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BACKGROUND KNOWLEDGE

Structure and Function of the Cell

Introduction to the cell

Both living and non-living things are composed of molecules made from chemical elements such as Carbon, Hydrogen, Oxygen, and Nitrogen. The organization of these molecules into cells is one feature that distinguishes living things from all other matter.

The cell is the smallest unit of matter that can carry on all the processes of life.

  1. Every living thing, from the tiniest bacterium to the largest whale, is made of one/more cells
  2. Before the C17th, no one knew that cells existed, since they are too small to be seen with the naked eye. The invention of the microscope enabled Robert Hooke, (1665) and Anton van Leuwenhoek (1675) to see and draw the first 'cells', a word coined by Hooke to describe the cells in a thin slice of cork, which reminded him of the rooms where monks lived.
  3. The idea that all living things are made of cells was put forward in about 1840 and in 1855 came 'Cell Theory' – i.e. 'cells only come from other cells' – contradicting the earlier theory of ‘Spontaneous Generation’.

Cell Theory consists of three principles:

  1. All living things are composed of one or more cells.
  2. Cells are the basic units of structure and function in an organism.
  3. Cells come only from the replication of existing cells.


Cell diversity

Not all cells are alike. Even cells within the same organism show enormous diversity in size, shape, and internal organization. Your body contains around 1013 to 1014 cells of around 300 different cell types, which we broadly classify into 4 groups.


Cell size

  1. A few types of cells are large enough to be seen by the unaided eye. The human egg (ovum) is the largest cell in the body, and can (just) be seen without the aid of a microscope.
  2. Most cells are small for two main reasons:
    1. The cell’s nucleus can only control a certain volume of active cytoplasm.
    2. Cells are limited in size by their surface area to volume ratio. A group of small cells has a relatively larger surface area than a single large cell of the same volume. This is important because the nutrients, oxygen, and other materials a cell requires must enter through it surface. As a cell grows larger at some point its surface area becomes too small to allow these materials to enter the cell quickly enough to meet the cell's need. (= Fick’s Law – something you need to learn well.)

Rate of diffusion α Surface Area x Concentration
                               Difference Distance

Cell shape

  • Cells come in a variety of shapes – depending on their function
  • The neurones from your toes to your head are long and thin
  • Blood cells are rounded disks, so that they can flow smoothly


Internal organisation

  1. Cells contain a variety of internal structures called organelles
  2. An organelle is a cell component that performs a specific function in that cell
  3. Just as the organs of a multicellular organism carry out the organism's life functions, the organelles
    of a cell maintain the life of the cell
  4. There are many different cells; however, there are certain features common to all cells
  5. The entire cell is surrounded by a thin cell membrane
    All membranes have the same thickness and basic structure
  6. Organelles often have their own membranes too – once again, these membranes have a similar structure
  7. The nucleus, mitochondria and chloroplasts all have double membranes, more correctly called envelopes
  8. Because membranes are fluid mosaics, the molecules making them up – phospholipids and proteins - move independently. The proteins appear to ‘float’ in the phospholipids bilayer
  9. Membranes can thus be used to transport molecules within the cell e.g. endoplasmic reticulum.
  10. Proteins in the membrane can be used to transport substances across the membrane
    – e.g. by facilitated diffusion or by active transport.
  11. The proteins on the outside of cell membranes identify us as unique.


Prokaryotes vs. Eukaryotes

Organisms whose cells normally contain a nucleus are called Eukaryotes; those (generally smaller) organisms whose cells lack a nucleus and have no membrane-bound organelles are known as Prokaryotes.

 ProkaryotesEukaryotes
Typical organismsBacteriaProtoctista, fungi, plants, animals
Typical size≈1-10 µm≈10-100 µm (sperm cells) apart from the tail, are smaller)
Type of nucleusNuclear body
No nucleus
Real nucleus with nuclear envelope DNA circular (ccc DNA) linear molecules (chromosomes) with histone proteins
Ribosomes70S80S
Cytoplasmatic structureVery few structuresHighly structured by membranes and a cytoskeleton
Cell movementFlagellae/cilia made of flagellinFlagellae and cilia made of tubulin
MitochondriaNone1 - 100 (though RBC’s have none)
ChloroplastsNoneIn algae and plants
OrganizationUsually single cellsSingle cells, colonies, higher multicellular organisms with specialized cells
Cell divisionBinary fission (simple division)Mitosis (normal cell replication)
Meiosis (gamete production)

 

Parts of the eukaryotic cell

The structures that make up a Eukaryotic cell are determined by the specific functions carried out by the cell. Thus, there is no typical Eukaryotic cell. Nevertheless, Eukaryotic cells generally have three main components: A cell membrane, a nucleus, and a variety of other organelles.


The cell membrane

  1. A cell cannot survive if it is totally isolated from its environment. The cell membrane is a complex barrier separating every cell from its external environment.
  2. This "Selectively Permeable" membrane regulates what passes into and out of the cell.
  3. The cell membrane is a fluid mosaic of proteins floating in a phospholipid bilayer.
  4. The cell membrane functions like a gate, controlling which molecules can enter and leave the cell.
  5. The cell membrane controls which substances pass into and out of the cell. Carrier proteins in or on the membrane are specific, only allowing a small group of very similar molecules through. For instance, α- glucose is able to enter; but β – glucose is not. Many molecules cannot cross at all. For this reason, the cell membrane is said to be selectively permeable.
  6. Cell membranes are made mostly of phospholipid molecules. They have only two fatty acid ‘tails’ as one has been replaced by a phosphate group (making the ‘head’).
  7. The head is charged and so polar; the tails are not charged and so are non-polar.
  8. The two ends of the phospholipid molecule have different properties in water. The phosphate head is hydrophyllic and so the head will orient itself so that it is as close as possible to water molecules.
  9. The fatty acid tails are hydrophobic and so will tend to orient themselves away from water.
  10. So, when in water, phospholipids line up on the surface with their phosphate heads sticking into the water and fatty acid tails pointing up from the surface.
  11. Cells are bathed in an aqueous environment and since the inside of a cell is also aqueous, both sides of the cell membrane are surrounded by water molecules.
  12. This causes the phospholipids of the cell membrane to form two layers, known as a phospholipid bilayer. In this, the heads face the watery fluids inside and outside the cell, whilst the fatty acid tails are sandwiched inside the bilayer.
  13. The cell membrane is constantly being formed and broken down in living cells.


Cytoplasm

  1. Everything within the cell membrane which is not the nucleus is known as the cytoplasm.
  2. Cytosol is the jelly-like mixture in which the other organelles are suspended, so cytosol + organelles = cytoplasm.
  3. Organelles carry out specific functions within the cell.
  4. In Eukaryotic cells, most organelles are surrounded by a membrane, but in Prokaryotic cells there are no membrane-bound organelles.


Fluid mosaic model of cell membranes

  1. Membranes are fluid and are rather viscous – like vegetable oil.
  2. The molecules of the cell membrane are always in motion, so the phospholipids are able to drift across the membrane, changing places with their neighbour.
  3. Proteins, both in and on the membrane, form a mosaic, floating in amongst the phospholipids.
  4. Because of this, scientists call the modern view of membrane structure the ‘Fluid Mosaic Model’.
  5. The mosaic of proteins in the cell membrane is constantly changing.


Membrane proteins

  1. A variety of protein molecules are embedded in the basic phospholipid bilayer
  2. Some proteins are attached to the surface of the cell membrane on both the internal and external surface. These may be hormone receptors, enzymes or cell recognition proteins (or antigens)
  3. Other proteins are embedded in the phospholipid bilayer itself. These are often associated with transporting molecules from one side of the membrane to the other and are referred to as carrier proteins.
  4. Some of these form channels or pores through which certain substances can pass (facilitated diffusion), whilst others bind to a substance on one side of the membrane and carry it to the other side of the membrane (active transport)
  5. Proteins exposed to the cell's external environment often have carbohydrates attached to them which act as antigens (e.g. blood groups A & B – group AB has both; group O has neither).
  6. Some viruses may also bind here too.


The nucleus (pl. nuclei)

  1. The Nucleus is normally the largest organelle within a Eukaryotic cell. But it is NOT the ‘BRAIN’ of the cell!!
  2. Prokaryotes have no nucleus, having a nuclear body instead. This has no membrane and a loop of DNA (and no chromatin proteins)
  3. The nucleus contains the cell’s chromosomes (human, 46, fruit fly 6, fern 1260) which are normally uncoiled to form a chromatinic network, which contain both linear DNA and proteins, known as histones. These proteins coil up (dehydrate) at the start of nuclear division, when the chromosomes first become visible.
  4. Whilst most cells have a single nucleus some cells (macrophages, phloem companion cells) have more than one.
  5. Fungi have many nuclei in their cytoplasm – they are coenocytic (= common cytoplasm throughout)
  6. The nucleus is surrounded by a double membrane called the nuclear envelope.
  7. The nuclear envelope has many nuclear pores through which mRNA, and proteins can pass. These dimples make it look like a golf ball.
  8. Most nuclei contain at least one nucleolus (plural, nucleoli). The nucleoli are where ribosomes are synthesised. Ribosomes, you remember, translate mRNA into proteins.
  9. When a nucleus prepares to divide, the nucleolus disappears.


Comparison of structures between animal and plant cells

 

Typical animal cell

 Typical plant cell

Organelles
  • Nucleus
    • Nucleolus (within nucleus)
  • Rough ER
  • Smooth ER
  • 80S Ribosomes
  • Cytoskeleton
  • Golgi apparatus
  • Cytoplasm
  • Mitochondria
  • Vesicles
  • Vacuoles
  • Lysosomes
  • Centrioles
  • Nucleus
    • Nucleolus (within nucleus)
  • Rough ER
  • Smooth ER
  • 80S Ribosomes
  • Cytoskeleton
  • Golgi apparatus
  • Cytoplasm
  • Mitochondrion
  • Vesicle
  • Chloroplast and other plastids
  • Central vacuole
    • Tonoplast (central vacuole membrane)
Additional structures
  • Cilia
  • Flagellae
  • Plasma membrane
  • Plasma membrane
  • Cellulose cell wall
  • Plasmodesmata


Mitochondria

  1. Mitochondria are found scattered throughout the cytosol, and are relatively large organelles (second only to the nucleus and chloroplasts)
  2. Mitochondria are the sites of aerobic respiration, in which energy from organic compounds is transferred to ATP. For this reason they are sometimes referred to as the ‘powerhouse’ of the cell
  3. ATP is the molecule that most cells use as their main energy ‘currency’
  4. Mitochondria are more numerous in cells that have a high energy requirement - our muscle cells contain a large number of mitochondria, as do liver, heart and sperm cells
  5. Mitochondria are surrounded by two membranes, indicating that they were once free-living organisms that have become mutualistic and then a part of almost every eukaryotic cell (not RBC’s and xylem vessels)
    1. The smooth outer membrane serves as a boundary between the mitochondria and the cytosol.
    2. The inner membrane has many long folds, known as cristae, which greatly increase the surface area of the inner membrane, providing more space for ATP synthesis to occur.
  6. Mitochondria have their own DNA, and new mitochondria arise only when existing ones grow and divide. They are thus semi-autonomous organelles.

 

Ribosomes

  1. Unlike most other organelles, ribosomes are not surrounded by a membrane
  2. Ribosomes are the site of protein synthesis in a cell
  3. They are the most common organelles in almost all cells
  4. Some are free in the cytoplasm (Prokaryotes)
    others line the membranes of rough endoplasmic reticulum (rough ER)
  5. They exist in two sizes: 70S are found in all Prokaryotes, chloroplasts and mitochondria, suggesting that they have evolved from ancestral Prokaryotic organisms. They are free-floating. 80S found in all eukaryotic cells – attached to the rough ER (they are rather larger)
  6. Groups of 80s ribosomes, working together, are known as a polysome.


Endoplasmic reticulum (ER)

  1. The ER is a system of membranous tubules and sacs
  2. The primary function of the ER is to act as an internal transport system, allowing molecules to move from one part of the cell to another
  3. The quantity of ER inside a cell fluctuates, depending on the cell's activity. Cells with a lot include secretory cells and liver cells
  4. The rough ER is studded with 80s ribosomes and is the site of protein synthesis. It is an extension of the outer membrane of the nuclear envelope, so allowing mRNA to be transported swiftly to the 80s ribosomes, where they are translated in protein synthesis
  5. The smooth ER is where polypeptides are converted into functional proteins and where proteins are prepared for secretion. It is also the site of lipid and steroid synthesis, and is associated with the Golgi apparatus. Smooth ER has no 80s ribosomes and is also involved in the regulation of calcium levels in muscle cells, and the breakdown of toxins by liver cells
  6. Both types of ER transport materials throughout the cell.


Golgi apparatus

  1. The Golgi apparatus is the processing, packaging and secreting organelle of the cell, so it is much more common in glandular cells.
  2. The Golgi apparatus is a system of membranes, made of flattened sac-like structures called cisternae.
  3. It works closely with the smooth er, to modify proteins for export by the cell.


Lysosome

  1. Lysosomes are small spherical organelles that enclose hydrolytic enzymes within a single membrane
  2. Lysosomes are the site of protein digestion – thus allowing enzymes to be re-cycled when they are no longer required. They are also the site of food digestion in the cell, and of bacterial digestion in phagocytes
  3. Lysosomes are formed from pieces of the Golgi apparatus that break off
  4. Lysosomes are common in the cells of Animals, Protoctista and even Fungi, but rare in plants.


Cytoskeleton

  1. Just as your body depends on your skeleton to maintain its shape and size, so a cell needs structures to maintain its shape and size
  2. In animal cells, which have no cell wall, an internal framework called the cytoskeleton maintains the shape of the cell, and helps the cell to move
  3. The cytoskeleton consists of two structures: a) microfilaments (contractile). They are made of actin, and are common in motile cells. b) microtubules (rigid, hollow tubes – made of tubulin).
  4. Microtubules have three functions:
    1. To maintain the shape of the cell
    2. To serve as tracks for organelles to move along within the cell
    3. They form the centriole


Centriole

  1. This consists of two bundles of microtubules at right-angles to each other
  2. Each bundle contains 9 tubes in a very characteristic arrangement
  3. At the start of mitosis and meiosis, the centriole divides, and one half moves to each end of the cell, forming the spindle
  4. The spindle fibres are later shortened to pull the chromosomes apart


Cilia and flagellae

  1. Cilia and Flagellae are structures that project from the cell, where they assist in movement
  2. Cilia (sing. cilium) are short, and numerous and hair-like
  3. Flagellae (sing. flagellum) are much longer, fewer, and are whip-like
  4. The cilia and flagellae of all Eukaryotes are always in a ‘9 + 2’ arrangement that is characteristic (see diagram)
  5. Protoctista commonly use cilia and flagellae to move through water
  6. Sperm use flagellae (many, all fused together) to swim to the egg
  7. Cilia line our trachea and bronchi, moving dust particles and bacteria away from the lungs


Plant cell structures

  1. Most of the organelles and other parts of the cell are common to all Eukaryotic cells. Cells from different organisms have an even greater difference in structure
  2. Plant cells have three additional structures not found in animal cells:
    1. Cellulose cell walls
    2. Chloroplasts (and other plastids)
    3. A Central Vacuole


Cellulose cell wall

  1. One of the most important features of all plants is presence of a cellulose cell wall.
  2. Fungi such as Mushrooms and Yeast also have cell walls, but these are made of chitin.
  3. The cell wall is freely permeable (porous), and so has no direct effect on the movement of molecules into or out of the cell.
  4. The rigidity of their cell walls helps both to support and protect the plant.
  5. Plant cell walls are of two types:
    1. Primary (cellulose) cell wall - While a plant cell is being formed, a middle lamella made of pectin, is formed and the cellulose cell wall develops between the middle lamella and the cell membrane. As the cell expands in length, more cellulose is added, enlarging the cell wall. When the cell reaches full size, a secondary cell wall may form
    2. Secondary (lignified) cell wall - The secondary cell wall is formed only in woody tissue (mainly xylem). The secondary cell wall is stronger and waterproof and once a secondary cell wall forms, a cell can grow no more – it is dead!


Vacuoles

  1. The most prominent structure in plant cells is the large vacuole.
  2. The vacuole is a large membrane-bound sac that fills up much of most plant cells.
  3. The vacuole serves as a storage area, and may contain stored organic molecules as well as inorganic ions.
  4. The vacuole is also used to store waste. Since plants have no kidney, they convert waste to an insoluble form and then store it in their vacuole - until autumn!
  5. The vacuoles of some plants contain poisons (eg tannins) that discourage animals from eating their tissues.
  6. Whilst the cells of other organisms may also contain vacuoles, they are much smaller and are usually involved in food digestion.


Chloroplasts (and other plastids)

  1. A characteristic feature of plant cells is the presence of plastids that make or store food.
  2. The most common of these (some leaf cells only!) are chloroplasts – the site of photosynthesis.
  3. Each chloroplast encloses a system of flattened, membranous sacs called thylakoids, which contain chlorophyll.
  4. The thylakoids are arranged in stacks called grana.
  5. The space between the grana is filled with cytoplasm-like stroma.
  6. Chloroplasts contain ccc DNA and 70S ribosomes and are semi-autonomous organelles.
  7. Other plastids store reddish-orange pigments that colour petals, fruits, and some leaves.


Multicellular organization

In a unicellular organism, one cell carries out all of the functions of life. In contrast, most cells in a multicellular organism are specialized to perform one or a few functions – more efficiently. Because of cell specialization, the cells of multicellular organisms depend on other cells in the organism for their survival.


Tissue, organs, and organ systems

  1. In most Multicellular Organisms, we find the following organization:
    • Cellular Level: The smallest unit of life capable of carrying out all the functions of living things.
    • Tissue Level: A group of cells that performs a specific function in an organism.
    • Organ Level: Several different types of tissue that function together for a specific purpose.
    • Organ System Level: Several organs working together to perform a function. The different organ systems in a multicellular organism interact to carry out the processes of life
  2. Plants also have tissue and organs, although they are arranged somewhat differently from those of animals – e.g. vascular tissue.
  3. The four plant organs are: Roots Stems Leaves and Flowers


Colonial organizations

  1. A colonial organization is a collection of genetically identical cells that live together in a closely connected group.
  2. Many of the cells of the colony carry out specific functions that benefit the whole colony.
  3. Colonial organisms (e.g. sponges, coral) appear to straddle the border between a collection of unicellular organisms and a true multicellular organism. They lack tissues and organs, but do exhibit the principle of cell specialization.
ORGANELLELOCATIONDESCRIPTIONFUNCTION
Cell wallPlant, not animalOuter layer rigid, strong, stiff made of celluloseSupport (grow tall) protection allows H2O, O2, CO2 to pass into and out of cell
Cell membraneBoth plant/animalPlant - inside cell wall animal - outer layer; cholesterol selectively permeable Support protection controls movement of materials in/out of cell barrier between cell and its environment maintains homeostasis
NucleusBoth plant/animalLarge, ovalControls cell activities
Nuclear membraneBoth plant/animalSurrounds nucleus selectively permeableControls movement of materials in/out of nucleus
CytoplasmBoth plant/animalClear, thick, jellylike material and organelles found inside cell membraneSupports /protects cell organelles
Endoplasmic reticulum (E.R.)Both plant/animalNetwork of tubes or membranesCarries materials through cell
RibosomeBoth plant/animalSmall bodies free or attached to E.R.Produces proteins
MitochondrionBoth plant/animalBean-shaped with inner membranesBreaks down sugar molecules into energy
VacuolePlant - few/large animal - small Fluid-filled sacsStore food, water, waste (plants need to store large amounts of food)
LysosomePlant - uncommon animal - common Small, round, with a membraneBreaks down larger food molecules into smaller molecules digests old cell parts
ChloroplastPlant, not animalGreen, oval usually containing chlorophyll (green pigment)Uses energy from sun to make food for the plant (photosynthesis)

This page has been written by Ian White.