Cell Function and Organisation

Cell tissues and organs

Cells à tissues à organs à organ system à organism

Cells- the basic structural and functional units of life, capable of performing all life processes.

Tissues- Groups of similar cells that work together to perform a specific function.

Organs- Structures made up of different types of tissues working together to carry out a particular function.

Organ system- A group of organs that work together to perform complex body functions

Organism- A living entity that can grow, reproduce, respond to stimuli, and maintain homeostasis

Tissues

All animal tissues arise from the 3 embryonic tissues namely:

·       Ectoderm- a type of embryonic tissue found on the outside

·       Mesoderm- the embryonic tissue that is found in the middle  

·       Endoderm- the embryonic tissue that is found on the inside

 There are 4 types of adult tissues:

·       Epithelial tissue

·       Connective tissue

·       Muscle tissue

·       Nervous tissue

Epithelial tissue- composed of cells that line the cavities and surfaces of structures throughout the body. It form layers of closely packed cells. No matter where it is found in the body, they have a polarity:

·       Apical side- faces away from other tissues and toward the environment

·       Basolateral side- faces the interior of the animal and is connected to other tissues.

Functions of Epithelial tissue:

·       Act as barrier between the inside and outside of body and between compartments

·       Control transport (in blood vessels, gut) and filtration (in kidneys)

·       Specialised into glands for secretion example hormones, milk, sweat, mucus and digestive enzymes

·       Those having cilia on their free and move substance over surface or those through tubes

·       Specialise as sensory cells as in smell and taste receptors.

Adaptations of epithelia:

·       Covering epithelia- found lining structures example the skin, alimentary canal, the respiratory tract, all ducts, the bladder and the kidney tubules

·       Glandular epithelia- responsible for synthesis and release of secretions

Types of epithelia

·       Simple- one cell thick

·       Pseudostratified- cells of different size all touching basement. Made up of one layer of cells. But as cells do not have the same shape, epithelium appears to have more than one layer. All cells rest on the basement membrane. Have large cells that may bear microvilli or cilia. It is found in the respiratory passages.

·       Stratified- many layers of cells- maybe squamous or cuboidal

Simple epithelia tissue:

·       Simple Squamous Epithelium- Made up of flattened cells with a characteristic building nucleus. The amount of cytoplasm is low. Edges of cells are irregular when observed in surface view. It provides a thin layer across which diffusion can occur readily. It also provides relatively friction-free surfaces for fluids to flow over them. It is found in endothelium in arteries and veins, capillary wall, alveoli, glomerulus and Bowman’s capsule in nephron

·       Simple cuboidal epithelium- Made up of roughly cube-like cells. Nucleus typically occupies centre of the cell. Cells generally not specialized. It is rich is specific transport channels. It functions in secretion and absorption. It is found in the proximal and distal convoluted tubule in nephron, line some glands such as salivary and sweat glands and covers the ovaries.

·       Simple columnar epithelium- Made up of elongated cells. Nucleus is at the basal end of the cell. Cell has a lot of cytoplasm. Modifications appear presence of microvilli, cilia and goblet cells. Microvilli- indicate increased surface area for secretion and absorption. Cilia- indicate movement of substances over the epithelium. Goblet cells- produce mucus. It is found in the small intestine, kidney, bronchi, trachea and oviduct.

Stratified epithelia tissue:

·       Stratified Squamous Epithelium- Made up of several layers of squamous cells. Upper layer of cells is continuously removed by friction. These are replaced by the germinal layer below. It is found in the mouth, skin, oesophagus, vagina, cornea.

·       Stratified cuboidal epithelium- made up of several layers of cuboidal cells. It lines ducts of sweat glands and urinary tract.

·       Stratified columnar epithelium- made up of several layers of columnar cells. Lines parts of pharynx and larynx and ducts of salivary glands.

·       Stratified transitional epithelium- consists of 3-4 layers of cells, all of similar size and shape except at the apical side where the cells are more flattened. All cells are able to modify their shape when placed under different conditions.  Found in place in the body that are subjected to considerable stretching. Cuboidal appearance is observed when the urinary bladder is empty but squamous appearance is observed when the urinary bladder is full. It is found in the unary bladder and pelvic regions of the kidney.

Connective Tissue- consists of cells suspended in a matrix. The cells secrete the matrix which can be solid to liquid consistency. The matric may contain different proportions of protein fibres collagen and elastin.

General functions of connective tissue:

·       Major supporting tissue of the body

·       Forms sheaths like bags around organs of the body, separating them so that they do not interfere with each other’s activities

·       Surrounds and protects blood vessels and nerves where they enter or leave organs.

Types of Connective tissue:

·       Loose connective tissue- consists of cells scattered within a matrix that contains a large amount of ground substance. This gelatinous material is strengthened by a loose scattering of protein fibres such as:

o   Collagen- which supports the tissue by forming a meshwork

o   Elastin- which makes the tissue elastic

o   Reticulin- which helps support the network of collagen

·       Dense connective tissue-  contains less ground substance. Collagen fibres are packed tightly, making it stronger than loose connective tissue.

o   Dense regular tissue- where collagen fibres line up in parallel. It is found in the tendons and ligaments

o   Dense irregular tissue- collagen fibres arranged irregularly, produces tough covering that package organs example around kidneys and adrenal glands, muscle, nerves and bones.

·       Specialised connective tissue

o   Cartilage- A firm and flexible tissue that does not stretch. Tougher than loose or dense connective tissue. Has great tensile strength. Cells called chondrocytes that are found in spaced called lacunae. Provides flexible support, shock absorption. Reduces friction on load-bearing structures. Found between vertebrae in vertebral column, in joints, nose, ear and trachea.

o   Bone- firmer extra cellular matrix. Provide structural support for the body and protective structures for the brain and the nervous system and internal organs. Provides a site for muscle attachment. Cells called osteocytes that are found in lacuna.

o   Blood- connective tissue with cells surrounded by a liquid extra cellular matrix called plasma. Its main function is to transport materials throughout the cell.

Muscle tissue- consists of elongated cells that can contract to generate a force and cause movement.

There are three types of muscle tissue:

·       Straited muscle- consists of long muscle fibres. They are packed with special proteins that move when ATP is broken. Straited muscle contracts in response to a complex series of events triggered by electrical signals arriving from nerve cells. Also called skeletal muscle because it attaches to bones. Is involved in locomotion, facial expressions, shivering and breathing. Its control could be conscious or unconscious. 

·       Cardiac muscle- makes up the wall of the heart. Similar to skeletal muscle but branched and makes direct cell-to-cell contact with other cardiac muscle cells. These connections help  transmit signals from one cardiac muscle to another during a heartbeat. It is under involuntary control. Its role is to pump blood

·       Smooth muscle- cells are tapered at each end and form a muscle tissue that lines the wall of the digestive tract and the walls of the blood vessels. Smooth muscle is responsible for involuntary movements such as passage of food down the digestive tract or the dilation of arteries near the skin in hot weather.

Nervous tissue- consists of nerve cells also called neurons and several types of supporting cells. Vary widely in shape. All have projections that contract other cells to deliver electrochemical signals.

Movement of molecules across membranes

Microtransfer- passive or active transport of small particles

Macrotransfer- movement of large particles or molecules. It involves fusion of the membrane and includes endocytosis and exocytosis

Passive transport- involves movement down a concentration gradient without any energy expenditure

Active transport- involves movement of a substance against a concentration gradient accompanied by the expenditure of energy in the form of ATP. Cells that carry out active transport have a high number of mitochondria and high concentrations of ATP. Active transport stops if ATP production is prevented by poisons such as cyanide, or by lack of oxygen.

Diffusion- refers to the net movement of substances from regions of high concentration to regions of low concentration. According to the kinetic theory, molecules and ions dissolved in water are constantly moving randomly. This random motion causes diffusion

Fick’s law- This law suggests that the rate of diffusion in a given direction across the exchange surface:

·       Is directly proportional to the surface area

·       Is directly proportional to the concentration gradient

·       Is inversely proportional to the distance (the length of the diffusion pathway.

Factors affecting the rate of diffusion:

·       Steep concentration gradient between extracellular and intracellular environment

·       Have a high surface area : volume ratio

·       Thin shape

·       Temperature- the higher the temperature, the higher the K.E. of the diffusing molecules, therefore the faster the rate of diffusion.

·       Electrical charge distribution- applicable only for charged particles that may be repelled or attracted

·       Size and mass of the diffusing particles- the larger and heavier the molecule, the slower the rate of diffusion

Factors affecting permeability:

·       Lipid solubility- plasma membrane is more permeable to molecules which are lipid soluble. These include small molecules example O₂, CO₂ and small uncharged polar molecules e.g. glycerol and alcohol

·       Molecular size- the cell membrane is more permeable to smaller molecules

·       Membrane thickness- the thinker the membrane, the less permeable

·       Temperature- the higher the temperature, the more permeable as phospholipid molecules move around each other faster.

·       Membrane chemical composition- the higher the amount of unsaturated fatty acids in the plasma membrane, the more permeable it becomes. This occurs as the fatty acid chains are less closely packed, thus reducing the strength of the hydrophobic interactions. The higher the amount of cholesterol in the plasma membrane, the less permeable it becomes. Cholesterol fills spaces between phospholipids, thus increasing the density of the hydrophobic region.

Selective permeability- When the membrane only allows some substances through but not others.

Factors contributing to selective permeability include:

·       Phospholipid bilayer- allow only lipid-soluble substances through

·       Ion channels- allow specific ions to diffuse through them.

·       Protein carriers- carry specific molecules across the membrane

Facilitated Diffusion- It have is specific that any given carrier transports only certain molecules. It is passive, meaning the direction of net movement is determined by the concentration gradient of the transported substance. It saturates, meaning if all protein carries are in use, increases in the concentration gradient do not increase the rate of transport. Protein carriers physically bind to the solute on one side of the membrane, and release them on the other. Facilitated diffusion can be slowed down if there is a second substance that is structurally similar to the normal substance.

Osmosis- The movement of water by diffusion across a semi-permeable membrane from a region of high concentration to a region of low water concentration. Osmosis can be taken as the movement of water molecules from a region where they have a higher kinetic energy because they are ‘free’ and not bound to solute molecules to a region of lower kinetic energy, across a selectively permeable membrane.

Water molecules pass through the plasma membrane as follows:

·       Through the phospholipid bilayer: as water molecules are very small compared to the large phospholipids. Although the fatty acid chains of the phospholipids are hydrophobic, water molecules manage to escape between the large phospholipid molecules as these move around each other

·       Through protein channels: water molecules pass along with the dissolved ions

·       Through aquaporins: these are specific protein channels that allow water molecules only through. They are found in plant cells and animal cells like 122 red blood cells and kidney cells. They facilitate the uptake of water and osmotic equilibrium is reached faster

Water potential- The tendency of water molecules to move from one place to another. This is actually a measure of the free kinetic energy of water molecules.
Pure water at atmospheric pressure has the highest water potential i.e. zero. All solutions at atmospheric pressure have negative values of water potential.
Solute potential- the ability of a solution to take in water from the other side of a semi-permeable membrane. The reduction in the free energy possessed by water molecules due to presence of solute particles. All solutions have a negative value of solute potential.  The more concentrated the solution, the lower the solute potential. (more -ve value).

Pressure Potential- It is a component of water potential in plants that results from the physical pressure exerted on water within the cell. It can be positive (when water is pushed against the cell wall, like in turgid cells) or negative (like in xylem during transpiration)

Points to remember:

·       Water always moves from higher water potential to a lower water potential

·       For two systems to be in equilibrium, they must have equal water potential

·       The solute potential of the cell tends to attracts water into the cell

·       The pressure potential of the cell opposes the entry of water into the cell

·       Animal cells do not have a pressure potential due to the absence of a cell wall

Behaviour of Animal cells in different solutions

a)       Red blood cells in a hypotonic solution i.e. a solution with a higher water potential. Water enters the cells causing them to swell and even burst releasing all haemoglobin

b)       Red blood cells in an isotonic solution i.e. a solution with the same water potential of the cell. Same amount of water moves in and out of cell. No changes to cell structure.

c)       Red blood cells in hypertonic solution i.e. a solution with a lower water potential than the cell. Water leaves the cells by osmosis. The cells shrivel up until the concentrations of water become equal.  

Behaviour of Plant cells in different solutions

a)       Plant cells in a hypotonic solution example pure water. As the water enters the cell its vacuole expands cytoplasm swells, pushing against the rigid cell wall. The cell wall opposes the exerting a pressure potential. At full turgor i.e. when a cell is in equilibrium with pure water

b)       Plant cells in an isotonic solution. New movement of water in and out cells is zero.

c)       Plant cells in a hypertonic solution. Water moves out of the cells as solution has a lower water potential. Cytoplasm shrinks and pulls away from the cell wall. This process is called plasmolysis.

Primary active transport- the process by which cells move molecules across a membrane against their concentration gradient using energy directly from ATP
How the sodium-potassium pump works:

·       Three Na⁺ ions from the cytoplasm bind to the Na⁺- K⁺ pump.

·       ATP molecule dissociates into ADP and P. The phosphate group joins to the Na⁺- K⁺ pump (phosphorylation).

·       Phosphorylation causes the Na⁺- K⁺ pump to change its shape. This causes the 3 Na⁺ ions to dissociate from the pump and are released into the extracellular environment.

·       Two K⁺ ions from the extracellular environment bind to the Na⁺ - K⁺ pump.

·       This causes the phosphate group to dissociate from the Na⁺- K⁺ pump.

·       The loss of the phosphate causes the Na⁺- K⁺ pump to regain its original shape.

·       Thus, the K⁺ ions are released inside the cytoplasm.

·       The Na⁺- K⁺ pump has its original shape, and the process can be repeated all over again

Directionality of active transport:

·       Uniport transport proteins- move a single solute in one direction example Ca²⁺ binding protein

·       Symport transport proteins- move 2 solutes in the same direction example uptake of Na⁺ and amino acids in the intestine

·       Antiport transport proteins- move 2 solutes in opposite directions, one into the cells and the other out of the cell example sodium potassium pump.

Secondary active transport- In this form of active transport the pumping of one substance indirectly drives the transport of another substance against the concentration gradient.
Example the entry of sucrose into phloem cells is couples to the entry of H+ ions.

·       H⁺ ions are actively pumped out of the cell by means of a proton pump. This lowers the H⁺ concentration inside the cell.

·       H⁺ ions will diffuse back into the cell down a concentration gradient. Diffusion of H⁺ occurs by means of a transport protein.

·       sucrose will also bind to the same transport protein.

·       Thus, as H⁺ moves in down its concentration gradient, sucrose is also ‘dragged’ along into the cell, against its concentration gradient

Endocytosis- During this process, materials that cannot penetrate through the membrane are taken up into the cell.

·       Phagocytosis- the engulfing of micro-organisms, foreign matter and other cells by a cell. The cell wraps pseudopodia around a large particle and packs it in a sac surrounded by part of the plasma membrane (i.e. A vacoule). Lysosomes fuse with vacuole and degrade the ingested material. This is used mostly by protozoa and some white blood cells to ingest large particles.

·       Pinocytosis- The uptake of fluid within minute vesicles originating from the cell surface membrane. The process is not specific, i.e. Any solutes dissolved in the droplet trapped in the small vesicle is transported into the cell.

·       Receptor mediated endocytosis- molecules are taken up by binding to receptor proteins on the cell surface membrane. When the appropriate molecule binds to the receptor the membrane folds inwards on itself forming a vesicle. The cell is able to take up specific molecules in bulk. Membrane regions having the receptor proteins are called coated pits. The inner side of the coated pits is lined by a protein called clathrin. Once the specific molecule binds to the receptor proteins, the internalisation process is similar to phagocytosis. Cholesterol is taken up in this way. When the cell needs cholesterol is integrates in its plasma membrane low density lipoprotein receptor proteins.

Exocytosis- this involves the elimination of wastes or secretion products e.g. Extracellular enzymes, out of cells through the fusion of a secretory vesicle with the plasma membrane.

The cell as the basic unit of living things

Unicellular-organisms that is made up of only one cell

Multicellular- organism that is made up of many cells

Prokaryotic cells- cells that lack internal membrane bound organelles

Eukaryotic cells- cells that have a prominent nucleus and numerous membrane- bound organelles.

Plasma membrane- consists of 3 major components: lipids, proteins and carbohydrates.

Lipids in the plasma membrane- the major component and structural backbone of membranes is a lipid bilayer. Most of the lipids are phospholipids i.e. Modified triglycerides attached to a phosphate group. The phospholipid molecule is amphipathic which has a hydrophilic part- the phosphate part is attracted to water and  hydrophobic part- the fatty acid part is not attracted to water. The phospholipids adjust themselves such that the hydrophilic part is always closest to water, whilst the hydrophobic part is always furthest away from water. As a plasma membrane is surrounded by water on both the cytoplasm side and the external environmental side, the phospholipids arrange themselves as a bilayer. The phospholipid bilayer actually forms a continuous sheet around the whole cell. This is due to the tendency of fatty acids to associate with one another and exclude water. Thus, structural damage in the membrane is sealed spontaneously. Other lipids present in the membrane include:

·       Glycolipids- triglycerides combined to a carbohydrate group instead of a phosphate group

·       Cholesterol- a lipid found in animal plasma membranes; related steroids (a group of lipids whose structure is similar to cholesterol) are found in the plasma membrane of plants. Cholesterol reduces the permeability of most plasma membranes

Proteins in the Plasma membrane- There may be:

·       Extrinsic (peripheral) proteins- these are attached to the surface of the plasma membrane by weak bonds

·       Intrinsic (integral) proteins- these are embedded in the plasma membrane

Carbohydrates found in plasma membrane- short carbohydrate chains may be attached to the lipid (form glycolipids) or protein molecules (forming glycoprotein). They are found only on the outer membrane surface. These carbohydrate chains may act as receptors and also allow cell – cell recognition. The unique arrangement of glycoproteins and glycolipids serves to fingerprint the cell at the:

·       species level

·       individual level

·       cellular level

Cytoplasm- The material enclosed by the plasma membrane between the membrane bound organelles. It has two components, the cytosol and the insoluble suspended particles, including the ribosomes. The cytosol consists mostly of water and dissolved ions (e.g. K⁺), small polar molecules (e.g. amino acids, monosaccharides, nucleotides) and soluble macromolecules like globular proteins. Many chemical reactions take place in the cytoplasm e.g. glycolysis (part of cellular respiration).

Functions of the Cytoplasm:

·       Store of vital chemicals

·       Site of chemical reactions

·       Supports and movement of membrane bound organelles.

Plant cell walls- Plant cells secrete cellulose and other polysaccharides to form rigid cell walls. It contains multiple layers of cellulose, formed into bundles of fibres. Cellulose cross links with other polysaccharides composing the cell wall. Layers of fibres run in different directions.

Primary cell wall- Thin and relatively flexible cell wall secreted by growing cell. Consists of cellulose fibres embedded in a polysaccharide matrix which protects them. Can expand with growth aa cellulose fibres are arranged randomly. After growth, it may be hardened.

Secondary cell wall- Formed after growth stops. Cellulose fibres are deposited in an orderly fashion. Multiple layers of different composition than primary wall formed between primary wall and plasma membrane. Has a strong and durable matrix that gives the cell protection and support example wood.

Middle lamella- A thin layer between primary walls of adjacent cells. It is rich in sticky polysaccharides called pectins.

Functions of the cell wall:

·       Protects plant cell

·       Maintains its shape

·       Prevents excessive uptake of water

Extra cellular matrix of Animal cells- Although animal cells do not have cell walls, some animal cells have a glycocalyx coat formed from carbohydrate regions of glycoproteins and glycolipids in the cell membrane. Some animal cells secrete other materials that bind to cell surfaces.

Functions of Extra cellular matrix:

·       Cellular recognition

·       Some secrete materials into intercellular spaces that increases mechanical strength of multicellular tissues.

·       Some secrete materials that bind to cell surfaces.

Nucleus- It is the largest organelle. Have a spherical or ovoid shape with a diameter of 5 mm. Typically, in the centre of animal cells. Present in all cells except red blood cells and phloem sieve tube cells. It is of prime importance as it stores the genetic information in DNA that determines the characteristics of the c ell and its functioning. DNA is found in the nucleoplasm. It is composed of the nuclear envelope, chromatin and nucleolus. 

Functions of the nucleus:

·       Chromosomes contain DNA, the molecule of inheritance.

·       Nuclear division is the basis of cell replication and hence reproduction.

·       The nucleolus manufactures ribosomes.

·       DNA is organised into genes which control the functioning and characteristics of the cytoplasm

Nuclear envelope- This is a double membrane that separates nucleus from the rest of cytoplasm. It is perforated with nuclear pores, which allow communication between the nucleus and the cytoplasm. The pore is formed by fusion of the outer and inner membranes of the nuclear envelope. The nuclear pores are channels. They are lined with proteins (pore complex) that control the passage of molecules between the nucleus and the cytoplasm. Generally:

·       Ribosomes pass out from the nucleus into the cytoplasm

·       Precursors of DNA and RNA move from cytoplasm into the nucleus.

·       mRNA passes out from the nucleus to the cytoplasm

Chromatin- It consists of coiled DNA bound to basic proteins called histones. During cell division chromatin condenses to from chromosomes. Chromatin is immersed in a semi fluid material called nucleoplasm. It consists mainly of water and dissolved substances Chromatin is attached to the nuclear lamina, a network of proteins (lamins) just below the nuclear membrane. The nuclear lamina maintains the shape of the nucleu. Two types of chromatin are:

·       Heterochromatin- tightly coiled, stains deeply and is thought to contain inactive DNA.

·       Euchromatin- loosely coiled, stains lightly and thought to contain active DNA.

Nucleolus- The site where ribosomes are synthesised. Generally, it acquires an intense stain due to large amounts of DNA and RNA in that region. It consists of ribosomal RNA. The number of nucleoli can vary depending on the species. The nucleolus is associated with a particular region on a specific chromosome called the nucleolar organiser. Its principal function is the synthesis of ribosomes.

Endomembrane system- Much of the volume of a eukaryotic cell is taken up by a series of folded internal membranes performing a large variety of functions.

Consequences of having an endomembrane system:

·       Compartmentalization allows incompatible chemical reactions to be separated. Potentially harmful reactants and/or enzymes can be isolated inside an organelle so as not to damage the rest of the cell.

·       Groups of enzymes that work together can be clustered on internal membranes instead of floating free in the cytoplasm. Organelles contain enzymes for a particular metabolic pathway. Hence the products of one reaction will always be in close proximity to the next enzyme in sequence. In this way the rate of metabolic reaction will be increased.

·       Organelles maintain high concentrations of molecules.

·       Compartmentalization makes large size possible. The molecules required for specific chemical reactions do not need to move long distances to interact.

·       Each compartment creates a microenvironment that is optimal for the activity of enzymes within it.

·       Rate of metabolic pathway inside an organelle can be regulated by controlling the rate at which first reactant enters

Endoplasmic Reticulum- It is an organelle found in all eukaryotic cells except the rad blood cells. Consists of a network of folded internal membranes forming sheets, tubes or flattened sacs in the cytoplasm. The cavities within the endoplasmic reticulum are called cisternae. These contain a variety of enzymes. Two types of endoplasmic reticulum are identified as the rough endoplasmic reticulum and the smooth endoplasmic reticulum.

Rough endoplasmic reticulum (RER)- It is continuous with the nuclear membrane as it originates from the outer membrane of the nucleus. It is abundant in protein secreting cells. It is lined with ribosomes. The amount of RER in a cell reflects how busy the cell is synthesising proteins for export

Functions of the RER:

·       Supports ribosomes from protein synthesis

·       Isolates proteins from the cytoplasm

·       Transports proteins- proteins built on the ribosome are threaded into the cisternae. They then move through the cisternae and are then generally packaged into transport vesicles.

·       Formation of membranes- the RER assembles lipids into bilayers from precursors found in the cytoplasm. Protein molecules are embedded into these lipid bilayer as well.

·       Modifies proteins- proteins in the RER may be chemically modified. This alters their function and destination.
Generally, the synthesised proteins are:

o   Protein to be used outside the cells

o   Proteins for plasma membrane

o   Hydrolytic enzyme to form part of lysosomes

Smooth Endoplasmic Reticulum (SER)- It usually tubular in structure and occupies less volume in the cell than RER. This is not lined with ribosomes.

Functions of SER:

·       Metabolism of lipids, phospholipids, fatty acids and steroids and their transport. SER is prominent in cells, which secrete steroid hormones

·       In liver cells it is concerned with detoxification and excretion of drugs, alcohol and hormones.

·       Plays a part in conversion of glycogen to free glucose in liver cells.

·       Involved in metabolism of lipids and cholesterol.

·       Chemical modification of proteins produced on the RER.

·       In muscle cells it forms a specialised structure called the sarcoplasmic reticulum. During muscle contraction it is involved in the uptake and release of Ca^2+.

Ribosome- Found in large numbers throughout the cytoplasm of both prokaryotic and eukaryotic cells. Numerous in cells that have high rates of protein synthesis example liver cells and pancreas cells. Consists of one large and one small sub-unit. Each sub-unit is made up of protein and ribosomal RNA. No membrane is present.

Functions of Ribosomes:

·       Site of protein synthesis

·       Free ribosomes are suspended in the cytoplasm (sometimes group together to from a polysome or polyribosome). They synthesize proteins needed within cell e.g. enzymes involved in glycolysis.

·       Bound ribosomes are associated with the endoplasmic reticulum. They synthesize proteins that are included in membranes, packed within lysosomes or secreted out of cell e.g. enzymes, hormones

Golgi apparatus- Consists of a stack of flattened membrane-bound sacs and associated vesicles. Here proteins are processed, stored and modified within saccules. It also packages substances in vesicles for secretion outside of the cell. It originates near the nucleus and / or endoplasmic reticulum and ends near the plasma membrane. Vesicles pinch off from this end of the Golgi apparatus.

Mode of action of the Golgi apparatus:

1.       Proteins synthesised on the RER are transported in vesicles to the Golgi apparatus.

2.       As the Golgi apparatus modifies and / or packages the protein, these are moved in vesicles between the cisternae.

3.       Vesicles leaving the maturing end of the Golgi apparatus either join to other organelles or to the plasma membrane, releasing their contents by exocytosis

Functions of the Golgi Apparatus:

·       Chemical modification of proteins synthesis by ribosomes on the RER:

o   Oligosaccharides may be added to proteins to form glycoprotein (important as receptors on plasma membrane)

o   Fatty acids may be added to proteins to form lipoprotein

·       In plants the Golgi apparatus produces:

o   Hemicellulose- a carbohydrate component of the cell wall

o   Pectates- compounds found in the middle lamella region cementing plants cells together.

·       In root cap cells produces a mucous polysaccharide which provides lubrication as the root passes through the soil.

·       In animal cells hyaluronic acid is formed which cements animal cells together to form tissues.  

Lysosome- Small membrane bound vesicles. Found in the cytoplasm of animal cells. Their presence in plant cells is still uncertain. Arise from Golgi apparatus or ER. Contain digestive enzymes involved in the intracellular breakdown of biomolecules, foreign substances and worn-out cell structures. The optimum pH within the lysosome is 5.5.

Functions of Lysosome:

·       Breakdown of substances taken up by cell:

o   The foreign substance, being either a necessary biomolecule or and invading microbe is enclosed in a vesicle derived from the plasma membrane.

o   A primary lysosome i.e. one which has just pinched off the Golgi apparatus fuses with the vesicle forming a secondary lysosome.

o   Digestive enzymes in the lysosome digest the foreign substance and digested products diffuse through the lysosome membrane into the cell. Waste products are released by exocytosis.

·       Breakdown of worn-out organelles

o   A primary lysosome fuses with a worn-out organelle forming an autophagosome. Digestive enzymes digest the organelle.

o   Most of the products formed diffuse into the cytoplasm where they are used in the synthesis of new organelles.

·       Autolysis- When the cell dies, enzymes from the lysosomes are released digesting the remains of the cell. This causes rapid deterioration of the cell. Lysosomes also play an important role in programmed cell death

·       Leaky lysosomes may cause some forms of tissue damage and ageing.

Microbodies- Membrane-bound organelles containing enzymes involves in metabolism. Could be peroxisomes or glyoxysomes.

Peroxisomes- A microbody that breaks down fats, producing toxic hydrogen peroxide. The enzyme catalase within the peroxisome immediately breaks down H2O2 to harmless water and oxygen. H₂O₂ is a highly poisonous substance within the cell. It is mainly produced as a by-product in:

·       Animal liver cells involved in the breakdown of fat and oxidation of lactic acid.

·       Plant mesophyll cells carrying out photorespiration.

Glyoxysome- A microbody containing enzymes that convert fats stored in plant seeds to sugars.

Vacuoles- These are membrane-bound sacs that perform various functions. Mature plant cells generally have a central vacuole that is usually very large. It is surrounded by a tonoplast and contains stored food, salts (e.g. K⁺ and Cl^- ), pigments (e.g. red and blue pigments in flowers), and wastes. In some plants the central vacuole store toxic or unpalatable substances as a means of defence against herbivores. The central vacuole enables plant cells to elongate. Protozoans have food vacuoles that digest food and contractile vacuoles that regulate water content.

Mitochondria- Organelle found in all aerobic eukaryotic cells. It is a double membrane-bound organelle involved in the conversion of food energy to energy found in ATP (adenosine triphosphate). May assume different shapes example spiral, rod or spherical. It is surrounded by 2 membranes the outer membrane which is smooth and permeable and the inner membrane which is folded. The christae increase surface area allowing more space for complexes of enzymes called stalk bodies. These include ATP-synthase, the enzyme involved in breaking down energy rich molecules during cellular respiration. The space between the two membranes is called the intermembrane space. The inside of the mitochondrion is filled with A semi-rigid matrix. The enzymes that break down food to release energy are found here. Circular DNA and 70S ribosomes are present within the mitochondrion

Functions of Mitochondria:

·       The site of aerobic respiration i.e. the chemical reactions that convert food energy into ATP.

·       Release of energy during respiration, thus cells which are very active are characterized with a large number of mitochondria.

Plastids- Organelles that produce and store food in algae and plant cells. All surrounded by a double membrane. There are four types: chloroplasts, chromoplasts, leucoplasts and amyloplast.

Chromoplasts- plastids containing carotenoids i.e. red, orange and yellow pigments. Associated with fruits (red pepper, tomato) and flowers (colour attracts insects)

Leucoplasts- colourless plastids adapted for food storage. Abundant in storage organs e.g. Roots and seeds. Classified according to the type of food stored :

·       amyloplasts - store starch

·       lipidoplasts - store lipids (fats and oils)

·       proteoplasts - store proteins.

Amyloplast- starch storage organelle found in non-photosynthetic tissue of plants, such as roots and storage tubers

Chloroplasts- It is the site of photosynthesis. It have an oval or disc shaped with a green appearance. Generally found in mesophyll cells of leaves or the outer cortex of herbaceous stems. It is bound by 2 membranes surrounding a semi-fluid matrix called the stroma. The stroma is the site of synthesis of carbohydrate from water and carbon dioxide.  The most common plastids containing several pigments including:

·       Chlorophyll A and B- green pigments that trap light energy for photosynthesis

·       Carotenoid pigments- yellow and orange pigments.

Within the stroma:

·       Ribosomes and DNA are present

·       Enzymes that synthesize sugars are present

·       Grana are present i.e. stacks of disc shaped structures called thylakoids. The thylakoid membranes are the sites where pigments (especially chlorophyll), enzymes and molecules needed to trap sunlight energy and convert it to chemical energy in ATP are located

Origin of Mitochondria and Plastids- The endosymbiotic theory suggests that MITOCHONDRIA and PLASTIDS evolved from free-living prokaryotes. When prokaryotes only were present somewhere photosynthetic whilst others fed directly on other prokaryotes. At some time, a small prokaryote was ingested but not digested. It started living inside the other cell, dividing at a similar rate. Eventually as this continued from one generation to another, an endosymbiotic relationship was established.

Proof for endosymbiotic theory:

·       The presence of a double membrane which may have arisen when the membrane of the larger cell stretched and formed a vesicle around the plasma membrane of the smaller cell.

·       Supported by existence of circular DNA similar in size and composition to bacterial DNA in both plastids and mitochondria.

·       Ribosomes in plastids and mitochondria are similar to those found in prokaryotes i.e. 70S type.

·       Similarities exist between the inner membrane of these organelles and the plasma membrane of prokaryotes.

·       Protein structure of enzymes in mitochondria and bacteria is similar.

·       Biochemical reactions in bacteria and these organelles similar

·       A few modern cells do contain smaller cells as endosymbionts.

·       Are able to divide by binary fission (same mechanism as in bacteria)

Cytoskeleton- A network of dynamic fibres extending gives cells their shape and physical texture; support; ability to move. It consists of three components: microtubules, microfilaments and intermediate filaments.

Microtubules- They are hallow, long and straight tubes. Wall consists of 13 columns of tubulin proteins. Diameter is about 25nm. Its monomer subunit are alpha tubulin and beta tubulin. Dissociate if colchicine is applied. They also dissociate at low temperatures and at high pressures.

Functions of Microtubules:

·       Resist compression forces which push on cell.

·       Basic structures in cilia, flagella & centrioles.

·       Control chromosome movements during cell division as they are the constituents of the spindle.

·       Movement of organelles within cell.

·       In plant cells control the arrangement of the fibrous components of the cell wall.

·       In the axon of nerve cells they transport material from the cell body to the synapse region

Microfilaments- They are solid rods consisting of 2 intertwined strands of actin. Diameter is about 7nm and its monomer subunit is actin. Are often associated with other proteins example myosin. Are often attached to the plasma membrane. May be single, grouped in bundles or form networks.  

Functions of microfilaments:

·       Resist tension forces which try to pull cell apart.

·       Muscle cell contraction

·       Cytoplasmic streaming i.e. flowing movement of cytoplasm inside cells

·       Cell motility e.g. pseudopodia

·       Increase S.A. of microvilli for greater absorption.

·       Cause cleavage of animal cells after cell division i.e. separation of daughter cell from mother cell.

Intermediate filaments- They are fibrous proteins supercoiled into thicker cables. Diameter is around 8-12 nm. Its monomer subunit is keratin family of proteins. Found only in mature cells of multicellular organisms. In some cells they end at the nuclear envelope, keeping the nucleus at the centre of the cell. Often permanent structures.

Functions of intermediate filaments:

·       Resist compression forces.

·       Provide strength & support.

·       Maintenance of cell shape.

Centrioles- It is a cellular structure made up of microtubules. A pair of structures lying at right angles to each other observed only in animal cells. The centrioles are found in the centrosome i.e. A region in the cytoplasm near the nucleus. A centriole is made up of 9 triplets of microtubules with a matrix in the middle. Triplets are kept in place by fibrils. No membrane is present. Their function may be to organize the build-up of microtubules.

Basal bodies- It is a cellular structure made up of microtubule. Similar in structure of the centrioles. At the bade of cilia and flagella. Seem to act as microtubule organising centres in cilia and flagella.  

Undulipodia- It is a cellular structure made up of microtubule. Structures found on the cell surface which allow movement or move substances over cells

Cilia- Short hair like projections, typically between 2-10µm long and numerous. Cilia are responsible for:

·       Movement in ciliates such as Paramecium

·       Movement of mucus and dust upwards in the trachea, preventing them from reaching the lungs.

·       Movement of the ovum along the oviduct into the uterus.

How a cilium works- They generate a force in a direction parallel to the cell. The microtubules in the cilium form an axoneme. Those on the periphery are DOUBLETS, each doublet has 2 short arms (dynein arms), which help to break down ATP and release energy needed for movement. The 2 central tubules in the central sheath act as a central skeleton i.e. keeping the cilium in place. The peripheral ones are important for movement. The cilium is said to be in the 9+2 pattern. Movement of cilia is due to contraction of microtubules, sliding of microtubules or a combination of both.

Flagella- Large, whip-like structures and fewer in number. Responsible for movement due to undulating movements i.e. wave-like motion generating a force perpendicular to the cell.

The Fluid Mosaic Model

All membrane molecules may spin on their own axis and move sideways. So membranes are fluid [not solid]. Fluidity increases with a greater concentration of unsaturated fatty acids in phospholipids. Flip flop movements seldom take place. Fluidity is due to the absence of covalent interactions amongst the molecules in the membrane. Yet they associate together due to hydrophobic and hydrophilic interactions. The mosaic pattern of cell membranes is due to the protein molecules present, that are floating about in a sea of phospholipids.

General functions of the Plasma membrane:

·       separates cellular contents from external surroundings

·       Controls exchange of substances between cell and environment.

·       Acts as a receptor site recognizing external stimuli such as hormones and other chemicals, either from the external environment or from other parts of the organism.

·       Provides surface area for enzymes.

·       In muscle cells it maintains a potential difference needed for contraction.

·       In nerve cells it maintains a potential difference needed for transmission of nerve impulses.

·       The membrane surrounding organelles separates the reactions within the organelle from those occurring in the cytoplasm.

Freeze Fracture Technique- It used to prepare the sample of the membrane. This reveals the membrane proteins embedded in the phospholipid bilayer. When the two lipid layers are separated the proteins can be seen as bumps protruding from the interior of each layer. These bumps are not observed when an artificial bilayer of pure lipids is freeze fractured.