(a) Name the parts labelled Y and Z.
Y : Carrier protein Z: Pore protein// Channel protein
(b)(i) State the component of structure X.
It is composed of two layers of phospholipids(ii) Explain the main function of X.
Acts as a barrier between the internal and external environment of the cell// Allows only specific molecules to pass through it
// provide the structural basis for all cell membrane(c) The plasma membrane is said to be semi-permeable. What is the meaning of ‘semi- permeable’?
A semi-permeable plasma membrane is a membrane that allows only certain substances to move freely across it.(d) The concentration of ions inside root cells is up to 100 times greater than in the soil. Anyway, the ions are still transported into the cells by active transport.
(i) Define active transport.
Active transport is a movement of substances / molecules / ions against the concentration gradient/from low to high concentration across the plasma membrane with the help of carrier protein and energy / ATP
(i) Explain what will happen to the uptake of the ions by root cells if the roots are immersed in a solution containing metabolic poisons such as cyanide.
P1 – there is no uptake of ions by root cells
P2 – metabolic poisons kill/ damaged the (root) cells
P3 – no energy/ ATP is produced
P4 – active transport does not occur(e) Using the mechanism of movement of a substance across the plasma membrane through osmosis, explain why food is preserved using concentrated sugar solution.
P1- The glucose with a high concentration acts as a hypertonic solution
P2 - causing water in food to move out through plasma membrane
P3 – This causes food to dehydrate
P4 – condition not favourable to microorganisms2. Diagram shows a plant cell. X, Y and Z are structures found in the cell.
(a) (i) Name the structures X and Y. X : Golgi Apparatus/body Y: Rough Endoplasmic Reticulum
(ii) State the function of Z. Z : site for cellular respiration // to generate / produce energy
(b) Explain the function of X and Y in the transportation of extracellular enzyme.
P1 : The nucleus / RNA instructs ribosomes to synthesized protein
P2 : The synthesized protein is transported in the Rough Endoplasmic reticulum / Y
P3 : to the transport vesicles
P4 : then the Golgi Apparatus / X packages /modifies /sorts / transports the synthesized proteins
P5 : to the secretory vesicles to be transported
P6 : out of the cell through the plasma membrane
All enzymes are protein. Enzymes are sensitive to temperature.
(c)(i) Explain why food is kept in the refrigerator?
P1 : Temperature in the refrigerator is very low
P3 : Rate of enzyme reaction decreases as temperature decreases
P3 : Enzymes are inactive at low temperature
(ii) A branded washing machine is provided with temperature regulator. A housewife uses the detergent containing enzyme at 40o C to wash the clothes. By using the information given, explain why?
P1 : The optimum temperature for enzyme reaction is about 40oC.
P2 : Low temperature makes the enzyme inactive
P3 : High temperature denatures the enzyme
(d) Figure below shows the structure of an enzyme and three substrates P, Q and R.
Based on the figure above, complete the schematic diagram below to show the mechanism of enzyme action on a suitable substrate.
3. A young spinach stem was cut into two longitudinal halves. Each was cut lengthwise again into two. This was repeated with another piece of spinach stem. The strips were of equal lengths and thickness. Two of these strips were placed into different sucrose solutions labelled A, B and C. The strips were observed 30 minutes later as shown below.
(a) Which of the above solution is :
(i) isotonic to the cell sap of the spinach? Solution B
(ii) hypotonic to the cell sap of the spinach? Solution A
(iii) hypertonic to the cell sap of the spinach? Solution C(b) Explain why the strip placed in solution A curve outwards.
Solution A is hypotonic to the cell sap; water moves into the vacuole/cut region by osmosis, the cuticle restricts intake of water from epidermal cells, parenchyma cells/cut region expands and become turgid, strip curves outwards.
(c) Explain why the strip placed in solution C curve inwards.
Solution C is hypertonic to the cell sap; water moves out of the vacuole/cut region by osmosis, the cuticle restricts loss of water from epidermal cells, parenchyma cells/cut region shrinks and become flaccid, strip curves inwards.
(d) Explain why the strip placed in solution B remained unchanged.
Solution B is of the same concentration as the cell sap of strips in B; movement of water in and out of cell is the same. Thus strip in B remain unchanged.
(e) If the epidermis is removed from the strip in solution A, would you expect any curvature? Give your reason.
No; all parenchyma cells become equally turgid as water moves in equally at all sides and the length increases equally on all sides
4. An experiment was done to study the effect of pH on the activity of amylase. Three test tubes labeled P, Q and R were filled with the same amount of amylase.
The solution in P was neutral.
The solution in Q was acidic.
The solution in R was alkaline.
The same amount of starch was put into each test tube and the contents were stirred. Every two minutes a drop of the mixture from each test tube was dropped into a drop of iodine solution on a white tile. Table 1 shows the results of the experiment.
(a) P = 8 MIN Q = 10 MIN R = 14 MIN
(b)(i) State two different observations made from the Table 1.
Observation 1: Time taken for complete hydrolysis of starch in test tube P is 8 minutes
Observation 2: Time taken for complete hydrolysis of starch in test tube R is 14 minutes
(ii) State the inferences from the observation in 1(b)(i).
Inference from observation 1: Time taken for complete hydrolysis of starch in test tube P is fastest because amylase works best in a neutral medium
Inference from observation 2: Time taken for complete hydrolysis of starch in test tube R is slowest because amylase is inactive in an alkaline medium
(c) Complete Table 2 based on this experiment.| Variable | Method to handle the variable |
| Manipulated variable pH | Use different medium of solution, acidic, alkaline and neutral |
| Responding variable Time taken for complete hydrolysis of starch | Measure and record the time taken for complete hydrolysis of starch using a stopwatch Measure and record the time taken for the dark blue colour to disappear using a stopwatch |
| Constant variable Volume of amylase Volume of starch Temperature |
Fix the volume of amylase at 1ml throughout the experiment Fix the volume of starch 5ml throughout the experiment Fix the temperature to be the same throughout the experiment |
(d) State the hypothesis for this experiment.
The enzyme amylase hydrolyses starch at optimal pH which is neutral.(h) State the operational definition for enzyme.
Enzyme/amylase hydrolysed/digest/act on starch and the time taken for the dark blue colour to disappear is affected by the pH of medium.
5. The figure shows the organelles involved in the production of extracellular enzymes.
(i) Extracellular enzyme is produced in a cell, then packed and secreted from the cell. It catalyses its reaction outside the cell. An example is amylase/pepsin/trypsin.
(ii) The instruction for making the extracellular enzyme is transcribed from the deoxyribonucleic acid (DNA) to ribonucleic acid (RNA) in the nucleus.
The RNA then leaves the nucleus through the nuclear pore and attaches itself to the ribosome located on the endoplasmic reticulum.
When the synthesis of the enzymes is completed, it is encapsulated in a transport vesicle which fuses with the golgi body.
In the golgi body, the enzyme is further modified before being packed in a secretory vesicle.
The secretory vesicle transports the enzyme to the plasma membrane, where it fuses with it and the enzyme is released outside the cell.
(b) The explanation of enzyme action is known as the ‘lock and key hypothesis’.
The substrate molecule fits into the active site of the enzyme molecule.
The substrate is the ‘key’ that fits into the enzyme ‘lock’. The reaction is specific.
Various types of bonds such as hydrogen and ionic bonds hold the substrate in the active site forming the enzyme-substrate complex.
Once the complex is formed, the enzyme changes the substrate to its product.
The product leaves the active site. The enzyme is not altered by the reaction and it can be reused.
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