1. Cell membrane

It is double membrane and encloses the entire cell; the cell membrane is compose mainly of the protein (55%), fats (40%) and carbohydrate (5%). The main cell unit of the cell membrane includes:

• Phospho-liquid: it is 75% liquid affecting the permeability and fluidity of the cell membrane
• Cholesterol: it is made up of mainly lipids and makes the membrane less fluid at high temperature and makes it fluid at low temperature. It reduces the entering and escape of polar molecule
• Glyco-lipids: there are carbohydrates combine with lipids which act as recognition sites for different substances
• Proteins: they provide structural supports to the cell membrane and aloud for active transport of materials across the membrane. They aloud cell to be recognized by different hormones and enzymes. They also saves as energy transducers and electron carriers
• Glyco-proteins: they are combination of carbohydrates and proteins.

2. Membrane Structure Fluid Mosaic Hypothesis

This model of the membrane structure was put forward by SINGER and NUCHDSON in 1972 in which protein molecules move or float about in the liquid phospholipids belayed. The phospholipids move rapidly by radiation within the belayed.

• The scattered protein molecule resemble a mosial (beads), since the phospholiquid is fluid or liquid, these add with the protein to form a fluid mosaic pattern
• The proteins do not form a continuous layer covering both sites of the membrane. They are insected at various points in the liquid belayer, some of which are attached to the surface membrane (extrinsic proteins). Other are attached within inner membrane as they penetrate (intrinsic proteins)
• The proteins have regions of hydrophobic which interact with the fatty acid tail to exclude water. The rest of the protein is hydrophilic
• Carbohydrates points or portions are attached to the outer surface of lipid molecules that is glycolipids and other on proteins that is glycoprotein
• Some of the fatty acid tails of the phospholipids are saturated and other unsaturated
• The more unsaturated the tail, the more fluid the membrane

3. Adaptation of the Cell Membrane to its Function

• Receptor proteins have very specific shapes, this help to make them move molecular across membrane
• The phospholipids belayer is made up of fats which give the cell its fluid method, this restricts the movement of polar molecule in and out of cells
• They are carrier protein molecule which help to move ions across cell membrane
• They are also channel proteins which involves the movement of molecules through the membrane
• Glycol-proteins with branched carbohydrates also help to move molecules across the membrane

1. Endoplasmic reticulum

It is a series of membrane that create many channels within the cytoplasm, this membrane channel are continuous with the nuclear membrane and also form sheets that enclose the cellular spaces called cisternae. There are two main types of endoplasmic reticulum

2. The rough endoplasmic reticulum (RER)

It contains many tiny granules called ribosome which can easily be seen with an electron microscope. The ribosome plays an important role in protein synthesis, the RER is more abundant in actively growing cells and those that secrets proteins

3. The smooth endoplasmic reticulum (SER)

It lacks ribosomes and is more abundant in cells that secrets mostly lipids e.g. the sebaceous glands in the skin.

Functions of the endoplasmic reticulum

-It acts as a path way for the transport and exchange of materials though out the cell.

-It synthesizes proteins and enzymes.                                            

-It also manufactures lipids and steroids.

-It also collects and stores synthesized materials.

-It forms a structural frame work which gives the shape of the cell

1. Mitochondrion

Function:

• It is a double membrane organelle which is found in cell in living thing
• The outer membrane enclosed the organelle and allow enzyme and metabolized to enter into it.
• The inner membrane is highly folded forming numerous cristae which increase the surface area for the breakdown of glucose.
• The cristae are surface over which the enzyme is concentrated for the breakdown of glucose to release energy.
• The ribosomes within the matrix are responsible for synthesis.

Function of the chloroplast

• It is a double membrane organelle found only in plant cell. It is concerned with chlorophyll formation and side for other photosynthetic pigments such as carotenoids.
• It forms the main side for photosynthesis made up of granar and stromat.
• It contains lamella which links up the granar within the organelles.
• It is also composed of small unit of DNA, lipids, starch grains and enzymes for CO₂ fireation.
• The lamella holds the chlorophyll to their right position to tap sunlight for photosynthesis.

Adaptation of chloroplast to their DNA

• The double membrane unit is permeable to h₂o and substances that take part in photosynthesis
• It contains numerous lamella with hold chlorophyll molecule to trap sunlight for photosynthesis
• The granar contains numerous granites called quantosomes for efficient of photosynthesis
• The stroma contains numerous enzymes for co2 fixation
• It also contains a loop of DNA replication

Adaptation of Mitochondrion

• It is a double membrane organelle which is found in cells of living thing
• The inner membrane is highly folled with cristae to increase the surface area for the attachment of muscle
• It’s membrane is permeable to permits the entering and exist of substances such as co₂
• The inner membrane is selectively permeable to solid
• The matrix contains soluble enzymes that are involve in the respiratory chain
• The matrix contains phosphate molecules which act as a reserve for the production of ATP for ADP.

Difference between chloroplast and mitochondria

2. The Nucleus

The nucleus is found in all eukaryotic cells except in mature red blood corpuscles of mammals and phloem sieve tube of plants. The is usually one pair of cells except in c: lrates e.g. paramecium conductor which has a micro and a mega nucleus

• It is the largest of all cell organelles and the first to be observed under a light microscope
• It is spherical or avoid in shape and seen to be bound by single membrane under the light microscope and a double membrane with pores under the electron microscopes
• The nuclear pore allows the exchange of materials between the nucleus and the cytoplasm
• The outer membrane is continuous with the endoplasmic reticulum which may be covered by ribosome
• Within the nucleus is jelly-like matrix known as the nucleoplasm (nuclear sap) which contains chromatin that is the euchromatin and the hetero chromatin and the nucleolus.

Functions of the nucleus

• It contains genetic information in the form of DNA and therefore responsible for hereditary
• Nuclear division is the bases of cell reproduction and hence replication
• Nucleolus manufactures ribosomes and RNA
• It is the control center of cell
• It carries instructions for the synthesis of protein in nuclear DNA

Golgi Bodies

• They are found in all eukaryotic cells, very prominent and well developed in secretary cells. It is seen with the light microscope but the structure can only be by electric microscope, it is similar to the endoplasm reticulum instructure.
• It is made up of stack (pile) flatten membrane bound sacs called cisternae together with the system associate vesicle called Golgi vesicles (small sacs) which are probably been derived from buds (vesicles) of the rough endoplasmic reticulum. In plant cells, they exist separate stacks called dictyosomes and the other end of the stock concave end. The cisternae break into Golgi vesicle if they are small or break up vacuoles if larger.

Functions of the Golgi apparatus

• They secret digestive enzymes of the pancreas e.g. the acinar cells of the pancreas which secrets trypsinogen etc… by reserve pinocytosis (exocytosis) such enzymes are secreted in their inactive form so that they don’t destroy cells that produce them. Inactive enzymes are generally called pro-enzymes or zymogens
• They function in the transport and storage of lipids
• They process and secrets carbohydrates use for making new plants cell walls, membranes and cuticles of insects
• They are involve in the manufacture of glycoprotein
• They process of combining carbohydrates with protein to form glycoprotein is called glycosylation e.g.mucin which form mucous in solution is secreted by Golgi bodies in the goblex cells of respiratory tract, at root tips to case penetration in the root cells.

3. LYSOSOMES

They are found in most eukaryotic cells particularly in animal cell, they are spherical bounded by a single membrane and are about 0.1 to 0.5 um in diameter

• It contains about digestive and hydraulic enzymes like proteases, lipases, nucleases etc which breaks down large molecules such as proteins fats, carbohydrate
• These enzymes are synthesized in the rough endoplasmic reticulum and transportated to the Golgi bodies. Golgi vesicle containing the process enzymes later but off to form lysosomes. It contents are homogenous and are acidic. His enzymes are separated from the rest of the cell so as not to destroy the cells, different form of lysosomes exist:

• Primary lysosomes: combine a variety of these digestive enzymes
• Heterophagosomes: enclosed material of extra cellular origin for digestion
• Autophagosomes: which enclosed cell component to be degraded to their simple unit

• When a primary lysosomes meet with the heterophagosome or autophagosome,a second lysosome is formed I which materials to be digested are in contact  with enzymes
• Primary lysosomes are referred to as a suicide bags because they self distractive (autolysis)
• Autophagy is the process whereby unwanted worn out structure within the cell are engulfed and digested within lysosomes. These worn out structures are enclosed by a single membrane from smooth endoplasmic reticulum and the whole structure then fuses with a primary lysosome to form a secondary lysosome or autophagy vacuole in which the worn out organelles is digested.

FUNCTIONS OF LYSOSOMES

• They digested materials taking in by phagocytosis or pinocytosis (endocytosis). After digestion, the products of digestion are absorbed in the cytoplasm while the residual bodies are digested by execytosis
• They destroy unwanted a worn out organelles within the cell
• They release enzymes outside the cell to digest external material e.g. sperms contains a special kind of lysosomes called acrosome  which release enzymes outside the cell to digest the layer of cells surrounding the egg to effect fertilization
• They are involved in autolysis that is self destruction of cells e.g. it occurs in muscles that are not exercised for these reason there are often called suicide bags.

4. RIBOSOMES

They are very small organelles made up of a small end large sub-units with a diameter of approximately 20mm. it is possible to unite this sub-units. The combination depends on the concerntration of magnesium ions in the cytoplasm, low magnesium ions help to separate them, there are made of rough equal amount of proteins and RNA. There have two types of RNA; one is found in the smaller and the other in the larger sub-units.

• They are two types of ribosomes: 70s and 80s (s=suedberg unit) this is related to the rate of sedimentation in a centrifuge. The greater the x number the higher the rate of sedimentation
• They are either bound on endoplasmic reticulum or may lie freely in the cytoplasm. Some may be found in the mitochondria or chloroplast of plants in this case they resemble prokaryotic ribosomes (smaller).
• Ribosomes lying freely in the cytoplasm are the site of synthesis of proteins to be retained within the cell whereas these bound to endoplasmic reticulum produce proteins that are subsequently outside the cell

NB: when several ribosomes occur along a common strand or RNA, it is called polyribosome or polysome

Fig:

Functions of Ribosomes

• They hold the various interacting molecules involve in protein synthesis in place e.g. messenger RNA, transfer RNA, amino acid etc… ensuring that
• Ensuring that amino acid are joined together to form polypeptide chains e.g. a protein made by a free ribosome is hemoglobin in young red blood cells.

PEROXISOMES (MICROBODIES)

These organelles are spherical bodies bounded by a single membrane. They are about 03-1.5um in diameter. They contain the enzyme catalase which catalyses the decomposition of hydrogen peroxide (h₂o₂) to water and oxygen hence the name peroxisome. Catalase is the fastest active enzyme known and it is actively can be demonstrated by dropping fresh ground liver in h2o2 where rapid release of o2 is observed. Liver cells contain a large number of these peroxisomes. Peroxisomes are concerned which metabolic activities involving oxidation plant peroxisomes are divided into three types:

• Glyoxysomes; which convert lipids to sucrose (carbohydrates) in some seeds that are lipid rich e.g. endoplasm of casto-oil seeds
• Leaf peroxisomes: are important in the process of photorespiration which chloroplast and mitochondrion
• Non-specialized peroxisomes are found in other tissues.

                      ₂h₂o₂    ----------------                 ₂h₂o + o₂

5. CENTRIOLES 

They are found in animal, fungal and algal cells but are not found in flowering plants. They are hollow cylinders of about 0.2um in diameter. Under the light microscope, it appears as two centrioles found at right angles to each other. Each centriole contains nine triplets of microtubules but no central microtubules are seen. At the beginning of nuclear division, they replicate themselves to become two pairs and each pair migrates to the opposite pole.

Fig:

Functions of centriole

• They produce a system of micro-tubes or spindle fibres during nuclear division which controls separation of chromatids and/or chromosomes
• They give rise to cilia and flagella.

6. CHLOROPLAST

• They are large, egg-shaped, double unit membrane bound structure found in the cytoplasm of cells in green plants only
• They contain chlorophyll and caroteroid pigments, they carry out photosynthesis and have the following parts: an external double unit membrane, a liquid stroma surrounded by the external membrane, grana (stacks of lamella o), interground lamella which links neighboring grana, oil or lipid droplets, starch grains, circular DNA, enzymes for reduction of carbon dioxide to sugar
• Lamella holds the chlorophyll molecules in the most suitable position for trapping a maximum amount of sunlight for photosynthesis. The chlorophyll molecules are laid on shelves, piled on top of each other offering a large surface for trapping of sunlight energy.

7. VACUOLE

• It is a fluid filled sac bounded by a single membrane a mature plant cell has a large central vacuole surrounded by a unit membrane called the tonoplast or vacuolar membrane
• It contains a fluid called cell sap which is a concerntrated solution of mineral salts, sugars, organic acids, oxygen co2, pigments and some waste products of metabolism
• Animal’s cells have relatively small temporary vacuoles e.g. food vacuole, contractive vacuole, autophagic vacuoles etc…
• The matrix also contains phosphate molecules which act as a reserve for the production of ATP when coupled with ADP