cell biology

# Cell structure and comparison This topic explores the fundamental building blocks of life, cells, by comparing the ultrastructure and functions of plant, animal, fungal, and bacterial cells, detailing their various cellular components [2](#page=2). ### 1.1 Cells as building blocks of life All living organisms are composed of one or more cells. Organisms made of a single cell are termed unicellular, while those with many cells are multicellular. The detailed organization of a cell as observed under a powerful electron microscope is known as its ultrastructure [2](#page=2). ### 1.2 Common cellular components Plant, animal, fungal, and bacterial cells share several key structures [2](#page=2). * **Cell membrane:** This selectively permeable barrier controls the movement of substances into and out of the cell [2](#page=2). * **Cytoplasm:** A jelly-like substance where most of the cell's chemical reactions occur [2](#page=2). * **Ribosomes:** The sites for protein synthesis, where amino acids are assembled into proteins [2](#page=2). ### 1.3 Components present in most cells (excluding bacteria) Apart from bacterial cells, most cells possess mitochondria and a nucleus [2](#page=2). * **Mitochondria:** These organelles are responsible for generating energy through aerobic respiration [2](#page=2). * **Nucleus:** This organelle contains the cell's DNA and directs all cellular activities [2](#page=2). ### 1.4 Bacterial cellular structures Bacterial cells, while lacking a true nucleus, possess a **nucleoid**, which is a large ring of DNA that controls some of the cell's activities. Additionally, bacteria contain **plasmids**, which are smaller rings of DNA that also regulate cellular activities [2](#page=2). ### 1.5 Cell walls With the exception of animal cells, all other cell types discussed are surrounded by a cell wall that provides structural support and strength [2](#page=2). * **Plant cell walls:** Composed of cellulose [2](#page=2). * **Fungal cell walls:** Composed of chitin [2](#page=2). * **Bacterial cell walls:** Have a chemical structure distinct from those of plants and fungi [2](#page=2). ### 1.6 Structures exclusive to plant cells Plant cells possess two additional organelles not found in animal, fungal, or bacterial cells [2](#page=2): * **Vacuole:** This organelle is filled with cell sap, a dilute solution of salts and sugars, which contributes to cellular support [2](#page=2). * **Chloroplasts:** These organelles contain chlorophyll, the green pigment essential for trapping light energy during photosynthesis [2](#page=2). ### 1.7 Summary of cellular components and functions | Cell structure | Organism (P, A, F, B) | Function (what it does) | | :------------- | :-------------------- | :---------------------------------------------------- | | Nucleus | P, A, F | Contains DNA. Controls the activities of the cell. | | Cytoplasm | P, A, F, B | Site of most chemical reactions. | | Cell membrane | P, A, F, B | Is selectively permeable. Controls entry/exit of materials. | | Chloroplasts | P | Contains chlorophyll. Involved in photosynthesis. | | Vacuole | P, F | Filled with cell sap; provides support. | | Cell wall | P, F, B | Strengthens the cell and gives it support. | | Ribosomes | P, A, F, B | Production of protein from amino acids. | | Mitochondria | P, A, F | Provide energy from aerobic respiration. | | Nucleoid | B | Large “ring” of DNA. Controls some cell activities. | | Plasmids | B | Small “ring” of DNA. Controls some cell activities. | *Tip: When comparing cell types, systematically consider each listed organelle and note its presence or absence in each cell type, along with its specific function.* > **Example:** A plant cell has a cell wall, a nucleus, mitochondria, chloroplasts, a vacuole, ribosomes, cytoplasm, and a cell membrane. In contrast, an animal cell lacks a cell wall, chloroplasts, and a large central vacuole, but contains the other common organelles [2](#page=2). ### 1.8 Comparative questions To solidify understanding, consider the following comparative points: 1. **Structures common to plant, animal, fungal, and bacteria cells:** These include the cell membrane, cytoplasm, and ribosomes [2](#page=2). 2. **Structures unique to plant cells:** Plant cells are distinguished by the presence of chloroplasts and a large central vacuole [2](#page=2). 3. **Organisms with similar cell structure to plants and animals but different cell walls:** Fungal cells share many similarities with plant and animal cells but have cell walls made of chitin, not cellulose or the absence of a wall [2](#page=2). 4. **Organisms with different cell wall structures to plant and fungal cells:** Bacterial cells have a cell wall with a chemical structure that differs from both plant and fungal cell walls [2](#page=2). --- # Transport across cell membranes Cell membranes, composed of lipids and proteins, regulate the passage of substances into and out of the cell through various transport mechanisms [5](#page=5). ### 2.1 Cell membrane structure and investigation The cell membrane is understood to be composed of both protein and lipid components. Experiments are designed to investigate the integrity and function of the cell membrane by observing the leakage of cell contents when these components are damaged. Proteins can be denatured by temperatures above 45 degrees Celsius, while lipids dissolve in alcohol [5](#page=5). #### 2.1.1 Investigating membrane composition using beetroot An experiment using beetroot cylinders can demonstrate the presence of lipids and proteins in the cell membrane. Beetroot cells contain a coloured pigment in their vacuole, which can be observed to leak out if the cell membrane is damaged [5](#page=5). **Experimental setup:** * Beetroot cylinders are placed in boiling tubes with either water or ethanol [6](#page=6). * One boiling tube with water is kept at 25 degrees Celsius, and another at 60 degrees Celsius [6](#page=6). * The boiling tube with ethanol is kept at 25 degrees Celsius [6](#page=6). * After 15 minutes, colour leakage is observed and compared using a colour chart [6](#page=6). **Expected Observations and Conclusion:** * Heating to 60 degrees Celsius denatures proteins, causing leakage [5](#page=5). * Exposure to ethanol dissolves lipids, causing leakage [5](#page=5). * The experiment confirms that the cell membrane consists of lipids and proteins [7](#page=7). **Evaluation points for the experiment:** * Washing cylinders removes surface pigments and potential contaminants [7](#page=7). * Washing in cold water prevents initial damage to the membrane before the experiment [7](#page=7). * Cylinders of equal length and diameter ensure a fair comparison of results [7](#page=7). * Beetroot is used due to its easily detectable coloured pigment in the vacuole [5](#page=5). * Increasing the number of beetroot cylinders and repeating the experiment can enhance reliability [7](#page=7). #### 2.1.2 The fluid mosaic model The cell membrane's structure is described by the fluid mosaic model. It consists of a phospholipid bilayer with embedded proteins. Some proteins extend across the entire membrane, forming channel proteins, which create pores for the movement of molecules. The phospholipid layers are considered fluid, and the proteins are interspersed within them, hence the "fluid mosaic" description [7](#page=7). > **Tip:** Remember that channel-forming proteins act like tunnels, allowing specific substances to pass through the membrane [7](#page=7). ### 2.2 Diffusion and its importance Diffusion is a fundamental process for the transport of materials across cell membranes [8](#page=8). #### 2.2.1 Definition and mechanism of diffusion Diffusion is defined as the net movement of molecules from an area of high concentration to an area of low concentration, moving down a concentration gradient. A concentration gradient is simply the difference in concentration of a substance between two areas [8](#page=8). * **Passive process:** Diffusion does not require metabolic energy to occur [8](#page=8). > **Tip:** Think of diffusion like a drop of ink spreading out in water – it moves from where there's a lot of ink to where there's none, without needing to be pushed [8](#page=8). #### 2.2.2 Role of the cell membrane in diffusion The cell membrane is selectively permeable, meaning it allows small, soluble molecules to pass through but restricts the passage of large, insoluble molecules [8](#page=8). #### 2.2.3 Importance of diffusion in organisms Diffusion is crucial for cellular survival and material exchange in both unicellular and multicellular organisms [8](#page=8). * **Unicellular organisms:** Rely on diffusion for uptake of essential substances like oxygen and glucose, and for the removal of waste products such as carbon dioxide [8](#page=8). * **Multicellular organisms:** Diffusion is vital for: 1. Exchange of respiratory gases between lungs and capillaries [9](#page=9). 2. Exchange of gases between capillaries and respiring cells [9](#page=9). 3. Movement of dissolved food from the small intestine into blood capillaries [9](#page=9). 4. Movement of dissolved food from blood capillaries to respiring cells [9](#page=9). 5. Movement of urea from cells into the bloodstream [9](#page=9). #### 2.2.4 Demonstrating diffusion and selective permeability Visking tubing can be used to model the selective permeability of cell membranes [10](#page=10). **Experiment with Visking tubing:** * Visking tubing, representing a selectively permeable membrane, is filled with a mixture of starch and glucose [11](#page=11). * The filled tubing is placed in a boiling tube containing distilled water [11](#page=11). * After 15 minutes, the surrounding water is tested for starch (using iodine solution) and sugar (using Benedict's solution) [11](#page=11). * **Results:** Glucose is detected in the water, while starch is not [11](#page=11). * **Conclusion:** This demonstrates that glucose molecules are small enough to pass through the visking tubing (cell membrane), but starch molecules are too large [11](#page=11). **Evaluation points for the Visking tubing experiment:** * Rinsing the filled tubing removes any external contamination [11](#page=11). * Leaving the experiment for 15 minutes allows sufficient time for diffusion to occur [11](#page=11). * Repeating the experiment with different sized molecules or varying concentrations can further explore diffusion principles [11](#page=11). ### 2.3 Osmosis and its effect on cells Osmosis is a specific type of diffusion involving water molecules across a selectively permeable membrane [12](#page=12). #### 2.3.1 Definition of osmosis Osmosis is the movement of water molecules from an area of high water concentration to an area of low water concentration, through a selectively permeable membrane [12](#page=12). #### 2.3.2 Osmosis in solutions of different concentrations When cells are placed in solutions with different concentrations, water movement occurs, affecting the cells [12](#page=12). * **Hypertonic solution:** Has a lower water concentration (higher solute concentration) than the cell. Water moves out of the cell, causing it to shrink or become plasmolysed [13](#page=13) [14](#page=14). * **Isotonic solution:** Has an equal water concentration to the cell. There is no net movement of water, and the cell remains unchanged [13](#page=13) [14](#page=14). * **Hypotonic solution:** Has a higher water concentration (lower solute concentration) than the cell. Water moves into the cell, causing it to swell or become turgid (in plant cells) or burst (in animal cells) [13](#page=13) [14](#page=14). #### 2.3.3 Effects on animal and plant cells * **Animal cells:** In a hypertonic solution, they shrink (crenation). In a hypotonic solution, they can burst (lysis). In an isotonic solution, they remain unchanged [14](#page=14). * **Plant cells:** In a hypertonic solution, they become plasmolysed (cell membrane pulls away from the cell wall). In a hypotonic solution, they become turgid (cell swells against the rigid cell wall). In an isotonic solution, they remain unchanged but are flaccid [14](#page=14). > **Example:** A red blood cell placed in pure water (a very hypotonic solution) will swell and burst because there is no cell wall to resist the influx of water. A plant cell, however, will become turgid as the cell wall prevents it from bursting [14](#page=14). #### 2.3.4 Demonstrating osmosis An osmometer can demonstrate osmosis. For instance, a setup with visking tubing filled with sucrose solution placed in water will show water moving into the tubing, causing the level of the sucrose solution to rise. This is because the water concentration is higher in the surrounding water than inside the tubing, and water moves across the selectively permeable membrane down its concentration gradient [12](#page=12) [13](#page=13). --- # Active transport Active transport is the movement of molecules and ions across the cell membrane against their concentration gradient, requiring energy and specific membrane proteins [15](#page=15). ### 3.1 Definition and mechanism of active transport Active transport involves the movement of molecules or ions from an area of low concentration to an area of high concentration, directly opposing the passive process of diffusion. This movement is facilitated by protein molecules, specifically carrier proteins, embedded within the cell membrane. These carrier proteins bind to the molecules or ions and, with the input of energy, transport them across the membrane [15](#page=15). > **Tip:** Remember that active transport is the "uphill" movement of substances, requiring biological work, unlike diffusion which is "downhill" and requires no energy input. ### 3.2 Energy requirement The energy required for active transport is supplied by cellular respiration, primarily in the form of ATP. Since respiration is the source of energy, factors that affect the rate of respiration will also affect the rate of active transport [15](#page=15). ### 3.3 Factors affecting the rate of active transport The rate of active transport is influenced by several factors, all of which are linked to the cell's ability to produce energy through respiration [15](#page=15): * **Temperature:** Higher temperatures generally increase enzyme activity involved in respiration, up to an optimal point, thereby increasing the rate of ATP production and active transport. Extremely high temperatures can denature enzymes and halt respiration [15](#page=15). * **Availability of oxygen:** Oxygen is essential for aerobic respiration, the primary method of ATP production in most cells. A lack of oxygen will limit respiration and thus reduce the rate of active transport [17](#page=17). * **Concentration of respiratory substrate:** Respiratory substrates, such as glucose, are broken down during respiration to release energy. A higher concentration of these substrates can increase the rate of respiration and consequently the rate of active transport, up to a certain limit [15](#page=15). > **Tip:** Understanding the link between respiration and active transport is key. If cellular respiration is inhibited, active transport will also cease. ### 3.4 Examples of active transport Several biological processes rely on active transport to function effectively: * **Ion transport in nerve cells:** Nerve cells actively transport sodium and potassium ions across their membranes to maintain specific ion gradients, which are crucial for nerve impulse transmission. This process helps to establish the resting potential and propagate action potentials [15](#page=15). * **Uptake in seaweed:** Seaweed cells actively transport iodine from the surrounding seawater into their cells. The concentration of iodine inside seaweed cells can be significantly higher than in the seawater, demonstrating movement against a concentration gradient [15](#page=15) [17](#page=17). > **Example:** In nerve cells, the sodium-potassium pump actively moves three sodium ions out of the cell for every two potassium ions pumped into the cell, using ATP. This creates electrochemical gradients vital for nerve signaling [15](#page=15). > **Example:** The experiment in Activity 2 on page 17 shows that seawater has a low concentration of potassium (0.02 units) and a high concentration of sodium (0.59 units), while seaweed cell sap has a high concentration of potassium (0.56 units) and a low concentration of sodium (0.03 units). This difference in concentration is achieved through active transport [17](#page=17). > **Example:** The graph in Activity 2 (page 17) illustrates how oxygen concentration affects the uptake of nitrate ions by barley roots. From 0 to 30% oxygen, the rate of nitrate uptake increases with oxygen concentration because more oxygen allows for higher rates of respiration and ATP production to fuel active transport. Above 30% oxygen, the rate levels off, suggesting that another factor, such as the number of available carrier proteins or nitrate transporters, becomes the limiting step [17](#page=17). --- # Cell division and DNA This topic explores the process of mitosis for cell division and the fundamental structure and function of DNA in carrying genetic information for protein synthesis [18](#page=18) [21](#page=21). ### 4.1 Mitosis: Producing new cells Mitosis is the process of cell division where new cells are produced from existing ones, controlled by the nucleus. Each species has a specific number of chromosomes in its cells, known as the chromosome complement. Most cells, excluding sex cells, have two matching sets of chromosomes, referred to as a diploid chromosome complement [18](#page=18). Chromosomes carry the genetic information essential for an organism's structure and function. To ensure vital genetic information is passed on without loss, new sets of chromosomes must be replicated before a cell divides [18](#page=18). #### 4.1.1 The sequence of events in mitosis The process of mitosis involves several distinct stages: * **Stage 1:** Chromosomes become visible within the nucleus. Two structures called centrioles are observed outside the nucleus [19](#page=19). * **Stage 2:** Each chromosome replicates, creating an identical copy (chromatid) joined at a centromere [19](#page=19). * **Stage 3:** The nuclear membrane disintegrates. Centrioles move to opposite poles of the cell and form spindle fibres. The replicated chromatid pairs then align at the cell's equator, attached to the spindle fibres [19](#page=19). * **Stage 4:** The chromatid pairs are separated and moved towards opposite poles of the cell [19](#page=19). * **Stage 5:** New nuclear membranes form around each group of chromatids [19](#page=19). * **Stage 6:** The cytoplasm divides, and new cell membranes form, resulting in two identical daughter cells. These cells then undergo a growth phase before mitosis can begin again [19](#page=19). > **Tip:** Mitosis ensures that the diploid chromosome complement of cells is maintained throughout this process [20](#page=20). ### 4.2 DNA and protein production #### 4.2.1 The structure of DNA Deoxyribonucleic acid (DNA) is located in the nucleus of every cell and is composed of two strands twisted into a helix. Each strand is a chain of bases, and the complete DNA molecule contains the entire genetic information of an organism. The two strands are connected by bonds between the bases [21](#page=21). DNA contains four types of bases: Guanine (G), Cytosine (C), Adenine (A), and Thymine (T). These bases pair specifically due to their shapes: Guanine always pairs with Cytosine, and Adenine always pairs with Thymine. This pairing is reversible, meaning Cytosine pairs with Guanine, and Thymine pairs with Adenine [21](#page=21). > **Example:** If one strand of DNA has the sequence A T C C G G T C T A A T G C T A T G A C C T T G C T, the complementary strand would be T A G G C C A G A T T A C G A T A C T G G A A C G A [22](#page=22). #### 4.2.2 Messenger RNA (mRNA) and protein synthesis Proteins are essential molecules composed of long chains of amino acids, with their specific sequence determining the protein's final structure. DNA carries genetic instructions on specific segments called genes, which can range from hundreds to thousands of bases long and code for particular proteins [23](#page=23). However, DNA is too large to leave the nucleus and travel to the ribosomes in the cytoplasm, where protein synthesis occurs. This challenge is overcome by messenger RNA (mRNA) [23](#page=23). * **mRNA function:** mRNA acts as a carrier molecule, creating a single-stranded "mirror image" copy of the genetic code from a gene in the nucleus. Because it is a single strand and smaller than DNA, mRNA can exit the nucleus and move to a ribosome in the cytoplasm [23](#page=23). * **Ribosomes and amino acids:** Ribosomes are structures in the cytoplasm that read the mRNA sequence. They match the mRNA "code words" to specific amino acids [23](#page=23). * **Protein assembly:** Amino acids are joined together in chains at the ribosomes to form proteins. The order of bases in the DNA sequence directly dictates the order of amino acids in the resulting protein chain [23](#page=23). Each group of three bases on the DNA strand forms a "code word" that specifies a particular amino acid [23](#page=23). > **Example:** Using the provided DNA code and table, a DNA strand can be translated into a chain of amino acids, forming a protein. For instance, the DNA code CCC corresponds to Amino Acid P, and GCA corresponds to Amino Acid M [24](#page=24). > **Activity:** Understanding the relationship between DNA base sequences and amino acid chains is crucial. For example, consider the DNA strand: C C C G C A C G C G G A A A C G G A C G C. Using a provided table, you can determine the corresponding amino acid sequence [24](#page=24). --- # Proteins, enzymes, and genetic engineering This section explores the diverse structures and functions of proteins, focusing on enzymes and the fundamental principles of genetic engineering [25](#page=25). ### 5.1 The variety of protein shapes and functions The specific sequence of amino acids in a protein dictates its folding and coiling, which in turn determines its unique three-dimensional shape and, consequently, its specific function. This process applies not only to enzymes but also to structural proteins, hormones, and antibodies [25](#page=25) [26](#page=26). #### 5.1.1 Functions of proteins Proteins perform a wide array of roles within living organisms, including: 1. **Structural:** These proteins provide support and form. * Examples include proteins in cell membranes that form channels for molecule transport and elastin in artery walls for flexible support [27](#page=27). 2. **Enzymes:** These are biological catalysts that accelerate biochemical reactions [27](#page=27). * Examples include amylase, which breaks down starch into maltose, and pepsin, which breaks down protein into peptides [27](#page=27). 3. **Hormones:** These are chemical messengers that regulate growth and metabolism, transported through the bloodstream. * Examples include insulin, which promotes the conversion of glucose to glycogen, and somatotrophin, which promotes the growth of long bones [27](#page=27). 4. **Antibodies:** These Y-shaped proteins are produced by white blood cells and are crucial for defending the body against antigens, which are foreign molecules recognized by the body [27](#page=27). > **Tip:** Understanding the link between amino acid sequence, protein shape, and protein function is a fundamental concept. ### 5.2 Enzymes Enzymes are essential biological catalysts manufactured by all living cells that significantly speed up cellular reactions without being consumed in the process [29](#page=29). #### 5.2.1 Enzyme activity and specificity The active site of an enzyme has a shape that is complementary to a specific substrate. This precise fit allows for the formation of an enzyme-substrate complex, facilitating the biochemical reaction. Enzymes are involved in two main types of reactions [29](#page=29) [30](#page=30): * **Degradation reactions (breaking down):** * Starch is broken down into maltose by amylase [30](#page=30). * Protein is broken down into peptides by pepsin [30](#page=30). * Fats are broken down into fatty acids and glycerol by lipase [30](#page=30). * Hydrogen peroxide is broken down into oxygen and water by catalase [30](#page=30). * **Synthesis reactions (building up):** * Glucose-1-phosphate is synthesized into starch by phosphorylase [30](#page=30). > **Example:** The lock-and-key model is often used to describe enzyme-substrate specificity, where the substrate (key) fits precisely into the enzyme's active site (lock). #### 5.2.2 Optimum conditions and denaturation Each enzyme functions optimally under specific conditions, known as its optimum. Deviating from these optimum conditions can lead to a decrease in enzyme activity, with significant deviations causing irreversible damage [31](#page=31). * **Optimum Temperature:** Enzymes have a temperature at which they are most active. For human enzymes, this is often around 37 degrees Celsius. High temperatures can cause enzymes to denature, while very low temperatures slow down their activity [31](#page=31). * **Optimum pH:** Similarly, enzymes have an optimal pH range for activity. Extreme pH values can alter the enzyme's shape and render it inactive. For instance, pepsin, found in the stomach, works best at a low pH (around pH 2) whereas enzymes in the small intestine operate at a more neutral pH [31](#page=31) [32](#page=32). An enzyme that has been permanently damaged and is unable to function due to unfavorable conditions is said to be **denatured** [31](#page=31). ### 5.3 Genetic engineering Genetic engineering involves the artificial transfer of DNA segments from one organism to another, a process that can also occur naturally [34](#page=34). #### 5.3.1 Principles and stages of genetic engineering The core principle of genetic engineering is to manipulate genes to achieve a desired outcome, such as producing a specific protein. The stages involved in this process, often exemplified by the transfer of the human insulin gene into bacteria, include [34](#page=34): 1. **Identifying the required gene on DNA:** Locating the specific gene of interest within the source organism's DNA (e.g., the insulin gene from human DNA) [34](#page=34). 2. **Extracting the required gene:** Isolating the identified gene from the source DNA. This often involves using specific enzymes [34](#page=34). 3. **Extraction of a bacterial plasmid:** Obtaining a circular piece of DNA called a plasmid from a bacterium. Plasmids serve as vectors for gene transfer [34](#page=34). 4. **Insertion of the required gene into a plasmid:** Inserting the extracted gene into the bacterial plasmid, creating recombinant DNA [34](#page=34) [35](#page=35) [36](#page=36). 5. **Insertion of the plasmid into a host cell:** Introducing the genetically modified plasmid into a host cell, typically a bacterium [34](#page=34). 6. **Growth of the genetically modified organism producing the required product:** Culturing the host cells, which will then replicate the inserted gene and produce the desired product (e.g., insulin) [34](#page=34) [35](#page=35) [36](#page=36). > **Tip:** Visualize genetic engineering as cutting a specific instruction manual page (gene) from one book (organism's DNA) and pasting it into another (bacterial plasmid) to make that second book do a new task. --- # Photosynthesis and respiration Photosynthesis is the process by which green plants use light energy to convert water and carbon dioxide into glucose, while respiration releases the chemical energy stored in glucose to generate ATP for cellular activities. ## 6. Photosynthesis and respiration ### 6.1 Photosynthesis: The process of energy capture Photosynthesis is an enzyme-controlled series of reactions occurring in green plants. Its primary function is to produce glucose from the raw materials of water and carbon dioxide. This process also requires light energy and chlorophyll [38](#page=38). #### 6.1.1 The role of light energy and chlorophyll Photosynthesis involves a 'light reaction' where light energy from the sun is trapped by chlorophyll, located within chloroplasts. This trapped energy is utilized in two key ways [39](#page=39): * **Splitting water:** Light energy is used to split water molecules into hydrogen and oxygen. Oxygen diffuses out of the cells and is released into the atmosphere. Hydrogen is a crucial component for the subsequent stage, where it binds to a hydrogen acceptor, reducing it [39](#page=39). * **ATP production:** Some light energy is converted into chemical energy in the form of adenosine tri-phosphate (ATP) by combining adenosine di-phosphate (ADP) and inorganic phosphate (Pi) [39](#page=39). #### 6.1.2 Carbon fixation: Building sugars The second stage of photosynthesis is known as carbon fixation. In this stage, carbon dioxide and hydrogen (from the light reaction) are used to build sugar, specifically glucose. This process requires energy, which is supplied in the form of ATP produced during the light reaction. The hydrogen acceptor, having delivered its hydrogen, becomes free to combine with more hydrogen. ATP is broken down into ADP + Pi, and this ADP + Pi can be reformed into ATP using energy, facilitating energy transfer between the two stages [40](#page=40). > **Tip:** ATP acts as an energy currency within the cell, providing instant energy for various cellular processes [39](#page=39). #### 6.1.3 The fate of sugar Once glucose is synthesized through photosynthesis, its chemical energy can be utilized in several ways. It can be immediately used for respiration, providing energy for cellular activities, or it can be converted into starch for storage or cellulose for structural purposes, such as in cell walls [41](#page=41). #### 6.1.4 Limiting factors in photosynthesis The rate of photosynthesis can be limited by factors that are in short supply. These limiting factors include [42](#page=42): * Carbon dioxide concentration [42](#page=42). * Light intensity [42](#page=42). * Temperature [42](#page=42). These factors can be monitored by measuring the rate of photosynthesis, for example, by observing changes in dry mass over time or counting the number of oxygen bubbles produced in a set period [42](#page=42). > **Example:** Graph 1 illustrates how the rate of photosynthesis increases with carbon dioxide concentration up to a certain point (A to B). Beyond this point (B to C), increasing carbon dioxide has no further effect, indicating that another factor, such as light intensity or temperature, is now limiting the rate [42](#page=42). Graph 2 further demonstrates the interplay of limiting factors, showing that at a lower temperature (15°C), the rate of photosynthesis is lower than at a higher temperature (25°C), even with sufficient carbon dioxide and light. Optimum conditions for plants would involve ensuring that none of these factors are limiting [43](#page=43) [44](#page=44). ### 6.2 Respiration: Releasing energy for cellular activities Respiration is an enzyme-controlled series of reactions that occurs in all living cells, releasing chemical energy stored in glucose. The energy released from respiration is used to generate ATP from ADP and Pi. This energy is vital for numerous cellular activities, including muscle contraction, cell division, protein synthesis, and the transmission of nerve impulses [45](#page=45). #### 6.2.1 Aerobic respiration Aerobic respiration occurs in the presence of oxygen and involves the breakdown of glucose into carbon dioxide and water, yielding a significant amount of energy. The complete breakdown of each glucose molecule in the presence of oxygen produces approximately 38 molecules of ATP [45](#page=45). Aerobic respiration begins in the cytoplasm with glycolysis, where glucose is broken down into two molecules of pyruvic acid. This initial stage occurs whether oxygen is present or not [46](#page=46). If oxygen is present, the pyruvic acid then moves into the mitochondria for further breakdown. In the mitochondria, pyruvic acid is broken down, releasing carbon dioxide and transferring energy-rich hydrogen. This energy is used to generate ATP from ADP + Pi. As two molecules of pyruvic acid are produced from each glucose molecule, this process happens twice, resulting in the generation of 36 ATP molecules from pyruvic acid breakdown. Combined with the 2 ATP molecules from glycolysis, this totals 38 ATP molecules per glucose molecule. The remaining hydrogen combines with oxygen to form water, with oxygen acting as the final hydrogen acceptor [47](#page=47). The overall word equation for aerobic respiration is: $$ \text{glucose} + \text{oxygen} + 38\text{ADP} + 38\text{Pi} \rightarrow \text{carbon dioxide} + \text{water} + 38\text{ATP} $$ [45](#page=45) [47](#page=47). #### 6.2.2 Anaerobic respiration Anaerobic respiration occurs in the absence of oxygen. This process still begins with glycolysis in the cytoplasm, yielding 2 molecules of ATP. However, pyruvic acid cannot be further broken down through the aerobic pathway [46](#page=46) [48](#page=48). * **In animals:** When oxygen is not available, pyruvic acid is broken down into lactic acid. This process is reversible, and the buildup of lactic acid creates an "oxygen debt" that is repaid once sufficient oxygen becomes available, allowing the pyruvic acid breakdown to recommence. The word equation for anaerobic respiration in animals is [48](#page=48): $$ \text{glucose} \rightarrow \text{lactic acid} + \text{energy (2 ATP)} $$ * **In plants and yeast:** In plants and yeast cells, anaerobic respiration, also known as fermentation, converts pyruvic acid into ethanol and carbon dioxide. This process is irreversible. The word equation for anaerobic respiration in plants and yeast is [45](#page=45) [48](#page=48): $$ \text{glucose} \rightarrow \text{ethanol} + \text{carbon dioxide} + \text{energy (2 ATP)} $$ [48](#page=48). In both types of anaerobic respiration, only 2 molecules of ATP are produced per glucose molecule, significantly less than in aerobic respiration [45](#page=45) [48](#page=48). --- ## Common mistakes to avoid - Review all topics thoroughly before exams - Pay attention to formulas and key definitions - Practice with examples provided in each section - Don't memorize without understanding the underlying concepts

| Term | Definition | |------|------------| | Ultrastructure | The detailed structure of a cell as seen with a powerful electron microscope, revealing fine internal components. | | Unicellular organism | An organism that consists of only one cell, performing all life functions within that single cell. | | Multicellular organism | An organism composed of more than one cell, often with specialized cells organized into tissues, organs, and systems. | | Cell wall | A rigid outer layer surrounding the plasma membrane of plant, fungal, and bacterial cells that provides structural support and protection. | | Cell membrane | A selectively permeable barrier that encloses the cytoplasm of a cell, controlling the passage of substances in and out. | | Cytoplasm | The jelly-like substance filling the cell, in which the organelles are suspended and where most of the cell's chemical reactions occur. | | Ribosomes | Small cellular structures responsible for protein synthesis, assembling amino acids into polypeptide chains according to genetic instructions. | | Nucleus | A membrane-bound organelle containing the cell's genetic material (DNA) and controlling the cell's growth and reproduction. | | Nucleoid | A region within a prokaryotic cell (like bacteria) that contains the cell's genetic material, typically a circular chromosome, not enclosed by a membrane. | | Plasmids | Small, circular, extrachromosomal DNA molecules found in bacteria, often carrying genes that confer advantageous traits. | | Mitochondria | Organelles known as the powerhouses of the cell, responsible for generating most of the cell's supply of adenosine triphosphate (ATP), used as a source of chemical energy. | | Chloroplasts | Organelles found in plant cells and eukaryotic algae that conduct photosynthesis, capturing light energy to convert carbon dioxide and water into glucose. | | Chlorophyll | The green pigment found in chloroplasts that absorbs light energy, essential for photosynthesis. | | Photosynthesis | The process used by plants, algae, and cyanobacteria to convert light energy into chemical energy, through a series of reactions that convert carbon dioxide and water into glucose and oxygen. | | Vacuole | A membrane-bound organelle present in plant and fungal cells, often large, that stores water, nutrients, and waste products, and maintains turgor pressure. | | Cell sap | The fluid contained within the vacuole of a plant cell, typically a dilute solution of salts and sugars. | | Selectively permeable | A property of cell membranes that allows certain molecules or ions to pass through it by means of active or passive transport. | | Aerobic respiration | The process by which organisms use oxygen to break down glucose and release energy in the form of ATP, producing carbon dioxide and water as byproducts. | | Denatured | A process where proteins lose their native three-dimensional structure (and thus function) due to external stress like heat, acid, or alkali. | | Lipids | A diverse group of organic compounds that are insoluble in water but soluble in organic solvents; includes fats, oils, waxes, and steroids. | | Ethanol | A volatile, flammable, colorless liquid alcohol, produced by fermentation of sugars by yeast or by synthesis from ethylene. | | Diffusion | The net passive movement of particles from a region of higher concentration to a region of lower concentration, driven by random thermal motion. | | Concentration gradient | The gradual difference in the concentration of a substance between two areas. | | Passive transport | The movement of substances across a cell membrane without the use of energy by the cell. | | ATP (Adenosine Triphosphate) | An energy-carrying molecule found in the cells of all living things. ATP captures chemical energy obtained from the breakdown of food molecules and releases it to fuel the cells activities. | | ADP (Adenosine Diphosphate) | A molecule that is formed when ATP is broken down to release energy; it can be re-energized to form ATP. | | Pi (Inorganic Phosphate) | A phosphate ion ($PO_4^{3-}$) that is not part of an organic molecule, often involved in energy transfer reactions. | | Osmosis | The net movement of water molecules across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration. | | Hypertonic solution | A solution that has a higher solute concentration, and therefore a lower water concentration, than another solution. | | Isotonic solution | A solution that has an equal solute concentration, and therefore an equal water concentration, to another solution. | | Hypotonic solution | A solution that has a lower solute concentration, and therefore a higher water concentration, than another solution. | | Turgid | The state of a plant cell when it is swollen due to the absorption of water, increasing internal pressure against the cell wall. | | Plasmolysed | The process in plant cells where the plasma membrane pulls away from the cell wall due to the loss of water by osmosis. | | Active transport | The movement of molecules across a cell membrane against their concentration gradient, requiring energy (usually from ATP) and transport proteins. | | ATP (Adenosine Triphosphate) | An energy-carrying molecule that powers cellular processes, formed from ADP and Pi using energy. | | Carrier protein | A type of membrane protein that binds to a specific molecule and facilitates its passage across the cell membrane, often used in active transport. | | Mitosis | A type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of growth and repair. | | Chromosome | A thread-like structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes. | | Diploid | Containing two complete sets of chromosomes, one from each parent. | | Chromosome complement | The characteristic number and arrangement of chromosomes in the nucleus of a species. | | Replicated | Copied; in genetics, referring to the duplication of DNA strands or chromosomes. | | Centrioles | Cylindrical structures found near the nucleus in animal cells, involved in the formation of spindle fibers during cell division. | | Spindle fibres | Fibers formed from microtubules that attach to chromosomes during cell division and pull them apart. | | Equator | The imaginary line across the middle of a cell where chromosomes line up during mitosis. | | Poles | The opposite ends of a cell towards which chromosomes move during mitosis. | | DNA (Deoxyribonucleic Acid) | A molecule that carries the genetic instructions for the development, functioning, growth, and reproduction of all known organisms. | | Double stranded helix | The characteristic twisted ladder shape of a DNA molecule, formed by two complementary strands. | | Bases (in DNA) | The four nitrogenous bases that form the "rungs" of the DNA ladder: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). | | mRNA (Messenger RNA) | A molecule that carries a copy of the genetic code from DNA in the nucleus to the ribosomes in the cytoplasm, where it directs protein synthesis. | | Gene | A specific segment of DNA that codes for a functional product, such as a protein or RNA molecule. | | Protein | Large complex molecules made up of chains of amino acids, performing a vast array of functions within organisms. | | Amino acids | The building blocks of proteins; small organic molecules that link together to form long chains (polypeptides). | | Ribosome | A cellular organelle responsible for protein synthesis, translating the mRNA sequence into a specific sequence of amino acids. | | Codon | A sequence of three nucleotides on an mRNA molecule that specifies a particular amino acid or signals the start or end of protein synthesis. | | Structural Proteins | Proteins that provide physical support and shape to cells and tissues, such as collagen in connective tissue. | | Hormones | Chemical messengers produced by endocrine glands that regulate various physiological processes, transported by the bloodstream. | | Antibodies | Y-shaped proteins produced by white blood cells that are part of the immune system; they bind to specific antigens to neutralize pathogens. | | Biological catalysts (Enzymes) | Proteins that speed up biochemical reactions without being consumed in the process. | | Active site | The specific region on an enzyme where a substrate binds and a chemical reaction is catalyzed. | | Substrate | The molecule upon which an enzyme acts. | | Enzyme-substrate complex | The temporary complex formed when a substrate binds to the active site of an enzyme. | | Degradation reactions | Chemical reactions that break down larger molecules into smaller ones. | | Synthesis reactions | Chemical reactions that build larger molecules from smaller ones. | | Optimum conditions | The specific set of environmental conditions (like temperature and pH) under which an enzyme functions most effectively. | | Optimum temperature | The temperature at which an enzyme exhibits its highest activity. | | Optimum pH | The pH at which an enzyme exhibits its highest activity. | | Denatured | When an enzyme loses its specific three-dimensional shape and thus its functional activity due to extreme conditions like high temperature or inappropriate pH. | | Genetic engineering | The deliberate modification of the characteristics of an organism by manipulating its genetic material. | | Plasmid | Small, circular DNA molecules found in bacteria that can be used as vectors to carry foreign genes into host cells. | | Host cell | A cell that is infected by a virus or a bacterium, or a cell that receives foreign DNA during genetic engineering. | | Genetically modified organism (GMO) | An organism whose genetic material has been altered using genetic engineering techniques. | | ATP (Adenosine Triphosphate) | An energy-carrying molecule essential for cellular processes, generated through respiration and used to power cellular activities. | | Photolysis | The splitting of water molecules by light energy during the light-dependent reactions of photosynthesis, producing oxygen, protons, and electrons. | | Limiting factor | A factor that restricts the rate of a process, such as photosynthesis, when it is in short supply. | | Carbon dioxide concentration | The amount of carbon dioxide present in the atmosphere or surrounding medium, which can limit the rate of photosynthesis. | | Light intensity | The strength or amount of light available, which can limit the rate of photosynthesis. | | Temperature | The degree of heat present, which can affect the rate of enzyme-controlled reactions like photosynthesis and respiration. | | Aerobic respiration | Cellular respiration that requires oxygen to break down glucose and release energy (ATP), producing carbon dioxide and water. | | Anaerobic respiration | Cellular respiration that occurs in the absence of oxygen, yielding less ATP than aerobic respiration. | | Lactic acid | A byproduct of anaerobic respiration in animal cells and some bacteria, produced during strenuous exercise when oxygen is limited. | | Fermentation | A metabolic process that converts sugar to acids, gases, or alcohol; it occurs in yeast and bacteria and also in oxygen-starved muscle cells, as a form of anaerobic respiration. | | Ethanol | An alcohol produced by fermentation of sugars by yeast and other microorganisms, a product of anaerobic respiration in plants and yeast. |