2 Movement of substances (1)

Chapter 1: Cell Structure and Organisation

1.1 What Are Cells?

• Learning Outcome(s):
    - Identify and state the functions of the following cell structures (including organelles) of typical plant and animal cells:
      - cell wall
      - cell membrane
      - cytoplasm
      - nucleus
      - cell vacuoles (large in plant cells; small in animal cells)
      - chloroplasts

Definition and Importance of Cells

• Cells: Basic building blocks of life.
    - Function as chemical factories carrying out necessary processes to maintain life.
    - Cells take in raw materials, which are transformed through chemical reactions into new molecules for various functions.

Historical Context

• Robert Hooke: Coined the term 'cells' in 1667 after examining tree bark slices with an early microscope.
    - Observed small, box-like structures resembling prison cells.

Methods of Studying Cells

• Most cells are microscopic and require specialized microscopes for observation:
    - Light microscopes: Magnification of about 1000x.
    - Electron microscopes: Magnification of up to 200,000x.
    - Some microscopes can be outfitted with cameras, providing images called micrographs.
• Different orientations of cutting cells may reveal distinct structures and functions.

Composition of a Cell

• Cells consist of protoplasm (Greek: protos = first; plasm = form), made up of three parts:
    - Cell membrane: Surrounds cytoplasm.
    - Cytoplasm: Site of cellular activities containing organelles.
    - Nucleus: Control center of the cell.

Key Cell Structures and Their Functions

• Cell Membrane:
    - Consists of lipids (lipid bilayer) and proteins.
    - Function: Partially permeable; allows only certain small and soluble substances to pass through.

• Cell Wall (Plant Cells Only):
    - Composed of cellulose.
    - Function: Fully permeable; provides protection against injury and maintains fixed shape.

• Cytoplasm:
    - Located between the cell membrane and nucleus.
    - Contains organelles; site of metabolic activities.

• Nucleus:
    - Function: Controls cellular activities and is essential for cell division. Cells lacking a nucleus (e.g., red blood cells) cannot divide.
    - Contains chromosomes which are structures made of DNA carrying hereditary information.
    - Nuclear Membrane: Separates nuclear contents from cytoplasm.

Organelles in the Cytoplasm

• Endoplasmic Reticulum (ER):
    - Rough ER (RER): Studded with ribosomes; synthesizes and transports proteins to the Golgi apparatus.
    - Smooth ER (SER): Tubular, synthesizes fats/steroids, and is involved in detoxification.

• Ribosomes:
    - Small structures, either attached to RER or free in cytoplasm; responsible for protein synthesis.

• Golgi Body (Golgi Apparatus):
    - Composed of flattened membranous spaces.
    - Function: Chemically modifies, stores, and packages substances from the ER for secretion.
      1. Vesicles pinch off from the ER and fuse with the Golgi, releasing substances.
      2. Modified substances are packaged in secretory vesicles which then move to the cell membrane.

• Mitochondria:
    - Oval or sausage-shaped organelles.
    - Function: Site of aerobic respiration, oxidizing food to release energy for cellular activities.

• Chloroplasts (Plant Cells Only):
    - Oval structures containing chlorophyll, necessary for photosynthesis.

• Vacuoles:
    - Fluids enclosed by membranes.
    - Large central vacuole in plant cells containing nutrients; small vacuoles in animal cells for temporary storage of food and water.

Differences Between Plant and Animal Cells

• Plant Cell:
    - Have a cell wall, large central vacuole, and chloroplasts.

• Animal Cell:
    - Do not have a cell wall, possess numerous small vacuoles, and lack chloroplasts.

1.2 How Are Cells Adapted to Their Functions?

• Learning Outcome(s):
    - Explain how the structures of specialized cells are adapted to their functions (e.g., energy provision in muscle cells, absorption in root hair cells, oxygen transport in red blood cells).

Specialisation and Differentiation

• Differentiation: The process by which cells become specialized for specific functions.
    - Examples include muscle cells, nerve cells, intestinal cells, red blood cells, and root hair cells.

Adaptations of Specialized Cells

• Red Blood Cells:
    - Adaptations: Contain hemoglobin for oxygen binding.
    - Shape: Biconcave, increases surface area for oxygen uptake, lacks a nucleus allowing more hemoglobin presence and flexibility for capillary travel.

• Muscle Cells:
    - Adaptations: Numerous mitochondria for energy supply during contraction.
    - Contains contractile proteins and multiple nuclei for division purposes.

• Root Hair Cells:
    - Adaptations: Long extensions increase surface area for water and nutrient absorption.
    - Maintains a lower water potential to facilitate water uptake via osmosis.

Stem Cells

• Stem Cells:
    - Possess the potential to differentiate into various specialized cells.
    - Scientists can cultivate these cells in labs for potential treatment of previously deemed untreatable diseases.

Chapter 2: Movement of Substances

2.1 What Is Diffusion?

• Learning Outcome(s):
    - Define diffusion and describe its role in nutrient uptake and gas exchange.

Definition of Diffusion

• Diffusion: The net movement of particles (atoms, molecules, or ions) from an area of higher concentration to an area of lower concentration (down a concentration gradient) until equilibrium is achieved.

Factors Affecting Diffusion

• Factors that influence the rate of diffusion include:
    - Concentration gradient: Steeper gradients lead to faster diffusion.
    - Diffusion distance: Shorter distances yield quicker rates.
    - Surface area-to-volume ratio: Larger surfaces accelerate absorption.

Example Scenarios of Diffusion

• The spreading of perfume in a room is a practical example of diffusion as gas particles move from concentrated sources to broader areas.

2.2 What Is Osmosis?

• Learning Outcome(s):
    - Define osmosis, investigate its effects, and discuss factors affecting its rate.

Definition of Osmosis

• Osmosis: The net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.

Relation to Diffusion

• Osmosis is a particular form of diffusion specific to water and involves movement across a selectively permeable barrier.

Water Potential

• Water Potential: Refers to the tendency of water to move through a solution.
    - Dilute solutions possess higher water potential than concentrated solutions.

Effects of Osmosis on Cells

• Plant cells in solutions with higher water potential will absorb water, become turgid due to their cell wall, preventing bursting.
• Animal cells in similar situations may swell and risk bursting due to lack of cell wall support.
• Conversely, in lower water potential solutions, plant cells become flaccid and animal cells may undergo crenation (shrinking).

2.3 What Is Active Transport?

• Active transport is a process through which cells move substances against their concentration gradient, necessitating energy expenditure (often from ATP).

Summary of Key Concepts

• Osmosis versus Diffusion:
    - Unique to water movement; across a semi-permeable membrane.
    - Diffusion can occur across barriers without restrictions.

Chapter 3: Biological Molecules

3.1 What Are Carbohydrates?

• Learning Outcome(s):
    - Identify the elements and main functions of carbohydrates, and perform tests for starch and reducing sugars.

Composition of Carbohydrates

• Carbohydrates consist of carbon, hydrogen, and oxygen with a hydrogens to oxygen ratio of 2:1.