(a)    Definition of a cell

A cell is the basic unit of structure and function in living organisms. It is bound by an outer membrane and contains definite organelles. A cell comes from pre-existing cells by division and can carry out all the activities of life including metabolism.

(b)    (i) Relationship between structure and functions of parenchyma

-Parenchyma has a thin cellulose wall which provides a pathway for water and mineral salts.

-The cells are unspecialized and serve as parking tissues. When turgid, they become tightly packed and provide support for the organs in which they are found,

-Air spaces exist between their cells which allow gaseous exchange to occur with the external environment.

-Their cells are metabolically active and provide sites for metabolic activities.

-They have large vacuoles and serve in food storage e.g. potato tubers where they store starch -Plasmodesmata between adjacent cells allow lateral transportation of substances between parenchyma cells.

(ii)  Tracheids

-Tracheids have dead cells at maturity.                    -.

-This prevents obstruction or usage of water being transported.

- It has pits for horizontal transportation of water

- The walls of their cells are lignified which enables them give support to other plant organs.

- Cells have tapering end walls which overlap, giving additional mechanical strength.

- Make a good labelled diagram of the tracheid.

(c)    Why specialization sometimes leads to loss of the ability of plant cells to divide

-Specialisation may lead to loss of some cell organelles like the nucleus. This may restrict metabolic activities and prevent synthesis of new materials and make them loss the ability to divide. Also, specialisation may cause the introduction of new materials like lignin, or suberine which blocks transportation of substances causing cells to loss their ability to divide.

(a)    (i) Reason why Xylem is a compound tissue

-Xylem is made up of four types of cells which are adapted to specific roles. A compound tissue is one with different types of cell layers. The various cells are: 

-Xylem vessels: They are long tubular structures formed by the fusion of several cells end to end in a row. This gives additional strength. The walls are lignified differentially into spiral, pitted, reticulate and scalariform. Lignification gives strength which supports plants Vessels have wide lumen to conduct water and mineral salts.

-Tracheids are single elongated, lignified cells. Their mature cells are dead with empty lumens.

-Xylem parenchyma: These have thin cellulose cell walls and their mature cells are living. They function for food storage, gaseous exchange through their intercellular air spaces and radial transport of food and water.

-Xylem fibres have short narrow cells with thick walls for mechanical strength.

(ii) Phloem

The phloem functions in the translocation of food, and is made up of five different types of cells.

-Sieve tubes: They are formed by the end to end fusion of cells called sieve elements. Their walls are made of cellulose and pectic substances and their mature cells lack nuclei.

-Companion cells: They occur near sieve elements and supply them with their needs. The sieve plates are pores to allow flow of substances from one sieve element to the other.

-Phloem parenchyma: They are cells with an elongated nucleus, are living and function in storage.

-Phloem fibres provide strength to resist pressure and occur only in dicots.

-Sclereids: They give additional mechanical strength and occur in older phloem.

(b)    Ways of harvesting Bark of Pygium for greater conservation of the tree

-The villagers should avoid removing a complete ring not to block translocation of food from the leaves to the roots and otherv parts.

-They should allow enough time' for the replacement of cut regions.

-They should remove part of the bark from the main trunk and not from the branches.

-Harvesting should be done when the tree is large and mature, not from small ones and planting of trees should be encouraged.

-Time the harvesting season well, not to kill the plant

-Minimise cutting down of the whole tree

-Develop techniques to tap the content of the bark.

-Cholesterol is present in the membrane which improves membrane flexibility, fluidity and stability.

Diagram of the fluid mosaic model of the plasma
membrane

(c)    Adaptation of cell membrane for movement of substances.

-The membrane contains proteins

-Some are channel proteins that allow an opening for simple diffusion of gases like O2 and CO2.

-Some are carrier proteins that help in the facilitated diffusion of glucose and amino acids, and in active transport.

-The membrane equally contains phospholipids, which facilitates the movement of fatty acids.

-Cholesterol plugs present restrict entry of' substances.

(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.