(a) (i) Structure of mitochondrion

-The mitochondrion is a small elongated organelle enclosed in a double membrane, a smooth outer and folded inner membrane.

-The folds give a large surface area for respiration reactions.

-Stalked elementary particles called oxysomes are present which provides sites for conversion of ADP and inorganic phosphate to ATP.

-The oxysomes and folds are known as the cristae. Oxidative phosphorylation and electron transport occurs in the cristae.

-     The space inside the membrane is called the matrix. It contains enzymes capable of catalysing the reactions of the Krebs cycle.

-     It equally contains DNA which enables the mitochondrion to replicate when the cell divides.

-     The mitochondrion functions mainly in energy production in the form of ATP.

-     The smooth outer membrane is permeable and allows exchange of substances.

inter cristal oxvsome

circular DNA ^matrix

Diagram of the mitochondrion

-    ii) Structure of the chloroplast

-The chloroplasts are bounded by a double unit membrane.

-The outer membrane is smooth while the inner membrane is extended inwards as a system of layers called lamellae. ’

-TJre lamellae are packed in some areas and appear as discs piled together known as the grana or thylakoid.

-They are connected by the integranal lamellae

-The grana contain chlorophyll and other pigments which traps solar energy used to excite the photosystem for the light dependent reactions of photosynthesis to occur, producing reduced NADPH and ATP.

-The space inside the membrane called stroma contains enzymes capable of catalysing the reactions of Calvin's cycle for production of energy rich molecules like glucose from low energy molecules like H2O and CO2.

-The arrangement of the thylakoids gives a large surface area for photosynthesis to take place.

Diagram of the chloroplst

(b) Similarities between chloroplast and mitochondrion    .

Similarities: Both of them;

-are enclosed in a double unit membrane

-contain circular DNA

-have enzymes in their internal aqueous matrices.

Differences

(a)    (i) Structure of the xylem vessel

-The xylem vessel contains four types of cells which are tracheids, vessel elements, parenchyma and fibres.

-Tracheids are elongated cells with lignified walls.

-They have tapering end walls that overlap. Mature cells are dead with empty lumens.

-Xylem vessels are very long tubular structures which are wider. They are formed by the fusion of several cells end to end. Their walls are lignified in several patterns.

-Xylem parenchyma has thin cellulose walls, large vacuoles and living contents. They contain intercellular air spaces.

-Xylem fibres are short narrow cells with thick walls. They have overlapping end walls with narrow lumens.

(ii)  Structure of bone

-Bone is a very rigid tissue whose matrix contains 70% mineral and 30% organic matter (collagen fibres).

-Bone cells called osteoblasts are present in the lacunae within the matrix.

-The matrix contains fine canals called canaliculli that connect the lacunae to each other

-Compact (dense) bone has a matrix arranged in Haversian systems. Each cylinder is made up of layers called lamellae.

-The exterior of bone is covered with a tough fibrous membrane called periosteum.

-Spongy (cancellous) bone occurs within dense bone. It is made up of thin bars of bone called trabeculae with spaces called bone marrow.

- The trabeculae are oriented in the direction in which the bone is stressed.

-They contain red bone marrow in the spaces at the head of long bones. The spaces of the shaft are filled with yellow bone marrow.

(b)    (i) Relationship between Tracheids and function of xylem

-Tracheids and vessels have lignified walls which enable the xylem to support the plant.

-The walls of vessels and tracheids overlap which gives additional strength and prevents the tubes from collapsing when they conduct water.

-Vessels have long wide empty lumens through which water is carried.

-The cells of tracheids are dead when mature. This prevents use of the water being transported and prevents any obstruction through their lumen.

(i)      Bone

-The matrix contains hard minerals like calcium and magnesium phosphate and collagen which enables it function in supporting the body.

-Heads of bones are covered with cartilage which prevents friction at joints during movement

 -The trabeculae are orientated in the direction in which bone is stressed which enables it withstand tension and compression.

-Red bone marrow found in the head of long bones help in the production of red blood cells.

(b) Differences between plant and animal cells

(c) How substances move in and out of cells

-Simple diffusion: Movement of substances across cells through pores following a concentration gradient e.g. CO? and O2 through channel proteins. -Facilitated diffusion: Movement of glucose and amino acids by the help of carrier proteins. The proteins pick up the substances and rapidly spin, dropping them inside the cells.

-Active transport: Carrier proteins use energy to transport substances against their concentration gradients e.g. uptake of glucose at the intestine, sodium-potassium pump during the transmission of impulses, and active loading of sugar to the phloem. Active bulk uptake of liquids (pinocytosis) or solids (phagocytosis) and bulk transport of substances out of cells using energy (exocytosis).

-Osmosis: Movement of water from an area of high water potential to those of low water potential through a selectively permeable membrane e.g. water uptake by root hairs, gain of turgidity by cells.

(c)    Why cardiac muscles are myogenic

-The impulses that cause their contractions originate from the heart muscle itself. They contract rhythmically from the time of formation till the death of the organism.

(a)    Definition of a unit membrane

-A unit membrane is the external boundary of cells and some organelles.

-All membranes have the same structure as revealed by electron micrograph.

-A unit membrane is trilamella with a phospholipid bilayer and protein molecules.

-The phospholipids are fluid with phosphate head that faces outwards and hydrocarbon tails facing inwards.

-Some proteins go through the membrane while others do not.

-The membrane is 7nm thick.

(b)     Functions of cell membranes

-Controls passage of materials in and out of cells by the use of channel and carrier proteins.

-Recognition of other cells by glycoproteins called antigens

-Reception site for hormones and neurotransmitters.

-Transmission of nerve impulses

-Insulation of nerves

-Forms barrier between cells and their surrounding membranes inside cells (intracellular membranes) act as reaction surfaces, act as intracellular transport system and provide separate compartments isolating different chemical reactions

(c)     The sequence of events between one cell division and the next

-     This sequence is called the cell cycle.

(a)    (i) Diffusion

It is the net movement of molecules or ions from a region of high concentration to that of low concentration.

It occurs in living and non-living systems where ever there is a concentration gradient and does not require energy e.g. movement of CO² and O² across respiratory surfaces such as gills of fish and alveoli in mammals.

(iii)  Active transport

-It is the movement of substances against a concentration gradient by the use of energy from ATP across cells.

-Cells that carry out active transports regularly have numerous mitochondria and high concentrations of ATP.

-Actively transported molecules can be carried by protons or protein molecules e.g. sodium/potassium pump which pumps Na⁺ out of the cell and K⁺ in, to generate impulses; active absorption of digested food in the gut and active absorption of mineral ions by root hairs.

(iv)     Pinocytosis

-Form of endocytosis by which a living cell engulfs minute droplets of liquid by the use of energy.

- Involves the infolding or invagination of the cell surface membrane forming vesicles e.g. uptake of nutrients from the follicles by human egg cells, pick up of thyroxin from the follicles of the gland.

(v)       Phagocytosis

-Process by which solid substance, are brought inside the cell by invagination of cell membranes using energy.

-The cell invaginates to form a depression containing the particles.

- The depression pinches off to form a vacuole, the inner surface derived from the outer surface of the cell membrane e.g. mode'of feeding of amoeba, action of phagocytes to destroy foreign particles.

(b)    (i) Features that affect the rate of diffusion

-Cell surface area, concentration gradient, type of molecule, length of diffusion path, temperature and Permeability of path

(ii)  Why active transport is affected by O² concentration while diffusion is not affected.

-Active transport uses energy derived from ATP.

-     ATP is produced during aerobic respiration.

-     A decrease in O² concentration will reduce the amount of ATP generated which will affect active transport.

-     Diffusion is a passive process which does not require energy in the form of ATP.

(a)    (i) Role of carbohydrates in the structure of plant and animal cells.

-The micro fibrils of plant cell wall contains cellulose molecules held by hydrogen bonds which makes it rigid and protect the internal structures from mechanical injury.

-Plant cells store reserved food as starch

-Glucose when in some cells alter the osmotic concentration which may lead to the influx of water enabling such cells attain turgidity.

-Carbohydrates combine with protein at cell membranes to form glycoproteins which are antigens.

-They help to distinguish between self and non­self.

(ii) Proteins

-Channel and carrier proteins at cell membranes help in the transport of substances across membranes.

-Some of the proteins go through the membrane and form pores where simple diffusion occurs while others float and transport substances by rapidly spinning.

-Some proteins in cell membranes are enzymes. They catalyse metabolic reactions at cell surfaces.

-The structure of some proteins at cell membranes gives a particular shape which acts as reception sites for some hormones.

-Plasma proteins like fibrinogen help to form a clot, thereby preventing excessive loss of blood when vessels are wounded.

-Some cells contain collagen and keratin which gives mechanical strength.

(ii) Lipids

-Cell membranes contain a phospholipid bilayer that controls the movement of polar molecules within cells.

-The layer is fluid with the phosphate heads facing outwards and hydrocarbon tails inward. This gives stability to the membrane.

-Some cells contain a waxy outer covering which prevents desiccation.

-Cholesterol present at cell membranes prevents close parking of phospholipids, thereby softening cell membranes, making them flexible.

-Cholesterol plugs also prevent the escape of polar molecules.

(b)    How antibiotics treat bacterial diseases

-Destroy pathogenic bacteria by digesting them (biostatic)

-Inhibit growth of the pathogenic bacteria

-Interfere in the metabolism of the pathogen and block protein synthesis, nucleic acid synthesis and disrupt the functioning of the cell membrane, capsing the pathogen to die.

(a)    Influence of Electron microscope on our idea of membrane structure.

-The electron microscope offers a higher resolution and magnification power.

-This has enabled the detailed structure of the

plasma membrane to be observed and studied.

-The various component molecules are distinctively seen to compose of proteins, phospholipid bilayer and molecules of cholesterol.

-It has also revealed that the phospholipids have phosphate heads facing outwards and hydrocarbon tails facing inwards.

-Some proteins go through the membrane while others do not.

(b)

 Ultrastructure of a plant cell

(i) -Drawn thickness = 6cm i.e. 6 x 10² = 0.06m

.'.Drawn thickness =(6 x IO^-2) m

 thickness » 7nm » (7 x 10^-9) m

Drawn thickness = actual thickness x magnification

Magnification =drwan thickness / actual thickness

Substitute:

Magnificaton = 0.06/7*10^-9 

Mag =  8.57 *10⁶

NB: Measure the thickness of the diagram drawn using your ruler if it is not given arid use the value obtained as the drawn thickness when necessary.

(ii)  Differences between magnification and resolution

-Magnification gives a larger image for clear viewing without distinguishing between components while the resolution shows distinct parts of the object viewed showing the various components as distinct entities.