Biology Form 1 Notes : Cell Physiology

Cell Physiology

Meaning of Cell Physiology

• Physiology refers to the functions that occur in living organisms.
• Cell physiology refers to the processes through which substances move across the cell membrane.
• Several physiological processes take place inside the cell, such as respiration.
• Oxygen and glucose enter the cell, while carbon (IV) oxide and water leave the cell through the cell membrane.

Structure and Properties of Cell Membrane

• The cell membrane is the protective barrier that shelters cellular contents.
• Movement of all substances into and out of the cells occurs across the cell membrane.
• It is made up of protein and lipid molecules.
• Lipid molecules have a phosphate group attached to one end.
• These are called phospholipids.
• The phospholipids are arranged to form a double layer.
• The ends with phosphate groups face outwards.
• Proteins are scattered throughout the lipid double layer.
• Some proteins act as carrier molecules that channel materials in and out of the cells.
• The cell membrane allows certain molecules to pass freely, others with difficulty, and some not at all.
• This selective permeability means the cell membrane is semi-permeable.

Properties of Cell Membrane

Permeability

• The cell membrane is semi-permeable.
• It allows small lipid-soluble molecules to pass through more easily than water-soluble molecules.
• This is due to the phospholipid double layer.

Polarity

• The cell membrane has electrical charges across its surface: positively charged ions outside and negatively charged ions inside. This contributes to electrical impulses in nerve cells.
• It is sensitive to changes in temperature and pH.
• Very high temperatures denature proteins, destroying semi-permeability. Extreme pH values have a similar effect.

Physiological Processes

• Some physiological processes include diffusion, osmosis, and active transport.

Diffusion

• Diffusion is the movement of molecules or ions from a region of high concentration to a region of low concentration, driven by a concentration gradient.
• Diffusion continues as long as there is a concentration difference between two regions.
• It stops when equilibrium is reached, i.e., when concentrations are equal.
• Diffusion occurs inside living organisms and in the external environment.
• It does not require energy.

Factors Affecting Diffusion

• Concentration Gradient: A steeper gradient increases the rate of diffusion.
• Temperature: Higher temperatures increase molecular kinetic energy, speeding diffusion; lower temperatures slow it.
• Size of Molecules or Ions: Smaller molecules or ions diffuse faster.
• Density: Denser molecules or ions diffuse more slowly.
• Medium: Diffusion is faster through gases and liquids than solids.
• Distance: Thinner surfaces allow faster diffusion.
• Surface Area to Volume Ratio: Larger ratios increase diffusion rate, as seen in small organisms like Amoeba.

Role of Diffusion in Living Organisms

Processes dependent on diffusion include:

• Gaseous exchange through respiratory surfaces.
• Absorption of materials into cells, e.g., glucose from blood and tissue fluid.
• Excretion of metabolic wastes like carbon (IV) oxide and ammonia.
• Absorption of digestion end-products from the intestines.

Osmosis

• Osmosis is the movement of water molecules from a region of high water concentration to a region of low water concentration through a semi-permeable membrane.
• It involves movement of water molecules only, not solute molecules.
• Osmosis occurs across cell membranes and non-living membranes like cellophane or visking tubing.
• It is a purely physical process.

Factors Affecting Osmosis

• Size of Solute Molecules: Osmosis occurs when solute molecules are too large to pass through the membrane.
• Concentration Gradient: Osmosis occurs between solutions of unequal solute concentration separated by a semi-permeable membrane.
• Temperature: Higher temperatures increase water molecule movement, enhancing osmosis; very high temperatures denature membrane proteins, stopping osmosis.
• Pressure: Increased pressure inside a plant cell decreases osmosis.

Roles of Osmosis in Living Organisms

• Movement of water into cells from surrounding tissue fluid and between cells.
• Absorption of water from soil into plant roots.
• Support in plants through turgor pressure resulting from water intake by osmosis.
• Absorption of water from the alimentary canal in mammals.
• Re-absorption of water in kidney tubules.
• Opening and closing of stomata.

Water Relations in Plant and Animal Cells

• The medium surrounding cells is described as hypotonic, hypertonic, or isotonic.

• Hypertonic: Solution with higher solute concentration than cell sap; cells lose water by osmosis.
• Hypotonic: Solution with lower solute concentration than cell sap; cells gain water by osmosis.
• Isotonic: Solution with the same solute concentration as cell sap; no net water movement.

Osmotic Pressure

Osmotic pressure is the tendency of a solution with high solute concentration to draw water into itself when separated from distilled water or dilute solution by a semi-permeable membrane.

• Osmotic pressure is measured by an osmometer.
• When plant cells are placed in distilled water or hypotonic solution, their osmotic pressure is higher than that of the medium.
• This causes water to enter cells by osmosis.
• Water collects in the vacuole, increasing its size.
• The cytoplasm is pushed outward, pressing the cell membrane against the cell wall.
• This builds up hydrostatic pressure inside the cell.
• The cell wall prevents further water entry when stretched to maximum.
• The cell is then said to be fully turgid.
• The hydrostatic pressure developed is called turgor pressure.

Plasmolysis

• When a plant cell is placed in a hypertonic medium, it loses water by osmosis.
• The cell’s osmotic pressure is lower than that of the medium.
• The vacuole shrinks and the cytoplasm contracts, causing the cell membrane to lose contact with the cell wall.
• The cell becomes flaccid; this process is called plasmolysis.
• Incipient plasmolysis is when the cell membrane just begins to lose contact with the cell wall.
• Plasmolysis can be reversed by placing the cell in distilled water or hypotonic solution.
• Full plasmolysis may not be reversible if prolonged.

Wilting

• Wilting is the drooping of leaves and stems of herbaceous plants after significant water loss through transpiration.
• It occurs in hot, dry conditions when water loss exceeds absorption by roots.
• Cells lose turgor and become plasmolysed, causing leaves and stems to droop.
• Wilting is corrected at night when root water absorption continues and transpiration stops.
• Prolonged wilting may cause plant death if soil water is not replenished.

Water Relations in Plants and Animals

Haemolysis

• Haemolysis is the bursting of red blood cell membranes, releasing haemoglobin.
• It occurs when red blood cells are placed in distilled water or hypotonic solution.
• The cell membrane cannot resist further water entry after maximum intake.

Crenation

• Crenation occurs when red blood cells are placed in hypertonic solution.
• They lose water by osmosis, shrink, and their shape distorts.
• Animal cells regulate salt-water balance (osmoregulation) to prevent such damage.
• An Amoeba in distilled water removes excess water using a contractile vacuole.
• The rate of contractile vacuole formation increases in hypotonic conditions.

Active Transport

• Active transport is the movement of solutes such as glucose, amino acids, and mineral ions from low to high concentration.
• It moves substances against a concentration gradient and requires energy.
• Active transport occurs only in living organisms.
• Energy comes from respiration.
• Specific proteins in the cell membrane, called carrier proteins or channel proteins, facilitate this movement.
• Each carrier protein has a shape specific to the substance it transports.
• The substance fits into a slot on the protein molecule.
• As the protein changes shape, the substance is moved across the membrane, expending energy.

Factors Affecting Active Transport

Availability of Oxygen

• Energy for active transport is provided by respiration.
• More oxygen increases respiration rate and active transport.
• Without oxygen, active transport stops.

Temperature

• Optimum temperature is required for respiration and active transport.
• Very high temperatures denature respiratory enzymes.
• Very low temperatures inactivate enzymes, stopping active transport.

Availability of Carbohydrates

• Carbohydrates are the main substrates for respiration.
• More carbohydrates increase energy production and active transport.
• Lack of carbohydrates stops active transport.

Metabolic Poisons

• Metabolic poisons, e.g., cyanide, inhibit respiration and stop active transport due to lack of energy.

Role of Active Transport in Living Organisms

Processes requiring active transport include:

• Absorption of mineral salts from soil into plant roots.
• Absorption of digestion end-products like glucose and amino acids into the bloodstream.
• Excretion of metabolic products like urea from cells.
• Re-absorption of useful substances and mineral salts into blood capillaries from kidney tubules.
• Sodium-pump mechanism in nerve cells.
• Re-absorption of useful materials from tissue fluid into the bloodstream.

Practical Activities

1. Experiment to Demonstrate Diffusion

• Use various coloured substances such as dyes, plant extracts, and chemicals like potassium permanganate.
• Introduce potassium permanganate (VII) crystals to the bottom of a beaker filled with water using glass tubing or a drinking straw, then remove the tubing.
• Observe the disappearance of crystals and the uniform colouring of water.

2. Experiment to Demonstrate Osmosis Using Visking Tubing

• Cut a strip of visking tubing 8–10 cm long and tie one end with strong thread.
• Place about 2 ml of 25% sucrose solution inside and tie the other end.
• Wash the tubing under running water and blot dry.
• Immerse it in a beaker containing distilled water and leave for at least one hour or overnight.
• Observe that the visking tubing increases in size and becomes firm.
• Set up a control experiment using distilled water inside the tubing instead of sucrose solution.

3. Experiment to Show Osmosis Using Living Tissue

• Peel Irish potato tubers and scoop out a hollow space at the center.
• Place sucrose solution inside the hollow and place the tuber in a beaker or petri dish with distilled water. Use a boiled potato as control.
• Set another control with distilled water inside the hollow instead of sugar solution.
• Leave the experiment for 3 to 24 hours.

4. Experiment to Demonstrate Turgor and Plasmolysis in Onion Epidermal Cells

• Obtain two strips of onion epidermis.
• Place one on a slide with distilled water and the other on a slide with 25% sucrose solution; place coverslips on both.
• Observe under a low-power microscope and leave for 30 minutes.
• After 30 minutes, observe again.

Cells in distilled water enlarge greatly, while cells in 25% sucrose shrink.