A2.2_B2.2 Structure and Organelles SL/HL Biology PDF

The concept of living cells… … developed over about three centuries (1600 – 1800) The discovery of cells was possible due to the invention of the microscope! Historical events in cell science 1665 Englishman Robert Hooke examines cork under a compound microscope He coins the ter...

The concept of living cells… … developed over about three centuries (1600 – 1800) The discovery of cells was possible due to the invention of the microscope! Historical events in cell science 1665 Englishman Robert Hooke examines cork under a compound microscope He coins the term “cells” to describe what he observed Historical events in cell science 1675 Dutchman Antonie van Leeuwenhoek was first to discover living microscopic single-cell organisms named “animalcules” Van Leeuwenhoek becomes the founder of microbiology Leeuwnhoek’s little animals in pond water. Historical events in cell science 1838-39 Two German friends, Mathias Schleiden (botanist) and Theodor Schwann (physiologist) realize that all plants and animals are made up of cells Schleiden Schwann Historical events in cell science 1855 Another German, the biologist and founder of pathology, Rudolf Virchow, built on the work of others to conclude that: “every cell comes from another existing cell” This refuted the popular idea that life was generated spontaneously! https://youtu.be/4OpBylwH9DU Virchow The Cell Theory Observations and experiments like these supported a fundamental theory in biology - The cell theory The principles of the cell theory are: 1) All living things are composed of cells 2) The cell is the smallest unit of life 3) Cells only arise from pre-existing cells The Cell Theory 1) All living things are composed of cells (or cell products) 2) The cell is the smallest unit of life Unicellular bacteria (prokaryote) Euglena (unicellular eukaryote) The Cell Theory 1) All living things are composed of cells (or cell products) 2) The cell is the smallest unit of life Plant cells in leaf. Animal cells in epithelia. The Cell Theory 3) Cells only arise from pre-existing cells Cell division - mitosis. The scale of typical cells Molecules ≈ 1nm (0.000 001 mm) Cell Membrane ≈ 10nm (0.000 01 mm) Mitochondria ≈ 1μm (0.001 mm) Bacteria ≈ 1μm (0.001 mm) Animal cell ≈ 20μm (0.02 mm) Plant cell ≈ 40μm (0.04 mm) Sea urchin embryo cells – large animal cells! The Functions of Life All living things, including single-cell organisms, carry out 7 basic Mr functions essential to survival: H Gr 1. Obtain food needed for energy & growth - Nutrition en 2. Run essential chemical reactions - Metabolism 3. Remove waste – Excretion 4. Maintain stable internal conditions - Homeostasis 5. Sense the environment - Responsiveness 6. Produce offspring - Reproduction 7. Grow and increase in size - Growth The Cell Theory: Living things consist of cells … and there are two major types of cells: Prokaryotic cells Eukaryotic cells Smaller cells having a simple Larger cells that have a structure without compartmentalised compartmentalisation structure Prokaryotes Microbiology = the study of microscopic organisms like bacteria (and viruses) Organisms whose cells lack a nucleus 'pro' = before, 'karyon' = nucleus Prokaryotes were the first to evolve and are by far the most abundant https://youtu.be/VzPD009qTN4 The ultrastructure of prokaryotic cells Prokaryotic cells typically contain a set of cellular components Flagella Slime capsule Cytoplasm ? Cell wall Nucleoid Cell membrane Plasmids 70S Ribosomes Pili Draw and learn the functions of these components! Eukaryotes Organisms whose cells contain a nucleus (‘eu’ = good / true ; ‘karyon’ = nucleus) Evolved from fusion between proto-eukaryotic and prokaryotic cells (endosymbiosis) Organelles? Discrete subunits of a cell with specific functions are called organelles Ex of eukaryotic organelles: nucleus, cell membrane, mitochondria, chloroplasts, ribosomes… In eukaryotic cells some organelles are membrane-enclosed Benefits of membrane-enclosed compartments The specific functions of a cell are performed by chemical reactions The reactions may require different conditions (sets of enzymes, solute concentrations, pH etc) Separating the cytoplasm in compartments with different internal conditions enable a diversity of functions to be carried out optimally Example: the lysosome The lysosome is enclosed by a single membrane Digestion of damaged cell components and material taken into the cell occurs in the lysosome Acidic internal environment activates specific digestive enzymes Example: the nucleus The double nuclear membrane protects DNA from cytoplasmic reactions Allows separation of transcription from translation In the nucleus, sections of mRNA can be recombined (alternative splicing) before being exported out to the ribosomes One gene could generate several protein products diversifying functions The ultrastructure of eukaryotic cells Eukaryotic cells typically contain a set of cellular subunits with some key differences between plant and animal cells Chloroplasts Mitochondria Centriole Cell wall Cytoplasm ? Nucleus Golgi apparatus Vacuole Cell membrane 80S Ribosomes Endoplasmic reticulum Lysosome Peroxisome Draw and learn the functions of these components! The cytoskeleton – the cells’ own skeleton! A cytoplasmic network of protein fibers Microtubuli fibres Microfilament fibres the cytoskeleton Microtubules Straight, cylindrical and dynamic fibres Built up and broken down when needed Provide strength and support cell shape Move chromosomes during mitosis Organelles like vesicles transported along microtubules by motor proteins Microtubules - kinesin motor protein Microfilaments Dynamic, thin fibre consisting of an actin protein polymer Maintain and change cell shape Involved in cell motility and muscle contractions Involved in splitting the cell membrane at the end of cell division Difference between animal and plant cell structure An external cellulose cell wall – enable high internal pressure Chloroplasts – double membrane organelle with internal thylakoids for photosynthesis A large fluid filled vacuole – maintain ”turgor” pressure No centrosome What about fungal cells? Neither animals nor plants Fungi are saprotrophic organisms that digest dead organic matter They can’t photosynthesise as they lack chloroplasts Cell walls made up of the polysaccharide chitin Some strange, atypical cells… Most eukaryote cells are ”normal” having one nucleus and a set of organelles enclosed by a membrane A few cell types are exceptional or atypical by having no nucleus (anucleate) or many nuclei (multinucleate) Multinucleate skeletal ”striated” muscle cells/fibres Some strange, atypical cells… When red blood cells develop in the bone marrow from blood stem cells, they lose their nuclei to make space for more hemoglobin to be stored Red blood cells are the only anucleate cell in our body! Some strange, atypical cells… Moulds (fungi) of the genus Mucor have thread-like, aseptate, multinucleate hyphae