Introduction to Cell Biology

Cell Biology Introduction

• Cell biology studies the structure and function of cells.
• Cells are the basic building blocks of all living things.
• Robert Hooke, a British scientist, discovered cells in 1665.
• C. Nageli and C. Cramer coined the term cell membrane in 1855.
• Minerals, water, and nutrients from our diet are absorbed by cells and provide energy.

Cell Membrane

• Factors affecting substance movement across the cell membrane:
  • Concentration gradient
  • Temperature
  • Energy (ATP)
  • Molecule size
  • Membrane surface area
  • Distance the substance travels
  • Stirring
• Cell membranes, also known as plasma membranes, surround the inner contents of cells.
• Organelles like mitochondria, endoplasmic reticulum, and the nucleus are bound by membranes.
• Membranes are composed of phospholipids and proteins.
• Lipids form a mobile phospholipid bilayer consisting of a hydrophobic tail (repelled by water) and a hydrophilic head (attracted to water).
• The cell surface membrane has a fluid mosaic structure.
• Lipids and proteins move within the membrane.
• Lipids move more rapidly than proteins.
• The mosaic refers to the scattered pattern of proteins.

Significance of Fluid Mosaic Structure

• Lipids facilitate the passage of small molecules like water.
• Membrane folding can increase the surface area of cells or organelles.
• Membranes can recover from minor physical damage due to fluidity.
• Membranes are easily damaged by heat, acids, and fat solvents due to their protein and lipid composition.

Functions of the Cell Membrane

• Acts as a physical barrier between the external environment, cytoplasm, and organelles.
• Selectively permeable, regulates molecule movement in and out of the cell.
• Facilitates communication and signaling between cells.
• Anchors the cytoskeleton, providing shape and structure to the cell.
• Transports materials across the membrane, using chemical energy in the form of ATP
• Forms pores to allow substances to pass

Functions of Membrane Proteins

• Enzymes are present for chemical reactions on the membrane surface, aiding protein synthesis, respiration, and photosynthesis.
• Receptor sites exist for hormones to influence cell activity.
• Proteins act as a skeleton, providing shape and enabling movement like phagocytosis.
• Proteins actively transport materials using ATP.

Movement of Substances Across the Cell Membrane

• Water, respiratory gases (O2 and CO2), nutrients, essential mineral ions, and excretory products move through the cell membrane.
• Movement occurs through osmosis, diffusion, plasmolysis, exocytosis, and endocytosis.

Diffusion

• Atoms and molecules of gases and liquids move randomly from high to low concentration areas, until evenly distributed.
• Concentration gradient is the difference in concentration between two regions.
• Diffusion rate is faster with a steeper concentration gradient.
• Higher temperatures increase the rate of diffusion because the medium gains kinetic energy.
• Smaller particle size allows for faster movement and diffusion.
• Thicker surface membranes slow the rate of diffusion.
• Stirring increases molecular collisions, speeding up diffusion.
• Larger surface areas increase the rate of diffusion.
• Shorter distances allow for faster diffusion.

Examples of Diffusion in Animals

• Gaseous exchange in the lungs involves the movement of oxygen and carbon dioxide between the lungs and blood.
• Absorption of food nutrients occurs in the villi/small intestine into the blood stream.
• Gaseous exchange in gills involves movement of oxygen and carbon dioxide between water and gill filaments.
• Oxygen moves from plant leaves into the atmosphere, and carbon dioxide moves from the atmosphere into leaves during photosynthesis.
• Mineral salts absorb from the soil into the hair of plant roots.
• Hormones move out of the endocrine glands into the blood

Osmosis

• Osmosis involves water molecule movement across a membrane from a weak solution (high water concentration) to a strong solution (lower water concentration).
• A weak solution has a high proportion of water molecules and is hypotonic.
• A strong solution has a low proportion of water molecules and is hypertonic.
• Solutions of the same strength are isotonic.
• Higher temperatures increase the rate of osmosis.
• Steeper concentration gradients increase the rate of osmosis.
• Larger and more permeable membranes increase the rate of water movement.

Applications of Osmosis in Living Things

a) Water is absorbed from the soil by root hairs. b) Water moves across cell membranes in leaves. c) Water moves from root hairs into the cortex of a plant. d) Water enters the Amoeba. e) Water is absorbed in the colon of mammals. f) Water is reabsorbed in the nephron of the kidney.

Plasmolysis

• Plant cells have strong cellulose walls that let substances pass through.
• Plant cells placed in salty water lose water, causing cytoplasm and vacuoles to shrink and cell membrane to pull away from the wall.

Active Transport

• Substances move against their natural flow, i.e., against the concentration gradient.
• This process requires energy supplied by the cell..
• Special proteins in cell membranes act as carriers using ATP.
• Involves the movement of ions like Na+, Cl-, K+.
• Affected by oxygen and temperature.

Examples of Active Transport

• Sugar and amino acids absorb into the bloodstream from the small intestine.
• Mineral salts absorb from the soil by root hairs in plants.

Endocytosis

• Transportation of large molecules into cells via vesicle formation.
  • Phagocytosis (cell eating); The cell takes in large solid materials by engulfing it.
  • Pinocytosis (cell drinking); The cell takes in liquid form of substance by forming vesicle.

Exocytosis

• Transportation of large molecules out of cells by forming vesicle.
• Vesicles form inside the cell, releasing contents to the exterior.
  • Removal of materials from the cytoplasm to build the cell wall.
  • Secretion of enzymes