Biological Processes and Cell Structures Flashcards

The Seven Characteristics of Life and Fundamental Biological Processes

The study of biology begins with identifying the seven core characteristics of life that define all living organisms. These processes include movement, respiration, sensitivity, growth, reproduction, excretion, and nutrition. Movement refers to an action by an organism or part of an organism causing a change of position or place. Respiration involves the chemical reactions in cells that break down nutrient molecules and release energy for metabolism; specifically, aerobic respiration requires oxygen. Sensitivity is the ability to detect or sense stimuli in the internal or external environment and to make appropriate responses. Growth is a permanent increase in size and dry mass by an increase in cell number or cell size or both. Dry mass is preferred for measuring growth because it excludes the fluctuating water content within an organism, which can vary significantly and do not reflect actual biological growth. Reproduction consists of the processes that make more of the same kind of organism. Excretion is the removal from organisms of the waste products of metabolism, such as chemical reactions in cells including respiration, toxic materials, and substances in excess of requirements. Finally, nutrition is the taking in of materials for energy, growth, and development.

Cell Structure, Function, and Organization

Cells are the basic functional units of all living organisms. Both plant and animal cells share several common structures, including the cytoplasm, where many metabolic reactions occur; the cell membrane, which controls the exit and entry of substances; the nucleus, containing DNA and genetic material; ribosomes, where protein synthesis occurs; and mitochondria, which are the site of aerobic respiration to release energy. However, plant cells possess unique structures including a cell wall made of cellulose for structural support, chloroplasts containing chlorophyll for photosynthesis, and a large permanent vacuole filled with cell sap. Specialized cells are adapted for specific functions: root hair cells possess a large surface area for the absorption of water and mineral ions; palisade mesophyll cells are packed with chloroplasts for photosynthesis; and red blood cells are designed to carry oxygen throughout the body. Ciliated cells use hair-like projections called cilia to move mucus.

Bacterial cells are distinct from plant and animal cells, lacking a nucleus and membrane-bound organelles like mitochondria. Their structure includes a cell wall, a cell membrane, cytoplasm, and ribosomes. Instead of a nucleus, their genetic material consists of circular DNA and smaller loops called plasmids. In multicellular organisms, the body is organized hierarchically to function efficiently: cells are grouped into tissues (groups of cells with similar structures working together to perform a shared function), tissues are grouped into organs (structures made up of a group of tissues working together to perform specific functions), and organs are grouped into organ systems (groups of organs with related functions working together to perform body functions).

Microscopy and Biological Calculations

The magnification of a specimen viewed under a microscope is determined by the relationship between the size of the image and the actual size of the object. The formula for magnification is expressed as:

$\text{magnification} = \frac{\text{image size}}{\text{actual size}}$

Conversely, the actual size of a biological structure is calculated by dividing the measured image size by the magnification:

$\text{actual size} = \frac{\text{measured image size}}{\text{magnification}}$

When performing these calculations, it is essential to use consistent units, typically millimeters ($mm$). For higher precision, biologists convert measurements between millimeters and micrometers ($\mu m$), where $1\,mm = 1000\,\mu m$.

Movement Into and Out of Cells

Substances move across cell membranes via diffusion, osmosis, and active transport. Diffusion is the net movement of particles from a region of their higher concentration to a region of their lower concentration down a concentration gradient; it is a passive process that does not require energy. Factors affecting the rate of diffusion include the surface area available, temperature, the steepness of the concentration gradient, and the diffusion distance. Water is a vital solvent for life, essential for digestion, excretion, and transport. Osmosis is a specific type of diffusion referring to the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentrated solution) through a partially permeable membrane.

In plant tissues, osmosis affects cell state: a cell becomes turgid when it takes in water, exerting turgor pressure against the cell wall. Conversely, if a cell loses water, it becomes flaccid; extreme water loss leads to plasmolysis, where the cell membrane pulls away from the cell wall. Active transport is the movement of particles across a cell membrane from a region of lower concentration to a region of higher concentration (against a concentration gradient) using energy from respiration. This process is critical for organisms, such as for the uptake of mineral ions by plant root hair cells.

Biological Molecules and Food Testing

Living organisms are composed of biological molecules including carbohydrates, proteins, and fats (lipids). Carbohydrates, proteins, and fats all contain the elements carbon, hydrogen, and oxygen. Proteins uniquely contain nitrogen and sometimes sulfur. Large molecules are built from smaller subunits: starch, glycogen, and cellulose are polymers of glucose; proteins are made of amino acids; and fats/oils are composed of fatty acids and glycerol.

To identify these molecules, specific chemical tests are used. Starch is identified using biological iodine solution, which turns from orange-brown to blue-black. Reducing sugars (like glucose) are tested with Benedict's solution, which requires heating and changes color from blue to green, yellow, or brick-red depending on the concentration. Proteins are identified using the Biuret test, which turns purple in the presence of protein. Fats and oils are detected using the ethanol emulsion test; a white, cloudy emulsion indicates the presence of lipids.

Enzyme Action and Metabolism

Enzymes are proteins that function as biological catalysts, controlling the rate of metabolic reactions without being consumed. Their function depends on the active site, which has a specific and complementary shape to a particular substrate. When a substrate binds to the active site, an enzyme-substrate complex is formed, leading to the creation of a product. This specificity is often described as the lock-and-key model. Enzyme activity is heavily influenced by environmental factors. Increasing temperature or shifting pH away from the optimum can cause the enzyme's active site to change shape, a process known as denaturation, which prevents the substrate from fitting. At the optimum temperature, enzyme activity is highest due to increased kinetic energy and a higher frequency of effective collisions between enzymes and substrates.

Human Nutrition and Digestion

A balanced diet provides all necessary nutrients in the correct proportions, including carbohydrates, fats, oils, proteins, vitamins (especially C and D), mineral ions (calcium and iron), fibre (roughage), and water. Vitamin C is vital for skin and gums, Vitamin D and calcium are essential for bone health, and iron is necessary for hemoglobin in red blood cells. The human digestive system, or alimentary canal, consists of the mouth (and salivary glands), oesophagus, stomach, small intestine (duodenum and ileum), pancreas, liver, gall bladder, and large intestine (colon, rectum, and anus).

Digestion occurs in several stages: ingestion (taking food/drink into the body), digestion (breaking down food), absorption (nutrients moving into the blood), assimilation (cells taking up and using nutrients), and egestion (removing undigested waste as faeces). Physical digestion involves breaking food into smaller pieces to increase the surface area for enzymes, without changing the chemical nature of the food. Chemical digestion involves the breakdown of large insoluble molecules into small soluble molecules. Key enzymes include amylase, which breaks starch into reducing sugars; proteases, which break proteins into amino acids; and lipases, which break fats into fatty acids and glycerol. The stomach secretes gastric juice containing hydrochloric acid ($HCl$), which maintains an acidic pH to kill harmful microorganisms and provide the optimum environment for protease activity.