B2 - Membranes and Compartmentalization

Membrane Permeability

Membranes function as barriers between aqueous solutions so large molecules and hydrophilic particles do not pass easily.

Diffusion

Net movement of particles from a region of higher concentration to a region of lower concentration. It is passive and only in liquids and gases.

Simple Diffusion Across cell Membranes

• Small non-polar molecules diffuse easily

• Rate of diffusion depends on the concentration. Higher concentration on one side = higher diffusion

• If the concentration is equal particles will move in both directions evenly

• Oxygen and Carbon are able to enter and leave cells

Integral proteins

Hydrophobic surface so are embedded in the bilayer. Includes transmembrane proteins which stretch from one side to the other.

Peripheral Proteins

Attached to the surface of the proteins or lipids of the membrane on just one sides

Facilitated diffusion through channel proteins

Include a pore allowing particles to pass through the membrane in either direction. Specific to one type of substance decided by size and charge or the pore. Passive movement, no ATP used.

Osmosis

The passive movement of water molecules from a region of lower solute concentration to higher solute concentration across a permeable membrane. Aquaporins increase permeability by allowing the movement of water molecules.

Active Transport

The movement of substances across the membranes using energy from ATP. Against the concentration gradient.

Pump Proteins

They carry out active transport. Work in one direction. Lower solute to higher solute,

Semi-Permeability/Partial Permeability

Allowing some substances through but not others.

Facilitated diffusion and active transport allows for control over particles. Net movement is always down the concentration gradient

Active transport can generate concentration gradients

Gycoproteins and Glycolipids

Polypeptide with carbohydrate attached.

Lipid with carbohydrate attached

Roles of Gycoproteins and Glycolipids

• Cell adhesion: creating a carbohydrate rich layer on outer face of animal cells called glycocalyx. Can be fused together

• Cell recognition: The differences in types allow cells to recognize other cells. Part of the immune system to distinguish between self and non-self cells

Fluid Mosaic Model

Advantages of Compartmentalization

• Small volume of organelles allows enzymes and their substrates to be concentrated, speeding up enzyme activity

• pH can be kept at an ideal level

• Biochemical processes can be kept separated

Fertilization

The fusion of a male and female gamete to produce a zygote (single cell). In multicellular organisms this cel divides repeatedly to form an embryo. Early stage embryos are unspecialized.

Differentiation

The specialization of cells as they develop along different pathways

Morphogens

Signalling chemicals that indicate to a cell its position in the embryo and which pathway of differentiation it should follow. They are regulators of gene expression, determining which genes are transcribed.

Properties of Stem Cells

• Self-replicating - can divide endlessly

• Undifferentiated - has not developed specialized features so has the capacity to differentiate to different pathways

Types of Stem Cells

• Totipotent: early stage, can differentiate into any cell type. Stem cell therapies

• Pluripotent: Able to differentiate into many but not all cell types

• Multipotent: found in bone marrow, differentiate into several cell types

Stem Cell Niches

The precise location of stem cells within a tissue with the proper microenvironment or conditions to stay undifferentiated for a long period or proliferate rapidly.

Bone Marrow - Stem Cell Niche

• Soft, spongy tissue in large bones

• Contains haematopoietic stem cells that produce red and white blood cells and platelets

• Generous supply of blood carrying oxygen, amino acids, and nutrients through cardiac output

Hair Follicles - Stem Cell Niche

Stem Cells at the base of each hair divide repeatedly for hair growth. Blood capillaries supply nutrients

Specialization - Cell Size

Most efficient size for cell function:

Male Gametes - not wide but long for swimming to egg

Red Blood Cells - Small size with large surface-to-volume ratio for oxygen

White Blood Cells - Can grow to produce antibodies in bulk

Female Gametes - Very large volume of cytoplasm containing food to sustain embryo

Neurons - cell body + axon < 1m long! Allows for the carrying of signals

Striated Muscle Fibres - large cells that can exceed 10cm, large and powerful muscle contractions

Surface Area-to-Volume Ratio

• Decreases with size of cell

• Rate of materials entering or leaving the cell depends on surface area

• Rate of materials used or produced depends on volume

• Creates limits on max size of cells before division