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Cell structure & organisation

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Paula Mills

This document provides information on cell structure and organization, including cell theory, types of cells, cell organelles, transport across the cell membrane, and differences between prokaryotic and eukaryotic cells. It discusses how cells are the basic unit of structure and function in living things according to cell theory. The key components and functions of plant and animal cells are described, including the cell membrane, nucleus, mitochondria, chloroplasts, vacuoles, endoplasmic reticulum, and golgi bodies. Mechanisms of movement across the cell membrane such as diffusion, osmosis, active transport, and facilitated diffusion are also summarized.

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CELL STRUCTURE & ORGANISATION
CELL THEORY All organisms consist of cells or the product of cells and all cells arise from pre-existing cells. The cell is the unit of structure and function of most organisms – they are the building blocks of life. They : - respond to stimuli - reproduce - move - transform energy for perform functions - produce waste materials
While organisms vary in size and complexity. Large organisms are multicellular while the smallest are unicellular. 1665 – Robert Hooke used the first recorded magnifying device to observe cork cells. 1667 – Leeuwenhoek used a small bead of glass to examine material from his mouth which he called “animalcules” or bacteria.
Two main types of microscopes Electron microscope Beams of electrons pass through a vacuum and then through a thin layer of tissue. Electrons have a much shorter wavelength compared to visible light they provide much better detail or resolution (ability to separate objects) X 100,000 magnification
Light microscope Relies on light passing through a very thin section of tissue. Combination of “eye piece” and “objective” lens X 1500 magnification Micrometre (μm) unit of length used to measure cells. If structures are measured within a cell scientists use nanometre (nm) Bacteria - 1 μm Human red blood cell - 10μm Human liver cell - 50μm Plant mesophyll cell - 100μm Viruses – 20-400nm (not considered a cell)
Animal Cell – Light Microscope
Plant Cell – Light Microscope
Animal Cell – Electron Microscope
Plant Cell – Electron Microscope
Some sizes of different cells
Surface Area To Volume Ratio The SA:V ratio explains the exchange of materials between external and internal environment. Cells which are microscopic in size allow the required substances and waste products to easily diffuse.
As a cell grows in size it will have relatively less surface area compared to its volume and is thought to be a stimulus for it to divide
The shape of a cell also affects SA:V ratio. The human red blood cell is a bi-concave disc which maximises surface area to volume increasing its capacity to exchange oxygen and carbon dioxide as well as allowing for flexibility in the capillaries.
A Simplified Classification Living things Cellular Non-Cellular Eukaryotes Prokaryotes Plant Animal Protist Fungi Bacteria Viruses
Prokaryotic cells Do not have defined nucleus, DNA is found as a concentrated mass or a small circle of DNA known as a plasmid Have existed for millions of years, relatively unspecialised and do not contain membrane bound organelles Eg bacteria
Some prokaryotes have extensively folded plasma membranes using their large surface areas for respiration, photosynthesis (chemosynthesis) and DNA replication. They contain ribosomes enabling protein synthesis. They are very small and as such have a very good SA:V ratio.
Eukaryotic cells Cells that make up large complex multicellular organisms are eukaryotic. Highly specialised, larger in size and contain a membrane bound nucleus and specialised membrane organelles
General animal cell
General plant cell
Nucleus Membrane bound and contains chromosomes which controls activities of the cell Mitochondria Site of aerobic respiration, enables cell to release energy from organic molecules for cellular purposes
Photosynthesis Glucose + Oxygen Carbon Dioxide + Water + Energy C6 H12 O6 + 6O2 6CO2 + 6H20 Carbon Dioxide + Water Glucose + Oxygen 6CO2 + 6H2O C6 H12 O6 + 6O2 Aerobic Respiration
Chloroplasts Contain the green pigment chlorophyll which allows for photosynthesis, converting inorganic materials to organic molecules using light
Vacuoles Membrane bound structures filled with fluid which contains dissolved sugars or salts. In plants cells contain large vacuoles surrounded by membrane. They are a storage site for solutes, cell turgidity and maintaining water balance. Unicellular cells that live in freshwater have “contractile” vacuoles which pump out water which enters via osmosis Animal cells contain “food vacuoles” that store food particles acquired by phagocytosis
Endoplasmic Reticulum Internal folded membranes that connect nucleus with cell membrane: Two types 1. Smooth ER – no attached ribosomes, involved with synthesis and metabolism of fluids 2. Rough ER – attached ribosomes, synthesised proteins are modified by ER.
Golgi Bodies (Apparatus) Found in all eukaryotic cells, consist of flattened discs surrounded by spherical vesicles. Involved in packaging and transporting sugar- coated proteins (glycoproteins) to the outside of the cell.
Cell structure & organisation
Cell Membrane The cell membrane is found in all living cells and controls the entry and exit of materials in out of the cell. It consists of two layers of phospholipid molecules embedded with proteins. There have been various models put forward over the last 100 years to explain its structure
Fluid Mosaic Model Fluid Refers to the ability of proteins and lipids to move sideways through within the structure, giving it a fluid nature Mosaic Refers to the variety of protein molecules that are embedded or attached to the phospholipid bi-layer. These proteins determine the function of the cell membrane
Types of proteins inside Mosaic layer 1. channel type proteins which span the membrane Passive – diffusion Active – active transport 2. receptor type proteins bound to the surface that are involved in cellular communication - recognition of antigens and hormones 3. proteins on underside of membrane held in place by cytoskeleton
Fluid Mosaic model explains the “semi- permeable” nature of the cell membrane.
The Fluid Mosaic Model • The phospholipid bi-layer consists of a double layer of phospholipid molecules. • The phospholipid molecules have a hydrophilic end (water loving) and a hydrophobic end (water hating) Hydrophilic Phosphate head Hydrophobic hydrocarbon tail
The Fluid Mosaic Model of a Cell Membrane Ref: Biology, Roberts
Cell Membrane Proteins Ref: Biology for the IB Diploma, Allott
Endocytosis & Exocytosis Movement of larger molecules such as proteins or polysaccharides and fluids. Exocytosis Substances passing out of cell. Molecules packaged by Golgi bodies and ER such as hormones, enzymes. Endocytosis Substances passing into the cell. Two types 1. Phagocytosis - solid particles “eating” 2. Pinocytosis – liquid particles “cell drinking”
Exocytosis Endocytosis
Cytoskeleton Internal framework of microtubules and micro fibres. Organised by the centrosome or “micro-tubule organising centre” (MTOC)located near the nucleus. Three functions 1. Support Maintains shape of animal cells in absence of cell wall. Organelles are supported and attached to cytoskeleton in the cytoplasm
2. Movement -movement of vesicles in side cell as in exocytosis -movement of chromosomes during cell division -movement of cilia and flagella -contraction and expansion of muscle cells 3. Regulation Relay of messages from external environment to cells interior
Cell structure & organisation
Prokaryotes vs Eukaryotes Prokaryotes • Smaller (1 – 10um) • Circular DNA • No Nucleus (nuclear Membrane) • Little internal organisation • No membrane-bound organelles • Single chromosome • Most have a cell wall (made of peptidoglycan) • Some can obtain energy by photosynthesis Eukaryotes • Larger (10 – 100um) • Linear DNA • Contains a nucleus • High level of internal organisation • Contains membrane bound organelles • Two or more chromosomes • Cell wall (if present) made of cellulose • Obtain energy by photosynthesis or ingesting other living organisms
Cells ensure that the composition of their internal environment is carefully regulated to ensure that chemical concentrations are kept relatively stable. Cells manufacture organic molecules from their interaction with inorganic molecules (and other organic molecules) The cell membrane regulates and controls this exchange
Cell structure & organisation
Living cells require a constant supply of energy to carry out processes. Inside cells chemical reactions are occurring at all times and these require a stable internal environment (Homeostasis) i.e. concentrations of substrates (O2) and products (urea, CO2) Reactions inside cells will also be adversely affected with changes in pH or temperature
Several characteristics will determine movement across the membrane 1. Size of the molecule 2. Chemical composition of the molecule. Chemicals which dissolve are more likely to move easily across the membrane 3. Shape of the molecule – cell membrane can discriminate and select specific molecules i.e. glucose
Examples of selective exchange: CO2 / O2 easily move through cell membrane because of their small size Glucose and water do not readily move across, they require specific protein channels or carrier proteins Proteins and polysaccharides are too large to cross the membrane unless they move via endocytosis or exocytosis.
Movement of molecules across a cell membrane
Diffusion The random movement of a molecules from a region of high concentration to a region of low concentration. It is a passive process which requires no energy.
Diffusion
Osmosis Diffusion of water across a semipermeable membrane from a high concentration to a low concentration. Water is a solvent, the materials dissolved in it, is the solute and the combined mixture is referred to as the solution
Isotonic – same solute concentration as the cell Hypotonic- a lower concentration as the cell Hypertonic – a higher solute concentration as the cell
Osmosis
Osmosis has a more drastic effect on animal cells than plant cells Why? Animal cells do not have a cell wall and therefore are prone to gaining (bursting) or losing water (shrivel), particularly if they exist in fresh water. E.g. Protozoa – Euglena (contractile vacuole expels excess water) Plant cells are strengthened by the cell wall-rarely destroyed by osmosis.
Active Transport In some situations molecules are required to move against a concentration gradient – this requires energy Transport proteins in the cell membrane actively move specific substances across the membrane from a high to low concentration ATP provides this energy
Active Transport
Active Transport Transmission of nerve signals involves exchange of sodium and potassium ions by diffusion across the cell membrane. When fatigued it is necessary to pump these ions against a concentration gradient back to the other side
Active Transport Another example is the absorption of glucose. If the concentration in the gut is higher than the blood it will diffuse, but if it is lower, it will only move by active transport. Why? Fish actively absorb ions through their gills to replace solutes lost in large volumes of their urine
Active Transport
Facilitated Diffusion This is where some larger molecules which cannot diffuse through the membrane are helped through the membrane by special proteins. They are facilitated through. Substances like: • Glucose • Amino acids Facilitated diffusion is still passive (substances move along a concentration gradient from high to lower concentrations)
Facilitated Diffusion

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Cell structure & organisation

  • 2. CELL THEORY All organisms consist of cells or the product of cells and all cells arise from pre-existing cells. The cell is the unit of structure and function of most organisms – they are the building blocks of life. They : - respond to stimuli - reproduce - move - transform energy for perform functions - produce waste materials
  • 3. While organisms vary in size and complexity. Large organisms are multicellular while the smallest are unicellular. 1665 – Robert Hooke used the first recorded magnifying device to observe cork cells. 1667 – Leeuwenhoek used a small bead of glass to examine material from his mouth which he called “animalcules” or bacteria.
  • 4. Two main types of microscopes Electron microscope Beams of electrons pass through a vacuum and then through a thin layer of tissue. Electrons have a much shorter wavelength compared to visible light they provide much better detail or resolution (ability to separate objects) X 100,000 magnification
  • 5. Light microscope Relies on light passing through a very thin section of tissue. Combination of “eye piece” and “objective” lens X 1500 magnification Micrometre (μm) unit of length used to measure cells. If structures are measured within a cell scientists use nanometre (nm) Bacteria - 1 μm Human red blood cell - 10μm Human liver cell - 50μm Plant mesophyll cell - 100μm Viruses – 20-400nm (not considered a cell)
  • 6. Animal Cell – Light Microscope
  • 7. Plant Cell – Light Microscope
  • 8. Animal Cell – Electron Microscope
  • 9. Plant Cell – Electron Microscope
  • 10. Some sizes of different cells
  • 11. Surface Area To Volume Ratio The SA:V ratio explains the exchange of materials between external and internal environment. Cells which are microscopic in size allow the required substances and waste products to easily diffuse.
  • 12. As a cell grows in size it will have relatively less surface area compared to its volume and is thought to be a stimulus for it to divide
  • 13. The shape of a cell also affects SA:V ratio. The human red blood cell is a bi-concave disc which maximises surface area to volume increasing its capacity to exchange oxygen and carbon dioxide as well as allowing for flexibility in the capillaries.
  • 14. A Simplified Classification Living things Cellular Non-Cellular Eukaryotes Prokaryotes Plant Animal Protist Fungi Bacteria Viruses
  • 15. Prokaryotic cells Do not have defined nucleus, DNA is found as a concentrated mass or a small circle of DNA known as a plasmid Have existed for millions of years, relatively unspecialised and do not contain membrane bound organelles Eg bacteria
  • 16. Some prokaryotes have extensively folded plasma membranes using their large surface areas for respiration, photosynthesis (chemosynthesis) and DNA replication. They contain ribosomes enabling protein synthesis. They are very small and as such have a very good SA:V ratio.
  • 17. Eukaryotic cells Cells that make up large complex multicellular organisms are eukaryotic. Highly specialised, larger in size and contain a membrane bound nucleus and specialised membrane organelles
  • 20. Nucleus Membrane bound and contains chromosomes which controls activities of the cell Mitochondria Site of aerobic respiration, enables cell to release energy from organic molecules for cellular purposes
  • 21. Photosynthesis Glucose + Oxygen Carbon Dioxide + Water + Energy C6 H12 O6 + 6O2 6CO2 + 6H20 Carbon Dioxide + Water Glucose + Oxygen 6CO2 + 6H2O C6 H12 O6 + 6O2 Aerobic Respiration
  • 22. Chloroplasts Contain the green pigment chlorophyll which allows for photosynthesis, converting inorganic materials to organic molecules using light
  • 23. Vacuoles Membrane bound structures filled with fluid which contains dissolved sugars or salts. In plants cells contain large vacuoles surrounded by membrane. They are a storage site for solutes, cell turgidity and maintaining water balance. Unicellular cells that live in freshwater have “contractile” vacuoles which pump out water which enters via osmosis Animal cells contain “food vacuoles” that store food particles acquired by phagocytosis
  • 24. Endoplasmic Reticulum Internal folded membranes that connect nucleus with cell membrane: Two types 1. Smooth ER – no attached ribosomes, involved with synthesis and metabolism of fluids 2. Rough ER – attached ribosomes, synthesised proteins are modified by ER.
  • 25. Golgi Bodies (Apparatus) Found in all eukaryotic cells, consist of flattened discs surrounded by spherical vesicles. Involved in packaging and transporting sugar- coated proteins (glycoproteins) to the outside of the cell.
  • 27. Cell Membrane The cell membrane is found in all living cells and controls the entry and exit of materials in out of the cell. It consists of two layers of phospholipid molecules embedded with proteins. There have been various models put forward over the last 100 years to explain its structure
  • 28. Fluid Mosaic Model Fluid Refers to the ability of proteins and lipids to move sideways through within the structure, giving it a fluid nature Mosaic Refers to the variety of protein molecules that are embedded or attached to the phospholipid bi-layer. These proteins determine the function of the cell membrane
  • 29. Types of proteins inside Mosaic layer 1. channel type proteins which span the membrane Passive – diffusion Active – active transport 2. receptor type proteins bound to the surface that are involved in cellular communication - recognition of antigens and hormones 3. proteins on underside of membrane held in place by cytoskeleton
  • 30. Fluid Mosaic model explains the “semi- permeable” nature of the cell membrane.
  • 31. The Fluid Mosaic Model • The phospholipid bi-layer consists of a double layer of phospholipid molecules. • The phospholipid molecules have a hydrophilic end (water loving) and a hydrophobic end (water hating) Hydrophilic Phosphate head Hydrophobic hydrocarbon tail
  • 32. The Fluid Mosaic Model of a Cell Membrane Ref: Biology, Roberts
  • 33. Cell Membrane Proteins Ref: Biology for the IB Diploma, Allott
  • 34. Endocytosis & Exocytosis Movement of larger molecules such as proteins or polysaccharides and fluids. Exocytosis Substances passing out of cell. Molecules packaged by Golgi bodies and ER such as hormones, enzymes. Endocytosis Substances passing into the cell. Two types 1. Phagocytosis - solid particles “eating” 2. Pinocytosis – liquid particles “cell drinking”
  • 36. Cytoskeleton Internal framework of microtubules and micro fibres. Organised by the centrosome or “micro-tubule organising centre” (MTOC)located near the nucleus. Three functions 1. Support Maintains shape of animal cells in absence of cell wall. Organelles are supported and attached to cytoskeleton in the cytoplasm
  • 37. 2. Movement -movement of vesicles in side cell as in exocytosis -movement of chromosomes during cell division -movement of cilia and flagella -contraction and expansion of muscle cells 3. Regulation Relay of messages from external environment to cells interior
  • 39. Prokaryotes vs Eukaryotes Prokaryotes • Smaller (1 – 10um) • Circular DNA • No Nucleus (nuclear Membrane) • Little internal organisation • No membrane-bound organelles • Single chromosome • Most have a cell wall (made of peptidoglycan) • Some can obtain energy by photosynthesis Eukaryotes • Larger (10 – 100um) • Linear DNA • Contains a nucleus • High level of internal organisation • Contains membrane bound organelles • Two or more chromosomes • Cell wall (if present) made of cellulose • Obtain energy by photosynthesis or ingesting other living organisms
  • 40. Cells ensure that the composition of their internal environment is carefully regulated to ensure that chemical concentrations are kept relatively stable. Cells manufacture organic molecules from their interaction with inorganic molecules (and other organic molecules) The cell membrane regulates and controls this exchange
  • 42. Living cells require a constant supply of energy to carry out processes. Inside cells chemical reactions are occurring at all times and these require a stable internal environment (Homeostasis) i.e. concentrations of substrates (O2) and products (urea, CO2) Reactions inside cells will also be adversely affected with changes in pH or temperature
  • 43. Several characteristics will determine movement across the membrane 1. Size of the molecule 2. Chemical composition of the molecule. Chemicals which dissolve are more likely to move easily across the membrane 3. Shape of the molecule – cell membrane can discriminate and select specific molecules i.e. glucose
  • 44. Examples of selective exchange: CO2 / O2 easily move through cell membrane because of their small size Glucose and water do not readily move across, they require specific protein channels or carrier proteins Proteins and polysaccharides are too large to cross the membrane unless they move via endocytosis or exocytosis.
  • 45. Movement of molecules across a cell membrane
  • 46. Diffusion The random movement of a molecules from a region of high concentration to a region of low concentration. It is a passive process which requires no energy.
  • 48. Osmosis Diffusion of water across a semipermeable membrane from a high concentration to a low concentration. Water is a solvent, the materials dissolved in it, is the solute and the combined mixture is referred to as the solution
  • 49. Isotonic – same solute concentration as the cell Hypotonic- a lower concentration as the cell Hypertonic – a higher solute concentration as the cell
  • 51. Osmosis has a more drastic effect on animal cells than plant cells Why? Animal cells do not have a cell wall and therefore are prone to gaining (bursting) or losing water (shrivel), particularly if they exist in fresh water. E.g. Protozoa – Euglena (contractile vacuole expels excess water) Plant cells are strengthened by the cell wall-rarely destroyed by osmosis.
  • 52. Active Transport In some situations molecules are required to move against a concentration gradient – this requires energy Transport proteins in the cell membrane actively move specific substances across the membrane from a high to low concentration ATP provides this energy
  • 54. Active Transport Transmission of nerve signals involves exchange of sodium and potassium ions by diffusion across the cell membrane. When fatigued it is necessary to pump these ions against a concentration gradient back to the other side
  • 55. Active Transport Another example is the absorption of glucose. If the concentration in the gut is higher than the blood it will diffuse, but if it is lower, it will only move by active transport. Why? Fish actively absorb ions through their gills to replace solutes lost in large volumes of their urine
  • 57. Facilitated Diffusion This is where some larger molecules which cannot diffuse through the membrane are helped through the membrane by special proteins. They are facilitated through. Substances like: • Glucose • Amino acids Facilitated diffusion is still passive (substances move along a concentration gradient from high to lower concentrations)