experiment to test Haldane and Oparin's hypothesis that the macromolecules of life could have spontaneously generated on a prebiotic Earth. Miller and Urey attempted to simulate the conditions on a prebiotic Earth in a laboratory setting. The gases methane, ammonia, and hydrogen along with water vapor represented the prebiotic atmosphere. Water was used to model the oceans. A condenser allowed water vapor to return to the liquid water. Energy was supplied through electrical sparks, simulating lightning, and a heat source to evaporate the water. After one week the water became a brownish black. Analysis of the water showed that many complex organic molecules, including amino acids, had been produced. The experiment showed that at least some of the organic molecules required for life could be spontaneously generated under certain conditions. Evaluate Miller and Urey's Experiment ---------------------------------------------------------------------------------------------------------- -------------------------------------------------------------------------------- Modelled prebiotic Earth and its atmosphere. There remains debate on the actual atmosphere of prebiotic Earth. Demonstrated that molecules such as amino acids can be generated spontaneously under certain conditions. The experiment did not produce all of the organic molecules required for life. The design of the experiment allows it to be replicated by other scientists. The simulation could not account for all conditions on the prebiotic Earth. Cell membranes: Cell metabolism requires the separation of the cytoplasm from the external environment. Cell membranes serve this function in modern cells. Cell membranes are formed of phospholipid bilayers. Phospholipids are **amphipathic**, with hydrophilic phosphate heads, and hydrophobic fatty acid tails. Fatty acids have formed spontaneously in experiments similar to Miller and Urey's experiment. Fatty acids spontaneously coalesce to form spherical bilayers (vesicles) when mixed with water. It has been hypothesized that the first genetic material was trapped within a phospholipid vesicle forming a protocell. RNA (First Genetic Material): RNA is hypothesized to the first genetic material because: - RNA can store genetic information - RNA is capable of self-replication - RNA can catalyse reactions **Ribozymes** (RNA molecules) in the ribosome catalyse the formation of peptide bonds during protein synthesis. LUCA (Last Univericial Common Ancestor): The last universal common ancestor (LUCA) is the most recent common ancestor for all organisms on Earth. There may have been other forms of life at the same time as LUCA, but these forms of life became extinct as LUCA and its descendent outcompeted them. There are no fossil remains for the last universal common ancestor (LUCA). However, the fact that all organisms share the same genetic code, strongly suggests that all organisms have evolved from LUCA. All living things share a number of genes, which are assumed to have been inherited from LUCA. Cladistics allows scientists to estimate the age of common ancestors by analysing changes in the genetic code. Earth is 4.6 Billion years old LUCA appeared approximately 4 billion years ago. Life has continued to evolve for the last 4 billion years producing the biodiversity currently found on our planet. LUCA involved near Hydrogen vents Evidence suggests that LUCA evolved in the vicinity of hydrothermal vents. Hydrothermal vents are rich in organic compounds, such as methane, carbon dioxide, and hydrogen. (it's just the building block, like with carbon dioxide, methane, and hydrogen are the building blocks of organic compound) Experiments similar to Miller and Urey's show that the building blocks of life, such as proteins and nucleotides can form from these chemicals. Fossils of cyanobacteria have been discovered from ancient seafloor hydrothermal vent precipitates. An analysis of genes common to all the three domains of life has identified genes that are highly conserved. Some of these genes are involved in thermophilic metabolism, suggesting that LUCA may have evolved in a high temperature environment such as hydrothermal vents. **\ ** **A2.2 Cell Structure** Guiding Question: What are the features common to all cells and the features that differ? How is microscopy used to investigate cell structure? **Nature of science - Deductive reasoning:** Deductive reasoning is a logical approach where you progress from general ideas to specific conclusions. The general idea is that all known organisms are composed of at least one cell. The conclusion (prediction) is that all newly discovered organisms will be composed of at least one cell. **Structure of microscope:** [**https://www.sciencelearn.org.nz/labelling\_interactives/6-label-the-microscope**](https://www.sciencelearn.org.nz/labelling_interactives/6-label-the-microscope) **how to measure sizes of objects viewed under a microscope:** [**https://www.savemyexams.com/as/biology/cie/25/revision-notes/1-cell-structure/1-1-the-microscope-in-cell-studies/eyepiece-graticules-and-stage-micrometers/**](https://www.savemyexams.com/as/biology/cie/25/revision-notes/1-cell-structure/1-1-the-microscope-in-cell-studies/eyepiece-graticules-and-stage-micrometers/)  A black text with black text Description automatically generated with medium confidence  **1000μm = 1mm 1μm = 1 x 10^-6^m** **1000nm = 1 μm 1 nm = 1 x 10 ^-9^m** **Other types of microscope:** - **Cryogenic Electron Microscopy** **Cryogenic electron microscopy allows scientists to view proteins and other biomolecules which do not readily crystalise.** - **Freeze Fracture Electron Microscopy** Freeze-fracture electron microscopy is a technique used to examine the ultrastructure of rapidly frozen biological samples, such as plasma membranes. Membranes are rapidly frozen, and then fractured in area of weakness, such as separating the phospholipid bilayer, and through integral proteins. The procedure has allowed scientists to analyse the structure of plasma membranes, and allowed the identification of integral proteins, leading to the development of the Singer-Nicolson model of membrane structure **Fluorescent Stains and Immunofluorescence in Light Microscopy:** Immunofluorescence is a technique used to visualize a specific protein or antigen in cells or tissue by binding a specific antibody chemically attached to a fluorescent dye. The specific antibodies attach to specific proteins within biological tissue. The sample can then be analysed using a fluorescence microscope. Fluorescent stains are specific, so scientists are able to study the location, distribution and quantity of specific biomolecules. Fluorescent stains can be used with living tissue, allowing scientists to study dynamic processes such as cell division. Fluorescent stains can be used to detect molecules at low concentrations Different coloured fluorescent stains can be used to label different molecules allowing the study of the interactions between molecules. **Features Shared by All Cells:** All cells share the following features: - A phospholipid plasma membrane which controls what enters and exits the cell. - Cytoplasm composed of mainly water, which is where most metabolism occurs. - DNA as the genetic material. - Ribosomes for protein synthesis. **Prokaryotic Cells (박테리아 같은):** Prokaryotes have a simple cell structure without compartmentalism. Prokaryotes are a diverse group of organisms with a wide variety of structures. +-----------------------------------+-----------------------------------+ | Cell Wall | The cell wall (composed of | | | **peptidoglycan**) provides the | | | cell with strength and support. | | | It prevents the cell from | | | bursting. | +===================================+===================================+ | Pili | Pili allow bacteria to adhere to | | | each other and other surfaces. | | | | | | Pili also allows the exchange of | | | genetic material between | | | bacteria. | | | | | | *Bacillus* bacteria have pili, | | | but *Staphylococcus* do not. | +-----------------------------------+-----------------------------------+ | Plasma membrane | The plasma membrane controls what | | | enters and exits the cell. The | | | plasma membrane is composed of | | | phospholipids. | +-----------------------------------+-----------------------------------+ | Cytoplasm | Most of the metabolism that | | | occurs in the cell occurs in the | | | cytoplasm. The cytoplasm is | | | mostly composed of water. | +-----------------------------------+-----------------------------------+ | 70S Ribosomes | The 70S ribosomes are responsible | | ([\$\\frac{50S}{30S}\$]{.math | for protein synthesis. | |.inline}*)* | | +-----------------------------------+-----------------------------------+ | Nucleoid Region | The nucleoid region contains a | | | single **circular** chromosome. | | | The chromosome contains DNA (no | | | protein) contains the genetic | | | information for the growth and | | | development of the cell | +-----------------------------------+-----------------------------------+ | Flagellum | The flagellum is responsible for | | | locomotion. *Bacillus* bacteria | | | have flagella, but | | | *Staphylococcus* do not. | +-----------------------------------+-----------------------------------+ **Eukaryotic Cells:** Eukaryotic cells have chromosomes located in a **nucleus**, as well as a variety of membrane bound organelles. All animals, fungi and plants are eukaryotes.  +-----------------------------------+-----------------------------------+ | Plasma Membrane | The plasma membrane controls what | | | enters and exits the cell. The | | | plasma membrane is composed of | | | phospholipids. | +===================================+===================================+ | Cytoplasm | Most of the metabolism that | | | occurs in the cell occurs in the | | | cytoplasm. The cytoplasm is | | | mostly composed of water. | +-----------------------------------+-----------------------------------+ | 80S Ribosomes | The 80S ribosomes are responsible | | ([\$\\frac{60S}{40S}\$]{.math | for protein synthesis. | |.inline}) | | +-----------------------------------+-----------------------------------+ | Nucleus | Chromosomes associated with | | | histone proteins are located in | | | the nucleus. The nucleus has a | | | double membrane with pores which | | | allow mRNA to enter the cytoplasm | +-----------------------------------+-----------------------------------+ | Chromosomes | Chromosomes are composed of DNA | | | wrapped around histone proteins. | | | The DNA is the genetic material | | | with information for growth and | | | development of the cell. | +-----------------------------------+-----------------------------------+ | Mitochondrion | Aerobic respiration, producing | | | ATP. | +-----------------------------------+-----------------------------------+ | Vaculoes | Plants have large vacuoles | | | involved in storing nutrients. | | | | | | Small vacuoles are found in | | | animal cells and are involved in | | | the removal of waste. | +-----------------------------------+-----------------------------------+ | Lysosome | Lysosomes are specialised | | | vesicles, which contain enzymes. | | | They are involved in the | | | digestion of large molecules. | +-----------------------------------+-----------------------------------+ | Rough Endoplasmic Reticulum | The rough endoplasmic reticulum | | | is a membrane structure with | | | ribosomes attached. It is the | | | site of protein synthesis and is | | | involved in transporting proteins | | | to the Golgi Apparatus. | +-----------------------------------+-----------------------------------+ | Smooth Endoplasmic Reticulum | The smooth endoplasmic reticulum | | | is a membrane structure without | | | ribosomes attached. It is | | | involved in lipid synthesis, and | | | detoxification. | +-----------------------------------+-----------------------------------+ | Golgi Apparatus | The Golgi apparatus modifies and | | | packages proteins to be exported | | | from the cell. | +-----------------------------------+-----------------------------------+ | Cytoskeleton | The cytoskeleton is composed of | | | protein microtubules, and is | | | involved in maintaining cell | | | shape, moving organelles, and | | | nuclear division (mitosis and | | | meiosis). | +-----------------------------------+-----------------------------------+ **Similarities:** Both have: - A phospholipid plasma membrane which controls what enters and exits the cell. - Cytoplasm where most metabolism occurs. - DNA as the genetic material. - Ribosomes for protein synthesis. **Difference:** +-----------------------+-----------------------+-----------------------+ | Feature | Prokaryotic Cell | Eukaryotic Cell | +=======================+=======================+=======================+ | Membrane bound | Not Present | Present | | organelles | | | +-----------------------+-----------------------+-----------------------+ | Mitochondria | Not Present | Present | +-----------------------+-----------------------+-----------------------+ | Location of | Nucleoid region in | Nucleus | | Chromosomes | the cytoplasm | | +-----------------------+-----------------------+-----------------------+ | Number of Chromosomes | One | Many | +-----------------------+-----------------------+-----------------------+ | Shape of Chromosomes | Loop of DNA | Linear | +-----------------------+-----------------------+-----------------------+ | Protein associated | None | DNA wrapped around | | with Chromosomes | | histone protein | +-----------------------+-----------------------+-----------------------+ | Ribosomes | 70S ribosomes | 80S ribosomes | +-----------------------+-----------------------+-----------------------+ | Cell Wall | Cmoposed of | Plant have a | | | peptidoglycan | cellulose cell wall | | | | | | | | Fungi have chitinous | | | | cell walls | | | | | | | | Animals have no cell | | | | wall | +-----------------------+-----------------------+-----------------------+ | Cell Size | Smaller (Usually 2-8 | Larger (Usually | | | nm in length) | 10-100 nm in length) | +-----------------------+-----------------------+-----------------------+ **Process of Life:** Homeostasis Maintenance of internal conditions within a narrow range --------------------- ---------------------------------------------------------------------------------------------------- Metabolism Complex network of interdependent and interacting chemical reactions occurring in living organisms Nutrition Processes that organisms use to obtain and use food (nutrients) for growth and development Movement Changing of the position of the organism Excretion Removal of metabolic waste Growth Increase in mass or size of an organism Response to stimuli Ability of organisms to respond to internal or external stimuli Reproduction Productino of offspring **Differences between animal, fungi, and plant cells:** +-----------------+-----------------+-----------------+-----------------+ | Features | Animal | Fungi | Plant | +=================+=================+=================+=================+ | Cell Wall | Not present | Chitin cell | Cellulose cell | | | | walls | walls | +-----------------+-----------------+-----------------+-----------------+ | Vacuoles | Small scattered | Fungi can have | Large central | | | vacuoles | small or large | vacuole | | | involved in | vacuoles | involved in | | | storing | depending on | storing | | | materials and | the species | nutrients and | | | waste products | | waste. The | | | | | vacuole also | | | | | helps maintain | | | | | turgor pressure | +-----------------+-----------------+-----------------+-----------------+ | Centrioles | Presents and | Not present | Not present | | | involved in | | | | | mitosis and | | | | | meiosis | | | +-----------------+-----------------+-----------------+-----------------+ | Plastids | Not present | Not present | Plastids found | | | | | in plant cells | | | | | include | | | | | | | | | | - Chloroplast | | | | | s | | | | | | | | | | - Chromoplast | | | | | s | | | | | | | | | | - Amyloplasts | +-----------------+-----------------+-----------------+-----------------+ | Cilia and | Present in some | Not present | Not present | | flagella | animal cells | | | +-----------------+-----------------+-----------------+-----------------+ **Atypical cells and nuclei:** Most eukaryotic cells have a single nucleus, but there are some exceptions: **Aseptate fungal hyphae**: Fungi have filamentous structure called hyphae, which may be separated by internal cell walls called septa. Some fungal hyphae are not separated by septa, forming one long **multinucleate** cell. **Skeletal muscle cells**, also known as muscle fibers, are **multinucleate**. **Red blood cells**, also known as erythrocytes, do not have a nucleus. **Phloem sieve tube elements** do not have a nucleus. Prokaryotic and Eukaryotic Cell: +-----------------------+-----------------------+-----------------------+ | Prokaryotic cells | Animal Cells | Eukaryotic plant cell | | | | | | There is a clear | The cells are | The cells are | | nucleoid region, and | eukaryotic as nuclei | eukaryotic as nuclei | | no nucleus present. | are clearly visible. | are clearly visible. | | | | | | | The cells are animal | The cells are plant | | | cells as they do not | cells as they have a | | | have a cell wall | fixed regular shape | | | surrounding them. | with a clear cell | | | | wall. | +-----------------------+-----------------------+-----------------------+ **Prokaryotes:** Nucleoid Region: The chromosome of prokaryotes is located in the nucleoid region. The prokaryotic chromosome is naked (not associated with proteins) and contains the genetic information for the growth and development of the cell. The nucleoid region is visible on micrographs as a lighter irregularly shaped region within the cytoplasm of the cell. Cell wall: The peptidoglycan cell wall of prokaryotes surrounds the cell. The cell wall is seen as a dark line around the outside of the cell. The plasma membrane is pushed against the cell wall. **Eukaryotes:** Nucleus: The nucleus is a large structure found in the centre of animal cells and pushed up against the cell wall in plant cells. The nucleus has a double membrane (envelope) with pores. The pores are highlighted by the short arrows in the micrograph The nucleus contains chromosomes. The chromosomes contain the genetic information for the growth and development of the cell. The RNA required for translation is produced within the nucleus. Mitochondrion: A mitochondrion has a double membrane. The outer membrane is smooth. The inner membrane, cristae, is highly folded, allowing mitochondria to be identified in micrographs. Mitochondria produce ATP by aerobic respiration Chloroplast (Plant): The chloroplast has a double outer membrane with many membranes within the chloroplast. Chloroplasts will only be found in plant cells capable of carrying out photosynthesis. Chloroplasts contain chlorophyll and carry out photosynthesis for the plant. Sap vacuole (Plant): The sap vacuole in plants is a large vacuole with a single membrane. It is located in the centre of the cell, pushing all other organelles against the cell wall Sap vacuoles are found in plant cells, and store nutrients and wastes, and maintain turgor pressure. Turgor pressure is the force within the cell that pushes the plasma membrane against the cell wall. RER: Rough endoplasmic reticulum is a network of tubules within eukaryotic cells. The rough appearance is as a result of many ribosomes attached to the surface of the rough endoplasmic reticulum. The rough endoplasmic reticulum is a site of protein synthesis. The proteins are then transported to the Golgi apparatus. SER: Smooth endoplasmic reticulum is a network of tubules within eukaryotic cells. There are no ribosomes present on the tubules, so they have a smooth appearance. Production of lipids and detoxification of harmful substances Golgi Apparatus: The Golgi apparatus is a series of stacked, flattened membranes. Vesicles (small vacuoles) are seen around the Golgi apparatus. The Golgi receives proteins from the rough endoplasmic reticulum. The Golgi apparatus modifies the proteins and packages them in vesicles for secretion. Chromosome: Chromosomes become visible during mitosis and meiosis. Chromosomes consist of two elongated DNA molecules (chromatids) held together until anaphase. Chromosomes are located in the nucleus, with DNA wrapped around histone proteins. The Chromosomes contain the genetic information for growth and development of the cell. Ribosomes: Ribosomes are found floating in the cytoplasm of prokaryotic and eukaryotic cells. Ribosomes are also found attached to the rough endoplasmic reticulum in eukaryotic cells. Ribosomes appear as spherical dots with a dark centre on micrographs Cell wall (Plant): Plant cell walls surround the cell. The plasma membrane is located on the inside of cell wall. Plasma Membrane: In plant cells the plasma membrane is pushed against the inside of the cell. The plasma membrane controls what enters and exits the cell. In animal cells, the plasma membrane is the outer boundary of the cell Microvilli (Animals): Microvilli are found on epithelial cells of the small intestine, and in the proximal convoluted tubule of nephrons in the kidney. Microvilli appear as long finger like extensions of a cell, significantly increasing the surface area. Microvilli increase the surface area of a cell, increasing the available surface area for transport of materials. Secretory Vesicle: Secretory vesicles transport proteins from the Golgi apparatus to the plasma membrane, where the proteins are secreted by exocytosis. Endosymbiotic theory: Evidence suggests that all eukaryotes evolved from a common unicellular ancestor that had a nucleus and reproduced sexually. Mitochondria and chloroplasts later evolved by endosymbiosis. The endosymbiotic theory proposes that a large cell engulfed a small aerobic prokaryotic cell. The small cell was not digested but developed a mutualistic relationship with the host cell. The host cell provided protection and received ATP energy from the aerobic cell. The aerobic cell evolved into mitochondria. Later the same process occurred with a photosynthetic prokaryote, which evolved into chloroplasts. The endosymbiotic theory is a good example as it explains how eukaryotic cells evolved from prokaryotic cells and is supported by a wide range of evidence. Evidence: Chloroplasts and mitochondria are a similar size to modern prokaryotes and share many characteristics which support the endosymbiotic theory. These characteristics include: - A single circular chromosome with naked DNA. - 70S ribosomes (larger 80S ribosomes are present in eukaryotic cells cytoplasm) for synthesising proteins. - Reproducing in the same manner through a process called binary fission. Chloroplasts and mitochondria also have double membranes with the outer membrane resulting from the endocytosis of the original, small prokaryotic cell. **Stem cells:** Stem cells are: - Undifferentiated cells. - Capable of differentiating into specialized cells. - Capable of endlessly reproducing. **Stem Cells Differentiate into Specialized Cells:**  The basis for stem cell differentiation is different patterns of gene expression, often triggered by changes in the internal or external environment. The genes that are expressed (turned on) determine the structure and function of the specialized cell. **Meristematic Tissue in Plants:** Plant cells contain meristematic tissue. Meristematic tissue contains cells that are: - Undifferentiated cells. - Capable of differentiating into specialized cells. - Capable of endlessly reproducing. **Multicellularity:** Multicellularity has evolved independently at least 25 times. Many fungi, eukaryotic algae and all animals and plants are multicellular. Advantages: Multicellular organisms evolved specialized tissues to carry out a range of functions, resulting in more efficient organisms, leading to longer life spans. The specialized tissue allows more efficient use of resources. Multicellular organisms are larger than unicellular organisms, so are better protected from predators, and are capable of consuming smaller organisms **B2.1 Membranes and membrane transport** **Guiding Question:** How do molecules of lipid and protein assemble into biological membranes? What determines whether a substance can pass through a biological membrane? **Phospholipid:** Two fatty acid chains and a phosphate are bonded to a glycerol molecule. The fatty acid tails are nonpolar and are hydrophobic. The phosphate head is charged and is hydrophilic. Phospholipids are amphipathic, as they have hydrophobic and hydrophilic regions. **Phospholipid bilayer:** Phospholipids naturally form bilayers when added to water. The hydrophilic phosphate heads face water. The hydrophobic fatty acid tails are in the middle of the bilayer. All membranes in cells are composed of a phospholipid bilayer. **Lipid Bilayer:** The phospholipid bilayer of plasma membranes separates the cytoplasm and cell contents from the environment. The bilayer acts as a barrier for materials entering and exiting the cell. Only hydrophobic (uncharged) particles can pass through the hydrophobic fatty acid tails at the centre of the bilayer. Large particles and hydrophilic, charged particles cannot pass directly through the phospholipid bilayer.  **Kinetic energy:** Kinetic theory states that particles are in constant motion. Particles in gases, liquids and solutes in aqueous solutions move in random directions. The random movement of particles allows diffusion and osmosis to occur. Diffusion and osmosis are passive processes in cells, as the cell does not provide any energy to move particles. **Simple Diffusion:** Diffusion is the passive transport of particles from a region of high concentration to a region of low concentration. It is called passive because it uses no energy from the cell. Small uncharged particles (such as O~2~ and CO~2~) and fat-soluble molecules (such as hydrophobic steroids) can diffuse across plasma membranes. Oxygen diffuses directly from the alveoli into the blood. Carbon dioxide diffuses directly from the blood into the alveoli of the lungs. Hydrophilic, charged particles cannot pass directly through cell membranes **Integral Proteins:** Integral proteins are permanently attached to the plasma membrane and penetrate into the centre of the phospholipid bilayer. Integral proteins contain a hydrophobic section (within the fatty acid tails) and two hydrophilic sections (one at each surface of the bilayer). The hydrophobic section anchors the protein within the bilayer. Integral proteins can be transmembrane or only partially penetrate the bilayer. Integral proteins can be glycoproteins, channels, or protein pumps.  **Peripheral protins:** Peripheral proteins are temporarily attached to one side of the membrane. They are attached to the membrane surface or to integral proteins, through electrostatic interactions. The charged peripheral proteins are attracted to the charged sections of the integral proteins and phosphate heads. Peripheral proteins are hydrophilic and do not penetrate the phospholipid bilayer Integral proteins can be receptors and enzymes. **Osmosis:** Osmosis is the passive transport of water molecules from a region of low solute concentration to a region of high solute concentration through a semipermeable membrane. Water is polar, but it is so small that it can move through a phospholipid bilayer. Solutes in water are charged (polar molecules or ions) and cannot pass through the phospholipid bilayers found in membranes. **Osmosis and Aquaporins:** Osmosis directly through membranes is a slow process. Aquaporins are integral channel proteins that selectively transport water rapidly through membranes. The presence of aquaporins in a plasma membrane significantly increases membrane permeability to water. Osmosis through aquaporins is an example of facilitated diffusion.  **Facilitated diffusion:** Charged particles cannot diffuse directly through cell membranes. Charged particles enter and exit cells through protein channels. Facilitated diffusion is the passive transport of molecules from a region of high concentration to a region of low concentration through channel proteins. Channel proteins are specific to the molecule that can pass through them, making cell membranes selectively permeable. **Channel protein:** Channel proteins have a central pore which allows specific particles to move through. The pore is lined with hydrophilic R groups from amino acids, that allow one type of molecule to pass through. Some protein channels are gated and will only open to allow facilitated diffusion to happen in response to a stimulus. Sodium and potassium voltage-gated channels open and close based on the potential difference across membranes.  **ATP:** ATP provides the energy required to change the shape of protein pumps for active transport in cells. **Active transport:** Active transport is the movement of particles from a region of low concentration to a region of high concentration using protein pumps and ATP energy. Active transport uses ATP energy to transport particles across cell membranes against the concentration gradient. Active transport involves the following: 1. A specific particle binds to a binding site on a specific protein pump. 2. ATP binds to the protein pump and hydrolyses to become ADP. 3. A phosphate remains attached to the protein pump and causes the protein pump to change shape. 4. The particle is moved against the concentration gradient and released. 5. The phosphate is released, causing the protein pump to return to its original shape. **Passive and active transport:**  **Membrane selectivitt:** Facilitated diffusion is a selective process, as only specific particles can pass through the protein channels. Active transport is a selective process as protein pumps are specific to the particles that they can transport. Simple diffusion is [not] a selective process, as any small or hydrophobic particle is able to pass through the phospholipid bilayer. **Glycoproteins and glycolipids:** Phospholipids and membrane proteins can have carbohydrate chains attached by a process known as glycosylation. Glycoproteins are membrane proteins with a carbohydrate chain attached. Glycolipids are phospholipids with a carbohydrate chain attached. The carbohydrates of glycoproteins and glycolipids are on the outside surface of the cell. **Functions of Glycoprotein and Glycolipid:** Roles of glycoproteins and glycolipids include: - Receptors: Glycoproteins act as receptors for hormones. When a hormone binds to a specific glycoprotein receptor, it changes metabolism within the cell. - Cell to Cell Communication: Neurotransmitters bind to glycoproteins allowing communication between cells. - Immune Response: Glycoproteins act as markers on cells allowing the immune system to distinguish between self and non-self-cells. - Cell to Cell Adhesion: Glycoproteins interact with glycoproteins on neighbouring cells, allowing the formation of tissues. **Cell to Cell adhesion:** Some glycoproteins are cell adhesion molecules and are responsible for direct attachment between neighbouring cells. The carbohydrates of glycoproteins and glycolipids can form an extracellular matrix with the glycoproteins and glycolipids of neighbouring cells, leading to stable cell to cell adhesion. The matrix provides structural support for neighbouring cells and plays an important role in the formation of tissues. **Cell recognition:** The carbohydrate chains that form on glycoproteins and glycolipids have specific shapes allowing the immune system to recognise the cells as self. Glycoproteins and glycolipids act as antigens if the carbohydrate chain is not recognized as self by the immune system. Antigens are substances which stimulate an immune response and the production of antibodies.  **Fluid Mosaic membrane:** Students should be able to draw a two-dimensional diagram of the fluid mosaic model of membrane structure. Cholesterol molecules should be added between the fatty acid chains. **Fluid phospholipid:** The phospholipids in cell membranes are fluid as they are not in a fixed position, and they move around. The fluidity of the membrane is affected by the types of fatty acids present in the phospholipids. **Saturated and unsaturated fatty acid:** Saturated fatty acids have single bonds between the carbons on the hydrocarbon chain. Unsaturated fatty acids have at least one double bond between carbons on the hydrocarbon chain. Saturated fatty acids are linear. Unsaturated fatty acids bend at the position of the double bond. Triglycerides with saturated fatty acids have higher melting points than triglycerides with unsaturated fatty acids. **Saturated fatty acid and fluidity of membranes:** Saturated fatty acids have no carbon-carbon double bonds resulting in straight fatty acid tails allowing close packing of the phospholipids These phospholipids have higher melting points, decrease the viscosity of membranes. Cell membranes with more saturated fatty acid chains have higher viscosity and higher melting points. Saturated fatty acids make membranes stronger at higher temperatures.  **Unsaturated Fatty acid and fluidity of membranes:** Saturated fatty acids have carbon-carbon double bonds, resulting in kinks in the fatty acid tails, preventing the close packing of phospholipids. These phospholipids have lower melting points, and the viscosity of membranes is increased. Cell membranes with more unsaturated fatty acid chains are more fluid (have lower viscosity) and have lower melting points. **Fatty Acids in the Phospholipid Bilayer of Steelhead Trout (Ex):** As the temperature decreases, the concentration of unsaturated fatty acids increases in all tissues of steelhead trout. At low temperatures, the high concentration of unsaturated fatty acids maintains the membranes' fluidity. At higher temperatures, the higher concentration of saturated fatty acids increases the stability of membranes. **Cholesterol:** Cholesterol is located in the membranes of animal cells and helps regulate the fluidity of the membrane. **Location of Cholesterol in the Phospholipid Bilayer:** Most of the cholesterol molecule is hydrophobic (nonpolar) and is located between fatty acid tails of phospholipids in cell membranes. The hydroxyl group (-OH) on the cholesterol is hydrophilic (polar) and forms a hydrogen bond with the phosphate of a phospholipid.  **Cholesterol Regulates Membrane Fluidity:** Cholesterol regulates the fluidity of cell membranes in animal cells. At higher temperatures, cholesterol reduces fluidity and increases melting point of phospholipids, resulting in stable membranes. At lower temperatures, the presence of cholesterol between phospholipids maintains fluidity of the membrane and prevents crystallization of the phospholipids. **Membrane Fluidity and Vesicles:** Vesicles are small membrane bound structures involved in transporting materials within cells. The fluid nature of cell membranes allows the formation of vesicles from membranes and the fusion of vesicles with membranes. Bulk transport (endocytosis and exocytosis) is possible due to the fluid nature of the plasma membrane allowing the formation of vesicles and the fusion of vesicles with the membrane.  **Protein Transport Within the Cell:** Proteins and other materials are transported within vesicles around the cell. 1. Proteins to be secreted from the cell are synthesised by ribosomes attached to the rough endoplasmic reticulum. 2. The rough endoplasmic reticulum forms a vesicle containing protein which is sent to the Golgi apparatus. 3. The vesicle fuses with the Golgi apparatus. The Golgi modifies the protein. 4. The Golgi apparatus packages the protein in a secretory vesicle. 5. The secretory vesicle moves towards the plasma membrane. 6. The secretory vesicle fuses with the plasma membrane (exocytosis), releasing the protein outside of the cell. **Exocytosis:** Exocytosis is the release of large particles from a cell. Exocytosis involves the fusion of a vesicle with the plasma membrane, releasing the content outside of the cell. This is an active process which requires ATP Energy. Endocytosis is the process by which large particles enter the cell. The large particles are surrounded by the plasma membrane, which buds off inside the cell to form a vesicle. This is an active process which requires ATP Energy. **Gated ion channels:** Ion channels are integral proteins which allow specific ions to pass through by facilitated diffusion. The pore in the ion channels is hydrophobic, allowing specific ions to enter and pass through. Ion channels may be gated, allowing the movement of ions under controlled conditions. Gated ion channels play a number of roles in the transmission of nerve impulses. Examples of gated ions include: - Voltage gated channels, which respond to changes in membrane potential difference. - Ligand gated channels, which respond to a ligand attaching to the channel. **Voltage gated channels:** Voltage gated channels open and close in response to changes in the potential difference (voltage) across a membrane where the channel is located. Sodium and potassium voltage gated channels are involved in the movement of action potentials along neurons. Calcium voltage gated channels are involved in synaptic transmission, the transfer of a nerve impulse from one neuron to another.  **Neurotransmitter Gated Ion Channel:** **Acetylcholine is a neurotransmitter that opens sodium channels in the postsynaptic membrane of neurons** **Acetylcholine is a ligand which attaches to a sodium ion channel.** **When acetylcholine is attached to the channel, the channel opens, allowing sodium ions to enter a neuron through the postsynaptic membrane.** **Sodium Potassium pumps:** **The sodium potassium pump actively transports sodium ions out of a cell, and potassium ions into a cell.** **The sodium potassium pump maintains resting potential in neurons.** **The sodium potassium pump is an exchange transporter, as Na^+^ and K^+^ travel in opposite directions.**  **Resting potential and sodium potassium pump:** Neurons are at resting potential (-70mV) when a nerve impulse is not being transmitted. The potential difference is maintained by sodium ions (Na^+^) being outside the axon of a neuron, and potassium ions (K^+^) and chlorine ions (Cl^-^) being inside the axon. The sodium potassium pump is involved in transporting Na^+^ out of the axon and transporting K^+^ into the axon. The sodium potassium pump transports Na+ and K^+^ against their concentration gradients and is an example of active transport. The action of the sodium potassium pump involves the following steps: 1. **Three Na^+^** attach to the sodium ion binding sites on the sodium potassium pump protein. 2. ATP attaches to the sodium potassium pump. 3. ATP is hydrolysed, with a phosphate remaining attached to the protein pump. ADP is released. 4. The phosphate causes the pump to change shape, moving the sodium across the axon membrane, releasing Na^+^ outside the axon. 5. **Two K^+^** attach to the potassium ion binding sites on the sodium potassium pump protein. 6. The phosphate is released from the pump. 7. The pump returns to its original shape moving the K^+^ into the axon. The process can be repeated. The sodium potassium pump is an example of an exchange transporter, as the sodium ions and potassium ions are transported in opposite directions. **Glucose transport:** Glucose is transported by two mechanisms from the small intestine into the epithelial cells that line the intestine. - **Facilitated Diffusion**: Glucose is passively transported through glucose channels from the small intestine into the epithelial cells. - **Sodium Dependent Glucose Cotransporters**: Cotransport links the movement of an ion (Na^+^) down its concentration gradient with the movement of a solute (Glucose) against its concentration gradient. Cotransport is an example of indirect active transport, as ATP energy is required to create a concentration gradient for the ion. **Sodium depends Glucose cotransporters:** During the indirect active transport of glucose from the small intestine into epithelial cells the following happens: 1. Sodium ions (Na^+^) are actively pumped out of epithelial cells by the sodium potassium pump, resulting in a low concentration of Na^+^ in epithelial cells. 2. Na^+^ and glucose bind to the sodium-dependent glucose cotransporter protein. 3. The attachment of sodium and glucose causes the protein to change shape, moving both glucose and sodium into an epithelial cell. The transport of glucose depends on the active transport of sodium out of the epithelial cells. **Cells, Tissue, Organs and Organ Systems:** Tissues are groups of cells that work together to carry out a function. If a tissue is to carry out its function, the cells in the tissue must be able to adhere to each other. **Cell to Cell Adhesion:** Cell adhesion is the process by which a cell uses a combination of an extracellular matrix and cell-adhesion molecules (CAMs) to adhere to other cells. Cell adhesion is required for development and maintenance of tissues, cell to cell communication, and cell regulation. The extracellular matrix is composed of chemicals released by the cell, which form a gel-like material that supports cells. CAMs are a range of proteins which are used in different cell junctions. Cell junctions are protein complexes that provide adhesion between animal cells. **Cell Adhesion Molecules:** There are a range of cell adhesion molecules (CAMs) which are used for different types of cell junctions: - Tight junctions form a seal between cells, preventing substances leaking between the cells. - Gap junctions are channels between cells that allow molecules to pass between cells, allowing cell communication. - **Adherens junctions** use protein complexes to connect cells together - **Desmosomes** use protein complexes to form strong connections between cells, providing tissues with structural integrity.  **\ ** **B2.2 Organelles and compartmentalization** **Guiding Question:** How are organelles in cells adapted to their functions? What are the advantages of compartmentalization in cells? **Cell Organelles:** Organelles are compartmentalized subcellular structures found within a cell, which have a specific function. The following cell structures are not considered organelles: - Cell wall which is an extracellular structure - Cytoplasm is the gel-like fluid spread throughout the cell. It does not have a compartmentalized structure - Cytoskeleton is found throughout the cell but does not have a compartmentalized structure. **Plazma membrane:** The plasma membrane is a phospholipid bilayer which surrounds and encloses the cell. The plasma membrane controls what enters and exits the cell. The IB considers the plasma membrane to be an organelle, even though it is not a subcellular structure. The plasma membrane is not normally considered an organelle by scientists. **Cell Fractionation:** Scientists prepare cells for ultracentrifugation by cell fractionation. Cell fractionation is a process which separates cell organelles while preserving their functions. Cell Fractionation involves the following steps: 1. **Homogenization**: Tissue, containing cells, is broken up in a blender. The cells are blended in a cold, buffered solution which is isotonic to the cytoplasm of the cells. 2. The blended solution is filtered to remove large cell debris. **Ultracentrifugation:** Ultracentrifugation uses a fast centrifuge to separate the cell organelles according to density, through the following steps: 1. The filtered solution containing the cell organelles is spun at low speed by an ultracentrifuge. 2. The densest organelles (nucleus) form a pellet at the bottom of the centrifuge tube. 3. The pellet is removed, and the process is repeated at faster speeds, producing a series of pellets containing one type of organelle each time. **Nucleus:\ **Eukaryotic cells store their chromosomes in a nucleus. The chromosomes contain the genetic information, in the form of DNA, for the growth and development of the cell. Genes in chromosomes are transcribed into mRNA within the nucleus. The mRNA leaves the nucleus to be translated into polypeptides by ribosomes. **Ceperation of nucleus and cytoplasm:** DNA is protected in the nucleus from potentially harmful reactions in the cytoplasm. The processes of transcription of DNA to mRNA occurs in the nucleus. The mRNA is modified before entering the cytoplasm. The nucleus separates the process of transcription from metabolism occurring in the cytoplasm of the cell. The mRNA leaves the nucleus to be transcribed into polypeptides by ribosomes. **Translation and trascription of prokaryote:** Prokaryotes do not have a nucleus, which means there is no separation of the processes of transcription and translation. As mRNA is being transcribed, ribosomes attach to the mRNA and translation can begin. **Compartmentalization:** Organelles are specialized structures which compartmentalize different processes, allowing for greater efficiency. Organelles provide a protective environment for specialized activities in the cell. Organelles are able to store and use concentrations of metabolites and enzymes which are not compatible with metabolism occurring within the cytoplasm of a cell. **Lysosomes:** Lysosomes are membrane bound organelles which contain digestive enzymes. The main function of lysosomes is the digestion of macromolecules within the cell. The digestive enzymes within a lysosome are capable of digesting other cell components and are kept separate from the cytoplasm and other cell structures by the membrane surrounding lysosomes. When a cell is damaged, the enzymes are released from lysosomes, resulting in the digestion of the cell. **Phagocytosis:** Phagocytosis is process by which solid materials, such as bacteria, are taken into a cell by endocytosis. A bacterium is taken into a cell by endocytosis, forming a phagocytic vacuole. Lysosomes fuse with the phagocytic vacuole to form a phagosome. The lysosome's enzymes digest the bacterium.  **Mitochondria:** Mitochondria are organelles whose function is the production of ATP through aerobic respiration. Mitochondria are recognisable on electron micrographs by the folded inner membrane forming cristae.  **Adaptation of mitochondria:** +-----------------------------------+-----------------------------------+ | Outer Membrane | The outer membrane is not | | | permeable to protons (H^+^), | | | allowing a high concentration of | | | protons to build up in the | | | intermembrane space. | | | | | | Channels for pyruvate to enter | | | the mitochondrion | | | | | | The outer membrane contains | | | protein channels, which allows | | | pyruvate to enter the | | | mitochondrion from the cell's | | | cytoplasm. | +===================================+===================================+ | Inner membrane | The space between the membranes | | | is small, allowing the rapid | | | accumulation of protons. | | | | | | Contains the proteins of the | | | electron transport chain and ATP | | | synthase | +-----------------------------------+-----------------------------------+ | Intermembrane space | The inner membrane contains the | | | electron transport chain which | | | actively transports protons into | | | the intermembrane space. | | | | | | Small space allowing rapid | | | accumulation of protons | | | | | | The inner membrane also contains | | | proton channels and ATP synthase. | | | Protons travel through the | | | channels, providing the energy | | | for ATP synthase to convert ADP | | | and P~i~ to ATP. | +-----------------------------------+-----------------------------------+ | Cristae | The inner membrane has many | | | folds, known as cristae, which | | | increase surface area | | | | | | The inner membrane is highly | | | folded into cristae. The cristae | | | increase the surface area of the | | | inner membrane. | +-----------------------------------+-----------------------------------+ | Matrix | Contains DNA, ribosomes, and all | | | of the enzymes involved in the | | | link reaction and the Krebs cycle | | | | | | Contains the enzymes and | | | metabolites required for the link | | | reaction and Krebs cycle. | +-----------------------------------+-----------------------------------+ | Chromosomes | Mitochondria have a single | | | circular chromosome. The | | | chromosomes contain the genetic | | | information for making all of the | | | proteins, including enzymes, | | | involved in aerobic respiration. | +-----------------------------------+-----------------------------------+ | 70S ribosomes | Synthesize the proteins required | | | for aerobic respiration. | +-----------------------------------+-----------------------------------+ **Chloroplast:** Chloroplasts are organelles which carry out the process of photosynthesis in plants and algae. Photosynthesis is the production of organic compounds (such as glucose) from carbon dioxide and water, using light energy. The pigment chlorophyll is found within chloroplasts and absorbs light energy, which is used in photosynthesis  +-----------------------------------+-----------------------------------+ | Thylakoid membrane | Contains chlorophyll for | | | absorption of light, electron | | | transport chain, and ATP synthase | | | | | | The stroma contains DNA, | | | ribosomes and all of the enzymes | | | required for the | | | light-independent reactions, the | | | Calvin cycle, of photosynthesis. | | | | | | The folded nature of the | | | thylakoid membrane increases the | | | surface area for absorption of | | | light. | +===================================+===================================+ | Thylakoid space | The folded nature of the | | | thylakoid membrane increases the | | | surface area for absorption of | | | light. | +-----------------------------------+-----------------------------------+ | Granum | Grana are flattened discs of | | | thylakoid membrane which increase | | | surface area. | | | | | | There is a small thylakoid space | | | inside the grana, allowing the | | | rapid accumulation of protons | +-----------------------------------+-----------------------------------+ | Stroma | The stroma contains DNA, | | | ribosomes and all of the enzymes | | | required for the | | | light-independent reactions, the | | | Calvin cycle, of photosynthesis. | | | | | | The stroma contains a chromosome, | | | 70S ribosomes and all of the | | | enzymes required for the Calvin | | | cycle. | +-----------------------------------+-----------------------------------+ | Chromosomes | The chloroplast's chromosome | | | contains the genetic information | | | for synthesizing all of the | | | proteins, including enzymes, | | | involved in photosynthesis. | +-----------------------------------+-----------------------------------+ | 70S ribosomes | The ribosomes, located in the | | | stroma, synthesize the proteins | | | required for photosynthesis. | +-----------------------------------+-----------------------------------+ | Enzymes | All of the enzymes required for | | | the Calvin cycle are present in | | | the stroma of the chloroplast. | +-----------------------------------+-----------------------------------+ **Nuclear membrane:** The nucleus has a double membrane with pores. The membrane separates the production of mRNA within the nucleus, from the cytoplasm. The nucleus maintains ideal conditions for transcription. The mRNA is too large to pass through the nuclear membrane and enters the cytoplasm through the nuclear pores. The nuclear membrane breaks down at the beginning of mitosis and meiosis. The double membrane allows the nucleus to break down into many small vesicles. At the end of mitosis and meiosis the vesicles fuse to reform the nucleus. **Ribosomes:** Ribosomes synthesize polypeptides (proteins) by translating mRNA. Ribosomes are composed of: - A large ribosomal subunit - A small ribosomal subunit. Both subunits are composed of proteins and ribosomal RNA (rRNA). **RER:** In eukaryotic cells, ribosomes can be floating free in the cytoplasm or attached to the rough endoplasmic reticulum. The free-floating ribosomes synthesize proteins to be used in the cell. The ribosomes attached to the rough endoplasmic reticulum synthesize proteins to be transported out of the cell, or for the production of lysosome enzymes. The membrane of the rough endoplasmic reticulum is an extension of the nuclear membrane and is covered in ribosomes. Proteins, produced by ribosomes attached to the rough endoplasmic reticulum, enter the lumen of the rough endoplasmic reticulum. The proteins are packaged into vesicles which are transported to the Golgi apparatus.  **Golgi Apparatus:** The Golgi apparatus is located between the rough endoplasmic reticulum and the plasma membrane. The Golgi apparatus is composed of a series of flattened sacs, known as cisternae. Vesicles, containing proteins from the rough endoplasmic reticulum, move to and fuse with the Golgi apparatus. The Golgi apparatus modifies the proteins and packages them into secretory vesicles. The secretory vesicles move towards the plasma membrane and secrete the protein by exocytosis. **Vesicles:** Vesicles are composed of a phospholipid bilayer, and transport materials around the cell. A protein, clathrin, is involved in the formation of some vesicles which transport a specific molecule. Clathrin is a triskelion-shaped protein which attaches to recruiter proteins in a membrane, forming a vesicle. The clathrin proteins polymerize to form a clathrin cage, forcing the membrane to form a rounded bud. This bud is cleaved off to form a clathrin-coated vesicle. The clathrin cage is removed.  **\ ** **B2.3 Cell specialization** **Fertilization:** The fusion of gametes is called fertilization, which leads to the development of a zygote. The zygote is a totipotent stem cell. **Totipotent** stem cells can develop into all other cell types or develop into an embryo. **Embryotic stem cells:** The zygote develops into a blastocyst through cell division over five days. The blastocyst contains embryonic stem cells. Embryonic stem cells are **pluripotent**, meaning that they can differentiate into all other cell types but not into an embryo. **Cell Differentiation:** All cells from a multicellular organism have the same genome. Stem cells differentiate into specialized cells by expressing some genes and not expressing others. **Adult stem cells:** The blastocyst develops into a foetus, as the embryonic stem cells differentiate into specialized cells. Some adult stem cells remain to replenish dying cells and repair damaged tissue. Most adult stem cells are **multipotent**, meaning that can form a range of closely related cells. Hematopoietic stem cells are multipotent as they can differentiate into all types of blood cells, but not other cell types. **Embryotic development:** The embryonic stem cells differentiate into specialized cells and tissues in a controlled manner through a group of gene regulating chemicals, known as morphogens. Morphogens are a group of gene regulating chemicals (Transcription factors) which determine the specialized cell that develops according to their concentration. Morphogens are produced and released from embryo cells and diffuse through tissues. The morphogens spread, creating a concentration gradient across tissue, with the highest concentration near the source cells. Morphogens bind to receptors on cells, resulting in the activation or repression of genes. The concentration of the morphogen determines which genes will be expressed, determining the type of specialized cell. **Stem Cells:** Stem cells are **undifferentiated** cells which have an **unlimited capacity to divide** and can **differentiate** into specialized cells.  **Stem cell niche:** Adult stem cells are multipotent stem cells that replenish dying cells, and repair damaged tissue. Adult stem cells are located in stem cell niches. Stem cell niches are locations in the body where stem cells can be maintained or promoted to proliferate and differentiate. Examples of stem cell niches include: - **Bone marrow** is a niche for hematopoietic stem cells which can differentiate into all types of blood cells - **Hair follicles** contain various pools of stem cells, such as epithelial, melanocyte, and mesenchymal stem cells that self-renew, differentiate, regulate hair growth, and maintain skin homeostasis **Totipotent, puloripotent, multipotent:** **Totipotent** stem cells can differentiate into all types of cells or develop into an embryo. Zygotes are totipotent. **Pluripotent** stem cells can differentiate into all types of cells, but not develop into an embryo. Embryonic stem cells are pluripotent. **Multipotent** stem cells can differentiate into a limited range of cells. Adult stem cells are multipotent. **Cell size:** There is a wide variety of cell sizes. Stem cells differentiate into specialized cells with a range of sizes. Specialized human cells include: - **Sperm** are the smallest cells in the body with a length of 50 to 70µm, and a width of 2 to 3µm. - **Ova** (egg cells) are the cell with the largest volume with a diameter of approximately 100µm. - **Neurons** have a width ranging from 4 to 100µm and can have a length of over 1m. - **Red blood cells** have a diameter of 6 to 8µm, allowing them to move through capillaries. - **White blood cells** range from 6 to 20µm depending on the type of white blood cell. - **Striated muscle fibres** are atypical multinucleate cells forming from the fusion of several cells. Striated muscle fibres can range in length from a few millimetres to several centimetres and have a diameter of 10 to a 100µm. **Implications of the Surface Area to Volume Ratio:** The surface area of a cell is the plasma membrane, which is responsible for the exchange of materials and heat between the cell and its environment. The volume of the cell is mainly cytoplasm, which is the site of most of the metabolism within the cell. As the cell grows the rate of metabolism increases faster than the cell's ability to transfer materials in and out of the cell. Growing cells tend to divide when the surface area of the plasma membrane is not efficient enough to exchange materials for metabolism. **Adaptation to Increase the Surface Area of Cells:\ **Specialized cells involved in the exchange of materials have adaptations to increase surface area. Erythrocytes (red blood cells) are thin and flat with a biconcave shape, which increases their surface area for the exchange of gases. The cells lining the proximal convoluted tubule include extensions of the cytoplasm known as microvilli. The microvilli increase the rate of absorption of nutrients and water as filtrate passes through the nephrons of the kidney. **Alveoli:** Gas exchange happens at the alveoli in the lungs. Alveolar epithelial tissue is composed of two types of cells which facilitate the exchange of gases. Alveolar tissue is adapted for the rapid exchange of gases, and consists of two types of cells: - **Type I Pneumocytes** are long and extremely flat cells adapted for gas exchange. - **Type II Pneumocytes** are cuboid-shaped cells which secrete a surfactant that reduces surface tension in the alveoli and provides a liquid for rapid diffusion of gases. The type II pneumocytes contain many secretory vesicles (lamellar bodies), which release the surfactant by exocytosis. **Muscle tissues;** Muscles contract and are involved in movement. There are three types of muscle cells: 1. Cardiac Muscle **Cardiac muscle** is located in the heart and is responsible for the continued rhythmic beating of the heart. Both muscle types contain contractile **myofibrils**. Cardiac muscle cells are branched and connected by intercalated discs to other muscle cells. The branched nature of the cardiac muscle and the intercalated discs allows the rapid transmission of electrical impulses through heart muscle tissue. Electrical impulses trigger contraction of the cardiac muscle cells. Intercalated discs are gap junctions between cells and allow ions to flow between two cells. 한번에 contract 한다. 2. Striated Muscle (also known as skeletal muscle) **Striated muscle** is attached to the skeleton and is also known as skeletal muscle. Striated muscle is involved in the movement of bones 3. Smooth Muscle Striated muscles are composed of long multinucleated fibres that are formed by the fusion of cells. Since muscle fibres are formed by the fusion of cells, it is debatable if muscle fibres should be considered cells. **Gametes:** **Gametes** are reproductive cells. Male gametes are sperm. Female gametes are eggs (ova). Gametes have haploid nuclei (half amount of DNA). When gametes fuse during fertilization, a diploid zygote is formed. **Sperm (Male gametes):** Sperm are small cells with a streamlined shape, adapted to swimming. Sperm are composed of three sections, a head, a midpiece and a flagellum (tail)  **Adaptation of sperms:** The head of sperm contains a haploid nucleus and an acrosome. The **haploid nucleus** contains the paternal genetic information which combines with the maternal genetic information at fertilization. The **acrosome** contains hydrolytic enzymes, which help the sperm to fertilize the egg. The midpiece contains many mitochondria that provide the sperm with ATP energy to swim The **flagellum** allows the sperm to swim through the female reproductive system to reach the egg. **Egg cells (Female gametes):** Eggs (Ova) are large cells that contain a haploid nucleus with the maternal genetic information which is combined with the male genetic information at fertilization. **Adaptation of Egg cells:** The ovum is surrounded by a glycoprotein matrix, known as the zona pellucida, which prevents polyspermy after fertilization. The ovum contains many vesicles known as cortical granules which make the zona pellucida impenetrable to sperm after fertilization. The cytoplasm of the egg contains many lipid droplets which provide the developing embryo with energy after fertilization.