Biology UNIT 1

• Cell membrane


  • Transports materials in and out of the cell, acts as a receptor to some chemicals (not large molecules like proteins)
  • Maintains the cell's shape
• The cell membrane is also known as the plasma membrane
• Cell membrane


  • Acts as a selectively permeable barrier, controlling the movement of substances into and out of the cell
• Structure of the cell membrane


  • Composed of a phospholipid bilayer
  • Bilayer consists of two layers of phospholipids with their hydrophilic heads facing the internal and external fluid environments
  • Phospholipids have a negatively charged phosphate "head" (hydrophilic) and two fatty acid "tails" (hydrophobic)
  • Arrangement of phospholipids as a double layer is due to the surrounding water-based mixture (extracellular fluid and cytoplasm)
• Fluid mosaic model


  • Describes how molecules are arranged in the cell membrane
  • "Mosaic" refers to the various types of molecules that make up the membrane, including phospholipids
  • "Fluid" refers to the constant movement of phospholipids and other molecules within the membrane
• Selective permeability


  • The cell membrane allows some substances to pass through easily while excluding others
  • Small substances, such as water, can diffuse through the membrane easily
  • Larger molecules, like proteins, are prevented from moving through the membrane
• Protein channels


  • Embedded within the cell membrane
  • Form channels that allow the passage of specific substances through the membrane
  • Some proteins have a structural function, while others act as carrier molecules for transporting substances
• Glycoproteins and glycolipids


  • Proteins and lipids that have carbohydrate portions on their surfaces
  • Carbohydrates act as identifiers on the outside of the cell
  • Play a role in cell-to-cell adhesion and can act as specific markers during immune responses
• Cholesterol


  • Found between phospholipid molecules in the cell membrane
  • Helps regulate the fluidity of the membrane
  • Ensures that the membrane is neither too rigid nor too fluid, allowing for proper cell functioning
• Functions of the cell membrane


  • Active transport of materials in and out of the cell
  • Provides mechanical support to maintain the cell's shape
  • Acts as a receptor for chemical materials, such as hormones, maintaining the specificity of the cell type
• Phospholipid bilayer


  • The cell membrane consists of a double layer of phospholipids
  • Each phospholipid molecule has a hydrophilic (water-loving) head and hydrophobic (water-repelling) tails
  • The hydrophilic heads face outward towards the aqueous environments, while the hydrophobic tails face inward, creating a stable barrier
• Protein channels


  • Embedded within the phospholipid bilayer
  • Act as gateways, allowing specific molecules and ions to pass through the membrane
  • Protein channels are essential for the facilitated diffusion of certain substances across the cell membrane
• Cholesterol


  • Interspersed among the phospholipids in the membrane
  • Provides stability and regulates the fluidity of the membrane
  • Helps maintain the proper consistency of the membrane under different temperature conditions
• Glycoproteins


  • Proteins with attached carbohydrate chains
  • Found on the outer surface of the cell membrane
  • Play important roles in cell recognition, cell signaling, and immune responses
• Passive transport


  Movement of molecules across the membrane without the expenditure of energy
• Diffusion


  • Spontaneous movement of molecules from an area of higher concentration to an area of lower concentration due to the random motion of molecules
  • Small, non-polar molecules like oxygen, carbon dioxide, and ethanol can diffuse directly through the phospholipid bilayer
  • Aims to establish equilibrium, where molecules are evenly distributed
• Osmosis


  • Specific type of diffusion that involves the movement of water molecules across a selectively permeable membrane
  • Water moves from an area of higher water concentration (hypotonic solution) to an area of lower water concentration (hypertonic solution) to balance the solute concentration
  • Crucial for maintaining proper water balance in cells
• Factors affecting diffusion


  • Temperature
  • Particle size
  • Concentration gradient
• Active transport


  Movement of molecules or ions against their concentration gradient, requiring the expenditure of energy
• Facilitated diffusion


  • Passive transport that utilizes carrier proteins to facilitate the movement of specific molecules across the cell membrane
  • Carrier proteins are highly specific and transport molecules from areas of high concentration to low concentration without requiring energy
• Active transport


  • Energy-consuming process of transporting molecules across a membrane against a concentration gradient
  • Energy is required because the substance needs to be moved against its natural tendency to diffuse in the opposite direction
  • Active transport is unidirectional, unlike passive transport such as osmosis and diffusion that occur from high to low concentration without requiring energy
• Endocytosis


  • Energy-requiring process in which the cell engulfs particles and brings them into the cell
  • "Endo" means "to bring in"
  • A form of active transport that allows the movement of large polar molecules that cannot pass through the hydrophobic cell membrane
• Phagocytosis


  Type of endocytosis where a phagocyte, such as a white blood cell, engulfs a particle like a microorganism and breaks it down using enzymes
• Exocytosis


  • Process in which the cell releases particles
  • "Exo" means "to take out"
  • Active movement of materials out of cells
• Phagocytosis


  • Type of endocytosis where the cell membrane surrounds and engulfs large solid particles
  • Often referred to as "cell eating" as the cell forms an extension of the membrane around the particle, creating a food vacuole
• Pinocytosis


  • Type of endocytosis where the cell membrane forms channels that take in fluid at their base
  • Often called "cell drinking" as it involves the uptake of liquids
• Elements required for cell survival and functioning


  • Energy source
  • Matter
  • Nutrients
  • Waste removal
• According to the Cell Theory: All living things are composed of one or more cells, cells arise from pre-existing cells through cell division, cells are the fundamental units of structure and function in all living organisms, the activity of an organism depends on the collective activity of its individual cells, energy flow and metabolism occur within cells, hereditary information (DNA) is passed from cell to cell, all cells share a basic chemical composition
• Common features of cells

  • Chemical reactions within cells occur in the cytosol, the colorless fluid inside the cell
  • The cell membrane surrounds the cell, separating its internal reactions from the external environment
  • Ribosomes, which are not bound by a membrane, synthesize proteins in all cells
  • Genetic material (DNA) carries hereditary information within cells
• Prokaryotic cells


  • Simple cells lacking a nucleus and membrane-bound organelles
  • Examples include bacteria and archaea
  • Smaller in size, typically ranging from 1 to 10 μm in length
  • Genetic material is present in a single circular chromosome and lacks a nucleus
  • Prokaryotes have smaller rings of DNA called plasmids
• Eukaryotic cells


  • Complex cells with a nucleus and membrane-bound organelles
  • Examples include animals, plants, fungi, and protists
  • Larger and more structurally complex than prokaryotic cells
  • Genetic material is enclosed within a nucleus, separated from the cytoplasm
  • Possess various organelles that carry out specific functions within the cell
• Endosymbiotic hypothesis


  • Suggests that eukaryotic cells evolved through a symbiotic relationship between prokaryotic cells
  • Smaller prokaryotic cells were engulfed by larger prokaryotes, leading to the integration of the smaller cells as organelles within the larger cell
  • Organelles such as mitochondria and chloroplasts are believed to have originated from symbiotic relationships between different prokaryotic cells
• Nucleus


  • Controls cell activity through DNA, which is the genetic material responsible for protein production and cell division
  • Surrounded by a double membrane with pores that regulate the movement of molecules in and out of the nucleus
  • In non-dividing cells, DNA and proteins form a network called chromatin, which condenses into chromosomes during cell division
  • The nucleolus, located within the nucleus, is involved in the production of ribosomes
• Endoplasmic reticulum (ER)


  • Consists of a network of flattened, membrane-bound sacs
  • Involved in synthesizing and transporting molecules within the cell
  • Smooth ER synthesizes and transports carbohydrates, lipids, and steroids
  • Rough ER has ribosomes on its surface and is responsible for protein synthesis
• Mitochondria


  • Rod-shaped organelles bound by a double membrane
  • The inner membrane is folded into structures called cristae, increasing the surface area for metabolic reactions
  • Mitochondria are the sites of cellular respiration, where energy is released in a usable form for the cell
  • They contain their own genetic material
• Nucleus


  • Surrounded by a double membrane with pores that regulate the movement of molecules in and out
• DNA and proteins in non-dividing cells


  1. Form a network called chromatin

  2. Condense into chromosomes during cell division
• Nucleolus


  Located within the nucleus, involved in the production of ribosomes
• Endoplasmic reticulum (ER)


  • Consists of a network of flattened, membrane-bound sacs
  • Involved in synthesizing and transporting molecules within the cell
• Smooth ER


  Synthesizes and transports carbohydrates, lipids, and steroids