Lesson (4) Cell processes

Cells perform many vital processes such as respiration, movement, nutrition, reproduction…etc. In this lesson, we'll study some of these cellular processes

 Cells get substances from the surrounding medium, and get rid of wastes by expelling them to the surrounding medium.

 Cell membrane has a property called selective permeability. This means that some substances (such as water, oxygen, carbon dioxide) can pass through it, while other substances (such as big molecules and salts) cannot do so. This depends on many factors such as:-

1- Size of particles

2- Charge of particles

3- The difference of concentration between the two sides of cell membrane

Substances are transported to and from cells by the following methods:-

1- Passive transport

Passive transport: The movement of substances through cell membrane where cell doesn't consume any energy.

Passive transport has the following methods:-

1- Diffusion

Diffusion: The movement of molecules through membrane from a region of high concentration to a region of low concentration, which makes the concentration of molecules on both sides equal.

E xample:-

 The exchange of carbon dioxide and oxygen

gases between the inner and outer mediums

of the cell during respiration process.

 If the concentration of molecules in cell is

greater that that of the external medium,

molecules move from the cell to the external

medium to balance the concentration of both sides

Fig. (35) diffusion process

2- Osmosis

Osmosis: The movement of pure water molecules through a membrane from a region of low concentration to a region of high one, which makes the concentration of both sides equal.

 Water transports from and to cell due to the difference between the concentration of cytoplasm (fluid inside the cell) and the external medium.

Fig. (36) Red blood cells and osmotic pressure

1- When we put cells in a solution of low concentration, water transports from the external medium (low concentration) into the cell (high concentration), which tears up cells.

2- When we put cells in a solution of equal concentration, water transports equally between the two mediums, and cells sizes don't change

3- When we put cells in a solution of higher concentration, water transports from the cell (low concentration) to the external medium (high concentration), which makes cells shrink.

3- Facilitated transport

Facilitated transport: The transport of molecules through a membrane by means of carrier protein (which carries molecules), where the cell doesn't consume any energy.

Example:-

Glucose is being transported to cells carried by carrier proteins.

Fig. (37) How facilitated transport occurs.

2- Active transport

Active transport: The transport of big molecules and ions through cell membrane against their concentration gradient (from low to high concentration) using energy

Active transport balance the concentration of ions inside cells

Active transport and nerve cells

Active transport allows nerve cells control the concentration of sodium and potassium ions in them, which allows sending nerve impulses to muscle cells.

Active transport and plant cells

It allows root cells absorb the ions of soil salts (though the concentration of ions in these cells is lower than that of soil)

3- Bulk movement

Bulk movement: Transporting relatively big molecules (such as wastes and protein particles) through cell membranes.

There are two kinds of bulk movements, which are:-

1- Exocytosis

Exocytosis: Transporting substances out of the cell through plasma membrane

How do Exocytosis take place:-

1- Golgi bodies store wastes in its vacuoles (Golgi vacuoles)

2- They move through cytoplasm to plasma membrane to fuse with it

3- They empty these wastes outside the cell.

Fig. (38) Golgi body and Exocytosis

2- Endocytosis

Endocytosis: Transporting substances to the cell through plasma membrane

How does Endocytosis take place

1- A part of plasma membrane bends to surround the particle forming a sac.

2- The sac containing the particle moves to cytoplasm.

 If the solid substances enter the cell by means of endocytosis process, we call it phagocytosis process

 If liquid substances enter the cell by means of Endocytosis process, we call it pinocytosis process.

Photosynthesis process: A process by which green plant cells get energy required for the plant to perform its vital processes

Photosynthesis process takes place inside the chloroplasts of plant cells, where they change light energy to chemical energy. Photosynthesis process occur in two stages:-

1- First stage

1- Granum inside chloroplasts catch energy from sunlight by chlorophyll pigment

2- The plant use a part of energy to split water molecules to hydrogen and oxygen gases.

3- The leaves of plants release oxygen gas

4- Stroma in chloroplasts store hydrogen gas

5- The rest of the energy is stored in the form of ATP compound

2- Second stage

It occurs in Stroma, where the energy stored in ATP is used to bind carbon dioxide gas and hydrogen gas together forming glucose sugar.

The equation of photosynthesis process:-

Light energy

Carbon dioxide gas + water ------------ Glucose + oxygen

Chlorophyll

Fig. (39) Photosynthesis process

We can summarize photosynthesis process in the following points:-

1- Plants use light energy of sun to perform photosynthesis process

2- Chloroplasts of plant cells absorb light energy

3- A part of energy is used to split water molecules into hydrogen and oxygen gases

4- Oxygen gas gets released from leaves, while hydrogen stays inside chloroplasts

5- The rest of energy is used to bind carbon dioxide gas with hydrogen forming glucose sugar.

We studied in chapter (1) that energy is stored in glucose sugar. In order to release energy from it, cells break it into simpler molecules by oxidizing it.

Cell respiration: Group of chemical reactions which occur inside cells to glucose in order to release the energy stored in it.

Cell respiration occurs in two stages:-

First stage: it occurs in cytoplasm releasing a small amount of energy.

Second stage: It occurs in mitochondria releasing a great amount of energy

How cell respiration occurs:-

1- Glucose molecules break into carbon dioxide and water molecules

2- Energy is stored in mitochondria in the form of ATP compound.

(N.B: Cell respiration is a catabolic process - involves catabolism- because glucose molecules are broken into simpler ones to release energy)

Fig. (40) Cell respiration (in mitochondria)

1- Stem cells

Stem cells: They are cells which have the ability to form any kind of specialized cells (muscle cells, liver cells, nerve cells…etc) according to certain conditions in laboratories.

Scientists discovered that these cells are formed during the early stages of fetus.