The cell – Basic unit of life: Chapter 5 

• Cell is the basic structural and functional unit of all living organisms
• All living organisms consists of one (unicellular organisms – bacteria) or more cells
• Each of these cells has the basic characteristics of life

The study of cells: 

• Robert Hooke; English scientist
• Robert coined the term ‘cell’
• Antonie van Leeuwenhoek; Dutch optician
• He was the first person to study animal cells
• Robert Virchow said that all cells develop from already existing cells
• Lwoff stated that when living organisms are studied on a cellular level, concept of unity is visible.
• Unity of structure because all cells consist of a nucleus in cytoplasm
• Unity of function because all cells have the same basic metabolism
• Unity of composition because the same macromolecules are found in all cells

How are cells studied?

• Light microscope: Make light shine through the object; lenses enlarge and focus the image
• Magnification: The ratio between the size of the image and the real size of the object
• Resolution: The sharpness
• The transmission electron microscope uses material embedded in plastic; thin sections are made of this plastic. Electrons move through the material and hit a photographic film
• Scanning electron microscope is used to study the surface structure of objects

The Cell: 

• Living matter is known as protoplasm
• A cell is the smallest unit of protoplasm
• Protoplasm is made up of nucleus and cytoplasm
• A cell consists of the nucleus and the cytoplasm
• Cytoplasm is everything but the nucleus
• Cell membrane encloses the liquid contents of the cell,
• the ground plasma (hyaloplasm)

Chemical composition of the protoplasm: 

• The protoplasm consists of inorganic as well as organic components

Inorganic components: 

• Water: Main component of the protoplasm
• 90% of an organism can consist of water
• Water serves as a solvent, a medium for chemical reactions, reagent in chemical reactions
• Gases: Oxygen/Carbon dioxide are dissolved in the ground plasm
• Mineral salts: Ionic form; E.g Sodium chloride, phosphate, potassium ions

Organic components: 

• Proteins: Important structural component of cell membranes
• All enzymes are proteins -/ some hormones are proteins
• Carbohydrates: Sugars and starch -/ most important source of energy
• Liquids (fats): Structural component of cell membrane -/ stored as reserve energy
• Nucleic acids: DNA and RNA

Physical nature of protoplasm: 

• Water is the solvent in biological systems

Protoplasm is really a composition of different types of solutions:

• True solution: The solution is homogenous
• All the particles have the same size and cannot be seen with the naked eye
• Salts, sugars and gases dissolved in water -/ examples of true solutions
• Suspension: Solution is heterogeneous
• Particles can be seen with the naked eye/ will often sink to the bottom when the solution is not stirred
• Cell organelles floating in the ground plasm form a suspension
• Emulsion: Solvent and the dissolved substance are liquids
• Oil droplets dispersed in water form an emulsion
• Colloidal solution: Particles of the dissolved substance are too big (heavy) to float, but too small (light) to sink to the bottom
• Colloidal particles sometimes have similar electrical charges and repel each other
• Remain in suspension
• Particles can form strings or clumps (aggregates)
• Aggregates can hold water in the spaces between particles
• Proteins form colloidal solutions
• In a more solid form the solution is in a gel state
• A more liquid state is described as a sol
• Endoplasm is ground plasm in the central area of the cell
• Endoplasm is usually in a sol state
• While the ground plasm bordering the cell membrane (ectoplasm) is in a gel state
• Sol and gel states are irreversible
• Temperature, pressure and pH play a role in the conditions of the protoplasm

Cellular wall structure: 

Cell wall: 

• Are found in plant cells only
• Not part of the living protoplasm
• Found directly outside the cell membrane
• Formed by the cell membrane
• Cell walls consists of cellulose fibres impregnated with carbohydrates

Primary cell walls: Consists of fibres arranged in disorderly manner

• The fibres are permeated with pectin (secondary carb), sugary substances

Secondary cell walls: Consists of neatly arranged fibres permeated with lignin (woody substances)

• Secondary cell walls also contain suberin (water proof substance)  
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Functions of the cell wall: 

• Lend support to the plant wall
• Responsible for the shape of the plant cell
• Offer protection against mechanical injury to the plant cell
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• Plasmodesmata are thin, cytoplasmic threads, they reach through the cell wall
• They connect the cytoplasm of adjacent cells
• In plant cells with secondary wall gaps occur from place to place in the cell wall
• Gaps are known as pits
• When a pit is surrounded by a raised part or ‘collar’, it is known as a bordered pit
• Pits allow lateral transportation between cells

Cell Membrane: 

• Consistency of olive oil
• Fluid structure
• A very tough barner
• The part that encloses the cell contents
• Part of the living protoplasm
• Cell membrane plays an important protective role, being differentially permeable  
• Cell membrane controls the movement of substances into and out of the cell
• The most acceptable theory about the structure of the cell membrane is the floating mosaic model 
• The cell consists of two layers of phospholiphydrophylic (fats with phosphate ions) -/ protein molecules move around in here
• Cell membrane is not a static structure and can change continuously
• The protein part of the cell membrane is hydrophylic -/ allows water soluble molecules to move through the cell membrane
• The phospholipid part is hydrophobic -/ only allows substances that are not water soluble to penerate the membrane
• Proteins often act as carrier molecules
• Carrier molecules take substances through the cell membrane, against concentration gradient -/ this is known as active transport
• Substances such as ions and other small molecules move through the tiny pores in the cell membrane

Functions of the cell membrane: 

• Enclose the cell contents
• Differentially permeable/ controls everything that enters or leaves the cell
• Forms an important part of the cellular  immunity system
• Membranes inside of the cell play an important role in bringing parts of the cell in contact with one another 

Transport in cells: 

• Transport inside cells (intracellular transport) takes place through diffusion
• Osmosis allows transport from cell to cell (Intercellular transport)
• Dissolved substances can therefore spread evenly through the ground plasm
• Substances can also dissolve from cell to cell

Tempo of diffusion is influenced by: 

• Concentration gradient in the system, bigger the difference in concentration, faster the diffusion will be
• Temperature in the system, a high temperature promotes diffusion
• Pressure in the system, when the pressure on the system increases, tempo of diffusion will increase

Active transport:

• Substances move through a membrane against the diffusion gradient (low to high concentration)
• It is called active because energy is needed
• High water potential arises from many free water molecules in a solution that can do work ( a solution with very little dissolved molecules)
• Pure water has the highest possible water potential

Osmosis definition: Osmosis is the movement of a solvent (water in the case of cells) from a high to a low water potential through a differentially permeable membrane

Nucleus: 

• Most important organelle in any cell
• Largest of the cellular structures
• Often be seen when using a light microscope
• All cells have a nucleus; even red blood corpuscles that do not have nuclei
• Nucleus is surrounded by a double differentially permeable nuclear membrane -/ Outer one being continuous with the ER
• Pores occur in the nuclear membrane
• Bringing nuclear contents into direct contact with the ground plasm
• Space between the two membranes is known as the perinuclear space
• Nuclear plasma is the liquid contents of the nucleus
• The chromatin network is found in the nuclear plasma
• The chromatin network consists mainly of DNA
• One or more solid bodies are found in the nuclear plasm
• The Nucleolus consists mainly of RNA and proteins -/ plays a role in protein synthesis

Functions of the nucleus: 

• Control of the cell membrane
• Differentiation/ specialisation of cells
• Transfer of hereditary information
• Reproduction of cells/ organisms

All organisms can be divided into two groups

• Prokaryotes
• Eukaryotes
• ‘Karyo’ refers to the nucleus

• Prokaryotes do not have true nuclei (chromatin is not surrounded by a membrane)
• Eukaryotes do have proper nuclei (nuclear membrane present)
• Bacteria are prokaryotes
• Unicellular/ multicellular organisms are eukaryotes
• Chromatic network consists of the macromolecule DNA
• During cell division the chromatic network shortens/ thickens to become visible as chromosomes
• Chromosomes bear the hereditary factors, known as genes
• Number of chromosomes per cell is a unique characteristic -/ differs from species to species
• Each cell in the human body contains 46 chromosomes
• Egg cells/ sperm cells have 23 chromosomes each

Mitochondria: 

• Regarded as the ‘power station’ of the cell
• The organelles are responsible for cellular respiration
• Cellular respiration is the gradual release of energy from an energy-rich fuel molecule in the presence of oxygen, with water/ carbon dioxide as waste products
• Energy is temporarily stored in the energy carrier of the cell, ATP
• Mitochondria are small, oval-shaped, surrounded by a double membrane
• Inner membrane is folded to increase the surface/ surrounds the liquid part of the organelle (the matrix)
• Folds of the inner membrane are called cristae
• Cristae serve as a basis for enzyme systems
• Enzyme systems are responsible for cellular respiration
• Mitochondria also contains DNA
• The DNA is in a specific ring structure, known as plasmid
• Mitochondrial DNA is transferred from the mother via the egg cell to the next generation
• Genetic history of a person can be studied by using this DNA, maternal side
• Mitochondrial DNA can also be used to identify somebody/ to connect a person to a crime

Mitochondrial disease: Genetics with a single parent 

• Mitochondria is the furnace of the cell
• Oxygen and food materials are burned there to produce energy
• Mitochondria have their own piece of DNA; small enclosed circle
• Sperm do not transmit mitochondria; mitochondria are passed down from the egg cell
• Some diseases lead to blindness because the optic nerve is damaged
• Some diseases lead to muscle weakness; several causes brain damage
• All are rare, but all show the characteristics patter of inheritance through the females

Endoplasmatic Reticulum: 

Endoplasmatic reticulum is a system of membranes inside the cell that:

• Brings cell contents in contact with the outside environment (extracellular environment)
• Enlarges the internal surface of the cell
• Acts as an internal transport system
• Serves as a basis for attachment for the ribosomes, playing a role in protein synthesis
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• ER with ribosomes is also known as granular (GER)/ coarse ER
• Smooth or agranular ER (AER) do not have ribosomes

Ribosomes: 

• Small, roundish; found in ground plasm
• Bases of protein synthesis
• The genetic code for the various proteins is brought from the DNA in the nucleus to the ribosomes by the RNA
• Amino acids are the building blocks of proteins
• Amino acids are arranged and bound in a certain sequence on the ribosomes

Golgi apparatus: 

• A membrane system
• Consisting of a number of dictyosomes
• System of stacked, flat membranous

Functions of Golgi apparatus: 

• Associated with secretion in the cell
• Plays a role in the forming of other membrane systems
• Changes/ sorts proteins
• Plays role in formation of lysosomes

Plastids: 

• Organelles found in plant cells only
• Three types of plastids: Chloroplast; Chromoplasts and Leucoplasts

Chloroplasts: 

• Oval shaped
• Surrounded by a double membrane
• Liquid contents is known as stroma
• Layers of membranes, the lamellae, are found in the stroma
• Thickenings known as thylakoids occur on the lamellae
• A stack of lamellae is known as granum
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• Thylakoids contain enzymes, co-enzymes, Cytochromes and various pigments
• Most important pigments are the green chlorophyll/ carotenoids, the yellow, the orange and red
• The pigments absorb light for photosynthesis
• Give colour to plant structure
• In chloroplasts the carotenoids are masked by chlorophyll = green chloroplast -/ cause of the green colour of plant parts
• Photosynthesis takes plants in the chloroplasts
• Photosynthesis is the conversion of radiant energy to chemical potential energy
• Chemical potential energy is bound with water/ carbon dioxide to from the energy-rich organic molecule glucose with oxygen as a by-product

Chromoplasts: 

• Structure is the same as that of chloroplasts; chloroplasts do not contain chlorophyll  
• Chromoplasts are found in most plant parts -/ are responsible for the colour of flowers, leaves that change colour
• Chloroplasts lose their chlorophyll/ change to red, orange or yellow

Leucoplasts: 

• Colourless plastids responsible for food storage in plant cells
• The reserve food can be oil droplets, proteins or starch

Vacuoles: 

• A fluid filled cavity in the ground plasma -/ surrounded by a membrane
• Common in plant cells but not all animal cells

Vacuoles in plant cells: 

• Membrane in a plant cell is called a tonoplast/ the liquid contents the cell sap
• Tonoplast is differentially permeable
• Cell sap consists of water with dissolved substances (sugar and salts)
• Turgor pressure is the pressure that the contents of a cell exert on the cell wall -/ caused by the uptake of water into the vacuole until it is full/stiff
• When a vacuole is full to the maximum is it turgid
• Cell loses a large amount of water and becomes soft it is flaccid

Functions of the vacuole:

• Storage of dissolved substances
• Lend support to the plant cell – turgor pressure –
• Promote osmosis – creating a low water potential inside the plant cell –

Vacuoles in animal cells: 

• Small
• More than one of the same type will occur
• Phagosomes: In unicellular organisms. Food vacuoles fuse with the lysosomes and digestion takes place
• Contractile vacuoles: Unicellular animals. Play a role in excretion of excess water (osmoregulation). Refers to the ability to contract rhythmically, move to the surface
• Lysosomes: small vesicles (bubbles) in the ground plasma – contains hydrolytic enzymes; digestive enzymes –
• Lysosomes protect the cell against foreign substance/ structures / play a role intracellular digestion
• Lysosomes can fuse with foreign substances/ food particles
• Digestive enzymes destroy foreign substances and break down food particles into smaller substances
• Membrane of a lysosomes is resistant to the digestive enzymes, does not have the typical membrane structure

Centrioles: 

• Consist of microtubules (small tubes)
• Two centrioles are found next to the nucleus in animal cells – area known as the centrosome –
• Centrioles lie at a 90 degree angel with one another
• Exact function of the centriole is not known
• It may play a role in the motility of some cells

Differences between plant and animal cells: 

Plants cells: 

• Contains plastids
• Have cell walls
• No centrioles
• One, large vacuole

Animal cells: 

• No plastids
• No cell wall
• Have centrioles
• No or few, small vacuoles

Differentiation and Specialisation: 

• Unicellular organisms – all cell functions are performed by organelles/ molecules
• Multicellular organisms – cells specialise; different cells perform different functions – division of labour takes place –
• When cells specialise = performed more effectively
• Cells must also differentiate; their structure must be suitable for the functions

Specialisation and differentiation: 

• Voluntary muscle cells: Cylindrical with myofibrils – can shorten when muscles contract
• Neurons: (nerve cells) – outgrowths that conduct impulses
• Wood vessels segments: Xylem are hollow tubes – water is transported –
• Root hair cells: Finger like outgrowths to increase the surface for water absorption

• In plants the meristematic cells differentiate/ specialise  (perform specific functions)
• In animals the stem cells will differentiate/ specialise

The relative size of cells: 

• Cells are generally very small (there are exceptions)
• Cells have to ingest food and get rid of waste products – occur via the cell membrane
• Various substances must be transported inside the cell
• Surface of the cell must be large enough to let enough substances move through the cell membrane to provide for the total volume of the cell – this is known as the surface-volume-ratio of the cell