Cells - Basic Unit of Life

• Cell


  Building block of living organisms
• Cells


  • Have different shapes and are microscopic
  • Seen as tiny, colourless, translucent units
• Cells discovered


  • Plant cells (Robert Hooke)
  • Bacteria and single-celled animals (Antonie van Leeuwenhoek)
• Cells first seen


  1. With simple lens microscope

  2. Chemical stains revealed internal structure

  3. Good light microscopes became available in early 1900s

  4. New techniques were needed to find out more about cells

  5. Electron microscope developed in 1940
• Cell theory


  • Developed in 1839 by microbiologists Schleiden and Schwann
  • All living things are made of cells and their products
  • New cells are created by old cells dividing into two
  • Cells are the basic building blocks of life
• Modern understanding of cell theory

  • Activity of organism depends on total activity of independent cells
  • Energy flow occurs in cells through breakdown of carbohydrates by respiration
  • Cells contain information necessary for creation of new cells – information known as 'hereditary information' contained within DNA
  • Contents of cells from similar species are basically the same
• Cell wall


  • Rigid outer layer of plant cells that surrounds cell membrane
  • Non-living tissue made up of cellulose
  • Completely permeable to water and mineral salts
  • Plasmodesmata = opening in cell walls – connect adjacent cells
  • Cell walls joined by the middle lamella

  
• Functions of cell wall

  • Protect the inner parts of the plant cell
  • Give plant cells a more uniform and regular shape
  • Provide support for the plant body
• Cell membrane


  • Surrounds cytoplasm
  • Consist mostly of lipids and proteins
  • Selectively permeable

  
• Functions of cell membrane


  • Controls movement of substances into and out of the cells
  • Fluid mosaic model describes arrangement of lipids and proteins
  • Proteins embedded in phospholipid bilayer – contribute to strength
  • Proteins shift around in phospholipid bilayer

  
• Nucleus


  • Largest organelle in cells
  • Contains genetic information (DNA)
  • Primary factor that distinguishes eukaryotes from prokaryotes
  • Double nuclear membrane – enclose nucleus/have small pores
  • Nucleoplasm – jelly-like fluid
  • Nucleolus – contains free nucleotide bases and produce ribosomes
  • Chromatin network – contains DNA which forms chromosomes
• Cytoplasm


  • Jelly-like substance
  • Up to 90% water
  • Contains dissolved nutrients and waste products
• Functions of cytoplasm


  • Hold together organelles
  • Nourishes cell
  • Supply it with salts and sugars
  • Provides medium for metabolic reactions
• Mitochondria


  • Rod-shaped, cylindrical organelles
  • Surrounded by double membrane
  • Fluid-filled matrix
  • Inner membrane folded inwards form cristae
  • Contain its own DNA

  
• Functions of mitochondria


  • Cellular respiration – produces energy (ATP)
• Endoplasmic reticulum


  • System of tubes enclosed by membranes
  • Found in plants and animals
  • Membranes appear to be continuous with cell membrane and nuclear membrane
  • Two types: Rough ER (has ribosomes attached) and Smooth ER (has no ribosomes attached)
• Functions of endoplasmic reticulum

  • Provides surfaces for attachment of ribosomes
  • Transports ribosomes throughout cell using rough ER
  • Synthesizes lipids and steroids (cholesterol) on smooth ER
• Ribosomes


  • Very small grain-like structures present in all plant, animal and bacterial cells
  • Each ribosome about 15-20nm in diameter
  • Consist of smaller and larger subunits
• Functions of ribosomes


  • Sites of protein synthesis
  • Found on ER, in cytoplasm, plastids and mitochondria
  • May also occur singly or as free clusters in cytoplasm (polyribosomes or polysomes)
  • Each ribosome made up of protein and rRNA
• Golgi apparatus (Golgi body)


  • Stack of flattened sacs (cisternae) and many spherical vesicles broken off from the cisternae
  • Found in both plant and animal cells
  • 7 nm (nanometers) across
  • Same structure as smooth ER
• Functions of Golgi apparatus


  • Makes and processes secretions, such as saliva and mucus
  • Transports proteins from ER to different parts of cell
  • Ribosomes, the ER and Golgi apparatus related to each other through involvement in protein synthesis and transport
• Vesicles and vacuoles (animal cells)

  • Vesicles = small vacuoles
  • Filled with fluid
  • Surrounded by membrane
• Functions of vesicles and vacuoles

  • Water vesicles – control amount of water in cytoplasm
  • Food vesicles – contains food particles
  • Vesicles from Golgi bodies – contain protein/lipids
• Lysosomes


  • Tiny sacs formed from vesicles from Golgi body
  • Contain enzymes – break down lipids and proteins
• Functions of lysosomes


  • Destroy foreign material – i.e. bacteria
  • Digest food particles
  • Break down dead cells and organelles
• Vacuoles (plant cells)


  • Fluid-filled organelles in the cytoplasm of most plant cells
  • Selectively permeable single membrane, the tonoplast, surrounds the vacuole
  • Contains cell sap: a liquid that consists of water, mineral salts, sugars and amino acids
• Functions of vacuoles


  • Plays a role in digestion and excretion of cellular waste
  • Storage of water and organic/inorganic substances
  • Maintaining shape of plant cells
  • When full of water, exerts pressure outwards, pushing the cell membrane against the cell wall = turgor pressure
• Centrioles


  • Found in animal cells
  • Cylindrical tube-like structure
  • Composed of 9 microtubules
  • Arranged in a very particular pattern
  • Two perpendicular centrioles = centrosome

  
• Functions of centrioles


  • Cell division – Mitosis
• Plastids


  • Structurally, double membrane-bounded
  • Found only in plant cells
  • 3 types: Chloroplast, Chromoplast, Leucoplast

  
• Functions of plastids


  • Chloroplasts - Responsible for photosynthesis, contains chlorophyll

  Chromoplasts - Contain pigments and give fruit, vegetables and flowers their colour

  Leucoplasts - Colourless, store starch
• Chloroplast


  • Made up of gelatinous substance = stroma
  • Contains enzymes essential for photosynthesis
  • Stroma surrounded by double membrane
  • Thylakoids suspended in stroma
  • Each stack = granum
  • Chlorophyll and other photosynthetic pigments located in thylakoids = site of photosynthesis
• Similarities in plant and animal cells

  • Mitochondria
  • Ribosomes
  • Golgi apparatus
  • Cell membrane
  • Endoplasmic Reticulum

  
• Differences in plant and animal cells

  • Regular shape (plant) vs Regular or irregular shape (animal)

  Large vacuoles (plant) vs Small/no vacuoles (animal)

  Cell wall (plant) vs No cell wall (animal)

  No lysosomes (plant) vs Lysosomes (animal)

  Plastids (plant) vs No plastids (animal)

  Chloroplasts (plant) vs No chloroplasts (animal)

  No centrioles (plant) vs Centrioles (animal)

  Nucleus towards the edge of the cell (plant) vs Nucleus towards the centre of the cell (animal)

  
• Movement across membranes: Cell membranes are selectively permeable, only allowing certain substances to pass through freely. Molecules enter or leave cells by diffusion, osmosis, and active transport.
• Similarities in plant and animal cells

  • Mitochondria
  • Ribosomes
  • Golgi apparatus
  • Cell membrane
  • Endoplasmic Reticulum

  
• Differences in plant and animal cells

  • Regular shape (plant)
  • Regular or irregular shape (animal)
  • Large vacuoles (plant)
  • Small/no vacuoles (animal)
  • Cell wall (plant)
  • No cell wall (animal)
  • No lysosomes (plant)
  • Lysosomes (animal)
  • Plastids (plant)
  • No plastids (animal)
  • Chloroplasts (plant)
  • No chloroplasts (animal)
  • No centrioles (plant)
  • Centrioles (animal)
  • Nucleus towards the edge of the cell (plant)
  • Nucleus towards the centre of the cell (animal)

  
• Cell membranes are selectively permeable
• Only allow certain substances to pass through freely
• Chemical nature and size of a substance

  Determines if it can pass through the cell membrane