The cell

 

 The cell

Cell structure	Properties	Functions
Cell membrane / Plasma membrane	Thin membrane consisting of a lipid layer sandwiched between two protein layers   This arrangement is known as the unit membrane and the shows two lipid layers with proteins within    Substances are transported across the membrane by active transport and diffusion	It controls the movement of substances into and out of the cell
Cytoplasm	Jelly-like substance in which are small structures scattered are called organelles	Chemical processes are carried out.
Vacuole	Cell sap where sugars and salts are stored
Cell wall	·         Made of cellulose ·         Between the cells is a middle lamella made of calcium pectate	Maintain the shape and rigidity of the cell
Chloroplast	Found in only plants   Consists of an outer unit membrane enclosing a series of interconnected membranes called lamellae   At various points along their length the lamellae form stacks of disc like structures called grana   The lamellae are embedded in a granular material called the stroma	Site for photosynthesis
Endoplasmic reticulum	This is a network of tubular structures extending throughout the cytoplasm of the cell   It serves as a network of pathways through which materials are transported from one part of the cell to the other	Smooth Endoplasmic Reticulum: transports lipids.   Rough Endoplasmic Reticulum: transports ribosomes.
Ribosomes	Consist of protein and ribonucleic acid (RNA)	Site for protein synthesis
Golgi bodies	Thin, plate-like sacs arranged in stacks and distributed randomly in the cytoplasm	Packaging and transportation of glycol-proteins   Produce lysosomes
Mitochondrion	Rod-shaped organelle   The foldings of the inner membrane are called cristae to increase the surface area for respiration   The inner compartments called the matrix	Site for respiration
Nucleus	Contains proteins and nucleic acid deoxyribonucleic acid (DNA) and RNA   The chromosomes are found in the nucleus	Carriers of the genetic information of the cell
Lysosomes	Vesicles containing hydrolytic enzymes	Involved in the breakdown of micro-organisms, foreign macromolecules and damaged or worn-out cells and organelles

 

Cell Physiology

• Cell physiology refers to the process through which substances move across the cell membrane e.g.  Oxygen, water, glucose

The cell membrane

·         Protective barrier that shelter cellular contents

·         Controls movement of all substances into and out of the cells takes place across the cell membrane

 

• It is made up of a lipoprotein where lipid molecules have phosphate group attached to it on one end (phospholipids) which are arranged to form a double layer. The ends with phosphate group face outwards while the proteins are scattered throughout the lipid double layer.

• Some of these proteins act as carrier molecules that channel some material in and outside the cells

• The cell membrane allows certain molecules to pass through freely while others move through with difficulty and still others do not pass through at all

• This is selective permeability and the cell membrane is described as semi-permeable. It allows small molecules that are soluble in lipid to pass through with more ease than water soluble molecules

• The cell membrane has electrical charges across its surface : it has positive charged ions on the outside and negatively charged ions on the inside. This property contributes to electrical impulses sent along nerve cells

• Sensitivity to changes in temperature and pH : Very high temperatures destroy the semi-permeability nature of the cell membrane because the proteins are denatured by extreme pH values have the same effect on the membrane permeability

 

Physiological processes

Diffusion

• Diffusion is the movement of molecules or ions from a region of high concentration to a region of low concentration aided by a concentration gradient. It requires a difference in concentration between two regions (concentration gradient) for it to occur and stops when the gradient ceases.

• Occurs inside living organisms as well as the external environment

• Does not require energy

 

Factors Affecting Diffusion

·         Concentration Gradient: increase in the concentration of molecules at one region results in a steeper concentration gradient which in turn increases the rate of diffusion

·         Temperature: high temperature increases kinetic energy of molecules. They move faster hence resulting in an increase in rate of diffusion, and vice versa

·         Size of Molecules or Ions: The smaller the size of molecules or ions, the faster their movement hence higher rate of diffusion

·         Density: The denser the molecules or ions diffusing, the slower the rate of diffusion, and vice versa

·         Medium: The medium through which diffusion occurs also affects diffusion of molecules or ions

·         Distance: This refers to the thickness or thinness of surface across which diffusion occurs. Rate of diffusion is faster when the distance is small i.e, thin surface

·         Surface Area to Volume Ratio: The larger the surface area to volume ratio, the faster the rate of diffusion.

 

Role of Diffusion in Living Organisms

Some processes that depend on diffusion include the following:

• Gaseous exchange: Movement of gases through respiratory surfaces is by diffusion

• Absorption of materials into cells: Cells obtain raw materials and nutrients from the surrounding tissue fluid and blood through diffusion, e.g, glucose needed for respiration diffuses from blood and tissue fluid into cells

• Excretion: Removal of metabolic waste products like carbon (IV) oxide, and ammonia out of cells is by diffusion

• Absorption of the end-products of digestion from the intestines is by diffusion

 

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

• Osmosis is a special type of diffusion that involves the movement of water molecules only and not solute molecules

• Osmosis takes place in cells across the cell membrane as well as across non-living membranes e.g cellophane or visking tubing which are also semi-permeable.

• It is purely a physical process

 

Factors Affecting Osmosis

·         Size of solute molecules : Osmosis' occurs only when solute molecules are too large to pass through a semi-permeable membrane

 

·         Concentration Gradient: Osmosis occurs when two solutions of unequal solute concentration are separated by a semi-permeable membrane

 

·         Temperature: High temperatures increase movement of water molecules hence influence osmosis.

 

·         Pressure: Increase in pressure affects movement of water molecules. As pressure increases inside a plant cell, osmosis decreases

 

Roles of Osmosis in Living Organisms

The following processes depend on osmosis in living organisms:

• Movement of water into cells from the surrounding tissue fluid and also from cell to cell

• Absorption of water from the soil and into the roots of plants

• Support in plants especially herbaceous ones, is provided by turgor pressure, which results from intake of water by osmosis

• Absorption of water from the alimentary canal in mammals

• Re-absorption of water in the kidney tubules

• Opening and closing stomata

• The medium (solution) surrounding cells or organisms is described by the terms hypotonic, hypertonic and isotonic

 

• A solution whose solute concentration is more than that of the cell sap is said to be hypertonic. A solution whose solute concentration is less than that of the cell sap is said to be hypotonic. A solution which has the same solute concentration as the cell sap is said to be isotonic

• The term osmotic pressure describes the tendency of the solution with a high solute concentration to draw water into itself when it is separated from distilled water or dilute solution by a semi-permeable membrane

 

Osmotic pressure

• When plant cells are placed in distilled water or in a hypotonic solution, the osmotic pressure in the cells is higher than the osmotic pressure of the medium. This causes the water to enter the cells by osmosis. The water collects in the vacuole which increases in size

• As a result the cytoplasm is pushed outwards and it in turn presses the cell membrane next to the cell wall. This builds up water pressure (hydrostatic pressure) inside the cell. When the cell is stretched to the maximum, the cell wall prevents further entry of water into the cell. Then the cell is said to be fully turgid. The hydrostatic pressure developed is known as turgor pressure

 

Plasmolysis

• When a plant cell is placed in a hypertonic medium, it loses water by osmosis. The osmotic pressure of the cell is lower than that of the medium. The vacuole decreases in size and the cytoplasm shrinks as a result of which the cell membrane loses contact with the cell wall. The cell becomes flaccid and the whole process is described as plasmolysis. Incipient plasmolysis is when a cell membrane just begins to lose contact with the cell wall.

 

Wilting

• The term wilting describes the drooping of leaves and stems of herbaceous plants after considerable amounts of water have been lost through transpiration. It is observed in hot dry afternoons or in dry weather

• This is when the amount of water lost through transpiration exceeds the amount absorbed through the roots. Individual cells lose turgor and become plasmolysed and the leaves and stems droop

 

Water Relations in Plants and Animals

Haemolysis

• Haemolysis is the bursting of cell membrane of red blood cells releasing their haemoglobin. It occurs when red blood cells are placed in distilled water or hypotonic solution

• This is because the cell membrane does not resist further entry of water by osmosis after maximum water intake

 

 

Crenation

• Takes place when red blood cells are placed in hypertonic solution. They lose water by osmosis, shrink and their shape gets distorted. Animal cells have mechanisms that regulate their salt water balance (osmoregulation) to prevent above processes that lead to death of cells

• An Amoeba placed in distilled water, i.e hypotonic solution, removes excess water using a contractile vacuole. The rate of formation of contractile vacuoles increases

 

Active Transport

• Active transport is the movement of solutes such as glucose, amino acids and mineral ions. From an area of their low concentration to an area of high concentration. It is movement against a concentration gradient and therefore energy is required

• As such it only takes place in living organisms. The energy needed comes from respiration. Certain proteins in the cell surface membrane responsible for this movement are referred to as carrier proteins or channel proteins

• The shape of each type of carrier protein is specific to the type of substances conveyed through it

• It has been shown that the substance fits into a particular slot on the protein molecule,

• As the protein changes from one form of shape to another the substance is moved across and energy is expended

 

Factors Affecting Active Transport

·         Availability of oxygen: Energy needed for active transport is provided through respiration

·         Temperature: Optimum temperature is required for respiration, hence for active transport

·         Availability of carbohydrates: carbohydrates are the main substrates for respiration

·         Metabolic poisons: Metabolic poisons e.g. cyanide inhibit respiration and stops active transport due to lack of energy

 

Role of Active Transport in Living Organisms

Processes requiring active transport:

• Absorption of mineral salts from the soil into plant roots

• Absorption of end products of digestion e.g. glucose and amino acids from the digestive tract into blood stream

• Excretion of metabolic products e.g. urea from the cells

• Re-absorption of useful substances and mineral salts back into blood capillaries from the kidney tubules

• Sodium-pump mechanism in nerve cells

• Re-absorption of useful materials from tissue fluid into the blood stream

 

 Study Questions

Look for the cell structure viewed on a light and electron microscope for both animal and plant cells